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Tracing deep-sea calcite dissolution; agreement between the Globorotalia menardii fragmentation index.. Francois, Roger 2006-12-31

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Tracing deep-sea calcite dissolution: Agreement between the Globorotalia menardii fragmentation index and elemental ratios (Mg/Ca and Mg/Sr) in planktonic foraminifers Figen Mekik1 and Roger Franc¸ois2 Received 17 March 2006; revised 22 October 2006; accepted 26 October 2006; published 20 December 2006. [1] Accurately quantifying deep-sea calcite dissolution is crucial for understanding the role of the marine carbonate system in regulating atmospheric pCO2 over millennia. We compare a foraminifer-fragmentation- based calcite dissolution proxy (Globorotalia menardii fragmentation index (MFI)) to Mg/Ca, Sr/Ca, and Mg/Sr in several species of deep dwelling planktonic foraminifers. We conducted microfossil and geochemical analyses on the same core top samples taken at different depths on the Ontong Java Plateau to maximize the dissolution signal and minimize the temperature overprint on our data. We also compare elemental ratios from planktonic foraminifer tests to modern bottom water CO3= undersaturation and model-derived estimates of percent calcite dissolved in deep-sea sediments. We find clear linear decreases in Mg/Ca or Mg/Sr in G. menardii and Pulleniatina obliquiloculata with increasing (1) bottom water CO3= undersaturation, (2) percent calcite dissolved in sediments calculated with biogeochemical modeling, (3) MFI, and (4) percent calcite dissolved derived from MFI. These findings lend further support to MFI as a calcite dissolution proxy for deep-sea sediments. In contrast, we find no significant correlation between Sr/Ca and independent dissolution indicators. Our results suggest that Mg/Ca and Mg/Sr from deep dwelling foraminifers could potentially be used as calcite dissolution proxies in combination with independent water temperature estimates. Likewise, establishing the relationship between MFI and dissolution-induced changes in the Mg/Ca of surface-dwelling foraminifers could provide a tool to correct Mg/Ca–derived sea surface temperature reconstructions for calcite dissolution. Citation: Mekik, F., and R. Franc¸ois (2006), Tracing deep-sea calcite dissolution: Agreement between the Globorotalia menardii fragmentation index and elemental ratios (Mg/Ca and Mg/Sr) in planktonic foraminifers, Paleoceanography, 21, PA4219, doi:10.1029/2006PA001296. 1. Introduction [2] The marine carbonate system acts to buffer changes in atmospheric pCO2 over thousands of years. It is a complex system with many components including (1) the input of ions into the ocean through weathering on land; (2) air-sea exchange of CO2; (3) the marine biological pump; (4) the ratio of organic carbon to calcite flux (rain ratio) at the seabed; and (5) the dissolution of carbonates in deep-sea sediments. Accurate estimation of the latter is thus one of the keys to understanding the evolution of the carbon cycle over geological time. [3] Cycles in calcite content in deep-sea sediments have been observed as far back as the Challenger Expedition (J. Murray and A. Renard in 1891 cited by Berger [1975]). These variations can represent changes in multiple factors, such as calcite flux, fluxes of other sedimentary components and, especially, calcite dissolution. However, a reliable quantitative calcite dissolution proxy is still elusive. Arrhenius [1952] was the first to use foraminifer fragmen- tation as a dissolution indicator. Many calcite dissolution indices have been proposed since then, such as the relative abundance in sediments of foraminifer species with differ- ent susceptibilities to dissolution [Berger, 1968, 1970; Thompson and Saito, 1974], the ratio of benthic to plank- tonic foraminifers in sediments [Thunnell, 1976], and the number of whole foraminifer shells per dry sediment weight [Le and Shackleton, 1992]. Comparing several of these proxies in the same cores, Le and Shackleton [1992] concluded that foraminifer fragmentation is a more robust indicator of dissolution than proxies based on foraminifer species assemblages. Oba [1969] found a good statistical relationship between the ratio of fragmented to whole Globorotalia menardii shells and benthic/planktic ratios in deep-sea sediments suggesting that G. menardii fragmenta- tion may be a good indicator of calcite dissolution. Subse- quent laboratory experiments [Ku and Oba, 1978] confirmed that dissolution damage in this species is pro- gressive. G. menardii fragmentation was one of the six proxies used by Peterson and Prell [1985a, 1985b] to quantify calcite dissolution (see Mekik et al. [2002] for a detailed summary of the history of calcite dissolution proxies). Most of these early proxies were calibrated against water depth or the calcite content of dry bulk sediment. [4] Recently, three dissolution proxies based on the mor- phology and fragmentation trend of planktonic foraminifer shells have been proposed: (1) carbonate size index PALEOCEANOGRAPHY, VOL. 21, PA4219, doi:10.1029/2006PA001296, 2006 1Department of Geology, Grand Valley State University, Allendale, Michigan, USA. 2Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada. Copyright 2006 by the American Geophysical Union. 0883-8305/06/2006PA001296 PA4219 1 of 12[Broecker and Clark, 1999], (2) size normalized shell weight [Lohmann, 1995; Broecker and Clark, 2001] and (3) the G. menardii fragmentation index (MFI) [Mekik et al., 2002]. The first two of these were calibrated against independent estimates of bottomwater [CO3=]. Emerson and Bender [1981] and Archer and Maier-Reimer [1994] established that in addition to CO3= undersaturation of bottom waters, seabed organic carbon flux significantly affects deep-sea calcite dissolution. Accordingly, MFI was calibrated with indepen- dent, model-derived estimates of percent calcite dissolved where changes in both bottomwater [CO3=] and seabed organic carbon flux were taken into account. [5] Although MFI has provided geographically coherent calcite rain rate reconstructions in subsequent studies [Loubere et al., 2004; Mekik et al., 2006; also Is carbonate compensation from the last deglaciation responsible for the late Holocene increase in atmospheric pCO2?, submitted to Paleoceanography, 2006, hereinafter referred to as Mekik et al., submitted manuscript, 2006], it is still empirical in nature and needs further corroboration from an independent calcite dissolution indicator. This is our main objective here. We choose Mg/Ca, Sr/Ca and Mg/Sr in deep dwelling plank- tonic foraminifer shells for comparison with MFI because Brown and Elderfield [1996], Rosenthal et al. [2000], Rosenthal and Lohmann [2002] and Dekens et al. [2002] established the potential of these element ratios as dissolu- tion indicators. As foraminifer Mg/Ca has a strong temper- ature dependence, we restrict our study to a depth transect (1977–4441 m) on the Ontong Java Plateau (OJP) in order to minimize the variability in temperature (28.4–28.9C) of our foraminifers’ habitat waters and to isolate the effect of calcite dissolution on element ratios in foraminifer shells. 2. Background 2.1. Globorotalia menardii Fragmentation Index [6] The Globorotalia menardii fragmentation index builds on Ku and Oba’s [1978] work and quantifies the amount of fragmentation of G. menardii shells in sediment. MFI is calculated as follows [after Mekik et al., 2002]: MFI ¼ D= DþWð Þ ð1Þ where W is number of whole G. menardii specimens and D is (number of specimens with holes) plus (number greater than half intact) plus (number less than half/3) plus (keels/5). The choice of denominators for the various components in the calculation of D is based on averaged visual approximations of the number of fragments within a specific category needed to make up a complete G. menardii test. Mekik et al. [2002] derived a transfer function for calculating percent calcite dissolved from a regression between measured MFI values and percent calcite dissolved estimated with the biogeochemical model, Muds [Archer et al., 2002] (R2 = 0.88): % calcite dissolved ¼ 5:111þ MFI * 160:491ð Þ  MFI2 * 79:636   ð2Þ [7] Muds calculates calcite dissolution rates using seabed organiccarbonflux(mmol/cm2/yr), percent calcite in dry bulk sediment, water depth (m) and bottom waterDCO3= (= [CO3=] of in situ seawater less [CO3=] at calcite saturation, in mmol/kg).DCO3= at each site was obtained from Archer’s [1996; personal communication, 2001] global gridded database. Organic carbon fluxes were estimated from surface ocean productivity compilations of Berger et al. [1987], Berger [1989] and BehrenfeldandFalkowski [1997]and theattenuationoforganic carbon flux with water depth (using Berger et al.’s [1987] equation). Mekik et al. [2002] also used Jahnke’s [1996] benthic oxygen flux data to independently estimate organic carbon fluxes for each sampling location. See Mekik et al. [2002] for further details on the modeling of percent calcite dissolved values used in the calibration of MFI. [8] Core top calibration samples for MFI were chosen from depth transects on the OJP and the East Pacific Rise (EPR) outside of the upwelling region in order to isolate DCO3= as the dominant control on calcite dissolution and to minimize the variability in the rain ratio [Mekik et al., 2002] which is more difficult to adequately constrain in the modern ocean [e.g., Klaas and Archer, 2002; Mekik et al., 2002, 2006]. MFI’s applicability in areas and oceanographic conditions beyond its calibration has been shown in subsequent studies, both in regions with variable seabed rain ratios [Mekik et al., 2006] and in down core work [Loubere et al., 2004; Mekik et al., submitted manuscript, 2006]. 2.2. Elemental Ratios in Planktonic Foraminifers [9] Mg/Ca in foraminifer shells carries both a water temperature signal [Nu¨rnberg, 1995; Nu¨rnberg et al., 1996; Rosenthal et al., 1997; Mashiotta et al., 1999; Lea et al., 1999; Elderfield and Ganssen, 2000; Lea et al., 2000; Rosenthal and Lohmann, 2002; Dekens et al., 2002; Lear et al., 2002; Anand et al., 2003] and a calcite dissolution signal [Brown and Elderfield, 1996; Rosenthal et al., 2000; Dekens et al., 2002; Rosenthal and Lohmann, 2002]. The dissolution signal is easier to detect in shells of deeper dwelling species (such as Neogloboquadrina dutertrei), but is also present to some extent in shells of surface dwellers, such as Globigerinoides ruber and Globigerinoides saccu- lifer [Dekens et al., 2002]. [10] McCorkle et al. [1995] reported a decrease of 15% in Sr/Ca in benthic foraminifers with increasing water depth below 2500 m on the OJP which they attributed to shell dissolution. Brown and Elderfield [1996] observed decreas- ing values of Sr/Ca in G. tumida with increasing water depth. They attributed this dissolution trend to chamber walls having higher Sr/Ca than keels, with the latter being more resistant to dissolution. Elderfield et al. [2000] also reported higher Sr/Ca in Globorotaliids in shallower water compared to those in deeper water. Mortyn et al. [2005] indicated that unlike non-Globorotaliids, Sr/Ca in Globor- otaliids show significant variability from place to place. Thus we investigate Sr/Ca as a possible tracer of calcite dissolution in two Globorotaliid species: G. menardii and G. tumida. 3. Methods 3.1. Sampling Method [11] We present new Mg/Ca, Mg/Sr and Sr/Ca data for several planktonic foraminifers from a subset of the same PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 2 of 12 PA4219core top samples with which MFI was originally calibrated. In order to limit temperature variability among samples, we limited our sampling to MFI’s calibration samples from the OJP only (Table 1). All samples are from gravity cores. [12] Considering that deep dwelling planktonic foramini- fers are subject to a smaller range of temperatures than surface dwellers and that their elemental ratios may be more sensitive to calcite dissolution, we chose deep dwellers to better isolate the dissolution signal recorded in their shell chemistry. According to Anand et al. [2003], N. dutertrei and Pulleniatina obliquiloculata live at 50–100 m water depth. According to Schweitzer and Lohmann [1991], Globorotalia tumidas grow their chambers above 50 m but their keels between 50–100m.G.menardiis are known to have symbionts, so they spend part of their life cycle close to the surface but are mostly thermocline dwellers [Be´, 1960]. [13] Foraminifers were picked individually in a given size range for each species, and all the picking was done by the same person (F. Mekik). N. dutertrei was picked from a 400–500 mm size range, P. obliquiloculata from 420– 510 mm, G. menardii from 700–800 mm and G. tumida from 900–1000 mm. These size ranges (within 100 mm) are similar to those used by Brown and Elderfield [1996] and Dekens et al. [2002], but our specimens are somewhat larger. Our results might have been more reproducible had we been able to find a sufficient number of foraminifers in a narrower size range [Elderfield et al., 2002]. On the other hand, the comparatively larger size of our specimens may have increased our precision, since analytical reproduci- bility appears to increase with increasing foraminifer shell size (J. Bijma, personal communication, 2006). 3.2. Sample Preparation [14] Samples were prepared using the Barker et al. [2003] cleaning protocol because it has been shown that more rigorous cleaning with a reductive step causes dissolution of planktonic foraminifer test material and biases results [Rosenthal et al., 2004]. However, we modified this cleaning protocol in two ways: (1) by using a scalpel to cut each foraminifer into two under the microscope instead of breaking foraminifers between glass plates, minimizing sample loss during cleaning and also ensuring that foreign particles are completely removed from each chamber, and (2) by adding a 5-min centrifuging step between each rinse with H2O2 to collect foraminifers at the bottom of the tubes and to minimize sample loss during cleaning. Table 1. Core Information Core Latitude Longitude Water Depth, m DCO3=,a mmol/kg T at 75 m,b C MFI Dissolved,c % ERDC 98 2.833 158.473 1977 1.19 28.7 34.9 ERDC 116 0.998 159.468 2272 4.51 28.6 42.1 ERDC 118 0.982 158.8 2163 3.28 28.6 39.5 ERDC 121 0.183 158.713 2245 4.21 28.6 41.5 ERDC 126 0.018 160.983 3328 16.43 28.5 64.1 ERDC 127 0.003 161.418 3724 20.89 28.8 70.3 ERDC 130 0.04 161.917 4123 25.39 28.8 76.9 ERDC 132 0.043 162.683 4441 28.98 28.9 77.4 ERDC 90 0.865 157.48 1903 0.35 28.4 32.8 ERDC 89 0.033 155.865 1932 0.68 28.4 33.9 ERDC 115 1.645 159.198 2157 3.22 28.6 39.5 ERDC 109 1.743 160.783 3636 19.9 28 68.7 ERDC 110 1.738 160.487 3003 12.76 28.7 57.4 ERDC 111 1.71 159.917 2667 8.97 28.7 51.3 ERDC 114 1.637 159.2 2151 3.15 28.6 38 aSource is Archer [1996]. bSource is Levitus and Boyer [1994]. cSource is Mekik et al. [2002]. Table 2. Number of Foraminifers in Each Measurement Core N. dutertrei G. menardii P. obliquiloculata G. tumida Measured Duplicate Measured Duplicate Measured Duplicate Measured Duplicate ERDC 98 22 7 4 40 24 ERDC 116 20 25 28 9 ERDC 118 10 6 8 ERDC 121 8 ERDC 126 26 19 7 ERDC 127 7 4 43 27 9 10 ERDC 130 5 4 ERDC 132 4 35 13 14 ERDC 90 8 34 26 ERDC 89 25 4 11 30 17 ERDC 115 13 6 27 22 5 ERDC 109 33 28 6 6 ERDC 110 31 5 ERDC 111 27 6 ERDC 114 5 5 14 2 PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 3 of 12 PA42193.3. Analytical Method [15] The Element 2 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at the University of British Colum- bia (Vancouver, British Columbia) was used for the simul- taneous determination of Mg/Ca, Sr/Ca and Mg/Sr from our foraminifer samples following the method described by Rosenthal et al. [1999]. Standard solutions were prepared gravimetrically: a set for Mg and Sr and another set for Ca alone. All standard and sample solutions were made with 0.075M nitric acid. 43Ca was measured to determine Ca concentrations in all our standards and samples. Polyatomic interferences on Ca and matrix effects on elemental ratios are insignificant in the concentration range used in our analyses [Rosenthal et al., 1999]. [16] We report Mg/Ca and Sr/Ca in N. dutertrei, P. obliquiloculata, G. menardii and G. tumida shells. Table 2 lists the number of foraminifers used for each measurement and Table 3 lists elemental ratios, duplicate measurements and the analytical error associated with each measurement. Analytical errors (66% confidence interval) are calculated from counting statistics for the ratio of each element to our internal standard (indium) and the uncertainties on the intercept and slope of the linear regressions obtained with our standard solutions. Duplicate measurements were made Table 3. Element Ratio Dataa Core N. dutertrei G. menardii P. obliquiloculata G. tumida Measured Duplicate Measured Duplicate Measured Duplicate Measured Duplicate Mg/Ca ERDC 98 2.95 ± 0.17 2.9 ± 0.17 2.53 ± 0.17 2.59 ± 0.15 2.73 ± 0.16 ERDC 116 2.16 ± 0.16 2.57 ± 0.15 2.76 ± 0.16 1.75 ± 0.1 ERDC 118 2.37 ± 0.14 2.26 ± 0.12 2.34 ± 0.13 ERDC 121 1.98 ± 0.18 ERDC 126 2.44 ± 0.14 2.47 ± 0.14 1.91 ±0.11 ERDC 127 1.4 ± 0.09 1.2 ± 0.09 2.24 ± 0.16 2.07 ± 0.12 1.56 ± 0.11 1.57 ± 0.09 ERDC 130 1.66 ± 0.17 1.4 ± 0.15 ERDC 132 1.46 ± 0.09 1.94 ± 0.12 1.69 ± 0.11 1.32 ±0.13 ERDC 90 2.41 ± 0.24 2.67 ± 0.2 2.75 ± 0.17 ERDC 89 2.36 ± 0.23 2.31 ± 0.23 2.46 ± 0.16 3.05 ± 0.19 2.97 ± 0.17 ERDC 115 2.73 ± 0.28 2.64 ± 0.22 2.84 ± 0.17 2.1 ± 0.15 ERDC 109 2.21 ± 0.14 2.3 ± 0.14 1.6 ± 0.13 1.79 ± 0.11 ERDC 110 2.57 ± 0.23 2.15 ± 0.21 ERDC 111 2.66 ± 0.15 2.05 ± 0.17 ERDC 114 2.68 ± 0.22 2.66 ± 0.16 2.64 ± 0.15 2.83 ±0.23 Sr/Ca ERDC 98 1.08 ± 0.05 1 ± 0.04 1.26 ± 0.05 1.48 ± 0.06 ERDC 116 1.05 ± 0.05 1.26 ± 0.05 1.55 ± 0.06 1.12 ± 0.05 ERDC 118 1.21 ± 0.05 1.16 ± 0.05 1.13 ± 0.05 ERDC 121 1.32 ± 0.08 1.15 ± 0.08 ERDC 126 1.25 ± 0.05 1.4 ± 0.06 1.12 ± 0.05 ERDC 127 1.14 ± 0.05 1.2 ± 0.05 1.28 ± 0.08 1.39 ± 0.06 1.08 ± 0.07 1.26 ± 0.05 ERDC 130 1.13 ± 0.07 1.41 ± 0.08 1.48 ± 0.06 1.18 ± 0.09 ERDC 132 1.21 ± 0.05 1.3 ± 0.06 1.31 ± 0.06 1.13 ± 0.07 ERDC 90 1.15 ± 0.08 1.24 ± 0.08 1.45 ± 0.06 ERDC 89 1.29 ± 0.09 1.3 ± 0.01 1.21 ± 0.06 1.33 ± 0.06 ERDC 115 1.24 ± 0.08 1.28 ± 0.05 1.33 ± 0.09 1.52 ± 0.06 1.15 ± 0.06 ERDC 109 1.29 ± 0.11 1.44 ± 0.06 1.15 ± 0.08 1.29 ± 0.05 ERDC 110 1.3 ± 0.09 1.2 ± 0.08 ERDC 111 1.44 ± 0.06 1.16 ± 0.07 ERDC 114 1.16 ± 0.08 1.49 ± 0.06 1.06 ± 0.06 Mg/Sr ERDC 98 2.73 ± 0.16 2.53 ± 0.20 2.06 ± 0.13 1.84 ± 0.11 ERDC 116 2.06 ± 0.17 2.04 ± 0.13 1.78 ± 0.11 1.55 ± 0.10 ERDC 118 1.96 ± 0.14 1.95 ± 0.13 2.11 ± 0.14 ERDC 121 1.72 ± 0.17 ERDC 126 1.95 ± 0.12 1.76 ± 0.11 1.71 ± 0.12 ERDC 127 1.23 ± 0.08 1 ± 0.08 1.75 ± 0.12 1.49 ± 0.09 1.44 ± 0.10 1.25 ± 0.09 ERDC 130 1.47 ± 0.14 1.19 ± 0.12 ERDC 132 1.21 ± 0.08 1.49 ± 0.10 1.29 ± 0.09 1.17 ± 0.10 ERDC 90 2.09 ± 0.20 2.15 ± 0.14 1.9 ± 0.10 ERDC 89 1.82 ± 0.18 1.78 ± 0.18 2.03 ± 0.15 2.29 ± 0.15 ERDC 115 2.22 ± 0.23 1.98 ± 0.15 1.87 ± 0.12 1.83 ± 0.14 ERDC 109 1.71 ± 0.19 1.6 ± 0.10 1.39 ± 0.11 1.39 ± 0.11 ERDC 110 1.98 ± 0.18 1.79 ± 0.17 ERDC 111 1.85 ± 0.11 1.65 ± 0.13 ERDC 114 2.31 ± 0.17 1.94 ± 0.09 1.75 ± 0.11 2.67 ± 0.23 aValues are given in mmol/mol for Mg/Ca and Sr/Ca and mmol/mmol for Mg/Sr. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 4 of 12 PA4219on a subset of our samples where enough foraminifer material was available (Table 3). 4. Results [17] Figure 1a shows the Mg/Ca, Mg/Sr and Sr/Ca ratios of foraminifers plotted against water temperature at 75 m depth [from Levitus and Boyer, 1994]. The effect of temperature variability must be minimal on Mg/Ca measurements in our samples from the OJP because all our sampling locations fall within a tight temperature range at the mean habitat depths of the foraminifer species under consideration (75 m). All foraminifer samples have a large range of variability in Mg/Ca, Mg/Sr and Sr/Ca at similar temperatures on the OJP (Figure 1a). [18] Figure 1b shows the relationship of Sr/Ca in each of our species with DCO3= from Archer’s [1996] global gridded database. Although Elderfield et al. [2000] found a decrease in Sr/Ca with water depth in G. tumida, we find no relationship in our samples between any of our foraminifers’ Sr/Ca and DCO3=, including G. tumida. This may not be surprising since Sr appears to be homoge- neously distributed throughout most foraminifer shells [Bender et al., 1975]. However, the discrepancy between Elderfield et al.’s [2000] results and ours remains Figure 1. (a) Temperature at 75 m water depth [Levitus and Boyer, 1994] plotted against foraminifer Mg/Ca, Mg/Sr, and Sr/Ca. (b) DCO3= [Archer, 1996] plotted against foraminifer Sr/Ca. All samples are from the Ontong Java Plateau. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 5 of 12 PA4219unexplained. Since Sr also appears to be homogeneous in our foraminifers, we investigated a dissolution trend using Mg/Sr as well as Mg/Ca. [19] Our N. dutertrei samples are limited in number and in their range of DCO3=. We therefore complemented them with a subset of Dekens et al.’s [2002] Mg/Ca data set. We show a subset of the data from Dekens et al.’s [2002] corresponding to the MFI calibration sites on the OJP to minimize temperature variability among samples. The open symbols correspond to our Mg/Ca data. Note that Dekens et al. [2002] used a 250–350 mm size range for N. dutertrei, while we used 400–500 mm. The trend of our data seems to mesh moderately well with theirs when plotted against DCO3= (values from Archer [1996]) (R2 = 0.61) (Figure 2). The relationship between Mg/Ca in N. dutertrei and DCO3= would likely be stronger had we used N. dutertrei tests of the same size as those used by Dekens et al. [2002]. [20] In Figure 3, we present Mg/Ca and Mg/Sr in G. menardii, P. obliquiloculata and G. tumida plotted against DCO3=. There is a clear decrease in Mg/Ca in both G. menardii and P. obliquiloculata with increasingly negative DCO3= (R2 = 0.87 – 0.88); and a similar but somewhat lower correlation between Mg/Sr and DCO3= (R2 = 0.68–0.82). These relationships in G. tumida are less robust (R2 = 0.64–0.71) and better fitted with an exponential curve, suggesting that changes in elemental ratios in this species may be more sensitive to the low end of calcite dissolution. [21] Figure 4 shows the same data plotted against model- derived estimates of percent calcite dissolved per sample location. The relationship between Mg/Ca or Mg/Sr and modeled percent calcite dissolved is statistically similar to those with DCO3= (Figure 3). Thus we are able to illustrate that there is a good linear trend between geochemical measurements from foraminifer shells and both modeled estimates of percent calcite dissolved and DCO3=. [22] Plotting Mg/Ca and Mg/Sr from the three foraminifer species against the fragmentation trend of G. menardii (MFI) in the same sediment samples (Figure 5) and against percent calcite dissolved using the MFI transfer function of Mekik et al. [2002] (Figure 6) reveals equally strong relationships for G. menardii and P. obliquiloculata (R2 = 0.74–0.94 for MFI; R2 = 0.69–0.88 for percent dissolved). Again, the relationships with G. tumida are less robust (R2 = 0.64–0.69 for MFI; R2 = 0.58–0.63 for percent dissolved). 5. Discussion [23] Our data indicate that both MFI and elemental ratios from our deep dwelling foraminifer species, Mg/Ca and Mg/Sr, covary in response to decreasing DCO3= and increas- ing calcite dissolution in the sediments. Other element/Ca ratios from foraminifer shells have been shown to also decrease with increasing dissolution, such as F/Ca [Rosenthal and Boyle, 1993], V/Ca [Hastings, 1994] and U/Ca [Russell, 1994]. Since we carefully chose our samples so as to limit the effect of temperature and other possible confounding variables (such as surface water [CO3=] and nutrient concentration) on foraminifer elemental ratios, this finding provides independent corroboration for MFI as a dissolution proxy. [24] The prospect of using foraminifer Mg/Ca as a calcite dissolution indicator is hindered by its strong temperature dependence in foraminifer shells [Nu¨rnberg, 1995; Nu¨rnberg et al., 1996; Rosenthal et al., 1997; Mashiotta et al., 1999; Lea et al., 1999; Elderfield and Ganssen, 2000; Lea et al., 2000; Rosenthal and Lohmann, 2002; Dekens et al., 2002; Lear et al., 2002; Anand et al., 2003]. Foraminifer Mg/Ca from samples with similar MFI values but collected over a wider range of temperature would establish the temperature dependence of Mg/Ca in P. obliquiloculata, G. menardii and G. tumida. This temperature signal combined with independent estimates of water temperature at their habitat depths could then be used to estimate down core variations in calcite dissolution fromMg/Ca in deep dwellers, although finding appropriate temperature proxies for such a purpose may prove difficult. Alternatively, if we can confirm and more precisely constrain the correlation between dissolution induced changes in Mg/Ca from deep dwelling foraminifers and MFI that we have documented in this study, we could combine these two measurements in down core samples to assess past changes in subsurface seawater temperature. [25] As DCO3= decreases from 0 to -30 mmol/kg, there is a 55% drop in Mg/Ca and Mg/Sr of G. tumida; 50% drop inG.menardii; and a30%drop inP. obliquiloculataMg/Ca and Mg/Sr. This DCO3= range corresponds to an increase in modeled percent calcite dissolved from 45%–80%. A closer examination of Figures 3 through 6 suggests that elemental ratios in P. obliquiloculata may be more sensitive to percent dissolved less than 50%, while elemental ratios in G. tumida Figure 2. Plot of DCO3= against Mg/Ca in N. dutertrei. Solid diamonds show data from Dekens et al. [2002] (250– 350 mm). Open diamonds show new data (400–500 mm). Samples are from the Ontong Java Plateau. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 6 of 12 PA4219Figure 3. DCO3= plotted against foraminifer Mg/Ca and Mg/Sr data. Bars show analytical error margins as listed in Table 3. Samples are from the Ontong Java Plateau. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 7 of 12 PA4219Figure 4. Modeled estimates of percent calcite dissolved plotted against foraminifer Mg/Ca and Mg/Sr data. Bars show analytical error margins as listed in Table 3. Samples are from the Ontong Java Plateau. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 8 of 12 PA4219Figure 5. MFI plotted against foraminifer Mg/Ca and Mg/Sr data. Bars show analytical error margins as listed in Table 3. Samples are from the Ontong Java Plateau. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 9 of 12 PA4219Figure 6. Estimates of percent calcite dissolved with the MFI transfer function [Mekik et al., 2002] plotted against foraminifer Mg/Ca and Mg/Sr data. Bars show analytical error margins as listed in Table 3. Samples are from the Ontong Java Plateau. PA4219 MEKIK AND FRANC¸OIS: DEEP-SEA CARBONATE DISSOLUTION 10 of 12 PA4219may be more sensitive to percent dissolved greater than 50%. If confirmed, such trends would point to combining the analysis of different species of foraminifers to optimally reconstruct calcite dissolution over the entire water depth range. [26] Dekens et al. [2002] showed that Mg/Ca in deep dwelling foraminifers (N. dutertrei in their work) is more susceptible to dissolution than Mg/Ca in surface dwellers. Surface dwellers (e.g., G. ruber and G. sacculifer) thus appear to be better suited to record sea surface temperature (SST) variations. Dekens et al. [2002] also state that though Mg/Ca from G. ruber and G. sacculifer primarily carry a temperature signal, they also have a DCO3= overprint, though to a lesser degree than Mg/Ca from N. dutertrei. Since the temperature dependence of Mg/Ca in G. ruber and G. sacculifer has been well established [e.g., Lea et al., 2000; Dekens et al., 2002; Rosenthal and Lohmann, 2002], it may be possible to tease out the effect of calcite dissolution on the Mg/Ca of these two surface dwellers (as we have done with other species herein) by analyzing samples representing a wide range of calcite dissolution but limited temperature variability. If a correlation between dissolution induced changes in the elemental ratios of surface dwellers and MFI can be found, then down core Mg/ Ca–derived SST reconstructions could be corrected for calcite dissolution in areas where foraminifer material for both proxies (Mg/Ca and MFI) are available in the same cores. 6. Conclusions [27] We compared a foraminifer fragmentation proxy (G. menardii fragmentation index (MFI)) with elemental ratios from foraminifer shells in the same samples. Our sample locations were carefully chosen so as to provide sediment samples subjected to a wide range of seabed calcite dissolution and surface water conditions with a narrow range of temperature, [CO3=] and nutrient concentra- tions. We studied deep dwelling planktonic foraminifers based on the work of Dekens et al. [2002] which indicated that deeper water foraminifers are more sensitive to dissolution than are surface dwellers. The correlations between MFI, elemental ratios, modeled percent calcite dissolved and DCO3= lend support for the use of MFI as a reliable calcite dissolution proxy. Mg/Ca or Mg/Sr in P. obliquiloculata, G. menardii and possibly G. tumida would also have the potential for use as calcite dissolution indicators after the temperature dependence of their signal is established. Alternatively, combining Mg/Ca in deep dwell- ing foraminifers with MFI, could provide a means for accurately estimating past changes in water temperature at these foraminifers’ habitat depths. Likewise, finding a correlation between dissolution-induced changes in the elemental ratios of surface dwellers and MFI could provide a tool to correct sea surface temperature estimates for calcite dissolution. [28] Acknowledgments. Many thanks to Paul Loubere for valuable discussions and for giving us the idea to cut foraminifers with a scalpel and to Maureen Soon for her impeccable lab support every step of the way. Special thanks are due to Jelle Bijma for sharing his unpublished data with us. We extend our gratitude to Heather Stoll, Gerald Dickens, and an anonymous reviewer for their thoughtful and constructive reviews that improved our manuscript. 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