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On the glacial-interglacial variability of upwelling, carbon burial and denitrification on the Northwestern… Ganeshram, Raja S. 1996

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ON THE GLACIAL-INTERGLACIAL VARIABILITY OF UPWELLING, CARBON BURIAL AND DENITRIFICATION ON THE NORTHWESTERN MEXICAN CONTINENTAL MARGIN by RAJA S. GANESHRAM B.Sc. The University of Madras 1983 M.Sc. The University of Madras 1985 M.S. Indian Institute of Technology, Madras 1987 A THESIS IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August 1996 ©Raja S. Ganeshram, 1996 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of EftETH frMD b^fttt S c ? E ^ C 6 S The University of British Columbia ; Vancouver, Canada I Date DE-6 (2/88) ABSTRACT Glacial-interglacial variability in upwelling on the NW Mexican margin is assessed by reconstructing the history of organic carbon and biogenic opal deposition and measuring the Ba/Al ratio in three piston cores that span the upper to the lower continental slope. Rates of accumulation of organic carbon, opal and to some degree biogenic barite are higher in interglacial intervals, indicating that upwelling-induced productivity was higher during the warm periods over the last 140,000 years. Despite cyclic changes in organic carbon accumulation, matrix-corrected HI values in the mid- and lower- slope cores are invariant and are similar to values in the laminated intervals from the oxygen-minimum site. This suggests that changes in organic carbon content are controlled by productivity variations and are not due to differential preservation induced by variations in bottom water oxygen concentrations. The lowest HI values in Mexican Margin sediments occur concurrently with large increases in grain size. Thus, increased degradation resulting from winnowing is offered as the leading explanation for the hydrocarbon impoverishments in the bioturbated upper slope deposits. Late Quaternary records of denitrification in the oxygen-deficient subsurface water masses of the Eastern Tropical North Pacific (ETNP) are constructed using 15N/14N ratios measured on bulk sediments. The profiles show a synchronous decrease in denitrification during the glacial periods over the last 140 kyrs. It is suggested that, because nitrate is a limiting nutrient in the modern ocean, a consequent increase in the oceanic nitrate inventory could have contributed to the observed decrease in glacial atmospheric pC02 by enhancing the fertility of the ocean. The glacial decreases in denitrification in the ETNP are attributed to large reductions in upwelling-induced fluxes of organic detritus on the margin in response to glacial shifts in the wind field off NW Mexico associated with the growth of Laurentide ice on northern North America, the establishment of a resident high pressure cell over the ice sheet, and the bifurcation of the Jet Stream. iv T A B L E O F C O N T E N T S Abstract ii Table of Contents iv List of Tables vii List of Figures viii List of Plates x Acknowledgments xi Chapter 1 General Introduction 1 1.1 COASTAL UPWELLING 1 1.2 OBJECTIVES 2 1. 2. 1 The response of coastal upwelling to glacial-interglacial climatic change 2 1. 2. 2 Factors controlling the burial of organic matter on continental margins 3 1. 2. 3 Glacial-interglacial variations in oceanic denitrification and their impact on the oceanic-nitrate budget and atmospheric C O 2 contents 4 Chapter 2 Glacial-interglacial variability in upwelling off N W Mexico.. 6 2.1 INTRODUCTION 6 2. 2 STUDY AREA 9 2. 3. STRATEGY 10 2. 4 RESULTS 14 2. 4.1 Chronostratigraphy 14 2. 4. 2 Biogenic records 22 2. 5 DISCUSSION 25 2. 5.1 Biogenic Opal 25 2. 5. 2 Organic carbon 27 2. 5. 3 Ba/Al ratios 29 2. 5. 4 Implications for the paleoclimate of western North America 31 2. 5. 5 Implications for atmospheric p C 0 2 37 2. 6 CONCLUSIONS 39 Chapter 3 Glacial-interglacial Variability i n Carbon Burial off N W Mexico 40 3.1 INTRODUCTION 40 3. 2 LATE QUATERNARY RECORDS OF CARBON BURIAL 44 3 .2.1 Organic carbon records ; 44 3. 2. 2 HI records .....49 3. 2. 3 Source of organic matter 56 3. 2. 4 Influence of sediment texture 66 3. 3 DISCUSSION 67 3 .3. 1 Factors controlling the burial of organic carbon in the Late Quaternary 67 3. 3. 2 Factors controlling the burial of organic carbon in Recent sediments 76 3. 3. 3 The role of winnowing in the spatial distribution of organic carbon 85 3. 3. 4 Implications for the global distribution of organic carbon on continental margins 87 3. 3. 5 The role of oxygen minima in the burial of hydrocarbons 89 3. 3. 6 The role of oxygen minima in the burial of organic carbon 95 3. 3. 7 Implications for the interpretation of hydrogen indices in geological records 97 3. 3. 8 The influence of winnowing on the preservational quality of organic material 99 3.4 CONCLUSIONS ..103 Chapter 4 Glacial-interglacial variability in denitrification off NW Mexico 104 4.1. INTRODUCTION 104 4. 2 NITROGEN ISOTOPES AND WATER COLUMN DENITRIFICATION 106 4. 3 RESULTS *. 107 4. 4 DISCUSSION 113 4. 4. 1 Possible causes for down-core shifts in S^N values 113 4. 4. 2 The cause of diminished denitrification during the glacial periods 117 4. 3. 3 Implications for the oceanic-nitrate inventory and pC02 during the glacial periods 120 4. 4 CONCLUSIONS 123 Chapter 5 Closing Remarks 124 References 129 Appendix I Analytical Methods 151 I-lTotal carbon, nitrogen and carbonate analysis 151 1-2 Biogenic opal 152 1-3 Elemental analysis by X-ray fluorescence 154 1-4 Rock-Eval pyrolysis 156 1-5 Stable isotope measurements . 156 I- 6 Determination of salt and porosity 157 Appendix II Data Tables 159 IJ-1 Concentrations of biogenic components and mass accumulation rates from the Mexican margin piston cores .159 H-2 Major elements from the Mexican Margin piston cores 168 JJ-3 Minor elements from the Mexican Margin Piston cores 182 JJ-4 Carbon and nitrogen isotope data from the Mexican Margin piston cores 196 H-5 Foraminiferal stable isotope data from the Mexican Margin piston cores 200 II- 6 Rock-Eval pyrolysis data from the Mexican Margin piston cores 206 H-7 Chlorinity data from the Mexican Margin piston cores 212 IJ-8 Box core data 216 LIST OF TABLES Table 1 Mexican Margin core locations 13 Table 2. Age picks used in the construction of timescales for Mexican Margin cores 15 Table 3. Results from Lignin Biomarker (A) analysis.. 64 Table 4. Surface area determination in Core NH15P 101 Table 5. Silica measured in pure clays and clays spiked with 3 wt. % pure diatomaceous opal after extracting for 3 hours in 2 M Na2C03 solution at 85 oc 155 V l l l LIST QF FIGURES Figure 1. Location map of the study area showing bathymetry and the core locations 8 Figure 2. A typical oxygen profile from the Eastern Tropical North Pacific adjacent to Northwestern Mexico 11 Figure 3. Benthic-foraminiferal oxygen isotope records from NW Mexican Margin piston cores 17 Figure 4a-c. Biogenic records from NW Mexican Margin cores 18-21 Figure 5. Age-depth plots for NW Mexican Margin piston cores 23 Figure 6. Generalized illustration of the large-scale atmospheric circulation and seasonal coastal upwelling on the west coast North America 32 Figure 7. A schematic of the postulated glacial-interglacial contrast in atmospheric circulation, precipitation and ice cover over the Western Hemisphere 35 Figure 8a-c. Records of % organic carbon and hydrogen indices (HI) from NW Mexican Margin cores 45-48 Figure 9. S2-TOC plots from Mexican Margin piston cores 51 Figure 10. Matrix correction for Core NH8P 53 Figure 11. Records of % organic carbon, hydrogen index (HI) and matrix-corrected HI from Core NH8P 54-55 Figure 12a-c. Records of Sl^Corganic a n c * Corganic/Ntotal r a t i o from NW Mexican Margin cores. 58-61 Figure 13. The relationship between Corganic/Ntotal and S^^Corganic in NW Mexican Margin piston cores............. 63 Figure 14. Relationships among grain size, Zr/Al and Sinonbio/Al ratios in Core NH15P 68 Figure 15a-c. Records of grain size proxies and % organic carbon from NW Mexican Margin cores 69-72 Figure 16. Relationship between organic carbon contents and Zr/ Al ratio in NW Mexican Margin piston cores........ 73 Figure 17. Organic carbon profiles from the box cores and upper 30 cm of the piston cores 77 Figure 18. Relationship between organic carbon contents and Zr/Al ratios in the Recent sediments from NW Mexican Margin 79 Figure 19. Relationship between Corganic/Ntotal ratios and S^Corganic values in Recent sediments from NW Mexican Margin 80 Figure 20 Hydrogen Index (HI) profiles from the box cores and the upper 30 cm of the piston cores 83 Figure 21. S2-TOC plots form the Recent Mexican Margin sediments 84 Figure 22 I/C0rganic profiles from the box cores and the upper 30 cm of the piston cores 86 Figure 23. The relationship between S2 and TOC in sediments from continental margins and euxinic basins 91 Figure 24. Down-core profiles of % total organic carbon (squares), HI (circles) and corrected-HI (triangles) from Northern California Margin Core GC117 93 Figure 25. Matrix correction for N. California Margin Core GC117. 94 Figure 26. Nitrogen-isotope profiles from the box cores 108 Figure 27a-c. Records of 8^N in bulk sediments and benthic-foraminiferal Sl^O from NW Mexican Margin cores... 109-112 Figure 28. Profiles of Corganic/Ntotal ratios from the box cores 114 Figure 29. Percent Opal vs. Time plots for 85 °C 2M Na2C03 sequential timed extractions of two powdered sediment samples 153 LIST OF PLATES Plate 1. X-radiographs (positive) of Core NH15P showing laminations typical of the Holocene intervals (A) and mottles (bioturbation) in sediments deposited during the Last Glacial Maximum (B) 24 ACKNOWLEDGMENTS This project would not have been successful without the generous contributions of several individuals. First, I would like to express my profound gratitude to Dr. Tom Pedersen for his excellent supervision, unfailing support, captivating enthusiasm and encouragement all through the study. It has certainly been an extremely enjoyable association. I am very grateful to Dr. Steve Calvert for his keen interest and able advice throughout the research. Dr. Calvert's work on carbon burial in continental margin sediments inspired me to choose this Ph.D. research topic in the first place. I am grateful to Drs. Allen H. Devol and James W. Murray for the opportunity to collect cores during the NSF-supported cruise 90-5 of RV New Horizon to the Mexican Margin, and to M. Soon, G. Jewsbury and G. Hargreaves for their assistance at sea. I am indebted to Bente Nielsen for carrying out isotopic determinations with extreme care, and an army of summer students for providing a helping hand in the laboratory over the years. The science presented here has benefitted immensely from discussions with Drs. Rainer Zahn, John Farrell, Mark Altabet, Roger Francois, Greg Cowie, John Crusius, Natalie Waser and Lionel Pandolfo. My thanks are due to the members of the faculty and staff of the Department of Oceanography for providing a congenial learning environment. I am obliged to Dr. A. G. Lewis for his able guidance and support while I taught sections of Oceanography 310; to Chris Mewis, Carol Leven and late Olive Lau for helping with the paperwork; and to Drs. Kristin Orians, Paul Leblond, Steve Pond, Max Taylor, Dick Chase and Paul Harrison for teaching me various aspects of oceanography. I would also like to thank all those who have participated in the Geochemistry group meeting and Paleoceanography discussion group for their enlightening contributions throughout the duration of my study. Thanks are due to the old "Biosciences Room No. 1325" Gang for their delightful company. In particular, Jay McNee and Bert Mueller must take credit for helping me acquaint with the finer aspects of North American culture. On a personal note, my appreciation is due to all the members of my family. I am eternally grateful to my wife Bhuvaneswari for her understanding and patience while I wrote this thesis and for postponing our honeymoon indefinitely until I defend. I am also very grateful to my Dad and Mom for their unwavering confidence in me, and my late grandmother Anniamma Patti who saw me through some very difficult periods of my childhood. My late grandfather Ramanatha Mudaliar has always been my role model. My aunts Dr. Indira, Dr. Shymala, Dr. Yasodha, Seetha and Ambika have all contributed variously as financiers, tutors and mentors throughout my life. My Uncle Dr. Eneyasivam took care of my responsibilities in India during my absence. Finally, my sister Rama and brother-in-law Balu helped me with family obligations in my absence and also kept me in touch with the music scene in India; for that, I thank them. Chapter 1 General Introduction 1.1. COASTAL UPWELLING Coastal upwelling is a major oceanic process, where wind stress on the sea surface forces surface water to move offshore allowing nutrient-rich waters to surface from below. This large scale supply of nutrients to the sun-lit surface layers fuels high rates primary production (Smith, 1983). As a result, coastal upwelling zones host some of the most biologically productive waters in the world (Berger et al., 1989) and underlying sediments are typically rich in organic carbon, opal and other biogenic constituents (e.g. Piper and Codispoti, 1975; Calvert and Price, 1983; Sarnthein et al., 1988; Wilson, 1989; Summerhayes et al., 1992; Pedersen and Calvert, 1995; Nelson et al., 1995). One biochemical consequence of intense coastal upwelling is the severe oxygen depletion and denitrification in underlying water masses due to the large respiratory oxygen demand exerted by the sedimenting organic particles (Codispoti, 1989). Where these oxygen-deficient waters intersect the continental margin, bioturbating macrobenthos are typically eliminated, resulting in laminated deposits (Calvert, 1987). The enhanced preservation of organic material in such depositional settings is thought to favour the accumulation of organic-rich sedimentary facies and petroleum source rocks (Demaison and Moore, 1980). Thus, coastal upwelling regimes play a pivotal role in increasing the oceanic fluxes of organic carbon, fostering the formation of petroleum source rocks, removing nutrients from the ocean, and modulating atmospheric C O 2 , thereby influencing climate. 1. 2 OBTECTIVES The intent of this study is to decipher the late Quaternary history of upwelling, primary production, sedimentary organic matter accumulation, and bottom water oxygenation on the continental margin off NW Mexico. The principal objective is to shed light on the nature of the link between climate and coastal upwelling and in particular, to contribute to the understanding of the mechanisms that could control the amplitude of climate change globally during the late Quaternary. The three specific problems described below are addressed in separate chapters, each of which has been written in a near "stand-alone" form to facilitate publication. This has unavoidably introduced some redundancy, but this has been minimized wherever possible. 1. 2. 1 The response of coastal upwelling to glacial-interglacial climatic change Because coastal upwelling is wind driven it responds sensitively to changes in climate. Sediments accumulating beneath upwelling zones thus can provide high resolution records of local responses to variability in Earth's climate. Deciphering such records, through space and time, is crucial in understanding the nature of various forcing and feedback mechanisms that link changes in climate, ocean circulation and biological productivity (Summerhayes, 1995). Toward this end, the history of upwelling off NW Mexico is reconstructed in Chapter 2, using productivity proxies (organic carbon, opal and Ba/Al ratios). Sarnthein et al. (1988) and Sarnthein and Winn (1990) have argued that increased upwelling in the low- and mid-latitude ocean during glacial periods, as evidenced off Northwest Africa, played a key role in lowering atmospheric pC02- This begs the following questions: "What has been the response of upwelling on the NW Mexican Margin to glacial-interglacial variability in climate?"; and, "Is the glacial increase in coastal upwelling seen by Sarnthein et al. universal? If not, then what can account for the variability?" We seek answers to these questions by describing changes in the strength and direction of the local and regional wind fields that influence upwelling off NW Mexico. 1. 2. 2 Factors controlling the burial of organic matter in continental margins Biologically productive coastal upwelling zones exert an important control on the mass balance of carbon in the atmosphere, oceans, biosphere and cryosphere on time scales ranging to millions of years (Summerhayes, 1995). Therefore, it is imperative to understand the factors that control the accumulation of organic carbon in margin sediments. Furthermore, although a large supply of organic detritus to sediments is recognized as a factor in enhancing the burial of organic matter on upwelling margins, its relative importance compared to influences on the preservation of organic carbon is widely debated (e.g. Emerson and Hedges, 1988; Calvert and Pedersen, 1992). In Chapter 3, a critical test is presented of the widely held view that oxygen-depleted conditions enhance both the quantity and quality (in terms of hydrocarbon yield) of organic matter buried in sediments. Central to this investigation is the question, "How important is the oxygen minimum zone in fostering enhanced preservation of organic carbon in sediments?" The organic carbon records from laminated sediments deposited within the footprint of the modern minimum are compared with records obtained from deeper bioturbated deposits underlying well oxygenated waters. In addition, both the source (marine versus terrestrial, determined by o*13c0rganio Corganic/Ntotal ratios and lignin biomarkers) and the type (determined by Rock-Eval pyrolysis) of the organic matter buried are assessed in oxic and anoxic settings. Factors other than bottom-water oxygen concentrations that could make a possible contribution to the variability in temporal and spatial distributions in organic carbon in continental margin sediments are also identified. 1. 2. 3 Glacial-interglacial variations in oceanic denitrification and their impact on oceanic-nitrate budget and atmospheric CO? contents Changes in both ocean chemistry and circulation have been invoked to explain the lower glacial atmospheric C O 2 concentrations observed in ice core records. An increase in the nutrient inventory of the glacial ocean is one mechanism that could have lowered atmospheric C O 2 levels by enhancing oceanic biological productivity and C O 2 storage (Broecker, 1982; Berger and Keir, 1984; Boyle, 1988). One way to accomplish this is to modulate the average nitrate content of the ocean on glacial-interglacial time scales by varying denitrification rates in oxygen-deficient surface water masses that underlie coastal upwelling zones (Berger and Keir, 1984; Codispoti, 1989). Denitrification in the oxygen-deficient water masses off western Mexico, accounts for at least a third of the oceanic-nitrate loss due to water column nitrate reduction (Codispoti and Christensen, 1985). Upwelling-induced changes in the fluxes of organic detritus through the water column in this area should produce simultaneous changes in denitrification rates. In Chapter 4, sedimentary nitrogen isotope measurements are used to test the hypothesis that oceanic productivity and attendant changes in CO2 may have been modulated on glacial-interglacial timescales by variations in the oceanic nitrate inventory due to changes in the rate of denitrification. Chapter 2 Glacial-interglacial variability in upwelling off NW Mexico 2. 1 INTRODUCTION Maps of biological production reveal that the surface productivity in the modern ocean is unevenly distributed (Koblentz-Mishke et al., 1970; Berger et al., 1987). Globally, oceanic primary production is concentrated in narrow belts along the equatorial divergence regions of the Atlantic and Pacific oceans, in the subpolar divergence in the southern ocean, and along continental margins. Among these high-productivity areas, the coastal upwelling zones associated with eastern boundary currents are particularly important as they contain some of the most biologically productive waters in the world (Berger et al., 1989). Because coastal currents and the associated offshore Ekman transport are wind driven, they are intimately tied to prevailing climatic and oceanographic patterns and are therefore extremely sensitive to changes in climate (Summerhayes, 1992). The combination of high biological production, shallow depths which reduce recycling of biogenic constituents in the water column, and high sedimentation rates which ensure rapid burial of biogenic components, make upwelling margins important locations for the removal of biologically-important elements from the ocean. The sedimentary fades in such settings are typically rich in organic carbon and opal (Berger et al., 1989). This is best illustrated by the composition of sediments accumulating in modern upwelling zones off Peru-Chile (Reimers and Suess, 1983 a,b) and off Namibia (Calvert and Price, 1983). Modern biogenic sediments on these continental margins are therefore expressions of prevailing climate. It follows that historical climatic variations should be recorded by changes in the compositional nature and rates of accumulation of sedimentary biogenic components. The burial of biogenic sediments on upwelling continental margins constitutes an important sink for nutrients (Piper and Codispoti, 1975; Christensen, et al., 1987; Wilson, 1989; Nelson et al., 1995). Such that coastal upwelling and associated biological productivity act to regulate oceanic nutrient inventories on time scales of thousands to tens of thousands of years. Even on shorter timescales (several thousands of years), upwelling and productivity play an important role in the cycling of biologically-important elements in the ocean. For example, recent work by Altabet et al. (1995) and Ganeshram et al. (1995) has highlighted the role of upwelling continental margins in modulating the oceanic NO3" inventory on glacial-interglacial time scales. Furthermore, because primary production largely determines the amount of carbon exported to and stored in the deep ocean, it exerts a large control on atmospheric CO2 concentrations (Broecker, 1982; Walsh, 1991). Thus, model scenarios constructed to explain the lower glacial atmospheric pCC>2 invariably involve enhanced oceanic primary production (Berger and Keir, 1984; Berger and Wefer, 1991). Interestingly, biogenic sedimentation on upwelling margins can be both cause and consequence of climate change. Clearly, then, deciphering biogenic sedimentary records from such settings promises to enhance our understanding of both the cause and effect of global climate change. In this thesis, a suite of chemical, isotopic and compositional variables is first used to unravel glacial-interglacial history of upwelling and productivity on the continental margin off Northwestern Mexico (Figure 1). Like much of 8 2 3 N 2 2 N 107 W 106 W Figure 1. Location map of the study area showing bathymetry and the core locations. Piston and box cores are shown as squares and circles, respectively. the temperate-latitude coast of western North America this region is today marked by upwelling-induced high average annual production in surface waters, but it will be shown that this has not been the case in the recent geological past. In that context we address three main questions: A) What has been the history of organic carbon and biogenic opal burial off NW Mexico during the late Quaternary?; B) How do variations in the rates of accumulation these phases relate to both global and regional climate change?; and C) Are variations in organic carbon accumulation most related to primary productivity, changes in preservation, or both? In the second part of the chapter, climatic interpretations arising from the new marine records and from terrestrial data sets from Mexico and the southwestern U.S. are compared to the glacial paleoclimate predicted for the region by General Circulation Models (GCMs). It will be shown that the marine and terrestrial records are fully consistent with the model predictions. The three sets of information collectively point to a glacial climatic regime for northwestern Mexico and adjacent waters that was dramatically different from that in the area today. 2. 2 STUDY AREA Coastal upwelling on the NW Mexican Margin is typical of eastern boundary current regimes in the northern hemisphere (Smith, 1983, 1992). Shore-parallel northwesterly winds result in Ekman transport and large scale vertical movement of subsurface nutrient-rich waters to the surface (Roden, 1972a). Biological and chemical consequences of this upwelling have been described by Griffiths (1968) and Stevenson (1970) respectively, while Codispoti (1973) has described the associated nutrient dynamics. A large standing stock of phytoplankton is a common feature in surface waters in this area (Stevenson, 1970). Santamaria-del-Angel and Alvarez-Borrego (1994) studied seasonal and temporal variations in the satellite-imaged pigment concentrations in this region and made two principal observations. First, there is a large shore-normal gradient. The highest pigment concentrations coincide with the locus of upwelling over the continental shelf and decrease offshore. Second, standing stocks of phytoplankton are highest in winter months corresponding with the upwelling season. The satellite-derived net annual primary production in this area is estimated to be ca. 330 g C m~2 yr~l (Longhurst, 1995), comparable to other coastal upwelling zones off southern California and Western Africa. These observations confirm earlier depth-integrated l^C productivity estimates of 0.9 g C/m^/day made by Zeitzschel (1969) off Mazatlan during upwelling seasons. The rapid offshore depletion in N O 3 " and pigment concentrations in upwelled waters in this area indicate that primary production is not limited by micronutrients. Oxygen-deficient subsurface and intermediate depth water masses of the Eastern Tropical North Pacific impinge on the upper slope of the Mexican Margin. Together with the high production and large flux of organic detritus through the water column, this maintains a thick oxygen minimum zone near the coast extending from ~150 to ~800 metres water depth. Oxygen concentrations are almost unmeasurable (<10 fxM) between 200 and 600 metres depth (Figure 2). Denitrification is a predominant respiration process in the upper part of the oxygen minimum; high rates of nitrate depletion occur in the water column between 200 and 600 m (Codispoti, 1973). Modern sediments 11 N W Mexican Margin O Z (LLM ) 0 50 100 150 200 250 0 500-1000J a. P 2000H 2500J 300-• • ^ ^ShelP • Jf " | Oxygen^Micient Zone • \ • \ • \ • \ • • \ • \ Lower Slope m • < Figure 2. A typical oxygen profile from the Eastern Tropical North Pacific adjacent to Northwestern Mexico. 12 accumulating on the Mexican Margin are organic-rich (organic carbon concentrations of >6 wt. % are common on the continental slope) and are typically laminated on the upper slope (see Chapter 3). 2. 3. STRATEGY This study focusses on a suite of box and piston cores raised in 1990 from the NW Mexican Margin south southwest of Mazatlan, while aboard the R/V New Horizon (Figure 1, Table 1). The cores comprise a shore-normal transect across the continental slope and rise. In order to assess glacial-interglacial variations in biogenic sedimentation across the margin, we selected three piston cores raised respectively from the upper slope in the oxygen minimum (see Fig. 2) (NH15P, 420 m water depth), the mid-slope below the oxygen minimum (NH8P, 1018 m depth) and the continental rise in well oxygenated water (NH22P, 2025 m depth). The reason for using multiple cores is to reconstruct a synoptic picture of sedimentation so that spatial variations in production, redistribution and preservation of biogenic components on the continental margin can be resolved. Multiple tracers are used in the reconstruction, including organic carbon, biogenic opal and Ba/Al ratios.The latter is used to define relative concentrations of biogenic barium. The barium contained in lithogenic particles yields a typical Ba/Al ratio of -.0075 (Dymond et al., 1992). Values higher than this "baseline ratio" are attributed here to contributions from biobarium. The sedimentary accumulation of organic carbon and biogenic opal as well as Ba/Al weight ratios are commonly controlled by biological production Table 1. Mexican Margin core locations. Latitude Longitude Depth (m) Piston cores NH15P 22° 41.0' N 106° 28.8' W 420 NH8P 22° 23.3' N 107° 04.5' W 1018 NH22P 23° 31.1' N 106° 31.1' W 2025 Box Cores NH1BC 22° 56.3' N 106° 26.2' W 110 NH2BC 22° 43.2' N 106° 21.6' W 133 NH3BC 22° 43.5' N 106° 17.4" W 107 NH6BC 22° 36.7" N 106° 31.1' W 620 NH12BC 22° 41.7' N 106° 27.8' W 322 NH15BC 22° 41.3' N 106° 29.0' W 425 NH11BC 22° 30.5' N 106° 17.5' W 135 NH19BC 22° 22.2' N 106° 15.2' W 97 NH7BC 22° 42.9' N 106° 25.7' W 190 NH17BC 22° 18.0' N 106° 33.1' W 785 and export of organic components from surface waters (Dymond et al., 1992; Francois et al., 1995). Thus, productivity variations through time should result in synchronous changes in these profiles. In contrast, however, preservation of individual components should foster incoherence between these records. We shall consider this here and will explore the preservational effects on organic carbon by comparing spatially the organic carbon accumulation history at the oxygen minimum site with a deeper core located below the minimum in well oxygenated waters. 2. 4 RESULTS 2. 4. 1 Chronostratigraphy The chronostratigraphy for the Mexican Margin cores was constructed using the age control points presented in Table 2. The ages for Isotope Stages 1, 2 and 3 are exclusively based on accelerator mass spectrometry (AMS) ^^C measurements made on bulk organic carbon. The ages are corrected for reservoir age (400 yr) and calibrated to U/Th age to derive calendar age (Bard et al., 1993) following the unpublished equation provided by E. Bard (personal communication, 1996): Calendar age = -5.85 X IO"6 ( 1 4 C age)2 + 1.39 ( 1 4 C age) - 1807 The published U/Th age calibration (Bard et al., 1993) is only valid for the last 20 kyr. While the new equation provided by Bard extends the range of calibration by several thousand years. Table 2. Age picks used in the construction of timescales for Mexican Margin cores. Depth (cm) Calendar Age (yrs) Age Picks NH22P NH8P NH15P 2 3105 AMS-14C age 32 6787 tt 47 10361 tt 71 13246 tt 91 16803 it 151 21152 tt 344 58960 Isotope Stage 4.0 416 73910 Isotope Stage 5.0 546 123820 Isotope Stage 5.5 596 129840 Isotope Stage 6.0 2 1571 AMS-14C age 32 3192 tt 77 6489 I I 97 9890 tt 142 15709 tt 167 16112 tt 202 17191 tt 382 23329 M 452 31864 tt 749 50210 * 2 955 AMS-14C age 62 3965 tt 112 5225 tt 142 6290 II 172 9016 tt 192 10332 tt 202 11011 tt 222 14162 tt 234 14934 tt 242 19882 ti 285 29287 II 355 32095 tt 405 41423 ti 585 58960 Isotope Stage 4.0 655 73910 Isotope Stage 5.0 785 90950 * 985 110790 * * Age picks derived by correlation of organic carbon records. See text for details. 16 The timescale for intervals older than Isotope Stage 3 was based on foraminiferal oxygen-isotope stratigraphy and the age picks were determined by visual correlation with the normalized Martinson stack, except where indicated otherwise (Table 2). Benthic foraminiferal Sl^O profiles measured on Uvigerina spp. in NH22P, and Bolivina spp. in NH8P and NH15P are given in Figure 3. The positions of age control points derived from oxygen isotope profiles are indicated in Figure 3 and the ages for isotope stage boundaries (Table 2) were taken from Martinson et al. (1987) and Imbrie et al. (1984). In core NH22P, all major isotope stages and most significant substages are well defined through Isotope Stage 6. In core NH15P, isotope stage boundaries 3.0 and 4.0 were recognized; however, the scarcity of foraminifera severely limits the resolution of the oxygen-isotope profile in Stage 5. Here, the age control points were derived by correlating the organic carbon record of NH15P with that of core NH22P (Figures 4a-c). The two prominent minima in wt. % organic carbon in Isotope Stage 5 in NH15P core were correlated with similar minima in NH22P at approximately 90 and 110 kyr. Similarly, in core NH8P, the termination of the record at -50 kyr is determined by matching organic carbon profiles (Figures 4b, c) The ages for intervals between age picks were extrapolated by using the 'Ager Program' developed at Brown University. Mass accumulation rates (MAR) shown in Figures 4a-c were calculated using the formula: MAR A (mg cm' 2 kyr"1) = [2400 (mg cm~3)} X [sedimentation rate (cm kyr"*)] [1-porosity] X [fraction A] NH15P, 420 m Water Depth 20 40 60 80 100 120 Calendar Age (kyr) 2.0. so O to u JZ mm* C <u PQ 2.5 A 3.0-4 3.5 A 4.0 A 4.5. NH8P, 1 0 1 8 m Water Depth Bolivina spp. • i I i i • I • • • 10 20 30 40 50 Calendar Age (kyr) 60 NH22P, 2025 m Water Depth i i I i i i I i i i I i i i I i i i I i i i I i i 0 20 40 60 80 100 120 140 Calendar Age (kyr) Figure 3. Benthic-foraminiferal oxygen isotope records from NW Mexican Margin piston cores. The positions of stages and substage boundaries used in the construction of the time scales are indicated by vertical lines. 18 Figure 4a-c. Biogenic records from NW Mexican Margin cores. The textured bar near the left margin indicates the presence or absence of laminations. Oxygen-isotope stages are indicated near the right margin and glacial stages are shaded. where 2400 is the assumed grain density and porosity was calculated from water content (Appendix I and II). The age-depth relationships for the cores and the linear sedimentation rates used for calculating MARs are shown in Figure 5. 2. 4. 2 Biogenic records Biogenic records from NH15P (420 m water depth) for the last 120 kyr are given in Figure 4a. Modern sediments at this site accumulate under oxygen-deficient bottom waters and the Holocene intervals exhibit fine laminations (Plate 1). Down-core organic carbon concentrations show large changes that vary in close sympathy with glacial-interglacial stratigraphy; organic carbon contents are two to four fold higher in the interglacial sediments compared to glacial deposits. The carbon-rich interglacial intervals are finely laminated, while carbon-poor glacial intervals are homogeneous. Biogenic opal concentrations follow organic carbon quite closely. MAR records of organic carbon and biogenic opal confirm that the cyclic glacial-interglacial variations seen in the concentration profiles are not artifacts of dilution. Ba/Al ratios are invariant and close to lithogenic values (.0075) throughout the core. Core NH22P (2025 m water depth) was raised from the lower continental slope. The modern sediments at this site are bioturbated and the bottom water is well oxygenated (Figure 2). Organic carbon and opal concentration and MAR records show cyclic variations that conform to glacial-interglacial stratigraphy for the last 140 kyr, organic carbon and opal contents being higher in the interglacial than in the glacial deposits (Figure 4c). Ba/Al ratios essentially 23 120 Depth (cm) Figure 5. Age-depth plots for NW Mexican Margin piston cores. The linear sedimentation rates shown represent the slopes of the individual line segments (differentiated by solid and open circles) and are used for calculating mass accumulation rates. 24-0 cm 5 cm 10 cm i5 cm e) of Core N H 1 5 P siiuwing lauiwauuiis lypicui ui the Holocene interval (A) and mottles (bioturbation) in sediments deposited during the Last Glac ia l M a x i m u m (B). 25 follow these distributions. The core is bioturbated in Isotopic Stages 1, 2, 3, 4 and 6, while Stage 5 is weakly laminated. Biogenic records from the midslope core (NH8P) are given in Figure 4b. Both organic carbon and biogenic opal concentrations and MARs are lower in the glacial than in the interglacial periods, as seen in the other cores. Ba/Al ratios in this core are close to lithogenic values in Stages 1 and 2 , and show a pronounced maximum in the lower part of Stage 3. Thus Ba/Al records are decoupled from organic carbon and opal records. Laminated intervals in this core do not show any coherence with either the distribution of biogenic components or glacial-interglacial stratigraphy. Variations in the concentrations of organic carbon and opal are spatially coherent across the margin, and conform with glacial-interglacial stratigraphy. Both opal and organic carbon contents are relatively high in interglacial intervals and low in the glacial deposits over the last 140 kyr. The MAR records of both components are equally coherent and rule out the possibility that these variations were brought about by variable dilution. 2. 5 DISCUSSION 2. 5.1 Biogenic Opal Throughout the ocean, a close positive relationship is observed between productivity in surface waters and opaline silica fluxes (Takahashi, 1989). Opal-rich sediments are clearly associated with high primary production in coastal upwelling zones (DeMasters, 1981; Calvert and Price, 1983; Calvert,1983; Nelson et al. 1995) and sedimentary opal contents have consequently been used in paleoproductivity studies on many continental margins (e.g. Mortlock et al., 1991; Shimmield et al., 1994). Opal burial on upwelling continental margins is enhanced by physical and biological factors such as pulsed upwelling which favours diatom productivity, the domination of large sized, heavily-silicified diatom assemblages, the occurrence of diatom blooms that promote aggregation and rapid sinking of diatom frustules, and grazing by larger animals and subsequent packaging of rapidly sinking fecal pellets (Nelson et al.,1995). Because sea water at all depths is undersaturated with respect to opal (Archer, 1993), dissolution of silica occurs during transit in the water column and at the sediment-water interface. Opal will continue to dissolve during burial until either the pore waters are saturated or the altered chemical composition of opaline surfaces prevents further dissolution (Kamatani et al., 1988). On upwelling margins, the shallow water column and high sedimentation rates decrease the exposure time of opal to corrosive bottom waters. Furthermore, relatively high rates of opal accumulation foster rapid saturation of pore waters, enhancing preservation of the remaining biogenic silica (Nelson et al., 1995). Thus, it may be suggested that the production and settling flux of opal in continental margin settings will increase with increasing primary production (although not in a linear fashion), and that opal preservation in sediments will be enhanced under conditions of increased supply. It follows that the higher biogenic opal concentrations and accumulation rates seen in interglacial sediments in this study probably resulted from enhanced upwelling and productivity on the Mexican Margin during late Quaternary warm periods. 27 2. 5. 2 Organic carbon The overall positive relationship between high primary production, large export fluxes of organic detritus and the enrichment of organic carbon in sediments is well documented (Berger et al., 1989). However, the conditions under which organic carbon supplied to sediments is preserved have been intensely debated in the recent literature (Calvert, 1987; Emerson and Hedges, 1988; Pedersen and Calvert, 1990; Calvert and Pedersen 1992; Hedges and Keil, 1995). Some of the factors thought to promote preservation of organic carbon in sediments include the presence of oxygen-depleted bottom waters, high sedimentation rates and a lack of bioturbation (Emerson and Hedges, 1988; Dean et al., 1994; Schlanger and Jenkyns, 1976). It has been suggested that enhanced preservation of organic matter should occur where oxygen minimum zones impinge on continental margins (Demaison and Moore, 1980), but evidence for this is not compelling (see Pedersen et al., 1992; Calvert et al., 1992; and Calvert and Pedersen, 1992). High sedimentation rates could potentially contribute to preservation by increasing the mineral surface area available for adsorption of organic molecules (Hedges and Kiel, 1995). Alternatively, if low bottom water oxygen is a controlling variable, high sedimentation rates will promote preservation by decreasing the exposure time of organic material to oxygen near the sediment-water inteface. Anaerobic conditions close to the interface preserve laminations in sediments by preventing mixing by macrobenthos (Emerson and Hedges 1988); such deposits usually host abundant organic components, so the presence of laminations in organic-rich deposits has been offered as evidence for enhanced preservation of organic carbon under oxygen-deficient conditions (e.g. Dean et al., 1994; Schlanger and Jenkyns, 1976). In contrast to these views, Calvert and Pedersen (1992) have argued that the elevated organic carbon concentrations in sediments accumulating in contact with oxygen minima on several continental margins could be attributed to the combined effects of organic carbon supply, sediment winnowing and dilution. In their view, the relationship between bottom-water oxygen concentrations and elevated organic carbon concentrations is fortuitous since the outer shelf and upper slope regions, where the oxygen minimum zone impinges on the continent, also tend to provide the quiescent conditions that favour the accumulation of organic material. Thus, they suggest that primary production and the supply of organic carbon to sediments exert the first order control on the distribution of sedimentary organic carbon. On the Mexican Margin, the organic carbon record from the oxygen minimum core (NH15P) is most sensitive to potential preservation effects due to variations in bottom water oxygen. The presence of fine laminations in the organic carbon-rich interglacial periods imply that an intense oxygen minimum existed during warm intervals, as seen today. Bioturbated glacial intervals argue for enhanced oxygenation of the water column when carbon-poor sediments were accumulating. Thus, there is a clear negative relationship between organic carbon content and bottom water oxygenation. However, the biogenic opal record in NH15P implies that higher productivity characterized the interglacial periods. Thus, one could argue that both productivity variations and preservation have influenced carbon records in this site. The organic carbon record from the lower continental slope (NH22P, 2025 m water depth) refutes this inference. The sediments at this depth are bioturbated throughout Stages 1, 2 , 3 and 4 indicating that the sediment/water interface was oxygenated sufficiently to support a population of macrobenthic fauna, conditions similar to today. Thus, the higher organic carbon concentrations in the interglacial sediments in Stages 1 and 3 in NH22P cannot be attributed to differential preservational effects (Figure 4c). The coherence between the organic carbon and opal records at this site strongly implies that the increases in organic carbon concentration instead resulted from increased productivity. Furthermore, higher organic carbon and opal concentrations are seen in interglacial periods in core NH8P (1018 m water depth) (Figure 4b), and organic carbon concentrations are not particularly elevated in the laminated intervals, which are sporadic and decoupled from glacial-interglacial stratigraphy. Ba/Al ratios The sedimentary barium content has been used in recent years as a paleotracer of export productivity (Shimmield, 1992; Dymond et al., 1992). Biogenic barite is supplied to sediments mainly in the form barite (BaSG*4) crystals (Dymond et al., 1992), which are thought to precipitate in the water column, for example within reducing microenvironments in diatom frustules where dissolved barium apparently reacts with sulphate produced from decaying tissue material (Dehairs et al., 1980; Bishop, 1988). The size distribution of barite crystals in the water column suggests that their formation primarily occurs within large particles in shallow waters and that they are released as fine particles in the oxygen minimum zone (Bishop, 1988; Dehairs et al., 1992). Since the formation of barite is mechanistically linked to the presence of biogenic particles, there exists a predictable positive relationship between export productivity in surface waters and the Ba flux to the underlying sediments (Dymond et al., 1992; Francois et al., 1995). Deposited barite is reasonably stable under sulphate-replete conditions in interstitial waters, but significant dissolution will occur under highly reducing conditions when sulphate is depleted during sulphate reduction (Brumsack, 1987; Shimmield, 1992). Like organic carbon and opal contents, Ba/Al ratios in core NH22P on the lower slope are higher in interglacial and lower during the glacials. This correlation suggests that the Ba/Al record reflects variations in the fluxes of organic detritus to the sediments in response to changes in productivity. The Ba/Al profile in this core cannot be explained by variable preservation of barite because significant dissolution of barite would produce low Ba/Al ratios (close to lithogenic values of .0075) and would obliterate the coherence between the Ba/Al record and other records of productivity. Such obliteration does occur at the OMZ site (NH15P, 420m), where sedimentary Ba/Al ratios are close to the lithogenic value (.0075) and are completely decoupled from organic carbon and opal records. This implies that substantial sulphate reduction has prevailed at this upper slope location for the last 120 kyr, and that almost complete dissolution of barite has occurred. At the mid-slope site (core NH8P), the Ba/Al ratios are close to the lithogenic values in Stages 1 and 2 and increase sharply at the base of Stage 3. Thus, the Ba/Al record is decoupled from organic carbon and opal profiles at this location. This distribution indicates that barite has been subjected to partial dissolution at this depth. In summary, the three productivity paleotracers indicate collectively that the NW Mexican Margin experienced stronger upwelling and higher primary production in interglacial than in glacial periods during the last 140,000 years. 31 2. 5 .4 Implications for the paleoclimate of western North America The strong shore-parallel south-blowing winds that drive upwelling along much of the west coast of North America are sustained in large part by the differential heating of land and the ocean. As shown in Figure 6, the resulting strong atmospheric pressure gradient is manifested as a low pressure cell over the heated landmass and a high pressure centre over the cooler ocean (Bakum and Nelson, 1991). Locally, upwelling on the western Mexican Margin is influenced by the seasonal migration of the regional low pressure centre situated over the northern Mexican mainland (Schrader and Baumgartner, 1983). When the North Pacific High is located at its northernmost position (-38° N) during the summer months, the Mexican Low is situated at the head of the Gulf of California (Figure 6). The associated surface winds flow onshore and suppress coastal upwelling during this season (Schrader and Baumgartner, 1983). In the fall, the North Pacific High weakens and migrates south to -28° N. In response, the Mexican Low retreats southward and inland. This results in shore-parallel northwesterly winds that drive upwelling along the western Mexican continental margin in winter and spring (Figure 6). During summer, the northern position of the North Pacific High brings the Mexican mainland under the influence of moisture-laden southeasterly trade winds that originate from the southern margin of the Azores/Bermuda High. Much of eastern Mexico receives rain during this period (Mosino-Aleman and Garcia, 1974). The trade winds are intersected by the high lands of the Sierra Madre Occidental, whose orographic effect leaves NW Mexico in a rain shadow. The continental northwesterlies that prevail during winter are a 32 120 W 105 W 90 W 75W Figure 6. Generalized illustration of the large-scale atmospheric circulation and seasonal coastal upwelling on the west coast North America. meagre source of moisture, which renders NW Mexico and SW United States arid throughout the year (Mosino-Aleman and Garcia, 1974). The data presented here suggest that this modern picture was quite different during glacial episodes. Lower rates of accumulation of biogenic components in the glacial deposits argue for decreased upwelling. By analogy with the modern seasonal cycle, this suggests that during the glacials the northwesterly winds could have been suppressed and summer-type conditions could have dominated, reducing upwelling off Western Mexico. This scenario would require coincident wetter conditions on the eastern Mexican mainland. Several lines of evidence suggest that this analogy is invalid. First, pollen records from DSDP Site 480 in the Gulf of California document cooler and wetter conditions in northwest Mexico during the Last Glacial Maximum, along with reduced upwelling (Byrne, 1982). Second, extensive terrestrial paleoclimatic records from MesoAmerica (Thomson et al., 1993; Allen and Anderson, 1993; Bradbury, 1989; Bradbury, in press) comprising lake level, pollen and packrat midden histories, indicate that southern California, NW Mexico, Arizona and Texas in general experienced wetter conditions and enhanced winter precipitation during the LGM, while sites in eastern Mexico bordering the Gulf of Mexico and Caribbean Sea record drier conditions. This glacial-age geographic contrast cannot be explained by the presence of prevailing southeasterly winds and an enhanced Atlantic moisture source. Indeed, the terrestrial paleoclimatic data strongly argue for a winter moisture source from the Pacific. Considering the terrestrial information in unison with our marine records, it appears that wind patterns in the western Mexican region were fundamentally different during glacial periods compared to today. Modelled atmospheric circulation for the last glacial (Kutzbach et al, 1993; COHMAP members, 1988) implies that the cooling of the continent and the large Laurentide ice cap strongly influenced atmospheric circulation over North America. During this time, a "permanent" high-pressure cell was maintained over the continental ice sheet which reversed the atmospheric pressure gradient between the land and the ocean. Consequently, the North Pacific High was much weaker and almost absent in winter. Thus, the models suggest a reduction in shore-parallel northwesterly surface winds along the western margin of North America. In addition, the presence of the ice sheet split the jet stream into two branches, according to the model results. The southern branch of the split jet stream flowed on to the North American continent at ~30° N, some 20° south of its present position. The split jet stream would have resulted in onshore winds from the Pacific and precipitation in much of the SW United States and NW Mexico. In this postulated scenario, summarized in Figure 7, wetter conditions onshore would have occurred contemporaneously with reduced upwelling in the coastal waters off NW Mexico. Thus, our marine data are fully consistent with both the terrestrial records and these model simulations, implying that the waxing and waning Laurentide ice sheet over the last 140,000 years exerted a major influence on regional wind patterns as far south as NW Mexico. The GCM simulations suggest that areas off central and southern California, where today the winds are upwelling-favorable year-round due to the influence of North Pacific High, and areas north of California, where upwelling occurs in summer months when the Aleutian Low weakens (Figure 6), should have experienced reduced upwelling during the glacial periods. The sparse data available on the late Quaternary history of productivity and Figure 7. A schematic of the postulated glacial-interglacial contrast in atmospheric circulation, precipitation and ice cover over the Western Hemisphere. The 18Ka (Last Glacial Maximum) scenario integrates information from marine sediment records, terrestrial paleoclimate records and general circulation models. upwelling off western North America support this suggestion. For example Lyle et al. (1992) reported lower productivity off southern Oregon based on biogenic sedimentary records which they attributed to diminished upwelling-favorable winds during the Last Glacial Maximum. Reduced organic carbon and biogenic opal contents in glacial-age sediments compared to interglacial deposits have also been reported off Northern California (Dean et al., 1994). Thus, the emerging picture indicates widespread reduction in upwelling on the western continental margin of North America during glacial periods. In addition, late Quaternary sediments deposited on the upper continental slope off western North America often tend to be laminated during warm periods and massive (homogeneous) during glacials, indicating a less severe oxygen minimum and bioturbational mixing during cold periods. For example, high-resolution observations made on cores retrieved by the Ocean Drilling Program from Santa Barbara Basin (Site 893A, sill depth 475 m) and from the Gulf of California (Site 480, 655 m water depth) show such variations (Kennett and Ingram, 1995; Keigwin and Jones, 1990), which are strikingly similar to the alternating laminated interglacial and bioturbated glacial character of the deposits in Core NH15P at 420 m water depth. Collectively, these observations strongly suggest that paleo-oxygen fluctuations in the water masses that contact the upper continental slope off western North America have been regional in extent. The remarkable correspondence between higher rates of accumulation of biogenic components in the Mexican Margin sediments and the preservation of laminations in Core NH15P suggests that the historical bottom-water oxygen content may have been driven by local or regional variations in upwelling and primary production. Reduced export productivity during the glacial periods should have decreased the consumption of oxygen in subsurface water masses. Indeed, sedimentary nitrogen isotope studies suggest that water-column denitrification was diminished in the oxygen-deficient subtropical subsurface water masses of the Eastern Tropical North Pacific (Ganeshram et al., 1995). The inferred periods of reduced denitrification correspond very closely to intervals when bioturbated sediments poor in biogenic components were deposited on the upper continental slope (See Chapter 4). In apparent contrast to Core NH15P, NH8P retrieved from 1018 m exhibits laminations mostly in the glacial intervals, indicating that the history of oxygenation of the intermediate (mid-slope) waters is decoupled from that in the overlying subsurface waters. This argues against the contention made in previous studies that the inferred paleoxygen fluctuations are related to the source and age of intermediate water masses (Kennett and Ingram, 1995; Behl and Kennett, 1996). 2. 5. 5 Implications for atmospheric pCO? Several studies have highlighted the role of upwelling and associated productivity in influencing the carbon budget of the ocean and atmosphere, the burial fluxes of carbon in sediments and temporal changes in the chemistry of intermediate and deep waters in the ocean. Sarnthein et al. (1988) and Sarnthein and Winn (1990), for example, compiled synoptic data sets of sedimentary carbon accumulation and showed that glacial productivity in low- and mid-latitude upwelling zones was generally higher during the LGM. They argued that this resulted from elevated wind stresses associated with intensified meridional surface winds, and they suggested that the higher glacial productivity played a key role in lowering glacial pCC»2 by increasing the transfer of carbon from the surface to the deep ocean via sinking organic particulates. The new picture emerging from upwelling regimes, however, is that the response of upwelling zones to glacial-interglacial cycles is areally variable and may not have contributed so directly to the glacial pCC»2 drawdown. For example, export productivity increased on the western margin of the Americas during the last deglaciation (Reimers and Suess, 1983a; Schrader and Sorknes, 1991; Lyle et al., 1992 and this study) while decreasing at the same time in the eastern equatorial Pacific (Pedersen, 1983; Pedersen et al., 1988; Lyle et al., 1988). Furthermore, productivity during the LGM in the northwest Arabian Sea was similar to that today (Sarnthein et al., 1988) or even slightly lower (Shimmield, 1992). Thus, variations in regional forcing impose temporal changes in upwelling and export production that may well differ from global scale glacial-interglacial variability. The importance of regional forcing mechanisms is well illustrated by comparing the response of coastal upwelling regimes (eastern boundary currents) in the Atlantic and the Pacific to glacial climatic shifts. In sharp contrast to the Pacific, the eastern boundary current regimes in the Atlantic experienced increased productivity during the glacials, e.g. off NW Africa (Miiller et al., 1983). This difference may reflect the absence of a significant ice sheet (equivalent to the Laurentide) on the Eurasian landmass during the glacial that could have influenced regional wind fields in the Atlantic. Such geographic contrasts demand that determination of the response of upwelling zones to glacial climatic forcing be made on a regional scale. Although progress continues to be made, current geographic coverage is clearly insufficient to yield a globally integrated assesment of the glacial-interglacial change in export productivity and to estimate its influence on glacial-C02 drawdown. 2. 6 CONCLUSIONS In summary: 1) during the last 140 kyrs the record of biogenic sedimentation on the NW Mexican Margin indicates a decrease in upwelling and productivity during glacial periods; 2) regional changes in atmospheric circulation associated with the waxing and waning of ice on North America appear to have been fundamental in controlling upwelling and export productivity on the NW Mexican Margin; and 3) The regionally variable response of upwelling to glacial climatic forcing argues against the notion that a globally uniform increase in upwelling contributed directly to the glacial pC02 drawdown. Chapter 3 Glacial-interglacial Variability in Carbon Burial off NW Mexico 3. 1 INTRODUCTION The burial of organic matter in continental margin sediments is a key component of the global cycles of both carbon and oxygen as well as in the formation of petroleum source rocks. Therefore, it is imperative to understand the relative importance of the factors that control the sedimentary accumulation of organic matter in margin settings. The conditions under which organic matter supplied to sediments is preserved have been intensely debated in the recent literature (Calvert, 1987; Emerson and Hedges, 1988; Pedersen and Calvert, 1990; Calvert and Pedersen 1992; Canfield, 1994; Hedges and Keil, 1995). Central to this controversy is the question "How important are oxygen-deficient bottom waters in fostering enhanced preservation of organic carbon in sediments?" High rates of organic carbon accumulation in the past at shallow paleodepths have been widely attributed to preservation under oxygen-deficient conditions (e.g. Schlanger and Jenkyns, 1976; Thiede and Van Andel, 1977; Arthur et al., 1987). According to this view, the preservational effect of the oxygen minimum zone, where it impinges continental margin, is considered to be pivotal in enhancing organic carbon accumulation in sediments. The argument for enhanced preservation is based on the premise that anaerobic bacteria are not as efficient as aerobes in degrading organic matter. As a result, proportionately more organic matter escapes degradation under anoxic conditions to be preserved in sediments (Emerson and Hedges, 1988 and references therein; Canfield, 1994 and references therein). The inefficiency of anaerobic degradation of organic matter may be manifested by intrinsically slower anaerobic respiration rates and/or the inability of anaerobes to oxidize the full range of organic molecules. The traditional view of the depositional environment of petroleum source rocks requires enhanced preservation of hydrocarbons under oxygen-deficient settings (Demaison and Moore, 1980; Tissot and Welte, 1978). Here, the survival and subsequent burial of the relatively labile hydrocarbon-rich organic fraction during anaerobic degradation is viewed as an essential prerequisite for the development of petroleum-generating source beds. Sediments deposited in modern continental margins in contact with an intense oxygen minimum have been proposed as one modern analogue that meets the traditional criteria (Demaison and Moore, 1980; Demaison et al., 1984). It has also been suggested that such deposits should yield high Rock-Eval hydrogen indices (HI), a parameter that correlates with the elemental H / C ratio of organic matter. High HI values signify well preserved organic matter (Espitalie, et al., 1977; Tissot and Welte, 1978; Peters, 1986). Carbonaceous fades in geological records are often laminated and exhibit higher HI values than organic-carbon-poor bioturbated deposits that may be intercalated in the same sedimentary sections (Arthur et al., 1984; Pratt. 1984; ten Haven et al., 1990; Ingall et al., 1993). Laminations imply deposition under oxygen-deficient conditions that are hostile to benthic infauna. Thus, the combination of high carbon contents, laminations and high HI values has been typically interpreted as fundamentally reflecting enhanced preservation under anoxic conditions. A comparison of this view with the organic carbon distribution in the modern ocean reveals deficiencies, however. Recent investigations have shown that mid-depth carbon maxima on several continental margins either extend over a larger depth range than the regional oxygen minima or are in some cases completely decoupled (Calvert and Pedersen 1992; Pedersen et al., 1992). Thus, factors other than preservation, such as shore-normal gradients in primary production and water depth that affect the supply of organic matter, textural effects due to winnowing, and dilution by other sedimentary components have been invoked to explain the observed organic carbon distribution (Calvert and Pedersen 1992, Pedersen et al., 1992). As an alternative to fundamental control by preservation, it has been proposed that primary production and the associated supply of organic matter to sediments is the first order determinant on the distribution of sedimentary organic carbon contents on continental margins (Calvert, 1987). In support of this proposal, recent investigations have failed to reveal significant differences in hydrocarbon-richness (HI values) among contemporary sediments accumulating within, above and below the oxygen minimum. (Calvert et al., 1992; Pedersen et al., 1992; Calvert et al., 1995). Thus, there is an discrepancy between the depositional conditions typically inferred for carbonaceous facies in geological records and the modern environments that are suppose to provide an analogue. This chapter takes the debate about the factors that influence the burial of organic carbon on continental margins one step further by examining the quality of the organic matter preserved in NW Mexican Margin sediments. The specific questions to be addressed are: 1) What are the factors that are important in determining organic carbon burial in this area, where the oxygen minimum is exceptionally severe?; 2) Is the buried organic matter initially deposited in contact with the oxygen minimum zone relatively more enriched in hydrocarbons?; and 3) How important is preservation in influencing variations in organic carbon accumulation generally on continental margins? The oxygen mimimum in this area is one of the most severe in the world's oceans and extends over a depth range of ~150 to ~800 m (Figure 2). Molecular oxygen is often unmeasurable in the bottom waters that impinge the upper slope between 200 and 600 m. The sediments that accumulate between the shelf break at -200 m and about 2 km depth in this area are fine-grained muds, pale to olive green in colour that often exhibit a brown surface layer up to a few mm to several cm thick enriched in Fe and Mn oxides. These deposits grade from clayey silt to silty clay with increasing water depth and are classified as "slope fades" by Van Andel (1964). These sediments typically exhibit fine laminations on the upper slope, where they are in contact with the oxygen-deficient zone. The upper boundary of the oxygen minimum coincides approximately with the shelf break, which is situated at depths lightly shallower than 200 m. Inner shelf deposits, occurring at depths shallower than - 100 m, are olive green mottled sands containing significant amounts of glauconite and foraminifera in the sand fraction and are classified as "littoral fades" by Van Andel (1964).These littoral sands may contain at places coarse shell fragments, pumice and phosphorite nodules. These deposits do not represent the modern upwelling fades (Van Andel, 1964) and are similar to the 'relict or residual' deposits commonly reported on other upwelling continental margins (Suess and Thiede, 1983b). Because of their incohesive nature, these sand deposits do not provide good quality cores and therefore were not sampled for this study. Modern sedimentation on the NW Mexican shelf is therefore limited to the outer shelf in water depths deeper than ~100 m. Deposits in this setting are homogeneous to mottled, olive green clayey silts. These deposits are classified as "shelf fades" by Van Andel (1964). These sediments are characteristically poorer in organic carbon, richer in calcium carbonate and coarser in texture than the slope deposits. 3. 2 LATE QUATERNARY RECORDS OF CARBON BURIAL Organic carbon burial is described using three piston cores raised respectively from the upper slope in the oxygen minimum (NH15P), the mid-slope below the oxygen minimum (NH8P) and the lower slope in well oxygenated waters (NH22P) at water depths of 420 m, 1018 m and 2025 m respectively. 3. 2.1 Organic carbon records Profiles of wt. % organic carbon, hydrogen indices, and lithology (i.e. the laminated or bioturbated character of the deposits) are shown in Figure 8. On the upper slope (420 m water depth), variations in organic carbon closely correspond to the presence or absence of laminations (Fig. 8a); the laminated intervals are carbon-rich (> 2.5 wt. %) and bioturbated intervals are carbon-poor (< 2 wt. %). On the mid slope (1018 m water depth), variations in organic carbon are independent of texture and the laminated intervals occur sporadically (Fig. 8b). Similarly, on the lower slope (2025 m water depth) variations in organic 45 Figure 8a-c. Records of % organic carbon and hydrogen indices (HI) from NW Mexican Margin cores. Laminated intervals are shaded. 46 (IA>[) aSy J r e p u a r e ^ carbon are independent of the presence or absence of laminations (Fig. 8c). Here, laminations are confined to the organic carbon maxima that characterize much of Isotope Stage 5. Variations in organic carbon contents are largely synchronous in the three cores and closely correspond to glacial-interglacial stratigraphy, such that interglacial stages are organic carbon-rich and glacials are organic-carbon-poor. Mass accumulation rates of organic carbon indicate that the observed variations in organic carbon concentrations are largely independent of dilution effects (Chapter 2, Figures 4a-c). 3. 2. 2 HI records Down-core variations in hydrogen indices on the upper slope (420 m) correspond to changes in organic carbon records and lithology (Fig. 8a). Laminated carbon-rich intervals are enriched in hydrocarbons (HI values >300), while bioturbated intervals are carbon poor and relatively impoverished in hydrocarbons (HI values 200-250). In contrast, the HI profiles in the deeper cores are largely independent of the presence or absence of laminations. At the mid-slope site (Fig. 8b), down-core increases in organic carbon are matched by higher HI values but both are decoupled from lithology. On the lower slope (Fig. 8c), the HI profile is essentially invariant and is independent of both the organic carbon content and the presence or absences of laminations. Thus, hydrogen indices are not related to variations in organic carbon content and lithology in the same fashion in each of these cores. The Rock-Eval pyrolysis method involves programmed heating of 100 mg of sample from 300 to 550 °C in a helium atmosphere and measuring the evolved pyrolyzates (Espitalie, et al., 1977; Tissot and Welte, 1978; Peters, 1986). The first peak in the pyrogram (SI) represents milligrams of hydrocarbons that can be thermally distilled from one gram of sediments at 300 ° C and is proportional to the organic fraction that can be extracted from sediments with organic solvents. The second peak (S2) represents milligrams of hydrocarbons generated per gram of sample by pyrolytic break down of kerogen (the fraction of organic carbon that is insoluble in organic solvents, acids and bases) between 300 and 550 °C. SI is a small fraction and is shown to be roughly proportional to the amount of organic material present in sediments. S2 normalized to total organic carbon yields the hydrogen index (mg hydrocarbon per gram of total carbon), which is proportional to the atomic H / C ratio of organic material in the sample (Espitalie, et al., 1977). A set of samples with the same type and quality but varying concentrations of organic material will have the same HI value. Such a suite can be described by a single regression line in an S2 vs. TOC plot. The slope of the line times 100 will yield the mean HI value for the suite. S2 vs.TOC plots for the Mexican Margin cores are given in Figure 9. Samples from NH22P essentially define one regression line; there is no significant difference between laminated and bioturbated sediments, which is consistent with the invariant HI profiles shown in Figure 8c. This result implies that the composition of the organic material in these sediments, in terms of the hydrocarbon richness, is similar despite marked variations with time in the concentration of organic matter. The S2 vs. TOC plot for NH8P sediments (Figure 9) also shows that the hydrocarbon richness of organic material in the laminated and bioturbated 51 TOC (Wt. %) TOC (Wt. %) Figure 9. S2-TOC plots from Mexican Margin piston cores. S2 (hydrocarbons generated from kerogen during pyrolysis) and TOC (total organic carbon) are determined by Rock-Eval pyrolysis. Open and filled circles represent bioturbated and laminated intervals, respectively. intervals is similar over a wide range of organic carbon concentrations. Since this is the case, then the HI profiles from core NH8P should be invariant and independent of organic carbon concentration, as seen in core NH22P. The fact that this is not the case (Figure 8b) demands an explanation. The high HI values in organic carbon-rich intervals in NH8P samples are due to the fairly large negative y-intercept in the S2 vs. TOC plot. Regression lines in S2 vs. TOC diagrams should ideally pass through the origin because even a small amount of organic material should yield hydrocarbons during pyrolysis. However, negative y-intercepts are common in S2 vs. TOC plots and are attributed to matrix adsorption of hydrocarbons (Katz, 1983; Espitali£ et al., 1984; Langford and Blanc-Valleron, 1990). Clay minerals are known to adsorb hydrocarbon compounds released by pyrolysis, the amount adsorbed varying with the quantity and type of clays present in sediment samples (Dembicki, 1992; Espitalie et al., 1984). Since the y-intercept is a measure of the hydrocarbons sequestered by the mineral matrix for a particular suite of samples, it can be used to correct HI values for matrix adsorption (Langford and Blanc-Valleron, 1990). Such a correction is demonstrated in Figure 10 for the NH8P samples. The matrix-corrected HI profile is plotted in Figure 11, which shows that HI values are indeed invariant and independent of variations in organic carbon, as seen in Core NH22P. The matrix correction in Figure 10 does not change the slope of the regression line, which yields a mean HI of ~420 (100 times the slope) irrespective of the presence or absence of an intercept. Thus, matrix corrections are not necessary if a mean HI value is to be deduced from the slope of a S2 vs. TOC plot for any given suite of samples. 53 5 0 0 2 4 6 8 10 T O C ( W t . %) Figure 10. Matrix correction for core NH8P. Matrix-corrected hydrogen indices (HI) values, shown in open squares and represented by the solid regression line, has been deduced by removing the y-intercept from the dotted regression line representing the uncorrected HI values shown in circles. The bioturbated and laminated sediments are donated in open and closed circles, respectively. 54 Figure 11. Records of % organic carbon, hydrogen index (HI) and matrix-corrected HI from Core NH8P. Laminated intervals are shaded. The S2 vs. TOC plot for NH15P (Figure 9) contrasts with those of the other two cores. The laminated and bioturbated sediments in NH15P yield unique regression lines with mean HI values of 360 and 300 respectively. Thus matrix correction merely reduces the contrast in HI values between laminated and bioturbated sediments seen in Figure 8a. In summary, the hydrocarbon richness of sediments accumulating on the mid- and lower slope of the Mexican Margin below the oxygen minimum has remained fairly constant with time despite significant variations in bioturbation and organic carbon burial. In contrast, the hydrocarbon richness of sediments deposited on the upper slope in contact with the oxygen minimum has varied in concert with changes in organic carbon accumulation and the degree of bioturbational mixing. Here, laminated carbon-rich sediments host organic material that is enriched in hydrocarbons relative to the bioturbated carbon-poor intervals. The probable explanation for this will be discussed in the following sections. 3. 2. 3 Source of organic matter Variations in the source of organic matter, in terms of marine versus terrestrial, could affect the hydrogen-richness of organic material preserved in sediments. This is because organic matter derived from terrestrial vascular plants is relatively poor in hydrocarbons in comparison to marine-sourced lipid-rich algal remains. To evaluate how much land-derived organic material is present, C0rganic/Ntotal ratio determinations, S^Corganic measurements and lignin biomarkers assays have been made and are described in the following paragraphs. 57 Marine organic material produced in low to mid-latitudes yields S^Corganic values between -18 to -22 %o PDB, while the Sl^Corganic of terrestrial C3 plant material ranges between -26 and -28 (Rau et al., 1989, Deines et al., 1980, Fontugne and Duplessy, 1981; Freeman and Hayes, 1992). Downcore profiles of Sl^Corganic ^ o r t n e Mexican Margin cores are given in Figures 12 a-c. The mean S^^Corganic values fall within the marine range for both the laminated and bioturbated deposits, indicating an overwhelmingly marine origin of the organic matter in both of these sediment types. In addition, 8 l 3 c o r g a n i c variations in all of the three cores are largely independent of variations in organic carbon content. Corganic/Ntotal weight ratios in marine plankton are 6 to 7, while terrestrial plants typically exhibit values >20 (Hedges et al., 1986). However, Corganic/Ntotal ratios in sediments containing predominantly marine-derived organic material can range from 5 to 13 (e. g. Muller, 1977; Fontugne and Calvert, 1992; Calvert et al., 1995; Muller et al., 1994). The lower values are usually only found in organic-lean sediments, such as pelagic clays, where ammonium ions incorporated into clays, such as illite, make a significant contribution to N contents (Muller, 1977). Higher Corganic/Ntotal weight ratios of predominantly marine-derived organic matter are attributed to the preferential bacterial degradation during diagenesis of labile N-bearing organic compounds, such as proteins and amino acids (Rosenfeld, 1981; Hedges et al., 1986). Corganic/Ntotal weight ratio profiles from the Mexican Margin show a general secular increase down core (Figures 12a-c) which is consistent with the preferential degradation of N-bearing compounds during burial. The exception 58 Figure 12a-c. Records of 813C0rganic and Corganic/Nfotal ratio from NW Mexican Margin cores. Laminated intervals are shaded. 59 60 ^ I I I I J I I • I In. i,.,,,! »• I ,*,,„> I Si I - 2 rH rj 00 00 X pa 2 J D ^ PH P H 1 H o « cj ^ rH OO CM ^ O U •*< -a o •oo U _Q ft I .5 .2 § pa H J I I I I I I I I I I i I I I I I I I I I I o o o o © o rH <N CO ^ ITi vo n Ifi CM O PC n X op-, Q O w CM O U CD r-l o Z u o U •v V % l l s 3 'S pa I-J I I I I I I I I I I I I I I i i' o o o CM ^ V4D 1 1 1 I s § O CM o to this secular trend is the large peak between 120 to 140 kyr at the base of NH22P (Fig. 12c), where C0rganic/Ntotal ratios exceed the marine range. Plots of Corganic/Ntotal vs. S^Corganic exploit the compositional differences between terrestrial and of marine organic material, and can be used to distinguish their relative contributions to sediments (Jasper and Gagosian,1990; Fontugne and Calvert, 1992; Muller et al., 1994). In sediments that host variable proportions of marine and terrestrial organic material, these parameters should fall on a mixing line that shows a negative correlation between Corganic/Ntotal and Sl3corganic- Such plots for the Mexican Margin cores (Figure 13) show no such linear relationships indicating that variations in these two parameters are independent of each other. Note that the high Corganic/Ntotal values in the base of NH22P (Fig. 12c) do not show correspondingly lighter Sl^Corg^ic values. The dominant input of marine organic matter is further supported by lignin biomarker analyses (Table 3). Vanillyl and syringyl index phenols are exclusive components of land-derived lignins (Hedges and Parker, 1976). In the Mexican Margin, A (defined as the ratio of vanillyl and syringyl index phenols per 100 mg of organic carbon) ranges from <0.1 to ~0.5. However, even the maximum ratio observed represents a very small terrestrial component (see Hedges and Parker, 1976); thus the low relative levels indicate that inputs of terrestrially-derived organic matter must be insignificant in these deposits. This conclusion is fully consistent with the carbon isotope and Corganic/Ntotal data. Note that the high C0rganic/N total ratios at the base of core NH22P (Fig. 12c) are not associated with a corresponding increase in A values. 63 ^ 1 6 . o 1l2-J 2 10-U 16. 1 12-1 S 10-1 3> . % o (to PDB) organic NH8P, 1018 m water depth - i — | — i — I — i — | — i — | — i — | — i — | — i — r -23 -22 -21 -20 8 1 3 C . %c (to PDB) organic NH22P, 2025 m water depth 1—I—1—I—1—R 23 -22 8 1 3 C - i — | — i — I — r --21 -20 %c (to PDB) •19 -19 organic Figure 13. The relationship between Corganic/Ntotal and 5 Inorganic m Mexican Margin piston cores. 64 Table 3. Results from Lignin Biomarker (A) analysis. (Data provided by Dr. Greg Cowie) Depth(cm) Age (Kyr) 8^3Corganic (%c to PDB) A (mg/100 mg Corganic) NH15P 51 3.4 -20.803 0.12 112 5.2 -20.717 0.068 162 8.1 -20.970 0.078 212 12.6 -21.553 0.281 242 19.9 -21.156 0.523 242 19.9 -21.156 0.532 285 29.3 -20.669 0.297 335 31.3 -20.917 0.432 335 31.3 -20.910 0.537 NH22P 22 5.56 -20.790 0.109 61 12.0 -21.038 0.152 66 12.6 -21.178 0.169 126 19.3 -20.085 0.160 174 25.7 -19.819 0.204 484 100.2 -20.253 0.123 554 124.8 -21.242 0.223 669 138.6 -20.975 0.153 The highest and lowest A values are found respectively in bioturbated and laminated sediments in the upper-slope core NH15P. This implies a slightly higher proportion of terrestrial organic matter in bioturbated intervals. This might have resulted from either an increase in the supply of terrigenous organic material or a lower input of marine organic material during the glacial low sea stand. One explanation for the lack of a similar trend in 5^^Corganic values in this core may be the presence of C4 plant debris. C4 plants, such as sea grasses, have heavy 5^^Corganic values (typically -14; Deines et al., 1980). However, inputs from C4 plants are important only in limited areas, such as some mangroves, coastal lagoons and reef tracts. Coastal marsh lands and mangroves occur in NW Mexican Margin south of Mazatlan in areas adjoining Costa De Nayarit (Curray and Moore, 1964). These regions could have been the source of C4 plant material to nearshore areas during the glacial low sea stands. Alternatively, S^Corganic values may not be sensitive to small changes in relative proportions of marine and C3-terrestrial inputs. The subtle variability in the S^Corganic P r ° f u e s produced by the apparent small variation in sources may have been obliterated by factors such as isotopic fractionation due to variations in surface water temperature, photosynthetic uptake of C O 2 by marine algae and diagenetic alterations (Rau et al., 1989, Fontugne and Duplessy, 1981; Freeman and Hayes, 1992, Fontugne and Calvert, 1992). In summary, organic material preserved in the Mexican Margin cores is of predominantly marine origin and variations in concentrations seen in the organic carbon profiles cannot be attributed to variable inputs of terrigenous organic matter. In core NH15P, the lignin biomarkers indicate a slight increase in the relative proportion of terrestrial organic matter in bioturbated intervals, corresponding with low HI values. 66 3. 2. 4 Influence of sediment texture Negative correlations between grain size and organic carbon content are common in continental margin sediments (eg. Premuzic et al., 1982, Shimmield et al., 1990, Calvert et al., 1995). This association has been attributed to the hydrodynamic equivalence of organic material and fine grained clays (Trask, 1953) and to the increased adsorption of organic material on fine-grained detrital particles, which have larger surface areas (Muller at al., 1977, Keil et al., 1994; Mayer, 1994). Variations in grain size have also been shown to be related to the hydrogen-richness of organic material, such that fine grained sediments typically have higher hydrocarbon contents than coarse-grained deposits (e.g. Wiessner et al., 1990; Pedersen et al., 1992), Thus, variations in sediment texture can influence both the concentrations and hydrocarbon-richness of organic material accumulating on continental margins. Variations in texture are evaluated in this work by using Z r / A l ratios and Sinonbio/Al ratios. Zr is a geochemically incompatible element that does not substitute for any of the major cations present in the lattices of aluminosilicates. Zirconium occurs in sediments almost exclusively in zircon (ZiSiC»4), which is very stable and resistant to weathering. Zircon preferentially accumulates in coarse-grained detrital deposits (Krauskopf, 1979); thus, Zr /Al ratios in marine sediments can be used to evaluate the relative abundance of the coarse fraction (Calvert and Price, 1983). Zircon is common in the heavy mineral fraction of NW Mexican Margin sediments (Van Andel, 1964). As a result, the Zr/Al ratio correlates well with the sand content (> 63 |xm size fraction) (Figure 14). The provenance of the zircon grains is thought to be the granitic batholiths that crop out in and around the Mazatlan area (Van Andel, 1964). It would therefore be expected that the coarse fraction in these deposits should also be enriched in quartz. That this is evident is shown by the positive correlation between the sand content and Sinonbio/Al ratios, where Sinonbio is defined as the difference between Si total and Siopal-Zr /Al and Sinonbio/Al profiles from the Mexican Margin are shown in Figures 15a-c. On the upper slope (core NH15P), grain-size proxies vary remarkably in sympathy with changes in lithology and organic carbon concentrations. Carbon-rich laminated sediments have low ratios and are thus finer grained while carbon-poor bioturbated intervals are coarser grained (higher ratios). At the deeper sites, down-core variations in Z r / A l and Sinonbio/Al are minimal and show no systematic variations. Zr /Al vs. organic carbon plots (Figure 16) illustrate the negative correlation between grain size and organic carbon concentrations on the upper slope. Weight % organic carbon and Zr /Al are weakly correlated at the deeper sites, where the Zr /Al ratio is almost invariant. These data suggest that variations in grain size have influenced organic carbon burial only on the upper slope. 3. 3 DISCUSSION 3. 3. 1 Factors controlling the burial of organic carbon in the Late Quaternary The accumulation of organic carbon on the NW Mexican Margin has varied in close conformity to glacial-interglacial cycles. As noted earlier, organic 68 45 z i • 1 • 1 > 1 • 1 • 1 0 10 20 30 40 50 Wt. % Sand (>63 |im size fraction) Figure 14. Relationships among grain size, Zr/Al and Sinonbio/Al ratios in Core NH15P. 69 Figure 15a-c. Records of grain size proxies and % organic carbon from NW Mexican Margin cores. Laminated intervals are shaded. 70 71 C M 0> u s 00 PH 00 2 o 2 mO C i i o ti • rH CD CM-J I I l ,| I I • • • -P oo H 60 IN O o £ . O fN rH \- © <N N l • ' o I i i o fN O CO O o in o VO 5 a. m CM o CM CM CM X z o 2 * H lw1 •i J 1 * • • 1 • 1 • • • 1 ' T * J U . in-i 60 M CMH o * J CD > L in PQ "-J © CM i i i | i i i | i O o © GO O o © CM © 73 1 2 H NH22P, 2025 m water depth § 8 x> n U KJ 4 Ci «« 60 i-t O 0 10 'y = 3.8057 + -0.031407X R = 0.0036781 T T I ' I ' I 15 20 25 30 35 Zr/Al X 104 (Wt. Ratio) 40 Figure 16. Relationship between organic carbon contents and Zr/Al ratio in NW Mexican Margin piston cores. carbon mass accumulation rate profiles are similar to those of wt. % organic carbon indicating that dilution effects have not influenced these records to an appreciable degree. Down-core variations in organic carbon contents are largely synchronous in the three cores indicating that the fundamental factor(s) that produced the cycles operated simultaneously at all three sites. The previous comparison of wt. % organic carbon, biogenic opal and Ba/Al ratio profiles (Chapter 2) revealed that the three components varied synchronously, except where barite dissolution occurred (Chapter 2, Figures 4a-c). Since accumulation of organic carbon, biogenic opal and barium are related by the export flux of organic detritus, the observed synchrony can be fundamentally attributed to variations in primary production, forced by glacial-interglacial climatic change. The invariance in HI profiles at the the lower and mid slope sites (NH22P, 2025 m and NH8P, 1018 m ) rules out the possibility that variations in organic carbon burial have been induced by differential preservation of organic matter. The quality of the preserved organic material (i.e. its hydrocarbon richness) is similar over a wide range of organic carbon concentrations. Furthermore, in these cores the constancy of sediment texture indicates that winnowing processes did not cause the observed differences in organic carbon burial. Thus productivity-induced variations in the supply of organic carbon to sediments is the only factor that can explain the synchronous changes in carbon accumulation on the mid and lower continental slope. The organic carbon profile from the upper slope, within the oxygen minimum (NH15P, 420 m), shows changes in concentrations that are also synchronous with the deeper cores. However, in contrast to the deeper sites, the organic carbon concentrations vary in concert with the hydrogen index, sediment texture and the presence or absence of laminations. Although a decrease in the supply of organic carbon to the sediments during the glacial episodes is adequate to explain the lower organic carbon concentrations, it cannot account directly for the lower HI values. Assuming that the bioturbated character of these deposits indicates increased oxygenation of the bottom waters, it could be argued that the variations in organic carbon contents reflect differential preservation. The crux of such an argument would be that increased degradation of organic material under oxygenated conditions lowered both the quantity and quality of organic matter preserved during glacial periods. A comparison of the HI values from the oxygen minimum site with the deeper cores argues against the preservation hypothesis. For example, the mean HI values of laminated sediments from the oxygen minimum site (NH15P; HI = -360) are not substantially higher than mean HI values at the more oxygenated sites on the lower and mid slope: the mean HI value in NH8P is higher (-420) while that in NH22P is similar (-330) to that in NH15P. Thus, the highest HI values are found on the mid slope below the oxygen minimum, where bioturbated sediments are accumulating at present. This implies that the slightly lower HI values in the glacial sediments at the upper slope site resulted from some factor other than increased bottom-water oxygen concentrations. Variations in grain size are unique to the upper slope site. The negative correlation between grain size and wt. % organic carbon implies that sediment winnowing reduces organic carbon accumulation (Figure 16). Thus, during the glacial periods, sediment winnowing on the upper slope probably contributed to the lower organic carbon concentration in these sediments, in addition to decreased primary production. This is quite conceivable given that modern sediments deposited on the NW Mexican shelf are subject to intense winnowing; during the glacial low sea stand, the then-shallower upper slope may also have come under such an influence. Variations in matrix-corrected hydrogen indices are also unique to the upper slope site, where coarser-grained glacial deposits host hydrocarbon-impoverished organic matter. Thus, increased winnowing is one explanation for the lower HI values in the glacial deposits. In addition, a slight increase in the relative proportion of terrestrial organic matter as indicated by lignin biomarkers may also have contributed to the lower HI values in the glacial deposits. 3. 3. 2 Factors controlling the burial of organic carbon in Recent sediments Measurements made on a suite of box cores (location shown in Figure 1) are used here to study carbon burial in modern sediments. Because sedimentation rates on the margin are in general high (see Chapter 2), the short box cores (<30 cm) represent deposits that accumulated within the last few thousand years. Box cores were not collected from the lower part of the continental slope. To represent this region, the top thirty centimetres of piston cores NH8P and NH22P are used. The tops of the piston cores might represent intervals slightly older than the box cores due to surface sediment losses during coring. Organic carbon concentration profiles are shown in Figure 17. The outer shelf sediments have organic carbon concentrations between 2 and 4 wt.% that are invariant with depth. Slope sediments host significantly higher organic carbon concentrations ranging from 6-12 wt.%. The average organic carbon content increases down slope from just over 6 wt. % at 190 m in the shelf edge 77 Organic Carbon (Wt. %) o o NH19BC(97m) • NH3BC(107m) • NH1BC (110 m) • NH2BC (133 m) o NH11BC (135 m) a NH7BC (190 m) o NH12BC (322 m) NH15BC (425 m) NH6BC (620 m) NH17BC (785 m) NH8P (1018 m) NH22P (2025 m) Figure 17. Organic carbon profiles from the box cores and upper 30 cm of the piston cores. Core locations are shown in Figure 1. Solid symbols denote box cores raised from depths corresponding to the oxygen-deficient zone (shaded area in Figure 2). Open symbols denote box cores collected from the outer shelf. Symbols filled with dots and crosses denote cores collected from depths below the oxygen-deficient zone. The water depths for each core are shown in parentheses in the legend. to over 10 wt. % at about 785 m. On the lower part of the slope, concentrations decrease from over 9 wt. % at 1018 m to less than 5 wt. % at 2025 m. The organic carbon content is not related to the position of the oxygen minimum in any simple fashion. The sediments on the mid-slope (620, 785 and 1018 m depth), below the zone of severe oxygen depletion (Figure 2), are more enriched in organic carbon than sediments that have accumulated in direct contact with the core of the oxygen minimum on the upper slope (shaded area in Fig. 17). Thus, the zone of sedimentary organic carbon enrichment on the western Mexican Margin extends far deeper than the footprint of the oxygen minimum. Figure 18 shows that there is a general negative correlation between Zr /Al ratios and organic carbon concentrations on the shelf and slope. The carbon-poor shelf sediments are coarser grained than carbon-rich slope deposits. In addition, the increase in organic carbon content with water depth is matched by a decrease in proxy grain size over a broad area. This decrease in grain size with water depth inferred from Zr /Al ratios is consistent with grain size determinations made by Van Andel (1964). Thus, variations in sediment texture seem to have influenced the distribution of organic carbon contents in these sediments. Deposits in the deeper waters (2025 m) on the lower slope are an exception to this general conclusion. Here, organic carbon concentrations are lower despite fine grain sizes. S^Corganic v a ^ u e s m t n e R e c e n t deposits range from -21.5 to -20 across the margin, indicating the dominance of marine-sourced organic matter in this area (Figure 19). C0rganic/Ntotal ratios range from 8 to 11.5, with higher values (> 9.5) occurring mostly in the shelf deposits. Given the lighter 8 l 3 c o r g a n i c values, the higher ratios cannot be due to increased relative input of terrestrial 79 i « 1 « 1 • 1 — I 15 20 25 30 35 Z r / A l X 1 0 4 (Wt. ratio) * NH19BC(97m) • • NH3BC(107m) • * NHlBC(llOm) • o NH2BC(133m) o o NH11BC (135 m) H * NH7BC(190m) a NH12BC (322 m) NH15BC (425 m) NH6BC (620 m) NH17BC (785 m) NH8P (1018 m) NH22P (2025 m) Figure 18. Relationship between organic carbon contents and Zr/Al ratios in the Recent sediments from NW Mexican Margin. Deposits from the shelf (A), the oxygen-deficient zone (B), the lower slope (C), and Core NH22P (D), plot in separate labelled domains. Core locations are shown in Figure 1. The water depths for each core are shown in parentheses in the legend. 80 organic -20.5 %c (to PDB) o o NH19BC(97m) • NH3BC(107m) • NHlBC(l lOm) • NH2BC(133m) © NH11BC (135 m) H NH7BC(190m) a NH12BC (322 m) NH15BC (425 m) NH6BC (620 m) NH17BC (785 m) NH8P (1018 m) NH22P (2025 m) -19.5 Figure 19. Relationship between Corganic/Ntotal ratios and Sl^Corganic values in Recent sediments from NW Mexican Margin. The domains of the shelf and the slope deposits are denoted by different shadings. Core locations are shown in Figure 1. The water depths for each core are shown in parentheses in the legend. organic material. Note that shelf sediments on average host slightly heavier S^Corganic values (> -20.5) than slope deposits (< -20.5). This distribution is opposite to what one would expect if land-derived C3 organic matter made a significant contribution nearshore that progressively decreased offshore. The shore-normal trend in 8^3cGrganic is also contrary to what one would expect if the isotopic values were predominantly controlled by a surface-water pCC»2 gradient induced by upwelling on the shelf (Rau et al., 1989). In such a case, the high pCC>2 in the freshly upwelled waters over the shelf should produce lighter S^Corganic values than waters advected offshore. The data collectively rule out terrestrial organic matter as a contributor to the higher Corganic/Ntotal ratios in the nearshore region. Furthermore, the notion of a low terrestrial organic influx is supported by the lack of lignins (Table 3) seen in the Holocene intervals of piston cores as well as by the the arid and the sparsely vegetated nature of the adjoining land area. Therefore, a more plausible explanation for the higher C 0 rganic /N total ratios on the shelf is that N-bearing organic compounds have been preferentially degraded during early diagenesis. This may indicate increased degradation of organic material in these deposits. A Post-depositional shift towards heavier S^C-organic values could also occur due to degradation of organic matter (McArthur et al., 1992; Muller et al., 1994 and references therein). Thus, the slightly heavier 5l3Corganic values observed in the shelf sediments may also reflect decomposition of organic material in these deposits. This implies that the shelf deposits have been subjected to relatively intense reworking, which is consistent with the coarser grain sizes indicated by the high Zr/Al ratio. The postulated role of degradation on the shelf is supported by the HI data. The shallow-water deposits have HI values ranging from 200 to 250 while the slope sediments have hydrogen indices of 275-375 (Figure 20). This relationship could reflect matrix adsorption during pyrolysis leading to an underestimation of the HI in carbon-poor samples. To evaluate this possibility, S2 versus TOC plots are shown in Figure 21. These show that the shelf and slope sediments yield regression lines with distinctly different slopes. The organic material accumulating on the continental slope is richer in hydrocarbons (mean HI value 327 ) than that on the continental shelf (mean HI value of 240). Thus, the lower HI values in the shelf deposits cannot be attributed to matrix absorption alone. If deposition under oxygen-deficient bottom-water controls the hydrogen richness of sedimentary organic matter, then sediments accumulating both above and below the zone of severe oxygen depletion, at depths <200 and >600 m, should exhibit lower HI values. This is not the case. The hydrogen richness of the lower-slope sediments deposited under well oxygenated waters is similar to that in shallower sediments bathed in oxygen-deficient waters. The preservational state of organic matter on the Mexican Margin is therefore not controlled by the presence or absence of oxygen-deficient waters. Instead, sediment texture appears to be the fundamental variable associated with hydrocarbon-richness: fine-grained sediments host hydrogen-rich organic matter. Furthermore, the fact that the HI values of sediments on the lower slope (at 2025 m depth) are similar to those deposits that accumulated within the oxygen minimum indicates that the lower organic carbon contents in the deeper-water sediments (Figures 17 and 18) are not the result of increased degradation under oxygenated bottom-waters. Hydrogen Index (mg H C / g C) 150 200 250 300 350 400 A NH19BC (97 m) • NH12BC (322 m) • NH3BC(107m) • NH15BC (425 m) • NHlBC(llOm) • NH6BC(620m) o NH2BC(133m) © NH17BC (785 m) o NH11BC (135 m) a NH8P (1018 m) A NH7BC(190m) a NH22P (2025 m) Figure 20. Hydrogen Index (HI) profiles from the box cores and the upper 30 cm of the piston cores. Shelf sediments (shaded area) exhibit distinctly lower HI than the slope deposits. Core locations are shown in Figure 1. Solid symbols denote box cores raised from depths corresponding to the oxygen-deficient zone (the shaded area in Figure 2). Open symbols denote box cores collected from the outer shelf. Symbols filled with dots and crosses denote cores collected from depths below the oxygen-deficient zone. The water depths for each core are shown in parentheses in the legend. 50 TOC (Wt. %) Figure 21. S2-TOC plots form the Recent Mexican Margin sediments. It is possible that the higher HI values observed on the lower slope could be due to the downslope movement of sediments initially deposited in the oxygen minimum. If this is the case, then the preservational state of the lower slope deposits may not reflect in situ bottom-water oxygen concentrations. Iodine/Corganic weight ratio measurements are used to investigate this possibility. I/Corganic ratios in sediments accumulating under well oxygenated sea water are higher (>100 XI0"^ ) than in sediments accumulating under anoxic conditions (<20 X10"4) (Price and Calvert, 1977; Francois, 1987). I/C0rganic ratios of modern Mexican Margin surface sediments should therefore reflect the present degree of oxygenation of the bottom waters (Fig. 22). Sediments accumulating in the oxygen-deficient zone have ratios (20 to 40 X 10"^ ) that are lower than those accumulating on the shelf ( 50 to 100 x 10~4) and lower slope (> 100 xl0"4). This confirms that the position of the oxygen minimum has been reasonably stable during recent times and that the Recent sediments reflect in situ depositional conditions. Thus, the higher HI values in the slope deposits cannot be attributed to allochthonous sedimentation. Note that iodine is readily lost during early diagenesis, relative to bulk organic carbon (Francois, 1987). The intermediate I/C0rganic ratios observed in Recent shelf sediments therefore add further support to the conclusion that the organic matter in the shelf deposits has been significantly degraded. 3. 3. 3 The role of winnowing in the spatial distribution of organic carbon Upwelling-driven primary production on the Mexican Margin is highest over the shelf and decreases offshore as illustrated by the satellite-derived pigment distribution in surface waters (Santamaria-del-Angel and Alvarez-Borrego, 1994). The combination of higher production and a short settling 86 I/C _______ X10 4 (Wt. ratio) organic 50 100 150 200 250 35 Shelf and lower-slope sediments Minimum Well oxygenated o o NH19BC(97m) • NH3BC(107m) • N H l B C ( l l O m ) • NH2BC(133m) o NH11BC (135 m) H NH7BC(190m) a NH12BC (322 m) NH15BC (425 m) NH6BC (620 m) NH17BC (785 m) NH8P (1018 m) NH22P (2025 m) Figure 221/ Corganic profiles from the box cores and the upper 30 cm of the piston cores. Sediments from the oxygen minimum (shaded area) exhibit distinctly lower I/Corganic ratios than the shelf and lower-slope deposits. Core locations are shown in Figure 1. The water depths for each core are shown in parentheses in the legend. distance should lead to organic carbon being supplied to shelf sediments at a higher rate than to slope deposits. However, much of the shelf is covered with coarser-grained relict deposits poor in organic carbon, while organic carbon-rich fine-grained deposits mantle the slope. Thus, winnowing on the shelf must augment the supply of organic material and fine-grained sediments to the continental slope via cross-shelf transport. The continental slope provides the quiescent conditions required for the accumulation of carbon-rich fine-grained sediments and is thus a depocentre. The decrease in proxy grain size and the increase in organic carbon content between the outer shelf and mid-slope regions indicates that hydrodynamic sorting processes are at work. This indicates that the intensity of sediment winnowing decreases with increasing water depth. Thus, maximum organic carbon concentrations occur on the mid rather than the upper slope. On deeper parts of the slope, the decreased supply of organic carbon due to the greater distance from the productive continental shelf contributes to the lower sedimentary organic carbon contents. In summary, variations in the supply of organic detritus from surface waters coupled with hydrodynamic sorting can adequately explain the distribution of organic carbon on the NW Mexican Margin. 3. 3. 4 Implications for the global distribution of organic carbon on continental margins Calvert and Pedersen (1992) reviewed the distribution of organic carbon and fine-grained sediments on continental margins in the Gulf of Mexico, the Atlantic, the Arabian Sea, the South China Sea, the Gulf of California and off Oregon-Washington. In all of these examples, the sedimentary organic carbon maxima on the slopes do not show a consistent relationship with the position of oxygen minimum zone. They do, however, exhibit a close relationship to sediment texture. Calvert and Pedersen (1992) concluded that the distribution of organic carbon on continental margins could be better explained by the interplay of hydrodynamic sorting processes, nearshore dilution of organic carbon by other sedimentary components and the supply of organic carbon which decreases offshore. The relationship between bottom currents and sediment accumulation on upwelling continental margins has been summarized by Suess and Thiede (1983a). These authors suggested that the depocenters of organic-rich sedimentary facies in upwelling margins are located in current shadows determined by the interactions of shelf-slope morphology and bottom currents. The strong poleward undercurrents and subsurface onshore flow characteristic of coastal upwelling circulation (Smith, 1983,1992) play a key role in allowing or preventing the deposition of organic-rich facies. In shallow continental shelf-slope regions, these currents sweep clean much of the fine-grained sediments and organic material leaving the carbon-poor coarse-grained residual deposits (Suess and Thiede, 1983a) seen for example, on the shelf and mid-slope off Peru (Reimers and Suess, 1983a), the outer-shelf off Namibia (Calvert and Price, 1983), and the shelf off NW Africa (Futterer, 1983). In all of these upwelling continental margin settings, the accumulation of organic carbon-rich facies occurs under quiescent conditions at depths where the longshore and cross-shelf subsurface flow are relaxed (Suess and Thiede, 1983). Further support for the slope-depocentre model was provided by Premuzic et al. (1982), who summarized an extensive data-set on the distribution of organic carbon on the US Atlantic continental margin. On the continental shelf and slope shallower than 2000 m organic carbon concentrations strongly correlate with the clay content of sediments. Shelf deposits are coarse-grained and carbonate-rich and grade into fine-grained sediments richer in organic carbon down slope. At depths greater than 2000 m, the distribution of organic carbon is independent of the clay content. Detailed studies on the fluxes of organic carbon in this area made during the SEEP program (Biscaye and Anderson, 1994; Anderson et al., 1994) provide an explanation for the grain size control on the organic carbon distribution observed by Premuzic et al. (1982). In these studies a series of sediment trap and current meter moorings showed that the depocenter of organic carbon and clays occurred on the slope between 400 and 1000 m depth, corresponding to a minimum in current velocities. Much of the organic material deposited within the depocenter is derived by cross-shelf transport following resuspension on the continental shelf. Thus, the strong correlation between the organic carbon and clay contents reflects hydrodynamic sorting and differential accumulation of sedimentary components. 3. 3. 5 The role of oxygen minima in the burial of hydrocarbons Demaison and Moore (1980) and Demaison et al. (1984) summarized the prevailing geological view on the environment of deposition of organic-rich sediments and petroleum source rocks. In their model, the survival of the hydrocarbon-rich organic fraction during early sediment diagenesis depended on deposition under oxygen-deficient bottom waters. This was envisioned as the principal control and an essential prerequisite for the formation of hydrocarbon-rich source beds. Demaison and his co-workers also espoused that modern sediments deposited in contact with intense oxygen minima, along with sediments accumulating in euxinic basins, are modern analogues of the depositional environment of petroleum source rocks. They presented the eastern Pacific and the northern Indian Ocean, where oxygen-depleted waters intersect the continental margin, as being type-settings for petroleum source rock formation on continental margins. They suggested that the organic matter accumulating in these settings is sparingly remineralized due to the oxygen deficiency in bottom waters, is consequently enriched in hydrogen, and therefore constitutes the precursor material for oil-prone Type I & JJ kerogens. This hypothesis is critically evaluated in the following section. It was shown earlier in this chapter that the quality of the organic matter currently accumulating on the NW Mexican Margin is independent of the bottom-water O 2 concentrations. But does this observation apply to similar environmental settings elsewhere? Organic carbon and S2 pyrolysis data from modern and late Quaternary sediments deposited under differing bottom-water oxygen levels on several continental margins are shown in Figure 23. The slopes of the lines yield the mean hydrogen index for each sample suite, while fields I, II and III indicate kerogen type (Langford and Blanc-Valleron, 1990). Lines F and Fl respectively represent laminated (suboxic bottom water) and bioturbated (oxygenated bottom water) surface sediments from the Gulf of California (data from Calvert et al., 1992). The variance about both lines is minor (R2 = 0.76 and 0.90, respectively), and the HI is slightly higher in the bioturbated deposits (321 vs. 240). Thus, there is no evidence that oxygen levels have influenced the hydrogen richness of either set of samples. The same conclusion applies to surface sediments from the West Indian Margin (Line E in Figure 23, R2= 0.99) which yield a similar HI (-280) that is invariant over a 91 tN 10 CD A NH8P(420) B NH22P(360) C NH15P Lam. (361) C 1 NH15P Bio. (298) D Mexican Slope (327) D 1 Mexican Shelf (241) 6 8 10 T O C (Wt. %) E E. Arabian Sea (281) F G. Calif. Lam. (240) F 1 G. Calif. Bio. (321) G N. Calif. Marg. (653) H Black Sea Unitl BSK14 (399) Figure 23. The relationship between S2 and TOC in sediments from continental margins and euxinic basins. Regression lines A-H represent different data sets as described in the text. broad range of organic carbon and bottom-water oxygen levels (data from Calvert et al., 1995). Similarly, HI values from euxinic Black Sea deposits (calcareous marl (Unit 1), core BSK 14; described by Calvert, 1990 and Calvert et al., 1991), are represented by regression line H (R2 = 0.88) (Calvert, unpublished data). The mean HI values (399) deduced for these deposits are similar to many continental margin sediments. In contrast to these results, Dean et al. (1994) came to a different conclusion in a study of bioturbated Holocene/Isotope Stage 2 and laminated mid-Stage 3 sediments deposited under the oxygen minimum off northern California (Figure 24). The bioturbated younger sediments there host ~1 % organic carbon, while the laminated older deposits contain about twice this amount. Hydrogen indices vary in sympathy and positively with the organic carbon concentrations. Dean et al. (1994) therefore concluded that the higher organic carbon content and hydrogen richness in the laminated deposits represented enhanced preservation under oxygen-depleted conditions. Reexamination of their data suggests that this conclusion may be invalid. Plotting their analyses as S2 vs TOC (Figure 25) shows that a significant negative intercept exists on the y-axis. The sympathetic variation between HI and % organic carbon downcore discussed by Dean et al. (1994) is therefore an example of the analytical artifact (low HI) that results from matrix adsorption of hydrocarbons during pyrolysis of carbon-poor samples. In fact, the excellent linear relationship (R2= 0.95) between S2 and TOC shown in Fig. 25 (and plotted in Figure 23 as Line G) indicates that the hydrocarbon richness and preservational quality of the organic matter in their sample set is the same in both laminated and bioturbated deposits. HI values corrected for matrix adsorption (Figure 25) yield an invariant HI profile (Figure 24). This 93 N . California Margin, Core GC117 TOC (Wt. %) 0.5 1 1.5 2 2.5 3 0 -J • L — • L 20 H 40 -J S 60 A CM 80 H 100H 120 200 300 400 500 600 700 800 HI (mg HC/g C) Figure 24. Down-core profiles of % total organic carbon (squares), HI (circles) and corrected-HI (triangles) from Northern California Margin Core GC117. The shaded area denotes the presence of laminations. Raw data from Dean et al. (1994). 94 Figure 25. Matrix correction for N. California Margin Core G O 17. Matrix-corrected hydrogen index (HI) values, shown in squares and represented by the solid regression line, have been computed by removing the y-intercept from the dotted regression line representing the uncorrected HI values, shown in circles. reevaluation argues against the contention of Dean et al. (1994) that organic matter is better preserved in laminated sediments. A better explanation for the higher organic carbon contents in the laminated deposits would appear to be that productivity was higher at the time of deposition. Indeed, enhanced export production is implied by higher concentrations of biogenic opal in the laminated intervals (see Fig. 3 in Dean et al., 1994). Three major conclusions can be drawn from the variety of data sets evaluated above. First, hydrogen indices of predominantly marine organic matter deposited in a variety of continental margin settings and euxinic basins correspond to Type II oil-prone kerogen. Second, there is no systematic variation in hydrocarbon richness with bottom water oxygen concentrations. Third, the accumulation of hydrocarbon-rich organic material is not limited to oxygen-deficient environments or to laminated sediments as argued by Demaison and Moore (1980) and Demaison et al. (1984). 3. 3. 6 The role of oxygen minima in the burial of organic carbon The observation that organic matter deposited on continental margins is relatively rich in hydrogen irrespective of variations in bottom-water oxygen concentrations is consistent with our understanding of organic matter diagenesis. Deposited organic matter is subject to bacterially-mediated oxidative degradation using a sequence of oxidants, in order, O2, NO3", M n 0 2 (s), FeOOH (s), and SO42- which are supplied from overlying water either by diffusion or in the case of the oxyhydroxides, on settling particles (Froelich et al., 1979). A large settling flux of organic matter in combination with high sedimentation rates ensures the burial of large proportions of relatively fresh organic material by reducing the residence time of organic material at the sediment-water interface. Under such conditions, oxygen is consumed quickly in the top few mm to cm of the sediments and secondary oxidants, especially S O 4 2 " , are used in the oxidation of a large proportion of organic matter (Jorgensen, 1983). Thus labile organic material is only briefly exposed to oxygen, following which degradation continues under anoxic conditions. Lower bottom-water oxygen concentrations in O2 minima restrict the diffusive influx of oxygen decreasing the thickness of the oxic veneer. Thus, in continental margin settings, the effect of variations in bottom-water oxygen concentrations is confined to the proximity of the sediment-water interface. It follows that, if oxygen-deficient bottom-water conditions are to play a role in enhancing organic matter preservation, then aerobic processes must be more efficient than anaerobic mineralization in degrading relatively labile organic matter which resides only briefly near the sediment-water interface. Field studies invariably conclude that in contemporary deposits, the organic carbon degradation rates are similar under oxic and anoxic conditions (eg. Jahnke, 1990; Reimers et al., 1992). These results are also consistent with laboratory experiments showing that organic matter degradation rates using oxygen and sulphate reduction are similar (example, Westrich and Berner, 1984; Kristensen and Blackburn, 1987; Lee et al., 1992). In addition, Henrichs and Reeburgh (1987) examined the positive relationship between the burial efficiency of organic carbon and sediment accumulation rates. They found that burial efficiency is similar for oxic and anoxic sediments that have similar sediment accumulation rates. In support of this, Canfield (1989, 1994) showed that burial efficiency of organic carbon is independent of bottom-water oxygen concentrations under the high rates of sedimentation (>0.1 g cm"2 y-1) often found on continental margins. Since the majority of field and laboratory studies involve reasonably fresh organic matter, it follows that there is no clear evidence that anaerobic degradation is less efficient than aerobic mineralization in degrading relatively labile organic material, and there is therefore little support for the notion that oxygen-deficient zones are exclusive areas of enhanced sedimentary carbon preservation. 3. 3. 7 Implications for the interpretation of hydrogen indices in geological records It has often been argued that Recent sediments and organic materials used in laboratory studies are relatively fresh, representing ages of a few hundred days to several hundred years, and as a result they may not show differences in degradation rates under oxic and anoxic conditions (Emerson and Hedges, 1988; Van Cappellen and Canfield, 1993; Canfield, 1994; Hedges and Kiel, 1995). This argument highlights the similar efficiencies of aerobic and anaerobic degradation provided that relatively labile organic substrates are available, and it envisions that anaerobic processes may not be efficient in degrading relatively refractory organic material. Thus, small differences between aerobic and anaerobic degradation rates would lead to large variations in the quality and concentrations of organic matter preserved in sediments after long-term diagenesis. Indeed, Canfield (1989, 1994) has shown that at low sediment accumulation rates (<0.1 g cm"2 y"l), the burial efficiency of organic carbon is higher for sediments deposited under oxygen-depleted conditions than oxygenated sediments with comparable sedimentation rates. This has been interpreted as evidence for less effective anaerobic degradation of refractory organic material. On this basis, Van Cappellen and Canfield (1993) and Canfield (1994) questioned the geological application of the approach used in the study of Calvert et al. (1992), which showed that the hydrogen-richness of laminated and bioturbated sediments is similar in the Gulf of California. Van Cappellen and Canfield (1993) and Canfield (1994) argued that the results from modern sediments may not provide a valid comparison for carbonaceous shales in geological records, because short-term diagenesis may not have permitted the biochemical impact of bottom-water oxygen to be fully expressed as differences either in the concentration or the composition of buried organic matter. In this context, a close correspondence between high organic carbon concentrations, high HI values and the presence of laminations is a common occurrence in geological records (e.g. Arthur et al., 1984; Pratt. 1984; Ingall et al., 1993; ten Haven et al., 1990, Dean et al., 1994). The common interpretation of this congruence is that laminations indicate deposition under oxygen-deficient bottom waters hostile to burrowing infauna, and the preservational effect of such conditions, as inferred by increased hydrocarbon richness (higher HI values), resulted in increased organic carbon accumulation. The data shown in Figure 23 do not support this contention; HI values in buried deposits that span tens of thousands of years have values very similar to surface sediments. For example, Recent sediments deposited on the Mexican continental slope (line D, Fig. 23), have HJ values similar to NH22P and NH8P sediments (lines B and A, Fig. 23) which were deposited over periods of 140 kyr and 50 Kyr respectively. The hydrocarbon-richness of the organic material deposited over such time spans is adequately described by a single regression line in each case. In contrast to the invariant HI records, C 0rganic/Ntotal ratios (Fig. 12) show exponential increases with depth indicating progressive and preferential diagenetic loss of labile nitrogen. These observations suggest that the hydrogen-rich fraction of organic matter is relatively stable during early sediment diagenesis over the broad range of depositional settings encountered on margins. Thus, the variability in hydrogen-richness observed in geological records is unlikely to be the result of compositional differences imposed by progressive diagenesis, as envisioned by Van Cappellen and Canfield (1993). Interestingly, this further suggests that the factors that produce variations in HI values of sediments accumulating on the continental margins operate either in the water column and/or at very close proximity to the sediment-water interface and are not sensitive to the oxidants used in the post-depositional degradation of organic matter. 3. 3. 8 The influence of winnowing on the preservational quality of organic material Variations in sediment texture imparted by winnowing processes seem to be fundamentally related to the spectrum of HI values observed both in late Quaternary and Recent sediments on the Mexican Margin. Several lines of evidence suggest that winnowing enhances degradation of organic material. For example, the decreased hydrocarbon richness of winnowed coarse grained sediments observed on the Oman Margin was attributed by Pedersen et al. (1992) to extensive degradation during reworking of the deposits. Using a different approach, Weisner et al. (1990) observed that in size-separated North Sea sediments, organic matter associated with the coarse fraction was hydrogen poor relative to the fine moeity. The latter contained organic matter similar in composition to freshly produced phytodetritus captured in the water column. These studies have concluded that sediment reworking prevents the accumulation of relatively fresh organic material that settles to the sea floor. Recently, coupled measurements of organic carbon contents and mineral surface area in margin sediments revealed that organic matter is often present in monolayer-equivalent concentrations on mineral surfaces (Kiel et al., 1994; Mayer, 1994). Mayer (1994) suggested that sorption of organic molecules on mineral surfaces enhances preservation, either by isolating organic compounds from enzymatic attack or by catalyzing condensation reactions that render organic material refractory to microbial remineralization. As a result, fine-grained sediments with higher surface area should preserve proportionately more organic material than coarse-grained sediments (Kiel et al., 1994; Mayer, 1994). However, organic-rich sediments in productive continental margins show two to four times the organic carbon concentration that can be accounted on the basis of mineral surface area (Hedges and Keil, 1995). Hedges and Keil (1995) have hypothesized that such "super-monolayer" coverage may reflect enhanced preservation under oxygen-deficient bottom-water conditions. Indeed, organic carbon concentrations in winnowed sediments from the continental shelf show monolayer equivalent coverage, while deposits from the slope fall into the super-monolayer category. Similarly, in core NH15P, fine-grained organic-rich interglacial deposits exhibit super-monolayer coverage, while the coarse-grained carbon-poor intervals yield monolayer equivalence (Table 4) (J. W. Murray, unpublished data). 101 Table 4. Surface Area determination in NH15P core. (Data provided by Dr. J.W. Murray) Depth Surface Organic OC/SA Depth Surface Organic OC/SA (cm) Area (m-2/g) Carbon (Wt. %) (cm) Area (m-2/g) Carbon (Wt. %) 11 20.63 8.72 4.23 603 15.6 1.85 1.19 31 22.78 7.74 3.4 623 13.83 1.51 1.09 51 21.5 8.02 3.73 643 15.44 2.07 1.34 71 21.49 7.74 3.6 663 16.9 2.73 1.62 91 25.41 7.29 2.87 683 24.67 5.17 2.1 126 20.95 8.6 4.11 713 24.78 4.27 1.72 135 20.33 9.05 4.45 733 17.17 2.23 1.3 156 16.28 8.82 5.42 753 17.4 2.25 1.29 176 22.79 8.9 3.91 773 15.3 2.18 1.42 196 20.53 6.72 3.27 793 22.87 5.15 2.25 216 20.14 2.95 1.46 813 23.58 5.45 2.31 236 18.93 2.41 1.27 833 18.65 4.79 2.57 263 11.37 1.06 0.932 862 24.49 4.88 1.99 283 16.16 2.62 1.62 884 21.94 5.37 2.45 303 14.2 2.32 1.63 903 16.12 2.62 1.63 323 16.55 2.97 1.79 923 31.18 4.78 1.53 343 22.56 2.42 1.07 943 12.75 1.23 0.964 363 18.87 2.37 1.26 963 22.44 2.26 1.01 383 19.6 2.75 1.4 983 16.91 1.53 0.905 413 25.59 2.8 1.09 433 19.56 2.05 1.05 453 21.33 3.08 1.44 473 25.72 3.56 1.38 493 23.81 5.65 2.37 513 27.04 4.08 1.51 533 28.49 3.73 1.31 563 17.1 1.97 1.15 583 23.48 3.58 1.52 An alternative explanation for monolayer equivalence in some margin deposits, that does not involve oxygen, may be winnowing. Winnowing will remove discrete hydrodynamically-mobile particles, leaving behind only organic matter that is sorbed to coarser-grained mineral surfaces. If the sorbed organic molecules are relatively depleted in hydrocarbons then this would explain the lower HI values of coarse reworked sediments. One mechanism that could satisfy this requirement is the sorption of dissolved organic carbon by mineral grains in the water column. Although speculative, this suggestion is supported by the observations that a major fraction of dissolved organic matter is refractory (Trumbore and Druffel, 1995) and that surface organic coatings are ubiquitous on settling mineral grains in the water column (Hunter and Liss, 1979; 1982). Redepositional processes are known to enhance the degradation of organic material that has been otherwise stable and preserved under conditions of long-term burial and diagenesis. Interesting examples include the organic-rich distal turbidites emplaced in pelagic sediments off NW Africa (Colley et al., 1984, Wilson et al., 1986). Here organic material transported from the continental margin off NW Africa by gravity flows was subjected to renewed remineralization once redeposited on the abyssal plain. Hulthe et al. (in press) report that organic matter exposed to repeated oxic and anoxic remineralization in laboratory experiments was more extensively degraded than organic material subject to degradation under either oxic or anoxic conditions alone. Thus, repeated burial and resuspension of organic material in sediments with a shallow oxic-anoxic interface should produce a more refractory organic component than simple long-term single-cycle burial. Finally, it has been suggested that refractory organic compounds could be more readily degraded when mixed with fresh organic material, which stimulates bacterial enzymatic activity (Canfield 1994 and reference therein). Such co-metabolism, due to cyclic deposition and resuspension of labile organic material resulting from winnowing, could aid the degradation of an immobilized mineral-adsorbed organic fraction. 3.4 CONCLUSIONS Several major conclusions can be drawn from the data presented in this chapter: 1) late Quaternary cycles in carbon burial on the NW Mexican Margin are mainly controlled by glacial-interglacial variations in export production; 2) the quality of organic matter buried on the Mexican Margin is essentially of Type II "oil-prone kerogen", similar to marine organic matter deposited in a variety of continental margin settings and euxinic basins; 3) the preferential accumulation of organic matter with fine-grained moieties in the slope depocentre, and the offshore decrease in the settling flux of organic detritus, adequately explain the the mid-slope maximum in sedimentary organic carbon content on the modern NW Mexican Margin; 4) a down-slope shift in the depocentre in response to glacial low sea stands is identified as a secondary factor influencing organic carbon burial on the upper slope; 5) the oxygen minimum zone seems to have no particular influence on either the quantity or on the quality of organic matter buried on the margin and; 6) the degree of winnowing and the variable presence of terrigenous organic material seem to be the fundamental factors that influence variations in the preservational quality of organic matter in this area. 104 Chapter 4 Glacial-interglacial variability in denitrification off NW Mexico 4.1. INTRODUCTION Records from ice cores have revealed a remarkable coupling between atmospheric pC02 and glacial-interglacial cycles. Changes in ocean chemistry and circulation have been identified as being the key governors in modulating atmospheric pC02 over these time-scales (Berger and Keir, 1984; Berger and Wefer, 1991). However, there is no unequivocal agreement on the relative importance of the mechanisms that drive such changes. An increase in the oceanic nutrient inventory is one mechanism that could contribute to the drawdown of glacial CO2 levels by increasing biological production in surface waters, leading to an increase in the pool of inorganic carbon stored in the glacial ocean (Broecker, 1982; Boyle, 1988). Broecker (1982) postulated that the phosphorus locked in shelf sediments that accumulated over interglacial high-stands would have been released by erosion during the glacial low-stands, resulting in an increased P inventory in the glacial ocean. This hypothesis was disproved when it was later recognized that late Quaternary sea level transgressions and regressions followed rather than led changes in atmospheric PCO2 (Berger and Keir, 1984). Furthermore, Cd/Ca measurements of benthic foramininfera (a good proxy for phosphorus) showed that the glacial ocean phosphate inventory was not elevated significantly (Boyle, 1988). Alternatively, it has been suggested that fixed nitrogen, which has a shorter residence time (lO^-lO4 years) than phosphorus (10^  years), is more likely to fluctuate on glacial-interglacial time scales (McElroy, 1983; Codispoti and Christensen, 1985; 105 Christensen et al., 1987). Berger and Keir (1984) contend in their Denitrification Model that a glacial increase in N O 3 " of 30% due to reduced oceanic denitrification would produce the pC02 increase observed in the ice cores. Fixed nitrogen is supplied to the ocean by rivers, in situ N-fixation by cyanobacteria, and atmospheric fallout. The major sinks are denitrification in oxygen-deficient waters and in margin sediments (Codispoti and Christensen, 1985). In the absence or near absence of oxygen, N O 3 " is used as an electron acceptor instead of O2 in the bacterially-mediated degradation of organic matter, resulting in denitrification (Goering, 1968; Cline and Richards, 1972). The gaseous products of denitrification (N2O and N2) are to a large extent lost to the atmosphere; this process constitutes a net loss of fixed nitrogen (which occurs mostly as nitrate) from the ocean (Codispoti and Richards, 1976). Water-column denitrification, which accounts for almost half (60-90 Tg N yr~l : Codispoti, 1995) of the total oceanic N-loss, occurs primarily in three areas: the Eastern Tropical North Pacific (ETNP), the Eastern Tropical South Pacific and the Arabian Sea (Codispoti and Christensen, 1985; Codispoti, 1989). Each of these regions is characterized by an intense oxygen minimum zone with near-zero oxygen and upwelling-induced primary production which supplies copious quantities of settling organic detritus that sustain high rates of denitrification in subsurface waters (Summerhayes, 1983; Suess and Thiede, 1983b; Codispoti, 1989). As a consequence, denitrification rates in these zones are sensitive to climate-induced changes in upwelling intensity and hydrography, which are known to vary on glacial-interglacial time scales (e.g. Reimers and Suess, 1983 a&b; Molina-Cruz, 1988; Clemens and Prell, 1990; Chapter 2, this study). For example, Altabet et al. (1995) recently concluded that denitrification in the Arabian Sea was reduced substantially during glacial periods. A simultaneous decrease in dentrification in the ETNP would extend this conclusion globally, because the ETNP hosts the largest pool of oxygen-deficient waters in the oceans and accounts for at least a third of the global N loss due to water column denitrification (Codispoti and Christensen, 1985; Christensen et al., 1987). To test this idea, we assess in this chapter glacial-interglacial variability in denitrification in the ETNP for the last 140 kyr. The principal tool used to estimate the extent of denitrification is 15N/14JSJ ratios measured downcore on bulk sediments. 4. 2 NITROGEN ISOTOPES AND WATER COLUMN DENITRIFICATION 15JSJ/14N ratios in organic matter produced in the near-surface ocean are a function of the 8^$N of the source NO3" supplied from up welled subsurface waters and the isotopic fractionation of NO3" during uptake by phytoplankton (Altabet and Francois, 1994). In the oxygenated waters of the open ocean, the isotopic composition of the source nitrate is ca. 5-6 %o (Liu and Kaplan, 1989) and therefore the I ^ N / M N ratio of the organic matter produced in near-surface waters is determined predominantly by the preferential phytoplanktonic uptake of 14]\JC>3" (Altabet and Francois, 1994). However, in oxygen-depleted waters, denitrification renders the isotopic composition of source NO3" heavy. Isotopic fractionation associated with denitrification discriminates against 15]sjC>3~ (fractionation factor ca. 1.03-1.04) and as a result 14jsJ03~1S preferentially converted to gaseous products of denitrification (Cline and Kaplan, 1975). As denitrification proceeds, the residual nitrate in the oxygen-deficient subsurface waters becomes progressively enriched in 15^03". Nitrate highly enriched in in (8^5N values in excess of +18%o; see Cline and Kaplan, 1975) is found in the subsurface waters of the ETNP, particularly near the highly productive continental margin, where denitrification is the predominant biological respiration process in the oxygen minimum zone (Garfield et al., 1983; Codispoti, 1989). The heavy nitrate is supplied to the surface waters by upwelling and utilized by biota resulting in unusual 8*5N enrichments (in excess of ~ 10 %o) in suspended particulate organic material (Saino and Hattori, 1987). In addition, nitrogen in modern bulk sediments in this area yields S^^N values of 9 to 10 %o (Figure 26) which are unusually high compared with upwelling areas not affected by denitrification in the water column (Altabet and Francois, 1994, Farrell et al., 1995). 4. 3 RESULTS Profiles of sedimentary o^$N and the 8^ 0^ of benthic foraminifera in three piston cores raised from the northwestern Mexican continental slope are shown in Figures 27a-c. The o^-^N profiles of all three cores vary in general sympathy with glacial-interglacial stratigraphy: heavy values (mean 9-10 %o) characterize interglacials (Isotope Stages 1, 3 and 5), while values 2 to 3%o lighter occur during the glacial Isotope Stages 2, 4 and 6. These glacial-interglacial variations in S^^N occur almost synchronously in these cores during the last 60 kyr, indicating that the signal is regional in nature. The offsets between o^N records are on average less than l%o and lack consistent temporal patterns in intervals younger than 60 kyr. Some larger offsets in older intervals (•> 60 Kyr), especially in Isotope Stage 5, are probably artifacts of age control which is poorly constrained in core NH15P. 108 515N (%c to air) O o NH19BC(97m) * NH3BC(107m) • NHlBC(llOm) • NH2BC(133m) • NH11BC (135 m) o NH7BC (190 m) NH12BC (322 m) NH15BC (425 m) NH6BC (620 m) NH17BC (785 m) Figure 26. Nitrogen-isotope profiles from the box cores. Core locations are shown in Figure 1. The water depths for each core are shown in parentheses in the legend. 109 Figure 27a-c. Records of b^N in bulk sediments and benthic-foraminiferal S^O from NW Mexican Margin cores. The textured bar near the left margin indicates the presence or absence of laminations. Oxygen-isotope stages are indicated near the right margin and glacial stages are shaded. I l l ( NH8P, 1018 m water depth ) 6 l s O smoothed (%c to PDB) 4 3 2 6 7 8 9 10 5*5N (%c to Air) 112 c NH22P, 2025 m water depth 5 l s O smoothed (%c to PDB) 5 4 3 £ 100 Bioturbated Laminated 7 8 9 10 11 5 1 5 N (%c to Air) 113 4. 4 DISCUSSION 4. 4. 1 Possible causes for down-core shifts in Sl^ -N values Mixing of marine organic matter with isotopically lighter terrestrial organic matter (ca. S^N = 2%o; Sweeney and Kaplan, 1980) can produce lighter 515N values in marine sediments (Peters et al., 1978). The possibility of significant admixture of terrestrial organic matter in Mexican Margin cores was evaluated in Chapter 3 by using Sl^Corganic v a l u e s ' Q>rganic/Ntotal ratios and lignin biomarkers. This confirmed that the organic material preserved in these sediments is overwhelmingly of marine origin both in glacial and interglacial intervals. Only a slight increase in terrestrial organic material was implied by the presence of lignin biomarkers in the glacial intervals of core NH15P (see Chapter 3 for detailed discussion). Since the decreases in S ^ N values occur uniformly in all three cores during the glacial intervals, the S^N shifts cannot be explained by mixing of terrestrial organic material alone. Diagenetic alteration of organic material is another possible cause of variations in the sedimentary S^N signal. Increases in 5^^N values of 5 to 10 %o between sediment trap material and surface sediments have been reported in the Southern Ocean (Altabet and Francois, 1994). It has also been observed that anaerobic diagenesis leads to increased S^N values (Nadelhoffer and Fry, 1988; Benner et al., 1991). This does not appear to be the case on the NW Mexican Margin. C0rganic/Ntotal ratios (Figure 28) in box cores show a general secular increase with depth, while the 8^N profiles are invariant (Figure 26). Thus the preferential release of nitrogen during degradation of organic matter seems to have no fractionation effect despite the broad spectrum of depositional 114 C . /N , (Wt. ratio) organic total 3 5 * NH19BC(97m) * NH7BC(190m) • NH3BC(107m) • NH12BC(322m) v NHlBC(l lOm) • NH15BC (425 m) o NH2BC(133m) • NH6BC(620m) o NH11BC (135 m) o NH17BC (785 m) Figure 28. Profiles of Corganic/Ntotal ratios from the box cores. The water depths for each core are shown in parentheses in the legend. Core locations are in Figure 1. conditions encountered on the margin. A similar conclusion appears to apply to sediments farther north:.Sweeney and Kaplan (1980) observed no significant shift in 8^^N during diagenesis in anaerobic Santa Barbara Basin sediments. Since diagenesis does not seem to produce a significant isotopic shift towards heavier values in Recent sediments, it cannot be invoked to explain the lighter values seen in the glacial intervals of the cores studied here. Similarly, heterotrophic processes can cause an increase in the 5^^N of settling organic detritus, due to the excretion of lighter nitrogen. The increase in 8 1 5 N is about 3.5 %o for each trophic level (DeNiro and Epstein, 1981; Fry, 1988). Thus, changes in the length of the food chain with time could produce down core variations in sedimentary S^N. However, suspended particulates collected in the ETNP below 200 m in the water column have on average S ^ N values exceeding 9 %o (Saino and Hattori, 1987), similar to 8^^N values in Recent and Holocene sediments (Figures. 26 and 27). Thus, the available evidence indicates that heterotrophic processes have no significant effect on the sedimentary S^N in this area today. This could be attributed to the short food chain associated with the highly productive upwelling regime off NW Mexico, where a relatively large fraction of the primary production is exported to sediments without undergoing recycling in the water column (cf. Eppley and Peterson, 1979). Given that heterotrophic processes have no significant effect on sedimentary b^N values today, it is unlikely that such processes could have contributed to the lighter shifts during the glacial periods, because a longer food chain resulting from decreased upwelling and productivity during the glacial periods should have produced sedimentary 8^N values that are heavier than those of Recent and Holocene sediments. Finally, nitrogen isotope fractionation due to the preferential uptake of 14NC>3" by phytoplankton in near-surface waters has been observed in continuous culture experiments, where the 8 1 5 N of the photosynthate follows the integrated product equation of first-order Raleigh fractionation kinetics (Montoya, 1990, 1994). Such a fractionation coupled with lateral advection of a residual nitrate pool that becomes progressively enriched in could produce a spatial pattern in sedimentary 8^^N values. This phenomenon is well documented in the equatorial Pacific and the Southern Ocean, where the residence time of upwelled nitrate is long (Altabet and Francois, 1994; Farrell et al., 1995). In the case of the Pacific equatorial divergence zone, incomplete use of nitrate in upwelled waters results in the lateral advection of relatively nitrate-rich waters north and south of the equator. The continuous utilization of this nitrate by phytoplankton results in the progressive l^N enrichment in the advecting nitrate pool and in the organic material sinking to the sea floor (Altabet and Francois, 1994). The resulting spatial pattern in sedimentary 8l$N mimics the meridional trend in the advected nitrate, with lighter values at the divergence and progressively heavier values north and south of the equator (Farrell et al., 1995). The advection of upwelled waters offshore coupled with progressive nitrate utilization should produce shore-normal gradients in sedimentary S^N on the Mexican Margin, with lighter values nearshore and heavier values offshore. Mexican Margin box cores do not show such a trend (Figure 26). This is probably because upwelled nutrients are rapidly and completely utilized on this continental margin (Pena et al., 1994) and therefore the mean isotopic composition of particulate organic nitrogen and surface sediments matches that of the upwelled nitrate. The lighter glacial S ^ N values could have resulted from enhanced upwelling and a plentiful supply of nutrients leading to incomplete utilization and advection of isotopically lighter nitrate into offshore areas. This postulated scenario is an unlikely explanation for shifts to lighter glacial S^N values because it entails a simultaneous increase in upwelling and productivity on the Mexican Margin which is contrary to evidence presented in Chapter 2. The heavier interglacial S^^N values (9 to 10 %o; Fig. 27) seen in the trio of NW Mexican Margin piston cores are similar to values observed in Recent sediments that underlie areas influenced by denitrification. In contrast, the lighter glacial values are common in sediments where subsurface waters are well oxygenated. For example, values in the range of 6 to 7 %o are commonly found in sediments underlying much of the equatorial Pacific where surface waters have moderate nutrient concentrations (Alatabet and Francois, 1994; Farrell et al., 1995). By analogy, and having ruled out varying terrestrial inputs and diagenetic and food chain effects, it appears that reduced water column denitrification is the most probable cause for the lighter §l$N values observed in glacial-age sediments on the continental margin off NW Mexico. 4. 4. 2 The cause of diminished denitrification during the glacial periods The oxygen minimum zone off NW Mexico extends from -150 to -800 m water depth and spans two distinct water masses: subtropical subsurface water (SSW) characterized by a salinity maximum at -250 m and Pacific intermediate water (PIW) distinguished by a salinity minimum at -700 m (Roden, 1972b Warsh et al., 1973). A secondary NC»2" maximum and the isotopically-heavy nitrate peak coincide with the salinity maximum of the SSW between 150 and 500 m (Cline and Kaplan, 1975; Garfield et al, 1983). This implies that the SSW hosts intense denitrification and supplies isotopically heavy nitrate to surface waters during periods of upwelling. Core NH15P was raised from these waters and exhibit a tight coupling between the presence of laminations and heavy 8 1 5 N values (Fig. 27). Thus, the SSW seems to have been well oxygenated during periods of reduced denitrification. On the other hand, in core NH8P the laminated intervals are decoupled from heavy 8 1 5 N values indicating that changes in oxygen concentrations at this depth did not influence denitrification rates on the margin. This site, situated at 1018 m water depth, is influenced by the PIW. These waters originate from higher latitudes in the northwestern Pacific Ocean (Wyrtki, 1967) and therefore, it is conceivable that the paleoxygen fluctuations in these water masses are remotely forced from outside the subtropical eastern Pacific. Thus, it appears that the oxygenation of SSW may play a key role in controlling denitrification rates off NW Mexico. The major source of SSW is the Equatorial Undercurrent or 13°C water (Tsuchiya, 1981), which flows west to east below the equatorial divergence zone and splits into north- and south-flowing arms in the eastern Equatorial Pacific around the Galapagos (Lukas, 1986). The north-flowing arm feeds into the subsurface water masses of the ETNP, which progressively lose oxygen and gain salt as they flow along the Central American coast (Wyrtki, 1967). The Equatorial Undercurrent is well oxygenated until it reaches the coast of Central America owing to ventilation related to the equatorial divergence (Lukas, 1986; Toggweiller and Carson, 1995). Therefore, subsurface waters of the ETNP are initially supplied with well-oxygenated water. The residence time of the SSW is thought to be relatively short (on the order of 10 years) due to extensive upwelling in the ETNP (Wyrtki, 1967). Given these characteristics, the depletion of oxygen in the subsurface waters can be mainly attributed to the high biological production in the surface waters off the Central American coast (Wyrtki, 1967). It follows that the diminished denitrification and increased oxygenation in the SSW during the glacial periods, implied by the sedimentary record, reflect decreased productivity and a reduced settling flux of organic detritus along the margin. This inference is supported by the reduced accumulation of organic carbon, opal and barium in the glacial deposits compared to the interglacial sediments off NW Mexico (Chapter 2, Figures 4a-c ). These cyclic variations in biogenic sedimentation appear to have been induced by glacial-interglacial variations in upwelling-supported productivity. This is also consistent with general circulation model predictions and terrestrial paleoclimatic data that indicate decreases in upwelling-favourable winds along this margin during glacial episodes in response to the growth of the Laurentide ice sheet on North America. The heavy b^^N values in the interglacial sediments (9 to 10 %o), combined with high organic carbon and opal concentrations and relatively high barium enrichments reflect intense upwelling and the delivery to the mixed layer of 15N_ e n r j c n ed nitrate from the underlying zone of denitrification, conditions similar to those existing today in the ETNP. Conversely, the decreases in the organic-carbon and opal concentrations in the glacial-age deposits, along with lower Ba/Al ratios argue for diminished productivity and a reduced supply of organic detritus to the ocean floor. This should have decreased the severity of the oxygen-depletion in subsurface waters during glacial episodes. The lighter b^$N values in glacial stages imply that denitrification and the associated supply of 15jsj_e nriCned nitrate to the surface waters were substantially reduced during such times. Thus, the concurrence in glacial intervals of lighter values, reduced accumulation of biogenic components and the lack of preservation of laminations on the upper slope provide strong support to the thesis that reduced denitrification on the NW Mexican Margin during the glacial episodes was forced by decreased upwelling-induced productivity. 4. 3. 3 Implications for the oceanic-nitrate inventory and pC02_during glacial periods The diminished glacial-age denitrification implied by our results has profound global ramifications. Fixed N is supplied to the ocean by rivers, in situ N-fixation by cyanobacteria, and atmospheric fallout, with the major sinks being denitrification in oxygen-deficient waters and in shelf sediments (Codispoti and Christensen, 1985). Previous investigations have revealed that the fixed-nitrogen budget of the ocean is unbalanced, and as a result, the ocean is losing fixed-nitrogen at a rate faster than it is being supplied (McElroy, 1983). The estimated supply of N to the oceans is 90 Tg N yr"1 and the total loss is thought to be 175-205 Tg N y r 1 , yielding a deficit of 85-115 Tg N yr"1 (Codispoti 1989; Devol, 1991). The feedback mechanisms that could potentially modulate variations in oceanic N contents do not seem to be tightly coupled. For example, contributions by N-fixing cyanobacteria are thought to be small at the present time, ca. 20 Tg N yr"1 of which only 5 to 10 Tg N yr"1 are fixed in open ocean oligotrophic areas, the rest being contributed by N fixation in estuaries, marshes and coral reefs (Capone and Carpenter, 1982). These observations have led to the suggestion that the oceanic N content oscillates on glacial-interglacial time scales such that the ocean gains N during times of glacial advances and loses it when glaciers retreat (Christensen et al., 1987; Codispoti 1989). Christensen et al. (1987) have argued that the supply of N to the oceans increases during glacial advances by way of reduction in terrestrial biomass and N release from erosion of shelf sediments. The latter could contribute 4.2 x 10^  Tg N, which if supplied uniformly over 10,000 years, would amount to a supply rate increase of 4.2 Tg N yr"1. The enhanced nitrogen supply resulting from a decreased terrestrial biomass is of the same order. By contrast, Christensen et al. (1987) assert that the sink for nitrogen increases during interglacials due to growth of the area of the submerged continental shelf that hosts sedimentary denitrification. Reducing the shelf area by 75%, as during the Last Glacial Maximum (Hays and Southam , 1977), would decrease modern shelf denitrification rates from 100 (Allen H. Devol, pers. comm., 1995) to 25 Tg N yr" 1, assuming that the decrease in denitrification is proportional to the shelf area. This "source", when added to the N that could be potentially derived from diminished terrestrial biomass and shelf-sediment erosion, would at best balance the glacial nitrogen budget, but the sum would be far from sufficient to cause a buildup of fixed N in the glacial ocean. From these considerations, it is apparent that an increase in the nitrogen inventory requires an additional decrease in the sink. Our results from the ETNP agree remarkably well with those of Altabet et al. (1995) in the Arabian Sea, suggesting that denitrification in both of these regions was reduced substantially during glacial episodes. Furthermore, studies from the Eastern Tropical South Pacific also show a general decrease in productivity accompanied by lighter 8^N values during the LGM (Reimers and Suess, 1983a; Reimers, 1981). Since these three areas account for almost all of the 60-90 Tg N yr - 1 lost due to water-column denitrification in the present ocean, global denitrification must have been greatly diminished during glacial periods. Assuming that water column denitrification was completely absent during the LGM and that shelf denitrification was reduced by 75%, the total annual N loss would have been reduced to ~40 Tg N yr"*, pushing the glacial oceanic-N mass balance to a substantial surplus. This scenario is consistent with previous model results (Altabet and Curry, 1989) that suggest that a 75% reduction in the area of oxygen-deficient water column is enough to increase the oceanic N O 3 " inventory by 25%. Primary production in the modern ocean is thought to be nitrogen-limited (Codispoti, 1989, Fanning, 1992) although in some areas production may be constrained by a dearth of micronutrients such as iron (Martin, 1990; Kolber et al., 1994). Nitrogen has a much shorter residence time (of order 10^ -104 years) than phosphorus (-10^ years), and its abundance in the ocean is more likely to fluctuate on glacial-interglacial time scales (McElroy, 1983). Thus, relative to nitrogen, phosphorus does not appear to have been important as a marine productivity regulator during the Late Quaternary. Indeed the molar N:P ratios in shallow waters in large areas of the Pacific and Indian Oceans where denitrification occurs are much lower than that predicted by Redfield Ratio (N:P = 16:1; ref. Redfield et al., 1963) (Codispoti, 1989, Fanning, 1992). Given the observed relative N-deficiency in the modern ocean, it is suggested that reduced denitrification in the glacial ETNP in concert with a similar reduction in the northern Arabian Sea and eastern Tropical South Pacific may have played a key role in lowering glacial atmospheric C O 2 levels by enhancing oceanic productivity in areas that are today considered to be oligotrophic. Via this 123 mechanism, a significant increase in the glacial N O 3 " budget could theoretically account for much of the observed pC02 decline seen in the ice-core records (Berger and Keir, 1984). 4. 4 CONCLUSIONS Nitrogen isotope measurements indicate a substantial decrease in dentrification in the oxygen-deficient water masses of the ETNP during late Quaternary glacial periods. Large synchronous decreases in biogenic fluxes suggest that the decreases in denitrification are forced by reduced upwelling, decreased export productivity, and consequent lower oxidant demand along the continental margins bordering the ETNP. These results provide strong support to the hypothesis that oceanic productivity and attendant changes in atmospheric C O 2 may have been modulated on glacial-interglacial time scales by changes in denitrification which caused variations in the oceanic nitrate inventory. 124 Chapter 5 Closing Remarks The late Quaternary history of upwelling, carbon burial and denitrification on the continental margin off NW Mexico has been discussed in this thesis, the overall objective being to explore the nature of the link between climate and coastal upwelling. A summary of the key contributions that have arisen from this study, their implications and some suggestions for future research are presented in this chapter. The history of biogenic sedimentation off NW Mexico was discussed in Chapter 2. It was concluded that the accumulation of biogenic components (organic carbon, opal and barium) on the continental margin declined during the glacial periods of the last 140 kyrs. This decrease was attributed to a shift in upwelling-favorable winds along the continental margin. A scenario describing the regional wind fields during glacial episodes was synthesized from published GCM results, published and forthcoming terrestrial paleoclimatic evidence and the new sedimentary records of upwelling discussed in this work. It is proposed that the waxing and waning of ice on the North American continent may have been fundamental in controlling upwelling-favorable winds and export productivity on the NW Mexican Margin. The inferred glacial decrease in upwelling off NW Mexico has profound implications for the existing view on the response of coastal upwelling zones to glacial climatic forcing. Low- and mid-latitude upwelling is thought to have increased globally during glacial periods owing to an increase in meridional wind strength, as evidenced in records from the equatorial divergence zones and upwelling regions off NW Africa. The consequent increase in export productivity has been directly implicated in the drawdown of pCC»2- The upwelling history off NW Mexico contradicts this view and strongly suggests that the response of coastal upwelling zones to glacial climatic forcing has been areally variable. This brings into question, indeed weakens, the hypothesis that increased low- and mid- latitude upwelling effected a pC02 drawdown during the Last Glacial Maximum. Significantly better information on the areal variability of export productivity through the late Quaternary is required before a realistic assessment can be made of the role of upwelling in modulating atmospheric pC02- In this respect, one region that clearly requires further attention is the western margin of North America. The COHMAP GCM predictions for the Last Glacial Maximum, namely a stronger Aleutian Low and a weaker North Pacific High, should have resulted in widely decreased upwelling along the margin. The marine records off NW Mexico, northern California and Oregon point in this direction, as do limited terrestrial paleoclimate data. Indeed, it is highly encouraging that model predictions, terrestrial data and marine records were able to be integrated effectively in this thesis in reconstructing the regional paleoclimate off northwestern Mexico. Future studies off the western Canadian Margin as well as continuing work off Washington, Oregon and California should further this progress by allowing a better description to be made of the wind fields that prevailed over the eastern North Pacific during glacial periods. Chapter 3 critically examined the factors that control organic carbon burial off NW Mexico and in particular evaluated the relative importance of the role played by the oxygen minimum in enhancing sedimentary carbon accumulation. Productivity, and consequently the supply of organic carbon, has been earmarked as providing the first order control on organic carbon burial. The organic matter buried in laminated sediments under the intense oxygen minimum is not enriched in hydrocarbons compared to homogeneous sediments that accumulated under well oxygenated conditions. Instead, hydrodynamic sorting has been identified in this work as a factor that influences both the spatial distribution and the preservational quality of sedimentary organic matter. Attempts to use organic carbon records to assess historical productivity variations on continental margins have sometimes been discouraged due to the possibility of differential preservation induced by variations in bottom-water oxygen concentrations. The multitracer approach used in this study, however, clearly demonstrates that the downcore organic carbon distributions retain the productivity signal despite large changes in past bottom-water oxygen concentrations and sedimentary redox conditions. Hydrogen index measurements made on the Mexican margin core suite clearly illustrate that the preservation of Type II 'oil prone' kerogen is not exclusive to sediments deposited under an intense oxygen minimum. This result broadens the spectrum of conditions under which petroleum-generating source beds are deposited on continental margins. Matrix adsorption during Rock-Eval pyrolysis is shown in Chapter 3 to be potentially responsible for the co-occurrence of high organic carbon concentrations and hydrogen indices seen commonly in geological records. This possibly artifactual congruence has often been used to support the argument that enhanced preservation holds the key for elevated organic carbon contents in sediments. Finally, overemphasis on the role of low oxygen concentrations in promoting organic matter preservation seems to have diverted attention from factors that could prove to be quantitatively more important, for example sediment winnowing which may affect the hydrocarbon-richness of preserved organic material. The influence of such reworking mechanisms definitely warrants further assessment. Nitrogen isotope measurements were used in Chapter 4 to show that denitrification in the Eastern Tropical North Pacific was substantially diminished during glacial periods in response to decreases in upwelling and productivity. Contemporaneous decreases in denitrification have also been inferred in the Arabian Sea. Thus, at least two of the three continental margins where significant water-column denitrification is occurring today apparently witnessed less nitrate loss during late Quaternary glacial episodes. Since denitrification in these regions accounts for the largest sink of fixed nitrogen in the modern ocean, it is apparent that oceanic productivity and attendant changes in C O 2 may well have been modulated on glacial-interglacial time scales by variations in the oceanic nitrate inventory. This phenomenon requires further documentation and quantification. First, it has to be establish with certainty whether denitrification decreased during the glacial periods in all major oceanographic regions where significant water-column denitrification is occurring today, and whether denitrification was occurring in different regions during the glacial periods. The planned study of sediments deposited under the OMZ off Peru will address the first objective, while the pending analysis of cores from the continental margins off California and Vancouver Island will start to address the second. Second, variations in denitrification rates should impart variations in the "whole ocean" nitrate 8l^N signal. One promising line of investigation would be to focus attention on the central gyre regions, where sediments may record changes in the nitrogen isotopic composition of "whole ocean" nitrate. 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Oxidation fronts in pelagic sediments: diagenetic formation of metal-rich layers. Science, 232, 972-975. Wyrtki, K., 1967. Circulation and water masses in the eastern equatorial Pacific Ocean. International Journal of Oceanology and Limnology, 1, 117-147. Zeitzschel, B., 1969. Primary productivity in the Gulf of California. Marine Biology, 3,201-207. Appendix I Analytical Methods 1-1 Total carbon, nitrogen and carbonate analysis Total carbon and nitrogen were determined by flash-combustion gas chromatography on a Carlo-Erba NA 1500 CNS elemental analyzer. The analytical operation of this instrument and the composition of the combustion column are similar to those described by Verardo (1990). Five to ten mg samples were precisely weighed into small tin cups using a microbalance and then loaded into the sample changer of the instrument. The samples are flash combusted in a stream of oxygen and helium at a temperature >1800 °C. The combustion products are quantitatively oxidized by passing them through a column of granulated Cr203 overlying silvered cobaltous cobaltic oxide (CO3O4 + Ag) maintained at 1050 °C. The excess oxygen is removed by passing the gas stream through heated copper at 650 °C. This treatment also converts N O 2 to N 2 . H 2 O is removed by passing the stream through a water-absorbing filter containing magnesium perchlorate. Gas separation is done on a Porapak QS chromatographic column maintained at 54 °C. Detection is by thermal conductivity. Acetanalide (CH3 C O N H C 6 H 5 ) and sediment standards were used to calibrate the instrument. The analytical precision (1 a) was estimated at + 1.25 % for total carbon and + 2 % for nitrogen. Carbonate carbon was directly determined by coulometry. Carbonates are decomposed with 10 % HC1 in a heated reaction vessel and the CO2 produced is swept by a C02-free air stream into ethanolamine solution containing a colorimetric indicator. C O 2 reacts with ethanolamine to form a strong titratable acid, which causes the %Transmittance to increase. The titration current is then automatically turned on, and OH- ions are electrically generated by reducing H 2 O at a silver electrode until the solution returns to its original colour. The total amount of current required is integrated and the results displayed as fig C. The precision was estimated at + 3 % (lo). Organic carbon was estimated by subtracting carbonate carbon from the total carbon measured using the CNS analyzer. 1-2 Biogenic opal Biogenic opal determination was based on a procedure modified from Mortlock and Froelich (1989). Powdered sediment samples (~50 mg) were treated with 10% H 2 O 2 and 10% HC1 to remove organic and carbonate carbon. Excess acid and peroxide were removed with distilled water and the sediments were dried overnight in an oven maintained at 60 °C. Opal was extracted with 20 ml of 2N Na2C03 solution in a constant temperature bath maintained at 85 °C for a total of 3 hours. Dissolved silica concentrations in the extract were determined by molybdate-blue spectrophotometry as described in Mortlock and Froelich (1989). % opal is calculated as 2.4 x % Si0pal- Care was taken to avoid clay contamination by optimizing the extraction time. Mortlock and Froelich (1989) reported that a single 5 hour extraction provided an accurate measure of biogenic silica and extraction beyond this duration increased the possibility clay contamination. However, timed extraction experiments in our laboratory showed that biogenic opal extraction is complete within the first 3 hours (Figure 29). The difference seems to be mainly because of the use of powdered sediment samples in our studies. Furthermore, by using powdered sediment samples we hoped to eliminate the problem of incomplete dissolution of radiolarians (due 153 10-*i 6-1 a. O Mexican Margin, Core NH15P (100-102 cm) 0- T 0 T 1 T 2 3 4 Time (hr) Figure 29. Percent Opal vs. Time plots for 85 °C 2M Na2C03 sequential timed extractions of two powdered sediment samples. The opal concentrations become asymptotic after 2 to 3 hours indicating the completion of the dissolution of biogenic opal. to their large size) reported by Mortlock and Froelich (1989). Accordingly, extraction time used in this study was reduced to 3 hours. In addition, Mortlock and Froelich (1989) have shown that the presence of opal in sediments inhibits day dissolution during extraction and clay contamination occurs in the absence of biogenic silica. Therefore severe clay contamination is a concern when measuring samples with low opal concentrations using this technique. However, a series of experiments with pure clays and clays spiked with opal showed that clay contamination is only severe in samples with less than 3 % opal (Table 5). The elevated 'opal' concentrations seen in the pure clay samples are mainly due to the dissolution of clays, which is inhibited in the presence of opal in the spiked samples. Note that the opal values reported in this study are in most cases higher than 3 %. The precision of this modified method was estimated to be +5 % (la) based on an internal sediment standard having > 5 % opal. The precision is expected to be lower for samples with less than 5 % opal. 1-3 Elemental analysis by X-ray fluorescence Major and minor element concentrations were determined using a Philips PW 1400 wavelength-dispersive sequential automatic spectrometer. Details description of instrumental conditions and corrections are given in Calvert et al. (1985). Determination of the major elements Fe, Mn, Ti, Ca, K, Na, Si, Mg and P were carried out on fused glass disks. To make a disk, a powdered sample is mixed with Spectroflux 105 (Li2B407, La2C>3 and TJ2CO3) in a Pt-Au crucible and heated in the muffle furnace at 1100 ° C for 15 min. Weight (volatiles) loss is compensated with Spectroflux 100 (Li2B407). Subsequently, the mixture is fused on a burner and the resulting molten glass cast as a bead in Table 5. Silica measured in pure clays and clays spiked with 3 wt. % pure diatomaceous silica after extracting for 3 hours in 2 N Na2CC»3 solution at 85 °C. Unspiked Clay* Spiked Clay* Bentonite (Montmorillonite) 2.1 3.12 from Upton, Wyoming. Illite-bearing shale 2.6 3.26 from Rochester, New York. *Averaged from 3 replicate extraction a metal platen. Determination of the minor elements, Ba, Co, Cr, Cu, Ni, Pb, Rb, Sr, V, Y, Zn, Zr, Mo, I, and Br was carried out on pressed pellets. Powdered samples were backed with borax and pressed into rigid discs in a hydraulic press. International rock standards were used for the calibration of the spectrometer. The precision and accuracy involved in the determination of various element by this technique are given in Calvert et al. (1985) and Francois (1988). 1-4 Rock-Eval pyrolysis Hydrogen indices and total organic carbon were determined using a Rock-Eval II instrument following the procedures described by Espitali£ et al. (1977) and (Peters 1986). The Rock-Eval pyrolysis method involves step-wise programmed heating of 100 mg of sample from 300 to 550 °C in a helium atmosphere. The pyrolyzates evolved are measured using a flame ionization detector. 1-5 Stable isotope measurements Stable isotope ratio determinations were made using a VG Isotech Prism mass spectrometer. For foraminiferal analysis, samples were wet sieved on a stainless steel 150 u.m mesh using distilled water following disaggregation. Foraminifera were picked at the microscope from the >150 u.m fraction, briefly rinsed with methanol and roasted in vacuo to remove organics. Stable isotope ratio determinations were made using a VG Isocarb common-bath sample preparation system and microinlet. A detailed description of the analytical procedure is provided in Pedersen et al (1991). Analytical precision (lo) was estimated to be 0.04 and 0.03 %o for b^O and b^C respectively based on internal standards. Results were reported in the 5-notation relative to the Pee Dee Belemnite (PDB) standard. Carbon isotope ratios in organic matter were determined using a Carlo-Erba 1106 CHN elemental analyzer coupled to the Prism mass spectrometer, as described by Pedersen et al (1991). Carbonate carbon was first removed by adding 10% HC1 to the finely powdered sample in a watch glass. A portion of decarbonated sample was packed in a tin capsule for introduction into the CHN analyzer. The analytical precision of these determinations was better than 0.1 %o (la). 8l3c values were reported relative to the PDB standard. Nitrogen isotope ratios were determined using a Fisons NA-1500 CNS analyzer directly coupled to the Prism mass spectrometer. Powdered sediments were packed in tin cups and flash combusted as described above. The N2 gas is separated chromatographically and introduced into the mass spectrometer inlet online in a stream of helium. Masses 28, 29 and 30 are monitored in their entirety as the N2 pulse passes through the source. Analytical precision based on internal acetanalide standards was better than 0.2 %o (la). b^N values are reported relative to air. 1-6 Determination of salt and porosity The amount of chlorine, and thereby the salt content of the dried sample, was determined volumetrically using a method adapted from Strickland and Parsons (1968). Salt in 100 mg of dried sediments was dissolved in 5 ml of distilled water by shaking in a vortex stirrer. The supernatant was titrated against silver nitrate (AgN03) solution using potassium dichromate (K"2CrC)4) 158 as an indicator. Sea salt abundance was computed as 1.82 x %C1. All elemental analyses reported in this study were corrected for dilution by salt using the formula: [EUSalt free = [EL] x 100/(100-%Salt). Porosity was calculated from water content using the formula proposed by Crusius and Anderson (1991): Porosity = fraction H20/(fraction H 2 O + (1-fraction H20)/2.5)), where 2.5 was the assumed grain density in g/cm^ and the water content was estimated from % Salt using the empirical equation suggested by Doff (1969): % H2O=(100 x % Salt)/(3.5+(0.985 x % Salt)). 159 Appendix II Data Tables Table JJ-1 Concentrations of biogenic components and mass accumulation rates from the Mexican margin piston cores Depth Cal. C 0 rg N Opal CaC03 CQrg Opal CaC03 cm Age kyr wt. % wt. % wt. % wt. % MAR mg/cm 2/kyr MAR mg/cm 2/kyr MAR mg/cm 2/kyr NH15P 2 0.955 9.54 0.96 8.74 2.76 631 578.2 182.5 12 1.46 9.21 0.94 8.66 1.08 645 606.5 75.53 22 1.96 9.28 0.92 9.43 1.66 667 677.3 119.1 32 2.46 8.30 0.85 8.22 3.04 533 528.1 195.1 42 2.96 8.44 0.85 7.33 3.06 655 569.5 237.5 51 3.41 8.60 0.87 7.10 4.15 684 564.5 329.8 62 3.96 8.42 0.82 7.86 6.74 683 638.6 547.7 72 4.22 9.74 0.96 7.86 3.06 859 693.2 270.2 81 4.44 9.16 0.88 7.50 7.48 744 609.0 607.1 92 4.72 9.60 0.91 7.55 4.96 739 580.7 381.9 101 4.95 8.77 0.84 7.87 6.68 834 748.3 634.9 112 5.22 8.65 0.83 7.85 8.42 469 425.4 456.5 122 5.58 7.21 0.66 7.52 1.57 427 445.0 92.94 132 5.93 8.41 0.79 7.53 5.64 455 407.6 305.3 142 6.29 8.98 0.85 7.10 8.39 447 353.4 417.3 150 7.02 8.68 0.83 4.52 431 224.6 152 7.20 9.68 0.90 7.28 7.51 480 361.3 372.7 160 7.93 7.56 0.71 5.24 378 262.1 162 8.11 8.82 0.81 7.87 5.28 444 396.5 265.9 170 8.83 7.84 0.74 5.99 392 299.6 172 9.02 8.08 0.73 8.99 7.30 401 446.1 362.3 181 9.61 6.37 0.74 20.3 330 1050 182 9.67 8.03 0.73 7.70 8.62 433 414.6 464.1 190 10.2 7.29 0.67 6.83 409 383.5 192 10.3 6.94 0.62 7.61 6.28 406 445.2 367.1 200 10.9 6.59 0.58 6.78 396 407.9 202 11.0 5.74 0.50 6.89 8.26 355 425.8 510.8 206 11.6 1.35 0.11 5.89 0.580 160 695.7 68.45 210 12.3 3.08 0.26 11.9 367 1417 212 12.6 1.76 0.16 5.98 15.6 212 719.1 1876 220 13.8 3.54 0.32 7.25 348 713.8 222 14.2 3.79 0.34 6.11 5.61 291 469.3 430.5 229 14.6 2.98 0.25 4.58 250 384.8 234 14.9 3.06 0.28 6.08 3.62 265 526.3 313.6 240 18.6 0.819 0.073 13.2 23.7 382.5 242 19.9 0.984 250 21.6 1.45 255 22.7 1.10 265 24.9 1.27 275 27.1 1.71 285 29.3 2.17 295 29.7 1.93 305 30.1 2.39 315 30.5 2.68 325 30.9 2.32 335 31.3 1.88 345 31.7 2.26 355 32.1 1.49 365 34.0 2.37 375 35,8 3.09 385 37.7 3.28 395 39.6 2.24 405 41.4 2.88 415 42.4 2.50 425 43.4 3.18 435 44.3 2.98 445 45.3 2.99 455 46.3 3.26 465 47.3 3.15 475 48.2 3.66 485 49.2 3.94 495 50.2 4.54 505 51.2 4.01 515 52.1 4.22 525 53.1 3.87 535 54.1 3.63 545 55.1 3.72 555 56.0 2.44 559 56.4 2.09 565 57.0 2.33 575 58.0 3.35 585 59.0 3.35 595 61.1 1.87 605 63.2 1.69 615 65.4 1.85 625 67.5 1.86 635 69.6 2.00 645 71.8 2.52 655 73.9 2.54 665 75.2 4.88 675 76.5 5.39 0.091 3.83 14.0 0.094 10.1 0.098 4.10 13.4 0.12 3.92 12.3 0.15 3.61 10.4 0.19 3.86 8.45 0.17 3.68 11.8 0.21 4.02 5.23 0.24 3.90 3.30 0.20 4.37 5.70 0.16 4.36 9.26 0.20 4.19 6.43 0.13 4.16 6.77 0.22 4.98 3.16 0.26 5.84 3.39 0.28 5.36 3.11 0.20 7.05 9.78 0.25 5.92 1.28 0.21 5.53 1.77 0.27 5.89 0.335 0.21 5.65 2.05 0.24 6.02 0.723 0.25 6.46 0.0606 0.25 6.54 0.370 0.30 6.96 0.232 0.32 6.85 0.0966 0.35 6.84 2.35 0.34 7.17 1.19 0.35 7.19 0.437 0.32 6.95 0.156 0.30 6.81 0.112 0.30 6.95 0.197 0.21 5.90 3.03 0.18 4.24 4.17 0.20 4.47 2.15 0.33 4.69 0.658 0.28 4.59 0.762 0.16 4.85 3.79 0.14 4.20 0.224 0.15 4.00 2.55 0.16 4.73 3.45 0.17 4.79 5.76 0.20 5.18 5.90 0.19 6.13 5.87 0.40 7.13 0.283 0.43 7.17 0.160 36.9 143.9 523.7 55.7 387.6 43.1 160.6 526.0 46.7 144.6 452.2 55.6 117.7 339.5 63.1 112.3 245.7 115 218.5 699.1 138 232.3 301.8 142 207.0 174.8 132 249.8 325.9 120 279.8 594.1 126 233.8 ' 358.6 104 289.5 471.1 128 269.4 170.8 156 295.0 171.4 165 270.1 156.8 150 307.7 66.78 140 309.2 99.05 150 278.3 15.83 133 303.7 110.3 152 306.2 36.74 167 331.5 3.112 164 340.2 19.26 179 339.7 11.32 184 320.1 4.514 208 314.0 107.8 184 328.0 54.39 188 320.9 19.52 182 327.7 7.340 173 325.8 5.364 179 334.2 9.492 145 350.4 180.3 131 265.3 261.3 144 277.1 133.2 193 241.5 33.89 180 246.7 40.94 127 329.0 256.9 118 293.7 15.64 127 274.7 175.1 128 326.5 238.3 133 318.7 383.0 158 326.1 371.3 161 388.2 372.2 233 340.9 13.55 249 330.9 7.402 685 77.8 5.44 695 79.2 6.28 705 80.5 4.71 715 81.8 3.39 725 83.1 3.38 735 84.4 2.66 745 85.7 2.26 755 87.0 2.57 765 88.3 2.87 775 89.6 2.45 785 90.9 2.97 792 91.6 5.36 805 92.9 4.94 815 93.9 6.14 825 94.9 6.26 835 95.9 5.36 845 96.9 4.25 855 97.9 3.75 865 98.9 5.44 875 99.9 5.69 885 101 5.62 895 102 5.78 905 103 3.69 915 104 5.62 925 105 5.33 935 106 1.37 945 107 1.50 955 108 1.57 965 109 2.64 975 110 1.96 985 111 1.67 995 112 1.31 NH8P 2 1.57 9.92 7 1.75 9.78 12 1.94 9.52 17 2.12 9.47 22 2.30 9.57 27 2.48 9.56 32 2.67 9.51 37 2.85 9.82 42 3.03 10.2 47 3.22 10.2 52 3.40 10.2 57 3.58 10.3 62 3.76 9.97 0.43 7.12 0.190 0.42 7.32 1.08 0.37 7.42 0.0443 0.27 5.00 0.120 0.26 5.17 0.503 0.19 5.25 0.581 0.18 5.17 4.12 0.19 5.27 3.02 0.22 4.91 4.85 0.18 5.32 3.41 0.23 7.18 4.46 0.41 7.17 0.563 0.40 7.34 1.85 0.48 7.60 0.0776 0.50 7.82 0.0550 0.43 7.55 0.0235 0.33 7.88 0.0303 0.30 7.36 0.0820 0.43 8.11 0.103 0.46 7.97 0.0574 0.45 8.13 0.128 0.47 7.94 0.0791 0.30 7.86 0.241 0.46 7.17 0.145 0.44 7.32 0.0920 0.12 6.11 6.15 0.13 4.76 5.89 0.13 4.51 6.08 0.22 4.59 3.01 0.18 4.50 6.04 0.15 4.89 6.68 0.11 4.67 6.16 1.1 8.43 10.4 1.1 7.84 10.7 1.1 7.75 11.5 1.0 7.82 11.7 1.0 7.71 11.3 1.0 7.55 11.6 1.0 7.44 12.4 1.1 8.25 12.4 1.1 7.23 11.9 1.1 6.78 11.8 1.1 8.68 11.9 1.1 8.56 10.6 1.1 8.46 12.2 255 353.5 8.890 287 354.0 49.27 232 337.2 2.182 198 311.8 6.982 197 321.7 29.34 173 340.9 37.69 151 346.7 276.3 171 351.8 201.5 176 300.7 296.8 163 354.0 227.0 177 428.2 265.9 246 329.1 25.85 244 361.6 91.12 280 346.8 3.542 281 350.8 2.467 258 363.4 1.129 220 408.3 1.570 208 407.4 4.539 266 396.1 5.047 267 374.3 2.699 272 393.3 6.181 275 378.8 3.773 200 425.1 13.03 267 340.7 6.905 262 359.6 4.522 104 462.9 466.2 109 345.8 428.4 113 324.8 438.4 146 253.2 166.1 119 274.6 368.6 115 337.8 461.4 101 360.0 475.2 361 307.0 379.4 364 291.6 397.1 348 283.0 420.5 350 289.3 432.2 351 283.2 416.0 362 285.8 437.8 362 283.6 471.6 362 304.4 458.0 373 265.6 436.1 396 262.9 457.9 392 334.6 458.9 416 344.2 425.9 388 329.1 473.2 67 4.67 10.2 72 5.58 11.2 77 6.49 12.2 82 7.36 9.77 87 8.24 8.29 92 9.12 8.43 97 9.99 8.53 102 11.3 7.69 107 12.6 6.72 112 13.9 4.48 117 14.2 4.47 122 14.5 4.45 127 14.8 4.87 132 15.1 5.61 137 15.4 4.10 142 15.7 3.83 147 15.8 3.63 152 15.9 3.46 157 16.0 3.42 162 16.0 3.39 167 16.1 3.66 172 16.3 3.51 177 16.5 3.56 182 16.7 3.74 187 16.9 3.76 192 17.2 3.84 197 17.4 3.77 202 17.6 4.58 207 17.8 4.32 212 18.0 4.24 217 18.1 4.57 222 18.3 4.94 227 18.5 4.45 232 18.7 4.73 237 18.9 5.14 242 19.1 4.83 247 19.2 4.57 252 19.4 4.48 257 19.6 4.21 262 19.8 4.30 267 20.0 4.97 272 20.2 4.77 277 20.4 4.80 282 20.5 4.55 287 20.7 4.66 292 20.9 4.61 1.1 7.75 12.4 1.2 8.41 7.47 1.2 8.29 3.80 0.98 7.41 17.9 0.83 6.90 17.7 0.84 6.69 20.8 0.85 6.30 18.6 0.75 5.71 19.9 0.64 6.15 20.6 0.43 5.87 14.9 0.44 5.66 15.0 0.44 4.21 16.3 0.48 3.23 16.3 0.54 3.03 18.3 0.40 3.09 16.1 0.37 3.55 14.6 0.35 3.47 14.0 0.33 3.67 14.8 0.34 3.62 14.5 0.33 4.23 14.1 0.34 3.69 13.5 0.34 3.76 . 12.1 0.34 3.90 12.5 0.36 3.81 11.0 0.37 4.06 10.7 0.37 3.93 10.1 0.36 4.01 12.5 0.43 3.99 16.7 0.41 3.63 17.1 0.40 4.02 18.9 0.43 4.17 19.0 0.46 4.21 18.5 0.43 4.23 18.1 0.45 4.12 17.8 0.49 3.97 14.4 0.46 3.24 15.1 0.44 3.92 15.9 0.43 3.22 14.2 0.40 3.27 17.4 0.41 3.52 14.2 0.47 3.27 12.5 0.45 3.45 12.5 0.46 3.46 12.5 0.44 3.68 13.0 0.44 3.83 12.2 0.44 3.69 11.8 399 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ON CO l-H T t CN CO i n CO i n CO 1-H T t $ CO T t od co co T t oq CM o CO t N CO t N CO ON CM CM CO CM CO oq CM l-H CO CM CO ON CM CM CO r n CO ON CM NO CO CO CO o CO ON CM ON CM v q CM CN CM a a o CO o CO NO CM o CO r H ON O o T t o T t CO r H vo l-H o CM t-H l-H CO o O l-H t-H 1-H i n o CM 1-H CO 1-H NO o t N l-H m t-H 00 o NO o CO o l-H l-H CO o 3 1-H o o 1-H 1-H O i n 1-H 00 o l-H o t N o r H r H CO ON 00 t-H NO t-H i n t-H o l-H t-H t N CO r H l-H t-H i n od t-H 00 T t l-H 00 ON CM 1-H t-H l-H CO l-H CM 1-H T t 1-H t-H l-H t N ON CN l-H i n 1-H T t ON i n 1-H CO t-H t-H v d 1-H T-H T t l-H T t t N CO l-H CO o t-H 93.6 00 o t-H 00 o t-H 1-H O t-H 99.5 98.6 l-H o l-H 96.0 97.0 95.9 98.7 97.0 93.9 93.4 96.5 93.4 96.5 98.7 r H O NO o 3 NO O 1-H O i n o 1-H o i n © 98.7 97.6 o o I-H ON a CO r H cO CM m t N SQ it CM i n co CO t-H i n co CO i n NO co 1 i 1 l-H NO CO CO i n CO i n CO CO oq CM co 00 r H co o l-H co CO o co i n ON CM o CM CO i n o CO CO CM 00 1-H CM 00 cQ 00 NO CM § R CM a a NO CM C ^ cQ T t CM 1-H CM n c^ cQ T t CM Nq CM a a a a T t CM c^ NO CM T t CM t N co 00 i n r H NO t-H i n t N T t 55 t N T t C N T t t N T t o i n i n T t i n T t CM T t T J CM m $ o i n 3 l-H NO ON m NO NO i n NO CM NO CM NO ON i n CO i n ON i n t N r H OV 00 CM t-H O t-H o od m od l-H l-H CO NO od od O 1-H 00 ON o t-H NO CM CO od od i n o oo NO ON t N NO CN CN T t od CM l-H 1-H 1-H ON ON CO CO ON C N 00 CN 00 v d 00 00 CM ON i n ON CM t-H m t N t-H i n m CM CO i n t N CO i n CM t N 3 3 CM i n i n t N i n m CM C N i n i n CM 00 i n CN 00 i n CM ON i n CN ON i n CM O NO CN O NO CM r n NO CN l-H VO NO CM NO CM CO NO C N CO NO 3 3 CM i n NO CN i n NO CM ^ N N i n N O ^ Q O O N O o d n r i f f i t O L O o oo I N T*< T * L O CM VO NO NO VO O r i r-i O ^ CO IN IN IN IN CO r-H T-H o 00 t-H l—I t-H t-H OV ON Ov oo Ov o o IN 1-H 1-H LO 1-H rH CO CO o CO t-H CO r-H r H o o t-H ON v d LO o o Ov Ov Ov Ov t-H r n r H 1-H Ov oo ON ON 1-H t-H CM oo CO T t cs LO t-H Ov 1-H es LO T t £N Ov © LO es CO cs LO v d 00 1-H CS LO L N LO LO vo VO VO VO VO LO LO LO LO LO LO Ov LO Ov LO r n 88 t N O 2° es 1-H IN cs o v o oo IN Ov Ov oo O IN oo oo CC] NO cs CS cs cs CS es cs CO CS es cs cs o o 1-H t-H CO CO T t CO LO NO O T t C S O r - H r t l N L O L O r H C O L O O N N p ^ N ^ ^ N p S [ N ^ H l C N ON ON ON ON 00 L N L N NO NO ON ON CO CO I O ON LN o r l c d r i i n o H ' N n <*>. *i i - H i - H O i - H O O O ' s t v O L O L O O N T t r H T-H I - H I - H I - H I - H I - H ON ON ON 00 LN LN v O v O L O L O v O O v L O C O C O l N e S O T t L O L O C O I - H O N O N O O O I - H L O C O C O T t T t ^ ^ CO CO T t T t T t T t T t T t T t O T t L O l N O N O T t c O N O v O r ^ C O T t C O C O C O NO 00 IN LN 00 LN LN LN NO NO 00 LN LN LN NO NO O O v O L O v O O O v O L N L N v O N O L O T t L O CO T $ T ? ^ 9 L O r H C S C S N O N O N O T t N O O N C S O O N t N T | ^ a N d d L N l N ' L O CO _ _ LO LO T t T t LO LO LO LO LO LN T t T t L O ON CS ON NO L O CO CO LN* CS L N L N L N L N N O l N v O v O v O v O v O L O L O r H L N O O N O O l N O N L O N O l N C S O C O L O C S c o c s c o c s c o c o e s c s e s c s c s c o c o c o c o c o i N l N C S e S C O O O O T t ^ O Q C S O Q r H NO NO NO ^O LO NO LO LO LO T t LO T t T t C O C O C O N O l N O r H v O O v O O v O l N e S C O C O v O 1 - H r H r H r H t — i C S r H r H O O t - H i - H r H C S C S r H T t T t C O T t L N O V O C S T t O O L N I N I N N O L N N O N O N O es I N oo c o LO NO LO T t T t LO ON °0 1-H «5 CO 1-H r H 1-H 00 r H 00 r H o o O r H T t ^ CO t -H NO ON T-H t -H t - H ON I - H L O vO T t ON CO CS T t NO LO T t T t ON CO 00 1 - H r H r H r H i - H r H r H r H i - H r H i - H C S i - H LN ON es co es _ O 00 00 o o I - H ON ON ON L N IN* ON CO O L O t-H ON O NO o o NO ON p C S 00* C S I N C S LN* ON ON ON ON 00 L 0 0 o o 0 L q p L q c N L O L q p O N o q co co NO co NO NO T t NO LO* od T t co* LO* I - H I - H ON I - H ON ON ON ON ON 00 ON ON ON I N T t I - H ON T t ON es IS NO. oo co es o * es cs es cs es CM eQ cs cQ O O l N O O C S O O O O O N r H - T t O O O ON r n T * es es co co es co co ON LO I - H 00 NO I N T - H CO t N O "<t NO T+< CM I - H ON ON I N C O ON ON CO i—< CO T t T t T t T t C O C O C O T t C O C O T t T t T t T t T t C M C O T t L O C S L Q T t T t C O T t C O i - H C M C S C O i - H N r H O \ v O v O 0 Q i n v O l f i " * l f ) l f ) ^ c M c s c s c s c s c s c s c s c s c s c s c M c s H c s c s e s c o e M e M e M c M e s e s e s e s e s e s c s L O L O O O L N N O O O N O l N O O N p c O O N l N L O L O v O v O L O L O L O v O L O L O L O T t T t T t T t N O T t o O N C M c O C S C O N O T t O O O O C S LO LO LO T t T t T t T t T t CO CO CO CO T t T-H I - H ON rH 00 ON I - H T-H t-H CS ON ON CO ON ON ON ON LO CO LO es I - H es es es 1-H 1-H 1-H t - H 00 L N c s L N L N e s L N e s L N e s t N N O L N K C O O N O N O O v O v O v O v O v O v O L N L N e s t N C S l N e s t N C S l N l r H r - i f M C M C O C O T t T t i S e S L N C S l N C S L N C M L N r H vO T-H L N l N L N L N L N l N L N L N ^ C N l L N r H r H C M C M CO CO T t T t LO LO NO O N N O C O r H L O C O I D O O L O C O O O O N N O N O L O L O C O i - H C O N O O L O L N K O N r H O ^ O N ^ O O O O O l N O O r H i - H r H i - H t - H t - H i - H C N r H C N C N i - H r H r H O O O N T f LO IN NO CN NO NO LO LO LO ON c o L O c o I N T ^ CN o q ^ f L N r n \ q L O ON 00 C N C N C C N O O O O O N O K r - i o d r - ' c N T f L O L O L O L O L O NO NO NO NO NO L O NO NO NO NO L O NO NO L O NO NO I N NO NO r H C O O O N O l N O N l N O O C O N O l N r - i C N C N L O O O O IN CO r-J ON CO L O O CO NO NO ON IN LN 00 LN NO NO NO NO NO C N 00 T f LN ON LO NO NO 3 T f CO T f T f ON 00 LN CO O O T f 00 ON LN IN LN LN LN LN NO LN 00 LN CN CO ON r-J LO CO LO T f NO ON ON LN IN LN IN LN LN LN LN 00 NO 38 00 ON CN O ON LN 00 00 LN t N t N CN L O L O L O T f T f T f 00 O LN O CO CO r H CO T f T f T f T f CO T f - . r H CN T f T f T f CO r H T f C<] T f O N O O O O T f O O T f L O O O O O L N L N O r H O C N . . 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T t T t T t T t T t T t T t T t CO T t T t T t T t T t T t T t T t T t T t T t TJ* IT) U J U ) TJ* *3* TJ* CO TJ< C M C M C M C O C O T t T t T t C M C O C M C M c O C O C O C O T t C O C O C O C O T ^ c O T ^ NO CM N© T t CO CM ON r n CO CO t N 00 CM I N CO ON t N NO i n 00 r H ®Q T t T t NO NO i n NO CO NO r H r H r H r H CM CN r H CM r H CM r H r H CM r H CM r H r H r H r H r H C M N O r H r H r H r H r H r H r H r H CM T t CM CO NO T t O N O N i n O N r n CM CO r H NO T t t N i n O O O O O O O O O O O O r n O O O O O i n c o o ^ T t c M i N i n c o c M o o O O O N O O O O O O O O NO ON VO 00 i n r H r H I N NO r H r H CO i n r H I N NO ON CO 00 i n CO CO r H CM CO CO CO CM T t i n T t T t T t CM r H CM CM r H CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO o o i n T t O N c o c M N O i N c o o o r H c o i n i n c M c o m T t ^ H c N c o c o c o c o c o c o c o c o c O c O c o T t NO T t NO i n t N I N NO I N i n t N NO T t i n i n NO NO i n vo i n L N NO i n NO i n i n I N OO i n I N N N C N i N d N N N N N N C M N N C N l N N d C O n O t l ' O f O o N H N O C O H m N i n n N i a ' t ^ M t ^ O N N v O N O N T t i n i n i n i n v o v o i n v o v o v o i n N o i n N O N O v o v o v o t N v o i n i n i n v o v o v o v o v o N O r H r H r H CO CO T t T t T t ON T t N O r H C O C M C N C O C O r H C O C O C O C M r H C O r H C M C N r H r H C M O N T t O N T t O N T t O N T t O N T t O N T t O N T t O N T t O N T t O N T t O N T t O N "rt O N T t O N T t O N r n CN CN, co co T t T t i n W"N vo vo C N I N oo oo ov ON o o r H r n C N CN co co TJ* T t i n i n N N CT\ N H LO ON ^ . o m v o v D ^ ^ r t r H i r j o o i n c s f o i n ^ N N H O N o LO LO LO O l < C » C O L O L N O a \ L O l > r 4 o o 6 » \ O v O \ O \ O \ O \ O N v O N N 0 0 0 O N N 0 0 C O » N \ O N N ( » O N O A » ( » 0 0 0 0 C O ( » Os r - i 00 ro 00 fN fN NO i n c ^ o > N N o \ m i H (NJ oq rN T f v q ^ ^ ^ C O K C O C O L O N O O O O O O C O f N L O f N CN t—t L O f N NO CO CN O L O L O f N L O C*s OS SO SO NO O O r H r H O O O O t—t r H N N t » C O < ^ Q O O N 0 0 O > O > H f f \ O N O N O N a \ O N O > O > O N H H r H r H H H H r H H H LO LO CO 3 s L O r n L O CO CN ^ 1 0 3 8 3 cO ro r o so T f CN CN CN CN" CO* CN ro ro ro ro ro co CN CN r n T f LO CO CO CO CO 00 T f O T f fN © r n Os . . _ . CO CO CO CN CO CO CO O LO CN f N CO " " CO C N r H f N O f N f N r H r H ON c O C N c o c o c O r O r O T f T f L O C N T f c O T f c o c N r O T f T f L O r O cocovqrHiocoiN,oqiopcNco v d v O v p O O N O N v p L O L N i r H C O C O O N O N T f T f T f LO T f T f T f T f T f LO LO LO LO NO NO vO NO CN L O LO NO NO CN CN CN CN VO NO NO L O C O » 0 > O O O N l f t v O O N ^ O N i r ) N i r ) c N N O r ) N ^ O i n N \ O r t N t N » 0 r H T f C N L O O \ C O L O O O C N C N L O C N C O C N O O L O C N O O O O C O f N C O O N C O C O C O C N T f L O N O L O L O N O N O v O v O C N v © v O V O O O C N C N L O C N O O C N \ O L O V O 0 0 C N C N V O L O LO T f l O 0 0 L O Q 0 0 0 0 O N 0 0 T * . 0 0 L O Q O N Q L N , r O ) \ O L O 0 0 0^ cQ CN 00 00 CO r H r H LO r H CN ON T f r H QN CN r H VO 00 CN LO ON OS CO 00 r H T f CO r H f N CO T f O O O r H r H r H r H r H r H O r H r H O O r H O r H r H r H r H r H r H O r H t - H r H r H r H r H r H 00 T f vO LO CO O T f LO CN ON SO LO O O ON ON t N CN CN T f vO NO 00 CN T f O T f vO O CN r H ON O CN CO IN, LO CO LO LQ N O v O C N O C N V O T f v O L Q T f v O C N ON O O O ON 00 CN LO C O C N C O C O C N C N f N f N f N C N f N f N f N C O f N f N f N C N f N f N f ^ N O L O O r H O O N O N O O N r - H O N O O N O O N O O O N O N r H O N l N . O O \ O O Q O Q C N r H O f N C N C O C O C O f N f N c O f N c O C N t O r N c O f N r O f O f N f N c O r N f N c O C N c O f N O O N O O N C O C O O O C N N O C O r H C O T f C O f O N ^ N V D N \ 0 » 0 N C 0 X 0 0 C 0 N N N C 0 0 N N 0 0 ( » ! » K N N K O V 0 V 5 N N r H fN CN fN CO T f T f T f LO LO T f CO LO T f LO LO rO T f T f LO r n fN CO LO rO CN CO CN CO CO ON T f ON \ b N N _ _ C O C O C O C O C O C O C O T f T f ON T f ON T f 00 00 ON N o ON T f ON T f ON Q r H r H fN fN '"""J"' ON 5? ON T f ON ^ LO LO T f T f T f ON T f ON NO CN CN T f T f T f ON T f ON T f ON T f op ON ON CO O t—t T f T f T f LO LO LO o tN IN IN O r H ID CN CO NO T t O CD ON ON tN ON IN CO ON r n T t ID CD r H L O ID IN NO CN NO CN ON IN ON r n r H r H r H ON co co 00 ON ON ON ON I D C D C D C D C N r H O O r H ^ O N C N r H r t . i N t N l D O O O r H O N O O t N t N O N l D O N O O O r H O O r H r H O O O O O O O l N r H r H r H r H r H r H r H r H r H r H r H r H r H r H O N I N I N O I N O NO I N LN CD ON CN . _ . LN ON - f 00 00 ON LN ID ID O N O N C N C N " * I O C N C O N O TS 2 J« T TJ< r H CN CO CN ON ON CN ON ID ID ID T t T t LD r H NO ID O T t T t ID T t T t c o i D p v q i D O N O o q O O O r H r H O r H O O O N T t C J ._ - - - - - - - ^ ^ L D N O N O N O N O N O N O N O L D ID NO CD ON NO © CO © LN Tt CD ID r H CO Tt CD NO I D O N C D L N O N O O T t O O L N ID TJ< ON CD CN r—t CN l—I r H fN NO tN tN 00 CO ID CN CD 00 N r - i N 00 O ON r H CO O O r-H r-i T-H CN © H H H r i H O N X S N N N N N r t i O i n v o m ^ N v O N N N N N N N N N U ^ C O N O C O L N l D l D l N C O T t l D C N O O O O O N r - H T t O r H i r N C N L N l D O N C O C O C O C O O r H C ^ T t N O O N l O O O r H C N C N O N O O N O N " • U") T t T t T t T t ID T t T t CD Tt " " C D T t T t l D l D l D l D l D l D T t S CO ON IN ON LN IN 00 NO CO CO CO CD CD CD CO C O T t L N N O T t l D t N C O O O r H r H T - H NO T t ID CN NO NO 00 00 00 ON LN LN °. © °) © ON ON T-H 00 00 r H IN NO LN NO ON 00 00 ON 00 ON L N c O O O N O O O O O T t O N O O N O t N T t LN CO CO CN r-i T t ON CN ON CN T t LD NO LN 00 NO ID T t CO T t T t T t TJ* O O N r H O O C N O t N O q c N . w . • v , N CN W _ T t C N C O C O C O C O C O T t T t T t rH rH vO CN LN O d r ^ C O K c N o d t D C O C N C N C N O N K K T t K c O CN O IN ID TtTtcOCDCo3cOCO T t O O O c D c O C N i D T t N D L N L N O O L N O N N O N O l N N O U ^ T t C O C O C O C O C O C O C O r H O O O N r H O N O O r H O C O C D C O C O C N C D T t T t T t T t CO ON NO Q T^  00 ID CO CO T t CO CO I f i N a N N ^ r t N r t r t f O N N O O H r H O O N r t t O t N l N t N i H r H H O r i O ON 00 T t NO NO L N ON T-H ON L N T t LD CO 00 ID NO O CN ID 00 CN NO 00 CO r—* L N Tt NO NO ID T t C O r H V O r H l N ^ ^ C N ^ ^ ^ ^ ^ ^ P ^ ^ P c O N O C N C N C D N O ^ C D r H V O T t r H r H r H O O O O N N O O l N l D T t C D r H C O C N C N I D O O O O O O O O N O O O O rH rH rH rH rH rH ON ON rH ON ON ON ON NO ON 00 ON 00 00 rH r H rH rH T-H T-H ON r H r H T-H T-H ON CD O Q 3 ?D CD C O CD c O C N C N c O O N O N r H O N O N C O O O C N O O I D r H v o o O I D t D O N O r H O O T t l D NO C N C N NO rn Co 00 ON ON O p rn p p ON ON ON 00 C N NO ID ID ID ID T * C N C N C N C N c D c O c O c O c O T t T t T t T t T t C O C D C D c D C O C D C D C D C O C D C D ON ON ON NO C N 00 LD T t CD NO CD T t T t NO CD T t CD CN CO ID NO C N T t T t ID T t C N ID ID OQ C N C N C N C N C N C N N c N C N C N C N C N C N r H C N C N C N C N C N C N N O O N O N C O O N v o a N 0 6 ^ n N n » t O « t t ^ N N O N n O ^ O i n O LN C N NO LD LD ID LD T t ID T t ID LD LD LN NO NO NO LD NO NO NO NO NO NO NO NO NO NO NO NO CD CN ON rH r H O C O r H r H O C O r H C O r H CN CN CN O CO H N O h N 1 T t ON ON T t ON T t ON r H CN CN CO CO ID ID LD ID LD Tt 3 LD ON T t ON T t ON T t O N T t O N T t O N T t O N T t O N . . . _ e O O O O N O N C O O r H r H C N C N C D C D L D L D L O I D L D I D L D L D L D L D L D N O N O N O N O N O N O N O N O NS S tN K T t ON ID ID co T-H CN O r n CN VO CO sO vo T f cQ CN vO vO ov CO co CO CN LO OV as CO LN LO CN CO T f LO T f CO T f IN o o as CN s VO LO CO CO CO CO cQS r n vO VO VO « r n OS r n as T f IN vO Table TI-4. Carbon and nitrogen isotope data from the Mexican Margin piston cores. Depth 8 1 5 N 5"C Depth 5 1 5 N 8l3 C Depth 5 1 5 N 8l3C cm air %o PDB%o cm air %o PDB %o cm air %o PDB %o NH15P NH8P NH22P 2 8.87 -21.06 2 8.58 -20.87 2 9.02 -20.78 12 8.75 -20.92 7 8.39 -20.82 7 9.01 -20.84 22 8.94 -21.53 12 8.50 -20.93 12 8.45 -20.79 32 8.53 -20.96 17 8.46 -20.98 17 8.78 -20.78 42 8.81 -20.97 22 8.49 -21.02 22 8.93 -20.79 51 8.66 -20.80 27 8.31 -20.97 27 8.66 -20.75 62 8.97 -20.88 32 8.36 -20.96 32 8.68 -20.72 72 8.64 -21.37 37 8.28 -20.86 37 8.90 -20.89 81 8.91 -21.15 42 8.67 -20.89 42 8.96 -20.69 92 8.93 -21.09 47 8.60 -20.89 47 9.47 -20.91 101 8.82 -20.87 52 8.47 -20.84 51 9.55 -20.94 112 8.71 -20.72 57 8.43 -20.80 56 9.40 -21.00 122 8.55 -21.01 62 8.44 -20.93 61 9.11 -21.04 132 8.74 -20.90 67 8.51 -20.88 66 9.80 -21.18 142 8.54 -20.81 72 8.48 -20.93 71 9.73 -21.09 150 8.40 -20.73 77 8.76 -20.81 76 9.20 -20.97 152 9.38 -20.75 82 8.68 -21.23 81 8.28 -20.93 160 9.02 -21.24 87 8.48 -20.97 86 8.28 -20.89 162 8.98 -20.97 92 8.21 -20.69 91 7.99 -20.79 170 8.86 -20.91 97 8.25 -20.75 96 7.85 -20.63 172 8.90 -20.68 102 8.46 -21.55 101 8.14 -20.53 181 -20.99 107 7.95 , -21.50 106 7.54 -20.36 182 8.62 -20.56 112 8.56 -21.18 111 8.00 -20.34 190 8.77 -20.84 117 8.65 -21.27 116 7.56 -20.35 192 8.79 -21.03 122 8.65 -21.29 121 8.17 -19.95 200 -21.59 127 8.42 -21.38 126 7.81 -20.08 202 8.93 -21.82 132 7.93 -21.87 131 8.39 -20.31 206 9.01 -22.84 137 7.83 -21.28 136 8.41 -20.23 210 9.15 -21.47 142 7.64 -21.16 141 7.46 -19.83 212 9.47 -21.55 147 7.70 -20.78 146 7.19 -19.75 220 -21.73 152 7.51 -20.94 151 7.32 -19.78 222 9.86 -21.88 157 7.27 -21.07 156 7.39 -19.97 229 -22.09 162 7.40 -20.90 164 7.07 -19.83 234 9.89 -21.88 167 7.25 -20.90 169 7.23 -19.76 240 7.86 -21.48 172 6.97 -20.72 174 7.29 -19.82 242 7.81 -21.16 177 7.22 -20.65 179 6.78 -19.65 250 6.94 -21.42 182 7.07 -20.67 184 7.17 -19.65 255 7.31 -21.09 187 7.04 -20.68 189 7.81 -19.65 265 7.47 -20.75 192 7.26 -20.62 194 7.96 -19.80 275 7.38 -20.63 197 7.00 -20.52 199 7.75 -19.82 285 7.97 -20.67 202 7.05 -20.64 204 8.15 -19.87 295 7.65 -20.69 305 7.93 -20.72 315 7.97 -20.66 325 9.54 -20.79 335 9.48 -20.82 345 9.29 -20.92 355 9.01 -20.91 365 9.04 -20.98 375 9.77 -20.91 385 9.77 -21.25 395 9.23 -20.98 405 8.54 -20.78 415 8.79 -20.73 425 8.75 -20.68 435 8.82 -20.65 445 8.86 -20.81 455 8.44 -20.60 465 8.53 -20.61 475 8.68 -20.74 485 8.48 -20.48 495 8.99 -20.31 505 8.74 -20.95 515 8.43 -20.89 525 9.57 -20.89 535 9.74 -21.10 545 9.59 -21.07 555 8.69 -20.86 559 7.64 -20.78 565 7.52 -20.86 575 8.40 -20.91 585 8.27 -20.99 595 7.24 -20.82 605 7.17 -21.02 615 7.03 -20.97 625 7.60 -20.95 635 7.39 -21.18 645 7.01 -20.87 655 7.29 -21.00 665 7.67 -20.26 675 8.29 -20.38 685 9.75 -21.00 695 10.3 -21.21 705 10.1 -21.22 715 9.48 -21.21 725 8.17 -20.70 735 7.91 -20.83 207 7.32 -20.66 212 7.34 -20.71 217 7.26 -20.58 222 7.09 -20.65 227 6.81 -20.49 232 6.91 -20.57 237 7.14 -20.51 242 6.97 -20.49 247 7.10 -20.47 252 7.29 -20.35 257 7.10 -20.47 262 7.31 -20.49 267 7.13 -20.27 272 6.49 -20.17 277 5.97 -20.54 282 5.72 -20.43 287 6.10 -20.52 292 6.60 -20.36 297 6.33 -20.36 302 7.11 -20.55 307 6.85 -20.56 312 6.83 -20.49 317 6.84 -20.31 322 6.75 -20.19 327 6.76 -20.30 332 6.87 -20.25 337 6.65 -20.22 342 -20.22 347 6.67 -20.30 352 6.90 -20.58 357 6.39 -20.46 362 6.54 -20.45 367 6.07 -20.40 372 6.46 -20.38 377 6.41 -20.32 382 6.95 -20.18 387 7.07 -20.27 392 7.59 -20.55 397 7.52 -20.61 402 -20.64 407 7.82 -20.70 412 7.97 -20.75 417 7.91 -20.74 422 7.47 -20.31 427 -20.56 432 7.46 -20.45 209 7.59 -19.91 214 7.73 -19.81 219 7.79 -19.69 224 8.47 -19.68 229 8.71 -19.75 234 9.36 -19.81 239 9.22 -19.81 244 9.40 -19.90 249 9.22 -19.99 254 9.11 -20.04 259 8.94 -20.04 264 9.28 -20.01 269 9.22 -19.89 274 9.01 -19.99 279 8.63 -19.87 284 9.12 -19.90 289 9.21 -20.00 294 8.98 -20.10 299 8.82 -20.17 304 8.60 -20.15 309 8.26 -19.93 314 8.71 -19.90 319 9.41 -19.86 324 9.46 -19.76 329 9.33 -19.86 334 9.20 -19.94 339 9.21 -20.06 344 9.73 -20.09 349 9.70 -20.43 354 8.92 -20.29 359 8.16 -20.34 364 7.95 -20.07 369 8.16 -19.94 374 7.25 -19.72 379 7.33 -19.90 384 7.21 -20.05 389 6.76 -19.76 394 6.75 -19.78 399 6.92 -19.97 404 7.45 -20.04 409 7.45 -20.03 414 8.36 -20.16 419 8.22 -20.22 424 8.71 -20.25 429 8.20 -20.06 434 7.85 -19.85 745 8.34 -20.79 755 7.89 -20.81 765 7.83 -20.67 775 7.92 -20.84 785 7.39 -20.62 792 7.55 -20.42 805 8.06 -20.10 815 8.39 -19.81 825 9.86 -19.82 835 9.66 -20.04 845 9.49 -20.35 855 9.35 -20.48 865 9.82 -20.08 875 9.43 -20.26 885 9.71 -20.31 895 9.17 -20.39 905 9.95 -20.62 915 9.90 -20.86 925 9.83 -20.89 935 8.35 -20.80 945 8.82 -20.70 955 8.89 -20.79 965 8.57 -20.72 975 8.25 -20.81 985 8.14 -20.79 995 8.76 -20.66 437 7.23 -20.32 442 6.91 -20.27 447 6.92 -20.31 452 7.12 -20.45 457 7.24 -20.58 462 7.64 -20.33 467 7.30 2^0.23 472 7.21 -20.13 477 7.47 -20.15 482 7.83 -20.18 487 8.53 -20.24 492 8.69 -20.28 497 8.54 -20.66 502 7.71 -20.49 507 7.49 -20.39 512 7.52 -20.36 517 7.99 -20.24 522 8.08 -20.23 532 7.94 -20.16 537 7.78 -20.12 542 -20.26 547 7.86 -20.44 552 -20.36 557 8.27 -20.35 562 8.66 -20.38 567 -20.35 572 8.38 -20.34 577 8.54 -20.40 582 8.52 -20.46 587 8.97 -20.50 592 9.46 -20.55 597 9.34 -20.59 602 9.29 -20.66 607 9.25 -20.72 612 8.65 -20.62 617 8.31 -20.56 622 7.90 -20.56 627 7.87 -20.55 632 7.68 -20.50 637 8.08 -20.47 642 8.25 -20.57 647 8.64 -20.60 652 8.51 -20.27 657 8.31 -20.18 662 8.56 -20.36 667 8.34 -20.40 439 8.23 -19.61 444 8.44 -19.62 449, 9.67 -20.00 454 9.80 -20.07 459 8.89 -20.31 464 8.40 -20.13 469 9.13 -19.92 474 10.1 -19.89 479 9.86 -20.15 484 9.49 -20.25 489 10.1 -20.22 494 8.65 -20.19 499 8.66 -20.30 504 7.77 -20.13 509 8.72 -20.26 514 8.87 -20.48 519 9.84 -20.79 524 10.4 -21.05 529 10.4 -21.45 534 10.3 -21.45 539 9.89 -21.68 544 8.35 -21.51 549 8.11 -21.33 554 8.77 -21.24 559 8.99 -21.17 564 9.88 -21.25 569 10.0 -21.17 574 9.94 -21.11 579 9.87 -21.08 584 8.98 -21.08 589 9.18 -21.07 594 9.16 -20.86 599 9.83 -20.89 604 9.47 -20.96 609 9.82 -21.03 614 9.13 -20.94 619 8.76 -20.82 624 8.64 -20.74 629 8.95 -20.70 634 8.74 -20.69 639 7.92 -20.69 644 8.32 -20.61 649 8.23 -20.93 654 8.19 -20.90 659 8.39 -20.95 664 8.26 -21.02 199 672 8.27 -20.58 669 8.72 -20.98 677 8.22 -20.34 674 8.03 -21.02 687 7.71 -20.36 692 8.53 -20.41 697 8.30 -20.45 702 8.47 -20.33 707 8.02 -20.18 712 8.02 -20.02 717 7.62 -20.26 722 7.88 -20.25 727 8.18 -20.26 732 8.48 -20.46 737 8.74 -20.72 742 9.21 -20.75 747 9.19 -20.80 o o CN US , U to to PH S to CQ Q OH CU 0 0 < -a u 1 ex, IQ , U C Q to PH , O pa t O P H CU -a u Q t-H CO J? t-H to PQ Q PH cn Q Pu <u oo < U N N O CN CO ON CO _^ » ^ O N ^ O N O N l O * » N v O t N C O O N N r i v O ^ O N l N l N O O O O N O N c O C N C N r - i r - i O O t - H O O O O O t - H LO O NO T-H ON l -H O O O i o o I I o o I I O O L O N O C O O N O N O N r H C N C O T f C N T f C N C N C O i - H L O p C N o q CO C O C O C O C O C O C O C O C O C O C O C O Q 00 LTN T f L O O C O L O r H t N r H O N O N i H t O ^ T f v O N O I N CO LO CN ON CN CN NO NO T f CO CO ^ f T f T f T f T f T f T f T f T f T f CN NO NO LO NO " T f NO IH 60 •r» LO NO 00 0 \ I - H . . _ > C O O N L O r - ' 0 0 T f C N P c O C O T f L O L O N O C N O O O N PH CN CN X z O r i C O O N N O L O T f O N C O L N j O T f O O r - J L O O O C N C N C O T f L O N O l S l N c d o O O O O N O N O N T f ON T f ON T f ON i - H T-H CN CN T f ON T f CO CO T f O N C O C O O O C O O O C O C O O O T f L O NO NO C N C N 00 ON ON CO 00 CO 00 CO 00 CO ~ t-H l-H CN ~ ' ~ o o CN CO NO CO ON ON NO LO 00 LO NO IN r-H T-H v q CO ON ^ C N L O L O O O N O N O C N C N l N N O T f T - i e N O O O O O O O C S T - H C N C N L O O O L O C N o o o N O O o p o o o o o o p o s L o o o C N O o c N I — I t - H t - H O r - H r H t - H l - H t — I I — I o o o o o o o I I I I I I I o o o o o o o I I I I I I I L O l N O O T - i C N N O C O O N L O C N T f O O O L O T f O O O O N O T f C N c O C O L O p i - i C O C N N O N O L O N O O \ T f L O L N N O L O T f l N C N T f N O C N p O N \ q c O C O C N T f C N C N C N C N C N C N C N C N C N C N C N C N C N C N C N C N C N .s > 3 T f co O NO 00 r-H CO IN . ON 00 NO T f cN T-H ON C N L O CO T f T f T f CN ON C N L O 00 CN C O i - H t - H C N C M C N C N C N C N C O C O C O C O T f L O L O v d l N PH 00 X z _ T f ON T-H ON T f ON i -H CN CN T f ON T f ON T f ON T f CO CO T f T f LO LO NO ON T f ON NO IN C N 3 ON T f ON 00 ON ON 3 ON o 00 T f I - H CN vq C N L O ON C N C N L O ON ON o o o N O N L O N O L o o T f o o L O v q cSoqpoNT-HpcscNjpoOTf l - H O l - H O r r ° 1 - H l - H r H t " H T - " r H I T I T I I I I I I I 9 9 9 9 9 9 9 9 9 CO T f vO L O LN I - H L O O t-H CN t-H <N 8 t N L O L O N O i r N C N p c O C O O N O O t N o q o q i N p i N p o q C N p T f i N o q T-H f H r H CN f H CN T-H T-H CN T-H T"H f H C N T f oo oq CN oq r i H r i CM C N CO O CN CN r i r i IN ON 3 CO NO NO NO O r l LO LO p C0 r i r i CN CO CO n CN C N ON C N L O O CN r f E N T f T f T f T f C N CN 00 CO ON T f p CO ON CN LO LO LO NO NO C N C N C N 00 00 ON ON ON I - H I - H r i O O LO n O CO NO o CO L O T f 00 n L O n CN LO ON oo o o o PH LO rH X z C O T f T f T f T f O T f O T f O T f O T f O T f O CO C M r - J r H C M O r - J C O p p O O t-H r-J i -H r-i T-H T-H Q r H r -I l l l I I I I I T t ON CO R r H CO NO CO ON ON ON p ON d d d l -H d IN o oo cs CO O N C S O O r H i — i t - H t - H \ © C O T f O O N T f l O i n ^ O O O T t i r O O O O H ^ ^ n H O N O N r H r i M ^ O n O O N H O O NO CO r H CO NO r H vq T t T t r H LO CO CO r H CS CS CS T t ON 00 r H t-H T-H CO r H CO CO CS CO T?"J< T^JH Tj^ < TJ^ < Tj^4 T>^ < Tj^4 T "^J< Tj^ "J Tj"J< ^ < T ^ 4 Tl^4 T ^ H Tj^ < Tj^4 c S N O O N L O L O T - H T - H o o o o o N O N O N O N O O o q o q o o o q i N i N t N i ^ d d d r H C S T j J L o v d r N o S d d r H c O T t L O N d L N c c o ^ cs cs cs cs TJ» T ? 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T t LO LO LO LO LO N   r H CS CO T^H C O v O © v O C O O T t O N O N C S . T t l N O v O v O c O t N L O v O L N C O L O L N C O C S © C S r H O L O © © © 0 0 r H © © L N O O T t L N O O C S T t v O L N O v L O C S O O v O v v O L O L O T t c O O O r H i - H CS r H I I O v 0 0 © v O O N O v L N O \ 0 q L N l N O v C O l N r H i - H C q T t vo oo p cs d © © r-i r-i cs CS cs CS cs O N T t O V T t O V T t O V T t O N T t O V T t O V L N O O O O O N O v O C D r H r H C S C S C O C O c s c s c s c s c s c o c o c o c o c o c o c o c o O V T t O N T t O V T t O V T t O N T t O N T t O V O V T t O V ^ L O L O v O v O L N L N O O O O O N O v O Q r H C S C S cO co co co co co co co co co co T t T t T t T t T t © Ov O T t CO CO ON C S O C S O T t T t L N C O O N C S L N O O C O r H O O T t . - . L N c q o O L N i - H C S C O v S c O C N O q c O O O O O H H O O ' H LO LO L D O v O N L O t N v p i O v O v O L N T t C O O T t v O O N t N O l N © v o o o o v L N T t L O i - H c s i N L O v q c q L ^ c o c s c s c s c s c s c s c s c s c s c s c s c s c s C S C N T t L O C S C S C O L N C O O V r H L O v O l N O O O N O r H C S L N O O O V r H C S T t C O T t L N O N r H O O © © O r H r H r H L O L O L O V O K L N O O O O O O O O O V t N t N L N l N l N l N l N l N L N v O L N O O O N T t L O C S C O L O L O L O L O l O v O v O L N L N L N N K N N N N N N N O O C S C O T t L O v o K o O O N LN Ov Ov ON Ov Ov Ov Ov Ov CO r—* I N ON O CN L Q O ON 00 L O 00 CN O ON T-H CO T-H NO L O TJ* O C O r H O N O N O C O O N C O L N C O C O C N O N N o q q H r i q v q q ' t i n v q CN CO CO T t T t T t T t C O T t T t T t T t T t T t N O N O t N . 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IN 00 c o CN NO a m vO CO TJ< r-H CM CN NO i n oo a r-H CO NO ON CN CN IN ON m CN d d ON i n ON CN t N T t i n t N ON ON oo ON r-H r-H r-H r-H r-H r-H ON o d ON 00 ON ON 00 IN CO i n CO r H d 0 0 r H OO ON O N ' * O N , t O N ' < } l O N 4 0 N ' ! t l O N T t l O N ^ O N N O t N l N C O C O O N O N O O r H r H C N C N C O C O C N C N C N C N C N C N C N C O C O C O C O C O C O C O C O ON ^ 1 ON ^ ON ^ 1 ON Tt ON N O t N t N O O O O O N O N O O C O C O C O c O c O c O C O T t T t ON IN IN CO IN IN i n NO NO S3 00 ON CN CO CN CN coinooincooNONON NO oo IN i n m IN vo O ON T-H O C O C O C O C O C O C N C O C O C O C O C O C N C N C N C O C O C O C N C O C O O O N O O N r H T * v p c O C O O O N O O N N O ^ ^ H O N c O C O C O T j S l r i T j i r j i c O ON O in NO CN O ON O ON r n T-H ON 00 CN CO CN CO CO CN CN ON in c o cO r-H r H ON r H 00 NO NO CO r H CO CN r - H C N r - H O O O r H l N O O O O C N C O C O C O C O C O C O C O C N C N C N C N i n O N O O C N C N r H O O c o c o c o c o c o c o c o c o oo oo ^ c o o c o i n NO T t c o oo H r i 6 r i r - i t ^ T-H T-H T-H T-H r H 00 ON r H vO r H T ? CN ON CN r H CO d d NO ON CN CN CN i n NO NO O r - i T-H T-H r H ON 00 ON r H r H CN NO T t CO ON CO od od od od a CN CN 00 CO CN C N CO CN K NO CN i n T t i n r - i CN i n ON i n C N CN C N C N C N C N C N C N C N C N KCSOOOOONONOO CN CN CN C N CN CN _ _ _ r—• r H CN CN CO CO C N C N C N C N C N C N C O C O C O C O C O C O C O C O CN 3 3 CNCNtNCNCNCNCNCNtNCNCNCNtNCN ' i n i n N O N O t N C N O O o o o N O N O O r H C O C O C O C O C O C O C O c O C O C O T t T t T t T t ON NO^  ON _ CN NO ON oq oq vq r i NO CO CO CO r i r i CN CO CO n i n 00 ON ON C N CN v q v q v q i n v q p o N o q r - H r - i r i r - i r i c N r i T t l r i i n i n CO tN CO CN ON O O ON VO CO T t n i n T t NO i n CN T t O O r H O O r H C O p i n p T t c O C N C N r H C N C N C N C N l r i r i r i O O O N T t v O C O C N i n c O N O T t N O O i n N o i n c N t N i n c N T t N O r i C N C O C O C O N c O C N C N C N C N C N T t C N IN CO n T t CN i n n CN CN CN T t CN ON NO T t T t o o i n i n c N N O c N c N t N i n CN i n r l r l n i n n c o i n CO CN oo CN n i n i n c o NO o oo o CN ON aa T t N O n r i T t i n T t T t C O i n i n n r H C N 00 00 CO CN NO NO c o i n 0 N 0 N r H L f J 0 0 C N O r 0 -r i r l ON i n a T t a 3 $ * c o R T t i n T t vo CN a m i n i n i n o N i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n c O T t i n i n N O C N O o o N O r H C N c O T t i n N o t N O O O N © ~ L n i n i n i n i n i n i n N o v o v o v o v o v o v o - -i n i n i n i n i n i n i n v o v o v o v d NO NO C N CN m i n i n m i n i n C N CO T t LO vq C N 00 ON CN IN C N CN C N C N CN C O O O L O L O N O C N r H C N C O ~. "~. ~ , « fO vO O rtto'ro^^Tjito^tO^dto^^^^'J^tri L O C N S O N O I N C O L O C N L O ~ L O CO T f T f . . O C N C N T - H C O O O N L O N O ON NO CO T f ON O N O 00 C N T f O T f T f C O C N C N C O C O C N r H r H r H LO t N LO C N ON vO C N LO CN T-H CO CO CO CO CO Cs| C N O N CO T-H 00 N O CN CO N O N O LO CO C O C O C O C O C O C O C O C O 3 LO LO LO CO O NO O NO CO CO CO CO T-H C N T-H C N O 00 LO CO T—i O C N vO CO CO CO CO CO CO LO LO LO ON t N r-H NO CO LO CO T f CN 00 T-H CN T-H CO ON CN LO O ON T-H CN NO CO T f ON © T - H C N C O C O L O T f C N O T - H T ^ ' ^ C N C O L O L O T f L O C O T - ' CN LO C N C N N O CO N O O N CQ O N T-H O O O O C N T f T f NO T f T f LO T f ON T f ON r H r-H C N CN _ _ _ ""•"J"' s^f^  ^ vf^  N^J^  3 ON cO CO ON T f ON ^ LO LO T f T f T f ON T f ON vo C N C N T f T f T f ON T f ON T f ON T f ON T f ON T f ON C " O O N O N O © T - H T - H C N C N C O C O T f T f T f L O L O L O L O L O L O L O L O T f r H T-H C N T f C N ON L O ON CO ON C N L O L O 3 N O CN LO O N 3 CO 00 LO C N C N 00 00 CO O N r n O N O N CO LO C N N O _ r H C N L O L N , N q C q C N c O p N O \ N O ^ r | n O N O \ T f T f T f CO CO CO CO T f T f T f T f LO LO T f T f T f LO LO LO LO LO T f T f LO LO LO T f CO CO CN LO 38 ON LO O CO CN CO CO CO CO CO CO 8 2 § LO CN C N ON 88 " ' CO CO CO CO CO LO NO CN LO LO LO CN • LO LO T f CN T f CO CO CO CO CO NO t N ON ON LO CO CN CO LO NO CO CO NO LO 00 CN C N LO 00 CO T f 00 CN NO r - i c O C N C N c O C N N O C X ) 0 0 O N C O C ^ C O 0 q c N CO ON T f LO LO t N CN NO CO CN N O ON LO T f LO CO CO CN T-H T-H CN LO I N NO 00 ON LO L N L N T-H OO 00 L N ON 00 NO OO* O N 00 T f CN CO L N CN C N CN C N CN C N T-H CN CN CO CO s^t'' 3 3 N N f N N f N N N N t N N ^ "^NT *NT v^f"' C N C N C N C N C N C N C N C N . O O r H r H C N C O C O T f T f LO LO LO LO LO LO LO LO L N C N L N C N T f LO LO NO LO LO LO LO C N T-H L O T f C N L N L O L O O N L O T f N O r H N O L N T f L O O N L O 00 N O C N O N O N T f C O T-H T-H C N C N T f oq C O T-H T-H C O L O C N O N T f L O L O T f C O C O L O L O L O L O C O L O T f r H T-H T-H C N T-H T—i C O O T f L O O N 00 00 CO CO CO CO NO T f C N CN 0Q C N T f 00 I N 00 00 CN T-H CO CO CO C O CO T f T f C O T-H C N C N T-H CO CN CN cQ CN 3 T f T f C N T-H cO T-H ON LO ON LO CN 00 CO L N NO O O CN O CN 00 T f LO ON CO O O r - i C N i N C O T ^ O N C ^ O O N O C N O O ' ^ T-H CN CN T-H T-H r H T-H T-H CN CN T-H C N T-H T-H CN CN NO CO CN T-H LO LO LO LO LO LO LO LO LO LO LO LO LO O T - H C N C O T f L O N O t N O O O N O r H C N oooooooooooooooooo LO LO LO LO LO LO LO u i v j r - i T t O T f l f l v O N O O O N O O O N O N O N O N O N O N O N O N O N O N CO t N VO NO CN LO LO LO LO 3 d i-i CO LO vO IN CN CN LN CO r n L O t - H ^ ^ v O C N r H l N r - H C O O T > C N L O T j < C N T j t C N C N C N C N C N C N C N C N C N C O T-H 00 vO IN LN TJ< LO C N C N C N C O O O v v O T - H L O T t C T v O v O O N O L N C T v O V T - i C O r t C T v C N L N L N v O C N L N C O O t N O O O O L Q l N C O C N C N C N C N C O C N C N C N C O C O C N C N C N C N C N t-H LN LN CO N 0 \ O 00 — C O C N C O C N C N C N C N C N CO CO LO CO CO Ov COOv L N Ov T-H CO Ov CN O t - H O v O p T t c O c O c O TJ* c o T t c o T t t-H c o CN T t T t c o L O NO L O vo vo oc3 L N v d v d L O I N L N L N T t 00 LO CO 00 CN T-H LO C N C O C N O O C O L N l N L O v O O O v O O v C O r H O V T t O N T t O V T t O V T t O V T t O V T t O V T t O V T t O V L O V O V O L N L N O O O O O V O V O O T - I T - H C N C N C O C O L O L O L O L O L O L O L O L O L O v O v O v O v O v O v O v O v O T t OV T t OV T t LO LO vO vo LN vO vO vO vO vO CO T t LO I N 00 T-H CO LO Tt 00 ^ CN LO LO I N VO CN CO T t T t T t T t T t T t T t CO CO Ov IN oq T t CN p co T t CO CO CO CO CO CO O N 00 00 t-H LN CN CO CO T t T t T t T t CO 00 CN CO LO T-H T-H T t T t T t T t CN LN NO CO c o c o CN v q c o T t IO LO LO T t T t T t 00 LN T-H LO CO CO T t CO CO CO CO o LO CO LN CN 3 " . C Q O L N v p c O C N C N O O v O e O C N O O N O O N C N r - H C O C N C O C O C O C N C O C N C O C O C O C O 00 CN T t r H C N O C O C O L O C O T t C O C O C O C O C O C O C O C O C O 00 NO C N L O O O C O N O N O N O L O O O T - I c N c O T t T t c o N O T t o o N C O r o c o T t L o i o i O L O v d N o c o c N c d T t NO CN NO LO ON T-H 00 CN CO CO oo o T t T t CO IN T t vO CN T t T t CO ON ON ON T t C N C O T t O O L N l N N O T t N f ^ N f N l N r v l N r ^ N N N f S N C - I K r ^ N N N N N N N N N N r N l f s f N * N N C 0 C 0 0 N C \ O O H H N N t 0 t 0 1 l 1 l i n i f ) N 0 v 0 N K 0 0 u N 0 N p p r i I D L O L O L O L O L O L O v O v O v O v O v O v O v O v O v O v O v O v O v O v O v O v O v O v O v O L N L N l N 3 3 Ov o CN r H CN r H T t - t T t LO LO LO o T-H CO cs CN O LO vO vO LO CO CO CO CO CO CO 1-H LO T-H CN co r H CS LO vO T t Ov 1-H LO T-H vd T-H CN T-H CO T-H CN T-H CO 1-H CN CS CN CS CN CS CN c t g R R R c t c t Table H-7. Chlorinity data from the Mexican Margin piston cores. Depth %C1 Depth %C1 Depth %C1 cm cm cm NH15P NH8P NH22P 2 3.86 2 4.87 2 3.03 12 3.61 7 4.75 7 2.92 22 3.5 12 4.85 12 2.92 32 3.99 17 4.78 17 3.88 42 3.19 22 4.82 22 3.88 51 3.1 27 4.66 27 3.68 62 3.02 32 4.63 32 3.83 72 2.73 37 4.8 37 3.77 81 3.02 42 4.82 42 3.75 92 3.23 47 4.54 47 3.75 101 2.48 52 4.57 51 3.61 112 4.83 57 4.36 56 3.7 122 4.38 62 4.53 61 3.45 132 4.84 67 4.49 66 3.49 142 5.31 72 4.57 71 3.52 150 5.32 77 4.5 76 3.49 152 5.33 82 4.42 81 3.25 160 4.28 87 4.21 86 3.36 162 5.24 92 4.36 91 3.39 170 5.28 97 4.25 96 3.52 172 5.33 102 4.42 101 3.39 181 5.1 107 4.25 106 3.49 182 4.87 112 3.75 111 3.44 190 4.65 117 3.8 116 3.52 192 4.44 122 3.77 121 3.5 200 4.3 127 3.73 126 3.75 202 4.17 132 3.92 131 3.7 206 1.86 137 3.7 136 3.94 210 1.84 142 3.66 141 3.56 212 1.82 147 3.59 146 3.5 220 2.53 152 3.54 151 3.52 222 3.23 157 3.25 156 3.47 229 3.01 162 3.43 164 3.46 234 2.79 167 3.33 169 3.41 240 1.99 172 3.3 174 3.36 242 1.19 177 3.39 179 3.47 250 1.05 182 3.33 184 3.5 255 1.11 187 3.44 189 3.44 265 1.22 192 3.5 194 3.56 275 1.48 197 3.44 199 3.59 285 1.75 202 3.72 204 3.63 295 1.69 207 305 1.76 212 315 1.99 217 325 1.79 222 335 1.52 227 345 1.85 232 355 1.34 237 365 1.93 242 375 2.12 247 385 2.13 252 395 14.1 257 405 2.04 262 415 1.84 267 425 2.31 272 435 1.95 277 445 2.1 282 455 2.07 287 465 2.04 292 475 2.22 297 485 2.35 302 495 2.4 307 505 2.41 312 515 2.49 317 525 2.32 322 535 2.28 327 545 2.26 332 555 1.69 337 559 1.57 342 565 1.6 347 575 2.07 352 585 1.95 357 595 1.39 362 605 1.33 367 615 1.36 372 625 1.36 377 635 1.44 382 645 1.56 387 655 1.54 392 665 2.28 397 675 2.38 402 685 2.34 407 695 2.42 412 705 2.19 417 715 1.74 422 725 1.74 427 735 1.49 432 3.66 209 3.61 3.49 214 3.58 3.59 219 3.63 3.68 224 3.55 3.68 229 3.58 3.77 234 3.57 3.77 239 3.41 3.68 244 3.46 3.81 249 3.44 3.7 254 3.46 3.7 259 3.41 3.72 264 3.44 3.79 269 3.46 3.82 274 3.47 3.9 279 3.49 3.68 284 3.59 3.68 289 3.49 3.65 294 3.45 3.36 299 3.45 3.49 304 3.52 3.61 309 3.52 3.54 314 3.65 3.49 319 3.31 3.34 324 3.52 3.46 329 3.52 3.44 334 3.5 3.44 339 3.57 3.65 344 3.57 3.58 349 3.33 3.57 354 3.31 3.44 359 3.44 3.59 364 3.22 3.5 369 3.25 3.54 374 3.22 3.58 379 3.21 3.38 384 3.28 3.58 389 3.21 3.77 394 3.1 3.79 399 3.13 3.73 404 3 3.56 409 2.93 3.58 414 2.89 3.61 419 2.33 3.7 424 2.22 3.95 429 2.22 3.52 434 2.41 745 1.42 755 1.43 765 1.62 775 1.44 785 1.68 792 2.4 805 2.19 815 2.42 825 2.47 835 2.26 845 2.05 855 1.87 865 2.22 875 2.33 885 2.24 895 2.29 905 1.93 915 2.3 925 2.2 935 1.17 945 1.25 955 1.27 965 1.88 975 1.63 985 1.36 995 1.13 437 3.52 442 3.44 447 3.44 452 3.57 457 3.65 462 3.68 467 3.85 472 3.81 477 3.84 482 3.81 487 3.72 492 3.7 497 3.8 502 3.79 507 3.7 512 3.72 517 3.85 522 3.59 532 3.59 537 3.56 542 3.84 547 3.66 552 3.55 557 3.31 562 3.3 567 3.21 572 3.28 577 3.31 582 3.38 587 3.47 592 3.47 597 3.52 602 3.44 607 3.55 612 3.43 617 3.24 622 3.28 627 3.21 632 3.22 637 3.31 642 3.46 647 3.55 652 3.41 657 3.43 662 3.44 667 3.44 439 2.27 444 2.34 449 2.25 454 2.17 459 2.21 464 2.21 469 2.39 474 2.31 479 2.34 484 2.22 489 2.39 494 2.17 499 2.41 504 2.37 509 2.3 514 2.35 519 2.37 524 2.4 529 2.44 534 2.23 539 2.09 544 2.27 549 2.09 554 2.33 559 2.35 564 2.4 569 2.47 574 2.54 579 2.61 584 2.81 589 2.63 594 2.68 599 2.51 604 2.48 609 2.51 614 2.48 619 2.5 624 2.5 629 2.64 634 2.61 639 2.59 644 2.47 649 2.47 654 2.5 659 2.55 664 2.49 672 3.31 669 2.49 677 3.21 674 2.49 687 3.52 692 3.57 697 3.59 702 3.45 707 3.54 712 3.43 717 3.47 722 3.5 727 3.46 732 3.45 737 3.34 742 3.55 747 3.49 vo C N •M res C O 00 t-H CU •s PH bo 3 I* <Tj CJ o Ov r-t vo t N oo CN 00 rH vO Ov 3 LO T f Ov vO C^ OV LO IN Ov ro LO 3 CN r H O CN r n r n CN rH CN CN CN rH CN CN rH o oo r H 00 r H ro LO T f T f T f T f T f T f T f T f T f T f T f T f T f T f T f T f T f ro T f CN ro ro ro ro O* O* O* o* O* O* O* o* O* o o* O* O* O* O* O O o* o* O O* o* o* o O Cf) IS z & 00 IH o n U * s CJ ra CJ PH CU as Tfrorooococooooo T f ro oo oo Ov ro T f r H t - H O v O v O v O V O V O V O V O V OS Ov Ov Ov 00 Ov Ov r H r H r H r H O O O O O O O O r H O O O O O O O O O O O O O O O O O O O O O O O O O VO T f Ov LO LO r H r H r H O O r H L n O v r O O v O L O v O r H C N T f v O r O L O C N L O v O L O O O O T f L O O O O O v O r o N c N c o c N c o r o r o r o r o r o r o r o r O T f r o r o r o r o r O r H r H C N C N C N . CN i—i LN LO CN Ov CO IN f N VO CN LO rH t N 00 r—t 00 f O T f Ov GO rH w d i n ^ v o N o o N v o o N ^ N q o o o N v o m q i f l O N ' * 0 6 0 6 0 6 0 6 0 6 0 6 0 6 0 6 06 oS o v o 6 o 6 o v o v o 6 o v o 6 o 6 o v v b v d K c < L N o ov 00 vo 00 0 00 vO Ov LO O T f CN OV K I-H CO LO 00 vO Ov O LO r n O Ov LO 3 CO CO T f CN CO CN T f CN T f CN CN CN T f CN CN CN CN T f CN CN CN CN cll VO LO IN LO IN LO 00 LO LO LO £8 vO vO vO vO 00 vO r n IN CN vO Ov vO 3 3 LO vO R 00 vo LO VO CO CN O r H r H CN CN CN CN CN CN O O L O O O r H Q T f O C O L O O v O v O C N T f T f L O L O L O LO T f OV LO OO CO T f LN 00 T f T-H \ 0 rH 00 00 VO IN IN VO K VO r H C s CO r H LO 00 OV CN LN vO VO T f CN CNCN r H O 00 LO O LO CO vO T f T f CO CO CO CO CO CN CN r H r H rH r H r-H CN rH rH r H r H CO CO r H VO LO CO CN CN N i f l N r n c o o v o v v o o i s C N C N C O C N r H r H C N O V l N L N v O v O C N t N v O L O T f v O LO LO VO *'•© VO VO L ">  > W > W L M M * U J NJ* N W J > W V*W >—' > W C O C O C O C O C O C O C O C O C O C O 3 IN LN 00 as so 00 o ov O L O O O v O v O C O C N L O L O r H O C N l N O V L O O v v O O O r H v O O CN vO LN IN T-H t N T f LO T-H CN O O0 00 LO T f O v q 00 LN LO Ov CO CN CN CO CO CN CO CO T f T f T f T f T f CO T f T f LO LO vO LN t N O * O * O * O * O O O O O O O O O O O O O O O O O O V O O C N O C N O O V O C N V O T - H C O O V C N L O V O L O C O c O O O c O O O V O O O C O r H T f O v L O O V O O O T f T f c O T f T f T f c O L O T f c O C O C O O O O O O v O v o v 00 O vO Ov 00 O T f CN OV CN r H CN CN IN "^s}"^  *^J^ *N"J"^  *^N^  4^"^  s^f^  ' 'N^ O O O O O O O O O O O O O O O O O O O O O O O O O O O T-H O 00 T f T-H OV O IN IN v q LN OV co T f ro ro co co ro 00 LO CO r O T f LN 00 vO O Os 3 8 2 CN CN T f O V O CO T f C O C O C O T f C O T t C O T f T f \ c O C O C O t N t N t N CN 00 CN CN N v O 00 LN 00 OV LO LO CO T f LO T J J v q - - • - - • - - -i N * v O v b v O L O T ^ C O C N C N C N C N C M C O C N C N C M 00 CN T-H O V CN O L O C O L Q v O O O O v O C O r H O O V O O C O T f L O r H O C O _ _ . 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OS N t o r i (O (O C M C M C C M C M C M C M C M C M i n i n T t C O C O C O C M T t r H C O C M O v v O v O O l N O v L O O v l O c q p c O C O C M l N ; i r i d r H C M r H O V O v O v O T t t N d L r i r - H d d l O v d v O T t r H C O C Q C Q C Q C Q C Q C Q C N T t T t T t c O T t T t C Q C O C Q C M C Q V O C N C N O O O O C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M N C M C M C O C N O V V O T t C O L O O r H O O C O l N C N T t - r H f N j _ c M r H c o C N c o o v t N . v o v o T t v o C N C o i n i n v q T t L Q c v J T j t N T t c Q c O C Q i n v d i n i n i O L Q v d c Q C O v d H ^ H ^ C M C M C M C M C M H C M H C O C M C M C M C M C M C M C M C M LO a 5 i n CM ov oo co CO vO 00 O r H C M C O T t i n v O C N O O O v O C M a a s a r H r H r H r H r n r H r H r H r H r H C M ^ ^ ^ ^ ^ r H C M C O T t L O v O C N O O O v | J 4 | i 4 h H » H I H I H > H h H ^ 4 4 4 H t H h H ^ ) H ) H 4 4 4 4 4 4 » H ) H > H > H ^ O S T t O N O C N r H L O O O C O C ^ T*rtrCT*tDfOT*cocOTt(riON C O C O C O C O C O C O C O C O C O C O C O C M C N C N C M C O C M C M C M N C M C O C M C O C ^ T-H IN LO d O co CO r n 9s CM LO as O CO T-H vO d O CO o LO ts. CO o LN LN d CO CM T t r H r H r H as CO as as as ov as as T-H CO T-H T-H T t LN L O cd vd O O L O C M O C M T t L O O v O O v L O C O O v T t v O C O T - H p O v p v q r H "oOOvodtNLNLNOOvd LQ Ov O CM LO Ov CO Ov r-i VO r t v d P © as CO CO IN IN LN LN LN VO vO IN LN Tt OV CM Ov v d CM r-i LN CO IN r-i C O C O O O v O O v v O v O v O L O O v C O tt LN IN OV O VO vO LN CM vo v O C O T - H C O C O L O C M i O O V T - j v O C O T t c o c d v d v d c o c M T t ^ o v o v c M v d c d C M r H r H r H r H r H r H O N r H T - H r H C S C M CO r n as O T t C M C M v O C M C O C M C M l N r H C O L Q T j * LN v d CO LO r H r H H T - i l N O V r - i c O L N C O r - i d T t N r H C O r H T t C M r H r H C M C S r H T t CO vO oo $ Ov T t LO T-H O R vO §8 00 T t LO LO CO LO CO oo IN VO O CM CM LN vo CO CM o O IN co LO LN CO Ov VO T j i 00 LN IN r - i r - i O v v d od Ov 00 IN 00 VO CM d CM v d LO T t oo LN CM r n r H I-H T-H I-H O o r n r n o r H r H T-H LO 00 T-H o T-H od CO T-H r H LN T t VO t> T t O O LN r H VO T t LO OV O CO O O I-H O O N L Q C O L N C M L N r - H O N v O C O v O r H O C O v q v q O N L O O v O O L O L O v O T t t N O v L O v O v d v O O O C M H r H r H I-H r H T-H T-H T-H T-H I-H I-H r H T-H I-H T-H LO T t CM CM CM CM CM r H CM r H r H r H r H T-H T-H 99.06 98.25 101.8 100.0 100.2 98.86 101.8 98.36 100.7 95.83 96.77 88.46 92.50 89.72 91.18 103.9 84.10 88.39 85.36 84.82 90.06 91.51 92.64 92.07 93.12 89.25 88.60 86.92 88.10 305.6 286.4 289.0 279.6 282.8 293.9 280.7 293.0 292.7 315.0 321.7 536.1 466.9 439.2 491.0 526.6 413.0 375.3 399.3 388.3 355.6 300.4 369.9 319.9 319.2 342.9 341.6 358.9 371.4 37.5 34.6 33.1 31.2 32.2 30.4 29.7 29.8 32.8 33.6 34.8 43.7 39.7 41.6 21.1 25.2 16.9 18.1 19.2 15.3 20.6 19.0 20.7 20.5 18.7 18.5 17.6 19.3 16.9 L85.6 L78.2 L79.2 L78.7 L76.4 177.1 176.3 L75.7 178.4 L77.1 L67.8 158.6 161.3 164.3 L29.9 157.3 L36.6 L42.2 L43.8 L32.0 L44.3 140.3 L40.6 141.2 138.6 L30.3 L29.6 L28.8 120.7 c M - c M - S S f c S L N - T l S ^ 00 IN v d LN T t v d LO T t LO T t T t T t LO CO OV CM v d LO T t T t LO T t LO LO CO r H CM r H O r H r H T-H r H T-H T-H T-H I-H I-H I-H I-H I-H I-H I-H T-H CM T-H I-H I-H r H i -H r H I-H T-H I-H I-H T-H I-H r H S R a a S c o ^ c o - c o ' c o - c S c o ^ C M C M C N C N C M C M C N C M C M C M C M C M C M C M r H r H i -H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H xxxxxxxxxxxxxxxxxxxxxxxxxxxxx r f o q c q c M r H f D i ^ o q v q r n vO IT) VD ON ON VO CO oo r n NO NO p go ON ON CO vO N N 8§8 T f O . 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NO o o IN T-H CN ON CO T-H O O  O O O CO r-H CO LO 00 LO ON O r f 00 00 00 LO IN 00 00 O O C O L O r H N O C O L O L O O s C N O O r H LO O . r H 00 O T f CO O ON 00 ON 00 ON ON 8 T f T f r< ON 8 NO CO T f T-H ON T-H ON ON T-H r n ON I N T-H ON ON 00 T-H CO c$ 00 ON 00 ON T-H CN LN ON NO T f CN O N O L O C S L O L O T f O N N O N O I N N O N O L N I N C N C N C O p i N c o o q c N c N N O L q c N c s O ON T f CN CO NO LO NO CN CO CO cO CO cO CO T f   f CO CO CN CO C O r H T f c O p N O l N l N L O C O NO r ^ ' o O C O C N O a O T f C N t N T f c O C N C O C O C O C N C O C O C O T f H i f ) N o \ o c o o i n r t * i n o o p N N i n N » c o N 2 o o S o ^ H ^ C N l N L O C O C O L O 0 0 . * ^ 9 ^ ^ * ' ^ T f C N T - H C N T - H C N C N C N C N I N C N ^ C O N O L O L N 00 CN CO C N T f NO 00 O O ^ M ^ O v O r t H \ O n N C O N v O C ^ t O r t O O * H » O O N N r t f O N O N n ON T f T-H r H IN ON T f ON OO T-H CO LO 00 CN T-H LO CO NO LN 0 \ LO T-H CO T-H LO t N NO CN 00 I N O O ^ C N T f T f ^ T f ^ ^ ^ r - ' P c N C N O O O O N r H r H O N O O t N O O l N O T f O O O O L O L O O O O O O N O O T f r H T—H T-H ON T-H r-H r H ON T-H ON 00 ON T-H ON T-H T-H T-H T-H T-H T-H T-H T-H T-H T-H r-H ON T-H ON T-H s LO O ON T f T f CO CO LO T-H T-H T-H ON T-H r ^ C O r O O L O O O p c O T f O O p T f c O L O l N r H C N C O 1—< NO NO T f CN ON NO CO T f 00 NO CO NO LO ON t N CO LN LO i—< r H 00 00 T f C N C N C N C N ^ C N C N ^ C N C M C N L O N O C N C N C N C N C N C N C O c N c N o q N O O N C O C O C O r H U - i N O L O I N T f LO LN T f LO ON LO O O N O C O r H O O O C O O O O r H N O L O T f L O N O L N O N C N O ^ O O C N r H C N T f C N r - H C N C N C O . . _ . - - - - - — — — O O O N O O T-H r H ON T-H T-H L O L N t N T - H C O C O T f O O O N O N O O O O O O O O r - H r H O O O 00 00 LN 00 00 00 00 T-H T-H ON ON T-H T-H ON T-H T-H T-H T-H T-H T-H T-H T-H T-H CN CN NO ~ ~ O 00 ON 00 LO CO r-H CN CO ON LN LN CN CO CO T f T f LN CO O 00 ON CO LO ON CO NO CO T f NO NO T f O O T-H co co T f co co co ON ON CO L O T-H 00 CO L O LN T f N O V v O t N C O N O l N H l N t N T f C O o d ON T-H CN C N T-H ON ON L O NO LN L Q L O C N C O C O C O C O C N C N C O C O C N C N C N O N O N N O C O O N N O I N L O coodcotNtNOONbod " ' IT) T f T f NO LO LO C N C N CN C N CN C N ON LO LN ON NO LO NO LO r-J LO T—H CN CN LO v£) 00 T f LO LO CO CN © ON 00 T f ON T-H LN LN NO LO NO T f ON ON 00 00 LN NO NO NO LN LN 00 NO 00 t N ON 00 ON 00 t N NO NO LO 0Q NO ON C N C N C N C N t O C O C N C N C N C N C N C N C N T-H T-H r H T-H T-H T-H CN CN CN C N CN CN C N C N CN CN CO T f _ CN CN O T-H p C O 00 C N NO ON IN C N CO LO T-H T-H CO CO 00 O CN T f CO T-H C O CN T f 00 C O © 00 L O T f LN NO T f ON T-H L O C N ON CN T f vO NO CO C O ON © © NO NO ON NO L Q T f ON L Q ON t N L/S LN © CM CN T—H T-H CN CN T-H CN CN CN CN T f T f C N CN C N CN r H T-H C N T-H CO C N C N C N C N C N C N C N C N C N C N C N C N C N C N C N N H C N C N C N C N C N r H NO NO T-H LO 00 T-H cN LN NO CO O O CN T-H 3 8 ON O LO CO NO CO C s 3 CN C s 00 © oo C s I N co C s NO LO r-H C s CO LO 8 CN © NO T-H ON NO C s ON N N C N J t N i t N i d r i N n n N m m i r i N T - H C M C N C M N C M C M H C N C M H C M C M C N C M C N J C M CN 00 ON CN © CN C N ? 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T t co co co co co O IN S 3 NO CM T t co co o LN T t 00 LQ CM CM CM CM O LO CO NO LO ON O CO CO LO CO T t CO T t co co 00 ON 00 ON 00 LO CO CO CO T t CO CO T t IN CO CM CM IN CO t-H CO T t T t 1-H CM NO O CM NO CM 00 L O L O CM CO 3 * L O r-H LQ 00 IN T t NO 8 CO r H CO NO NO LN tt v© LN Op ON O T-H CM CO T t LO NO IN 00 ON p I - H CM CO T t LO NO IN 00 ON CO t-H CM CO T t C M C M C ^ C M C O C O C O C O C O C O C O C O C O C O T t T * T t T t T t T t T ^ ' ' ' C M ^ C M C M C ^ C N I C M C N C M C ^ C ^ I N , ^ t-H 1-H 1-H t-H t-H 1-H t-H t-H T-H 1-H 1-H T-H t-H t-H 1-H t-H r H T-H t-H 1-H t-H 1-H T-H r H T-H t-H K K K K f f i K f f i f f i f f i K K f f i f f i K K K K K D H p H ' D H ' p M ' D H ' r M ' r C H ' H Ov CN CO vO CO © CO CO M W O v i n o o N i O N n ^ ^ o o o v o o ^ v o o o o o i f i C V | H N n S r i H 0 0 r i O 0 \ » 0 \ N C > N 0 6 0 \ N N M N N H M N H N N H H p i H r i r i r i r i O O C N C N T f K K K 00 OV O LO r—i 00 O Ov 00 IS, C O O r H r H \ O O O C v | O O V O O c O c O O O T f O O v O v O C O T f T f Q L O T - H T - H C N C O C O C N C O C O T - H r H C N C O C O T f V O C S C N C N H C N C N C N C N C N C N C N C N C N C N C N H O ^ O M N r H C O N M N O i r H T t o p ^ O v O M H O M f l O N ' I ' O t N v O O fxj N TJH ^  ^ CTl OV CO O CN TT 00 tN OO CN r—t O CO LN tS O LO CO T f tN CV C N C N C N C N C N C N L O L O L O L O L O L O L O L O C ^ O OV 00 oo o OV OV CN IN Ov IN T f 00 Ov Ov 00 Ov Ov IN Ov 3 Ov O CO T-H IN CN VO T f • CO LO LO vO T f T f LO Ov Ov Ov OV OV Ov Ov co VO vo OV 88 CO o 00 00 00 Ov vO as vO T-H T-H VO vO IN CO LO T-H as IN T-H IN vO "3! 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T f c O T f T f L O | N T f L O v p L o o q o q c N r H c o L O L o c o i N v q T - H K c ^ CO T f T f T f T f CN CN CO CO T f CO T f T f T f T f T f C O 0 0 T - H T - H L O T f 0 0 O V 0 0 O V 0 0 c O O v v O C O L O T f t N C s © O V T f O V T - H O V T - H C N O O r n ' r - H C O c K o O O O T - i c N L O T - i l O C O L O O V C N O O v O O O v O L O v O O O L O K C s L O L O t N L O l N l N O O C O r H r H r H r - i v O L O v O L O v O v O C O C O C O C O C O C O C O C O C O C O C O C O T f T f T f T f T f T f T f CN VO LO CO OV vp 00 00 — OV CO CO T f C N C N r ^ C N C O C O T - H ' c O v d O V L N L O 0 p c O C O v q « CN v q LO co Ov co © T-H CO LO 5 T f © CN v d LO op LO LO LO T f C s 00 vO CN 00 CO LO OV O LO T f LO 88 T - H T f C O © L O © V O C O 0 0 T f H r - i o c o K L O O v C N T - H O H e O T f L O L O L O C s O O L O v O v O v O v O v O C s CN LO Cs LO " r-J CN vO vo 3 LO LO X z 8 C s LO CN C s C s X z X z £8 IN T-H z Os LO CN CN T-H CN CO vo vO vo v S " 5 H W H H H H H H H 2 2 2 2 2 2 R £££ 2 2 2 LO so CN Cs IN 00 Ov CN C N 00 CN 00 2 2 2 2 2 2 S3 H O N X O N K O M N O \ » V O V O N O > X o o s o v o o o c o N i n o N N c r i O « H H O O M < * 0 0 O 0 M N n r v i n c o n t o f N N N r o i N n i n t n r c < J v O N f O O » M v O ( O N N O i » v o c c i n n n r H N N N t r i L f j o o N t s ^ a N N v o ^ a a c N o ' T > LO 00 _ " ^ " i ^ R ^ X O S O r H r i i N O s O H n N C O N O O L N L O O O L N L O r H O © C O N N N N N C N r - i H i N f N l C N n N 0 O C N T t O r 0 r - H \ © N © 0 \ N 0 0 0 T t L N c Q v q i n v q c q c q o N i ^ CTN ON ON ON ON ON ON ON ON ON ON ON ON ON o LO T-H 00 T-H t N t N o t N 8 CO LO o ON o cs T-H CO CO T-H CS cs cs T-H ON p ON p ON 00 IN oo 00 tN IN d d d d d r-i r-i d r-i d d r-i d d d d d d d CM cs cs cs cs cs cs cs cs cs cs cs cs cs cs cs cs cs cs NO NO r-H T-H O r H r t CO LO O O r H O 00 O O r - H r - H r H i - H r H r H O CS I oo oo oo ON LO CQ T f Tt* CS ON IN TI" O CS LO NO NO CO T t 3 NO £ T t d IN T t cs T t NO ON NO cs oo ON cs 1-H cs NO d " i 1-H 3 i IN LO CO cs CO LO O CS 00 T t CS NO LN ON t N NO LN IN IN IN IN NO NO NO CS CO ON o ON NO LO CS 00 T-H LO d CO LO r - i 95 NO LN CS CO ON r H o CO O oo LN NO NO oo CS CO CO CO CO cs CS CS cs cs oo O r H T t S 3 NO NO NO NO T t LN 00 r H CO r H T t r H T t 00 LN CO T t LN C S T t L O T t C S O N T t v o O N O l N O O L N L O " ' N O L O O O L N O N O O O N O N ON CS NO LO CO T-H £ £ ' ' ' ' cs T t co 00 LN NO O 4 00 CO NO NO CO LO ON Tt io d ON od co oO 3 LO 00 O CO CO CO o T t cs CO oo V 0 NO LO LO ON LO IN © I-H o oo LN NO LN 3> CO co L O T t T t T t r N K o o K K N O t N o S 3 co cs o i - H \ O O C O C S O N O O i - H O N N O T t L N d ^ r i r i r H O N ^ r i r i " ; ^ 1 ' ! . . . I-H vO r H T-H T-H I-H I-H I-H I-H I-H 00 LO 00 LN CO ON R S o § I N O O r H r H C S p L N L O p a q O N r H f O L O r H p LN 00 00 ON CO ON r-i T t LN T-H ON IN LO LN CO ON CO 00 00 00 LO CN ON LN © ON NO ON r H p L O C O T * T * L O O N l N O O O N T t C S N O O i - H O O N O p N p C S L ^ T t T t T t T t T t LO T t T t LO LO LO LO LO NO LO LO T t T t T t LO LO LO LO *5 LO CN LO 00 00 r H r N T t ^ r H O N O T t L O T j C N L O L O O O C S N O O N O L O C S O C S C O C O C O C O i - H T - H C S T t L O L O L O l O T t C O T t T t C O C O T t L ^ CN CN NO NO CN I-H T-H I-H I—H I-H I-H T-H I-H I-H I-H I-H T-H T-H I-H I-H I-H I-H r H I-H 00 I-H I-H T-H T-H r H c q O N C q T t L O O N L O C O O \ r H C O O O T t r H C O L O r H C O O Q L*N NO NO r H NO CN LO O N r H v f c c Q L Q C S C S C S O N O N L N L O ^ O O O O 00 © T t CS I-H O r H O r n CS CO T t LO NO 0 > O i - H C S C O T t L O N O l N C O O N O O O O O O c 3 L N O O C ^ O r - t C S T t LO NO LN 00 00 ON ON ON ON ON ON ON ON ON ON I-H I-H T-H I-H I-H I-H I-H © O © T-H T-H T-H y—i f—4 ^ r H r*H T-H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H r H \ Q v© V O V O V O V O z z z z z z z z z 2 z z z z z z z z z z z z z z z 2 z z z vo o as LO co -n t N f o n r t ' f c o r o co co cO cO cO cO VO O CO CO r-H CO LO T f T f T f CO CN CN CN CN LQ OV OV CN CN T f CN CN CN L Q C O V O C Q L O L N I - H C N C N r - i C N C N C Q r H C N C N C N C N C N C N C N C N C N C N 00 CN IN O O OV CN CN r-1 N \ o o \ i n N H r | H c o m H H i f i i n ' i r t N i f i o i r ) O H i r ) 0 \ o v O H i n o \ T f l N L O C O H L O T f O O C N O v O V r - i T - i O p r H p r ^ o v o v o v o v o v o v o v o v o v c d o d c o c o c o c o c o c o c o c o T f o v i N r s p i N O v o v p c N p r - * r - * r 4 r - * r - * C N r - 4 C N C N r H C Z > C O C O C O C O C O C O C O C O C O C O C O 00 vo CO LO IN VO T f CO IN IN IN IN* r—I CO IN VO CN i N v O l N C O C O r - H O O O O T f _ .O O V C O K v O v O l N C v l c O v O T f v q i N L O L N v d l N v b L O L O v d v d v d L O L O L O L O L O * vp CN OV IN CO as o vO Ov vO o Ov Ov t-H LO CN T f as as o vo ov o T f Ov o T f Ov LN Ov IN LO OV LO v o LO CN as as oo t-H as LO o Ov o* o* o L O 00 L N CN CO CN CO >£ L O I N I N L O O L O 00 L O vO vO EN L N vO Ov O 00 L O vO L O L O vO vO L O L O o* o* O * o* O * O * O * 1-H o* o* o o* O * © * O * o* o* CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN C N l O t - H L O t - H O V O v i - H C O O Q T f c O L O T f L O L O C O C O C O C N O O O O O O O O O O T f C N OV O as vci od I-H VO LO LO CN LN t-H tN OV O * O * - . LN LO CN Ov Ov IN T f ov LO CN co oo IN* as od CN CO CO LO CO C N -O vd OV CO r-H IN* . . - o LN o t-H CN LN LO* LO* v d o CO CO 00 IN CN 1-H Ov T f t-H LN CN CN CO LN LN LO* IN* CNCN Ov CN L O L N v q o v o q v q p o o OV vd OV CO t-H CN LO* O O Ov Ov vo O IN C N C N 1-H 1-H t-H C N t-H 00 LN I-H T f as o o I N o I N O vO IN vo C N r H t-H t-H O L O O O V O O T f C N O O O V v O C O L O O v O T f c O r H » H i ? ) \ o i f i O N ^ O N N O H O O \ N ^ O q n N O \ v O p N N O O ; C O O O v d C N v d C s O v O v r H O O O O C N C O T f T f T f T f c O C O T f O C N l O O v O v O t - H C N C N O V O OV O T f v q T f C N O O C N O O C Q T f T f O O L N l O O V r H L O C O p L O C O f > C N C O T > C O T f r H r H O O L O O C N O v O V r H L N C N C N O O C O OV I-H CO CN CN " " " CO CO CO LO IN vO ' o v O T f L O C O L Q i - H O O O S S C O T f L O C O C O T f c N C O C O C O T f CO CO CO CO CO CO CO CO CO L O v£) v£> i - H O O C O O O L O r H V O L N t - H L O O O v O L N L O v O v O L O L O T f L O L O O O v O r H C N O v O V L O t-H t-H 1-H f H t-H r H t-H t-H t-H t"H t-H CN CN 00 00 LN CN C N V O L N i - H t - H O V O O C - s C s O O C O O C O r H O O co as p T f t N vO 00 CN O T-H 1-H C s O T f LO T f LN tN Cs C N OV LO r H LO LO r i O LN CN CN I-H LO IN t N CO OP t-H LQ T f CN CN Ov T f L O v O l N O O O V O r H r H t - H r H r H t - H r H C N C N VO VO VO VO 2 2 2 2 z Z vO vo vo XXX zzz CNCN Ov o VO o vo C N OV C N C N C N 00 L O O T f T f 00 T f T f LO* CN p* LO* LN 00* 00* t-H CO* IN t-H OV Ov OV 00 IN L O vO CN vO vO vO vO L O L O IN L O LN T f T f L O L O L O t-H C N C O L O T f IN $ as o 1-H C N C O L O VO IN 00 T f T f T f T f T f T f L O L O L O L O L O L O L O L O 1 1-H 1 1-H i 1-H i 1-H 1 1-H 1 1-H 1 t-H i t-H 1-H 1 t-H 1 1-H 1 1-H 1 r H 1 as ds as o\ o\ X X X X X X X 2 2 2 2 2 2 2 ON r H R R © O N C N C N C M C H C Q C s O O O O C s O N O N O N O O N i - H O O O N O N O t - H r H C N ^ v o n o H N n i n N o o N o n o v o N ^ i n o o o m w N v p o o N N ON NO r n 00 r H (N CO H j r H » i n m i N H < q ^ N O N b L O l O N O N d N O N O N d ON ON r H CO LO CN NO NO ON CO LO NO CO, LQ Cs CD 00 LO LO NO NO t s o d o d od t s o d NO Cs NO N© s© \ © oc5 CO r n ON C s o ON O C s r n . Tj< Tj< CO ON ON ON C Q C M C Q 00 C N C O C N C N d o d o CQ OP r H cN <9" c o o o LO ON ON ON 00 ON NO LO ON C s CN CO CO 00 ON 00 o d c i d c J CN CN CN CN CN i i i i i LO NO ON 3 ON 00 ON Cs Cs ON ON o oo o o o o o o q t O i n v p O N r H N r H N q O O N r H a s v O ^ O O O N N O N O N O V O N O N O N O L O V O O t s 00 3 8 § 8 R 3 c o N q c N C s L O N q c o L o c N c N o O L Q C N o p c s C ^ r - i ON r - i LO CN NO NO LQ LQ « t r t CO CN CN CN CN CN CN CN CN CN CN CN CN CN C N C N NO ON ct ON ON NO NO CO NO ci CN i © CN i d CN i o CN NO LD ON CN d o CD CN T t oo ON CN 00 ct Cs d oo NO ON d LO NO 00 t s CO ID ON d T t ct S3 d CN ON £ CN O t - H CO NO L O N O L O T t N O O t s O O r H l D N O O C s O p Q O O C O O O i - H O N T t C ^ O P l S . L O i S . N D a N r H p r H C N C N K CO CD CO CO CO LO CO T t NO 00 OP 00 00 OO ON r H ON r H t-H t-H ON t-H t-H t-H r H r H O O O N C O r H L O C N L Q O p L Q T t T t r - i LO LD CN CN t—i CD t s d ID T £ ID ID ID NO * " ~ " CO CO CD CD ID t s C s oo CD CO CD CO o d ob 08 op c o c o cO c o 0P NO t-H ID T t CN T t CO ID C s LQ T t *N""P "^ "J' ""^ J^  c o T t t - H o p c s c o N q o c N LD O Cs CQ t s OP CN CN CN CN r H . . T t T t T t T t T t CO NO NO tS Q L O C S T t O C O C s C O l O C s r H t - H C N H c O C O C O C O C O T t T t C O C O C O C N r H C O C s K C s O O C s K C s C s K r H T - H r H r H r H r H r H r H r H r H r H t - H r H t - H r H r H r H O N O N C s C s C N T t N O L O L O r H O N ^ ^ ^ ^ ^ ^ ^ ^ C N C s C O C N L O O N C O t - H C N C N I D O O T t C O T t ^ c O T t C O C N C N T t C N C N T t LD LD ID NO V© LO LO LO LO ON ON t-H t-H t-H r H l-H r H t-H t-H t-H 1-H t-H t-H t-H t-H t-H ON O t-H CN CO Tt LD NO C s L O N O N O N O N O N O N O N O N O O P O N O r H C N C O T t L D N O C s O O O N O r H C N c O T t t - H r H t - H i - H i - H i - H t - H i - H i - H \ O N O t s t s C s C s t s C s C s N C s C s 0 0 00 00 00 0p I I I I I I I I I f-H r H 1-H t-H 1-H 1-H 1-H r H 1-H f-H t-H t-H 1-H 1-H t~H f-H f H ON ON ON ON ON ON ON ON ON i- i i i i i i i i i i i i i i i i 1 - H i - H t - H t - H r H i - H r H r H i - H C s C s C s C s C s C s t s C s C s p C r C r C P C r C r C r C n H D H X S S X S r C X r C r C r C r C 

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