Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
DSpace
UBCV
Colombo, Manuel
Doctor of Philosophy - PhD
University of British Columbia
This PhD thesis describes some of the first distributions of lead, iron and manganese in the Canadian Arctic Ocean and other multiple trace metals in remote rivers in the Canadian Arctic Archipelago (CAA), unraveling the processes which modulate their distributions. A clear spatial variability in the distributions of dissolved aluminum, iron, manganese, nickel, copper, zinc, cadmium and lead was observed in small CAA rivers, which reflects differences in bedrock geology and South-North environmental gradients (e.g., solar radiation, precipitation, temperature) in the Archipelago. The concentrations of many trace metals from these small CAA rivers are considerably higher than those measured in Canada Basin waters, and therefore they have the potential to be a significant source of metals to transiting waters to adjacent coastal areas. Dissolved lead concentrations in the Canadian Arctic Ocean were among the lowest reported concentrations in ocean waters, attributed to the isolation of this region from anthropogenic inputs. Nonetheless, distinctive high and anthropogenically derived lead features, advected from the northeast Subarctic Pacific and the Labrador Sea, were observed in this study. The natural cycling of lead in the Canadian Arctic Ocean is primarily controlled by an interaction of glacial and sea ice melt water inputs, and scavenging throughout the water column and especially at the sediment water interface. Contrary to lead distributions, dissolved iron and manganese (DFe and DMn) concentrations measured in the Canadian Arctic Ocean were within the range of those reported for major basins. In our study the highest DFe and DMn concentrations were generally found in surface and subsurface waters. This is attributed to large riverine inputs present in this region and to continental shelf sediment fluxes. A dynamic interplay between removal processes and inputs of these trace metals largely control their vertical distribution below 300 m. Advective sources are predominant in the low productivity Canada Basin, contrasting with the important role of regenerated DFe and DMn in the productive Baffin Bay and Labrador Sea. The findings presented in this thesis shed light on important biogeochemical processes and water masses circulation taking place in this unique and underexplored environment.
ON THE BIOGEOCHEMICAL PROCESSES CONTROLLING TRACE METAL DISTRIBUTIONS IN THE CANADIAN ARCTIC OCEAN AND ARCTIC RIVERS by Manuel Colombo B.Sc., National University of La Plata, 2012 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Oceanography) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) November 2019 © Manuel Colombo, 2019 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: On the Biogeochemical processes Controlling Trace Metal Distributions in the Canadian Arctic Ocean and Arctic Rivers submitted by Manuel Colombo in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Oceanography Examining Committee: Kristin Orians Supervisor Susan Allen Supervisory Committee Member Maria T. Maldonado Supervisory Committee Member Madjid Mohseni University Examiner Ulrich Mayer University Examiner Additional Supervisory Committee Members: Roger Francois Supervisory Committee Member iii Abstract This PhD thesis describes some of the first distributions of lead, iron and manganese in the Canadian Arctic Ocean and other multiple trace metals in remote rivers in the Canadian Arctic Archipelago (CAA), unraveling the processes which modulate their distributions. A clear spatial variability in the distributions of dissolved aluminum, iron, manganese, nickel, copper, zinc, cadmium and lead was observed in small CAA rivers, which reflects differences in bedrock geology and South-North environmental gradients (e.g., solar radiation, precipitation, temperature) in the Archipelago. The concentrations of many trace metals from these small CAA rivers are considerably higher than those measured in Canada Basin waters, and therefore they have the potential to be a significant source of metals to transiting waters to adjacent coastal areas. Dissolved lead concentrations in the Canadian Arctic Ocean were among the lowest reported concentrations in ocean waters, attributed to the isolation of this region from anthropogenic inputs. Nonetheless, distinctive high and anthropogenically derived lead features, advected from the northeast Subarctic Pacific and the Labrador Sea, were observed in this study. The natural cycling of lead in the Canadian Arctic Ocean is primarily controlled by an interaction of glacial and sea ice melt water inputs, and scavenging throughout the water column and especially at the sediment water interface. Contrary to lead distributions, dissolved iron and manganese (DFe and DMn) concentrations measured in the Canadian Arctic Ocean were within the range of those reported for major basins. In our study the highest DFe and DMn concentrations were generally found in surface and subsurface waters. This is attributed to large riverine inputs present in this region and to continental shelf sediment fluxes. A dynamic interplay between removal processes and inputs of these trace metals largely control their iv vertical distribution below 300 m. Advective sources are predominant in the low productivity Canada Basin, contrasting with the important role of regenerated DFe and DMn in the productive Baffin Bay and Labrador Sea. The findings presented in this thesis shed light on important biogeochemical processes and water masses circulation taking place in this unique and underexplored environment. v Lay Summary Trace metals are important constituent of fresh and seawater environments, acting as micronutrients for primary producers and tracers of key biogeochemical processes. In this thesis we present the concentrations of multiple trace metals in the Canadian Arctic Ocean and small rivers in the Canadian Arctic Archipelago, a region extremely sensitive to climate change. We also investigate the processes controlling the distributions of these trace metals, identifying freshwater, glacial and sediment inputs, as well as the degradation of organic matter as the main sources of trace metals, while they are principally removed from the aqueous phase by adsorptive/oxidative scavenging. vi Preface A version of Chapter 2 has been published: Colombo, Manuel, Brown, Kristina A., De Vera, Joan, Bergquist, Bridget A. and Orians, Kristin J., 2019. Trace Metal Geochemistry of Remote Rivers in the Canadian Arctic Archipelago. Chemical Geology, 525, 479-491. DOI: https://linkinghub.elsevier.com/retrieve/pii/S0009254119303742. Freshwater samples were collected by Kristina Brown and Roger Francois, and I performed the sample processing and analysis of particulate and dissolved trace metals. Lead isotopic analysis were carried out by Joan De Vera and Bridget Bergquist in the University of Toronto. I was responsible for writing the manuscript and producing the figures and data analysis. The manuscript was revised and edited by Kristina Brown, Bridget Bergquist and Kristin Orians. A version of Chapter 3 has been published: Colombo, Manuel, Rogalla, Birgit, Myers, Paul G., Allen, Susan. E. and Orians, Kristin J., 2019. Tracing Dissolved Lead Sources in the Canadian Arctic: Insights from the Canadian GEOTRACES program. ACS Earth and Space Chemistry, 3 (7), 1302-1314. DOI: https://pubs.acs.org/doi/abs/10.1021/acsearthspacechem.9b00083. I collected, processed and analyzed all the lead samples. Birgit Rogalla, Susan Allen and Paul Myers developed and ran the Ariane Particle Track Simulations. I did all the analysis and synthesis of the data, and the figures (Ariane Particle Track simulation figures were done in collaboration with Birgit Rogalla). I wrote this manuscript with valuable feedback from the co-authors. A version of Chapter 4 was submitted to a journal, submitted October 14th: Colombo, Manuel, Sarah Jackson L., Cullen, Jay T. and Orians, Kristin J., 2019. Dissolved iron and manganese in the Canadian Arctic Ocean: on the biogeochemical processes controlling their vii distributions. I collected, processed and analyze the iron and manganese samples for the UBC group and Sarah collected and analyze the UVic samples. Results from both universities were combined in a single dataset. I performed the data analysis, produced the figures and wrote the manuscript in collaboration with Jay Cullen and Kristin Orians. I collected and analyzed the gallium data presented in Appendix D, which is going to be published soon in a peer-reviewed journal, and it will be co-authored by Colombo, Manuel and Orians, Kristin J. viii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables .............................................................................................................................. xiii List of Figures ............................................................................................................................. xvi List of Symbols ......................................................................................................................... xxix List of Abbreviations .................................................................................................................xxx Acknowledgements .................................................................................................................. xxxi Chapter 1: Introduction ................................................................................................................1 1.1 The role of trace elements in the ocean .......................................................................... 1 1.1.1 Bioactive elements and their role in phytoplankton physiology ................................. 2 1.1.2 Trace elements as tracers ............................................................................................ 4 1.2 Sources, sinks and internal cycling of trace metals ........................................................ 6 1.3 Area of study ................................................................................................................. 10 1.3.1 The Arctic environment ............................................................................................ 10 1.3.2 The Arctic Ocean ...................................................................................................... 11 1.3.2.1 Arctic gateways ................................................................................................. 12 ix 1.3.2.2 General circulation of the Arctic Ocean ........................................................... 17 1.3.3 The Arctic fragile environment in a climate change context .................................... 18 1.4 The Canadian Arctic GEOTRACES program .............................................................. 20 1.5 Thesis research goals .................................................................................................... 23 Chapter 2: Trace Metal Geochemistry of Remote Rivers in the Canadian Arctic Archipelago ...................................................................................................................................25 2.1 Summary ....................................................................................................................... 25 2.2 Introduction ................................................................................................................... 26 2.3 Methods......................................................................................................................... 28 2.3.1 Sampling ................................................................................................................... 28 2.3.2 Sample processing and analysis ................................................................................ 30 2.3.3 Calculations and statistical analysis .......................................................................... 34 2.4 Results ........................................................................................................................... 35 2.4.1 Trace metal distributions in CAA rivers ................................................................... 35 2.4.2 Particulate trace metals: enrichment factors and lead isotopic analysis ................... 40 2.5 Discussion ..................................................................................................................... 42 2.5.1 Spatial variability of dissolved trace metals in CAA rivers: linking trace metal distributions with environmental variables and bedrock geology ........................................ 42 2.5.2 Constraining dissolved trace metal concentrations from coastal draining rivers in the Canadian Arctic Archipelago ................................................................................................ 47 2.5.3 Particulate trace metals: sources, enrichment factors, and lead isotopic analysis .... 53 x 2.6 Conclusions ................................................................................................................... 58 Chapter 3: Tracing Dissolved Lead Sources in the Canadian Arctic: Insights from the Canadian GEOTRACES Program ............................................................................................59 3.1 Summary ....................................................................................................................... 59 3.2 Introduction ................................................................................................................... 60 3.3 Methods......................................................................................................................... 62 3.3.1 Study area.................................................................................................................. 62 3.3.2 Water sampling and analysis .................................................................................... 65 3.3.3 Ariane particle track simulations .............................................................................. 67 3.4 Results and discussion .................................................................................................. 68 3.4.1 Dissolved Pb in the Canada Basin: influence of shelf interactions and freshwater inputsâ¦ ................................................................................................................................. 69 3.4.1.1 Sea ice melt contributions to the polar mixed layer .......................................... 69 3.4.1.2 Influence of Pacific derived waters on the Pb signature in the Canada Basin .. 71 3.4.1.3 Enhanced Pb removal in near-bottom winter Bering Sea Water (wBSW) ....... 73 3.4.1.4 Lead distributions in Atlantic and Canada Basin deep waters .......................... 74 3.4.2 Tracing the far-reaching footprint of Pb in the Labrador Sea and Baffin Bay ......... 75 3.4.3 Lead distributions in the dynamic Canadian Arctic Archipelago ............................. 81 3.5 Conclusion .................................................................................................................... 85 Chapter 4: Dissolved Iron and Manganese in the Canadian Arctic Ocean: On the Biogeochemical Processes Controlling their Distributions ......................................................87 xi 4.1 Summary ....................................................................................................................... 87 4.2 Introduction ................................................................................................................... 88 4.3 Study area and hydrography ......................................................................................... 90 4.4 Methods......................................................................................................................... 94 4.4.1 Sample collection ...................................................................................................... 94 4.4.2 Trace metal analyses ................................................................................................. 95 4.4.2.1 Magnesium-induced coprecipitation and isotope dilution method (UBC) ....... 95 4.4.2.2 Offline extraction and triple quadrupole ICP-MS/MS method (UVic) ............ 96 4.4.2.3 Quality control and accuracy ............................................................................ 97 4.5 Results ........................................................................................................................... 97 4.5.1 Dissolved Fe and Mn in surface waters .................................................................. 100 4.5.2 Dissolved Fe and Mn in shelf seas and subsurface waters ..................................... 102 4.5.3 The contrasting biogeochemistry of DFe and DMn in deep waters ....................... 107 4.6 Discussion ................................................................................................................... 109 4.6.1 Dissolved Fe and Mn in surface waters: the role of fresh water inputs and phytoplankton uptake .......................................................................................................... 109 4.6.2 Dissolved Fe and Mn in shelf seas and subsurface waters: shelf-ocean interactions, scavenging removal and advective transport ...................................................................... 115 4.6.2.1 Influence of Chukchi Sea derived waters on the Fe and Mn signature in subsurface waters in the Canada Basin ........................................................................... 115 xii 4.6.2.2 Benthic sources of dissolved Fe and Mn in the shallow Canadian Arctic Archipelago environment................................................................................................ 117 4.6.2.3 The importance of CAA waters modulating subsurface DFe and DMn in Baffin Bay and the Labrador Sea ............................................................................................... 122 4.6.3 The contrasting biogeochemistry of DFe and DMn in deep waters ....................... 124 4.6.3.1 Dynamic interplay of DFe inputs (advection and remineralization) versus scavenging removal in deep waters ................................................................................ 124 4.6.3.2 DMn distributions in deep ocean waters dominated by scavenging ............... 131 4.6.4 Conclusions ............................................................................................................. 137 Chapter 5: Thesis Conclusion ...................................................................................................139 5.1 Trace metal geochemistry of remote rivers in the Canadian Arctic Archipelago ....... 139 5.2 Dissolved lead, iron and manganese in the Canadian Arctic Ocean........................... 141 5.3 Future work ................................................................................................................. 146 Bibliography ...............................................................................................................................148 Appendices ..................................................................................................................................173 Appendix A - supplementary material for chapter 2 .............................................................. 173 Appendix B - supplementary material for chapter 3 ............................................................... 179 Appendix C - supplementary material for chapter 4 ............................................................... 184 Appendix D - gallium distribution in the Canadian Arctic Ocean: results and preliminary figures ..................................................................................................................................... 194 xiii List of Tables Table 1.1 Sampling stations during the 2015 Arctic GEOTRACES cruise expedition ............... 22 Table 2.1 Dissolved and particulate trace metal concentrations (dTM and pTM) and lead isotopic ratios measured in CAA rivers. Duplicate samples, when available, are displayed for each river. dTM concentration are expressed in nM for Al, Fe, Mn, Ba, Ni, Cu and Zn, and pM for Cd, Pb and Ga. pTM concentrations are expressed as µg/L. .................................................................... 36 Table 2.2 Pearsonâs correlation matrix for dissolved trace metals and dissolved organic carbon (DOC) from small CAA rivers (excluding the Glacial River). DOC data is reported in Table A.1........................................................................................................................................................ 46 Table 2.3 Partition coefficients (Kd, μg/gμg/mL) of trace metals in CAA rivers. Kd values were calculated as the ratio of dissolved trace metal concentrations to particulate concentrations. Values are expressed as log10 of Kd. ............................................................................................. 47 Table A.1 Ancillary data from sampled CAA rivers, including location, salinity, suspended particulate matter (SPM), and dissolved organic carbon (DOC). ............................................... 173 Table A.2 Accuracy determined from the analyses of SLRS-5, SLRS-2, Sample Spike, and JSd-1 reference materials (AVG±1SD). ............................................................................................ 174 Table A.3 NIST 981 standards were measured in two analytical runs at two different concentrations of Pb (run 1 and run 2) and compared to published values. Standards 1A and 2A (NIST 981) are treated as external process standards and went through the sample chemical purification methods as samples. Standards 1B and 2B (NIST 981) are treated as instrumental standards and were analyzed without column purification. ........................................................ 174 xiv Table A.4 BCR-2 as external process standard measured at two different concentrations compared to published values. .................................................................................................... 175 Table A.5 Ranges and averages of dissolved and particulate trace metal concentrations (dTM and pTM) and annual mean discharge (km3/yr) for Arctic rivers, the Danube delta and the Amazon basin. dTM concentration are expressed in nM for Al, Fe, Mn, Ba, Ni, Cu and Zn, and pM for Cd, Pb and Ga. pTM concentrations are expressed as µg/L. .......................................... 176 Table A.6 Linear regression analysis results of dissolved trace metal concentrations and DOC versus flow for Kolyma, Mackenzie, Yenisey, Ob, and Lena rivers. When a significant regression is established, the difference in trace metal concentrations between low and high flow season is calculated and expressed as percentage of the low value (bolded numbers). α = Significance F, Dif. (%) = percentage difference between concentrations measured during low flow versus high flow.................................................................................................................. 177 Table B.1 Dissolved Pb concentrations (pmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th - October 1st 2015). Those samples suspected of contamination* are flagged (red bolded numbers) and not discussed in the manuscript ................................................................................................................................... 179 Table C.1 Accuracy determined from the analyses of SAFe D1, SAFe D2 and GSP S GEOTRACES reference materials (AVG±1SD). ....................................................................... 184 Table C.2 Dissolved Fe concentrations (nmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th-October 1st 2015). ..................................... 185 xv Table C.3 Dissolved Mn concentrations (nmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th-October 1st 2015). ..................................... 187 Table D.1 Dissolved Ga concentrations (pmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th - October 1st 2015). ................................. 194 xvi List of Figures Figure 1.1 Schematic showing the inflow of the two Atlantic-derived waters masses: the Fram Strait Branch (FSB; red) and the Barents Sea Branch (BSB; blue), and the Pacific inflow (green) into the Arctic Ocean (after Rudels et al., 2004). The formation/modification area of FSB and BSB are displayed with solid colors and the circulation with colored arrows. Main landmarks and straits are also indicated in the figure: Bering Strait (BS), Davis Strait (DS), Fram Strait (FS) and Barents Sea Opening (BSO)................................................................................................... 14 Figure 2.1 Rivers sampled during the Canadian Arctic GEOTRACES program in the summer of 2015 (August). Northern rivers are displayed with black diamonds, central rivers with black squares, and continental rivers with black circles. The names of the sampled rivers are presented in Table 2.1. A simplified bedrock geology map is superimposed in the Figure; geological data was modified from Okulitch (1991) and Nunavut Geoscience (2017). ........................................ 30 Figure 2.2 Dissolved trace metal concentrations measured in 14 CAA rivers; results are expressed on a logarithmic colorbar. ............................................................................................ 38 Figure 2.3 Particulate Ba/Al and Pb/Al ratios collected from CAA rivers (a and b). Dashed lines indicate the average crustal metal/Al ratio (Rudnick and Gao, 2013) and bolded numbers indicate those rivers which are located in close proximity to mining exploration projects and/or communities. c) Particulate 208Pb/206Pb versus 206Pb/207Pb composition from CAA rivers and isotopic signatures from selected sources (Godwin and Sinclair, 1982; Graney et al., 1995; Millen et al., 1995; Sangster et al., 2000; Simonetti et al., 2003; Lima et al., 2005). d) Pb/Al versus 206Pb/207Pb plot showing the relative enrichment and changing Pb isotopic ratios of CAA rivers. The names of the sampled rivers are presented in Table 2.1. ............................................ 42 xvii Figure 2.4 PCA loadings and score plots for dissolved trace metals. The bedrock geology of each river is superimposed in the figure, with the names of the sampled rivers as presented in Table 2.1. ...................................................................................................................................... 43 Figure 2.5 Upper Panel: average daily discharge of the Kolyma River in 2004, featuring the dates for which trace metal data is available (green dots), and examples of regression analysis of Cd, Fe, and Cu versus flow; data retrieved from Peterson et al. (2016) and Shiklomanov et al. (2018). Full results of the regression analysis are presented in Table A.6. Middle Panel: daily discharge of the Mecham, Ellice, and Tree rivers in 2015, featuring the dates when rivers where sampled; data retrieved from the Water Survey of Canada (2017). Lower Panel: ranges of dissolved trace metal concentrations measured for northern and continental CAA rivers and projected concentrations during the high flow season (dashed lines). .......................................... 50 Figure 2.6 Mining exploration projects carried out in Nunavut from 2011 to 2016 (Costello et al., 2011; Senkow et al., 2012; Senkow et al., 2013; Bigio et al., 2014; Bigio et al., 2015). a) northern and central Arctic rivers and b) continental Arctic rivers. The sampled rivers for trace metals are indicated by filled black diamonds, squares and circles; bolded numbers indicate those rivers which are located in close proximity to active mining exploration projects and/or communities. The names of the sampled rivers are presented in Table 2.1. ................................ 56 Figure 3.1 Sampled stations for lead during the Canadian Arctic GEOTRACES cruises (GN02 and GN03), bathymetry and schematic of Pacific derived water circulation in the Canada Basin (after Timmermans et al., 2014; Kondo et al., 2016) and Baffin Bay and the Labrador Sea (after Yashayaev and Clarke, 2008; Curry et al., 2011; Lozier et al., 2017). CB: Canada Basin, CAA: Canadian Arctic Archipelago, BB: Baffin Bay, LS: Labrador Sea, ACW: Alaskan Coastal Water, xviii sBSW: summer Bering Sea Water, wBSW: winter Bering Sea Water, AW: Arctic Water, EGC: East Greenland Current, WGC: West Greenland Current, WGIW: West Greenland Intermediate Water, BC: Baffin Current, LSW: Labrador Sea Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. For a detailed view of the Canadian Arctic Archipelago, including the main landmarks and straits, refer to Figure 3.6d. .............................. 64 Figure 3.2 Interpolated contour section of dissolved Pb along the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay. Sampled depths are superimposed on the plot (white circles) and the individual Pb profiles are displayed above the contour plot. For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. Stations sampled are displayed in the inset map and the beginning of the transect is indicated by the red star. Note the break axis in the contour map (white line at 1050 m) and the logarithmic scale for the individual profiles. Stations were chosen to capture the flow of Arctic waters from the Canada Basin to Baffin Bay. ............................................................... 69 Figure 3.3 a) Potential density (ÏÎ¸) versus Pb, Potential Temperature, Oxygen, Fluorescence, and Silicate and N* (calculated from the following equation: N*= ([NO3- + NO2- + NH4+] â 16PO43- + 2.9) x 0.87; Gruber and Sarmiento, 1997). Orange shaded bands indicate the isopycnal surface where Pb peaks (ÏÎ¸, 24.8-25.8) and the purple dashed line indicates the winter Bering Sea Water (wBSW). Silicate and N* data from the Canadian GEOTRACES cruise were provided by ArcticNet (Jean-Eric Tremblayâs group). For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. b) Potential temperature (θ) versus salinity (S) diagram, gray profiles represent type II waters and black profile type III waters (for details refer to text). ACW: Alaskan Coastal Water, sBSW: xix summer Bering Sea Water, AL: Atlantic Layer, CBDW: Canada Basin Deep Water. c) Stations sampled, bathymetry and schematic of Pacific derived water circulation in the Canada Basin (after Timmermans et al., 2014; Kondo et al., 2016) .................................................................... 71 Figure 3.4 a) Potential temperature (θ) versus salinity (S) diagram from Baffin Bay stations with Pb concentrations for the analyzed samples (color scale). Water masses in Baffin Bay were identified following Tang et al. (2004) and Curry et al. (2011). WGIW: West Greenland Intermediate Water, TrW: transitional water, AW: Arctic Water, BBDW: Baffin Bay Deep Water. b) Potential Density (ÏÎ¸) versus dissolved Pb in Baffin Bay. c) Sampled stations, bathymetry and schematic circulation (after Yashayaev and Clarke, 2008; Curry et al., 2011; Lozier et al., 2017) in Baffin Bay and the Labrador Sea. EGC: East Greenland Current, WGC: West Greenland Current, BC: Baffin Current, LSW: Labrador Sea Water, ISW: Icelandic Slope Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. d) Dissolved Pb, θ, S, and oxygen profiles in the Labrador Sea. The water masses were identified following Yashayaev and Loder (2009) and Yashayaev and Loder (2016). For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. .............................................................................................................. 78 Figure 3.5 Water mass back trajectories from station BB3 (upper panel) and BB1 (lower panel) from 2015 back to 2011 over two depth ranges. Tracks were grouped according to their origin and path; this breakdown is shown in proportions in the bar plots on the right. The groups are, Nares Strait: originates from the Arctic Ocean and passes through Nares Strait. Parry Chanel: originates from the Arctic Ocean and passes through Parry Chanel. Coastal West Greenland Current: originates from the WGC and travels into the CAA, where it mixes with Arctic waters. xx Baffin Bay interior: circulates in the Baffin Bay interior for the entire time period. West Greenland Current: travels from the Atlantic via the WGC around Baffin Bay. Directly from Atlantic: travels directly from the Atlantic to the station without circulating around Baffin Bay........................................................................................................................................................ 79 Figure 3.6 a) CTD temperature (θ) versus salinity (S) diagram for all the stations in the CAA. Those stations influenced by Baffin waters (Atlantic signature) are displayed with red colors, while the stations influenced by Arctic waters are displayed with blue colors. The θ/S data from the Canada Basin endmember (CB1) and Baffin Bay endmember (BB2) are shown with blue and red solid lines, respectively. b) Pb concentration (color scale), θ and S data measured at CAA7-CAA9, CAA4. c) Pb concentration (color scale), θ and S data measured at CAA7-CAA9, CAA4. Note that temperature/salinity axes vary between plots. d) Sampled stations and conceptual scheme of water masses and circulation pathways in the CAA (after Michel et al., 2006; Wang et al., 2012). The inset includes a Terra- MODIS visible image of eastern Devon Island taken on July 25, 2015 (https://worldview.earthdata.nasa.gov/), illustrating the presence of glacial runoff along the Devon Island coast. Light blue: Arctic-derived waters (low Pb), Red: Atlantic-derived water (high Pb), Orange: Glacially modified surface waters (high Pb surface waters) and Light orange: mixing between glacial runoff, Atlantic and Arctic waters. MS: MâClure Strait, VS: Viscount Melville Sound, BS: Barrow Strait, PS: Penny Strait, LS: Lancaster Sound, CI: Cornwallis Island, SI: Somerset Island, DI: Devon Island, BI: Bylot Island, EL: Ellesmere Island. Parry Channel is the main pathway in central CAA connecting MâClure Strait with Lancaster Sound. ........................................................................................................................................... 83 xxi Figure 4.1 Sampled stations for Fe and Mn during the Canadian Arctic GEOTRACES cruises (GN02 and GN03), bathymetry and schematic of water circulation in the Canada Basin (after Aksenov et al., 2011; Kondo et al., 2016) and Baffin Bay and the Labrador Sea (after Yashayaev and Clarke, 2008; Curry et al., 2011; Lozier et al., 2017). CB: Canada Basin, CAA: Canadian Arctic Archipelago, BB: Baffin Bay, LS: Labrador Sea, ACW: Alaskan Coastal Water, wBSW: winter Bering Sea Water, FSB: Fram Strait Branch, BSB: Barents Sea Branch, AW: Arctic Water, EGC: East Greenland Current, WGC: West Greenland Current, WGIW: West Greenland Intermediate Water, BC: Baffin Current, LSW: Labrador Sea Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. For a detailed view of the Canadian Arctic Archipelago, including the main landmarks, straits and water circulation, refer to Figure 4.8a. ............................................................................................................................... 92 Figure 4.2 Potential temperature (θ) versus salinity (S) diagram for the Canada Basin (a), the CAA (b), Baffin Bay (c) and the Labrador Sea (d). For the CAA (b), the θ/S data from the Canada Basin endmember (CB2) and Baffin Bay endmember (BB2) are shown with solid green lines. The main water masses are indicated in the figure: ACW: Alaskan Coastal Water, wBSW: winter Bering Sea Water, LH: Lower Halocline, AL: Atlantic Layer, FSB: Fram Strait Branch, BSB: Barents Sea Branch, CBDW: Canada Basin Deep Water, AW: Arctic Water, TrW: transitional water, WGIW: West Greenland Intermediate Water, BBDW: Baffin Bay Deep Water, LSW: Labrador Sea Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. .................................................................................................................. 94 Figure 4.3 Interpolated contour section of dissolved Fe (a) and Mn (b) along the Canada Basin, the Canadian Arctic Archipelago, Baffin Bay and the Labrador Sea. Sampled depths are xxii superimposed on the plot (white circles) and the individual profiles are displayed above each contour plot. Stations included are displayed in the inset map and the beginning of the transect is indicated by the red star. Note the break axis in the contour map (white line at 1050 m) and the logarithmic scale for the individual profiles. Stations were chosen to capture the flow of Arctic waters from the Canada Basin to the Labrador Sea. ..................................................................... 99 Figure 4.4 Interpolated contour section of dissolved Fe (a) and Mn (b) in the Canadian Arctic Archipelago across Parry Channel; sampled depths are superimposed in the plot (white circles) and the individual profiles are displayed above each contour plot. c) The stations sampled at the three transects across Parry Channel are displayed in the maps, the beginning of the transect is indicated by the red star (Southern side of Parry Channel). Note the logarithmic scale for the individual profiles. ...................................................................................................................... 100 Figure 4.5 Average surface (< 40 m) DFe and DMn concentrations (nmol kgË1) for the sampled stations, Mn data and axis are displayed in blue and Fe data and axis are displayed in red....... 102 Figure 4.6 Profiles of Fe, Mn, Si, N*, Temperature, Oxygen and Fluorescence versus density (ÏÎ¸) for the deep stations in the Canada Basin (upper panel: CB2-CB4), Baffin Bay (middle panel: BB1-BB3) and the Labrador Sea (lower panel: LS2 and K1). N* was calculated from the following equation: N*= ([NO3- + NO2- + NH4+] â 16PO43- + 2.9) x 0.87; Gruber and Sarmiento (1997). Silicate and N* data from the Canadian GEOTRACES cruise were provided by ArcticNet (Jean-Eric Tremblayâs group). Note the surface water concentration of DFe for the station CB4 in the Canada Basin was extremely high (3.42±0.19 nmol kg-1), and is not displayed in the Figure to emphasize deep water features. Important water masses were superimposed in the plots to facilitate the interpretations and discussion of the trace metal data following Aksenov xxiii et al. (2011), Kondo et al. (2016), Tang et al. (2004), Curry et al. (2011), Yashayaev and Loder (2009 and 2016). wBSW: winter Bering Sea Water, LH: Lower Halocline, FSB: Fram Strait Branch, BSB: Barents Sea Branch, CBDW: Canada Basin Deep Water, WGIW: West Greenland Intermediate Water, TrW: transitional water, AW: Arctic Water, BBDW: Baffin Bay Deep Water, LSW: Labrador Sea Water, ISW: Icelandic Slope Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. ............................................... 106 Figure 4.7 a) Dissolved Mn concentrations versus practical salinity in the upper 40 m for all the stations. Linear regression lines for the Canada Basin (CB2 to CB4), Baffin Bay and Labrador Sea surface waters (green) and CAA surface waters (blue) are superimposed in the figure. The 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500), are also superimposed in the figure. The density of overlapping lines indicates increased confidence. The inset displays the Fe versus salinity for the same stations. b) Dissolved Mn concentrations (upper 40 m) versus distance from southern Lancaster Sound (CAA4 and CAA3), Baffin Bay coast (BB3), the Davis Strait (BB1) to the Labrador Sea (LS2 and K1), tracing the evolution of DMn in the Arctic outflow from southern Lancaster Sound to the Labrador Sea. Stations included in this plot are displayed in the map and the beginning of the transect is indicated by the red star. The inset shows the monthly-averaged log 10 Chl a concentration in the Labrador Sea (May 2015; data retrieved from MODIS-Aqua sensor: https://oceancolor.gsfc.nasa.gov/l3/), illustrating the intense phytoplankton bloom that took place approximately two months before the sampling. Monthly-averages from 2015, including the entire study area, are presented in Figure C.4. ..... 112 Figure 4.8 a) Sampled stations and circulation pathways (blue arrows) in the CAA (after Michel et al., 2006; Wang et al., 2012); red and light red shaded area: conceptual scheme displaying xxiv intense diapycnal and tidal mixing in Barrow Strait area and eastern CAA (modified from Hannah et al., 2009; Hughes et al., 2017; Hughes et al., 2018). Dissolved Fe (red profiles) and Mn (blue profiles) concentrations (nmol kgË1) for each CAA station are superimposed in the Figure panel 4.8a; the gray shading indicates the bottom floor, the pale green shaded area indicates those samples below 130 m, and the number within a white circle is the distance (m) from the deepest sample to bottom sediments. MS: MâClure Strait, VS: Viscount Melville Sound, BS: Barrow Strait, PS: Penny Strait, LS: Lancaster Sound, BaI: Bathurst Island, CI: Cornwallis Island, SI: Somerset Island, PWI: Prince of Wales Island, DI: Devon Island, ByI: Bylot Island, EI: Ellesmere Island, BI: Baffin Island. Parry Channel is the main pathway in central CAA connecting MâClure Strait with Lancaster Sound. b) and c) Dissolved Fe (red symbols) and Mn (blue symbols) versus transmissivity for CAA samples below 130 m (values greater than 100% indicate offset in the calibration). Linear regression lines and R2 values are displayed in the figure, as well as the 95% prediction limits and the 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500). The density of overlapping lines indicates increased confidence. d) Natural logarithm of dissolved Fe concentrations in the Arctic Waters (AW; ÏÎ¸= 26.28-27.12) versus distance from southern Lancaster Sound (CAA4 and CAA3), Baffin Bay coast (BB3) to the Davis Strait (BB1), showing an exponential decrease in DFe concentrations (DFe= 1.732 e -0.0006 Distance or Ln [DFe]= -0.0006 Distance+0.549 when it is linearized). Stations included in this plot are displayed in the inset map and the beginning of the transect is indicated by the red star and the linear regression line, R2 value and the 95% prediction limits and the 95% confidence intervals are displayed in the figure. ........................ 120 xxv Figure 4.9 a) AOU and DFe relationship in deep waters (>300 m) in Baffin Bay and the Labrador Sea. Linear regression lines and R2 values are displayed in the figure, as well as the 95% prediction limits and the 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500). The density of overlapping lines indicates increased confidence. b) Estimated mean ages for Canada Basin waters from chlorofluorocarbon data (Tanhua et al., 2009); age profiles from the stations 30015 (Lat./Lon.: 75.3/-144.2; JOIS97 cruise) and 10008 (Lat./Lon.: 75.5/-150.2; Beringia cruise) were chosen due to their close proximity to our sampled stations. c) DMn versus water mass age, including data from Statham et al. (1998) and from Canada Basin Deep Waters (this study). Pseudo-first order scavenging model solutions, using different scavenging constants, are superimposed in the plots (for details refer to text); the best fit described by Statham et al. (1998) and chosen in this study are displayed with dashed and solid lines, respectively. Green shaded bands indicate background DMn concentrations (0.10-0.15 nmol kg-1) measured in deep ocean waters (for details refer to text). ....................... 128 Figure 5.1 Conceptual diagram displaying how differences in the bedrock geology and North-South environmental gradients influence trace metal distribution in small CAA rivers. A simplified bedrock geology map is superimposed in the Figure (for reference refer to Figure 2.1). The photos displayed in the Figure were taken during field sampling, and evidence the differences in environmental settings between northern and southern CAA rivers. .................. 140 Figure 5.2 Conceptual scheme displaying the concentrations and key processes controlling the distributions of dissolved Pb in the Canadian Arctic Ocean. CB: Canada Basin, CAA: Canadian Artic Archipelago, BB: Baffin Bay, LS: Labrador Sea, ACW: Alaska Coastal Water, wBSW: xxvi winter Bering Sea Water, ACBC: Arctic Circumpolar Boundary Current, AW: Arctic Water, WGIW: West Greenland Intermediate Water, DSOW: Denmark Strait Overflow Water. ........ 143 Figure 5.3 Conceptual scheme displaying the concentrations and key processes controlling the distributions of dissolved Fe and Mn in the Canadian Arctic Ocean. CB: Canada Basin, CAA: Canadian Artic Archipelago, BB: Baffin Bay, LS: Labrador Sea, wBSW: winter Bering Sea Water, ACBC: Arctic Circumpolar Boundary Current, AW: Arctic Water, DSOW: Denmark Strait Overflow Water. ................................................................................................................ 144 Figure A.1 Metal/Al ratios in suspended particulate matter collected from CAA rivers. Dashed lines indicate the average crustal metal/Al ratio (Rudnick and Gao, 2013) and bolded numbers indicate those rivers which are located in close proximity to active mining exploration projects and/or communities. The names of the sampled rivers are presented in Table 2.1. ................... 178 Figure B.1 Upper Panel: Full depth profiles from CB3, CAA2 and BB3 stations comparing results from the University of British Columbia (this work; UBC) and the University of Victoria* (UVic). Lower Panel: Intercalibration exercises among UBC, UVic, the University of Alaska Fairbanks (UAF), the Texas A&M University (TAMU) and the Massachusetts Institute of Technology (MIT). ..................................................................................................................... 181 Figure B.2 Water mass back trajectories from BB2 station and water mass proportions at two selected depth ranges. Water tracks were grouped according their origin: Nares Strait: originates from the Arctic Ocean and passes through Nares Strait. Parry Chanel: originates from the Arctic Ocean and passes through Perry Chanel. Coastal West Greenland Current: originates from the WGC and travels into the CAA mixing with Arctic waters. Baffin Bay interior: circulates in the xxvii Baffin Bay interior for the entire time period. West Greenland Current: travels from the Atlantic via the WGC around Baffin Bay. ................................................................................................ 182 Figure B.3 Interpolated contour section of dissolved Pb in the Canadian Arctic Archipelago along and across Parry Channel; sampled depths are superimposed in the plot (white circles) and the individual Pb profiles are displayed above the contour plot. For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. Stations sampled are displayed in the maps aside the contour plots, the beginning of the transect is indicated by the red star (Southern side of Parry Channel). Note the logarithmic scale for the individual profiles. .................................................................................................. 183 Figure C.1 Upper Panel: Full depth profiles of dissolved Fe from CB4, CAA2 and BB3 stations comparing results from the University of British Columbia (UBC) and the University of Victoria (UVic). Lower Panel: Intercalibration exercises among UBC and the Texas A&M University (TAMU). ..................................................................................................................................... 190 Figure C.2 Upper Panel: Full depth profiles of dissolved Mn from CB4, CAA2 and BB3 stations comparing results from the University of British Columbia (UBC) and the University of Victoria (UVic). Lower Panel: Intercalibration exercises among UBC and the Texas A&M University (TAMU). ..................................................................................................................................... 191 Figure C.3 Dissolved Fe and Mn concentrations (nmol kgË1) for the sampled stations, Mn (blue profiles) axis is displayed above each plot and Fe (red profiles) axis is displayed below each plot. Note the logarithmic depth scale................................................................................................. 192 Figure C.4 Monthly-averaged Chl a concentration from January to September 2015 in the study region (data retrieved from MODIS-Aqua sensor: https://oceancolor.gsfc.nasa.gov/l3/). ......... 192 xxviii Figure C.5 Dissolved Fe versus water mass age for Canada Basin Deep Waters (>1200 m). The regression line and R2 value is displayed in the figure, as well as the 95% prediction limits and the 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500). The density of overlapping lines indicates increased confidence. The correlation is significant at a p-value< 0.001. ........................................................................................................................... 193 Figure D.1 Dissolved Ga concentrations (pmol kgË1) for the sampled stations. Note the logarithmic depth scale. .............................................................................................................. 196 Figure D.2 Interpolated contour section of dissolved Ga along the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay. Sampled depths are superimposed on the plot (white circles) and the individual Ga profiles are displayed above the contour plot. Stations sampled are displayed in the inset map and the beginning of the transect is indicated by the red star. Note the break axis in the contour map and the logarithmic scale for the individual profiles. Stations were chosen to capture the flow of Arctic waters from the Canada Basin to Baffin Bay. .................. 197 Figure D.3 Interpolated contour section of dissolved Ga in the Canadian Arctic Archipelago along and across Parry Channel; sampled depths are superimposed in the plot (white circles) and the individual Ga profiles are displayed above the contour plot. Stations sampled are displayed in the maps aside the contour plots, the beginning of the transect is indicated by the red star (Southern side of Parry Channel). Note the logarithmic scale for the individual profiles.......... 198 xxix List of Symbols Al AluminumBa Barium Cd Cadmium Cu Cooper Fe Iron Ga Gallium H2O2 Hydrogen peroxide H2SO4 Sulfuric acid HCl Hydrochloric acid HNO3 Nitric acid Mn Manganese µL Microliter µmol Micromole NH+4 Ammonium Ni Nickel nmol Nanomole NO2- Nitrite NO3- Nitrate O2 Oxygen P Phosphorous Pb Lead pmol Picomole PO4 Phosphate Si(OH)4 Silicic acid Zn Zinc xxx List of Abbreviations ACBC Arctic Circumpolar Boundary Current ACW Alaskan Coastal Water BC Baffin Current BSB Barents Sea Branch BSO Barents Sea Opening CAA Canadian Arctic Archipelago DOC Dissolved organic carbon DSOW Denmark Strait Overflow Water EGC East Greenland Current FSB Fram Strait Branch HNLC High-nutrient, low-chlorophyll ICP-MS Inductively coupled plasma mass spectrometers LDPE Low density polyethylene LSW Labrador Sea Water NADPH Nicotinamide adenine dinucleotide phosphate NEADW Northeast Atlantic Deep Water PCA Principal component analysis PML Polar mixed layer POC Particulate organic carbon POM Particulate organic matter sBSW Summer Bering Sea Water SPM Suspended particulate matter wBSW Winter Bering Sea Water WGC West Greenland Current WGIW West Greenland Intermediate Water WSC West Spitsbergen Current xxxi Acknowledgements These last years have been one of the most intense and rewarding of my life, both academically and personally. My Ph.D. journey would have not been the same without the support from many people to whom I will always be thankful. First of all, I want to greatly thank my supervisor Kristin Orians for her guidance and support throughout my Ph.D. Working under her supervision was a continuous pleasure; I truly appreciate the degree of autonomy she granted me and her confidence in my work. I have never imagined learning as much about trace metal biogeochemistry in the Arctic environment as I did in my Ph.D., without her support this would have not been possible. Beyond her academic guidance, I will be always in debt for her generosity and for opening the doors of her house and family, making me feel at home in Vancouver. In the same line, and after numerous delicious dinners, I can state that Kirk is the best chef in Kitsilano area (p-value< 0.00001 ï) and both are lovely hosts. I also want to extend my gratitude to my committee members: Maite Maldonado, Susan Allen and Roger Francois; my thesis would definitely not be the same without your advices and comments. Thanks for your support and time! Furthermore, I would like to thank to Matty Jeronimo, being a research assistant under his supervision in the Occupational and Environmental Health Laboratory during all these years has been an incredibly fun and rewarding experience, learning a lot about analytical chemistry, troubleshooting of instruments, and the analysis of many different environmental samples. Thanks for involving me in interesting projects and for being a key support before I started my Ph.D. Another important mentor in my academic life is Eric Speranza, to whom I thank for being the first one to teach me how to âdo scienceâ, work in the lab, and for guiding me during my first steps in scientific writing. It is always a pleasure to brainstorm about science with you! Many thanks to Jay Cullen, Philippe Tortell, Maureen Soon, Kristina Brown, Vivian Lai, David Semeniuk, Dave Janssen, May Ver, Lucy Porritt and Michael Lipsen for helping me to become an oceanographer and geochemist and supporting me as a TA. I learnt a lot from you all from data analysis to ocean sampling! I will always be in debt with my family; thanks Mom, Dad and Sofi!! I could not have done anything without you. It is much easier to undertake challenging endeavors knowing you xxxii will always be there encouraging me when I need itâ¦ You are my Cartesian origin point, my reference and stable place in the world! Mom and Dad, many many thanks for teaching me, since I was a kid, to think critically, to be curious, to enjoy the wonders of nature, and to inculcate the love of reading! No doubts I could have not accomplish much without your education and love. You will always be an inspiration for me! Thanks Sofi for your lovely messages, for your kindness, for laughing with me with our inside jokes as if we were still kids, and for teaching me to be âmore relaxedâ ï...difficult task! I wish to thank my Argentinean friends, who are always welcoming when I visit them, and with whom I spend amazing summer times making me feel like time has never passed. It is always comforting hanging out with all of you: the âsuper amigosâ friends (Belu, Juli, Luly, Lore, Maru, Mica and Pili), my long-life friends (Juancito, Ale, Matu and Javi), Lula, Noe, Clari, Juanpe, Ale and many others! Finally, my time in Canada would have not been what it was, if I have not had such amazing friends in Vancouver!! Your support, advices, care, and all the fun we have had together means a lot to me. Massive thanks to my RES friends (Ghazal, Dani, Jackie, Kevin, Gui, Nan, Tugce, Bas, Ale, Lucy, Kieran and Liz), to my EOAS friends (Kari, Dani, Artem, Jingxuan, Ashley, Genna, Ben, Cheng, Birgit, Sarah, Ana, Iselle, Saurav, Chen, Donaji, Sarai, Jason, Ania, Nari and Tereza), and to Matt and Bulmaro my tennis mates!1 Chapter 1: Introduction Seawater and freshwater are a complex mixture of solutes (chlorine, sodium, titanium, aluminum, manganese, iron, lead, etc.), solids (plankton biomass, fecal pellets, atmospheric dust, riverborne clay minerals, etc.) and colloids (polymetallic oxy-hydroxides, humic substances, etc.), and this thesis is focused on the study of those elements with concentrations lower than 10 µmol kg-1, also known as trace elements (Bruland et al., 2013). As result of their low concentrations, and the difficulty of collecting contamination-free samples, there were no accurate measurements of trace elements until the 1970âs. The development of more sensitive analytical instruments, like the atomic absorption spectrometer and the inductively coupled plasma mass spectrometers (ICP-MS), allowed the detection of trace elements at very low concentrations. The application of clean techniques during sampling, filtering and analysis of the samples has also had a great effect in obtaining accurate trace metal concentrations and isotopic ratios (Sohrin and Bruland, 2011). 1.1 The role of trace elements in the ocean Despite of their low concentration in seawater, trace elements play a significant role as regulators of ocean nutrient dynamics and in the marine carbon cycle (Brand et al., 1983; Morel and Price, 2003; Morel et al., 2013). In addition, their spatial and temporal distributions provide information on key biogeochemical and physical processes, helping to better understand the global circulation and anthropogenic impacts in the world oceans (SCOR Working Group, 2007; Klunder et al., 2012b; Anderson et al., 2014). 2 1.1.1 Bioactive elements and their role in phytoplankton physiology More than a dozen trace elements are known to have a biological role (Morel and Price, 2003), the most studied ones being manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn) and cadmium (Cd). These metals, also known as micronutrients, are important cofactors in enzymes and structural elements in proteins, and most of them are rapidly depleted from surface ocean by phytoplankton uptake (Morel and Price, 2003). As result of their importance in phytoplankton physiology, the concentrations of most of these elements are maintained at similar ratios among different phytoplankton taxa. If the Redfield ratio is extended to include these metals, the new stoichiometric ratio would be: (C106N16P1)x1000 Fe8Mn4Zn0.8Cu0.4Co0.2Cd0.2 (Morel et al., 2013); indicating that Fe and Mn are quantitatively the most important trace metals for most marine phytoplankton. The range of surface concentrations of these bioactive metals in most areas of the oceans is in the picomolar to nanomolar range. In order to enhance the uptake of some of these metals from near-depleted waters, some phytoplankton release strong metal-binding organic ligands to surrounding waters to facilitate the acquisition of these metals (Rue and Bruland, 1995; Gerringa et al., 2015; Buck et al., 2018). The most studied examples of these ligands are the siderophores produced by marine bacteria to bind Fe. There are different types of siderophores, but their general structure is amphiphilic with fatty acid tails and a head group that contains hydroxamate and catechol (Morel and Price, 2003). Some diatoms such as Thalassiosira oceanica, produce an extracellular reductase to reduce the Fe+3 to Fe+2 (dissolved and bioavailable); the reduction rates are faster in Fe-limited than in Fe-sufficient phytoplankton cultures (Maldonado and Price, 2001). The great diversity of organic ligands and transmembrane transport proteins is an example 3 of the adaptive mechanisms phytoplankton have developed through time to face the extremely low concentrations of bioactive metals in surface waters. Trace elements are involved in virtually all important physiological pathways such as light and dark reactions of photosynthesis, oxidative stress detoxification, respiration, nitrogen assimilation, nitrification, denitrification, nitrogen fixation, etc. For example, Fe and Mn are important elements in the light reaction, where light is absorbed by chlorophyll and other pigments and the energy harvested is used to drive electrons from water to generate nicotinamide adenine dinucleotide phosphate (NADPH), which is then used to reduce carbon dioxide (CO2) and to produce adenosine triphosphate by driving protons across a membrane (Lehninger et al., 2008; Morel et al., 2013). During light reactions some dangerous active oxygen species are created as superoxide radicals, which are rapidly converted to less noxious species (H2O2) by the superoxide dismutase, a Mn/Fe enzyme in marine phytoplankton or a Cu/Zn enzyme in other organisms (Morel et al., 2013). The series of reactions that include the reduction of CO2 by the reducing agent NADPH to produce simple sugars are known as dark reactions or the Calvin cycle (Lehninger et al., 2008). One of the most important enzymes in the dark reactions is ribulose l,5-bisphosphate carboxylase/oxygenase (rubisco), which catalyzes the incorporation of CO2 into an organic form. Although, no trace elements are used as enzyme cofactors in the Calvin cycle, they are involved in the CO2 concentration reactions (e.g. Co, Cd, Mn and Zn ) and in the activation of rubisco by Mn or Mg (Morel and Price, 2003). Trace metals are also required by heterotrophic bacteria, which play a key role in the ocean carbon cycle; Tortell et al. (1996) estimated that the requirements of Fe by oceanic heterotrophic bacteria are higher than those observed in phytoplankton, accounting for 20-45% of iron uptake in the subarctic Pacific. 4 Metalloenzymes are also involved in the nitrogen cycle, since Fe, Mo and Cu are required during the uptake of ammonium (NH4+), the reduction of nitrate (NO3-) to NH4+, nitrification and denitrification reactions and nitrogen fixation (Maldonado and Price, 1996; Morel and Price, 2003; Morel et al., 2013). After reviewing the importance of trace metals in the physiological pathways of marine phytoplankton, it is clear why the scarcity of these elements has a profound impact in the primary production of the oceans. In particular, Fe has been recognized as a limiting factor of primary production in high-nutrient, low-chlorophyll (HNLC) regions (Baar et al., 2005; Boyd et al., 2007). 1.1.2 Trace elements as tracers Trace elements not only play a crucial role in phytoplankton physiology, and hence in marine productivity, but also as tracers of biogeochemical and physical processes in the ocean (e.g. tracers of water masses, aeolian inputs, ocean circulation, scavenging), anthropogenic impacts and past environmental conditions (SCOR Working Group, 2007; Anderson et al., 2014). Continental dust has been recognized as an important pathway to transfer trace elements to open ocean waters, including the bioactive element Fe. Therefore, it is important to determine the geographic distribution and magnitude of dust deposition events. The distribution and concentration of some trace metals such as aluminum (Al), lead (Pb), and gallium (Ga) yield relevant information about atmospheric inputs. Dissolved Al, a reactive scavenged trace metal with short residence times (weeks to few years), has been extensively used as a proxy for dust deposition (Orians and Bruland, 1986; Measures and Vink, 2000; Measures et al., 2005; van 5 Hulten et al., 2013). Dissolved Ga has also been shown to be an excellent complementary proxy providing an averaged view of aeolian inputs, since it shares similar physicochemical properties (atomic radius, ionization energy, electronegativity, etc.) and crustal geochemistry to Al (Shiller, 1988; Shiller and Bairamadgi, 2006). Nonetheless, they have noticeable differences in their aqueous chemistry, Ga being less reactive with surface ocean residence times about two orders of magnitude greater than Al, 5-20 years vs. weeks to five years (Orians and Bruland, 1988; Shiller, 1998). The longer surface residence time of Ga allows for more accurate estimates of atmospheric inputs in highly productive waters, where the Al "footprints" are quickly removed by scavenging (Shiller and Bairamadgi, 2006). Most of the current knowledge about scavenging processes in the oceans has been derived from the study of three natural radioactive decay series: 238Uranium (U) series, the 232Thorium (Th) series and the 235U series. Uranium (parent isotope) concentrations and distributions in seawater are essentially conservative while those of thorium and protactinium, produced from the radioactive decay of the uranium, are dominated by scavenging. As the rate of radioactive decay of all naturally occurring radionuclides is constant, it is possible to predict precisely the expected concentration of Th at secular equilibrium. The observed deficits of Th are due to scavenging, allowing the scavenging rate to be estimated (Swarzenski et al., 2000; Chester and Jickells, 2012; Owens et al., 2015). The concentrations and distributions of trace metals have been also used to study the scavenging process; for instance, the concentrations of Pb, Mn, Ga and Al, as well as their molar ratio (Ga/Al) are used to characterize scavenging processes through the water column (Orians and Bruland, 1988; Statham et al., 1998; Shiller and Bairamadgi, 2006; Middag et al., 2011b). 6 Finally, the study of Pb concentrations and isotopic composition, besides providing useful information about as atmospheric inputs and water mass distributions (Véron et al., 1999; Bollhöfer et al., 2001; Ewing et al., 2010), have been mostly used as a tracer of anthropogenic pollution. The footprint of anthropogenic lead has been detected in almost all environmental matrices such as rivers, ocean, sediments and in the atmosphere, in particular in the northern hemisphere (Komárek et al., 2008; Callender, 2013; Boyle et al., 2014). 1.2 Sources, sinks and internal cycling of trace metals Trace metals are involved in intricate biogeochemical cycles from their introduction into the ocean environment, cycling throughout the biotic and abiotic compartments, to their final burial in ocean sediments. The main sources of trace elements are: atmospheric transport, river runoff, groundwater seeps, hydrothermal circulation, and diffusion from sediments. Atmospheric inputs: is the second largest source of trace elements to the ocean (Chester and Jickells, 2012), but represents the major source of key bioactive metals to open ocean waters where the influence of riverine discharges is limited (Jickells et al., 2005). Atmospheric inputs of trace elements are particularly important in surface waters immediately downwind of the arid regions of Africa, the Arabian Peninsula, Central Asia, Australia, and South America. Particles coming from the erosion of the land's surface are the major natural sources of aeolian inputs for most of the elements (Al, Cu, Pb, Ni, Zn, Fe, etc.), but volcanic emission and vegetal exudation also provide large quantities of natural trace metals to the atmosphere (Nriagu, 1979; Maring and Duce, 1987; Callender, 2013; Mahowald et al., 2018). Aerosols particles can be delivered to the ocean by dry and/or wet deposition; the solubility of the elements present in the aerosols is controlled by a number of factors such as aerosol mineralogy, atmospheric 7 chemical/photochemical processing, seawater-dissolved concentrations, concentrations of seawater ligands, etc. (Bruland et al., 2013; Fishwick et al., 2017; Mahowald et al., 2018). In addition, the solubility of the same element may change depending on its source; some studies have shown that some trace metals (e.g. Fe, Cu, Zn and Pb) are more soluble when they derive from urban-rich than from crust-rich aerosols, while other trace metals (e.g. Mn and Cd) are significantly soluble from both crust-rich and urban-rich aerosols (Chester et al., 1993; Bruland et al., 2013; Fishwick et al., 2017). River runoff: Relative to ocean waters, rivers have much higher concentrations of most trace metals as result of mechanical and chemical weathering of crustal minerals. The type and concentrations of elements found in rivers depend on the composition of the geology of the river basin, the temperature, pH, flow and other environmental conditions that influence the rate of weathering (Benoit and Rozan, 1999; Holmes et al., 2012; Barker et al., 2014; Bring et al., 2016; Shim et al., 2017). Rivers are the major sources of trace metals to the world oceans, however, most of this flux is in a particulate form that is trapped in estuaries via settling, and does not reach the open ocean. Estuaries are highly productive transition environments (Fichez et al., 1992; Arhonditsis et al., 2007) where, in addition to particulate settling, the increase of ionic strength in the river-water interface leads to complex processes such as flocculation, scavenging and precipitation, which affect the distribution and concentration of trace metals (Bruland et al., 2013). As result of these processes, non-conservative behaviors (deviations from linear elemental concentration as a function of salinity) between the two endmembers (freshwater-seawater) have been reported for many trace metals (Boyle et al., 1977; Tanguy et al., 2011; McAlister and Orians, 2012). 8 Hydrothermal vents: They are generally related to spreading centers where seawater infiltrates through cracks in the crust and its physicochemistry is altered as it approaches the magma chamber below the spreading center (German and Seyfried, 2013). As result of this interaction, the seawater becomes hot, anoxic, acidic and is enriched in reduced metals such as a Fe+2, Mn+2 and Zn, and to a lesser extent with Al, Co, Cu and Pb (German and Seyfried, 2013). Due to its buoyancy, the hydrothermal fluid ascends in the deep ocean carrying the dissolved metals. Previously it was thought that hydrothermal inputs did not extend far from the vent areas, where most of these metals experience redox reactions and precipitate as the hydrothermal fluid mixes with the cold, oxic and alkaline seawater. For example, Fe+2 (aq) and Mn+2 (aq) are oxidized into insoluble Fe+3 and Mn+4 oxides, forming colloids and particles which, along with the Fe sulfides, settle onto the sediments where they are no longer available for primary producers (Bruland et al., 2013). However, recent investigations have shed light on the transport of the trace metal rich hydrothermal plumes in deep ocean waters, which, due to the complexation with organic ligands, allow trace metals to be maintained in solution, enabling a wider dispersion than previously expected, especially for Fe, Mn, Cu and Zn (Yücel et al., 2011; Kleint et al., 2015; Fitzsimmons et al., 2017). Sediment diffusion: In recent years, sediments and continental-oceanic interfaces have been gaining increasing scrutiny as potentially important sources of trace metals, either by reductive and/or non-reductive dissolution processes (Jeandel and Oelkers, 2015; Charette et al., 2016; Jeandel, 2016; Abadie et al., 2017; Cheize et al., 2019). As was described above, natural fluxes represent the main input of trace elements to the ocean. However, due to anthropogenic influence (large-scale metal mining and smelting, fossil-9 fuel combustion, municipal waste incineration, cement production, technological waste, etc.) the emission rate of Cd, Mercury (Hg), Pb, Cu, Zn, Arsenic (As), Cr and Ni has increased dramatically (Nriagu and Pacyna, 1988; Nriagu, 1990). For example, during the high emission period, from 1970s to 1980s, estimates of anthropogenic emissions to the atmosphere were two times (Cu, Ni), five times (Zn, Cd), and 33 times (Pb) greater than the natural emissions (Nriagu, 1996). As these metals are mostly dispersed through the atmosphere, they can contaminate distant regions of the planet like the Arctic and Antarctic (Akeredolu et al., 1994; Nriagu, 1996; Callender, 2013). The ultimate fate of all trace elements is burial in sediments. After continuous cycling in the water column, trace elements sink to the deep ocean by adsorption onto particulate matter (scavenging) and/or the incorporation in biogenic material. The scavenging phenomenon occurs when dissolved cations are adsorbed to the surface of negatively charged particles such as particulate organic matter (POM), clay minerals and oxy-hydroxides present in the ocean. The rate of scavenging depends on the amount of particulate matter (higher fluxes of particlesâhigher rates of scavenging), the availability of adsorption active sites and the nature of the charged particles (Chester and Jickells, 2012). Even though scavenging takes place in the entire water column, it plays a preponderant role in the removal of metals in the deep ocean. The adsorption of metals to particulate matter is usually reversible; some metals are adsorbed and desorbed multiple times during their way to the sediments (Fitzsimmons et al., 2017; John et al., 2018; Weber et al., 2018). In the euphotic zone, the incorporation of trace elements in the biogenic material is the primary mechanism of removal. The bioactive elements, which are involved in important metabolic pathways in phytoplankton physiology, are rapidly removed 10 from solution and assimilated into phytoplankton soft tissues. At the same time, some metals (Ba, Cu, Ag, Zn, Pb, etc.) are actively incorporated into the hard exoskeleton made of biogenic silica and calcite (Yuan-Hui, 1991). When phytoplankton die, the organic fraction that is not remineralized, along with the undissolved inorganic fraction, sink to the bottom of the ocean and are buried in the sediments. 1.3 Area of study 1.3.1 The Arctic environment The Arctic is a vast region that extends across northern North America, northern Europe and northern Asia; it encompasses eight countries and the Arctic Ocean surrounded by continental tundra. This region is generally delimited by the Arctic Circle (66°32' N), nonetheless, there are other delimitations of the Arctic based on a floristic boundary and on an ecosystem marine boundary (AMAP, 1998). This region is characterized by low air temperatures, holding some of the most extreme habitats on Earth, with species and people that have adapted to its unique conditions. The Arctic is distinguished by its extreme seasonality, in the winter months, there is a total lack of incoming solar radiation, while in the summer, it receives higher levels of solar radiation than any other place on Earth. However, much of the solar radiation received is lost to space due to the reflection of short-wave radiation by extensive cloud, snow and ice cover (AMAP, 1998; CAFF, 2013). This radiation imbalance produces low temperatures and results in a redistribution of heat from southern latitudes via air and ocean currents. Other distinctive features of the Arctic landscape are: a) the presence of an extensive permafrost layer, where about 30% of the worldâs soil carbon is estimated to be stored (Hugelius et al., 2014), b) Arctic rivers, with approximately 10% of the global river runoff being delivered 11 to the Arctic Ocean, and c) the presence of sea ice cover in some regions of the Arctic Ocean (Bell and Brown, 2018). The rivers in the Arctic are classified in two main types, those that have headwaters within the Arctic and those with headwaters farther south (most of the larger rivers e.g. Ob, Yenisey, and Lena, from Siberia, and Mackenzie River from Canada). Most Arctic rivers exhibit a short flow season with main flows taking place during the period of spring and summer melt. When the melt is completed, water levels in the river drop to base flow. These rivers are very responsive to snow and ice melt events and rainfall due to the presence of relatively impervious permafrost and a shallow active zone, resulting in highly episodic flow regimes (AMAP, 1998). Arctic habitats are among the least anthropologically disturbed on Earth, where almost pristine tundra, mountain, freshwater and marine habitats still exist; more than 21,000 species of mammals, birds, fish, invertebrates, plants and fungi are found in the Arctic (CAFF, 2013). Arctic communities rely on these natural resources for their survival; for instance, the Canadian Arctic Inuit communities are particularly dependent upon the sea, which supplies them with necessary food. The unique fauna of the Arctic Ocean supplies many other ecosystem services such as hides and furs, heating oil, substrates for tools and sculpture (Bell and Brown, 2018). 1.3.2 The Arctic Ocean The Arctic Ocean is about 4000 km long and 2400 km wide, and is strongly stratified by salinity. Despite the fact that it constitutes less than 3% of the global ocean area and about 1% of the volume, it plays a crucial role in climate, global ocean circulation, and global distribution of nutrients and trace metals (Rudels et al., 1999; Melling et al., 2008; Klunder et al., 2012b). The Arctic Ocean is divided at a depth of 1500 m into the Canadian and Eurasian basins by the 12 Lomonosov Ridge; the Canadian Basin is further divided into the Canada and Makarov basins by the Alpha-Mendeleyev Ridge and the Eurasian Basin is further divided into the Amundsen and Nansen basins by the Nansen-Gakkel ridge. The global climate system is strongly influenced by the Arctic Ocean; the water vapor transported from the Atlantic to the Pacific by atmospheric circulation is returned to the Atlantic thorough a "downhill journey" of low salinity waters from the North Pacific, via Bering Strait, to the Arctic Ocean, and then into the North Atlantic primarily through Baffin Bay and the Labrador Sea (Jones, 2003; Carmack et al., 2010). Changes in the rate and path of the Arctic outflow of fresh waters through the Arctic Ocean have the potential to impact the global thermohaline circulation by changing the vertical density gradient, creating a stratified surface layer that disrupts the production of water dense enough for deep convection to occur (Aagaard and Carmack, 1989; Budikova, 2009; Carmack et al., 2010; Melling et al., 2012). 1.3.2.1 Arctic gateways The Arctic Ocean is surrounded by land and connected to the Pacific and Atlantic Oceans through narrow gateways (Figure 1.1). Scientists are monitoring the water exchange, trace element concentrations and isotope distributions and shelf interactions in these narrow connections between the Arctic Ocean and its adjacent ocean basins to have a better understanding of Arctic Ocean water fluxes, circulation, fresh water budgets, trace elements and isotope biogeochemistry. Barents Sea gateway: The Barents Sea Opening (BSO; Figure 1.1) is a 400 km wide passage between Bear Island and the North Cape by which the upper layers of the warm and salty North Cape Current (Atlantic origin) enters the Arctic Ocean (Beszczynska-Möller et al., 13 2011). The mean inflow of the North Cape Current is estimated as 1.8-2.0 Sv (1 Sv= 106 m3 s-1), with large interannual variations, and a fraction of it recirculates in the Barents Sea and returns to the Atlantic Ocean as a colder outflow (Beszczynska-Möller et al., 2011). In addition to the North Cape Current, the warmer and fresher Norwegian Coastal Current enters the Barents Sea along the Norwegian coast with an estimated volume flux of 1.8 Sv and a freshwater flux of 26 mSv, relative to a reference salinity of 34.8 (Loeng, 1997; Skagseth et al., 2011). After passing through the Barents Sea most of the Atlantic water leaves the Barents Sea into the Arctic Ocean via the strait between Novaya Zemlya and Frans Josef Land and then via St. Anna Trough (Jones et al., 1995; Loeng, 1997; Beszczynska-Möller et al., 2011). During its time in the Barents sea, the Atlantic water changes its characteristics due to the mixing with surrounding water, atmospheric cooling, ice formation and brine rejection becoming denser and cooler with temperatures below 0 °C (Jones et al., 1995; Rudels et al., 1996; Loeng, 1997; Aagaard and Woodgate, 2001; Beszczynska-Möller et al., 2011). This dense, cold and less saline water mass originated in the Barents Sea sinks through the St. Anna Trough and continues its eastward flow in the boundary current (Rudels et al., 2004; Aksenov et al., 2011). In the Canadian Basin the presence of less dense Pacific water prevents the Atlantic derived upper layers from flowing through the Canadian Arctic Archipelago (CAA) and their main exit is Fram Strait. Only some of the Barents Sea branch halocline water passes through Nares Strait and may occasionally renew the deep and bottom waters of Baffin Bay (Jones, 2003; Rudels et al., 2004). 14 Figure 1.1 Schematic showing the inflow of the two Atlantic-derived waters masses: the Fram Strait Branch (FSB; red) and the Barents Sea Branch (BSB; blue), and the Pacific inflow (green) into the Arctic Ocean (after Rudels et al., 2004). The formation/modification area of FSB and BSB are displayed with solid colors and the circulation with colored arrows. Main landmarks and straits are also indicated in the figure: Bering Strait (BS), Davis Strait (DS), Fram Strait (FS) and Barents Sea Opening (BSO). Fram Strait: It is a wide and deep (~2600 m) channel between Greenland and Svalbard (Figure 1.1), where Atlantic waters flow into the Arctic Ocean and the Arctic water and sea ice 15 are exported into the Nordic Seas (Rudels, 1995). The Atlantic water inflow is concentrated above the eastern shelf slope where the West Spitsbergen Current (WSC) splits into three separate branches. The easternmost branch follows the upper shelf slope north of Svalbard and constitutes the main source of Atlantic waters to the Arctic Circumpolar Boundary Current (ACBC). The second branch continues eastward and ultimately rejoins the eastern branch forming the Fram Strait Branch (FSB). Finally, the westernmost branch of WSC recirculates directly in the northern Fram Strait exiting again to the south (Aagaard et al., 1985; Rudels, 1995; Beszczynska-Möller et al., 2011). Unlike the deep Barents Sea branch, the FSB core waters are largely protected from interactions with the ice and the atmosphere, and they retain much of their initial heat. The FSB flows eastward along the Eurasian continental slope, and north of the Kara Sea meets the deep Barents Sea branch continuing in the ACBC current eastward (Jones et al., 1995; Rudels et al., 2005; Aksenov et al., 2011). The deepest Atlantic waters entering the Arctic Ocean though Fram Strait are the Norwegian Sea Deep Water and Greenland Sea Deep Water (Aagaard et al., 1985). The relatively fresh Arctic outflow is constrained to the western part of the Fram Strait in the East Greenland Current (EGC), which transports from 4 to 10 Sv (Rudels et al., 1999; Beszczynska-Möller et al., 2011). Bering Strait: It is the only connection between the Arctic ocean and the Pacific Ocean (Figure 1.1); it is a shallow (~50 m) and narrow (~ 85 km wide) passage which is divided into two channels by the Diomede Islands and the US-Russian border (Woodgate and Aagaard, 2005; Beszczynska-Möller et al., 2011). The long-term mean volume of Pacific water transported by the Bering Strait is 0.83 Sv with great seasonal variability, with a maximum in summer (1.3 Sv) and a minimum in winter (0.3 Sv; Roach et al., 1995). The Bering Strait throughflow is 16 distinguished from the previously described Atlantic inflows, since it provides a large source of nutrients, such as phosphate and silicate (Pacific water is relatively depleted in nitrate) and freshwater (~one-third) to the Arctic Ocean (Jones, 2003; Steele et al., 2004; Woodgate and Aagaard, 2005). Within the Arctic Ocean, Pacific waters have a range of salinity values from less than 30 to more than 33, which is substantially lower than the Atlantic inflow which has a well-defined salinity of 34.85 (Jones et al., 2008). Pacific waters entering the Arctic Ocean through Bering Strait contribute to the surface and halocline waters, producing three distinct water types: 1) a relatively fresh (31<S<32) and warm (1 <T< 6 °C) Alaskan Coastal Water (ACW) containing significant river influence. 2) A relatively saltier (32<S< 33) and colder (0 <T< 2 °C) summer Bering Sea Water (sBSW) occupying the bulk of the central Chukchi Sea, and 3) the winter Bering Sea Water (wBSW) with near-freezing temperature minimum and nutrient maximum at S= 33.1 (Steele et al., 2004; McLaughlin et al., 2005; Timmermans et al., 2017). In addition to the spatial heterogeneity, two different circulation patterns of halocline waters have been observed in the Canadian Basin; during the positive Arctic Oscillation state the Beaufort Gyre is small and is composed mostly of ACW, and the Transpolar Drift drives the sBSW directly from the Chukchi Sea to north of Ellesmere Island. The other circulation pattern occurs during the negative Arctic Oscillation state when the Beaufort Gyre expands, sweeping ACW into its anticyclonic circulation and providing an ACW signature to waters on the Chukchi Abyssal Plain and perhaps in the Transpolar Drift Stream. The strong recirculation within the Beaufort Gyre also brings sBSW to the southern Beaufort Gyre (Steele et al., 2004). An important fraction of the freshwater carried by the Pacific inflow flows from the Canada Basin through the CAA to Baffin Bay, and from there is exported to the North Atlantic through the wide (360 km) and deep (650 m) Davis Strait (Wang et al., 2012; Hill et al., 2013). There is also 17 a fraction of the Pacific waters exiting the Arctic Ocean through Fram Strait in the EGC (Jones, 2003; Aksenov et al., 2011). 1.3.2.2 General circulation of the Arctic Ocean As a general view, the Arctic circulation can be split into a low salinity surface loop and a deeper water loop. The low salinity surface loop is formed by the polar mixed layer (PML), a mixture of Pacific and Atlantic waters, river-runoff and sea ice melt water, which has an anticyclonic circulation dominated by the wind driven Beaufort Gyre and the transpolar drift which moves ice and low salinity surface waters out of the Beaufort Gyre and across the Lomonosov Ridge close to the north of Greenland (Jones et al., 1998). The surface circulation in the southern part of the Eurasian Basin is less certain, but there are indications of an eastward flow in the boundary current above the Atlantic Layer (Rudels, 1995). The deeper water loop consists of the ACBC, which originates when the FSB and deep Barents Sea Branch (BSB) meet north of the Kara Sea (Figure 1.1). These two water masses have different physical properties, the FSB is warmer and saltier than the colder and relatively fresher BSB (Aksenov et al., 2011). The ACBC current splits north of the Laptev Sea with a large fraction that recirculates in the Eurasian Basin along bathymetric features, particularly along the Nansen-Gakkel Ridge, the Lomonosov Ridge and the Mendeleyev Ridge, returning to Fram Strait. A small part of the boundary current crosses the Lomonosov Ridge and enters the Canadian Basin (Jones et al., 1995; Rudels et al., 2004; Aksenov et al., 2011). Besides the ACBC that enters to the Canadian Basin, some studies have suggested that there is a deep spill-over between the Amundsen and the Makarov basins through gaps in the Lomonosov Ridge away from the continental slope (Jones et al., 1995; Rudels, 1995). Through these gaps the colder 18 Amundsen Basin water sinks through the weakly stratified Makarov Basin water column, perhaps reaching the bottom. This phenomenon would explain the constant salinity and the decreasing temperature registered in the deep water above the isothermal and isohaline bottom water. In the Canadian Basin the return flow of the boundary current towards the Eurasian Basin is distinguished from the return flow via the Makarov Basin by slightly lower maximum temperature and by its higher silicate content (Rudels et al., 1999). After passing through the Makarov Basin, the modified boundary current re-crosses the Lomonosov Ridge north of Greenland reaching the Eurasian Basin and exiting the Arctic Ocean through Fram Strait. A detailed circulation description from the Canada Basin, the CAA, Baffin Bay and the Labrador Sea, where the trace metal samples for this study were collected, is presented in chapter 3 (page: 63) and 4 (page: 91) of this thesis, including figures displaying the main water masses. 1.3.3 The Arctic fragile environment in a climate change context Most of the observed increase in global average temperatures since the mid-twentieth century is attributed to the increase in anthropogenic greenhouse gas concentrations (>90% probability of occurrence), and not by natural external forcing factors (Rosenzweig et al., 2008). The response of anthropogenic warming in physical systems include shrinking glaciers in every continent, melting permafrost, lake and river warming with effects on thermal stratification, chemistry and freshwater organisms, and increases in coastal erosion. The anthropogenic warming has also altered biological systems shifting spring events (for example, leaf unfolding, blooming date, migration and time of reproduction), changing species distributions and community structure, and changing marine-ecosystem functioning and productivity (Macdonald et al., 2005; Ford et al., 2006; Barber et al., 2008; Mcguire et al., 2009). 19 Even though climate change is a global phenomenon, its effects are most extreme in the Arctic environment due to a complex series of interactions and positive feedback processes. Sea ice and snow reflect a much larger fraction of incident sunlight (high albedo) than water and soil, thus, a reduction of sea ice and snow causes a perturbation in the energy budget, amplifying warming in the Arctic. This warming is transferred globally and, at the same time, feeds back regionally to further reduce snow and ice extent (AMAP, 1998; Comiso et al., 2008; Overland et al., 2011; CAFF, 2013). Another positive feedback includes the thawing of permafrost because massive amounts of peat-based carbon and gas hydrate deposits bound in permafrost can be removed and exposed to the atmosphere releasing carbon dioxide and methane (CAFF, 2013). More recently it has been suggested that the recent warming of the Arctic is also related to an increase in the downward longwave flux, linked to an increase of poleward moisture transport and enhanced greenhouse effect (Orsi et al., 2017). Since 1980, the rate of increase of atmospheric temperatures in the Arctic has been twice that of the rest of the planet, and projections show that the Arctic will experience the largest future temperature changes on the planet (Overland et al., 2011). Changes associated with the Arctic sea ice cover are among the best quantified effects of climate change occurring in the Arctic. The amount of Arctic summer sea ice cover and multi-year ice is decreasing at an accelerating rate; from 1953â2006, Arctic sea ice extent at the end of the summer melt season in September has declined at a rate of 7.8% per decade. Over the period of modern satellite observations (1979 â2006) the trend is even larger (9.1% per decade), and the decline is faster than the model predictions (Serreze et al., 2007; Stroeve et al., 2007). For instance, in September 2007 the sea ice extent reached a minimum (second lowest minima) of 20 4.1 x 106 km2; this is 37% lower than the climatological averages (Comiso et al., 2008; Overland et al., 2011). The third lowest minimum sea ice extent was observed in September 2011, and in September 2012 the sea ice extent reached the all-time lowest minimum with only half the extent compared with the 1979-2000 average (CAFF, 2013). Moreover, the nine lowest extents in the satellite era have all occurred in the last 12 years (National Snow and Ice Data Center: https://nsidc.org/arcticseaicenews/charctic-interactive-sea-ice-graph/). Current projections suggest that the Arctic Ocean will become largely ice-free in summer within the next 30-40 years (CAFF, 2013). Multiple impacts in physical environments and in biological systems have been observed due to the warming of the Arctic. For instance, an increase in the wave activity and an intensification of the coastal erosion was observed as result of the reduction of the Arctic sea ice extent (Arrigo et al., 2008). In addition, a profound impact on the Arctic permafrost, resulting in increased export of organic carbon and sediments to the Arctic Ocean, along with increased primary production, impacting local marine ecosystems, and alterations of the thermohaline circulation are some of the expected effects due to the warming of the climate (Arrigo et al., 2008; McLaughlin et al., 2011). 1.4 The Canadian Arctic GEOTRACES program This research thesis is framed in a broader and multidisciplinary project: The Canadian Arctic GEOTRACES Program, which aims to study the distribution and modification of geochemical tracers and physical processes in the Canadian Arctic Ocean. In this thesis we present trace metal data collected from Labrador Sea, Baffin Bay across the Canadian Arctic Archipelago to the Canada Basin during the Arctic GEOTRACES cruise expedition on the 21 CCGS Amundsen. The expedition consisted of two consecutive legs one from July 10th to August 20th and another one from September 4th to October 1st, in 2015. Seawater samples were collected at 18 sampling stations throughout the transect targeting important physicochemical processes, water masses and key locations, Table 1.1. At each station, the targeted sampling depths were selected based on physicochemical data provided by the CCGS Amundsen's rosette which has a suite of sensors including oxygen, fluorescence, transmissometer, nitrate and photosynthetically active radiation (PAR), in addition to the classic CTD (conductivity, temperature, and depth) instrument. Through the analysis of information provided by these sensors the trace metal rosette sampling targeted specific water masses and processes such as maximum and minimum oxygen zones, highly productive areas, high particulate fluxes, etc. To monitor for potential contamination during sampling operations, dissolved Zn was measured onboard using flow injection analysis with established chemiluminescence and fluorescence detection methods. Zn is one of the more contamination prone metals, and therefore, the analysis of this element allows testing of the GO-Flo bottles for contamination and checking the consistency of the water profile for each station. In addition to the seawater samples, we have analyzed freshwater samples from remote rivers in the Canadian Arctic Archipelago (CAA), which were also sampled during the Canadian Arctic GEOTRACES program. The precise location of the stations as well as the sampling, sub-sampling, processing and analysis of seawater and freshwater samples are fully described in the following main chapters of this thesis. 22 Table 1.1 Sampling stations during the 2015 Arctic GEOTRACES cruise expedition Station Name Bottom Depth (m) Cast Depth (m) Target Process/Region Lat (°N) Lon (°E) Location K1 3310 3015 Cross over station 56.12 -53.36 Labrador Sea LS-2 3031 2800 North of Labrador Sea (capture deep western boundary current) 60.45 -56.56 Labrador Sea BB-1 1040 1000 Davis Strait (north of sill) 66.85 -59.06 Baffin Bay BB-2 2370 2310 Center Baffin Bay 72.74 -67.00 Baffin Bay BB-3 1243 1010 Western Baffin Bay (capture wedge of CAA water) 71.40 -68.59 Baffin Bay CAA-1 645 609 Transect across Lancaster Sound mouth 74.52 -80.57 Canadian Arctic Archipelago (CAA) CAA-2 700 600 Transect across Lancaster Sound mouth 74.31 -80.49 CAA CAA-3 676 610 Transect across Lancaster Sound mouth 73.81 -80.49 CAA CAA-4 181 181 Transect across Somerset and Devon islands 74.12 -91.51 CAA CAA-5 260 252 Transect across Somerset and Devon islands 74.53 -90.80 CAA CAA-6 260 245 Center strait between Cornwallis Island and Bathurst Island 74.75 -97.45 CAA CAA-7 219 198 West of Summerset Island (Peel Sound) 73.67 -96.52 CAA CAA-8 564 450 Western Parry Channel 74.13 -108.83 CAA CAA-9 350 334 Penny Strait (Tidal mixing) 76.33 -96.75 CAA CB-1 465 400 McClure Strait 75.01 -120.64 Canada Basin CB-2 1365 1200 1200 m bathymetry line (capture boundary current) 75.81 -129.21 Canada Basin CB-3 3658 3500 140 W line deep station Beaufort Gyre 76.98 -140.03 Canada Basin CB-4 3827 3500 Cross over station 74.99 -150.01 Canada Basin 23 1.5 Thesis research goals The Arctic is one of the most pristine habitats on the Earth and holds an incredible diversity of species which are highly adapted to this cold environment. In addition, the Arctic Ocean is involved in climate regulation, global ocean circulation, and global distribution of nutrients and trace metals. In recent decades the stability of this unique region has been threatened by global warming, contamination and by the increase of economic activities developed in the Arctic such as oil and gas exploration and sea transportation. Therefore, it is essential to study the biological, chemical and physical processes in the Arctic before further perturbations occur. The primary focus of my project is the study of dissolved Fe, Mn, Pb and Ga in the Canadian Arctic Ocean and multiple other trace metals in remote rivers in the CAA, where there is limited information about these trace metals compared to the Eurasian Basin or the Chukchi Sea. The overarching goal is: to describe the spatial and vertical distribution of selected trace metals from the deep Canada Basin to the Southern Labrador Sea, and to investigate the processes (e.g. biological uptake, scavenging, sediment resuspension, lateral advection, brine rejection, river inputs, continental shelf interaction, water mixing, etc.) which influence/alter the trace metal signature of the water masses studied in this region. Lastly, this research will also report for the first time the trace metal geochemistry of remote Arctic rivers in the CAA. This thesis provides an overview of the biogeochemical cycles of the studied trace metals in the Canadian Arctic Ocean. This work not only reports the transformations of the trace metal signature from the Pacific inflow waters to the North Atlantic, but also will help to investigate 24 tracers of the circulation of Arctic waters from Canada Basin, through the CAA, and into the Atlantic. 25 Chapter 2: Trace Metal Geochemistry of Remote Rivers in the Canadian Arctic Archipelago 2.1 Summary Trace metals are essential micronutrients for phytoplankton, as well as useful tracers of biogeochemical processes in aquatic systems. The largest Arctic rivers are known to play an important role in the cycling of trace metals in the Arctic Ocean; however, these systems account for little more than half of Arctic freshwater discharge, leaving a major gap in our understanding of the trace metal geochemistry from smaller river systems. This is especially problematic for rivers draining the Arctic coastal margins, as these continuous permafrost systems are anticipated to be particularly sensitive to climate change. This study presents the first observations of dissolved and particulate trace metal concentrations and lead isotopes in 14 rivers in the Canadian Arctic Archipelago (CAA). Dissolved concentrations of aluminum, iron, manganese, nickel, copper, zinc, cadmium, and lead display clear spatial variability in their distributions. Small rivers located in the central islands of the CAA have remarkably low concentrations of these trace metals, while the rivers draining the southern continental landmass have higher values. This geographic variability in dissolved metal concentrations reflects both differences in bedrock geology and the South-North gradients of environmental conditions (e.g. flow, solar irradiation, dissolve organic carbon distribution) in the Archipelago. We extrapolate these findings to estimate the dissolved trace metal concentrations for other rivers draining the CAA based on their geographic location. Elemental ratios measured in the suspended particulate matter in several rivers indicates a significant enrichment of nickel, copper, zinc, and lead over the crustal composition. In particular, lead isotope composition shows an extremely large range, 26 varying from the highly radiogenic Tree River (206Pb/207Pb: 1.5121) in the south, to the relatively unradiogenic Glacier River (206Pb/207Pb: 1.0153) in the central CAA. Lack of correlation between the Pb isotopic composition and Pb enrichment, suggests Pb is derived from within the watershed rather than from external sources (e.g. atmospheric transport). This work presents the first observations of dissolved and particulate trace metal concentrations in small coastal draining rivers from the CAA, an important first step in developing a baseline against which to evaluate future change in the high Arctic. 2.2 Introduction The study of trace elements in aquatic environments has increased since the 1970âs due to the advancement of analytical and sampling techniques, bringing about a better understanding of their key roles in nutrient dynamics and carbon cycling (Morel et al., 2013). For instance, manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) are involved in virtually all important physiological pathways of planktonic organisms (Morel and Price, 2003). Deficits of these biologically essential metals can limit phytoplankton growth, while at high levels certain metals can be toxic, especially lead (Pb), Cd, and Cu (Bruland et al., 1991). Furthermore, trace elements, such as barium (Ba), Pb, aluminum (Al), and gallium (Ga), are useful tracers of biogeochemical and physical processes as well as anthropogenic perturbations in aquatic environments (Anderson et al., 2014). The isotopic compositions of some of these trace elements can also provide useful geochemical information. For example, differences in lead isotopic signatures are extensively used to âfingerprintâ anthropogenic and natural sources of lead-containing particulates to the environment (Bollhöfer and Rosman, 2002; Komárek et al., 2008; Cheyne et al., 2018). 27 Arctic rivers play an important role in the biogeochemical cycles of organic carbon, nutrients, major ions, and trace metals in the Arctic Ocean, where approximately 11% of world's river flow is discharged (Lammers et al., 2001; Millot et al., 2003; Rember and Trefry, 2004; Hölemann et al., 2005; Dahlqvist et al., 2007; Guay et al., 2010; Pokrovsky et al., 2010; Bring et al., 2016). Initially, available data describing dissolved and particulate trace metal distributions was limited to the largest Arctic rivers. Trace metal concentrations have been reported for the Yenisey, Lena, and Ob rivers since early 90s (Martin et al., 1993; Dai and Martin, 1995; Guieu et al., 1996; Hölemann et al., 2005; Guay et al., 2010), and since 2002, trace metal data for the Yenisey, Lena, Ob, Mackenzie, Yukon, and Kolyma rivers have been collected through the Pan-Arctic River Transport of Nutrients, Organic Matter, and Suspended Sediments (PARTNERS) and Arctic-Great Rivers Observatory (Arctic-GRO) projects (https://arcticgreatrivers.org/). More recently, the dissolved and particulate trace metal geochemistry of small and medium sized permafrost dominated Arctic rivers has been investigated along the Alaskan, Russian, and Siberian coasts (Rember and Trefry, 2004; Dahlqvist et al., 2007; Pokrovsky et al., 2010; Barker et al., 2014). Despite these recent studies, little is known about the trace metal characteristics of smaller rivers draining the continuous permafrost regions of the high Arctic islands of the Canadian Arctic Archipelago (CAA). This region has an important role as a pathway through which Arctic waters of Pacific origin are exported to the North Atlantic (Jones, 2003; Beszczynska-Möller et al., 2011). During its flow from the Arctic Ocean to Baffin Bay and the North Atlantic, the geochemistry of transiting waters is altered due to interactions with extensive continental shelves and riverine inputs (Michel et al., 2015), enhancing the productivity of CAA 28 marine system, which is estimated to support up to 32% of the total primary production of the Arctic shelves (Hill et al., 2013; Michel et al., 2015). Even though the annual mean discharge of individual CAA rivers is small (Ë 15.2 km3; Water Survey of Canada, 2017) compared to the average annual discharge of the Mackenzie River (~283 km3; Water Survey of Canada, 2017), the combined outflow of all CAA rivers (201 to 257 km3/year; Lammers et al., 2001; Alkire et al., 2017) could have a considerable influence on the freshwater balance of Arctic Ocean waters transiting the CAA. In the context of climate change, where Arctic river discharge is expected to increase due to the intensification of the global hydrological cycle (Peterson et al., 2002; Bring et al., 2016), characterizing the major and trace element distributions of coastal draining Arctic rivers will be essential in determining the impacts of terrestrial warming on the marine environment. The aim of this paper is to determine the concentrations and distributions of dissolved and particulate trace metals and lead isotopes in coastal draining rivers across the CAA. We also investigate the sources of particulate trace metals in the CAA and explore the environmental variables affecting trace metal distributions. 2.3 Methods 2.3.1 Sampling Fourteen coastal draining Arctic rivers in the Canadian Arctic Archipelago (Table 2.1) were sampled in the summer of 2015 (August) during the Canadian Arctic GEOTRACES program (Figure 2.1). This paper details particulate and dissolved trace metal results, whereas observations of river dissolved inorganic (major ions, stable isotopes, inorganic carbon) and organic (dissolved organic carbon) composition are presented in Brown et al. (in review). Rivers 29 were accessed using the shipâs helicopter as the CCGS Amundsen travelled through the CAA. At each site, river water samples (250 mL) were collected by hand using gloves and clean low density polyethylene (LDPE) bottles while standing in the river and facing upstream (salinity measured for all samples was below 0.19, Table A.1). The LDPE bottles were opened and closed under water, just below the river surface, to avoid external contamination. River samples were collected into Bel Art LDPE bottles that were thoroughly cleaned by soaking in successive baths at 60 °C of Extran Liquid Detergent (VWR), 6 M environmental grade HCl, and 0.7 M trace metal grade nitric acid (HNO3, Fisher Scientific, Ontario, Canada) for 1 day, 1 week, and 1 week, respectively, as recommended by the GEOTRACES Protocol (http://www.geotraces.org/). Once back on the ship, trace metal samples were immediately filtered in a portable trace metal laboratory, equipped with HEPA filters, using an acrylic vacuum chamber with Teflon filter holder and tubing. The filters used were 0.2 μm supor polyethersulfone (PES) membrane filters (VWR) that were previously washed with 1 M trace metal grade hydrochloric acid (HCl; Seastar Chemicals Inc., Sidney, Canada) at 60 °C for 24 h, and then rinsed and stored in Milli-Q water. Between each sample, the filter holder and tubing were rinsed with 1% trace metal grade HCl and Milli-Q water to avoid cross-contamination. The filtered water was acidified to pH ~1.8 by the addition of 6 M HCl and stored in clean LDPE bottles. PES filters containing the particulate load were dried under the laminar hood and stored in cleaned plastic capsules. PES filters were always handled using pre-cleaned plastic forceps and clean gloves. For ease of comparison, rivers are grouped by their geographic location (Figure 2.1). The continental rivers are those that have their watersheds on the North American continent and include the Tree (#15), Ellice (#14), and Simpson (#13) rivers (black circles). The central CAA 30 rivers are the Pasley (#12), Le Feuvre Inlet* (#11), and Creswell (#10) rivers (black squares), and the northern rivers are the Mecham (#9), Cunningham (#7), Garnier (#8), Saaqu (#5), Marcil Creek (#4), Devon Island* (#6), Glacier* (#2) and Akpat Kuunga* (#1) rivers (black diamonds). Rivers with no official name on available charts were named according to distinctive features of the region, denoted by *. Figure 2.1 Rivers sampled during the Canadian Arctic GEOTRACES program in the summer of 2015 (August). Northern rivers are displayed with black diamonds, central rivers with black squares, and continental rivers with black circles. The names of the sampled rivers are presented in Table 2.1. A simplified bedrock geology map is superimposed in the Figure; geological data was modified from Okulitch (1991) and Nunavut Geoscience (2017). 2.3.2 Sample processing and analysis River samples were analyzed for dissolved and particulate trace metals in The Pacific Centre for Isotopic and Geochemical Research (PCIGR), an analytical center at the University of British Columbia (UBC). To prevent contamination, the processing and analyses of the samples were conducted in class 1000 laboratories, maintained in an overpressure environment by HEPA 31 filtered air, and under class 100 laminar flow fume hoods. All the plasticware used during the analysis was cleaned as described in section 2.3.1. Dissolved trace metals: In order to remove the organic matrix of the samples, which complicates the analysis by inductively coupled plasma mass spectrometer (ICP-MS), and to pre-concentrate the dissolved trace metals, filtered river water was poured in Teflon flat-bottom Savillex vials, dried down in a PFA-coated heating block (120 °C), and then 400 µL of trace metal grade HNO3 was added. After acid evaporation the samples were redissolved in 1.5 mL of 1% HNO3 with trace quantities of indium as an internal standard. Suspended Particle Matter analysis: Large volume river water samples were collected into pre-cleaned 10 L HDPE cubetainers and filtered through a specially designed filtration unit described in Voss et al. (2015). Waters were filtered through 90 mm PES membrane filters (Millipore, pore size 0.22 μm) and then stored frozen. Once thawed, collected sediments were rinsed from the filters with Millipore purified water, freeze dried, then weighed; following the methods described in Voss et al. (2015). Suspended particulate matter (SPM) is reported as the weight of material recovered divided by the volume of water filtered, in mg/L. Particulate trace metals: Filters containing the particulate fraction were digested following the protocol described by Ohnemus et al. (2014). Briefly, filters were placed into Teflon flat-bottom Savillex vials containing the piranha solution (trace metal grade sulfuric acid/hydrogen peroxide [3:1]; Fisher Scientific, Ontario, Canada) and heated at 110 °C for 60-120 min to completely dissolve the organic fraction and the PES filters. The mineral matrix was digested by heating the residues of the previous step under reflux using a mixture of trace metal grade mineral acids (HNO3, HCl and hydrofluoric acid, Fisher Scientific, Ontario, Canada). The 32 residues were heated for 3-4 h at 100-110 °C, and then the sample was taken to dryness at 100-110 °C. Once dry, 2 mL of HNO3/H2O2 was added to each vial, and taken to dryness a second time on the hotplate at 100-110 °C. The final dry residue was resuspended in a small volume (100 μL) of HNO3/H2O2 and then heated at 110 °C. After vigorous bubbling ceased, vials were uncapped and dried at 135 °C. Finally, the residues were re-dissolved in 2 mL of 1% HNO3 with trace quantities of indium as an internal standard. Particulate samples were diluted before the analyses, and concentration are expressed as µg of metals per liter of river water filtered. Trace metals measurement: Dissolved and particulate Fe, Mn, Al, Ba, Cu, Ni, Zn, Pb, Ga, and Cd were analyzed from a ten-point calibration curve prepared in 1% trace metal grade HNO3 from 1 ppm certified single element standards. The analyses were conducted by a high resolution Thermo Finnigan Element2 ICP-MS at the PCIGR. A medium mass resolution was selected for Fe, Mn, and Ga analysis in order to remove the isobaric interferences caused by 40Ar16O and 40Ca16O for 56Fe, 15N40Ar for 55Mn, and 138Ba+2 for 69Ga; the rest of the trace metals were analyzed using low mass resolution. Brand new low-flow PFA-Teflon nebulizer (Elemental Scientific Inc., Omaha, USA) and sample and skimmer cones (Fisher Scientific, Ontario, Canada) were used for this study. During sample analysis, procedural blanks, filter blanks, and quality control spikes were routinely run to ensure quality throughout the measurements. The accuracy was calculated by analyzing SLRS-2 and SLRS-5 (river water, National Research Council of Canada) and JSd-1 (stream sediment, Geological Survey of Japan) certified references materials multiple times (Table A.2). As these river water reference materials do not report Ga values, the accuracy for this element was evaluated base on spiked samples. All analyzed elements for both dissolved and 33 particulate phases were in good agreement with certified reference materials. The average and standard deviation values (1SD) for dissolved and particulate trace metals are presented in Table A.2. The relative standard deviation (1RSD) of reference material was 3.5% or better for most the dissolved elements, while the values for Zn, a highly contamination prone element, were higher (~14%). The RSDs of most of the particulate analyses ranged between 3-11%, and for Pb and Ba was 16 and 20%, respectively. Lead isotopic measurement: The measurement of Pb isotopes in the suspended particulate matter (SPM) of CAA rivers was done at the Department of Earth Sciences, University of Toronto, Ontario, Canada, using a Thermo Scientific Neptune Plus Multi-Collector ICP-MS with an APEX inlet system (Elemental Scientific Inc, Omaha, Ne, USA). Trace metal clean procedures were followed during sample preparation and all reagents and matrix solutions were made using acids that were Optima-gradeTM (Fisher Scientific, Ontario, Canada). The method of sample processing, column purification, and Pb isotope analysis was based on the methods of Cheyne et al. (2018) and Reuer et al. (2003). Sample filter digests were heated and allowed to reflux in a solution of 900 µL HNO3 and 100 µL HCl for 24 h to reduce any traces of organic matter remaining after sample digestion. The solutions were then dried and the solids redissolved in ~2N HBr prior to column separation in polypropylene microcolumns loaded with AG 1 x 8 anion exchange resin (chloride-form, 200-400 mesh). The resin was cleaned with three 1 mL aliquots of 6 N HCl and multiple aliquots of MQ water, and then conditioned with 4 mL 2N HBr. Sample solutions were loaded to the resin and the matrix washed through with 2 mL 2 N HBr and 2 mL 1.7 N HCl. Finally, Pb was eluted with 4 mL 6 N HCl, dried and redissolved in 2% HNO3 for isotope analysis. The MC-ICP-MS was run with X and Jet cones to enhance the 34 sensitivity of the instrument to about 0.2V/ppb for Pb 208. This configuration allowed isotope measurements on Pb concentrations as low as ~1ppb with an uncertainty of 1000 ppm (2SD) for 206Pb/207Pb. Due to persistent blank issues with Tl, the instrumental mass bias was corrected using sample-standard bracketing with the NIST 981 Pb standard as the bracketing standard and no internal Tl correction. Each sample was strictly bracketed by standards. The errors reported are the 2SD of replicate runs, or are the largest 2SD of the run if a sample was only run one time. Two process standards (NIST 981 and USGS basalt BCR-2), which went through the same purification method as the samples, had measured isotope values (presented in Tables A.3 and A.4) that were in agreement with the published values (Galer, 1998; Weis et al., 2006; Cheyne et al., 2018). 2.3.3 Calculations and statistical analysis In this study, the enrichment factors for the studied elements were calculated using the following equation: EF= [(Me/Al) river]/[(Me/Al) crustal], where (Me/Al) river is the metal to Al ratio measured in the rivers and (Me/Al) crustal is the metal to Al ratio reported for the average crustal composition. The average element concentrations from the upper continental crust were obtained from Rudnick and Gao (2013). The statistical analysis and graphics in this manuscript were developed using MATLAB 9.2 and Python 3.6.0. River discharge data for the Mecham, Ellice, and Tree rivers were obtained from the Water Survey of Canada (Water Survey of Canada, 2017). Multivariate analysis such as factor and principal component analysis (PCA) are widely used in environmental studies to estimate sources of trace and heavy metals and to spatially characterize multiple areas based on geochemical data (Loska and WiechuÅa, 2003; Chabukdhara and Nema, 2012). In this work, 35 dissolved trace metal data were used in the PCA to assist in the study of the geographical variability of trace metal distributions in CAA, in concert with bedrock geology data (Okulitch, 1991; Nunavut Geoscience, 2017). Trace metal data were z-score normalized before PCA (Chabukdhara and Nema, 2012). 2.4 Results 2.4.1 Trace metal distributions in CAA rivers Concentrations of dissolved and particulate trace metals, and lead isotopic ratios measured in 14 remote rivers in the Canadian Arctic Archipelago (Figure 2.1) are reported in Table 2.1; an extensive review of the published data on trace metals for Arctic freshwater systems is presented in Table A.5. Dissolved trace metals: Dissolved trace metal concentrations measured in this study show some general trends (Figure 2.2). Rivers with headwaters on the Canadian continent (Figure 2.1, black circles) had high values for most of the elements with the exception of Ba and Ga. The geochemistry and flow of these rivers differ from the other rivers in this study, which have their headwaters within the islands of the CAA (discussed in section 2.5.1). The Glacier River (#2) had high concentrations of Al, Fe, Mn, Ni, Cu, Pb, and Ga; and the northern and central CAA rivers had lower concentrations of trace metals in comparison to rivers in the south and the Glacier River (Figure 2.1, black squares and diamonds). 36 Table 2.1 Dissolved and particulate trace metal concentrations (dTM and pTM) and lead isotopic ratios measured in CAA rivers. Duplicate samples, when available, are displayed for each river. dTM concentration are expressed in nM for Al, Fe, Mn, Ba, Ni, Cu and Zn, and pM for Cd, Pb and Ga. pTM concentrations are expressed as µg/L. Rivers Al Fe Mn Ba Ni Cu Zn Cd** Pb Ga Pb Isotopes* dTM pTM dTM pTM dTM pTM dTM pTM dTM pTM dTM pTM dTM pTM dTM dTM pTM dTM pTM 206/207 208/206 Akpat Kuunga (1) 205/ 218 1377 1.23/ 2.30 688 6.31/ 6.50 10 170/ 173 9.81 0.132/ 0.099 2.58 2.19/ 2.29 3.59 0.398/ 1.58 2.51 8.08/ 11 1.53/ 3.20 0.779 85/92 0.507 1.1913 2.2383 Glacier (2) 616 19355 212 13980 164 151 44 168 7.31 17 1.60 17 2.42 42 11 35 4.71 381 6.66 1.0153 2.4238 Marcil Creek (4) 116/ 117 561/ 670 39/48 307/ 360 1.11/ 1.20 6.00/ 6.70 300/ 303 2.68/ 4.85 1.08/ 1.17 6.70/ 2.07 11/11 3.89/ 3.87 0.901/ 1.16 1.58/ 2.15 8.24/ 8.72 3.23/ 5.22 0.676/ 0.678 11/12 0.298/ 0.319 1.2520 2.0612 Saaqu (5) 107/ 112 78/88 0.592/ 2.23 60/57 0.632/ 0.693 0.756/ 0.760 16/16 0.701/ 0.663 0.603/ 0.601 0.091 0.998/ 1.02 0.10/ 0.10 0.407/ 0.653 0.173/ 0.663 5.12/ 5.87 1.01/ 1.26 0.045/ 0.035 48/48 0.034/ 0.023 1.3321 1.9030 Devon Isl. (6) 133 79 0.913 51 2.35 0.553 185 0.479 0.117 <LOD 0.657 0.11 1.12 0.861 5.51 1.74 0.027 54 0.020 1.2795 1.9738 Cunningham (7) 87/91 1.81 1.33/ 1.25 1.85/ 2.15 0.366/ 0.356 0.114/ 0.099 44/44 0.024/ 0.011 1.07/ 1.09 <LOD 2.26/ 2.31 0.07/ 0.08 0.007/ 0.005 0.221 3.59/ 5.25 0.601/ 0.670 0.010/ 0.011 56/58 0.003 - - Garnier (8) 82 952 1.07 674 0.218 11 95 6.14 0.527 4.44 1.81 3.5 0.402 2.32 4.56 3.53 0.793 62 0.335 1.2177 2.0286 Mecham (9) 63 1.87 0.638 1.66 0.067 0.104 27 0.028 0.777 <LOD 1.88 0.09 0.069 <LOD 9.99 1.49 0.013 36 0.003 - - Creswell (10) 49/51 86/ 171 6.02/ 5.30 53/ 104 6.72/ 7.18 0.787/ 2.02 241/ 243 0.601/ 1.65 1.17/ 1.21 0.100 3.62/ 3.63 0.14/ 0.17 0.908 0.546/ 0.227 9.28/ 12 4.48/ 6.07 0.031/ 0.059 38/43 0.020/ 0.044 1.2144 2.0682 Le Feuvre Inlet (11) 66/66 132/ 37 3.79/ 3.35 78/19 2.28/ 4.42 1.92/ 0.514 241/ 242 0.372/ 1.92 0.765/ 0.850 <LOD 5.80/ 5.89 0.10/ 0.09 1.74/ 3.44 0.274 10/10 1.59/ 1.78 0.039/ 0.020 45/46 0.027/ 0.014 1.1350 2.3648 Pasley (12) 54 1.42 4.67 2.05 0.923 0.115 82 0.042 2.05 <LOD 5.04 0.09 0.258 <LOD 9.17 1.72 0.014 69 0.003 - - Simpson (13) 321/ 857 1469/ 1638 258/ 506 765/ 865 51/54 22/23 16/17 11/18 5.59/ 5.69 1.11/ 3.60 9.09/ 9.28 3.58/ 3.43 1.18/ 2.38 2.69/ 3.86 11/14 32/73 0.855/ 0.975 18/41 0.417/ 0.519 1.2976 2.0246 Ellice (14) 551/ 631 510/ 530 1142/ 1053 401/ 411 11/11 10/11 25/25 4.98/ 4.87 10/10 2.05 12/13 3.50/ 3.57 4.83/ 5.35 2.93/ 2.29 14/20 60/64 0.715/ 0.705 24/32 0.256/ 0.230 1.2832 2.0464 Tree (15) 96/ 264/ 73/ 190/ 26/26 5.17/ 301/ 2.94/ 8.16/ 2.13 9.19/ 3.52/ 6.91/ 3.03/ 22/23 12/15 0.638/ 19/35 0.190/ 1.5121 1.6670 37 209 287 133 202 5.74 307 3.09 8.17 9.33 3.49 7.19 4.51 0.641 0.183 <LOD: concentrations that were below the limit of detection. Ni LOD: 0.086 µg/L and Zn LOD: 0.038 µg/L *: Pb isotopic ratios were measured in the particulate phase **: Particulate Cd was not analyzed in this study In addition to the general trends highlighted above, Marcil Creek (#4) had concentrations of dissolved Fe and Cu greatly exceeding the values of the other northern rivers. Unlike the distribution of most trace metals analyzed in this study, Ga showed slightly lower values (18-41 pM) in the continental rivers than most of the central and northern rivers (35-92 pM). Dissolved Ba concentrations displayed a large spatial variability across the sampled rivers showing an opposite pattern to the rest of the elements; similar Ba distributions have been observed by Alkire et al. (2017) in CAA rivers. Moreover, Ba concentrations spanned the range of values reported for Eurasian rivers (~20-141 nM) and North American rivers (~170-425 nM; Guay et al., 2009). 38 Compared with available data in the literature, the dissolved concentrations measured in most of the central and northern CAA rivers (excluding the Glacier River) were extremely low, especially for Cunningham (#7), Mecham (#9) and Pasley (#12) rivers. For instance, the concentrations of Al (~2-3 times), Fe (~3-34 times), Mn (~2-100 times), Ni (~2-40 times), Cu (~2-5 times) and Pb were ~3-32 times lower than the lowest reported values for Arctic rivers, including small Alaskan and Russian Arctic rivers and creeks with comparable flow and size (Table A.5 and references therein). On the other hand, the concentrations of the former trace metals (Al, Fe, Mn, Ni, Cu, and Pb) measured in continental CAA rivers lie in the lower end of the range reported for other Arctic rivers. The dissolved concentrations of Ba, Zn, and Cd for most of the studied CAA rivers were in the range of values reported for other Arctic rivers, Table A.5 and references therein. Figure 2.2 Dissolved trace metal concentrations measured in 14 CAA rivers; results are expressed on a logarithmic colorbar. Particulate trace metals: Particulate trace metal concentrations are expressed as µg per liter of water filtered, and, therefore, these values are largely controlled by the SPM 39 concentrations, which varied greatly across the rivers sampled (Table A1). The particulate concentrations of Al and Fe were ~2 to ~3 orders of magnitude higher than Ba, Ni, Cu, Zn, Pb, and Ga, reflecting the relative abundance of these metals in the continental crust (Figures 2.3a, 2.3b and A.1, Table 2.1). Glacier River (#2) had extremely high concentrations of all analyzed elements, reflecting its high SPM concentration (Table A1). The remaining rivers can be grouped into three different categories based on their concentrations of particulate trace metals (Figures 2.3 and A.1, and Table 2.1). The Akpat Kuunga (#1), Garnier (#8), Simpson (#13), Ellice (#14), and Tree (#15) rivers as well as Marcil Creek (#4) have concentrations ~1 to 2 orders of magnitude higher than the other two groups; the Saaqu (#5), Devon Island (#6), and Creswell (#10) rivers as well as the river in Le Feuvre Inlet (#11) had intermediate concentrations; and the Cunningham (#7), Mecham (#9), and Pasley (#12) rivers, had concentrations ~1 order of magnitude lower than the âintermediateâ rivers. Compared with available data in the literature, Glacier River (#2) exhibited concentrations of particulate trace metals which were ~3 to 94 times greater than those reported for other Arctic rivers, but comparable to those reported for major world rivers (Table A.5 and references therein). The particulate trace metal concentrations reported for the Akpat Kuunga, Garnier, Simpson, Ellice, and Tree rivers and for Marcil Creek were similar to or slightly higher than data published for other Arctic rivers (Mn: ~4-14, Cu: ~4 times higher). Particulate concentrations measured in the Saaqu, Devon Island, and Creswell rivers as well as the river in Le Feuvre Inlet were at the same level as values reported for major Arctic rivers, but lower than those reported for major world rivers for Mn, Ba, Ni, Zn, and Pb (Table A.5 and references 40 therein). Cunningham, Mecham, and Pasley rivers had the lowest concentrations for most of the analyzed trace metals (excluding Cu), with values that were lower than major Arctic rivers. 2.4.2 Particulate trace metals: enrichment factors and lead isotopic analysis Particulate Ba/Al and Pb/Al ratios measured during this study are displayed in Figures 2.3a and 2.3b, along with the estimated crustal metal/Al ratios, used to evaluate potential contributions of trace metals from external sources, or sources that differ from upper continental crust. Lead isotopic composition (208Pb/206Pb versus 206Pb/207Pb) and the relationship between the 206Pb/207Pb ratios and Pb/Al ratios are also displayed in Figures 2.3c and 2.3d. Particulate elemental ratios, normalized to Al, for the rest of the elements are presented in Figure A.1. Fe/Al, Mn/Al, Ba/Al, and Ga/Al ratios (0.538, 0.0078, 0.0087, and 0.0003, respectively) were in agreement to those reported in the literature for crustal ratio (Rudnick and Gao, 2013; Figures 2.3a and A.1). However, in the Cunningham (#7), Mecham (#9), and Pasley (#12) rivers Mn and Ga ratios were ~ 6 and up to 11 times higher than the average crustal ratios. In contrast, an enrichment of Ni, Cu, Zn, and Pb over the Al fraction compared to upper continental crust was observed for most of the sampled rivers (Figures 2.3b and A.1). High enrichment factors (EFs, section 2.3.3) for Ni were observed in Marcil Creek (#4), as well as Garnier (#8), Ellice (#14), and Tree rivers (#15) with values that were 7 to 13 times higher than the average crustal ratios. Similar results were found for Cu and Pb, with EFs ranging from 11 to 37 and 4 to 11 times higher than crustal ratios in the same rivers. The highest values were found in the Cunningham (#7), Mecham (#9), and Pasley rivers (#12) with extremely high EFs for Cu (128-177 times higher) and Pb (29-47 times higher). Zn/Al ratios were remarkably elevated in the Cunningham River (150 times higher) followed by the Devon Island, Saaqu, Ellice, and Tree 41 rivers (6-17 times higher). It is important to mention that the Cunningham (#7), Mecham (#9), and Pasley rivers (#12) had the lowest concentrations of particulate Al, which could have influenced the EF calculations producing artificially high EFs for these rivers. The Pb isotopic composition of SPM for 11 CAA rivers is presented in Figure 2.3c and Table 2.1. Generally, particulate Pb-isotopic composition was quite heterogeneous across CAA rivers in our study. For example, the Tree River (#15) exhibited the most radiogenic signature for 206Pb/207Pb (1.5121) and the lowest value for 208Pb/206Pb (1.6670), while the Glacier River (#2) had the most unradiogenic value of 206Pb/207Pb (1.0153) and the highest 208Pb/206Pb (2.4238). The isotopic composition of the other CAA rivers fell between these two extremes. The wide range in Pb isotopic composition observed in this study encompasses values reported from multiple studies from different regions and different natural sample types (Figure 2.3c). The Pb/Al versus 206Pb/207Pb plot revealed that there is no recognizable change in the Pb isotopic composition with increasing Pb enrichment (Figure 2.3d). 42 Figure 2.3 Particulate Ba/Al and Pb/Al ratios collected from CAA rivers (a and b). Dashed lines indicate the average crustal metal/Al ratio (Rudnick and Gao, 2013) and bolded numbers indicate those rivers which are located in close proximity to mining exploration projects and/or communities. c) Particulate 208Pb/206Pb versus 206Pb/207Pb composition from CAA rivers and isotopic signatures from selected sources (Godwin and Sinclair, 1982; Graney et al., 1995; Millen et al., 1995; Sangster et al., 2000; Simonetti et al., 2003; Lima et al., 2005). d) Pb/Al versus 206Pb/207Pb plot showing the relative enrichment and changing Pb isotopic ratios of CAA rivers. The names of the sampled rivers are presented in Table 2.1. 2.5 Discussion 2.5.1 Spatial variability of dissolved trace metals in CAA rivers: linking trace metal distributions with environmental variables and bedrock geology All dissolved trace metals, with the exception of Ba, displayed distinctive spatial patterns in their concentrations. For most elements, lower concentrations were measured in the northern and central CAA rivers, whereas high concentrations were observed in the southern continental rivers (Figure 2.2); for Ga this pattern is inverted. Marcil Creek (#4) stands out from these trends with noticeably elevated Fe (39/48 nM) and Cu (11 nM) concentrations compared with other northern rivers (1.3±0.6 and 1.7±0.6 nM, respectively). Marcil Creek has a community nearby 43 (Arctic Bay, ~900 people) and dissolved organic carbon (DOC) concentrations approximately 3 times higher than the other northern rivers (Table A.1); both of these distinctive features may help to explain the higher concentrations of Fe and Cu, especially the high DOC concentrations, as the distributions of these elements are strongly controlled by their complexation with organic substances (Rember and Trefry, 2004; Dahlqvist et al., 2007; Pokrovsky et al., 2010). To further investigate the geographic variability of dissolved trace metals in CAA rivers and understand their drivers, a PCA was applied to the dissolved trace metal data. Three principal components (PC) were selected which accounted for approximately 90% of the total variance (PC1: 52.9%, PC2: 25.2%, PC3: 13.7). Figure 2.4 PCA loadings and score plots for dissolved trace metals. The bedrock geology of each river is superimposed in the figure, with the names of the sampled rivers as presented in Table 2.1. PCA results support the contrasting spatial distributions of trace elements observed in CAA rivers (Figure 2.4). The northern and central rivers, which share similar trace metal signatures (lower concentrations of most trace metals) are distinctively clustered in the negative PC1 axis, differing from the continental rivers and the Glacier River grouped in the positive PC1 axis. The drainage basins of northern and central rivers generally have similar bedrock geology, characterized by sedimentary rocks such as limestones, siltstones, sandstones, dolomites, and 44 carbonates from the Ordovician to the Silurian periods (Figures 2.1 and 2.4). The bedrock geology of the drainage basins of Akpat Kuunga (#1) and the Glacier (#2) rivers as well as Marcil Creek (#4) differ from the other northern and central Arctic rivers; the region drained by these rivers is characterized by an older formation of anhydrites, carbonates, basalts, and clastic sediments from the middle Proterozoic (Figures 2.1and 2.4). Despite the difference in the bedrock geology, these rivers are clustered with the rest of the northern and central rivers in the PCA. Glacier River (#2) is the only northern river located in the positive PC1 and PC3 axes. This may be explained by the extremely high particulate concentration of this river (1-4 orders of magnitude higher than all analyzed rivers, Table A.1) which may influence the geochemistry of dissolved trace metals. The bedrock geology of the continental rivers is characterized by intrusive rocks (granites, monzonites, and syenites) from the Archean and lower Proterozoic for the Simpson (#13) and Ellice (#14) rivers and Proterozoic metasedimentary rocks for the Tree River (#15; Figures 2.1 and 2.4). Even though the distribution of trace metals is well correlated with the bedrock geology of CAA, other environmental variables are also expected to play an important role in controlling their spatial distributions. This is also evident in the PCA, where the northern rivers (diamonds) are tightly clustered on the PC1 axis, differentiating from the central rivers (squares) despite sharing similar bedrock geology. The latitudinal gradients of climatic conditions (e.g., solar radiation, precipitation, temperature) across CAA likely account for many of the differences between northern and continental CAA watersheds. For example, the development of permafrost soils and vegetation, soil organic carbon content, river discharge, and permafrost active layer thickness, all increase from north to south (e.g., AMAP, 1998; Hugelius et al., 2014; Bring et al., 45 2016; Water Survey of Canada, 2017; Tarnocai, 2018). These environmental gradients are expected to influence the geochemistry of major and trace elements in downstream freshwater environments, as well as the distribution of DOC, colloids, particulate organic carbon (POC), and SPM, which affect the solubility and partitioning of trace metals (Dai and Martin, 1995; Shiller, 1997; Holmes et al., 2012; Mann et al., 2012; Rosa et al., 2012; Barker et al., 2014). In particular, the S-N gradient in the thickness of the permafrost active layer, an important zone where most geomorphic, hydrological, soil microbial, and biogeochemical activities take place, will have important consequences for the interaction of river waters with both organic and mineral soils, thus strongly influencing their metal loads (Bring et al., 2016). Furthermore, unlike the northern CAA, the drainage basins of continental rivers are characterized by the presence of floodplains and wetlands (Millot et al., 2003); interactions of river water with these distinctive ecosystems can alter the fluvial transport of dissolved and colloidal nutrients, DOC, and trace elements (Millot et al., 2003; Rosa et al., 2012; Bring et al., 2016; Shim et al., 2017). Rivers from this study exhibit a clear latitudinal gradient in the distribution of DOC, where continental CAA rivers had notably higher values than northern rivers (360±145 and 37±17 µM C; Table A.1). Strong positive correlations (R> 0.80) were found for DOC versus dissolved Al, Fe, Ni, and Pb, followed by Cu (R= 0.77) and Mn (R= 0.70), and moderate correlations (0.44<R< 0.57) were found for Zn, Cd, and Ga (Table 2.2). The strong association of trace metals with organic complexes has been described for numerous freshwater systems in the Arctic and worldwide (Dai and Martin, 1995; Mitchell et al., 2001; Dahlqvist et al., 2007; Pokrovsky et al., 2010; Rosa et al., 2012; Shim et al., 2017). 46 Table 2.2 Pearsonâs correlation matrix for dissolved trace metals and dissolved organic carbon (DOC) from small CAA rivers (excluding the Glacial River). DOC data is reported in Table A.1. DOC Al Fe Mn Ba Ni Cu Zn Cd Pb Ga DOC 1.00 Al 0.88* 1.00 Fe 0.84* 0.86* 1.00 Mn 0.70* 0.71* 0.37 1.00 Ba -0.28 -0.38 -0.34 -0.07 1.00 Ni 0.81* 0.74* 0.83* 0.59* -0.09 1.00 Cu 0.77* 0.62* 0.69* 0.49* 0.22 0.78* 1.00 Zn 0.51* 0.43** 0.55* 0.45** 0.34 0.84* 0.67* 1.00 Cd 0.57* 0.44** 0.51* 0.56* 0.30 0.83* 0.69* 0.89* 1.00 Pb 0.95* 0.97* 0.92* 0.68* -0.35 0.84* 0.71* 0.52* 0.55* 1.00 Ga -0.44** -0.29 -0.37 -0.35 -0.21 -0.49* -0.68* -0.43** -0.46** -0.42** 1.00 * Correlation is significant at p-value<0.01 (2-tailed) ** Correlation is significant at p-value<0.05 (2-tailed) Partition coefficients (Kd, μg/gμg/mL) describing how trace metals distribute between aqueous and particulate phases can lend insight to trace metal distributions throughout CAA rivers, including their complexation with organic species and their relative solubility. Higher Kd indicates a greater tendency for metals to concentrate in the solid phase (particle reactive trace elements), whereas lower Kd indicates a greater affinity to the aqueous phase (Li, 2011). The average Kd from CAA rivers (not including Glacier River; discussed below) increases from Ba>Cu>Ni>Ga>Zn and Mn>Al>Fe>Pb (Table 2.3) following the general trend expected by their particle reactivity (Tang et al., 2002; Ho et al., 2007). In addition, three rivers stand out as having consistently low Kd values for most trace elements, including the Glacier (#2), Simpson (#13), and Ellice (#14) rivers (Table 2.3). The estimated Kd coefficients for the Glacier River, which had the largest SPM concentration (Table A.1), were notably low compared with other northern CAA rivers. This could be attributed to the particle concentration effect, which results in a decline in the Kd as SPM increases, and is observed in many fresh and estuarine environments 47 (Benoit and Rozan, 1999; Tang et al., 2002). Compared with central and northern CAA rivers, the continental rivers display relatively lower Kd values; these rivers have significantly higher DOC concentrations (Table A.1), and therefore, as result of the competitive colloidal-organic complexation, the trace metal solubility is enhanced yielding lower Kd values (Benoit and Rozan, 1999). Table 2.3 Partition coefficients (Kd, ðð/ððð/ðð³) of trace metals in CAA rivers. Kd values were calculated as the ratio of dissolved trace metal concentrations to particulate concentrations. Values are expressed as log10 of Kd. Rivers log Kd Al log Kd Fe log Kd Mn log Kd Ba log Kd Ni log Kd Cu log Kd Zn log Kd Pb log Kd Ga Akpat Kuunga (1) 7.5 8.9 6.5 4.7 7.7 6.5 6.7 8.3 7.0 Glacier (2) 6.6 6.7 4.8 5.0 5.2 5.8 6.0 3.4 3.0 Marcil Creek (4) 7.8 7.6 7.5 4.5 7.3 6.2 6.9 8.4 8.1 Saaqu (5) N/A N/A N/A N/A N/A N/A N/A N/A N/A Devon Isl. (6) N/A N/A N/A N/A N/A N/A N/A N/A N/A Cunningham (7) 5.9 7.5 6.8 3.5 5.7 5.8 8.6 8.0 5.7 Garnier (8) 7.5 8.9 7.8 4.5 7.0 6.4 6.8 7.9 6.8 Mecham (9) 6.9 8.5 8.3 4.7 6.8 6.7 7.7 8.5 6.9 Creswell (10) 7.6 8.0 6.2 4.1 5.8 5.4 6.4 7.3 6.7 Pasley (12) 7.5 8.5 7.9 5.1 6.9 7.0 7.6 9.2 7.4 Simpson (13) 6.7 6.3 5.6 5.5 5.6 5.5 6.2 6.6 7.1 Ellice (14) 6.8 6.1 6.6 5.5 5.8 6.0 6.2 7.1 7.4 Tree (15) N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A: Not available. The Kd from Saaqu (#5), Devon Isl. (#6), Le Feuvre Inlet (#11) and Tree (#15) rivers were not calculated due to the lack of SPM data from these rivers. 2.5.2 Constraining dissolved trace metal concentrations from coastal draining rivers in the Canadian Arctic Archipelago The distributions of trace metals in CAA rivers display a clear spatial pattern between northern and central rivers and those rivers draining the Canadian continent. This pattern can be employed to estimate the dissolved concentrations for other rivers in the archipelago, not sampled in this study, based on their geographic location and bedrock geology. The remote locations of many CAA rivers and their limited access impose significant constraints on sample 48 collection at different periods during the hydrological cycle, precluding the assessment of the seasonal variability of trace metals in much of this region. In high latitude rivers, major and trace element as well as DOC, colloids, and SPM, exhibit distinct seasonal variability in their distributions, in particular during the spring snowmelt (Rember and Trefry, 2004; Hölemann et al., 2005; Dahlqvist et al., 2007; Pokrovsky et al., 2010; Bagard et al., 2011; Holmes et al., 2012; Mann et al., 2012; Barker et al., 2014; Bring et al., 2016). During the spring snowmelt period the topsoil layers are intensively flushed, mobilizing organic substances and trace elements from the permafrost active layer interstitial water into freshwater systems (Rember and Trefry, 2004; Bagard et al., 2011; Barker et al., 2014). Insoluble trace elements (e.g., Al, Ba, Co, Cu, Fe, Mn, Ni, and Pb), in contrast with major ions, exhibit a sharp increase in their concentrations during the spring season, with values up to ~25 times higher at spring melt season than during baseline flow in the winter (Rember and Trefry, 2004; Hölemann et al., 2005; Bagard et al., 2011; Barker et al., 2014; Peterson et al., 2016). This spring peak is closely correlated with a concomitant increase in DOC, underscoring the importance of organic and organo-mineral colloids facilitating the mobilization of insoluble elements in Arctic freshwater systems (Dahlqvist et al., 2007; Pokrovsky et al., 2010; Bagard et al., 2011). Additionally, the seasonal variability of trace metals is known to be controlled by the reductive dissolution of Fe-Mn oxyhydroxides and adsorption/desorption reactions (Hölemann et al., 2005). In this study, CAA rivers were sampled in August during the low flow period (Figure 2.5, middle panel). As we were not able to collect samples in different seasons or flow periods, it is difficult to put our observations in context with the annual hydrological cycle. Therefore, we 49 attempt to account for the expected seasonal variability in trace metal concentration by comparing with observations from the PARTNERS and Arctic-GRO data sets that sampled the major Arctic rivers throughout the year (Peterson et al., 2016; Shiklomanov et al., 2018). With the assumption that trace metal concentrations in the small rivers sampled in this study behave like those in the largest Arctic rivers, we can use this comparison to estimate trace metal concentrations from CAA rivers at the high flow period. First, we applied a linear regression analyses of dissolved trace metal concentrations and DOC versus flow for the Kolyma, Mackenzie, Yenisey, Ob, and Lena rivers to determine the relationship between trace metal concentrations and daily average river discharge (Table A.6 and Figure 2.5, upper panel). We chose to use a linear regression analysis as it fit the general trends of the data well (Table A.6) and, with the limitations on our available CAA data set (one sample from one point in time), we cannot justify the use of a more complex concentration-discharge model to describe our CAA systems. For most of the rivers, the regression analysis showed strong significant relationships between Fe, Cu, Pb, and DOC versus discharge, while Mn and Ni showed strong significant relationships only in the Kolyma and Yenisey rivers (Table A.6). When the regression analysis showed a significant relationship (high R2 and p-value< 0.05), the percentage difference in trace metal concentration between the low and high flow periods was calculated, and this percent value was then used to estimate the trace metal concentrations during the high flow season in the CAA rivers. We chose to apply the percentage difference results from the Kolyma River only to our CAA data set as this river most closely resembles the environmental features of the CAA rivers studied. For instance, the Kolyma River has the smallest and northernmost watershed which is entirely underlain by continuous permafrost (Mann et al., 2012). Moreover, the Kolyma River has the lowest flow (101 km3/year), and relatively low values of POC (56±29 µM C), 50 DOC (500±172 µM C), and SPM (42±38 mg/L); features which are similar to the CAA rivers in our study (Table A.1) and other small CAA rivers (Alkire et al., 2017). Note that this exercise was not carried out for Ba due to its highly variable distribution, nor was the Glacier River (#2) included in this comparison due its anomalously high trace metal and SPM concentrations, Tables 2.1 and A.1. Figure 2.5 Upper Panel: average daily discharge of the Kolyma River in 2004, featuring the dates for which trace metal data is available (green dots), and examples of regression analysis of Cd, Fe, and Cu versus flow; data retrieved from Peterson et al. (2016) and Shiklomanov et al. (2018). Full results of the regression analysis are presented in Table A.6. Middle Panel: daily discharge of the Mecham, Ellice, and Tree rivers in 2015, featuring the dates when rivers where sampled; data retrieved from the Water Survey of Canada (2017). Lower Panel: ranges of dissolved trace metal concentrations measured for northern and continental CAA rivers and projected concentrations during the high flow season (dashed lines). The range of dissolved trace metal concentrations measured for northern and continental CAA rivers and the projected concentrations during the high flow season (spring freshet) are presented in Figure 2.5, lower panel. Based on the dissolved trace metal concentrations measured in this study, rivers located in northern and central CAA with bedrock characterized by Paleozoic sedimentary rocks (e.g., Melville, Bathurst, Cornwallis, Devon, Somerset, Baffin, Prince of 51 Wales islands; Figure 2.1) are predicted to have relatively low values of Al, Fe, Mn, Ni, Cu, Cd, and Pb, as well as high values of Ga (Figure 2.5, lower panel). On the other hand, rivers draining from the Canadian continent with bedrock characterized by old Archean intrusive rocks (e.g., continental Nunavut; Figure 2.1) are predicted to have relatively high concentrations of Al, Fe, Mn, Ni, Cu, Cd, and Pb, as well as low values of Ga (Figure 2.5, lower panel). As CAA rivers were sampled during the low flow season, we anticipate trace element concentrations to differ during the high flow season, as exhibited for many Arctic rivers. Based on the seasonal behavior of Kolyma River, we project high flow concentrations to be approximately 190, 1280, 160, 140, and 110 % higher than low flow concentrations for Fe, Mn, Ni, Cu, and Pb, respectively (Table A.6 and Figure 2.5, lower panel). Similar increases of dissolved trace metal concentrations (~100% to more than 1300%) during the high flow season have been reported for other small Arctic rivers draining coastal Alaska and Russia (Rember and Trefry, 2004; Guay et al., 2010; Pokrovsky et al., 2010; Barker et al., 2014), and the estimated high flow concentrations for the small CAA rivers in our study are within the ranges of those reported for other small Arctic rivers during the spring freshet period (Table A.5 and references therein). It is worth mentioning that the effect of the spring freshet in enhancing the concentrations of insoluble trace metals is expected to be larger in continental CAA rivers than in the northern ones. Watersheds of the small, coastal draining rivers in the southern CAA are characterized by the presence of wetlands, higher soil organic carbon content, as well as increased discharge and DOC concentrations compared to the central and northern CAA rivers, where watersheds are characterized primarily by exposed bedrock (AMAP, 1998; Hugelius et al., 2014; Bring et al., 2016; Alkire et al., 2017; Tarnocai, 2018). These differences are likely to have an influence on the complexation and sources of trace metals (Barker et al., 2014). 52 In order to provide some context to evaluate the influence of trace metal inputs from small CAA rivers into the marine environment, we compared the concentration measured in this study with those reported for Canada Basin surface waters that would be transiting through the CAA. For most of the CAA rivers, the concentrations of dissolved Al (~20-344 times), Ba (~2-8 times), and Ga (~3-22 times) are higher than those found in the Canada Basin, suggesting that these rivers could act as a potential source for these metals as they mix with Canada Basin derived waters within the CAA coastal system (Thomas et al., 2011; Giesbrecht et al., 2013; McAlister and Orians, 2015). Additionally, the Glacier River and rivers in the southern CAA have higher concentrations of Mn (~3-40 times), Fe (~40-550 times), Cu (~7 times), and Pb (~3-16 times) than those reported in the Canada Basin (Cid et al., 2012; Kondo et al., 2016; Colombo et al., 2019b), indicating these rivers could also increase metals loads to ocean waters transiting the CAA, provided these metals make it through the local estuarine zones. Estuaries are highly productive transition environments (Fichez et al., 1992; Arhonditsis et al., 2007) where, in addition to particulate settling, the increase of ionic strength in the river-water interface leads to complex processes such as flocculation, scavenging and precipitation which affect the distribution and concentration of trace metals (Boyle et al., 1974; Tanguy et al., 2011; Bruland et al., 2013). On the other hand, the concentrations of Ni, Zn, and Cd in Canada Basin surface waters are in the same range or higher than those found in small CAA rivers (Cid et al., 2012; Kondo et al., 2016), suggesting that CAA rivers would serve to dilute these trace metals in transiting waters. This study reports the first survey of dissolved trace metal concentrations for coastal draining rivers in the Canadian Arctic Archipelago, a necessary step in determining their 53 influence on the distribution of trace elements in CAA marine system. Due to the lack of seasonal observations, we are unable to estimate fluxes of trace metals to the CAA marine system, as any estimates would be highly uncertain. Further research is needed to establish budgets of trace metal fluxes from CAA rivers to the coastal marine environment, including a comprehensive investigation of the seasonal variability of trace metal concentrations and their behavior along the river-ocean mixing-gradients as they are transported into river estuaries. 2.5.3 Particulate trace metals: sources, enrichment factors, and lead isotopic analysis Unlike dissolved trace metals, particulate distributions did not show any spatial trends, as their concentrations are controlled primarily by river SPM. For instance, the anomalously high concentrations of particulate trace metals observed in the Glacier River (#2) can be explained by the high load of particles of this river. Glacier River has a high suspended particulate concentration likely composed of glacial flour, and had a SPM concentration of 261.65 mg/L, much higher than values measured for the other rivers (from ~0.03 to ~19 mg/L, Table A.1). Trace element concentrations in SPM are largely influenced by grain-size composition, as metals are usually concentrated on fine-grained materials such as clays (Windom et al., 1989; Covelli and Fontolan, 1997). In this study, trace element concentrations were normalized for mineralogical variability using Al; this element is one of the most important constituents of aluminosilicates, the main group of minerals found in fine sediment fractions. Al has been extensively used as a normalizer of particulate trace metal concentrations in numerous studies because of its high natural abundance in the earth's crust, the relative constant ratio of metal to aluminum in crustal rocks, and scarce anthropogenic sources (Windom et al., 1989; Covelli and Fontolan, 1997; Cécile Guieu and Martin, 2002; Yigiterhan et al., 2011). 54 Elemental to Al ratios measured in SPM were compared to average upper continental crustal (UCC) ratios in order to determine if external sources, or sources differing greatly from UCC elemental composition, are important in this region. Particulate ratios show two distinctive patterns. For Fe, Mn, Ba, and Ga, the ratios are in agreement with those reported for the UCC (Figures 2.3a and A.1), reflecting the crustal composition of the catchment basin. Conversely, the Ni, Cu, Zn, and Pb to Al ratios display an enrichment compared to the UCC for those rivers close to mining exploration projects (Figures 2.3b, 2.6 and A.1). Regional differences in the composition of the bedrock being weathered, potentially in combination with anthropogenic sources from mining activities, are proposed to be the main drivers of the high EFs of Ni, Cu, Zn, and Pb observed in the Tree (#15), Ellice (#14), Pasley (#12), Cunningham (#7), Garnier (#8), and Mecham (#9) rivers, as well as in Marcil Creek (#4; Figures 2.3b and A.1). Alternative explanations such as errors during sampling and/or sample processing and different particle distribution, characterized by more Fe/Mn oxyhydroxides than aluminosilicates, are not expected to influence the particle ratios. Contamination during sample processing is always a risk when working with trace elements; however, solution and filter blanks were always low and all the samples were corrected by blank subtraction. Some authors reported shifts in the SPM class spectrum that can alter the Me/Al values. A commonly studied case is the adsorption of trace metals onto Fe/Mn oxyhydroxide particles, increasing the ratio of these elements over Al (Hölemann et al., 2005; Yigiterhan et al., 2011). The fact that the only elements that show an enrichment over the crustal composition are Ni, Cu, Zn, and Pb, and not Ba or Ga, suggests that oxyhydroxides are not the main factor explaining the distinctive ratios observed in this study. 55 Although the average crustal composition used to calculate these EFs agree with those reported for the Canadian Precambrian shield (Shaw et al., 2008), regional differences do exist. Sediment-hosted Zn, Cu, Ni, and Pb deposits, metal-bearing volcanogenic massive sulphide (VMS) deposits, and gold deposits are present in the CAA (Savard et al., 2000; Morrison, 2004; Bigio et al., 2015). Owing to the economic importance of these deposits present in Nunavut (Nassichuk, 1987; Galley et al., 2007), this region has been a site of mining exploration and development since the1970âs (Nassichuk, 1987; Morrison, 2004). In 2015 alone, more than 40 mining projects were active and 132 prospecting permits were issued in Nunavut (Bigio et al., 2015; Figure 2.6). In addition to local point sources, long-range atmospheric transport of metals to the Arctic and Antarctic regions, including Pb, Cu, Zn, Cd, Ni, and Cr, whose anthropogenic emissions have largely surpassed natural emissions (Nriagu, 1996; Callender, 2013), have been widely reported in the literature and studied through the analysis of ice core samples (Nriagu, 1990; Boutron et al., 1994; Boutron et al., 1995; Sherrell et al., 2000; Johansson and Tyler, 2001; Shotyk et al., 2005; Callender, 2013). 56 Figure 2.6 Mining exploration projects carried out in Nunavut from 2011 to 2016 (Costello et al., 2011; Senkow et al., 2012; Senkow et al., 2013; Bigio et al., 2014; Bigio et al., 2015). a) northern and central Arctic rivers and b) continental Arctic rivers. The sampled rivers for trace metals are indicated by filled black diamonds, squares and circles; bolded numbers indicate those rivers which are located in close proximity to active mining exploration projects and/or communities. The names of the sampled rivers are presented in Table 2.1. In order to assess whether anthropogenic or non-anthropogenic related sources are driving the high EFs observed in some rivers, we further investigate the sources of particulate trace metals in this region by analyzing the Pb isotopic composition of SPM from 11 CAA rivers. These rivers were extremely heterogeneous in their Pb isotopic composition (Figure 2.3c), displaying a large range from the Tree River (206Pb/207Pb: 1.5121 and 208Pb/206Pb: 1.6670) to the Glacier River (206Pb/207Pb: 1.0153 and 208Pb/206Pb: 2.4238). Most of the remaining rivers have values ranging between 1.1913 and 1.3321 for 206Pb/207Pb and between 2.0612 and 1.9030 for 57 208Pb/206Pb. The large range of particulate lead isotope data observed may be related to the diversity in composition and age of the bedrock geology across CAA (Figure 2.1). Lead isotopes indicate that external sources of metals, coming from long-range atmospheric transport as well as from fossil fuel combustion by heavy machinery operations used in mines and communities, are not the main factors explaining the enrichment of Pb as well as other anthropogenic metals (e.g., Ni, Cu, and Zn) observed in some CAA rivers. If external sources of Pb were the major sources of Pb enrichment, it might be expected that they would have similar Pb isotopic compositions and loadings. This should lead to a correlation between Pb enrichment and Pb isotopes, however, this is not observed in our data (Figure 2.3d). In addition, CAA rivers that are closest to mining exploration projects (with the highest EFs) have similar Pb isotopic ratios to those rivers located far away from mines (with the lowest EFs), suggesting that Pb is from natural local sources (Figure 2.3d). Lead isotopic data lend an alternative explanation for the extremely high EFs of Pb, Ni, Cu, and Zn observed in CAA rivers located in closest proximity to mining exploration projects. Regional VMS and sediment-hosted mineral deposits are well known to be naturally enriched in base metals and gold (Galley et al., 2007), and hence, may explain the local enrichments of some metals for those rivers located close to mineral deposits. Detailed upstream and downstream sampling around mining areas would be needed to ascertain whether the preferential exploitation of VMS and sediment-hosted mineral deposits was contributing to local trace metal enrichment, or if the high EFs observed reveal the natural weathering processes of watersheds containing these rocks-types. 58 2.6 Conclusions Dissolved trace metal concentrations measured in 14 small, coastal draining rivers in the CAA are generally lower than values reported for other Arctic and world rivers. This pattern is particularly notable for rivers from the northern CAA, which have some of the lowest trace metal values ever reported for freshwater systems. This study demonstrates that the distribution of dissolved trace metals across the CAA is largely influenced by the South-North environmental gradients (e.g., DOC) and the bedrock geology of the region. Linking the spatial variability of trace elements to the seasonal data available from the Kolyma River, we estimate that spring peak discharge from CAA rivers could have dissolved trace metal concentrations on the order of ~200 % of the values measured during low flow in summer. Particulate trace metals show an enrichment of Pb, Ni, Cu, and Zn relative to the average crustal composition in rivers located near active mining exploration projects. Nonetheless, as this enrichment is not accompanied by any noticeable trend in Pb isotopic signatures, external sources are not considered responsible for the high EFs, but instead, enrichment of these elements appears to be due to local sources. This study establishes a baseline of dissolved and particulate trace metal data in small rivers across the CAA, and highlights the importance of continued investigations of spatial and temporal distributions of trace metals within Arctic rivers draining coastal permafrost systems, which are sensitive to both climate change and increased anthropogenic activities. 59 Chapter 3: Tracing Dissolved Lead Sources in the Canadian Arctic: Insights from the Canadian GEOTRACES Program 3.1 Summary This study addresses an important knowledge gap in the distribution of lead (Pb) in the Arctic Ocean, by presenting dissolved Pb concentrations from samples collected during the GEOTRACES Canadian cruise in 2015. Dissolved Pb showed an increase in concentration from the Canada Basin towards Baffin Bay and the Labrador Sea. Canada Basin was the most isolated region from anthropogenic Pb inputs with low background concentrations (1.4-6.2 pmol kgË1) across most of the basin, although higher Pb features from the advection of Pacific derived waters and sea ice meltwater were present. Likewise, high Pb peaks (13.5-26.6 pmol kgË1) along narrow isopycnal surfaces (ÏÎ¸ 27.4-27.6) in Baffin Bay were attributed to the advection of North Atlantic waters spreading a high Pb signature into the bay, contrasting with the low values (2.5-7.4 pmol kgË1) present in the mixed layer, Arctic and Baffin Bay Deep waters. The Labrador Sea, largely influenced by the recirculation of North Atlantic waters, had the highest Pb concentrations (~17-34 pmol kgË1) measured in this study. The Canadian Arctic Archipelago (CAA) represents a transition environment influenced by Arctic waters imprinting a low Pb signature in the western CAA and the southern side of Parry Channel, while Baffin Bay waters prevailed in the eastern CAA recirculating westward along the northern side of Parry Channel and progressively losing their relatively high Pb signature. Extremely low concentrations were measured in Canada Basin and CAA waters, which reflect the remoteness of this region from anthropogenic inputs as well as the old ventilation age of deep waters masses, providing a baseline for assessing future Pb studies. Finally, we successfully integrate modelling data with 60 field observations demonstrating that dissolved lead, due to its distinctive anthropogenic signature in North Atlantic waters, can be a useful complimentary tracer of water masses in the Canadian Arctic Ocean. 3.2 Introduction The geochemical cycle of lead (Pb) has been largely altered due to anthropogenic emissions, primarily by coal and leaded gasoline combustion, which have surpassed its natural sources (Nriagu and Pacyna, 1988; Nriagu, 1990; Callender, 2013; Mahowald et al., 2018). The large spread of Pb contamination has been studied from numerous environmental matrices such as sediments (Graney et al., 1995; Lima et al., 2005; Cheyne et al., 2018), ice cores (Osterberg et al., 2008; Gross et al., 2012) and corals (Shen and Boyle, 1987; Kelly et al., 2009), as well as with direct measurements of ocean waters (Boyle et al., 2014, and references therein). The North Atlantic Ocean was heavily impacted by Pb pollution due to early industrializing economies surrounding this basin, and it is certainly the most studied region with accurate Pb data from 1979 to the present (Shen and Boyle, 1988; Véron et al., 1994; Véron et al., 1998; Véron et al., 1999; Boyle et al., 2008; Schaule and Patterson, 2013; Boyle et al., 2014; Noble et al., 2015; Zurbrick et al., 2018). Pre-anthropogenic (mid-1800s) surface Pb concentrations in the North Atlantic Ocean, reconstructed from the study of corals, ranged from 10 to 15 pmol kgË1, rising to about 90 pmol kgË1 in the 1920s due to coal combustion and ore smelting (Shen and Boyle, 1987; Kelly et al., 2009). After the 1940s, concentrations increased significantly reaching a maximum value of about 200 pmol kgË1 in the 1970s due to the use of leaded gasoline. A steady decrease of dissolved Pb has been observed since the 1980s from the phasing out of leaded gasoline in 1970 and 1990 in North America and Europe, respectively (Wu 61 and Boyle, 1997a; Boyle et al., 2014). The dissolved Pb surface concentrations in major ocean basins are decreasing towards pre-industrial values (~10-30 pM; Pinedo-González et al., 2018) due to the reduction of anthropogenic emissions. The higher concentrations, remnants from previous decades, have been declining due to scavenging removal and mixing with recently ventilated waters, and have been deepening during thermocline ventilation, resulting in relatively high concentrations in intermediate waters of the North Atlantic Ocean (Shen and Boyle, 1988; Noble et al., 2015; Schlitzer et al., 2018; Zurbrick et al., 2018). Therefore, Pb distributions, despite not being conservative such as traditional tracers, e.g. salinity and temperature, lend complementary transient information which can help trace and characterize water masses and their circulation (Bridgestock et al., 2018). National and international research programs such as GEOTRACES (www.geotraces.org) have been rapidly expanding our knowledge about the temporal and spatial distributions of Pb, and other trace elements, along with the internal cycling and equilibrium exchange of Pb between dissolved, colloidal and particulate phases (Chen et al., 2016; Rusiecka et al., 2018). Recent studies report the distributions of Pb in the northwest Pacific Ocean (Zurbrick et al., 2017), eastern tropical Atlantic Ocean (Bridgestock et al., 2018), North Atlantic Ocean (Noble et al., 2015), and the North Subarctic Atlantic including the Iceland Basin and the Irminger and Labrador seas (Zurbrick et al., 2018). Nonetheless, the geochemistry of Pb in other regions, such as the Arctic Ocean, is still poorly characterized. This manuscript addresses this knowledge gap, unraveling the spatial and vertical distribution of dissolved Pb in the Canadian Arctic Ocean, a key pathway whereby the fresher and cooler Pacific derived waters flow from the Canada Basin to the Labrador Sea. This study 62 provides new insights into the sources and fate of Pb in the Canadian Arctic Ocean which, together with recent publications from the Subarctic region (Zurbrick et al., 2017; Zurbrick et al., 2018), will help improve our understanding of the geochemical cycle of Pb in this changing environment. 3.3 Methods 3.3.1 Study area The circulation, water mass structure and key features of the studied area are outlined below. The Canada Basin is strongly stratified, with a multi-layered halocline (~30-400 m) which effectively insulates the polar mixed layer (PML, ~0-30 m) from the underlying heat of the Atlantic layer (~400-1500 m). Below the Atlantic layer (> 1500 m), lies the cold and more saline Canada Basin Deep Water (Timmermans et al., 2003; Steele et al., 2004; McLaughlin et al., 2005; Timmermans et al., 2014). The halocline in the Canada Basin is unique due to the presence of relatively fresh Pacific-origin waters and due to its spatial heterogeneity (McLaughlin et al., 2004; Steele et al., 2004; Shimada et al., 2005). The Alaskan Coastal Water (ACW) and the winter Bering Sea Water (wBSW) are advected from Bering Strait and modified during their flow across the Chukchi Sea, contributing to the upper and middle halocline; the lower halocline consists mostly of Atlantic-origin waters (McLaughlin et al., 2005; Shimada et al., 2005; Timmermans et al., 2014; Figure 3.1). The Canadian Arctic Archipelago (CAA) is a complex network of islands and channels, connecting the Arctic Ocean to Baffin Bay. This shelf dominated region is an important export conduit of the fresh and nutrient rich Pacific origin waters, enhancing productivity downstream (Michel et al., 2006; Beszczynska-Möller et al., 2011; Wang et al., 2012; Hill et al., 2013), and 63 potentially impacting deep convection in the North Atlantic (Proshutinsky et al., 2002; Wang et al., 2012; Figure 3.1). Baffin Bay is connected to the Arctic Ocean through Lancaster Sound, Jones Sound and Nares Strait, and to the Labrador Sea through the wider and deeper (300 km and 650 m) Davis Strait (Rudels, 1986; Tang et al., 2004; Wang et al., 2012). The overall circulation is cyclonic, with an Atlantic-Arctic derived northward flow on the eastern side of Davis Strait, the West Greenland Current (WGC), and a southward flow on the western side, the Baffin Current. The Labrador Sea plays a key role in deep convection, contributing intermediate depth Labrador Sea Water (LSW) to the Atlantic Meridional Overturning Circulation (Tang et al., 2004; Yashayaev and Loder, 2016; Lozier et al., 2017). The Labrador Sea subsurface (> 500 m) circulation is cyclonic and consists of two components: the WGC and the Labrador Current (LC). LSW is formed in late winter through deep convection, becoming colder, denser, thicker and deeper, and is then isolated from the surface (Yashayaev et al., 2007a). A diagram displaying the main water masses discussed here is presented in Figure 3.1. 64 Figure 3.1 Sampled stations for lead during the Canadian Arctic GEOTRACES cruises (GN02 and GN03), bathymetry and schematic of Pacific derived water circulation in the Canada Basin (after Timmermans et al., 2014; Kondo et al., 2016) and Baffin Bay and the Labrador Sea (after Yashayaev and Clarke, 2008; Curry et al., 2011; Lozier et al., 2017). CB: Canada Basin, CAA: Canadian Arctic Archipelago, BB: Baffin Bay, LS: Labrador Sea, ACW: Alaskan Coastal Water, sBSW: summer Bering Sea Water, wBSW: winter Bering Sea Water, AW: Arctic Water, EGC: East Greenland Current, WGC: West Greenland Current, WGIW: West Greenland Intermediate Water, BC: Baffin Current, LSW: Labrador Sea Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. For a detailed view of the Canadian Arctic Archipelago, including the main landmarks and straits, refer to Figure 3.6d.65 3.3.2 Water sampling and analysis Seawater samples were collected during the Canadian Arctic GEOTRACES project aboard the CCGS Amundsen during two consecutive cruises (GN02 and GN03) from Labrador Sea to Beaufort Sea from July 10th to October 1st, 2015. Samples were collected at 17 stations (Figure 3.1), targeting important physicochemical processes, water masses and key locations (Table 1.1). At each station, seawater was collected with the use of a powder coated trace metal rosette attached to a Kevlar line (Giesbrecht et al., 2013); the rosette was equipped with 12 L, Teflon coated GO-Flo bottles (General Oceanics, Miami, USA) and a SeaBird 911 CTD/SBE 43 oxygen sensor instrument package. Subsamples were filtered on board in a portable trace metal clean laboratory equipped with 2 HEPA filters, benches and GO-Flo racks. To minimize external contamination, the samples were processed following the Sampling and Sample-handling Protocols for GEOTRACES Cruises (Cutter et al., 2010). Briefly, sea water was filtered through 0.2 μm AcroPak Supor polyethersulfone capsule filters (VWR); subsamples (250 mL) were stored in Bel Art low density polyethylene (LDPE) bottles and acidified to pH 1.8 using 500 μL of 6 M trace metal grade hydrochloric acid (HCl; Seastar Chemicals Inc., Sidney, Canada). LDPE bottles were thoroughly cleaned by soaking in successive baths at 60 °C of Extran Liquid Detergent (VWR), 6 M environmental grade HCl and 0.7 M trace metal grade nitric acid (HNO3; Fisher Scientific, Ontario, Canada) for 1 day, 1 week, and 1 week respectively, as recommended by the GEOTRACES Protocol (http://www.geotraces.org/). Dissolved Pb was preconcentrated 50-fold from 50 mL acidified seawater using a modified magnesium-induced coprecipitation method and quantified by isotope dilution (Wu and Boyle, 1997b; McAlister and Orians, 2015). The seawater sample was transferred into clean 66 Falcon centrifuge tubes (Fisher Scientific, Ontario, Canada) by pouring directly from the bottle and allowed to equilibrate overnight with 207Pb enriched spikes obtained from Oak Ridge National Laboratories. High-purity concentrated NH4OH (Seastar Baseline Chemicals Inc., Sidney, Canada) was then added to precipitate the trace metals. After allowing ~10 min of Mg(OH)2 precipitation to occur, the sample was mixed and centrifuged at 4,000 rpm for 2 min. The sample was then decanted and re-centrifuged under the same conditions. After being centrifuged, the pellet was redissolved in 1 mL of 1% HNO3. The analyses were conducted by a high resolution Thermo Finnigan Element2 Inductively Coupled Plasma Mass Spectrometry in the Pacific Centre for Isotopic and Geochemical Research at the University of British Columbia. Brand new low-flow PFA-Teflon nebulizer (Elemental Scientific Inc., Omaha, USA) and sample and skimmer cones (Fisher Scientific, Ontario, Canada) were used for this study. To prevent contamination, the processing and analysis of the samples were conducted in class 1000 laboratories, maintained in an overpressure environment by HEPA filtered air, and under class 100 laminar flow fume hoods. All the plasticware used during the analysis were cleaned as previously described. The advantage of this method lies in the low reagent blank used, which minimizes the potential for contamination. During the analysis, procedural blanks and quality control spikes (acidified Milli-Q water equilibrated with 207Pb enriched and natural Pb standards) were routinely run to ensure quality of the measurements. Additionally, duplicate seawater samples were run for each profile approximately every six samples. All sample concentrations reported here are corrected for the analytical blank by subtracting the average blank on the corresponding analytical day. The counts per second (cps) of the blanks run during the analysis (n=56) were 67 549±263, representing only 3.0±1.2% of sample cps (~0.45±0.36 pmol kgË1). Results from the analysis of SAFe D1 (25.2±0.9 pmol kgË1; n=5) and GSP (63.0±5.7 pmol kgË1; n=5) GEOTRACES reference material were in the range of the reported values by the oceanographic community (Noble et al., 2015; 27.7±2.6 and 62.3±4.8 pmol kgË1, respectively; GSP data: Jim Moffett pers. comm., GEOTRACES standard and reference trace metal coordinator). The relative standard deviation (1RSD) of SAFe D1 and GSP analysis yield precisions of 3.8 and 9.1%, and the RSDs of replicate analysis of control spikes (n=40) were 4% or better. Additionally, the intercalibration exercises carried out with colleagues from numerous universities were in close agreement (Figure B.1). The statistical analysis and graphics in this manuscript were developed using Python 3.6.0. 3.3.3 Ariane particle track simulations The sources and paths of the water measured at the stations in Baffin Bay were traced backwards from 2015/08/08 to 2011/01/01 using Ariane (Blanke et al., 1999) with five-day averaged velocity fields from a 1/12 degree coupled ocean-ice model of the Arctic and Northern Hemispheric Atlantic (Hu et al., 2018), based on the Nucleus for European Modelling of the Ocean (NEMO; Madec, 2008). The model is forced by hourly atmospheric data from the Canadian Meteorological Centreâs global deterministic prediction system (Smith et al., 2014), Global Ocean Reanalyses and Simulations data for the boundary conditions (Ferry et al., 2012) and with monthly river runoff climatology (Dai et al., 2009). Water parcels at the eight grid points surrounding each station were also tracked to assess their spatial sensitivity and the distributions in resulting paths. Based on runs from 2014/08/08 to 2010/01/01 and from 2016/08/08 to 2012/01/01, there is little interannual variability in the tracks. 68 3.4 Results and discussion This study reports some of the first observations of dissolved Pb concentrations in the Labrador Sea, Baffin Bay, the Canadian Arctic Archipelago and the Canada Basin. The full dataset is presented in Table B.1, including those samples that are suspected of contamination, which are flagged in the table and excluded from the discussions below. In general, low surface concentrations were measured, suggesting that atmospheric inputs are not a main source of Pb in the Canadian Arctic, especially in summer when Arctic aerosol concentrations are at their minimum (Marsay et al., 2018b). Dissolved Pb in the Canada Basin, especially in wBSW and Canada Basin Deep Water, and in the Canadian Arctic Archipelago were among the lowest reported concentrations in ocean waters (Boyle et al., 2014), contrasting with the elevated values commonly found in the North Atlantic and the northwest Pacific oceans (Boyle et al., 2014; Zurbrick et al., 2017). We observed a clear increase in dissolved Pb from the Canada Basin towards Baffin Bay and the Labrador Sea, as well as distinctive advective features of high Pb (Figure 3.2). We first discuss the Pb distributions in the Canada basin followed by Baffin Bay and the Labrador Sea. The Pb geochemistry in the Canadian Arctic Archipelago is discussed last, since this region is influenced by the Canada Basin and Baffin Bay endmembers. 69 Figure 3.2 Interpolated contour section of dissolved Pb along the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay. Sampled depths are superimposed on the plot (white circles) and the individual Pb profiles are displayed above the contour plot. For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. Stations sampled are displayed in the inset map and the beginning of the transect is indicated by the red star. Note the break axis in the contour map (white line at 1050 m) and the logarithmic scale for the individual profiles. Stations were chosen to capture the flow of Arctic waters from the Canada Basin to Baffin Bay. 3.4.1 Dissolved Pb in the Canada Basin: influence of shelf interactions and freshwater inputs 3.4.1.1 Sea ice melt contributions to the polar mixed layer The lowest concentrations (4.6±0.6 pmol kgË1) of Pb in surface waters measured during the cruise, were in the Canada Basin polar mixed layer (PML) and at some of the stations in the Canadian Arctic Archipelago (CAA2, CAA7-CAA9). At CB4, however, the PML had almost double (8.5 pmol kgË1) the surface concentrations measured at CB1 to CB3 (Figure 3.3a). The PML at CB4 also yielded the lowest surface salinity (S, 25.2), compared to the other stations (S, 26.7-27.6), which reveals the Beaufort Gyre influence on the distribution and storage of 70 freshwater at this station (Proshutinsky et al., 2009; Jackson et al., 2010). Surface waters in the Canada Basin contain significant amounts of meteoric water, dominated by Eurasian runoff, except in the southern Beaufort Sea where low surface salinities are the result of the entrainment of North American river runoff which does not extend far beyond the shelf break (Guay et al., 2009; Lansard et al., 2012). Eurasian and North American Arctic rivers have high Pb concentrations (~50-500 pM; Colombo et al., 2019a), nevertheless, this high signature is removed from the dissolved phase along the salinity gradient due to complex estuarine processes such as particulate settling, flocculation of organomineral complexes and scavenging (Dai and Martin, 1995; Tanguy et al., 2011; Bruland et al., 2013). Therefore, offshore transport of continental runoff is not considered the primary source of the relatively high Pb found in the PML at CB4. In addition to meteoric water, sea ice meltwater represents an important fraction of surface waters (~5-20%), especially in southern Canada Basin where the Beaufort Gyre accumulates freshwater and sea ice (Guay et al., 2009; Proshutinsky et al., 2009; Yamamoto-Kawai et al., 2009; Lansard et al., 2012). The stations in the Canada Basin were sampled in September, when the release of fresh water from sea ice peaks (Proshutinsky et al., 2009; Lansard et al., 2012). A water mass analysis performed in this region (Alexis Beaupré-Laperrière, pers. comm., following methods similar to Lansard et al. 2012 using salinity, total alkalinity and isotopic composition of seawater) reveals a higher contribution of sea ice meltwater to surface waters at CB4 (20%) compared with CB1-CB3 (~5-10%), whereas the contribution of meteoric water at CB4 was 7%. The release of trace metals during sea ice melting has been previously suggested as an important mechanism that supplies trace elements to the surface waters (Huang et al., 1992; Measures, 1999; Klunder et al., 2012a; Giesbrecht et al., 2013). Recently, dissolved Pb concentrations as high as 33 pM have been measured in Arctic sea 71 ice melt ponds sampled in the Makarov and Canada basins (Marsay et al., 2018a), indicating that sea ice meltwater may be a plausible source of the higher Pb in the PML at CB4. Figure 3.3 a) Potential density (ÏÎ¸) versus Pb, Potential Temperature, Oxygen, Fluorescence, and Silicate and N* (calculated from the following equation: N*= ([NO3- + NO2- + NH4+] â 16PO43- + 2.9) x 0.87; Gruber and Sarmiento, 1997). Orange shaded bands indicate the isopycnal surface where Pb peaks (ÏÎ¸, 24.8-25.8) and the purple dashed line indicates the winter Bering Sea Water (wBSW). Silicate and N* data from the Canadian GEOTRACES cruise were provided by ArcticNet (Jean-Eric Tremblayâs group). For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. b) Potential temperature (θ) versus salinity (S) diagram, gray profiles represent type II waters and black profile type III waters (for details refer to text). ACW: Alaskan Coastal Water, sBSW: summer Bering Sea Water, AL: Atlantic Layer, CBDW: Canada Basin Deep Water. c) Stations sampled, bathymetry and schematic of Pacific derived water circulation in the Canada Basin (after Timmermans et al., 2014; Kondo et al., 2016) 3.4.1.2 Influence of Pacific derived waters on the Pb signature in the Canada Basin Below the PML, Pb concentrations remained low (2.5-5.5 pmol kgË1) down to about 40 m, where the local temperature minimum and an increase in salinity mark the remnant winter mixed layer (Jackson et al., 2010). The concentrations progressively increased, reaching the highest values measured in the Canada Basin (8.3-15.9 pmol kgË1), with a marked shoaling of the peak from CB4 to CB1 (~100 to 40 m) due to the deepening of isopycnal surfaces in the central 72 Beaufort Gyre (Figure 3.2). Interestingly, when dissolved concentrations are plotted against density, the peak is aligned in a narrow layer between ÏÎ¸ 24.8 and 25.8 isopycnal surfaces (Figure 3.3a), and its top (ÏÎ¸, 24.8) coincides with the temperature maximum of the ACW (upper halocline layer) transitioning to the wBSW (middle halocline layer) by its bottom (ÏÎ¸, 25.8). This high Pb feature extends as far as the western region of the Canadian Arctic Archipelago (CAA8; Figure 3.2). In the same density range, the fluorescence peak lies just below the ACW and at the top of the Si, phosphate and nitrate nutricline, where solar irradiation and nutrient availability triggers primary production in subsurface waters (McLaughlin et al., 2005; Ardyna et al., 2013). Following McLaughlin et al. (2004), two different water mass structures were distinguished from the potential temperature (θ) versus salinity (S) plots (Figure 3.3b). First, the type II waters (CB2 to CB4), characterized by a distinct temperature maximum (> -1.0 °C) in the upper halocline (ACW) with salinity profiles displaying large regional variability in the top 300 m (29 <S â¤ 32.2). The Pb concentrations in/or just below the ACW water from CB2 to CB4 had similar values (10.0±1.2 pmol kgË1). Slightly higher Pb concentrations (15.9 pmol kgË1) were measured in the type III waters at CB1 west of Banks Island; these waters are distinguished by a warm surface halocline in the range 27<S<31.5, probably developed by solar heating of ice-free surface waters. Below S= 31.5, the θ-S shape of wBSW and lower halocline water were distinct from those of the central Canada Basin (McLaughlin et al., 2004; Timmermans et al., 2014). The ACW occupies most of the Beaufort Sea and the North American continental slope, and regardless of the water type, high Pb was observed at the four stations. This high Pb signature can be traced from the northeast Subarctic Pacific (GEOTRACES cruise GP02), where concentrations in the upper 200 m range from approximately 40 to 50 pmol kgË1 (Schlitzer et al., 73 2018), as a result of the increased industrial emissions from Asian countries and Russia over the last four decades (Zurbrick et al., 2017). During its transit through the Bering and Chukchi Seas, this high Pb signal is weakened, with concentration ranging 13-20 pmol kgË1 on the Bering Sea slope and 3-13 pmol kgË1 in the Bering Strait (Robert Rember pers. comm.; 2015 US GEOTRACES Arctic cruise GN01), values that are in the same range to those observed in the ACW across the Canada Basin. The decrease of this high signature from the northeast Subarctic Pacific to the Canada Basin is attributed primarily to the efficient scavenging of Pb, a highly particle reactive element, in the shallow and productive Bering and Chukchi shelves (Brown et al., 2011; Hill et al., 2013), yielding short residence times in this region (< 1 year; Lepore et al., 2009; Chen et al., 2012; Kadko et al., 2018). 3.4.1.3 Enhanced Pb removal in near-bottom winter Bering Sea Water (wBSW) Beneath the upper halocline Pb peak, low concentrations were found across the wBSW and Atlantic-derived lower halocline layer (~100-300 m), with lower concentrations at the stations CB2-CB4 (type II waters) than at CB1, type III water (2.2±0.7 vs 6.0±0.1 pmol kgË1). Although ACW and wBSW are both of Pacific origin, they exhibit distinctive physicochemical properties, aside from those observed for Pb. Compared with ACW, the wBSW is distinguished by a weak temperature minimum near S= 33.1, high nutrient and nutrient-like trace metals concentrations, high dissolved organic matter and low oxygen concentrations (Hioki et al., 2014; Kondo et al., 2016; Figure 3.3a). These distinctive properties provide evidence that wBSW undergoes notable organic matter remineralization in the Bering and Chukchi shelves (Hioki et al., 2014; Granger et al., 2018). In summer the surface waters are freshened by sea ice melt, isolating the wBSW to a 74 deeper layer in close proximity with the sediment. Nutrients are rapidly drawn down from surface waters due to summer phytoplankton blooms, increasing the particle flux, and prompting organic matter remineralization and oxygen consumption in the underlying wBSW (Granger et al., 2018). Dissolved Pb distributions, unlike nutrient-like trace metals, are low in the cold, near-bottom wBSW revealing a potential enhanced removal of Pb at the sediment-water boundary by sediment resuspension events over the extensive continental shelves (Lepore et al., 2009; Chen et al., 2012; Kuzyk et al., 2013). The negative N* values of wBSW (Figure 3.3a) indicate that this water was influenced by sedimentary denitrification and/or anammox in the Bering and Chukchi shelves (Hioki et al., 2014; Kondo et al., 2016; Granger et al., 2018; Vieira et al., 2019). Under reducing sediment conditions, which trigger the nitrogen benthic loss, an increase of Mn and Fe in near-bottom waters has been observed in the Chukchi Sea (Vieira et al., 2019). These redox sensitive elements are mobilized from the sediments, diffusing into the bottom water where they become oxidized and precipitate back to the sediments. In this context Pb can be further removed from the dissolved phase in near-bottom waters during the Mn redox cycle either by its coprecipitation with Mn-oxides or the adsorption onto Mn-oxides coated particles (Nozaki et al., 1997), a phenomena which is anticipated to co-occur with the precipitation of Fe-oxides under the same circumstances. 3.4.1.4 Lead distributions in Atlantic and Canada Basin deep waters A slight increase in Pb (4.6±0.8 pmol kgË1) is observed in the warm Atlantic layer (~300 to 800 m; ÏÎ¸= 27.9), reflecting the circulation of the Fram Strait Branch (FSB) in the boundary current along the continental slope (Figures 3.2 and 3.3a). Lead has a short residence time (20-80 years) in thermocline and deep waters (Shen and Boyle, 1988; Schaule and Patterson, 75 2013). As a result, the Pb signature of the FSB, advected from the Norwegian Sea, has undergone significant scavenging and mixing with surrounding waters during its flow from Fram Strait to Canada Basin (20-50 years; Tanhua et al., 2009), and thus its initial high Pb signature has almost vanished. Underlying the Atlantic layer, low concentrations (3.2±0.3 pmol kg-1) were measured in Canada Basin Deep Waters (1500-3500 m). In addition to scavenging removal of Pb, this old (~500 years) and isolated water mass (Macdonald et al., 1993; Tanhua et al., 2009) was ventilated before the peak in anthropogenic Pb emissions, yielding a pre-anthropogenic low signature. 3.4.2 Tracing the far-reaching footprint of Pb in the Labrador Sea and Baffin Bay A common feature in Baffin Bay is the low concentration of Pb found in the surface mixed layer (~11.5 m), with lower values measured at the central stations (BB2 and BB3) than at southern BB1 (5.8±1.1 vs 13.6 pmol kgË1; Figure 3.4b and 3.4c). Underneath the mixed layer, four water masses can be identified by their temperature and salinity properties: The West Greenland Intermediate Water (WGIW) of North Atlantic origin, the Arctic Water (AW) produced by winter convection and mixing between WGC and Arctic-derived waters, the transitional water (TrW) laying between the AW and WGIW, and the Baffin Bay Deep Water (Cuny et al., 2005; Curry et al., 2011; Figure 3.4a). The concentrations of Pb in the AW, characterized as subzero water with a salinity range of 32.0-33.7, were relatively low at BB2 and BB3 (6.3±0.7 and 5.3±1.5 pmol kgË1), and slightly higher at BB1 (10.8±3.0 pmol kgË1). Underlying the AW and TrW, lies the WGIW (~300-800 m) distinguished by its warm temperature (θ> 1.3) and high salinity (Sâ¥ 34.2). The highest Pb concentrations were found in the narrow WGIW isopycnal range (ÏÎ¸ 27.4-27.6), representing the densest water that crosses the 76 Davis Strait sill from the Labrador Sea (Figure 3.4b). As was noted for Pb concentrations in the mixed layer and AW, the Pb peak in the WGIW at BB2 and BB3 was lower than at BB1 (13.5 and 14.1 vs 26.6 pmol kgË1, respectively). The higher Pb at BB1, located in the central part of Davis Strait, is associated with the first recirculation of the northward WGC inflow which has uniformly high Pb concentration (23.6±1.0 pmol kg-1) in the upper 1000 m at the western tip of the Greenland slope (Zurbrick et al., 2018). These high values are attributed to the Irminger Sea Water (ISW), a remnant of the North Atlantic Current spreading northwestward along the Reykjanes Ridge, circulating cyclonically in the Irminger and Labrador seas, and advecting warm, salty and anthropogenically elevated Pb waters (Véron et al., 1999; Cuny et al., 2005; Noble et al., 2015; Lozier et al., 2017; Zurbrick et al., 2018). The subsurface layers of the WGC are mixed with neighboring cold, fresh and low Pb waters advected from the Arctic Ocean, leading to the cooling, freshening and weakening of the Atlantic Pb signature in AW and TrW in western Baffin Bay, especially at BB3. Nonetheless, this mixing is small in the warmer and saltier WGIW core, where high Pb was found in all Baffin Bay stations (Figures 3.4b). The relatively high concentrations (12.9±1.2 pmol kg-1) in TrW (~150-200 m) at BB2 are comparable to those observed in WGIW at BB2 and BB3 (13.5 and 14.1 pmol kg-1, respectively), and illustrate the shallow mixing of the WGC with Arctic derived waters in central Baffin Bay. In the eastern and central region of Baffin Bay the AW extends to a depth of approximately 100 m progressively deepening during its cyclonic circulation due to the inflow of fresher waters from the Arctic Ocean in the northwest region of the bay (Tang et al., 2004). In order to assist with the interpretation of Pb distributions in Baffin Bay, water mass back trajectories were calculated from ANHA12 simulations. This analysis supports our 77 interpretations above, showing that surface waters (< 450 m) are greatly influenced by Arctic derived waters, while below ~450 m, waters tend to originate from the Baffin Bay interior and WGC. Additionally, the trajectories illustrate the augmented Arctic outflow at station BB3, where from approximately 190 to 400 m, the waters are directly exported from the Arctic Ocean to Baffin Bay through Parry Chanel and Nares Strait (~60% of surface water), and the water advected by the WGC travels into Smith and Lancaster Sounds, likely mixing with Arctic waters with lower Pb concentrations (~20% of surface water; Figure 3.5 and B.2). The trajectories also show that the addition of Arctic waters at BB3 is mostly restricted to the upper 400 m; below this depth, water masses that have not been significantly mixed with Arctic derived waters, such as from the WGC and Baffin Bay interior, are prevalent (~60%). This coincides with the depths where the high Pb feature is observed (Figures 3.4b, 3.5). A similar pattern is observed at BB1, although the surface waters (< 450 m) receive an augmented contribution from the Baffin Bay interior and some water is directly advected from the Labrador Sea, likely explaining the relatively high Pb values measured at BB1 compared with the central Baffin Bay stations (Figures 3.4b, 3.5 and B.2). The Baffin Bay Deep Water (BBDW; > 1200 m) is characterized by a small change in salinity and a constant decrease in potential temperature (Figure 3.4a); this deep and old water has no direct access to the Arctic and Atlantic Oceans and its origin is under discussion (Tang et al., 2004). The BBDW had the lowest Pb concentration measured in Baffin Bay (2.5-7.4 pmol kgË1), attributed to the ventilation age of this old water, presumably before times of higher anthropogenic Pb deposition, and the scavenging throughout the deep water column. 78 Figure 3.4 a) Potential temperature (θ) versus salinity (S) diagram from Baffin Bay stations with Pb concentrations for the analyzed samples (color scale). Water masses in Baffin Bay were identified following Tang et al. (2004) and Curry et al. (2011). WGIW: West Greenland Intermediate Water, TrW: transitional water, AW: Arctic Water, BBDW: Baffin Bay Deep Water. b) Potential Density (ÏÎ¸) versus dissolved Pb in Baffin Bay. c) Sampled stations, bathymetry and schematic circulation (after Yashayaev and Clarke, 2008; Curry et al., 2011; Lozier et al., 2017) in Baffin Bay and the Labrador Sea. EGC: East Greenland Current, WGC: West Greenland Current, BC: Baffin Current, LSW: Labrador Sea Water, ISW: Icelandic Slope Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. d) Dissolved Pb, θ, S, and oxygen profiles in the Labrador Sea. The water masses were identified following Yashayaev and Loder (2009) and Yashayaev and Loder (2016). For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. The surface concentration in the Labrador Sea (16.9 pmol kgË1) agrees with that reported by Zurbrick et al. (2018) during the 2014 GEOVIDE cruise (12.1-16.2 pmol kgË1). This concentration is higher than the levels found in central Baffin Bay (5.8±1.1 pmol kgË1), but similar to those found in Davis Strait (13.6 pmol kgË1) supporting the recirculation of the WGC observed at BB1. A sharp increase in Pb (30.5±0.3 pmol kgË1) is observed below the mixed layer around 100 m, as result of the advection of modified ISW carried by WGIW from Baffin Bay, where comparable concentrations (26.6 pmol kg-1) were measured at Davis Strait in a similar density range (ÏÎ¸, 27.5-27.6; Figure 3.4d). 79 Figure 3.5 Water mass back trajectories from station BB3 (upper panel) and BB1 (lower panel) from 2015 back to 2011 over two depth ranges. Tracks were grouped according to their origin and path; this breakdown is shown in proportions in the bar plots on the right. The groups are, Nares Strait: originates from the Arctic Ocean and passes through Nares Strait. Parry Chanel: originates from the Arctic Ocean and passes through Parry Chanel. Coastal West Greenland Current: originates from the WGC and travels into the CAA, where it mixes with Arctic waters. Baffin Bay interior: circulates in the Baffin Bay interior for the entire time period. West Greenland Current: travels from the Atlantic via the WGC around Baffin Bay. Directly from Atlantic: travels directly from the Atlantic to the station without circulating around Baffin Bay. In the deep water column (~500-2500 m), Pb was high (26.4-34.2 pmol kgË1) and uniformly distributed across multiple hydrographic features (Figure 3.4d), as observed by Zurbrick et al. (2018). In 2015, when the Canadian Arctic GEOTRACES cruise took place, two LSW classes were clearly distinguished by the Conductivity-Temperature-Depth (CTD) 80 observations. The newly ventilated and fresher LSW formed during the winters of 2014 and 2015 (LSW-2015), and the old, saltiest and least oxygenated LSW produced in the winters of 1987-1994 (LSW-87/94), which was the densest and most voluminous LSW class formed in recent years (Yashayaev and Loder, 2016). The thin, relatively saline and warm layer that lies between the two LSW classes is the Icelandic Slope Water (Yashayaev et al., 2007b; Yashayaev and Loder, 2009). Underlying the LSW-87/94, lies the saline, warm (θ ~3 °C, S ~34.92) and less oxygenated Northeast Atlantic Deep Water (NEADW). The deepest water is the less saline, colder (Î¸â¤ 2.6 °C, S ~ 34.9), and more-recently oxygenated Denmark Strait Overflow Water (Cuny et al., 2002; Yashayaev et al., 2007a; Yashayaev and Loder, 2016; Figure 3.4d). Numerous processes are likely to account for the Pb maximum spreading across the LSW-2015, LSW-87/94 and NEADW. Among them, the deep convection taking place in 2015 mixing the ISW with older LSW formed during previous years (Yashayaev et al., 2007b; Yashayaev and Loder, 2009; Yashayaev and Loder, 2016), scavenging removal of Pb in the old LSW-87/94 waters, with approximately 45% reduction between the concentrations measured in the LSW in 1993 during the IOC II cruise (Véron et al., 1999) and those measured in the residual LSW-87/94 in 2015, and the advection of NEADW from Norwegian Sea are the most significant processes modulating Pb distributions below ~500 m. The Pb concentration dropped from the NEADW to the deep Denmark Strait Overflow Water (DSOW) reaching 17.6 pmol kgË1 at ~2800 m. During deep convection, surface and subsurface waters are subducted, entraining the Pb signature to the deep ocean. Therefore, differences in the ventilation time and place between these two water masses may explain the disparity in Pb distributions. Even though the DSOW was ventilated more recently than the 81 NEADW (5-8 vs. 11-13 years; Azetsu-Scott et al., 2005), DSOW Pb concentrations have remained relatively low (< 20 pmol kg-1) during the last 20 years (Véron et al., 1999), indicating that the ventilation time is not the main driver of Pb concentrations in DSOW. The NEADW and DSOW originate in different regions; the DSOW is produced in the Greenland Sea, receiving the fresh and low Pb Arctic outflow (Yashayaev and Loder, 2009; Lozier et al., 2017), whereas the NEADW is the result of dense overflow waters produced in the Norwegian Sea, where the Iceland Scotland Overflow Water evolves into the NEADW. The Norwegian Sea is influenced by the North Atlantic Current bringing warm and high Pb upper-ocean waters from the subtropical western North Atlantic (Noble et al., 2015; Pinedo-González et al., 2018). As observed for Pb, the concentrations of dissolved 230Th, a highly particle reactive radionuclide, decreased below ~2500 m in DSOW at LS2 (Grenier et al., 2018), suggesting that enhanced bottom scavenging of Pb by sediment resuspension could be an additional process affecting Pb distribution in DSOW. Similarly, in the North Atlantic, from Massachusetts to the Bermuda Atlantic Time Series station, the depletion of 230Th in DSOW was associated with bottom scavenging throughout the thick nepheloid layer, where a concomitant decrease in dissolved Pb was observed (Hayes et al., 2015; Noble et al., 2015). 3.4.3 Lead distributions in the dynamic Canadian Arctic Archipelago Two main domains are recognized in the Canadian Arctic Archipelago (CAA) based on CTD and Pb data. Arctic waters of Pacific origin, entering through MâClure and Penny Straits, travel predominantly along the southern side of Parry Channel (CAA7-CAA9 and CAA4). These waters are cooler (Ë1.6 <θ< 0.8 °C) and fresher (25.1 <S< 34.8) compared to Baffin waters of Atlantic origin recirculating in the eastern CAA (Ë1.5 <θ< 4.9 °C; 28.2 <S< 34.5). Likewise, Pb 82 has distinctive distributions and lower concentrations in the stations influenced by the Arctic outflow, relative to those influenced by Atlantic waters (Figure 3.6b, 3.6c and B.3). The vertical Pb distribution in Viscount Melville Sound (CAA8) resembles that of CB1, with low concentrations in the PML (1.7 pmol kgË1) and a sharp increase between 40-100 m (6.4-12.5 pmol kgË1) linked to the advection of the upper halocline layer (ÏÎ¸ 24.8-25.8) from Canada Basin (Figure 3.2). The middle and lower halocline waters (100-300 m; Pb: 3.5±0.8 pmol kgË1) as well as the Atlantic waters (>300 m; 5.7±0.7 pmol kgË1) are transported from Canada Basin along western Parry Channel, one of the deepest regions (~500 m) of the archipelago, as far as Viscount Melville Sound. Only the surface and Pacific-derived waters (ACW and wBSW) are able to cross the shallow Barrow Strait, and their properties are modified throughout the eastward journey to Baffin Bay. The Pb distribution in Barrow Strait (CAA7) and north of Somerset Island (CAA4) were consistent to those found in Canada Basin, showing low concentrations in the upper 50 m (4.6±0.4 and 6.2±0.1 pmol kgË1, respectively), a maximum between ~60-100 m (7.3±0.9 pmol kgË1) and returning to lower values below 100 m (3.3±0.7 and 5.2±0.8 pmol kgË1, respectively). The slightly higher concentrations found at CAA4 may indicate the recirculation of Baffin waters traveling westward along the northern side of Parry Channel (discussed below). The station in Penny Strait (CAA9) captures the southward flow from the Arctic Ocean to the CAA interior; the near linearity of the θ-S profile (Figure 3.6b) is consistent with the strong tidal mixing in this region (Hannah et al., 2009; Hughes et al., 2018). Despite the fact that Canada Basin θ-S features have faded, the relatively low Pb concentrations measured in Penny Strait (3.2-7.4 pmol kgË1) indicate the prevailing Arctic origin of these waters. 83 Figure 3.6 a) CTD temperature (θ) versus salinity (S) diagram for all the stations in the CAA. Those stations influenced by Baffin waters (Atlantic signature) are displayed with red colors, while the stations influenced by Arctic waters are displayed with blue colors. The θ/S data from the Canada Basin endmember (CB1) and Baffin Bay endmember (BB2) are shown with blue and red solid lines, respectively. b) Pb concentration (color scale), θ and S data measured at CAA7-CAA9, CAA4. c) Pb concentration (color scale), θ and S data measured at CAA7-CAA9, CAA4. Note that temperature/salinity axes vary between plots. d) Sampled stations and conceptual scheme of water masses and circulation pathways in the CAA (after Michel et al., 2006; Wang et al., 2012). The inset includes a Terra- MODIS visible image of eastern Devon Island taken on July 25, 2015 (https://worldview.earthdata.nasa.gov/), illustrating the presence of glacial runoff along the Devon Island coast. Light blue: Arctic-derived waters (low Pb), Red: Atlantic-derived water (high Pb), Orange: Glacially modified surface waters (high Pb surface waters) and Light orange: mixing between glacial runoff, Atlantic and Arctic waters. MS: MâClure Strait, VS: Viscount Melville Sound, BS: Barrow Strait, PS: Penny Strait, LS: Lancaster Sound, CI: Cornwallis Island, SI: Somerset Island, DI: Devon Island, BI: Bylot Island, EL: Ellesmere Island. Parry Channel is the main pathway in central CAA connecting MâClure Strait with Lancaster Sound. Baffin waters enter the CAA through Lancaster Sound, traveling westward along the northern side of Parry Channel, joining the Arctic outflow waters near Barrow Strait, and exiting the archipelago on the southern side of Lancaster Sound (Wang et al., 2012; Figure 3.6d). On the northern side of Lancaster Sound, an unanticipated difference in most of the physicochemical properties and Pb concentrations was observed between CAA1 and CAA2, especially in the upper 60 m. Although these two stations are ~20 km apart from each other, surface waters at 84 CAA1 were noticeable fresher (31.8-32.9 vs 32.5-33.4) and cooler (-0.7 Ë 2.7 vs -1.2 Ë 4.9 °C) than at CAA2 (Figure 3.6c). Similarly, Pb concentrations in the surface were significantly (p-value< 0.05) higher at CAA1 than CAA2 (12.4±1.0 vs 4.8±0.5 pmol kgË1). The aforementioned discrepancies suggest that an additional freshwater source is present along the Devon Island coast near CAA1. The CAA has one of the largest volumes of glacier ice on Earth with approximately 10% of its surface covered by glaciers, which are mainly distributed along the western sides of Ellesmere, Devon and Baffin islands, and on Bylot Island (Lenaerts et al., 2013). Station CAA1 was located in close proximity (~5 km) to the Devon coast, receiving the meltwater runoff from Devon glaciers during the summer season (Figure 3.6d). Sediment-laden meltwater runoff from these glaciers would explain not only the fresher and cooler surface waters at CAA1, but also the decrease in transmissivity measured at CAA1, as observed in subsurface glacial plumes (ÏÎ¸, 25.0-26.5) in Greenland fjords close to northern Baffin Bay (Kanna et al., 2018). Concentrations of dissolved Pb in glacial streams from CAA are ~ 6 to 50 times greater (35 pM) than those found in other central CAA rivers (Colombo et al., 2019a). Hence, the mixing of glacial runoff with Baffin waters could explain the high Pb observed in the subsurface plume (ÏÎ¸, 25.5-26.6) traveling along Devon Island (CAA1 and CAA5). Below this plume, concentrations dropped (5.3±1.1 pmol kgË1) from 120 m till the bottom (Figure B.3). From the θ-S diagram it can been seen that waters at CAA2 do not contain glacial runoff waters. The distinctive physical properties and Pb signature from Baffin waters are preserved, with low surface concentrations (4.8±0.5 pmol kgË1), peaking in TrW from 80-150 m (ÏÎ¸, 26.9-27.2; 9.1±1.0 pmol kgË1), as observed at BB2, and returning to lower values below 200 m (4.8±0.4 pmol kgË1; Figures 3.6c and B.3). 85 Finally, CAA3 and 6 represent transition stations influenced by multiple water sources such as Arctic derived waters, Baffin Bay waters and glacial inputs. CAA3 station, located in southern Lancaster Sound (Bylot Island coast), receives the Arctic outflow flowing along southern Parry Channel, Baffin Bay waters recirculating in eastern Parry Channel as well as glacial inputs (Figure 3.6d). The concentrations in the upper 50 m at CAA3 were higher (8.7±0.9 pmol kgË1) than those observed in surface waters at CAA4 and CAA7-CAA9 reflecting the mixing of low Pb Arctic origin waters with the recirculation of the glacial subsurface plume from Devon Island, albeit local glacial inputs from Bylot Island cannot be excluded. The lower values (4.4±0.8 pmol kgË1) measured in intermediate waters (~60-200 m) were comparable to those found in AW in Baffin Bay and Canada Basin waters in western CAA, and the slight Pb increase from ~200-400 m corresponds to TrW. The CAA6 station, located in Barrow Strait west of Cornwallis Island, receives the remnants of Baffin Bay waters and glacial inputs from Devon Island, as well as Arctic waters flowing southward from Penny Strait and entering through MâClure Strait (Figure 3.6d). Despite the Arctic influence, the relatively high Pb signature found at CAA1 and CAA5 was still traced at CAA6 (7.03-11.65 pmol kgË1). 3.5 Conclusion This study presents some of the first observations of dissolved Pb across the Labrador Sea, Baffin Bay, the Canadian Arctic Archipelago and the Canada Basin which were made during the GEOTRACES Canadian program in 2015. This extensive data set not only provides a baseline for assessing future Pb studies in the Arctic, but also will contribute to a better understanding of the global Pb cycle. The Canada Basin and the Canadian Arctic Archipelago are relatively isolated from anthropogenic inputs, with the lowest Pb concentrations 86 (<7 pmol kgË1) measured in this study and among the lowest reported in the literature. Nonetheless, remnants of anthropogenic pollution in the form of higher Pb concentrations (~10-25 pmol kgË1) were clearly traced in the Canada Basin from the northeast Subarctic Pacific and in Baffin Bay from the North Subarctic Atlantic, overlying the pervasive low concentrations. Lead concentrations in the Labrador Sea yielded the highest concentrations (~17-34 pmol kgË1) measured in this study, reflecting the influence of the sub-polar gyre circulation advecting warm, salty and anthropogenically elevated Pb waters. In addition to the anthropogenic influence, the distribution of dissolved Pb in the Canadian Arctic is modulated by natural processes occurring at the continent-ocean interface such as glacial water runoff and sea ice melt (inputs), and at the sediment-water boundary (scavenging removal â sinks). Finally, we integrate modelling data along with field observations to unravel the Pb distribution in Baffin Bay, demonstrating that dissolved lead is a useful complimentary tracer of water masses in the Canadian Arctic Ocean. This new Pb data collected during the GEOTRACES Canadian program, along with Pb isotopes, major and micro nutrients data, will improve our knowledge about the dynamic Arctic ocean environment. 87 Chapter 4: Dissolved Iron and Manganese in the Canadian Arctic Ocean: On the Biogeochemical Processes Controlling their Distributions 4.1 Summary Dissolved iron (DFe) and manganese (DMn) are essential micronutrients involved in virtually all phytoplankton physiological pathways, and their deficit can limit primary production in major nutrient rich surface ocean waters. In this work we present the spatial distributions and biogeochemical cycling of these metals across the Canadian GEOTRACES transect in the Canadian Arctic Ocean during the summer and autumn of 2015. Surface concentrations are dominated by freshwater inputs showing a strong negative correlation with salinity, especially for DMn which behaves more conservatively than DFe. The highest surface concentrations were measured in the Canadian Artic Archipelago (Fe: 0.401-1.91 and Mn: 4.33-9.54 nmol kg-1) and the Canada Basin (Fe: 0.225-0.479 and Mn: 3.93-7.02 nmol kg-1), regions highly influenced by riverine inputs, whereas the lowest values were found in the Labrador Sea (Fe: 0.106-0.362 and Mn: 0.450-1.09 nmol kg-1) due to the combined effect of reduced freshwater inputs and phytoplankton uptake. Subsurface and deep water distributions are largely controlled by a complex balance between DFe and DMn sources (advective inputs and organic matter remineralization) and removal process. Noticeable subsurface peaks (~100-300 m) were observed in the Canada Basin (Fe: 0.541±0.060 and Mn: 1.38±0.42 nmol kg-1) and Baffin Bay (Fe: 0.753-1.025 nmol kg-1), as result of the advective transport from the Chukchi Sea and the Canadian Arctic Archipelago, shallow and shelf dominated environments where DFe and DMn are strongly released from the benthic boundary layer. Advective sources of DFe and DMn are also present in deep Canada Basin waters (>300 m), where the Arctic Circumpolar Boundary 88 Current influences the distributions of these metals in this low productivity basin. In the highly productive Baffin Bay and the Labrador Sea, organic matter remineralization is a relevant source of DFe and DMn to deep waters. In the deepest waters (>1000 m), scavenging of DFe and DMn governs their vertical distributions; a pseudo-first order scavenging model was found to best explain the continuous reduction of DMn in the Canada Basin, where the concentrations reach nearly uniform low concentrations (0.150±0.004 nmol kg-1) after ~400 years. Applying this DMn scavenging model we were able to estimate the ages (120-190 years) of deep Baffin Bay waters, which have been under discussion for many years. 4.2 Introduction Iron (Fe) and manganese (Mn) are key micronutrients, which along with major algal nutrients, regulate the biological productivity of global oceans and play a major role in the marine carbon cycle (Martin and Fitzwater, 1988; Bruland et al., 1991; Jickells et al., 2005; Boyd and Ellwood, 2010; Tagliabue et al., 2017). These micronutrients are involved in important phytoplankton metabolic pathways, such as photosynthesis (e.g. photosynthetic systems, the electron transport chain, activation of the ribulose l,5-bisphosphate carboxylase/oxygenase enzyme) and the nitrogen cycle (Maldonado and Price, 1996; Morel and Price, 2003; Morel et al., 2013), and Fe is also required by heterotrophic bacteria (Tortell et al., 1996). Understanding the mechanisms controlling the distributions of Fe and Mn in global oceans is therefore a primary endeavor of international programs such as GEOTRACES. Atmospheric dust (dry and wet) deposition has been long recognized as a main external source of Fe and Mn, as well as many other trace metals, especially to surface waters in the open ocean (Duce et al., 1991; Martin et al., 1991; Guieu et al., 1994; Jickells et al., 2005; Moore and 89 Braucher, 2008). However, in recent years riverine inputs, sediment-water interactions (reductive and non-reductive dissolution), continental margin inputs and hydrothermal vents are being recognized as important source terms in the biogeochemical cycles of Fe and Mn (Fung et al., 2000; Lam and Bishop, 2008; Middag et al., 2011b; Klunder et al., 2012a; Jeandel and Oelkers, 2015; Jeandel, 2016; Fitzsimmons et al., 2017; Milne et al., 2017; Tagliabue et al., 2017; Cheize et al., 2019). These extra sources are particularly relevant for the Arctic Ocean, a region characterized by significant freshwater inputs (receiving approximately 11% of world's river discharge) and extensive continental shelves, where atmospheric deposition is not a dominant source of Fe and Mn (Lammers et al., 2001; Hölemann et al., 2005; Measures et al., 2008; Pokrovsky et al., 2010; Bring et al., 2016; Shelley et al., 2017; Marsay et al., 2018b; Shelley et al., 2018). In seawater, Fe and Mn are usually found at sub nanomolar levels owing to rapid biological uptake in surface waters, precipitation of Fe+3 and Mn+3/4 oxides in oxygenated waters (enhancing the scavenging removal of other trace elements), and scavenging onto organic and inorganic particles throughout the water column (Landing and Bruland, 1987; Johnson et al., 1997; Sunda, 1997; Wu et al., 2001; Morgan, 2005; Klunder et al., 2012a; Bown et al., 2018). In spite of ferric ionâs low solubility, it can be maintained in solution by complexation with organic ligands, which account for approximately 99.9% of the DFe observed in ocean waters (Rue and Bruland, 1995; Gerringa et al., 2015). More recently, Oldham et al. (2017) has reported the presence of organic ligands stabilizing Mn+3, otherwise insoluble in ocean waters, and indicating that ligands may also play a greater role in DMn distributions. Since the launch of GEOTRACES in 2010 (http://www.geotraces.org/), our knowledge about the concentrations, biogeochemical cycles and processes shaping the distributions of DFe 90 and DMn has significantly increased. High resolution profiles of DFe and DMn have been sampled on transects across the major ocean basins, and in remote areas of the Southern Atlantic Ocean and the Arctic Ocean (Wu et al., 2014; Fitzsimmons et al., 2015; Hatta et al., 2015; Hulten et al., 2017; John et al., 2018; Pham and Ito, 2018; Schlitzer et al., 2018). Even though numerous studies on Fe and Mn biogeochemistry in the Arctic Ocean have been published in the last 10 years, they have focused on the Eurasian Basin (Middag et al., 2011b; Klunder et al., 2012a; Klunder et al., 2012b), the Chukchi Sea and adjacent shelf break areas of the Canada Basin (Nakayama et al., 2011; Aguilar-Islas et al., 2013; Hioki et al., 2014; Kondo et al., 2016). In this manuscript we report the concentrations of DFe and DMn in the Canadian Arctic Ocean (the Canada Basin, the Canadian Arctic Archipelago, Baffin Bay and the Labrador Sea) elucidating the mechanisms controlling the spatial distributions of these elements in this key region, where Arctic waters properties are modified and ultimately exported to the North Atlantic Ocean. 4.3 Study area and hydrography The Canada Basin is strongly stratified by salinity, with a polar mixed layer (PML), seasonally modified by freeze-thaw cycles of ice and snow, and a multilayered halocline (~30-400 m) insulating the PML from the underlying saltier and warmer Atlantic Layer (AL). The halocline assembly consists of the Alaskan Coastal Water (ACW), with a potential temperature (θ) of about ~0 °C and a salinity (S) range of 30 <S< 32, and the winter Bering Sea Water (wBSW), distinguished by a shallow temperature minimum near S=33.1, which are advected from Bering Strait and contribute to the upper and middle halocline layers (Steele et al., 2004; McLaughlin et al., 2005; Timmermans et al., 2017; Figures 4.1 and 4.2a). Unlike the upper 91 and middle halocline, the lower halocline (LH) consists mostly of Atlantic-origin waters, and is identified by a sharp increase in temperature at salinities between 33.3 and 34.6 (McLaughlin et al., 2005; Shimada et al., 2005; Woodgate and Aagaard, 2005). The AL (~400-1200 m) has two components: the Fram Strait Branch (FSB), distinguished by a temperature maximum and the Barents Sea Branch (BSB), which is deeper and colder (Smethie et al., 2000; McLaughlin et al., 2004; Aksenov et al., 2011). Underlying the AL lies the old, cold and more saline Canada Basin Deep Water (CBDW > 1200 m; Timmermans et al., 2003; Figures 4.1 and 4.2a). The Canadian Arctic Archipelago (CAA) is a complex network of islands and shallow straits, connecting the Arctic Ocean to Baffin Bay. This shelf dominated region is an important export conduit for fresh and nutrient rich Pacific waters to the North Atlantic, enhancing the productivity downstream (Michel et al., 2006; Beszczynska-Möller et al., 2011; Wang et al., 2012; Hill et al., 2013). The CAA links the Arctic Ocean with Baffin Bay by three main pathways, Parry Channel (~120 m), running from MâClure Strait to Lancaster Sound, Nares Strait (~ 220), and Jones Sound (~125 m). Two main domains are recognized in the CAA based on the conductivity, temperature and depth (CTD) data. The Arctic water of Pacific origin, which are cooler (-1.6<θ<0.8 °C) and fresher (25.1<S<34.8), dominate the western CAA region and the southern side of Parry Channel (CB1, CAA4 and CAA7-9), compared to Baffin waters of Atlantic origin (-1.5<θ<4.9 °C; 28.2<S<34.5) which recirculate in the eastern CAA (CAA1-3 and CAA5; Figures 4.1 and 4.2b). Baffin Bay is connected to the Labrador Sea and the North Atlantic Ocean through Davis Strait (~650 m). The overall circulation in this bay is cyclonic, with a northward flow on the eastern side of Davis Strait, the West Greenland Current (WGC), which consists of subsurface 92 fresh waters of Arctic origin (East Greenland Current), and the warm and salty West Greenland Intermediate Water (WGIW: ~300-800 m; θ> 1.3 °C and S> 34.2) of North Atlantic origin (Cuny et al., 2005; Curry et al., 2011; Lozier et al., 2017). This northward inflowing water is modified during its cyclonic circulation, and the upper layers of the WGC are mixed with Arctic waters, resulting in the fresh and cold Arctic Water (AW; θ< 0 °C and 32.0 <S< 33.7), which continues its flow to the Labrador Sea as the Baffin Current (Tang et al., 2004; Cuny et al., 2005). Underlying the WGIW, lies the Baffin Bay Deep Water (BBDW), which is characterized by a small change in salinity and a constant decrease in potential temperature (Tang et al., 2004; Figures 4.1 and 4.2c). Figure 4.1 Sampled stations for Fe and Mn during the Canadian Arctic GEOTRACES cruises (GN02 and GN03), bathymetry and schematic of water circulation in the Canada Basin (after Aksenov et al., 2011; Kondo et al., 2016) and Baffin Bay and the Labrador Sea (after Yashayaev and Clarke, 2008; Curry et al., 2011; Lozier et al., 2017). CB: Canada Basin, CAA: Canadian Arctic Archipelago, BB: Baffin Bay, LS: Labrador Sea, ACW: Alaskan Coastal Water, wBSW: winter Bering Sea Water, FSB: Fram Strait 93 Branch, BSB: Barents Sea Branch, AW: Arctic Water, EGC: East Greenland Current, WGC: West Greenland Current, WGIW: West Greenland Intermediate Water, BC: Baffin Current, LSW: Labrador Sea Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. For a detailed view of the Canadian Arctic Archipelago, including the main landmarks, straits and water circulation, refer to Figure 4.8a. The subsurface circulation in the Labrador Sea is cyclonic, consisting of two components: the WGC and the Labrador Current, which flows southward along the Labrador Shelf and Slope and is the extension of the Baffin Current (BC; Cuny et al., 2002; Fischer et al., 2004). The Labrador Sea Water (LSW) lies between the subsurface water and the deep west boundary current; two LSW classes were distinguished from the CTD in 2015: the newly ventilated and fresher LSW formed during the winters of 2014 and 2015 (LSW-2015), and the old, saltiest and least oxygenated LSW produced in the winters of 1987-1994 (LSW-87/94; Yashayaev and Loder, 2016). Underlying the LSW-87/94, lies the saline, warm (θ ~3 °C, S ~34.92) and less oxygenated Northeast Atlantic Deep Water (NEADW), and the deepest, less saline, colder (Î¸â¤ 2.6 °C, S ~ 34.9), and more-recently oxygenated Denmark Strait Overflow Water (DSOW; Yashayaev et al., 2007; Yashayaev and Loder, 2016; Figures 4.1 and 4.2d). 94 Figure 4.2 Potential temperature (θ) versus salinity (S) diagram for the Canada Basin (a), the CAA (b), Baffin Bay (c) and the Labrador Sea (d). For the CAA (b), the θ/S data from the Canada Basin endmember (CB2) and Baffin Bay endmember (BB2) are shown with solid green lines. The main water masses are indicated in the figure: ACW: Alaskan Coastal Water, wBSW: winter Bering Sea Water, LH: Lower Halocline, AL: Atlantic Layer, FSB: Fram Strait Branch, BSB: Barents Sea Branch, CBDW: Canada Basin Deep Water, AW: Arctic Water, TrW: transitional water, WGIW: West Greenland Intermediate Water, BBDW: Baffin Bay Deep Water, LSW: Labrador Sea Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water. 4.4 Methods 4.4.1 Sample collection Seawater samples were collected on the CCGS Amundsen as part of the GEOTRACES sections GN02 and GN03 (10/07/2015-1/10/2015) which covered an area from 56°N to 77°N and 53°W to 150°W in the Canadian Arctic. Vertical profiles of 8 to 25 discrete depths were obtained at 18 stations: four in the Canada Basin, nine in the CAA, three in Baffin Bay and two 95 in the Labrador Sea (Figure 4.1). A trace-metal clean sampling system consisting of a powder-coated aluminium frame, which held twelve 12 L Teflon coated GO-FLO bottles (General Oceanics, Miami FL USA) and a SeaBird 911 CTD/SBE 43 oxygen sensor instrument package (Seabird Electronics Inc, Bellevue WA USA), attached by a 4000 m 4-member conducting Vectran cable encased in polyurethane (Cortland Cable Co., Cortland NY USA) was used to collect seawater samples. On-board the ship, samples were gravity-filtered through 0.2µm Acropak filters (Pall Corporation) into 250 and 500 mL LDPE bottles (Bel Art and Nalgene) in a HEPA filtered air environment. The sampling devices and bottles were prepared and pre-cleaned according to established GEOTRACES protocols (Cutter et al., 2010). After filtration, samples were acidified to pH=1.7 using SeaStar trace metal grade hydrochloric acid (SeaStar Chemicals, Sidney, BC). 4.4.2 Trace metal analyses Dissolved Fe and Mn from the Canadian Arctic GEOTRACES program were both measured independently by labs at the University of British Columbia (UBC) and the University of Victoria (UVic) following different analytical procedures. To prevent contamination, the processing and analysis of the samples at UBC and UVic were conducted in class 1000 laboratories, pressurized with HEPA filtered air, and under class 100 laminar flow fume hoods. All the plasticware used during the sample preparation and analysis were cleaned according to GEOTRACES protocols (Cutter et al., 2010). 4.4.2.1 Magnesium-induced coprecipitation and isotope dilution method (UBC) Dissolved Fe and Mn were preconcentrated 50-fold from 50 mL acidified seawater using a modified magnesium-induced coprecipitation method and quantified by isotope dilution (Wu 96 and Boyle, 1998; Saito and Schneider, 2006; Colombo et al., 2019b). As the scavenging efficiency of Mn, a monoisotopic element, is similar to Fe, DMn concentrations can be quantified by normalization to 57Fe counts: ðð = [[ðð]ððð [57ð¹ð]ð¡ðð¡ðð ððð â[57ð¹ð]ð ððððð ððð ] ð¥ [ð¹ðð ðððð] ð¥ [ð ðððð ð£ððð¢ððð¡ðð¡ðð ð£ððð¢ðð] ð¥ [ð¹ð ð ððð ðð¡ðð£ðð¡ð¦ðð ð ððð ðð¡ðð£ðð¡ð¦] as described by Saito and Schneider (2006). The seawater samples were equilibrated overnight with 57Fe enriched spikes (Oak Ridge National Laboratories) and trace metals were then precipitated with high-purity NH4OH (Seastar Baseline Chemicals Inc., Sidney, Canada). The analyses were conducted by high resolution Thermo Finnigan Element2 Inductively Coupled Plasma Mass Spectrometry in the Pacific Centre for Isotopic and Geochemical Research at UBC. A medium mass resolution was selected for Fe and Mn analysis in order to remove isobaric interferences caused by 40Ar16O and 40Ca16O for 56Fe and 15N40Ar for 55Mn. The counts per second (cps) of the blanks run during the analysis (n=69) represented only 1.5±1.1% (~0.011±0.006 nmol kg-1) and 3.0±1.9% (~0.065±0.012 nmol kg-1) of sample cps for Fe and Mn, respectively. 4.4.2.2 Offline extraction and triple quadrupole ICP-MS/MS method (UVic) Dissolved Fe and Mn were extracted and preconcentrated (8x) using the seaFAST-pico SC-4 DX system (ESI, Omaha, NE, USA). For each sample 20 mL of seawater were loaded onto a column which contained Nobias PA-1 resin (containing the functional groups ethylenediaminetriacetic acid and iminodiacetic acid). The column was then rinsed with an ammonium acetate buffer solution (pH=6.0) prepared by bubbling high-purity anhydrous ammonia gas through twice-distilled acetic acid with the pH adjusted by additions of NH3 to remove the matrix. Samples were eluted in 2.5 mL of 1.6 M Seastar Baseline HNO3 (Seastar Chemicals, Sidney, BC, Canada). The preconcentrated samples were analysed using the Agilent 97 8800 ICP-MS/MS (Jackson et al., 2018). This method yielded a limit of detection of 0.029 and 0.002 nmol L-1 for Fe and Mn, respectively. 4.4.2.3 Quality control and accuracy During the analysis, procedural blanks and quality control spikes (acidified Milli-Q water equilibrated with natural trace metal standards) were routinely run to ensure quality of the measurements. The accuracy was calculated by analyzing the SAFe D1, SAFe D2 and GSP S GEOTRACES reference materials multiple times, and the results were all within the error of the reported values from the oceanographic community (Table C.1). The precision of the analysis, relative standard deviations (1RSDs) of analyzed reference materials, was 4.3±1.4 and 3.7±1.4 % for Fe and Mn, and the RSDs of replicate analysis of control spikes (n=40) were 4.4%. Additionally, the intercalibration exercises carried out with colleagues from Texas A&M University were in close agreement to our values (Figures C.1 and C.2). The sample concentrations reported here were corrected for the analytical blank by subtracting the average blank on the corresponding analytical day. The DFe and DMn results from UBC and UVic (replicate samples stored in different LDPE bottles) showed excellent agreement (Figures C.1 and C.2), and therefore the two data sets were combined (Tables C.2 and C.3). The statistical analysis and graphics in this manuscript were developed using Python 3.6.0. 4.5 Results This study reports the vertical and spatial distributions of dissolved Fe and Mn concentrations in the Labrador Sea, Baffin Bay, the Canadian Arctic Archipelago and the Canada Basin. The full dataset of DFe and DMn is presented in Tables C.2 and C.3. Interpolated contour sections of DFe and DMn across the studied basins are presented in Figures 4.3 and 4.4, along 98 with the full depth profiles for each station. All sampled stations are displayed in a single figure in the supplementary material (Figure C.3). In general, surface DFe and DMn concentrations were high in the Canada Basin and the Canadian Arctic Archipelago (CAA), decreasing their concentrations towards the Labrador Sea. Below surface waters, the lowest DFe concentrations were measured in the Canada Basin (0.238-0.616 nmol kgË1), followed by the Labrador Sea (0.114-0.827 nmol kgË1) and Baffin Bay (0.529-2.00 nmol kgË1), while the CAA displayed the highest Fe concentrations (0.436-6.87 nmol kgË1; Figures 4.3a, 4.4a and C.3). Similar to DFe, the highest subsurface DMn concentrations were measured in CAA (0.416-8.40 nmol kgË1), while deep water concentrations were low and similar across the Canada Basin, Baffin Bay and the Labrador Sea (Figures 4.3b, 4.4b and C.3). In this paper we first discuss the biogeochemical and physical processes affecting the distributions of DFe and DMn in the surface waters of the Canadian Arctic Ocean (<40 m), followed by subsurface (~40-300 m) and deep waters (>300 m). 99 Figure 4.3 Interpolated contour section of dissolved Fe (a) and Mn (b) along the Canada Basin, the Canadian Arctic Archipelago, Baffin Bay and the Labrador Sea. Sampled depths are superimposed on the plot (white circles) and the individual profiles are displayed above each contour plot. Stations included are displayed in the inset map and the beginning of the transect is indicated by the red star. Note the break axis in the contour map (white line at 1050 m) and the logarithmic scale for the individual profiles. Stations were chosen to capture the flow of Arctic waters from the Canada Basin to the Labrador Sea. 100 Figure 4.4 Interpolated contour section of dissolved Fe (a) and Mn (b) in the Canadian Arctic Archipelago across Parry Channel; sampled depths are superimposed in the plot (white circles) and the individual profiles are displayed above each contour plot. c) The stations sampled at the three transects across Parry Channel are displayed in the maps, the beginning of the transect is indicated by the red star (Southern side of Parry Channel). Note the logarithmic scale for the individual profiles. 4.5.1 Dissolved Fe and Mn in surface waters For most stations, the highest DMn concentrations were measured in the upper 40 m (3.10-10.78 nmol kgË1), sharply decreasing with increasing depth (Figures 4.3b, 4.4b and 4.5). Intermediate concentrations were found in Canada Basin surface waters (<40 m; 5.15±1.21 nmol kgË1), while distinctively high concentrations were present in MâClure Strait (CB1), Viscount Melville Sound (CAA8) and Penny Strait (CAA9) in the western region of the CAA (8.91±1.15 nmol kgË1). 101 The concentrations then decrease towards Barrow Strait (CAA4 to CAA7) and Lancaster Sound (CAA1 and CAA3) in the eastern CAA (5.32±0.66 nmol kgË1), and Baffin Bay (3.26±0.38 nmol kgË1; Figure 4.5). Lowest surface water concentrations were found in the Labrador Sea (0.795±0.263 nmol kgË1). Surface DFe concentrations were generally low (0.106-1.91 nmol kgË1) in the Canadian Arctic Ocean, with the exception of CB4 where extremely high concentrations (3.42±0.19 nmol kgË1) were measured in the polar mixed layer (Figure 4.3a, 4.4a and 4.5). Although there was more variability than observed for DMn, surface (<40 m) DFe distributions resemble the general trends described for DMn. The highest concentrations were measured in the CAA (1.12±0.34 nmol kgË1); however, stations CB1, CAA8 and CAA4 had lower surface DFe values (0.587±0.113 nmol kgË1) despite high DMn concentrations measured at these stations, reflecting either different sources and/or removal mechanisms for these metals (Figure 4.5). Intermediate DFe concentrations were measured in Baffin Bay (0.336-0.763 nmol kgË1), followed by the Canada Basin (0.225-0.479 nmol kgË1; not including PML value at CB4) and the Labrador Sea (0.106-0.362 nmol kgË1). 102 Figure 4.5 Average surface (< 40 m) DFe and DMn concentrations (nmol kgË1) for the sampled stations, Mn data and axis are displayed in blue and Fe data and axis are displayed in red. 4.5.2 Dissolved Fe and Mn in shelf seas and subsurface waters In this section, we present the distributions of DFe and DMn in the CAA shelf dominated region (stations CB1 and CAA1-CAA9) and in subsurface waters (40-300 m) in the Canada Basin, Baffin Bay and the Labrador Sea, focusing on the advective features found in these deep basins. The Canadian Arctic Archipelago (CAA). The highest concentrations of DFe and DMn in the Canadian Arctic Ocean were generally found in the CAA, though a marked spatial difference is noted between western and eastern CAA (Figures 4.3 and 4.4). In MâClure Strait and Viscount Melville Sound (CB1 and CAA8) the concentrations of DMn were lower than the rest of CAA stations, steadily decreasing from 40 to 200 m and remaining low approaching the seafloor 103 (0.549±0.092 and 0.545±0.145 nmol kgË1, respectively), whereas east of Barrow Strait (sill depth ~50 m) the concentrations were up to an order of magnitude higher. Likewise, DFe concentrations were lower in MâClure Strait and Viscount Melville Sound and the vertical profiles were different from the eastern CAA stations, with an increase of DFe below surface waters, peaking between 70-100 m (CB1: 1.10±0.05 and CAA8: 1.15±0.02 nmol kgË1), and steadily decreasing towards the bottom (CB1: 0.688±0.008 and CAA8: 0.447±0.036 nmol kgË1; Figure 4.3a). In the eastern CAA, below 40 m, DMn concentrations generally decreased with depth to near-bottom waters where a rapid rise of Mn was detected. The stations located along the southern edge of Parry Channel (CAA7, CAA4 and CAA3) exhibited lower subsurface concentrations (2.55±0.42, 3.06±0.83 and 2.57±1.03 nmol kgË1, respectively) than the stations located on the north side of Parry Channel and in Penny Strait (CAA6: 5.23±0.39, CAA5: 4.28±0.32, CAA1: 3.85±0.66 and CAA9: 3.84±0.40 nmol kgË1; Figure 4.4b). An increase of DMn was observed in near-bottom waters for most stations in the eastern CAA, with higher concentrations found at stations along the northern edge of Parry Channel (CAA6: 6.06±0.12, CAA5: 5.51±0.22 and CAA2: 8.40±0.23 nmol kgË1), compared with the southern edge stations (CAA4: 3.02±0.12 and CAA3: 1.78±0.04 nmol kgË1). However, CAA7 displayed concentrations comparable to the northern stations (6.84±0.27 nmol kgË1; Figure 4.4b). Contrasting with DMn distribution in the eastern CAA, DFe concentrations, below 40 m, generally increased with depth to near-bottom waters. As was observed for DMn in near-bottom waters, much higher DFe was measured in the northern Parry Channel (CAA6: 5.13±0.27, CAA5: 3.91±0.14, CAA2: 6.62±0.54 and CAA1: 6.87±0.60 nmol kgË1) compared to the southern stations (CAA7: 2.54±0.091, CAA4: 2.01±0.19 and CAA3: 2.24±0.18 nmol kgË1; Figure 4.4a). Station 104 CAA2 stands out for having significantly (p-value< 0.01) lower concentrations of DFe and DMn compared to CAA1, despite being only ~20 km distant from CAA1 (Figure 4.4). The Canada Basin. Below the surface maximum, DMn concentrations decreased sharply with depth, reaching a subsurface minimum at ~100 m (1.35±0.08 nmol kgË1), while DFe concentrations remained similar (0.396±0.060 nmol kgË1) from the surface down to 100 m (Figures 4.3 and 4.6). From about 100 to 300 m a distinctive feature of high concentrations for both DFe and DMn was observed in the density range of ÏÎ¸= 25.69-27.64, reaching values as high as 0.616 and 2.00 nmol kgË1, respectively. These subsurface peaks were consistent across the Canada Basin (Fe: 0.541±0.060 and Mn: 1.37±0.41 nmol kgË1), with no significant differences in the concentrations detected among the three stations in this basin (ANOVA; p-value> 0.05; Figures 4.3 and 4.6). Baffin Bay and the Labrador Sea. Dissolved Fe in subsurface Baffin Bay waters exhibited a distinctive peak within the AW, with the highest values observed at BB3 (0.884±0.108 nmol kgË1) along the Baffin Bay slope, while the peaks were less pronounced in central Baffin Bay (BB2: 0.624±0.079 nmol kgË1) and Davis Strait (BB1: 0.754±0.117 nmol kgË1). In the Labrador Sea this subsurface maximum was not observed, but a sharp rise of DFe was noted from the surface to 300 m (ÏÎ¸~ 27.7) at stations LS2 and K1 (0.287±0.148 to 0.652±0.033 and 0.106±0.038 to 0.652±0.037 nmol kgË1, respectively; Figures 4.3a and 4.6). In contrast, in Baffin Bay DMn did not show a subsurface peak, and the concentrations rapidly decreased with depth reaching consistently low concentrations at ~300 m (ÏÎ¸~ 27.4; 0.500±0.064 nmol kgË1). Dissolved Mn in Labrador Sea subsurface waters was slightly higher than surface waters, related to the pronounced reduction of freshwater inputs and 105 enhanced phytoplankton uptake of DMn in this basin (section 4.5.1), slowly decreasing its concentration with depth to approximately 300 m (LS2: 0.590±0.050 and K1: 0.665±0.018 nmol kgË1). 106 Figure 4.6 Profiles of Fe, Mn, Si, N*, Temperature, Oxygen and Fluorescence versus density (ÏÎ¸) for the deep stations in the Canada Basin (upper panel: CB2-CB4), Baffin Bay (middle panel: BB1-BB3) and the Labrador Sea (lower panel: LS2 and K1). N* was calculated from the following equation: N*= ([NO3- + NO2- + NH4+] â 16PO43- + 2.9) x 0.87; Gruber and Sarmiento (1997). Silicate and N* data from the Canadian GEOTRACES cruise were provided by ArcticNet (Jean-Eric Tremblayâs group). Note the surface water concentration of DFe for the station CB4 in the Canada Basin was extremely high (3.42±0.19 nmol kg-1), and is not displayed in the Figure to emphasize deep water features. Important water masses were superimposed in the plots to facilitate the interpretations and discussion of the trace metal data following Aksenov et al. (2011), Kondo et al. (2016), Tang et al. (2004), Curry et al. (2011), Yashayaev and Loder (2009 and 2016). wBSW: winter Bering Sea Water, LH: Lower Halocline, FSB: Fram Strait Branch, BSB: Barents Sea Branch, CBDW: Canada Basin Deep Water, WGIW: West Greenland Intermediate Water, TrW: transitional water, AW: Arctic Water, BBDW: Baffin Bay Deep Water, LSW: Labrador Sea Water, ISW: Icelandic Slope Water, NEADW: Northeast Atlantic Deep Water, DSOW: Denmark Strait Overflow Water.107 4.5.3 The contrasting biogeochemistry of DFe and DMn in deep waters Inter-basin differences of DFe and DMn were observed in the deep waters (>300 m; Figure 4.3), with the lowest values found in the Canada Basin (Fe-AVG: 0.361±0.086, Fe-range: 0.238-0.592 nmol kgË1; Mn-AVG:0.280±0.101, Mn-range: 0.147-0.565 nmol kgË1), followed by the Labrador Sea (Fe-AVG: 0.717±0.052, Fe-range: 0.647-0.827 nmol kgË1; Mn-AVG:0.428±0.095, Mn-range: 0.306-0.593 nmol kgË1), while Baffin Bay had the highest values (Fe-AVG: 0.942±0.151, Fe-range: 0.647-1.330 nmol kgË1; Mn-AVG:0.485±0.176, Mn-range: 0.239-0.776 nmol kgË1). The Canada Basin. An interesting difference in DFe distributions was observed in the Canada Basin below 300 m, where significantly (p-value< 0.01) lower concentrations were measured at CB3 station than at the stations CB2 and CB4 across FSB, BSB, CBDW (ÏÎ¸> 27.92), a phenomenon which was not seen for DMn (Figure 4.6). Below the subsurface peak of Fe and Mn in the wBSW, concentrations dropped within the Atlantic Layer (AL), from approximately 300 to 800 m, and the lowest levels of DFe (CB2 and CB4: 0.412±0.007; CB3: 0.284±0.001 nmol kgË1) and DMn (CB2-CB4: 0.327±0.012 nmol kgË1) coincided with the FSB warm core (θ= 0.7-0.8 ºC) at ~500 m (ÏÎ¸=27.92; Figure 4.6). In the lower AL, across BSB waters (~800-1000 m), concentrations increased slightly, while in the CBDW (>1200 m), DMn concentrations decreased steadily to the bottom with uniformly low concentrations below 2500 m (CB3: 0.149±0.002 and CB4: 0.158±0.006 nmol kgË1). Similarly, DFe at CB4 decreased steadily from 1000 to 3500 m (0.449±0.003 to 0.238±0.011 nmol kgË1), while at CB3, DFe distributions were maintained virtually uniform in CBDW (0.303±0.024 nmol kgË1; Figure 4.6). 108 Baffin Bay. Dissolved Fe concentrations rose from transitional waters (TrW, 0.669±0.019 nmol kgË1) across the warm (1.3-3.9 ºC) West Greenland Intermediate Water (WGIW) to ~1000 m, where similar concentrations were measured from 300 to 800 m at the stations BB3 and BB1 along the Baffin Bay slope (0.995±0.082 and 1.09±0.13 nmol kgË1), and slightly lower concentrations at BB2 station (0.866±0.062 nmol kgË1). The sharp DFe increase at 1000 m noted at BB1 (1.33±0.14 nmol kgË1) and BB3 (2.00±0.13 nmol kgË1) contrasts with the quickly decreasing concentrations observed at the central BB2 station in Baffin Bay Deep Waters (BBDW) from approximately 1000 to 2300 m (0.962±0.002 to 0.647±0.094 nmol kgË1, respectively; Figure 4.6). Dissolved Mn concentrations decreased below AW, remaining nearly uniform across the WIGW, with higher values at stations BB3 and BB1 (0.521±0.011 and 0.600±0.036 nmol kgË1, respectively), and increased concentrations towards the seafloor (4.20±0.008 and 0.768±0.057 nmol kgË1, respectively). Otherwise, at the central BB2 station the DMn in the WGIW was lower (0.416±0.005 nmol kgË1) than over the Baffin Bay slope (BB3 and BB1), noticeably decreasing in BBDW (0.254±0.016 nmol kgË1). As observed at BB3, a sudden increase of DMn occurred in the near-bottom waters (0.736±0.048 nmol kgË1); an increase not observed for DFe (Figure 4.6). The Labrador Sea. Below subsurface waters (>300 m; ÏÎ¸= 27.71), DFe continued to increase with depth, across the LSW-2015, ISW and LSW-87/94 water masses, reaching the highest concentrations measured in the Labrador Sea at ~2000 m (0.818±0.009 nmol kgË1; ÏÎ¸= 27.82) at the interface between the old LSW-87/94 and the NEADW. Underneath LSW-87/94, DFe concentrations declined across the NEADW, reaching uniformly low concentrations in the deep DSOW (0.677±0.023 nmol kgË1). The vertical distribution of DMn in deep waters, opposite 109 to DFe, generally decreased with depth, with considerably higher concentrations observed across LSW-15 (~400-1000 m; ÏÎ¸= 27.71-27.74) at K1 (0.665 to 0.528 nmol kgË1) compared with LS2 (0.519 to 0.479 nmol kgË1). Underneath LSW-15, no significant concentration differences were observed between LS2 and K1, where concentrations steadily decreased with depth reaching the lowest values (0.306-0.330 nmol kgË1) in the lower boundary of NEADW (~2500 m; ÏÎ¸= 27.86). In the bottom DSOW waters DMn concentrations were moderately higher (0.359-0.385 nmol kgË1) than NEADW (Figure 4.6). 4.6 Discussion 4.6.1 Dissolved Fe and Mn in surface waters: the role of fresh water inputs and phytoplankton uptake The relatively high surface concentrations of DMn (3.20-7.02 nmol kgË1) along with DFe (0.225-0.479 and 3.42 nmol kgË1 for the PML at CB4) in the Canada Basin agree with previous measurements (Mn: 3.44-6.74 and Fe: 0.2-0.85 nmol kgË1) made in the area (Nakayama et al., 2011; Aguilar-Islas et al., 2013; Kondo et al., 2016; Sim, 2018). Although there are no previous measurements in the CAA, the high DMn and DFe surface concentrations (4.33-10.78 and 0.401-1.91 nmol kgË1, respectively) in this region are in the range of those reported for the Canada Basin shelf break (Mn: 5.48-8.36 and Fe: 0.44-1.66 nmol kgË1) and the Chukchi Sea (Mn: 1.62-13.8 and Fe: 1.11-22.2 nmol kgË1). These regions are both characterized by shallow shelves and elevated riverine freshwater input (Cid et al., 2012; Kondo et al., 2016; Sim, 2018; Vieira et al., 2019). As noted in section 4.5.1, DMn and DFe decrease dramatically towards Baffin Bay (2.56-3.68 and 0.336-0.763 nmol kgË1, respectively) and the Labrador Sea (0.450-1.09 and 0.106-0.362 nmol kgË1, respectively) toward values that are similar to those 110 reported for surface waters in the north Subarctic Atlantic (Mn: 0.25-0.50 and Fe: 0.02-0.53 nmol kgË1; Measures et al., 2008; Achterberg et al., 2018). In order to identify the sources of Mn and Fe to surface waters (< 40 m) in the Canadian Arctic Ocean, we examine the distribution and relationship of these metals compared to other physicochemical properties. As observed in previous studies in the Arctic Ocean, there is a negative correlation between DMn and DFe (to a lesser extent) with salinity (Middag et al., 2011b; Nakayama et al., 2011; Klunder et al., 2012a; Nishimura et al., 2012; Aguilar-Islas et al., 2013; Kadko et al., 2018; Sim, 2018). The highest DMn concentrations are seen in the low salinity range (< 31), illustrating the prevailing role of freshwater inputs on the distribution of DMn, as well as DFe in the Canadian Arctic Ocean. Dissolved Mn behaves almost conservatively, exhibiting: 1) a strong negative linear correlation of DMn versus salinity in Canada Basin (CB2 to CB4), Baffin Bay and Labrador Sea surface waters (R2= 0.83 and p-value< 0.01), with concentrations spanning from 0.450 nmol kgË1 in the Labrador Sea to 7.02 nmol kgË1 in the Canada Basin (Figure 4.7a). 2) a moderate correlation (R2= 0.55 and p-value< 0.01) is found in CAA surface waters (including CB1). This correlation is much stronger if the CAA6 surface data point, which groups more closely with the Canada Basin samples, is excluded from the analysis (R2= 0.74 and p-value< 0.01; Figure 4.7a). These different trends are most likely attributable to distance from riverine sources; the CAA is a shallow (~300 m), shelf-dominated region highly influenced by freshwater runoff, glacial meltwater and sediment-seawater interactions (Lammers et al., 2001; Michel et al., 2015; Alkire et al., 2017; Colombo et al., 2019a), which explains the elevated DMn of this region (4.33-10.78 nmol kgË1) and the steeper slope of the linear regression for CAA surface samples (Figures 4.5 and 4.7a). At CAA2, the DMn concentrations 111 (3.72±0.21 nmol kgË1), as well as DFe (0.457±0.066), were slightly lower than the other stations in the archipelago, reflecting the recirculation of Baffin waters with lower DFe and DMn concentrations in northern Lancaster Sound (Wang et al., 2012; Figure 4.5). Although we did not analyze the relative contribution of meteoric water and sea ice meltwater to the âfreshwaterâ pool in surface waters (e.g. optimum multiparameter analysis), some inference can be drawn by estimating the concentration of DMn at zero salinity, extrapolated from the CAA regression line (Figure 4.7a) assuming conservative behavior, and comparing it with riverine and sea ice meltwater end-members. The projected DMn end-member from the regression line is approximately 45 nmol kgË1, which is substantially higher than the concentrations reported for sea ice and sea ice melt ponds (3-24 nM; nM ~ nmol kg-1) in the Arctic Ocean (Campbell and Yeats, 1982; Marsay et al., 2018a). Therefore, sea ice meltwater is not likely to be the main freshwater input of DMn and DFe to the CAA region. On the other hand, the projected DMn from CAA surface data is generally lower than the concentrations measured in remote continental rivers (11-54 nM, up to 750 nM during the spring peak discharge) and glacial meltwater (164 nM; Colombo et al., 2019a) in the Canadian Arctic Archipelago. Allowing for non-linear losses in estuaries, riverine inputs are presumably the main contributor to high DMn in CAA surface waters. 112 Figure 4.7 a) Dissolved Mn concentrations versus practical salinity in the upper 40 m for all the stations. Linear regression lines for the Canada Basin (CB2 to CB4), Baffin Bay and Labrador Sea surface waters (green) and CAA surface waters (blue) are superimposed in the figure. The 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500), are also superimposed in the figure. The density of overlapping lines indicates increased confidence. The inset displays the Fe versus salinity for the same stations. b) Dissolved Mn concentrations (upper 40 m) versus distance from southern Lancaster Sound (CAA4 and CAA3), Baffin Bay coast (BB3), the Davis Strait (BB1) to the Labrador Sea (LS2 and K1), tracing the evolution of DMn in the Arctic outflow from southern Lancaster Sound to the Labrador Sea. Stations included in this plot are displayed in the map and the beginning of the transect is indicated by the red star. The inset shows the monthly-averaged log 10 Chl a concentration in the Labrador Sea (May 2015; data retrieved from MODIS-Aqua sensor: https://oceancolor.gsfc.nasa.gov/l3/), illustrating the intense phytoplankton bloom that took place approximately two months before the sampling. Monthly-averages from 2015, including the entire study area, are presented in Figure C.4.113 The stations in the Canada Basin, Baffin Bay and the Labrador Sea, in contrast to the CAA, are much deeper (~1000-3500 m), and less influenced by continental shelves, and thus the influence of freshwater inputs is expected to be reduced. Among the deep stations, those in the Canada Basin (CB2 to CB4) had the lowest surface salinity values (25.1-29.2) and the highest DMn concentrations (3.20-7.02 nmol kgË1) compared with Baffin Bay and the Labrador Sea (S: 30.8-34.5; DMn: 0.450-3.68 nmol kgË1), as result of the entrainment and accumulation of freshwater and sea ice melt by the Beaufort Gyre (Guay et al., 2009; Proshutinsky et al., 2009; Lansard et al., 2012). The progressive loss of DMn from southern Lancaster Sound (CAA4: 6.07±0.57 and CAA3: 5.10±0.27 nmol kgË1) towards central Baffin Bay (BB2 and BB3: 3.48±0.21 nmol kgË1), Davis Strait (BB1: 2.83±0.27 nmol kgË1) and the Labrador Sea (LS2: 0.97±0.12 and K1: 0.450±0.12 nmol kgË1) may reflect less freshwater inputs from rivers in the CAA and adjacent shelves areas, and/or dilution from other freshwater sources (e.g. sea ice meltwater) with distance along the flow path (Figures 4.5 and 4.7b). Sinks for DMn and DFe may also be enhanced by intense phytoplankton uptake in the productive waters of the Labrador Sea, where the lowest concentrations of these trace elements are measured (Figure 4.5). Unlike the other stations sampled in this study, and typical surface DMn data reported from most other basins (Statham et al., 1998; Boyle et al., 2005; Wu et al., 2014; Hulten et al., 2017), surface concentrations in the Labrador Sea are lower than subsurface samples, not showing a DMn surface maximum (Figures 4.3b, 4.4b and 4.6). Similar profiles have been found in the Weddell Basin, where the near depletion of DMn in surface waters was attributed to fast biological Mn uptake under Fe limiting conditions, due to enhance oxidative stress in Fe-limited phytoplankton and the resulting expression of Mn-containing superoxide 114 dismutase, coupled with the lack of external Mn sources (Middag et al., 2011a; Middag et al., 2013). Intense phytoplankton spring blooms, triggered by increased irradiance and mixed-layer shoaling, occur in the Labrador Sea (Frajka-Williams et al., 2010). These Labrador Sea blooms are the most intense in the Canadian Arctic Ocean, typically peaking in May and slowly decreasing in intensity throughout the summer. When samples were collected in the Labrador Sea (July, 2015), an intense bloom had recently been observed (May, 2015; Figures 4.7b and C.3); this bloom was one of the largest in recent years. (https://oceancolor.gsfc.nasa.gov/l3/). Iron-limiting conditions in the Labrador Sea have not been described yet, mainly due to the scarcity of DFe data. However, some studies have shown that the Iceland and Irminger basins experience seasonal Fe-limiting conditions during the spring bloom, when relatively high macronutrient concentrations were still found in surface waters (Nielsdóttir et al., 2009; Ryan-Keogh et al., 2013; Achterberg et al., 2018). As a first approximation, to evaluate whether phytoplankton in Labrador surface waters (< 40 m) were potentially limited by Fe relative to PO4-3, we calculated the Fe* index (Fe*= DFe â 0.47[PO4-3]) developed by Parekh et al. (2005). Positive Fe* values indicate that DFe is sufficiently high to allow full utilization of PO4-3, while negative values suggest potential DFe deficiency. At station K1 (the furthest from Davis Strait) surface Fe* was close to 0 (0.009) whereas at LS2 station the values were slightly higher (0.196-0.330). Although no negative values were observed in this study, the low Fe* at K1 suggests potential Fe stress in the Labrador Sea phytoplankton, especially during the early bloom period. It is important to highlight that the macronutrient concentrations in Labrador Sea surface waters were extremely low at the sampling time (post-bloom), and hence Fe limitation could be attenuated. Therefore, increased biological uptake of DMn and DFe coupled with limited 115 freshwater inputs of these metals are likely to explain the low Fe concentrations and DMn minimum in Labrador Sea surface waters. The main sources of DFe to surface waters in the Canadian Arctic Ocean are expected to be similar to those of DMn. However, the distribution of DFe deviates from the trends observed for DMn in surface waters, which reflect its unique aqueous biogeochemistry. Dissolved Fe is less soluble, more scarce relative to its biological requirement for microbes, and has faster oxidation and complexation kinetics than Mn (Landing and Bruland, 1987; Bruland et al., 1991; Nico et al., 2002; Hansard et al., 2009; Mendez et al., 2010; Aguilar-Islas et al., 2013; Morel et al., 2013; Gerringa et al., 2015; Buck et al., 2018). Surface DFe concentrations in the Labrador Sea, Baffin Bay and most of the stations in CAA generally increase with decreasing salinity, but deviations from this relationship (lower DFe than predicted by surface salinity) are evident in the Canada Basin and CAA4, 6 and CAA8 stations. 4.6.2 Dissolved Fe and Mn in shelf seas and subsurface waters: shelf-ocean interactions, scavenging removal and advective transport 4.6.2.1 Influence of Chukchi Sea derived waters on the Fe and Mn signature in subsurface waters in the Canada Basin Noteworthy subsurface maxima of DFe (0.541±0.060 nmol kgË1) and DMn (1.37±0.41 nmol kgË1) were observed in the wBSW within a narrow depth range (100-300 m) across the Canada Basin (Figures 4.3 and 4.6). The highest concentrations coincided with a peak in major algal nutrient concentrations (Si: 34, PO4-3: 1.8 and NO3-: 16 µmol L-1), and minima in N*, temperature and oxygen values (-10, -1.5 °C and 6 mL L-1, respectively) at a density of ÏÎ¸= 26.5 (Figure 4.6). This characteristic wBSW signature is imprinted during its flow across the 116 shallow and highly productive Chukchi Sea, where intense organic matter remineralization on the seafloor promotes sedimentary denitrification/anammox, leading to negative N* values, and increasing the benthic flux of reduced species of Fe and Mn to the overlying wBSW as sediment porewater O2 and redox potential diminishes (Chang and Devol, 2009; Kondo et al., 2016; Granger et al., 2018; Vieira et al., 2019). Similar subsurface peaks (ÏÎ¸= 26.5) of DFe and DMn, along with a nutrient maximum and low N* values, were observed along the Chukchi slope (Fe: 4.53±1.01 and Mn: 37.7±18.3 nM) and the Canada Basin (Fe: 1.62±0.41 and Mn: 1.34±0.57 nM), with the metal concentrations decreasing exponentially with distance from the shelf break (Hioki et al., 2014; Kondo et al., 2016). It is not surprising that DFe concentrations in the wBSW reported by Hioki et al. (2014) and Kondo et al. (2016) were higher than those measured in this study (D5 station: 1.62±0.41 nM versus CB2-CB4 stations: 0.541±0.060 nmol kgË1), since their D5 station was located ~ 350 km from Chukchi shelf while CB2-CB4 stations were located more than 600 km away from the shelf. Nonetheless, DMn concentrations measured at D5 (1.34±0.57 nM) were comparable to those measured in this study (1.37±0.41 nmol kgË1). This contrasting behavior between DFe and DMn could be attributed to the more rapid oxidation kinetics and enhanced scavenging loss of Fe relative to Mn, that without organic ligand stabilization, Fe is rapidly removed from the water column (Sunda et al., 1983; Field and Sherrell, 2000; Thuróczy et al., 2011; Wu et al., 2011; Klunder et al., 2012b; Buck et al., 2018). 117 4.6.2.2 Benthic sources of dissolved Fe and Mn in the shallow Canadian Arctic Archipelago environment The concentrations of Fe and Mn measured in the Canadian Arctic Archipelago (CAA) were the highest observed in this study (Figures 4.3 and 4.4), with subsurface concentrations comparable to those found in other shallow shelf regions in the Arctic Ocean such as the Chukchi Sea (Fe: ~3-15 nM) and the Laptev and Barents shelves (Fe: ~2-12 and Mn: ~3-5 nM), albeit DMn concentrations in Chukchi near-bottom waters were considerably higher (~10-81 nM) than in the CAA (Middag et al., 2011b; Klunder et al., 2012a; Kondo et al., 2016; Vieira et al., 2019). Below the surface, the most striking feature in the distributions of dissolved Fe and Mn is the spatial variability observed between the western and the eastern CAA, with significantly lower concentrations and no evidence of benthic inputs of these trace metals in MâClure Strait (CB1) and Viscount Melville Sound (CAA8) compared with the stations located in the eastern Parry Channel (CAA1-CAA7; Figures 4.3, 4.4 and 4.8a). This discrepancy is attributed to the advection of Arctic waters with low Fe and Mn from the Canada Basin, less mixing in the western CAA, and enhanced sedimentary Fe and Mn inputs in the eastern CAA. Subsurface Arctic waters (wBSW and LH), advected from the Canada Basin, carry relatively low DFe and DMn (0.488±0.097 and 1.31±0.52 nmol kgË1) to western Parry Channel (~500 m) up to Barrow Strait, where the sill depth (~120 m) restricts the continued direct flow downstream (Wang et al., 2012; Colombo et al., 2019b). Mixing is intense throughout the shallow CAA as result of tidal forcing, shear instabilities and the breaking of internal waves over the rough topography in the small straits and conduits forming the archipelago (Hannah et al., 2009; Hughes et al., 2017; 118 Hughes et al., 2018). However, mixing is particularly strong in the central sills area (~120 m) near Barrow Strait (confined by Bathurst, Cornwallis, Devon, Sommerset and Prince of Wales Islands) and in Penny Strait (Figure 4.8a), where averaged diapycnal diffusivities and buoyancy fluxes are up to an order of magnitude larger than in the western CAA (Hughes et al., 2017). As expected, the Arctic outflow is greatly modified over the shallow Barrow Strait area and in Lancaster Sound by incorporating river and glacial runoff, mixing with Baffin Bay waters, and most importantly, by interacting with bottom topography (Wang et al., 2012; Hughes et al., 2017; Colombo et al., 2019b; Colombo et al., 2019a). In this context, the elevated DFe and DMn measured in subsurface waters and the sharp rise in their concentration towards near-bottom waters in eastern Parry Channel stations (CAA1-CAA7) are likely associated with sedimentary inputs. The release of Fe and Mn from sediments is identified as a key source that explains the elevated concentrations of these metals in shelf-slope regions worldwide (Johnson et al., 1999; Lam and Bishop, 2008; Cullen et al., 2009; Klunder et al., 2012a; Hatta et al., 2015; Milne et al., 2017; Cheize et al., 2019; Vieira et al., 2019). Although the release of Fe and Mn from anoxic sediments (reductive dissolution) cannot be neglected (Klar et al., 2017), it is not considered the primary mechanism taking place in the CAA. All studied CAA stations were well oxygenated, including the bottom waters which had oxygen concentrations above 200 µmol kg-1 (4.6 ml L-1) and N* values ranging from -6.3 to 2.5. Under the strong reducing sediment conditions present in the highly productive Chukchi shelves, near-bottom waters had much lower N* values (~ -15) and much higher DMn concentrations (~10-81 nM) than observed in the CAA (Hioki et al., 2014; Kondo et al., 2016; Vieira et al., 2019). This suggests stronger benthic reductive inputs in 119 the Chukchi Sea region compared to the CAA. Likewise, the release of nutrient-like trace metals during the remineralization of organic matter does not appear to be a main source in the CAA, given the poor relationship between DFe and apparent oxygen utilization (R2= 0.03 and p-value= 0.11). Thus, the rise of DFe and DMn from 130 m toward the seafloor along the eastern Parry Channel is likely related to the resuspension, desorption and/or dissolution from sediment particles during mixing, as indicated by the moderate (R2= 0.52 and p-value< 0.001) and strong (R2= 0.68 and p-value< 0.001) negative correlations found between DFe and DMn versus transmissivity (lower transmissivity values indicate greater suspended particle matter; Figures 4.8b and 4.8c). Other factors such as the distance from the deepest sample to bottom sediments do not explain the variability in metal concentrations observed in near-bottom waters. The distance from the deepest samples to the bottom floor for the stations sampled in western CAA (CB1 and CAA8), which do not show any Fe and Mn enrichment in near-bottom waters, was 61 and 113 m; however, the stations CAA1 and CAA3, located in eastern Parry Channel at a comparable distance from the seafloor (72 and 98 m, respectively), do show a clear enrichment of these trace metals (Figure 4.8a). Moreover, in eastern CAA, bottom inputs of DFe and DMn can be traced not only at the deepest samples, but also in overlying samples. The distributions of particulate Fe and Mn from the same CAA stations show similar trends (Li, 2017), with a sharp increase in near-bottom waters for those stations located in Barrow Strait and Lancaster Sound (CAA1-CAA7), with relatively low concentrations found in the western CAA (CB1 and CAA8), further supporting the importance of sediment resuspension and benthic inputs of both dissolved and particulate Fe and Mn in eastern Parry Channel. 120 Figure 4.8 a) Sampled stations and circulation pathways (blue arrows) in the CAA (after Michel et al., 2006; Wang et al., 2012); red and light red shaded area: conceptual scheme displaying intense diapycnal and tidal mixing in Barrow Strait area and eastern CAA (modified from Hannah et al., 2009; Hughes et al., 2017; Hughes et al., 2018). Dissolved Fe (red profiles) and Mn (blue profiles) concentrations (nmol kgË1) for each CAA station are superimposed in the Figure panel 4.8a; the gray shading indicates the bottom floor, the pale green shaded area indicates those samples below 130 m, and the number within a white circle is the distance (m) from the deepest sample to bottom sediments. MS: MâClure Strait, VS: Viscount Melville Sound, BS: Barrow Strait, PS: Penny Strait, LS: Lancaster Sound, BaI: Bathurst Island, CI: Cornwallis Island, SI: Somerset Island, PWI: Prince of Wales Island, DI: Devon Island, ByI: Bylot Island, EI: Ellesmere Island, BI: Baffin Island. Parry Channel is the main pathway in central CAA connecting MâClure Strait with Lancaster Sound. b) and c) Dissolved Fe (red symbols) and Mn (blue symbols) versus transmissivity for CAA samples below 130 m (values greater than 100% indicate offset in the calibration). Linear regression lines and R2 values are displayed in the figure, as well as the 95% prediction limits and the 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500). The density of overlapping lines 121 indicates increased confidence. d) Natural logarithm of dissolved Fe concentrations in the Arctic Waters (AW; ÏÎ¸= 26.28-27.12) versus distance from southern Lancaster Sound (CAA4 and CAA3), Baffin Bay coast (BB3) to the Davis Strait (BB1), showing an exponential decrease in DFe concentrations (DFe= 1.732 e -0.0006 Distance or Ln [DFe]= -0.0006 Distance+0.549 when it is linearized). Stations included in this plot are displayed in the inset map and the beginning of the transect is indicated by the red star and the linear regression line, R2 value and the 95% prediction limits and the 95% confidence intervals are displayed in the figure. The consistently higher concentrations of both DFe and DMn measured in the stations located along the northern edge of Parry Channel relative to the southern ones are potentially explained by increased sediment resuspension due to the recirculation of Baffin Waters in this region (Figure 4.8a). Additionally, the presence of extensive subglacial runoff from marine-terminating glaciers distributed along the western sides of Ellesmere, Devon and Baffin islands may be a source of more easily reducible Fe oxides and soluble Fe+2 containing minerals compared to sediments close to the ice-free coast of southern Parry Channel (Lenaerts et al., 2013; Henkel et al., 2018). Lastly, the significantly (p-value< 0.01) lower subsurface DFe and DMn concentrations present at station CAA2 compared to CAA1 (Figures 4.4 and 4.8a), despite their close proximity (~20 km apart), are related to the different water masses feeding these stations. Station CAA1, as well as CAA5 and CAA6, are located in close proximity to the Devon Island coast (~ 5km; Figure 4.8a), receiving inputs of glacial runoff which are recognized by having extremely high concentrations of DFe and DMn (212 and 164 nM; Colombo et al., 2019a). Conversely, the station CAA2 is influenced by the recirculation of low DFe and DMn Baffin waters; these differences can be also noted from the temperature and salinity profiles, with CAA1 waters noticeable fresher and colder than at CAA2 (Figure 4.2b), and from the distributions of dissolved Pb (Colombo et al., 2019b). 122 4.6.2.3 The importance of CAA waters modulating subsurface DFe and DMn in Baffin Bay and the Labrador Sea The CAA is an important pathway linking the Canada Basin with Baffin Bay, whereby Arctic derived waters are exported to the North Atlantic (Jones, 2003; Beszczynska-Möller et al., 2011). In Baffin Bay this Arctic outflow is mixed with the WGC evolving into the fresher (S=32.0-33.7) and colder (T< 0 °C) Arctic Water (AW; Figure 4.2c). During its flow through the shallow CAA shelves, Arctic derived waters are enriched in Fe (section 4.6.2.2), relative to Canada Basin concentrations, advecting this high signature to the Baffin Bay interior, where a DFe peak (0.753-1.025 nmol kgË1) is observed in subsurface AW (Figures 4.3a and 4.6). Dissolved Fe concentrations in AW (BB3>BB1>BB2) reflect the proportional contribution of Arctic derived waters, where BB3 is most influenced by this outflow followed by BB1 (~85 and ~60%), while BB2 station, located in the center of the bay, is more isolated from the fresh, cold and Fe rich coastal CAA outflow (~40%; Colombo et al., 2019b). The offshore transport of this AW DFe-rich plume along isopycnal surfaces (ÏÎ¸= 26.28-27.12) can be followed for more than 1400 km from the western Parry Channel in the CAA (station CAA4) to Davis Strait (station BB1), with the Baffin Current (Figure 4.8d), but no traces of this high DFe plume is found in the Labrador Sea (Figure 4.3a and 4.6). An exponential function (DFe= A e[-d/Lo], where A: 1.732 nmol kg-1, d: distance from station CAA4 [km] and Lo: scale length of offshore transport, ~1450 km; Figure 4.8d) best describes (R2= 0.92, p-value< 0.05) the offshore decrease of DFe from the shelf break (Figure 4.8d). We estimate the scale length of DFe transport as the distance over which concentrations drop to approximately 37% or 1/e of the initial values (Johnson et al., 1997; Klunder et al., 2012b; Aguilar-Islas et al., 2013). The scale length of 123 offshore transport of DFe in Baffin Bay (Lo= ~1450 km) is 4-5 times greater than those calculated for the Canada and Nansen basins (380 and 260 km), where the currents are predominantly parallel to continental slopes (Klunder et al., 2012b; Aguilar-Islas et al., 2013), and 4 times lower than the scale length of DFe observed from central California coast to the Pacific Ocean at 1000 m depth (5000 km ; Johnson et al., 1997). The differences in length scales presumably reflect both an increased strength of the horizontal advection of DFe relative to sampled stations, as well as lateral inputs from Baffin Bay slope compared with the Canada and Nansen basins. The enhanced biological and abiotic sinks of DFe over the productive, shallow and extensive shelves and slopes in Baffin Bay (< 1000 m deep) may explain the reduction in the advective transport of DFe compared with the transect sampled in the Pacific Ocean (> 4000 m deep). Subsurface (40-300 m) waters in the Labrador Sea do not exhibit a DFe peak, but the concentrations increase rapidly with depth and their vertical distributions are primarily controlled by remineralization rather than by continental advective fluxes, as suggested by the sharp increase in nutrient concentrations (Figure 4.6) and the very strong positive correlation between DFe and Apparent Oxygen Utilization (AOU; R2= 0.92, p-value< 0.001, n=8) in these waters. We expand upon the relationship of DFe and AOU and the importance of remineralization in controlling DFe distributions in section 4.6.3.1. Dissolved Mn concentration in subsurface Baffin Bay waters (40-300 m) sharply decreases with depth till 300 m (ÏÎ¸= 27.39; Figure 4.6), and its distribution appears to be largely controlled by particle scavenging and oxidative removal of Mn+2 below the surface waters (Landing and Bruland, 1980; Sunda and Huntsman, 1994; Statham et al., 1998). The AW peak in DMn concentrations is believed to be obscured in Baffin Bay due to the considerable high 124 surface and subsurface concentrations measured in this basin. However, the DFe/DMn ratio of AW (0.610±0.058) is clearly distinguished from overlaying (0.281±0.027) and underlying (1.06±0.18) waters, and is similar to the ratio found in CAA3-4 (0.606±0.076) in the same density range. The subsurface DMn concentrations in Baffin Bay are higher than those measured in the Labrador Sea (1.24±0.78 vs. 0.735±0.148 nmol kgË1; Figures 4.3b and 4.6). This could be explained by the differences in surface DMn concentrations between Baffin Bay (influenced by freshwater inputs) and the Labrador Sea (3.26±0.38 vs. 0.795±0.263 nmol kgË1), and the AW outflow in Baffin Bay advecting high DMn from the CAA. 4.6.3 The contrasting biogeochemistry of DFe and DMn in deep waters 4.6.3.1 Dynamic interplay of DFe inputs (advection and remineralization) versus scavenging removal in deep waters Dissolved Fe concentrations below 300 m in the Canada Basin are the lowest among the deep stations sampled in this study, and their distributions, contrary to what is observed in Baffin Bay and the Labrador Sea, are not easily explained by remineralization processes. We estimate the influence of remineralization of sinking organic particles on the deep water DFe distributions, using the relationship between apparent oxygen utilization (AOU) and DFe concentrations. The AOU is calculated as the difference between measured dissolved oxygen concentration and its saturation concentration, as result of organic matter respiration; DFe:AOU correlation analyses have been used extensively in the literature, providing insights about inputs of DFe from remineralization of organic matter (Bergquist and Boyle, 2006; Wu et al., 2011; Rijkenberg et al., 2014; Fitzsimmons et al., 2015; Hatta et al., 2015; Pham and Ito, 2018). In the Canada Basin, no significant correlations are found between DFe and oxygen (R2= 0.10, 125 p-value= 0.11, n= 26) or DFe and AOU (R2= 0.09, p-value= 0.13, n= 26) in the deep water column, which suggest that remineralization is not a significant source of DFe. This is not surprising as the Canada Basin has one of the lowest primary productivity levels in the Arctic Ocean, and extremely low vertical fluxes of particulate organic carbon (Hill et al., 2013; Varela et al., 2013; Brown et al., 2014; Crawford et al., 2015; Li, 2017). The vertical distributions of DFe in the AL (~300-1000 m) and the CBDW (>1200 m) are a result of the advection of Atlantic waters travelling in the Arctic Circumpolar Boundary Current (ACBC) and the subsequent scavenging within the CBDW. This was also observed in the Eurasian Basin where the Fe signature in Atlantic-derived waters is reduced due to scavenging along their cyclonic circulation from eastern Nansen Basin to the Makarov Basin (Klunder et al., 2012b). Underneath the wBSW, DFe distributions follow two different trends, one at stations CB2 and CB4 where the concentrations in the AL were slightly lower (0.405-419 nmol kgË1) in the warm FSB core (~500 m and ÏÎ¸= 27.92) than in the deeper, colder and fresher BSB (~600-1000 m and ÏÎ¸= 28.00; 0.467±0.012 nmol kgË1), with concentrations decreasing with depth in the CBDW (0.409 to 0.238 nmol kgË1). The distribution of DFe at CB3 appears different from the other two stations, having relatively uniform and significantly lower concentrations from 300 to 3000 m (0.293±0.017 nmol kgË1; Figure 4.6. Dissolved Fe in the AL (FSB + BSB) in the Canada Basin is lower than that measured in the eastern and western Nansen Basin (0.609±0.106 and 0.548±0.071 nmol kgË1) and comparable to the Amundsen (0.420±0.044 nmol kgË1) and the Makarov (0.362±0.066 nmol kgË1) basin concentrations (Klunder et al., 2012b). This difference in the DFe distributions, as well as in particulate Fe and dissolved 230Th (Li, 2017; Grenier et al., 2018), between CB2 and CB4 relative to CB3 is best explained by varying influence of the 126 ACBC. Due to their locations, both CB2 and CB4 are more directly impacted by the cyclonic circulation of the FSB and the BSB (ACBC components) around the basin compared to CB3 (Figure 4.1). The ACBC travels along the Siberian, Alaskan and Canadian continental slopes, where DFe is mobilized from the continental margin (sediment resuspension, reductive dissolution, etc.), entraining high concentrations to the Canada Basin interior (Nakayama et al., 2011; Klunder et al., 2012b; Kondo et al., 2016). However, as was observed for the wBSW, this high DFe signature is rapidly attenuated before reaching the more central and offshore CB3 station. The higher DFe measured in the BSB compared to FSB waters, reflects the circulation pathways of these water masses. The BSB flows around the rim of the Arctic Ocean in close proximity to the Fe-rich continental margin while the FSB flow diverts toward the deep basin earlier as it enters the Arctic Ocean (Aksenov et al., 2011). In the old and isolated (~200-500 years; Tanhua et al., 2009) CBDW, the losses of DFe by scavenging dominate over remineralization and/or advective inputs, which is indicated by the continuous decrease of DFe with depth at CB4 (Figure 4.6). In CBDW a strong linear relationship between DFe concentration and water mass age (Tanhua et al., 2009; Figure 4.9b) is found at CB4 (DFe= [(-0.0005 Age) / years + 0.473] nmol kg-1; R2= 0.96 and p-value< 0.001; Figure C.5), where DFe is lost at an approximate rate of 0.5 pmol kg-1 year-1. However, this relationship is not present at CB3, where uniformly low concentrations were measured from 1000 to 3500 m (0.303±0.024 nmol kgË1), presumably due to isolation of this station from Fe enriched waters advected in the ACBC. In Baffin Bay and the Labrador Sea, remineralization of sinking particles appears to have a predominant role in shaping the subsurface and deep distribution of DFe in the TrW, WGIW 127 and LSW, while scavenging dominates in the deeper BBDW, NEADW and DSOW. In Baffin Bay from 300 to 1000 m, DFe concentrations show a very strong positive linear correlation with AOU, with distinct trends for BB1 and BB3 stations (DFe= [(0.0054 AOU) / µmol L-1 + 0.463] nmol kg-1; R2= 0.84 and p-value< 0.001; Figure 4.9a) and BB2 (DFe= [(0.0030 AOU) / µmol L-1 + 0.498] nmol kg-1; R2= 0.82 and p-value< 0.05; Figure 4.9a). The deepest sample at BB3 is excluded from this analysis as it shows a clear sedimentary input evidenced by notably high DFe and DMn (Figures 4.3 and 4.6), and high particulate Fe and Mn (approximately one order of magnitude higher values than overlaying waters; Li, 2017). The difference in the DFe remineralization rates (slope of DFe:AOU plot) and the DFe concentrations between BB1 and BB3 versus BB2 is associated with enhanced scavenging at BB2. This station is located in the center of Baffin Bay, comparatively isolated from the cyclonic circulation of WGC and BC, and from ~300 to 800 m more than 90% of its waters have been circulating in the interior of the bay for at least 4 years, contrasting with the lower proportion of these old waters present at BB1 and BB3 (<50%; Colombo et al., 2019b). In the Labrador Sea, DFe and AOU also show a very strong correlation (DFe= [(0.0055 AOU) / µmol L-1 + 0.504] nmol kg-1; R2= 0.81 and p-value< 0.001) from approximately 300 to 2400 m, and the rate of remineralization and pre-formed DFe (slope and intercept of the DFe:AOU plot) are similar to those found at BB1and BB3 (Figure 4.9a). Despite the similar DFe:AOU trends, the concentrations are considerably higher in Baffin Bay than in the Labrador Sea (Figures 4.3a and 4.9a). As pre-formed DFe concentrations (other sources of Fe besides remineralization) are virtually the same for both basins (~0.48 nmol kgË1), the higher DFe (0.942±0.151 nmol kgË1), as well as the higher nutrient concentrations, measured in Baffin Bay are believed to be the result of increased remineralization (>AOU) of sinking organic particles 128 from the highly productive surface waters fueled by Pacific-derived nutrients (Varela et al., 2013; Lehmann et al., 2019). Figure 4.9 a) AOU and DFe relationship in deep waters (>300 m) in Baffin Bay and the Labrador Sea. Linear regression lines and R2 values are displayed in the figure, as well as the 95% prediction limits and the 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500). The density of overlapping lines indicates increased confidence. b) Estimated mean ages for Canada Basin waters from chlorofluorocarbon data (Tanhua et al., 2009); age profiles from the stations 30015 (Lat./Lon.: 75.3/-144.2; JOIS97 cruise) and 10008 (Lat./Lon.: 75.5/-150.2; Beringia cruise) were chosen due to their close proximity to our sampled stations. c) DMn versus water mass age, including data from Statham et al. (1998) and from Canada Basin Deep Waters (this study). Pseudo-first order scavenging model solutions, using different scavenging constants, are superimposed in the plots (for details refer to text); the best fit described by Statham et al. (1998) and chosen in this study are displayed with dashed and solid lines, respectively. Green shaded bands indicate background DMn concentrations (0.10-0.15 nmol kg-1) measured in deep ocean waters (for details refer to text). The DFe concentrations in Labrador Sea Waters (~300 to 2000 m; LSW-2015 and LSW-87/94) agree with the values reported for the Irminger Basin (Achterberg et al., 2018) and the North Atlantic Subarctic Gyre (NASG) in the GEOTRACES GA02 transect (Rijkenberg et al., 129 2014; Schlitzer et al., 2018), with the highest concentrations (0.818±0.009 nmol kgË1) observed at approximately 2000 m corresponding to the interface between LSW-87/94 and NEADW. These first measurements in the Labrador Sea support the hypothesis that the LSW is enriched in DFe by continental sediment resuspension during its southward flow along the North American continental slope in the deep western boundary current (Fitzsimmons et al., 2015; Hatta et al., 2015). The concentrations found in LSW at the offshore stations from Massachusetts to the Bermuda Atlantic Time Series station (line W) are higher than those measured in this study (1.1±0.2 nM vs. 0.716±0.056 nmol kgË1). Nevertheless, the extent of the DFe enrichment due to continental shelf interactions is smaller than previously inferred by Hatta et al. (2015) using estimates of preformed and regenerated DFe (35% vs. 55%). Lastly, we estimate the Fe:C ratios of sinking organic matter in Baffin Bay (8.6 μmol mol-1) and the Labrador Sea (8.8 μmol mol-1) converting AOU to remineralized organic carbon using a AOU:C ratio of 1.6 (Martin et al., 1987). Our estimated Fe:C ratios are similar to those reported for the Iceland Basin (12.1 μmol mol-1; Achterberg et al., 2018) and for the subtropical North Atlantic Ocean (7.1-12.8 μmol mol-1; Sunda, 1997; Bergquist and Boyle, 2006; Fitzsimmons et al., 2013), and slightly higher than estimates for Line W (5.69 μmol mol-1; Fitzsimmons et al., 2015). However, the inferred Fe:C ratios in Baffin Bay and the Labrador Sea are higher than the ratios reported in the North Pacific (2.5-4.4 μmol mol-1) and Southern Ocean (1.5-2.1 μmol mol-1; Sunda, 1997), and lower than the Fe:C ratio from the Irminger Basin (20.85 μmol mol-1; Achterberg et al., 2018). In the deepest waters of Baffin Bay and the Labrador Sea, the DFe:AOU ratios deviate from the trends previously described (Figure 4.9a). At BB2 station the concentrations suddenly 130 decrease across BBDW (>1000 m) from 0.916±0.043 to 0.647±0.094 nmol kgË1 (Figures 4.6 and 4.9a), and a large fraction of the remineralized DFe appears to be lost due to scavenging in this old and isolated water mass (Tang et al., 2004). In BBDW scavenging of DFe dominates over remineralization inputs, contrasting with nutrient distributions which increase steadily with depth reaching the highest concentrations measured in this study (Lehmann et al., 2019; Figure 4.6). Both DFe and NO3- are released to the water column during the remineralization of organic particles, but only DFe undergoes scavenging; therefore, the DFe/NO3- ratio provides information about scavenging regimens (lower valuesâstronger scavenging). DFe/NO3- ratios remained fairly constant (0.0495±0.0012) across the TrW and WGIW (remineralization > scavenging from 300-1000 m), decreasing notably with depth from the upper boundary of BBDW (0.0449) towards its base (0.0266; remineralization < scavenging). In the same way, DFe:AOU ratios in the Labrador Sea below ~2400 and ~2200 m at LS2 and K1, respectively, do not follow the trend described for LSW (Figure 4.9a), and the concentrations decreased to 0.677±0.023 nmol kgË1 in bottom DSOW (Figures 4.3a and 4.6). The DFe/NO3- ratios in the DSOW (> 2400 m) are lower (0.0447±0.0012) than in overlying waters (0.0507±0.0010), suggesting that scavenging of DFe is likely taking place, possibly as a consequence of the resuspension of sediments in this bottom water. The energetic flow of DSOW over bottom topography in the NASG is known to cause intense sediment resuspension, which results in elevated concentrations of dissolved Al in this water mass (Middag et al., 2015). Even though transmissivity data is not available for the deep casts (>1500 m) in the Labrador Sea, it is expected that the DSOW flow generates similar intense sediment resuspension events in this basin as suggested by the sharp decrease of particle reactive elements such as dissolved Pb and 131 230Th at LS2 and K1 stations below 2500 m (Grenier et al., 2018; Colombo et al., 2019b). The comparably low DFe measured in bottom DSOW in the Irminger Basin (0.630-0.740) and in the NASG (0.500-0.590) associated with a thick nepheloid layer would reinforce the importance of enhanced bottom scavenging of DFe in DSOW due to sediment resuspension events. However, this behaviour contrasts with the sharp increase of DFe in near-bottom waters described in this study for the Canadian Arctic Archipelago and the Baffin Bay Slope, and with results from previous studies highlighting the importance sediment resuspension events as a source of DF in shelf environments (Lam and Bishop, 2008; Cullen et al., 2009; Klunder et al., 2012a; Hatta et al., 2015; Milne et al., 2017; Cheize et al., 2019; Vieira et al., 2019). 4.6.3.2 DMn distributions in deep ocean waters dominated by scavenging In the Canada Basin below 300 m, DMn distribution are controlled primarily by the influence of the ACBC from 300 to 1000 m in the AL and scavenging removal in the CBDW. The concentrations measured in the AL (0.373±0.068 nmol kgË1) in the Canada Basin are considerable higher than Atlantic-derived waters (0.120-0.240 nM) in the Eurasian Basin (Middag et al., 2011b) as result of the addition of Mn to the ACBC waters as they flow along the shallow shelves of the Siberian and Chukchi seas (Kondo et al., 2016). The FSB and BSB, the two main components of the AL, display a clear difference in their concentrations, with the warmer FSB (~500 m and ÏÎ¸= 27.92) having lower values (0.327±0.012 nmol kgË1) than the BSB (0.396±0.074 nmol kgË1; ~600-1000 m and ÏÎ¸= 28.00). This is similar to the difference previously described for DFe (section 4.6.3.1). Nevertheless, the noted difference in DFe between stations CB2 and CB4 versus CB3 is not observed for DMn (Figure 4.6), which, as was 132 noted for the wBSW (section 4.6.2.1), DMn concentrations do not change significantly across the basin. In CBDW (>1200 m), DMn concentrations diminish with increasing depth, reaching uniformly low values 0.151±0.004 nmol kgË1 in bottom waters (>3000 m), the lowest concentrations measured in this study. The Canada Basin provides an excellent opportunity to study the scavenging dynamic of DMn, as well as DFe, especially within the CBDW. This old water is isolated from the ACBC by the Alpha-Mendeleev ridge complex, limiting advective trace metal inputs. In addition, the CBDW has not been recently ventilated, with an estimated water mass age of 500 years at ~4000 m (Macdonald et al., 1993; Timmermans et al., 2003; Tanhua et al., 2009; Figure 4.9b). As the potential sources of trace metals to deep waters (advective sources, hydrothermal inputs and benthic fluxes) are non-existent or greatly reduced in CBDW, we can effectively isolate and estimate the effects of scavenging on trace metal distributions. We plot DMn concentrations (below 1200 m) against estimated water mass ages from Tanhua et al. (2009; Figure 4.9b and 4.9c). A pseudo-first order scavenging model (âðð·ðð/ðð¡ = ðð·ðð) is applied to fit our data: ð·ððð¡ = ð·ðð0 ðâðð¡ + ð·ðð ð (1), where ð·ððð¡, ð·ðð0 ððð ð·ðð ð are the concentrations at time t, time 0 and steady-state, and k is the first-order scavenging constant (Yeats and Bewers, 1985; Statham et al., 1998; Richard et al., 2013). The calculated solutions of the scavenging model, using three different k values (0.015, 0.01, 0.008 years-1; Weiss, 1977; Statham et al., 1998) and an assumed DMn âequilibriumâ of 0.14 nmol kgË1, show an excellent agreement with the observed data in CBDW (Figure 4.9c). Various k values are chosen to fit the observed CBDW DMn data to account for different scavenging scenarios; high scavenging removal rates (e.g. 0.019 and 0.015 years-1) were used by 133 Weiss (1977) and Statham et al. (1998) to model DMn losses in the Galapagos Rift and in deep waters (~3000 m) from the Atlantic into the Indian and Pacific oceans, respectively. In these studies, most of the samples come from highly productive regions (e.g. Peruvian upwelling system, Greenland Sea, North Subarctic Atlantic) with high export production rates (Falkowski et al., 1998), rather than the extremely low productivity and export production rates typical of the Canada Basin (Hill et al., 2013; Varela et al., 2013; Brown et al., 2014; Crawford et al., 2015). Slightly lower scavenging removal rates (k= 0.010 and 0.009 years-1) were also used in the equation (1) to fit our Canada Basin data (Figure 4.9c). The best fit for the DMn data in CBDW is obtained with k= 0.010 years-1, with a curvilinear relationship which describes the observed results with less than 3% of error and flattens out at about 400 years at CB3 and CB4 (0.147 and 0.150 nmol kgË1; Figure 4.9c). Scavenging dynamics of DMn in the CBDW are similar to those described by Statham et al. (1998; Figure 4.9c); adding support to the hypothesis that DMn distributions in deep ocean waters approach a steady state concentration after approximately 400 years (if no external sources of Mn exist) reaching extremely low âbackgroundâ concentrations (0.10-0.15 nM) in deep ocean waters (Statham et al., 1998; Middag et al., 2011b; Middag et al., 2011a; Hulten et al., 2017). Finally, the slightly lower scavenging rate constant used in this study (k= 0.010 years-1), provides a better fit for Stathamâs model for the stations located in the Sargasso Sea and Southern Ocean (low productivity and export production regions) which were overestimated using a higher k value (0.015 years-1; Figure 4.9c). Dissolved Mn concentrations below 300 m in Baffin Bay and the Labrador Sea, contrary to DFe, do not show a significant correlation with AOU. This could be explained by the greater elemental stoichiometry of Fe relative to Mn in marine phytoplankton (2:1) and diminished Fe 134 scavenging owing to complexation by organic ligands (Ho et al., 2003; Gerringa et al., 2015). In the highly productive Baffin Bay waters, Mn is indeed released to the dissolved phase during the remineralization of sinking particles, but rapid scavenging of DMn potentially obscures any correlation with AOU in WGIW (~300-1000 m and ÏÎ¸= 27.35-27.63). The only station where DMn concentrations show a significant positive relationship with AOU is BB1 (DMn= [(0.0024 AOU) / µmol L-1 + 0.368] nmol kg-1; R2= 0.83 and p-value< 0.05). Interestingly the Mn:C ratio of sinking organic matter at BB1 (3.8 μmol mol-1) derived from the DMn:AOU relationship and the AOU:C ratio agrees with the Mn:C ratio (4.1 μmol mol-1) estimated using the Fe:C in Baffin Bay (8.3 μmol mol-1) and the 2:1 Fe/Mn stoichiometry. The nearly uniform distributions of DMn in WGIW (Figure 4.6) are associated with a dynamic equilibrium between remineralization inputs and scavenging removal. The lower concentrations measured at BB2 station (0.416±0.005 nmol kgË1) relative to the stations BB1 and BB3 (0.600±0.036 and 0.521±0.011nmol kgË1) are related with the greater proportion of old Baffin Bay water in the former station, as described in the section 4.6.3.1. The sudden increase of DMn at near-bottom waters at stations BB1 and BB2 (0.768±0.057 and 0.736±0.048 nmol kgË1) and the extremely high concentration measured at BB3 (4.20±0.008 nmol kgË1) is also accompanied by the spike of particulate Fe and Mn concentrations and a decrease of the particle reactive radionuclide 230Th (Li, 2017; Grenier et al., 2018), which strongly suggest that particle resuspension at the sediment-water boundary is driving the DMn increase. The DMn concentrations in BBDW (~700-2300 m and ÏÎ¸ >27.63) are low and steadily decrease with increasing depth from 0.314±0.103 to 0.244±0.032 nmol kgË1 at ~1900 m, distinguishing them from overlying WGIW concentrations (0.416±0.005 nmol kgË1; Figure 4.6). 135 Like DFe (section 4.6.3.1), the decrease of DMn in BBDW is attributed to scavenging removal in these old and isolated bottom waters. The origin of BBDW has been under discussion for many decades, and the ventilation age of these deep waters (77-1450 years) is still uncertain (Tang et al., 2004; Lehmann et al., 2019). Insights from the scavenging dynamic of DMn in the Canada Basin could help to better constrain the large uncertainties in the BBDW age. Dissolved Mn in deep waters reaches a background concentration of 0.10 to 0.15 nmol kgË1 after approximately 400 years, provided no external sources are present (Figure 4.9c). In Baffin Bay, external sources of Mn to BBDW are assumed to be negligible since this bay is isolated from Arctic and Atlantic outflows by shallow (< 150 m) and narrow passages in the Canadian Arctic Archipelago and by the Davis Strait (< 650 m), and the benthic fluxes of DMn are restricted to near-bottom waters (Figure 4.6). Additionally, the entrainment of high DMn from shallower waters is also unlikely considering that convective mixing is restricted to the upper 400 m (Colombo et al., 2019b), and below this depth the WGIW and BBDW temperature profiles can be clearly distinguished from each other (Figure 6). The lowest concentrations measured in BBDW (0.242±0.002 nmol kgË1) are far from the background level of Mn, which indicate that these waters have not reached the equilibrium concentration, and therefore they are younger than ~400 years. Considering that the concentrations in the upper boundary of BBDW (0.314±0.103 nmol kgË1) are comparable to those of CBDW (0.269-0.336 nmol kgË1), we can roughly estimate the age which will reproduce the DMn measured in BBDW using the equation (1). Based on the scavenging model previously described, using k values from 0.008 to 0.015 years-1 and assuming that external sources of Mn are negligible, an estimate of age 120-190 years would reproduce the 0.242±0.002 nmol kgË1 measured in BBDW, a similar range is obtained applying the Statham et al. (1998) model. These estimates are approximately one order of magnitude lower than the 136 upper limit range of estimated residence time (1450 years), and comparable to the lower range (77-455 years) estimated using a time dependent 3He/ Tritium two-box model (Top et al., 1980). Finally, the distributions of DMn in Labrador Sea deep waters (> 300m), are characterized by a steady decrease in concentration with increasing depth across multiple water masses (LSW-2015, ISW, LSW-87/94 and NEADW) until about 2500 m (ÏÎ¸= 27.86) where the lowest concentrations were measured (0.306-0.330 nmol kgË1; Figure 4.6). Differences in DMn concentrations across LSW-2015 waters (~400-1000 m and ÏÎ¸= 27.71-27.74) are noticed between LS2 and K1 stations. In these stations DMn mirrors the temperature and oxygen profiles (Figure 4.6), with higher DMn concentrations measured in the cooler, fresher and more oxygenated LSW-2015 waters at K1 than at LS2. Below the ISW, DMn distributions at stations LS2 and K1 follow similar trends across LSW-87/94, NEADW and DSOW waters, and their concentrations are a result of the combined effects of scavenging removal and the initial concentrations of these water masses at the time they were last ventilated. The old LSW-87/94 waters have slightly higher concentrations (0.367±0.023 nmol kgË1) than the NEADW (0.338±0.014 nmol kgË1), despite the fact that the LSW-87/94 was ventilated approximately 30 years ago (Azetsu-Scott et al., 2005; Yashayaev and Clarke, 2008) compared to the more recently ventilated NEADW (~11-13 years; Azetsu-Scott et al., 2005). This apparent inconsistency is related to differences in surface DMn concentration where these water masses were ventilated, with relatively higher concentration in the Labrador Sea (0.450-0.850 nmol kgË1) compared to the Irminger and Iceland basins (~0.20-0.50 nM; Achterberg et al., 2018). The modest increase of DMn (0.372±0.013 nmol kgË1) in bottom DSOW waters (>2500 m and ÏÎ¸ >27.86) at stations LS2 and K1 is potentially linked to sediment resuspension events generated 137 by the energetic flow of DSOW over bottom topography. It is important to highlight that DFe concentrations were uniformly low in this water mass (section 4.6.3.1); this difference between DMn and DFe in the DSOW has been previously observed in the Irminger Basin and the NASG (Rijkenberg et al., 2014; Hulten et al., 2017), reporting similar values of DMn (~0.35-0.40 nM) to those measured in this study. Over much of the sampled area in the Canadian Arctic Ocean, the noticeable rise of DFe concentrations in near-bottom waters are associated with intense sediment resuspension events (e.g. the Canadian Arctic Archipelago, the Baffin Bay slope region) and are accompanied by an increase of DMn. However, the contrasting behavior described for the DSOW, where the bottom waters were enriched in DMn, but not in DFe, is also found at the deep BB2 station. Consequently, further research, including iron isotopic measurements and size partitioning analysis (soluble, colloidal and dissolved fractions), is required to unravel the processes that result in the apparent decoupling of DMn and DFe in DSOW. 4.6.4 Conclusions We present distributions of dissolved Fe and Mn in the Canadian Arctic Ocean from the Canada Basin to the Labrador Sea. High resolution profiles were sampled during the Canadian GEOTRACES Arctic cruise in 2015, with seawater samples being analyzed independently with different methodologies by two universities. Fresh water sources, mostly riverine inputs, control the surface distributions of these metals in the Canadian Arctic Ocean; especially high concentrations were found in the Canadian Arctic Archipelago (CAA), a coastal region receiving a large flux of freshwater from numerous river systems. Surface waters in the Labrador Sea stand out with their extremely low DFe and DMn concentrations (0.106-0.362 and 138 0.450-1.09 nmol kg-1), that likely result from reduced freshwater inputs and increased phytoplankton uptake in the region. Relatively high DFe and DMn are present in subsurface waters (~100-300 m) across the Canada Basin, the CAA and Baffin Bay; these distinctive signatures are advected from the Chukchi Sea and the CAA, where increased concentrations reflect benthic fluxes and continental shelf-seawater interactions in these shallow environments. The distributions of DFe and DMn in deep waters are regulated by complex interactions between removal processes (e.g. scavenging, oxidative precipitation), inputs of these trace metals (e.g. remineralization, advection, adsorption/desorption equilibrium dynamics between particulate and dissolved phase) and the presence of organic-ligands enhancing their solubility. As a general trend, DFe distributions in Baffin Bay and the Labrador Sea below 300 m are regulated by an interplay between remineralization inputs and scavenging removal in the deepest waters, while in the Canada Basin, an extremely low productivity region, the remineralization of sinking organic particles does not appear to be a significant source of trace metals to deep waters. The distributions of DFe and DMn in the deep Canada Basin waters are greatly influenced by the advection of these metals in the Arctic Circumpolar Boundary Current, and the subsequent scavenging in deeper waters. The vertical distribution of DMn in the deep basins is predominantly controlled by scavenging removal, and a pseudo-first order scavenging model explains the uniformly low concentrations (0.151±0.004 nmol kg-1) measured in the deep and old (>3000 m and ~400 years) Canada Basin Deep Waters. The consistent scavenging behavior of DMn observed in deep world oceans and in the Canada Basin allows to better constrain the ages of Baffin Bay Deep Waters, which have been under debate since 1980âs. 139 Chapter 5: Thesis Conclusion Results presented in this thesis provide important information about the concentration, distribution and cycling of various trace metals in the Canadian Arctic Ocean and small rivers draining the continuous permafrost Canadian Arctic Archipelago (CAA). Additionally, this study establishes a baseline for dissolved lead, iron, manganese and gallium in the Canadian Arctic Ocean and numerous other dissolved and particulate trace metals as well as lead isotopes for remote CAA rivers. This first data set will help to elucidate how the trace metal biogeochemistry in the Canadian Arctic Ocean may be affected by climate change related disturbances and their potential effects in the physical and biological Arctic systems, as well as future anthropogenic impacts in this vulnerable environment. 5.1 Trace metal geochemistry of remote rivers in the Canadian Arctic Archipelago The combined outflow of all rivers in the archipelago is estimated to be comparable to some major Arctic rivers (e.g. Kolyma, Yukon, Mackenzie rivers), and therefore, small rivers in the archipelago have the potential to play an important role in the cycling of trace elements in the Arctic Ocean, as described throughout this thesis. Our findings reveal that the dissolved trace metals concentrations display a contrasting spatial distribution, where most of the rivers located in the northern region of the archipelago share a similar trace metal signature (extremely low concentrations) differing from continental rivers in the south (higher concentrations). These distributions are highly influenced by the bedrock geology of the drainage basins and the North-South gradient of important environmental conditions (e.g., solar radiation, precipitation, temperature, DOC distribution; Figure 5.1). Given this distinctive spatial distribution, we estimated ranges of concentrations of trace metals in other remote rivers based on their 140 geographical location and during the high flow season. Additionally, a high enrichment of particulate Ni, Cu, Zn, and Pb relative to Al is observed in CAA rivers located in close proximity to active mining exploration projects. However, as the enrichment of these metals is not accompanied by a noticeable shift in the Pb isotopic signatures, we suggest that the higher values of these metals relative to average upper continental crust ratios may be explained due to the local enrichments of those rivers close to mineral deposits. The lead isotopic signature of the suspended particulate matter from these rivers spans an extremely large range, from very non-radiogenic values of 206 Pb/207Pb (1.0153) to highly radiogenic ratios of 206Pb/207Pb (1.5121). To our knowledge such a large range in lead isotopic composition of samples coming from a same region has not been previously reported. Figure 5.1 Conceptual diagram displaying how differences in the bedrock geology and North-South environmental gradients influence trace metal distribution in small CAA rivers. A simplified bedrock geology map is superimposed in the Figure (for reference refer to Figure 2.1). The photos displayed in the Figure were taken during field sampling, and evidence the differences in environmental settings between northern and southern CAA rivers. 141 5.2 Dissolved lead, iron and manganese in the Canadian Arctic Ocean The trace metals studied in this thesis have distinctive aqueous biogeochemistry; on one hand Pb is a highly reactive element which has strong affinity with sinking particles, relatively short residence times, no significant biological drawdown, and strong anthropogenic influence. On the other hand, Fe and Mn are important micronutrients, closely related with the biogeochemical cycling of organic matter having fast assimilation by phytoplankton in the euphotic zone and remineralization of sinking biogenic detritus in the deep ocean; they also exhibit strong redox chemistry and ligand complexation. The contrasting behavior of these metals allows to better characterize and understand the processes shaping the trace metals distributions in the Canadian Arctic ocean. In the low productivity Canada Basin, the distributions of dissolved Pb, Fe and Mn are mainly controlled by advective inputs, while organic matter regeneration does not appear to have a significant role. In the ACW halocline water, the advection of anthropogenically high Pb waters from the Subarctic Pacific Ocean contrasts with the prevailing low concentrations, attributed to reduced atmospheric inputs to surface waters and enhanced scavenging removal in wBSW. Conversely, there is a rapid decrease of DFe and DMn concentrations with depth in halocline waters from surface waters (freshwater inputs) to approximately 500 m; however, the low background found in halocline waters is interrupted by the advection of wBSW from the Chukchi Sea transporting a high Fe and Mn signature (benthic and regenerated sources) from these shallow and highly productive shelves (Figures 5.2 and 5.3). Below the halocline waters, slightly higher DPb is found in the Atlantic Layer, as result of the advection of anthropogenically elevated waters traveling in the ACBC. Similarly, dissolved Fe and Mn show comparatively 142 higher concentrations in the Atlantic Layer, attributed to the remobilization and entrainment of these metals during the ACBC cyclonic circulation along the Siberian, Alaskan and Canadian continental slopes, an especially important phenomenon for the BSB (Figures 5.2 and 5.3). Scavenging removal dominates the distribution of Pb, Fe and Mn in the deep and old CBDW. The concentrations of Pb in the Canadian Arctic Archipelago (CAA) are predominantly low, aside from the moderately high glacial inputs in the northern Parry Channel region; this low Pb signature is exported from the CAA to Baffin Bay in the cold AW plume (Figure 5.2). Riverine and glacial inputs explain the very high DFe and DMn concentrations measured in CAA surface waters. A noteworthy feature is observed in the archipelago, where uniformly low concentrations in the western CAA contrast with high DFe and DMn observed in the eastern CAA due to increased diapycnal mixing and sediment resuspension (Figure 5.3). As was described for DPb, the AW outflow can be traced in Baffin Bay as a cold and high DFe subsurface peak. 143 Figure 5.2 Conceptual scheme displaying the concentrations and key processes controlling the distributions of dissolved Pb in the Canadian Arctic Ocean. CB: Canada Basin, CAA: Canadian Artic Archipelago, BB: Baffin Bay, LS: Labrador Sea, ACW: Alaska Coastal Water, wBSW: winter Bering Sea Water, ACBC: Arctic Circumpolar Boundary Current, AW: Arctic Water, WGIW: West Greenland Intermediate Water, DSOW: Denmark Strait Overflow Water. 144 Figure 5.3 Conceptual scheme displaying the concentrations and key processes controlling the distributions of dissolved Fe and Mn in the Canadian Arctic Ocean. CB: Canada Basin, CAA: Canadian Artic Archipelago, BB: Baffin Bay, LS: Labrador Sea, wBSW: winter Bering Sea Water, ACBC: Arctic Circumpolar Boundary Current, AW: Arctic Water, DSOW: Denmark Strait Overflow Water. 145 The advection of Atlantic-derived waters (WGC and WGIW) circulate cyclonically in Baffin Bay advecting their high Pb signature to nearby waters in this bay; this is especially evident for the WGIW, which is distinguished by its higher temperature and Pb concentrations. The concentrations of DPb drop in the deep and old Baffin Bay waters and in the more recently ventilated DSOW in the Labrador Sea waters, as a consequence of increased scavenging within the nepheloid layer created by the energetic flow of DSOW over the rough topography (Figure 5.2). While the advection of Atlantic-derived waters has a profound influence in the cycling of DPb in Baffin Bay and the Labrador Sea, it is not as important in shaping the distributions of DFe and DMn in these basins. The progressive loss of these micronutrients in surface waters from Baffin Bay to the Labrador Sea illustrates the combined effects of reduced freshwater inputs and intensified phytoplankton uptake of DFe and DMn in these highly productive waters (Figure 5.3). Underneath the euphotic zone, Fe is actively released to the dissolved phase, due to remineralization of sinking organic particles, steadily increasing its concentration with depth in Baffin Bay and the Labrador Sea. The vertical distributions of DMn in these basins do not follow the trends described for DFe, and their concentrations tend to decrease with depth. Although DMn is indeed released to the dissolved phase during the remineralization of organic matter, this metal is likely to be rapidly scavenged, and therefore removed from the dissolved phase (Figure 5.2). Dissolved Fe and Mn experience a sudden increase in their concentrations in near bottom waters along the Baffin Island slope. However, they display a contrasting behavior in near bottom waters in the Labrador Sea as well as at the deepest station in Baffin Bay, with decreasing DFe concentrations contrasting with the rise of DMn. 146 5.3 Future work Results from this work provide a first characterization of the biogeochemistry of numerous trace metals in the Canadian Arctic Ocean and remote CAA rivers. However, further research is needed to fully unravel their cycling in this unique environment. For instance, due to sampling constraints, we are currently lacking seasonal trace metal data, which is essential to gain insights about how their distributions change over time, and to establish accurate budgets of trace metal fluxes from CAA rivers to the coastal marine environment. A comprehensive study of the trace metal behavior along the river-ocean mixing-gradients is also key to constrain trace metal fluxes from CAA rivers. In addition to seasonal data, expanding the sampling area towards northern Baffin Bay, Nares Strait, southern CAA (e.g. Amundsen Gulf, MâClintock Channel, Gulf of Boothia), etc., would be useful to assess new sources of trace metals to the Arctic Ocean. New research on trace metal biogeochemistry in the Canadian Arctic Ocean should also include isotopic measurements and size partitioning analysis (e.g. soluble, colloidal and dissolved fractions). 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(2017) Historic and Industrial Lead within the Northwest Pacific Ocean Evidenced by Lead Isotopes in Seawater. Environ. Sci. Technol. 51, 1203â1212. 173 Appendices Appendix A - supplementary material for chapter 2 Table A.1 Ancillary data from sampled CAA rivers, including location, salinity, suspended particulate matter (SPM), and dissolved organic carbon (DOC). Rivers Lat (°N) Lon (°E) Salinity a (PSS78) SPM (mg/L) DOC a (µM C) Akpat Kuunga (1) 73.7638 -80.4468 0.06 8.31 13.1 Glacier (2) 73.6124 -78.6805 0.02 261 23.3 Marcil Creek (4) 72.9971 -84.9864 0.03 3.18 96.6 Saaqu (5) 73.7936 -86.9829 0.10 N/A 16.0 Devon Isl. (6) 74.5110 -88.4910 0.07 N/A 38.7 Cunningham (7) 74.0153 -93.6242 0.11 0.85 53.5 Garnier (8) 73.9445 -92.0369 0.13 13.6 39.0 Mecham (9) 74.6926 -94.7783 0.14 0.14 55.2 Creswell (10) 72.8491 -93.4162 0.16 2.43 80.5 Le Feuvre Inlet (11) 72.3412 -96.9172 0.19 N/A 162 Pasley (12) 70.4696 -95.5796 0.11 0.03 170 Simpson (13) 67.6763 -100.5683 0.09 19.1 458 Ellice (14) 67.8930 -104.0758 0.04 4.82 467 Tree (15) 67.6242 -111.9161 0.05 N/A 156 (a) Salinity and DOC data are from Brown et al., (in review) Geochemistry of Small Canadian Arctic Rivers with Diverse Geological and Hydrological Settings their table S2. N/A: Not available. 174 Table A.2 Accuracy determined from the analyses of SLRS-5, SLRS-2, Sample Spike, and JSd-1 reference materials (AVG±1SD). SLRS-5 (ug/L) SLRS-2 (ug/L) Sample Spike (ug/L) JSd-1 (ug/g)a Measured (n=4) Certified Measured (n=3) Certified Measured (n=3) Spiked Measured (n=3) Certified Fe 95.6±1.1 91.2±5.8 139.8±2.6 129±7.0 - - 3.76±0.24 3.46±0.16 Mn 4.44±0.03 4.33±0.18 11.5±0.2 10.1±0.3 - - 0.0733±0.0056 0.0726±0.0018 Al 49.5±0.4 49.5±5.0 90.3±1.6 84.4±3.4 - - 7.45±0.55 7.71±0.15 Ba 14.7±0.03 14.0±0.5 14.2±0.06 13.8±0.3 - - 500.4±99.1 520±16.6 Cu 16.0±0.1 17.4±1.3 2.84±0.03 2.76±0.17 - - 22.1±2.5 22±2.7 Ni 0.396±0.014 0.476±0.064 1.09±0.030 1.03±0.10 - - 7.42±0.52 7.04±0.73 Zn 1.06±0.152 0.845±0.095 3.96±0.39 3.33±0.15 - - 106.9±5.47 96.5±9.64 Pb 0.088±0.002 0.081±0.006 0.116±0.003 0.129±0.011 - - 9.8±1.06 12.9±3.15 Ga - - - - 0.0078±5.3E-6 0.0077 13.1±0.4 17.2±1.9 (*) Cd 0.0069±0.0002 0.0060±0.0014 0.0325±0.0002 0.028±0.004 - - - - (a) for Fe, Mn and Al particulate concentrations are expressed as % (*) Preferable value Table A.3 NIST 981 standards were measured in two analytical runs at two different concentrations of Pb (run 1 and run 2) and compared to published values. Standards 1A and 2A (NIST 981) are treated as external process standards and went through the sample chemical purification methods as samples. Standards 1B and 2B (NIST 981) are treated as instrumental standards and were analyzed without column purification. Ratios This study, run 1a (5 ppb Pb) This study, run 2b (1 ppb Pb) Cheyne et al. (2018) c Galer (1998) d Std 1A Std 1B Std 2A Std 2B AVG (n=2) 2SD AVG (n=2) 2SD AVG (n=2) 2SD AVG (n=4) 2SD AVG (n=5) 2SD AVG 2SD 206/207 1.0932 0.0001 1.0932 0.0001 1.093 0.001 1.093 0.001 1.0932 1.00E-05 1.093 N/A 207/206 0.9147 0.0001 0.9148 0.0001 0.915 0.001 0.915 0.001 - - - - 208/206 2.1677 0.0003 2.1677 0.0003 2.168 0.004 2.168 0.004 2.1678 N/A 2.1676 N/A 206/204 16.94 0.02 16.95 0.02 17 0.1 17 0.1 16.94 0.002 16.941 0.002 207/204 15.49 0.01 15.5 0.01 15.5 0.1 15.5 0.1 15.496 0.002 15.496 0.002 208/204 36.72 0.04 36.74 0.04 36.8 0.3 36.8 0.3 36.722 0.005 36.722 0.004 175 Table A.4 BCR-2 as external process standard measured at two different concentrations compared to published values. Ratios Run 1a (5 ppb Pb) Run 2b (1 ppb Pb) Cheyne et al. (2018) c Weiss et al. (2006) AVG (n=2) 2SD AVG (n=2) 2SD AVG (n=6) 2SD AVG 2SD 206/207 1.2009 0.0001 1.199 0.001 1.2 0.001 1.2 0.001 207/206 0.8327 0.0001 0.834 0.001 0.833 N/A 0.833 N/A 208/206 2.0645 0.0003 2.067 0.004 2.065 N/A 2.065 N/A 206/204 18.76 0.02 18.8 0.1 18.76 0.02 18.75 0.02 207/204 15.62 0.01 15.7 0.1 15.627 0.003 15.625 0.004 208/204 38.73 0.04 38.9 0.3 38.74 0.03 38.72 0.04 (a) Standards in run 1 were analyzed together with samples 1, 2, 4, 8, 13, 14, and 15 in 5 ppb solutions. (b) Standards in run 2 were analyzed together with samples 5, 6, 10, and 11 in 1 ppb solutions. (c) Pb isotope measurements were done in the same laboratory where this study was conducted but used regular cones using both a Tl internal spike and NIST 981 bracketing for instrumental mass bias correction. (d) Pb isotopes were measured using triple spiking for instrumental mass bias correction 176 Table A.5 Ranges and averages of dissolved and particulate trace metal concentrations (dTM and pTM) and annual mean discharge (km3/yr) for Arctic rivers, the Danube delta and the Amazon basin. dTM concentration are expressed in nM for Al, Fe, Mn, Ba, Ni, Cu and Zn, and pM for Cd, Pb and Ga. pTM concentrations are expressed as µg/L. Rivers Annual mean discharge Al Fe Mn Ba Ni Cu Zn Cd Pb Ga Ref. dTM pTM dTM pTM dTM pTM dTM pTM dTM pTM dTM pTM dTM pTM dTM dTM pTM dTM This Study Northern CAA - 49-218 1.42-1638 0.592-48 1.66-865 0.067-7.18 0.099-23 16-303 0.011-9.81 0.099-2.05 0.091-4.44 0.657-11 0.07-3.89 0.005-3.44 0.173-4.51 3.59-12 0.601-6.07 0.010-0.975 11-92 - Continental CAA - 96-857 73-1142 11-54 16-307 5.59-10 9.09-13 1.18-7.19 11-23 12-72 18-41 - Other Arctic Rivers Imnavait Creek* - 1453-6509 - 926 - 353-3007 - 41-112 - 19-34 - - - 26-67 - - - - - 1 Roche Mountanee Creek* - 145-1538 - 20-1916 - 49-76 - 110-503 - - - - - - - - - - - 1 Kuparuk 1.3 - - 185-4170 - - - 131-349 - - - 9.8-13 - 0.5-6.3 - - 25-288 - - 2,15 Sagavanirktok 1.6 - - 38-1290 - - - 200-288 - - - 3.5-15 - 0.5-6.7 - - 19-225 - 2,15 Onega 16 - - - - - - - - - - 6.2 0.49 1.5 1.8 39 190 0.24 - 3,16 Mezen 20 - - - - - - - - - - 7.8 0.31 1.7 1.4 36 210 0.39 - 3,16 Colville 20 - - 68-1825 - - 340-540 - - - 11-42 - 0.49-5.0 - - 56-321 - - 2,15 Kolyma 101 - - 394-2972 11-766 55-67 4.7-16 10-32 5.0-17 10-151 91-538 5,16 Severnaya Dvina 104 1508-5448 - 4153-7271 - 151-1529 - 110-323 - 18-97 - 17-137 - 79-245 - 71-338 681-1607 - 244-502 4,16 Pechora 146 - - - - - - - - - - 6.9 0.19 1.5 1.1 48 240 0.21 - 3,16 Mackenzie 283 - - 28-232 - 11-44 - 296-417 - 13-24 - 23-103 - 20-158 - 88-197 51-829 - - 5,16 Ob 409 - - 429-654 - 23-303 - 90-249 - 14-47 0.41-3.1 5.9-49 0.22-0.42 1.2 1.2 0.89-48 54-230 0.06-1.3 - 3,6,7,16 Lena 534 - 35-2906 387-5883 39-1432 71-1471 0.7-1.6 81-208 - 3.9-13 0.02-1.4 13-39 0.04-0.97 1.3-65 0.11-4.2 20-110 70-2220 0.05-1.8 - 8,9,10,16 Yenisey 596 - - 251-317 - 7-184 - 39-119 - 8.8-9.4 - 9.8-29 0.22 1.8 0.38 10-47 24-350 0.06 - 3,6,16 177 Major World Rivers Danube delta 281 211-539 - 21-156 - 26-55 35-113 - - - 2.1-6.8 61-120 2.2-51 11-41 4.7-18 224-302 145-220 1.6-5.2 - 11 Amazon basin 5298 - - 180-1300 - 6.7-618 3.1-169 38-409 4.5-191 1.9-23 0.24-12 2-29 0.10-12 0.30-84 0.87-43 18-60 - - - 12,13,14 *Annual mean discharge was not available for these rivers; however, their watersheds are extremely small (Imnavait Creek: 2.2 km2 and Roche Mountanee Creek: 89 km2) being smaller than CAA rivers sampled in this study 1-Barker et al. (2014), 2-Rember and Trefry (2004), 3-Guay et al. (2010), 4-Pokrovsky et al. (2010), 5-Peterson et al. (2016), 6-Dai and Martin (1995), 7-Moran and Woods (1997), 8-Hölemann et al. (2005), 9-Guieu et al. (1996), 10-Martin et al. (1993), 11-Guieu and Martin (2002), 12-Shiller and Boyle (1987), 13-Boyle et al. (1982), 14-Seyler and Boaventura (2003), 15-McClelland et al. (2014), 16-Whitefield et al. (2015) Table A.6 Linear regression analysis results of dissolved trace metal concentrations and DOC versus flow for Kolyma, Mackenzie, Yenisey, Ob, and Lena rivers. When a significant regression is established, the difference in trace metal concentrations between low and high flow season is calculated and expressed as percentage of the low value (bolded numbers). α = Significance F, Dif. (%) = percentage difference between concentrations measured during low flow versus high flow. Cd Cu Fe Mn Ni Pb Zn DOC R2 α Dif. (%) R2 α Dif. (%) R2 α Dif. (%) R2 α Dif. (%) R2 α Dif. (%) R2 α Dif. (%) R2 α Dif. (%) R2 α Dif. (%) Kolyma 0.128 0.430 - 0.928 4E-04 138 0.927 0.002 186 0.847 0.003 1285 0.945 2E-04 156 0.886 0.005 111 0.224 0.283 - 0.952 2E-04 162 Mackenzie 0.023 0.621 - 0.141 0.229 - 0.606 0.005 239 0.014 0.701 - 0.014 0.702 - 0.001 0.928 - 0.006 0.793 - 0.482 0.012 19 Yenisey 0.012 0.725 - 0.937 6E-08 240 0.918 3E-07 800 0.944 1E-07 1508 0.880 6E-06 154 0.682 9E-06 108 0.010 0.739 - 0.884 6E-07 334 Ob 0.138 0.211 - 0.504 0.009 136 0.037 0.528 - 0.046 0.501 - 0.089 0.300 - 0.003 0.860 - 0.531 0.005 - 0.229 0.084 - Lena 0.082 0.343 - 0.512 0.013 42 0.213 0.096 - 0.016 0.670 - 0.309 0.048 45 0.314 0.037 138 0.973 2E-04 99 0.620 0.001 116 178 Figure A.1 Metal/Al ratios in suspended particulate matter collected from CAA rivers. Dashed lines indicate the average crustal metal/Al ratio (Rudnick and Gao, 2013) and bolded numbers indicate those rivers which are located in close proximity to active mining exploration projects and/or communities. The names of the sampled rivers are presented in Table 2.1. 179 Appendix B - supplementary material for chapter 3 Table B.1 Dissolved Pb concentrations (pmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th - October 1st 2015). Those samples suspected of contamination* are flagged (red bolded numbers) and not discussed in the manuscript Depth LS2 Depth BB1 Depth BB2 Depth BB3 Depth CAA1 Depth CAA2 Depth CAA3 Depth CAA4 Depth CAA5 10.8 16.9 10.5 13.6 9.5 6.9 9.9 4.7 9.6 10.8 10.7 4.5 8.7 7.3 8.9 6.1 11.4 10.4 100.8 30.1 20.9 17.9 28.7 6.7 29.7 5.8 29.5 12.8 29.3 4.4 18.4 8.8 33.2 6.0 20.7 11.5 150.2 30.5 46.4 15.0 49.2 5.4 49.6 8.1 43.5 12.7 42.5 4.9 28.6 9.7 47.0 6.2 45.1 7.6 297.2 24.2 99.1 9.1 98.9 6.9 99.2 3.6 59.3 13.3 59.7 5.7 39.7 9.0 47.0 6.4 45.1 7.3 300.3 24.3 148.7 8.3 149.0 11.2 147.2 4.0 79.7 12.6 79.1 7.8 59.8 5.5 59.4 7.2 70.2 6.1 399.7 23.1 199.4 6.1 198.7 13.9 198.5 4.9 99.6 12.7 99.4 13.7 80.0 5.1 79.3 8.1 78.4 9.6 497.5 27.5 199.4 6.1 198.7 13.6 198.5 5.2 99.6 14.2 99.4 13.5 80.0 5.1 94.6 3.9 118.7 4.1 598.1 36.5 399.0 12.2 299.2 13.5 296.7 5.0 119.9 7.1 120.1 9.1 100.2 4.1 115.7 5.6 139.2 4.4 698.1 38.3 499.7 26.6 497.4 5.9 395.5 5.6 149.0 5.1 148.8 10.3 119.1 3.8 115.7 5.8 139.2 4.6 1000.5 32.3 598.2 19.4 596.1 8.3 494.3 9.8 199.1 5.8 199.1 4.1 149.3 3.2 148.4 5.8 158.3 6.5 1000.5 31.8 798.4 8.5 693.7 8.6 593.0 14.2 297.4 5.7 298.6 5.1 198.2 4.0 187.6 6.0 1397.4 30.6 798.4 9.1 798.5 8.1 790.3 5.3 397.8 3.2 399.0 4.7 298.1 7.8 219.1 6.1 1598.1 34.2 998.5 7.4 798.5 8.3 790.3 5.6 596.4 4.6 596.9 4.9 397.6 5.3 219.1 6.2 1997.1 28.4 993.6 3.4 987.4 2.5 596.4 4.8 596.9 5.1 397.6 5.0 2197.1 40.5 1191.2 4.1 597.6 3.8 2397.6 41.5 1392.6 3.1 2397.6 41.3 1593.0 5.8 2597.7 26.4 1891.8 5.8 2698.1 21.1 2087.4 5.7 2797.5 17.6 2087.4 5.8 2288.0 3.4 180 Depth CAA6 Depth CAA7 Depth CAA8 Depth CAA9 Depth CB1 Depth CB2 Depth CB3 Depth CB4 9.4 11.4 9.7 4.0 10.7 1.7 10.5 3.5 10.7 3.9 10.6 5.4 9.9 4.6 9.9 8.5 32.7 7.2 30.2 4.7 44.5 12.5 17.1 7.4 14.5 10.8 25.1 20.6 24.7 3.1 25.4 5.5 73.9 10.1 50.0 4.9 44.5 12.4 49.8 5.3 42.4 15.6 39.8 2.5 39.6 3.3 39.5 3.8 99.3 10.2 75.1 6.5 47.5 7.3 69.4 3.5 42.4 16.3 57.6 3.1 39.6 3.4 39.5 3.9 99.3 19.4 75.1 6.1 89.0 6.4 89.3 3.9 59.7 10.6 64.6 10.8 57.4 6.6 60.4 4.7 149.4 10.0 103.9 8.5 118.7 3.7 118.9 3.3 74.2 6.0 64.6 10.7 64.3 8.3 70.3 6.2 174.4 7.0 119.7 3.3 148.8 2.3 118.9 3.3 148.8 6.2 99.2 2.4 98.9 1.9 70.6 7.2 189.2 7.6 159.6 4.5 197.8 3.4 148.7 3.4 247.9 5.9 138.8 3.2 118.7 1.8 98.7 11.1 238.9 10.6 188.0 2.8 197.8 4.8 198.3 5.6 327.2 10.3 198.3 2.2 148.4 1.4 118.5 2.9 238.9 11.6 188.0 2.7 247.3 3.1 247.7 3.8 327.2 10.8 296.9 3.7 178.0 2.4 148.4 3.1 296.7 6.3 296.9 3.3 347.6 5.1 395.8 4.9 197.8 2.0 217.6 3.1 346.1 5.0 396.0 3.4 790.3 5.6 247.2 1.5 247.6 10.6 444.9 6.5 790.3 5.7 296.6 3.0 296.6 3.9 987.5 3.2 474.4 4.4 494.2 4.6 1184.6 2.6 790.1 4.3 592.9 4.7 987.2 3.2 790.2 5.4 1184.0 3.4 790.2 5.1 1380.7 3.5 987.3 3.2 1577.2 3.1 1216.1 3.1 1969.6 3.4 1381.6 3.1 2459.2 4.2 1577.6 3.2 2459.2 3.8 1970.4 3.4 2947.6 2.5 2459.7 3.7 3435.0 2.8 2459.7 57.5 2948.1 3.7 3435.3 3.1 * Samples with elevated concentrations which were not consistent with hydrographic features, adjacent samples in the profiles and/or replicate samples measured by other colleagues. Samples coming from our first station in the Labrador Sea (station K1) are neither 181 presented in this table nor discussed in the manuscript due to contamination. Lead data coming from K1 was high and scattered, with concentrations that did not match those found in the other Labrador Sea station (LS2) and those reported in the literature (Zurbrick et al., 2018). This suggests the GoFlo bottles did not have enough time to be cleaned; Pb concentrations returned to consistent values after soaking the bottles in the first four deep casts (~3000 m) at K1. Figure B.1 Upper Panel: Full depth profiles from CB3, CAA2 and BB3 stations comparing results from the University of British Columbia (this work; UBC) and the University of Victoria* (UVic). Lower Panel: Intercalibration exercises among UBC, UVic, the University of Alaska Fairbanks (UAF), the Texas A&M University (TAMU) and the Massachusetts Institute of Technology (MIT). * Lead was also measured by Cullenâs Lab at UVic during the Canadian Arctic GEOTRACES program. Although water samples came from the same GoFlo bottles, they were stored, processed and analyzed independently by UVic and UBC groups which allows for intercalibration of our results. Lead was analyzed by the UVic group using a recently published method2 by ICP-MS/MS following offline matrix removal and pre-concentration with the seaFAST system, while the UBC group used a modified isotope dilution-coprecipitation method (described in the manuscript). Despite the differences in the methods the agreement is excellent. 182 Figure B.2 Water mass back trajectories from BB2 station and water mass proportions at two selected depth ranges. Water tracks were grouped according their origin: Nares Strait: originates from the Arctic Ocean and passes through Nares Strait. Parry Chanel: originates from the Arctic Ocean and passes through Perry Chanel. Coastal West Greenland Current: originates from the WGC and travels into the CAA mixing with Arctic waters. Baffin Bay interior: circulates in the Baffin Bay interior for the entire time period. West Greenland Current: travels from the Atlantic via the WGC around Baffin Bay. 183 Figure B.3 Interpolated contour section of dissolved Pb in the Canadian Arctic Archipelago along and across Parry Channel; sampled depths are superimposed in the plot (white circles) and the individual Pb profiles are displayed above the contour plot. For those Pb samples for which replicate analyses are available, the error bars reflecting the standard deviation are smaller than the symbols. Stations sampled are displayed in the maps aside the contour plots, the beginning of the transect is indicated by the red star (Southern side of Parry Channel). Note the logarithmic scale for the individual profiles. 184 Appendix C - supplementary material for chapter 4 Table C.1 Accuracy determined from the analyses of SAFe D1, SAFe D2 and GSP S GEOTRACES reference materials (AVG±1SD). DFe (nmol kg-1) DMn (nmol kg-1) Measured Consensus* Measured Consensus* SAFe D1 (n=14) UVic 0.655±0.043 0.67±0.04 0.41±0.01 0.35±0.05 SAFe D2 (n= 9) UVic 0.972±0.038 0.956±0.024 0.46±0.03 0.36±0.05 SAFe D1 (n= 5) UBC 0.625±0.017 0.67±0.04 0.385±0.012 0.35±0.05 GSP (n= 5) UBC 0.159±0.006 0.184±0.043 0.740±0.024 0.777±0.034 * Consensus values for SaFe D2 were retrived from Jackson et al. (2018), SaFe D1 values from the GEOTRACES Standards and References Materials webpage (http://www.geotraces.org/sic/intercalibrate-a-lab/standards-and-reference-materials), and GSP values from Jim Moffett (personal communication, GEOTRACES standard and reference trace metal coordinator). UVic: reference material measured using an offline extraction and triple quadrupole ICP-MS/MS method, and UBC: reference material measured using a magnesium-induced coprecipitation and isotope dilution method . 185 Table C.2 Dissolved Fe concentrations (nmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th-October 1st 2015). Depth K1 Depth LS2 Depth BB1 Depth BB2 Depth BB3 Depth CAA1 AVG STD AVG STD AVG STD AVG STD AVG STD AVG STD 29.0 0.106 0.038 10.8 0.213 0.047 10.5 0.527 0.001 9.5 0.336 0.031 9.9 0.349 0.010 9.6 0.818 0.026 49.4 0.114 0.027 30.1 0.362 0.003 20.9 0.763 0.004 28.7 0.574 0.016 29.7 0.374 0.002 29.5 1.10 0.07 98.9 0.320 0.024 100.8 0.697 0.078 46.4 0.884 0.026 49.2 0.753 0.030 49.6 0.763 0.010 43.5 1.81 0.15 147.9 0.494 0.022 150.2 0.700 0.077 99.1 0.804 0.011 98.9 0.642 0.018 99.2 1.03 0.03 59.3 2.42 0.24 198.1 0.617 0.058 297.2 0.652 0.033 148.7 0.573 0.056 149.0 0.529 0.066 147.2 0.864 0.032 79.7 2.28 0.20 301.5 0.652 0.037 497.5 0.651 0.059 199.4 0.696 0.004 198.7 0.656 0.015 198.5 0.656 0.009 99.6 2.38 0.19 399.4 0.666 0.023 598.1 0.674 0.071 399.0 0.929 - 299.2 0.746 0.026 296.7 0.949 0.028 119.9 2.18 0.20 499.3 0.680 0.015 698.1 0.701 0.052 499.7 0.986 0.051 398.0 0.846 0.017 395.5 0.956 0.069 149.0 2.24 0.18 601.5 0.702 0.021 798.1 0.720 0.018 598.2 1.01 0.00001 497.4 0.855 0.023 494.3 0.971 0.071 199.1 2.95 0.20 699.1 0.706 0.007 1000.0 0.747 - 698.8 1.09 0.04 596.1 0.851 0.024 593.0 0.931 0.050 297.4 3.48 0.34 797.8 0.698 0.020 1397.4 0.808 0.055 798.4 1.18 0.03 693.7 0.889 0.034 691.7 0.988 0.036 397.8 3.16 0.39 1000.4 0.671 0.008 1598.1 0.788 0.062 998.5 1.33 0.14 798.5 0.911 0.017 790.3 1.17 0.07 596.4 6.87 0.59 1198.5 0.661 0.025 1997.1 0.827 0.062 993.6 0.962 0.002 987.4 2.00 0.13 1398.4 0.734 0.038 2397.6 0.807 0.035 1191.2 0.916 0.043 1598.2 0.772 0.036 2496.4 0.716 0.042 1392.6 0.835 0.024 1997.2 0.808 0.055 2597.7 0.664 - 1593.0 0.761 0.030 2217.6 0.748 0.041 2698.1 0.666 - 1891.8 0.721 0.064 2266.6 0.726 - 2797.5 0.672 - 2288.0 0.647 0.094 2413.6 0.724 - 2707.3 0.697 - 2951.7 0.647 - Depth CAA2 Depth CAA3 Depth CAA4 Depth CAA5 Depth CAA6 Depth CAA7 AVG STD AVG STD AVG STD AVG STD AVG STD AVG STD 10.7 0.391 0.019 8.7 1.01 0.09 8.9 0.601 0.082 11.4 1.27 0.01 9.4 0.914 0.040 9.7 0.766 0.065 186 29.3 0.523 0.014 18.4 0.857 0.012 33.2 0.401 0.015 20.7 1.26 0.08 32.7 1.00 0.07 30.2 1.29 0.06 42.5 0.861 0.024 28.6 0.693 0.022 47.0 0.436 0.008 45.1 0.910 0.047 73.9 2.63 0.12 50.0 1.51 0.09 59.7 0.934 0.010 39.7 0.697 0.038 59.4 0.800 0.067 70.2 1.53 0.12 99.3 2.96 0.21 75.1 1.36 0.04 79.1 0.897 0.015 59.8 0.750 0.038 79.3 1.79 0.24 78.4 1.81 0.15 149.4 4.64 0.33 103.9 1.31 0.05 99.4 0.812 0.039 80.0 1.13 0.04 94.6 1.89 0.14 118.7 2.28 0.12 174.4 4.75 0.33 119.7 1.17 0.08 120.1 0.925 0.034 100.2 1.50 0.09 115.7 1.76 0.01 139.2 2.45 0.15 189.2 4.86 0.37 159.6 1.55 0.14 148.8 1.06 0.01 119.1 1.60 0.11 148.4 2.01 0.19 187.6 3.51 0.22 238.9 5.13 0.27 188.0 2.54 0.09 199.1 1.31 0.10 149.3 1.23 0.06 219.1 3.91 0.14 298.6 1.51 0.14 198.2 1.28 0.10 399.0 2.28 0.27 298.1 0.936 0.045 596.9 6.62 0.54 397.6 1.32 0.07 597.6 2.24 0.18 Depth CAA8 Depth CAA9 Depth CB1 Depth CB2 Depth CB3 Depth CB4 AVG STD AVG STD AVG STD AVG STD AVG STD AVG STD 10.7 0.668 nan 10.5 1.91 0.12 10.7 0.729 0.063 10.6 0.249 nan 9.9 0.479 0.005 9.9 3.42 0.19 44.5 0.633 0.020 17.1 1.60 0.01 14.5 0.536 0.004 25.1 0.231 0.002 24.7 0.453 nan 25.4 0.312 0.033 47.5 0.604 0.022 49.8 1.06 0.10 42.4 0.702 0.022 39.8 0.416 0.047 39.6 0.332 0.014 39.5 0.225 0.031 89.0 1.15 0.02 69.4 1.12 0.05 59.7 0.819 0.040 57.6 0.431 0.033 57.4 0.394 0.015 70.3 0.269 0.002 118.7 1.03 0.02 89.3 1.26 0.09 74.2 1.10 0.05 64.6 0.455 0.021 64.3 0.411 0.004 98.7 0.519 0.006 148.8 0.796 0.008 118.9 1.79 0.11 148.8 0.933 0.030 99.2 0.510 0.002 98.9 0.614 0.009 118.5 0.544 0.030 197.8 0.772 0.031 148.7 1.73 0.09 198.1 0.699 0.007 138.8 0.518 0.022 118.7 0.616 0.026 148.4 0.556 0.017 247.3 0.605 0.003 198.3 2.38 0.11 247.9 0.948 0.022 198.3 0.479 0.002 148.4 0.595 0.046 217.6 0.589 0.020 296.7 0.526 0.008 247.7 2.01 0.05 327.2 0.829 0.006 296.9 0.444 - 178.0 0.573 0.007 247.6 0.602 0.042 346.1 0.508 0.005 296.9 2.00 0.08 347.6 0.743 0.013 395.8 0.405 0.019 197.8 0.502 0.017 296.6 0.587 0.009 444.9 0.447 0.036 396.0 0.688 0.008 790.3 0.470 0.023 247.2 0.405 0.010 494.2 0.419 0.004 987.5 0.463 0.005 474.4 0.284 0.001 592.9 0.468 0.062 1184.6 0.592 0.066 592.9 0.272 - 790.2 0.488 0.004 790.1 0.298 0.009 987.3 0.449 0.003 987.2 0.283 - 1216.1 0.409 0.005 187 1184.0 0.267 0.005 1381.6 0.391 0.005 1380.7 0.291 0.011 1577.6 0.376 0.0001 1577.2 0.311 0.013 1970.4 0.359 0.005 1969.6 0.319 0.018 2459.7 0.316 0.014 2459.2 0.317 0.001 2948.1 0.280 0.059 2947.6 0.285 0.020 3435.3 0.238 0.011 3435.0 0.348 0.018 Table C.3 Dissolved Mn concentrations (nmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th-October 1st 2015). Depth K1 Depth LS2 Depth BB1 Depth BB2 Depth BB3 Depth CAA1 AVG STD AVG STD AVG STD AVG STD AVG STD AVG STD 29.0 0.450 0.019 10.8 0.850 0.060 10.5 3.10 0.05 9.5 3.52 0.11 9.9 3.68 0.05 9.6 5.42 0.05 49.4 0.522 0.015 30.1 1.09 0.04 20.9 2.56 0.02 28.7 3.58 0.24 29.7 3.13 0.02 29.5 5.02 0.07 98.9 0.732 0.018 100.8 1.03 0.03 46.4 1.52 0.03 49.2 2.49 0.03 49.6 3.14 0.17 43.5 4.95 0.10 147.9 0.750 0.020 150.2 0.866 0.053 99.1 1.15 0.05 98.9 1.09 0.04 99.2 2.02 0.01 59.3 4.84 0.06 198.1 0.726 0.018 297.2 0.590 0.050 148.7 0.955 0.018 149.0 0.838 0.179 147.2 1.29 0.002 79.7 4.58 0.03 301.5 0.665 0.018 497.5 0.519 0.021 199.4 0.560 0.025 198.7 0.590 0.023 198.5 0.806 0.064 99.6 3.97 0.14 399.4 0.593 0.009 598.1 0.512 0.022 399.0 0.611 - 299.2 0.412 0.006 296.7 0.528 0.058 119.9 3.58 0.08 499.3 0.578 0.019 698.1 0.482 0.009 499.7 0.562 0.059 398.0 0.420 0.034 395.5 0.517 0.061 149.0 3.33 0.06 601.5 0.564 0.011 798.1 0.468 0.024 598.2 0.573 0.046 497.4 0.420 0.049 494.3 0.535 0.048 199.1 3.40 0.10 699.1 0.549 0.040 1000.0 0.479 - 698.8 0.653 0.029 596.1 0.410 0.045 593.0 0.502 0.045 297.4 3.30 0.05 797.8 0.558 0.033 1397.4 0.366 0.026 798.4 0.686 0.024 693.7 0.314 0.103 691.7 0.525 0.036 397.8 3.53 0.14 1000.4 0.528 0.027 1598.1 0.345 0.031 998.5 0.768 0.057 798.5 0.285 0.081 790.3 0.776 0.025 596.4 2.97 0.16 1198.5 0.492 0.020 1997.1 0.345 0.030 993.6 0.260 0.041 987.4 4.21 0.01 1398.4 0.478 0.027 2397.6 0.324 0.032 1191.2 0.256 0.034 1598.2 0.404 0.014 2496.4 0.306 0.035 1392.6 0.239 0.030 1997.2 0.354 0.011 2597.7 0.311 0.028 1593.0 0.242 0.028 188 2217.6 0.342 0.006 2698.1 0.321 0.025 1891.8 0.244 0.032 2266.6 0.347 0.008 2797.5 0.385 0.036 2119.8 0.611 0.027 2413.6 0.330 0.005 2288.0 0.736 0.048 2707.3 0.337 0.011 2951.7 0.359 0.018 Depth CAA2 Depth CAA3 Depth CAA4 Depth CAA5 Depth CAA6 Depth CAA7 AVG STD AVG STD AVG STD AVG STD AVG STD AVG STD 10.7 3.93 0.16 8.7 5.42 0.08 8.9 6.63 0.41 11.4 5.92 0.35 9.4 4.79 0.14 9.7 6.26 0.44 29.3 3.51 0.11 18.4 5.12 0.01 33.2 5.50 0.44 20.7 5.60 0.00 32.7 4.43 0.10 30.2 4.34 0.07 42.5 3.03 0.16 28.6 4.75 0.03 47.0 5.05 0.35 45.1 4.77 0.17 73.9 4.66 0.00 50.0 3.26 0.00 59.7 2.58 0.02 39.7 4.16 0.00 59.4 3.40 0.19 70.2 3.80 0.13 99.3 4.86 0.01 75.1 2.43 0.12 79.1 2.49 0.14 59.8 3.95 0.04 79.3 3.24 0.08 78.4 4.15 0.13 149.4 5.55 0.05 103.9 2.32 0.13 99.4 2.34 0.16 80.0 3.13 0.04 94.6 2.91 0.06 118.7 4.31 0.21 174.4 5.54 0.01 119.7 2.17 0.10 120.1 2.18 0.05 100.2 2.96 0.06 115.7 2.70 0.09 139.2 4.34 0.08 189.2 5.55 0.11 159.6 4.32 0.22 148.8 1.31 0.07 119.1 3.02 0.02 148.4 3.02 0.12 187.6 4.77 0.09 238.9 6.06 0.13 188.0 6.84 0.67 199.1 1.07 0.08 149.3 2.34 0.01 219.1 5.51 0.22 298.6 1.20 0.08 198.2 1.81 0.00 399.0 1.60 0.10 298.1 0.813 0.013 596.9 8.40 0.23 397.6 1.23 0.03 597.6 1.78 0.04 Depth CAA8 Depth CAA9 Depth CB1 Depth CB2 Depth CB3 Depth CB4 AVG STD AVG STD AVG STD AVG STD AVG STD AVG STD 10.7 7.50 0.72 10.5 9.55 0.20 10.7 10.78 nan 10.6 5.28 0.20 9.9 7.02 0.12 9.9 5.86 0.00 44.5 2.96 0.27 17.1 8.59 0.33 14.5 8.13 0.13 25.1 3.94 0.05 24.7 4.07 0.16 25.4 6.03 0.30 47.5 2.94 0.20 49.8 3.91 0.01 42.4 3.95 0.17 39.8 1.67 0.06 39.6 3.20 0.18 39.5 5.82 0.31 89.0 1.39 0.14 69.4 3.37 0.04 59.7 1.63 0.02 57.6 1.68 0.03 57.4 1.80 0.04 70.3 1.86 0.03 118.7 1.14 0.01 89.3 3.35 0.01 74.2 1.32 0.03 64.6 1.45 0.03 64.3 1.76 0.02 98.7 1.26 0.02 189 148.8 0.789 0.017 118.9 4.04 0.02 148.8 0.898 0.0001 99.2 1.49 0.05 98.9 1.33 0.00 118.5 1.30 0.09 197.8 0.811 0.047 148.7 3.95 0.03 198.1 0.463 0.021 138.8 2.01 0.02 118.7 1.53 0.01 148.4 1.52 0.04 247.3 0.586 0.037 198.3 4.69 0.03 247.9 0.689 0.009 198.3 1.23 0.01 148.4 1.73 0.20 217.6 1.76 0.06 296.7 0.475 0.023 247.7 3.78 0.18 327.2 0.604 0.008 296.9 0.418 0.027 178.0 1.75 0.11 247.6 1.17 0.06 346.1 0.436 0.029 296.9 3.64 0.10 347.6 0.548 0.003 395.8 0.311 0.002 197.8 1.75 0.00 296.6 0.690 0.018 444.9 0.416 0.023 396.0 0.439 0.026 790.3 0.348 0.017 247.2 0.958 0.007 494.2 0.338 0.057 987.5 0.306 0.020 474.4 0.331 0.039 592.9 0.374 0.075 1184.6 0.328 0.030 592.9 0.359 - 790.2 0.425 0.023 790.1 0.366 0.018 987.3 0.565 0.007 987.2 0.332 0.016 1216.1 0.336 0.010 1184.0 0.269 0.021 1381.6 0.293 0.030 1380.7 0.251 0.055 1577.6 0.248 0.025 1577.2 0.215 0.037 1970.4 0.195 0.030 1969.6 0.172 0.040 2459.7 0.165 0.011 2459.2 0.153 0.019 2948.1 0.158 0.012 2947.6 0.147 0.018 3435.3 0.150 0.013 3435.0 0.148 0.051 190 Figure C.1 Upper Panel: Full depth profiles of dissolved Fe from CB4, CAA2 and BB3 stations comparing results from the University of British Columbia (UBC) and the University of Victoria (UVic). Lower Panel: Intercalibration exercises among UBC and the Texas A&M University (TAMU). 191 Figure C.2 Upper Panel: Full depth profiles of dissolved Mn from CB4, CAA2 and BB3 stations comparing results from the University of British Columbia (UBC) and the University of Victoria (UVic). Lower Panel: Intercalibration exercises among UBC and the Texas A&M University (TAMU). 192 Figure C.3 Dissolved Fe and Mn concentrations (nmol kgË1) for the sampled stations, Mn (blue profiles) axis is displayed above each plot and Fe (red profiles) axis is displayed below each plot. Note the logarithmic depth scale. Figure C.4 Monthly-averaged Chl a concentration from January to September 2015 in the study region (data retrieved from MODIS-Aqua sensor: https://oceancolor.gsfc.nasa.gov/l3/). 193 Figure C.5 Dissolved Fe versus water mass age for Canada Basin Deep Waters (>1200 m). The regression line and R2 value is displayed in the figure, as well as the 95% prediction limits and the 95% confidence intervals, estimated by bootstrap resampling of residuals (n=500). The density of overlapping lines indicates increased confidence. The correlation is significant at a p-value< 0.001. 194 Appendix D - gallium distribution in the Canadian Arctic Ocean: results and preliminary figures Table D.1 Dissolved Ga concentrations (pmol kgË1) measured in the Labrador Sea (LS), the Baffin Bay (BB), the Canadian Arctic Archipelago (CAA) and the Canada Basin (CB) during the Canadian Arctic GEOTRACES program (July 10th - October 1st 2015). Depth K1 Depth LS2 Depth BB1 Depth BB2 Depth BB3 Depth CAA1 Depth CAA2 Depth CAA3 Depth CAA4 9.3 14.71 10.9 16.16 10.6 7.88 9.6 4.62 10 3.45 9.7 6.21 10.8 6.75 8.84 5.74 9 2.81 29.3 15.59 30.4 19.78 21.1 8.79 29 7.37 30 4.09 29.8 5.83 29.6 7.19 18.6 5.85 33.5 1.86 49.9 16.96 101.8 24.68 46.9 11.92 49.7 9.12 50.1 7.33 44 9.12 42.9 11.18 28.9 4.72 47.5 2.98±0.23 99.8 23.52 151.7 24.50 100.1 12.95 100 14.74 100.2 10.65 59.9 8.95 60.2 13.67 40.1 3.89 60 4.62 149.3 23.46 300.3 25.58±0.10 150.2 16.85 150.6 13.95 148.8 16.72 80.5 9.84 79.8 14.10 60.4 4.58 80.1 6.19 200 23.69±1.21 502.9 25.18 201.5 18.98±0.17 200.9 18.36±0.003 200.6 16.85±1.31 100.7 11.74±0.11 100.3 13.82±0.36 80.9 7.33±0.33 95.6 7.41 304.5 24.30 604.7 26.27 403.4 22.51 302.49 22.08 300 23.35 121.2 10.74 121.2 15.21 101.3 8.83 116.9 7.72 403.5 24.17 706 28.13 505.3 23.17 402.49 23.88 400 25.44 150.6 12.40 150.2 20.81 120.4 9.91 150 6.44 504.5 23.60 807.3 28.42 605.1 25.79 503.2 24.41 500 24.24 201.3 16.83 201 23.60 150.9 12.82 607.9 25.52 1415.5 26.28 707 24.15 603.2 23.65 600 24.21 300.7 22.47 301.5 25.01 200.4 18.83 706.7 25.46 1619.6 27.51 808 27.19±0.17 702.1 24.33 700 28.51 402.4 22.22 403 25.48 301.4 24.05 806.7 24.27±0.87 2026 29.72 1011 26.42 808.4 26.28±0.57 800 27.95±2.23 603.5 26.98±1.02 603.2 26.22±0.47 402.1 23.57±0.81 1000 25.13 2229.9 32.70 1006.4 25.81 1000 28.29 1200 26.65 2434.6 33.51±0.58 1207.1 25.20 1400 25.52 2535.5 30.35 1411.8 26.16 1600 27.10 2639 31.26 1615.8 29.13 2000 30.75 2741.6 30.11 1920.2 28.31 2250 30.16±0.95 2843.3 32.06 2119.79 39.51±0.03 2300 32.90 2324.6 37.20 2450 32.35 2750 32.82 3000 32.20 195 Depth CAA5 Depth CAA6 Depth CAA7 Depth CAA8 Depth CAA9 Depth CB1 Depth CB2 Depth CB3 Depth CB4 11.5 4.68 9.5 3.65 9.8 3.24 10.8 6.33 10.6 6.69 10.8 3.30 10.7 7.08 10 5.20 10 5.17 20.9 4.30 33 4.50 30.5 3.48 45 4.67±0.03 17.3 7.15 14.7 3.67 25.4 4.42 25 4.81 25.7 5.38 45.6 4.24±0.30 74.7 9.63 50.5 4.27 48 6.37 50.3 7.95 42.8 3.90±0.01 40.2 4.62 40 4.95±0.06 39.9 5.48±0.64 70.9 7.65 100.4 8.86±0.27 75.9 5.84±0.01 90 8.30 70.1 9.70 60.3 3.96 58.2 4.68 58 4.31 61 5.81 79.2 8.54 151 9.37 105 6.05 120 8.44 90.3 11.16 75 4.29 65.3 5.383±0.13 65 5.44 71.1 4.33 120 9.44 191.3 9.99 121 6.47 150.4 12.59 120.2 12.71 150.4 10.91 100.3 5.10 100 5.32 71.4 5.23 140.7 13.50±0.0005 241.5 10.01±0.09 161.3 6.55 200 14.80±0.04 150.3 12.80 200.3 15.14 140.3 5.60 120 5.59 99.8 5.42 160 15.53 190 9.10±0.76 250 16.88 250.5 13.25 250.7 18.12 200.5 12.78 150 5.46 119.8 6.36 189.7 19.09 300 22.53 300.3 13.63 330.9 21.55±0.25 300.2 23.42 180 5.32 150 5.91±0.14 221.5 19.57±0.33 350 23.33 351.5 23.90 400.4 26.65 200 8.89 220 7.56 450 24.40 400.5 24.00 800.2 29.77±0.23 250 14.94 250.4 11.99 1000.3 29.16 300 20.66 299.9 16.60 1200.5 28.60 480 21.89 500 21.63 600 24.66 600 22.36 800 23.94±1.51 800 26.20±1.00 1000 27.25 1000.1 28.26 1200 27.63 1232.5 29.59 1400 27.88 1600.3 29.60 1600 26.34 2000.6 26.80 2000 24.54 2500.3 26.81±0.10 2500 24.61±1.23 3000.2 26.62 3000 23.61 3498.5 26.30 3500 25.08 3500 27.32 196 Figure D.1 Dissolved Ga concentrations (pmol kgË1) for the sampled stations. Note the logarithmic depth scale. 197 Figure D.2 Interpolated contour section of dissolved Ga along the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay. Sampled depths are superimposed on the plot (white circles) and the individual Ga profiles are displayed above the contour plot. Stations sampled are displayed in the inset map and the beginning of the transect is indicated by the red star. Note the break axis in the contour map and the logarithmic scale for the individual profiles. Stations were chosen to capture the flow of Arctic waters from the Canada Basin to Baffin Bay. 198 Figure D.3 Interpolated contour section of dissolved Ga in the Canadian Arctic Archipelago along and across Parry Channel; sampled depths are superimposed in the plot (white circles) and the individual Ga profiles are displayed above the contour plot. Stations sampled are displayed in the maps aside the contour plots, the beginning of the transect is indicated by the red star (Southern side of Parry Channel). Note the logarithmic scale for the individual profiles.
Thesis/Dissertation
eng
Oceanography
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
On the biogeochemical processes controlling trace metal distributions in the Canadian Arctic Ocean and Arctic rivers
Text