- Email: [email protected]
Ca-temperature relationship of benthic foraminiferal calcite: New core-top calibrations in the < 4 °C temperature range
Earth and Planetary Science Letters 272 (2008) 523–530
Contents lists available at ScienceDirect
Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
The Mg/Ca-temperature relationship of benthic foraminiferal calcite: New core-top calibrations in the b 4 °C temperature range Stephanie L. Healey a,⁎, Robert C. Thunell a, Bruce H. Corliss b a b
Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA Department of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina 27706, USA
A R T I C L E
I N F O
Article history: Received 21 April 2008 Accepted 15 May 2008 Available online 7 July 2008 Editor: M.L. Delaney Keywords: paleoceanography Mg/Ca paleothermometry benthic foraminifera carbonate ion
A B S T R A C T Calcite magnesium/calcium ratios were determined for four species of benthic foraminifera (Cibicidoides wuellerstorfi, Cibicidoides mundulus, Oridorsalis umbonatus and Pyrgo murrhina) from core-top samples collected from the Atlantic, Indian, and Pacific Oceans where modern bottom water temperatures are less than 4 °C. Mg/Ca of all four species increase with increasing temperature, and the Mg/Ca:temperature relationship does not appear to be necessarily exponential. Mg/Ca of C. mundulus is consistently higher than C. wuellerstorfi, implying differences in the uptake of Mg by different species within this genus. The observed Mg/Ca for C. wuellerstorfi at temperatures less than 3 °C is lower than those predicted from published Mg/Ca: temperature relationships derived for Cibicidoides spp. at higher temperatures. Previous studies have attributed this change in the Mg/Ca-temperature relationship of C. wuellerstorfi to the effect of decreased carbonate ion saturation (Δ[CO 2− 3 ]) on Mg/Ca at low temperatures. A relationship between the Δ[CO2− 3 ] and Mg/Ca of 0.0083 ± 0.002 (mmol/mol)/(μmol/kg) has been established for C. wuellerstorfi from this study. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ocean circulation and temperature, along with continental ice volume are important elements of the global climate system and reconstructing past changes in these parameters are basic objectives of paleoclimatology. The change in δ18O composition of seawater due to the melting of continental ice sheets since the last glacial maximum (LGM) has been estimated using fossil coral reef terraces (Fairbanks, 1989) and sediment pore-fluid chemistry (Adkins and Schrag, 2001; Schrag et al., 2002, 1996). Although δ18O changes in benthic foraminiferal calcite in deep-sea cores have been used as a firstorder approximation of Late Pleistocene glacial–interglacial sea-level changes (e.g. Shackleton, 1967), most of these records also contain glacial–interglacial variability of a higher amplitude than what is expected based on independent sea-level estimates (e.g. Labeyrie et al., 1987; Martin et al., 2002; Skinner et al., 2003), suggesting that bottom water temperatures also vary significantly on these timescales. For example, it has been estimated that bottom waters were about 1.5–3 °C colder in the Pacific (Chappell and Shackleton, 1986; Shackleton, 1987; Martin et al., 2002) and 3–4 °C colder in the Atlantic (Schrag et al., 2002; Adkins and Schrag, 2001; Martin et al., 2002) during glacial times than today.
⁎ Corresponding author. E-mail address: [email protected] (S.L. Healey). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.05.023
The oxygen isotope composition (δ18O) of benthic foraminiferal calcite is a function of the temperature and the δ18O composition of sea water at the time of calcification. Likewise, the δ18O composition of sea water is a function of global ice volume and local variations in salinity. Thus, benthic foraminiferal δ18O measurements alone cannot provide quantitative estimates of past changes in any of these parameters. However, with the development of Mg/Ca paleothermometry, it is now possible to use benthic foraminiferal Mg/Ca to directly estimate bottom water temperatures (Rosenthal et al., 1997; Rathburn and De Deckker, 1997; Martin et al., 2002; Lear et al., 2002). Thus, paired Mg/Ca and δ18O measurements of benthic foraminifera offer the potential to identify how much of the total δ18O change of benthic foraminiferal calcite is due to changes in the isotopic composition of seawater. The majority of paleoclimate records utilizing benthic foraminiferal δ18O and/or Mg/Ca are based on sediment cores retrieved from water depths greater than 2000m. At present, over 95% of the global ocean deeper than 2000m has bottom water temperatures that are b5 °C (Fig. 1), and it is reasonable to assume that in most regions temperatures below this depth were colder during glacial periods. Thus, it is important that the relationship between temperature and the Mg/Ca of benthic foraminiferal calcite be well constrained at the low end of the temperature spectrum. Existing benthic foraminiferal Mg/Ca-temperature calibrations are based on data sets spanning bottom water temperatures of 0–18 °C and combine different species of benthic foraminifera, including species that live primarily in waters warmer than
524
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
Fig. 1. World map showing the average annual bottom water temperature at 2000 meters depth (World Ocean Atlas, 1998). The locations of all core-top samples referenced and/or presented in this study are also shown as red symbols: this study (open squares), Lear et al., 2002 (solid squares); Martin et al., 2002 (open circles); Russell et al., 1994 and Rosenthal et al., 1997, after Martin et al., 2002 (solid circles), Marchitto et al., 2007 (open diamond).
5 °C (Lear et al., 2002; Martin et al., 2002; Rosenthal et al., 1997; Elderfield et al., 2006). Martin et al. (2002) were the first to recognize that the Mg/Ca-temperature sensitivity in benthic foraminifera is higher
at cold temperatures (b3 °C) relative to that at warmer temperatures. Similarly, work on planktonic foraminifera (McConnell and Thunell, 2005) has shown that trace metal paleotemperature equations can vary
2 2− 2− Fig. 2. Calculated carbonate ion concentration (black), and ΔCO2− 3 (blue) vs. depth, where ΔCO3 = [CO3 ] sample − [CO3 ]calcite saturation, at the five GEOSECS stations nearest to the locations of core-top samples used in this study. Dashed lines represent the depths of core-top samples from each location. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
based on the temperature range used in the calibration. Most recently, Elderfield et al. (2006) have argued that the greater response of benthic foraminiferal Mg/Ca to temperature in the b3 °C temperature range is attributable to the effect of low carbonate ion saturation (ΔCO3) values on the uptake of Mg at low temperatures (Elderfield et al., 2006). The goal of this study is to examine the Mg/Ca-bottom water temperature relationship of individual benthic foraminifera species in the b4 °C temperature range, and further assess the potential of this proxy as a paleothermometer. The Mg/Ca of four different benthic foraminiferal species were analyzed from the Atlantic, Pacific, and Indian Oceans coretop samples from sites with bottom waters b4 °C. These results were compared to similar data from previous studies in the same temperature range. 2. Methods Core-top samples used for Mg/Ca-determinations in this study were obtained from five locations in the Southeast Indian Ocean, South China Sea, North Atlantic, Equatorial Atlantic, and the South Atlantic (Fig. 1, Table A1 in Appendix). The core-top samples cover a range of water depths from 1095–4548m and a bottom water temperature (BWT) range of 0.8–3.8 °C. Carbonate ion concentration has been estimated for each core location using the nearest GEOSECS station (Lewis and Wallace, 1998), and ΔCO3 has been calculated as: 2− ΔCO3 = [CO2− 3 ] sample location − [CO3 ] calcite saturation (Fig. 2). Sediment samples were disaggregated in buffered water and wetsieved through a 63μm mesh. Samples were dry sieved through a 250μm mesh to obtain typically between 5 and 30 benthic foraminiferal specimens. Species used in this study include Cibicidoides wuellerstorfi, Cibicidoides mundulus, Oridorsalis umbonatus, and
525
Pyrgo murrhina. The foraminiferal shells were cleaned following the rigorous procedure outlined in Martin and Lea (2002), without the alkaline chelation step. Cleaned samples were dissolved in 300–500μL 0.5N HNO3 and Mg/Ca ratios were measured using a Jobin Yvon Ultima C inductively coupled plasma atomic emission spectrometer (ICPAES). Duplicate analyses of a standard solution yielded an analytical precision of 0.4%. Duplicate analyses of samples resulted in an average precision of ~ 1.4%. 3. Results: benthic foraminiferal Mg/Ca-temperature calibrations 3.1. C. wuellerstorfi The new Mg/Ca results for C. wuellerstorfi from this study are shown in Fig. 3A. Exponential and linear regressions of this data yield the following equations: Mg=Ca ¼ 0:781F0:023 expð0:23F0:0134BWTÞ Mg=Ca ¼ ð0:295F0:0184BWTÞ þ 0:67F0:04
R2 ¼ 0:91 R2 ¼ 0:90
ð1Þ ð2Þ
As observed in previous studies (Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006), our data for the temperature range of b4 °C yield Mg/Ca ratios which exhibit a stronger dependence on temperature than the relationship established over a larger temperature range (0–18 °C; Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006; Fig. 3A). For this species, a linear fit describes the Mg/Catemperature relationship as well as an exponential equation (Fig. 3A). The standard errors of the linear and exponential regressions derived from this new data are ± 0.20 and ± 0.18 °C respectively.
Fig. 3. Mg/Ca ratios of A) C. wuellerstorfi, B) C. mundulus, C) O. umbonatus, and D) P. murrhina from core-top samples vs. bottom water temperature from this study. Both linear and exponential curves have been fit to each data set. Dashed lines bracket the 95% confidence interval about each regression.
526
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
set and the small temperature range (1–3 °C), further analyses will be necessary to adequately constrain the Mg/Ca-temperature relationship for this species. 4. Results: comparison with previous benthic foraminiferal Mg/Ca-temperature calibrations In this section, we briefly review previous studies of Lear et al. (2002), and Martin et al. (2002), which have examined the relationship between temperature and Mg/Ca in various species of benthic foraminifera (Figs. 5 and 6). Because the objective of our study is to examine the nature of this relationship at low temperatures, we specifically look at the data from these previous studies for samples that have present day bottom water temperatures of b5 °C. For each of these previous studies, we generate new Mg/Ca-temperature equations using this low temperature sample subset. The locations of the core-top samples used in these studies are shown in Fig.1. A detailed description of the sample selection for these data subsets from previous studies is available in Appendix A. Fig. 4. Comparison of the linear C. mundulus Mg/Ca-temperature equation from this study (red), Elderfield et al. (2006) (black) and the linear C. wuellerstorfi regression and data from this study (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Mg/Ca-temperature calibrations Exponential equations
3.2. C. mundulus
Mg/Ca = BB ⁎ e(A ⁎ BWT)
Mg/Ca of C. mundulus was measured in 21 samples and shows the expected increase with increasing temperature (Fig. 3B). The exponential and linear equations determined from this data are as follows: Mg=Ca ¼ 1:00F0:13 expð0:277F0:0594BWTÞ
R2 ¼ 0:54 R2 ¼ 0:54
Mg=Ca ¼ ð0:496F0:104BWTÞ þ 0:798F0:227
ð3Þ
Study
Published equations Martin et al. (2002) Lear et al. (2002)
ð4Þ
The exponential fit indicates an increase in Mg/Ca of ~ 27% per °C (Fig. 3B), while the linear fit to the same data suggests a dependence of the Mg/Ca ratio on temperature of ~ 0.496mmol/mol per °C (Fig. 3B). In both cases, the C. mundulus Mg/Ca change per °C is larger than the Mg/Ca change in C. wuellerstorfi from this study (Figs. 3A and B, 4, Table 1). However, the slopes of the exponential equations for C. wuellerstorfi and C. mundulus are not significantly different (Figs. 3A and B, 4, Table 1). 3.3. Other benthic foraminiferal species
Lear et al. (2002) Elderfield et al. (2006) Individual species only Lear et al. (2002) Martin et al. (2002) Russell et al. (1994) and Rosenthal (after Martin et al. 2002) This study This study Lear et al. (2002)
We also measured Mg/Ca ratios in the benthic foraminifera O. umbonatus and P. murrhina (Figs. 3C and D). The Mg/Ca:temperature relationship for O. umbonatus from this study is equally well described by the following exponential and linear regressions (Fig. 3C): Mg=Ca ¼ 0:988F0:08 expð0:252F0:0364BWTÞ
R2 ¼ 0:69
Mg=Ca ¼ ð0:449F0:0664BWTÞ þ 0:773F0:151
R2 ¼ 0:72
ð5Þ ð6Þ
P. murrhina exhibits much higher Mg/Ca ratios than any other species analyzed (Fig. 3D), which is consistent with previous results for this species (Boyle, 1985). The exponential and linear equations derived for this species are as follows: Mg=Ca ¼ 3:095F0:608 expð0:569F0:0824BWTÞ Mg=Ca ¼ ð6:312F1:1694BWTÞ−2:038F2:78
R2 ¼ 0:79 R2 ¼ 0:69
ð7Þ ð8Þ
Like other species of benthic foraminifera, the Mg/Ca ratio of P. murrhina increases with increasing temperature, indicating its potential as a paleothermometer. However, given the limited sample
This study
R2
Temperature range (°C)
0.11
0.95
− 1–18
0.867
0.109
0.94
0.8–18
1.008 0.9
0.114 0.11
0.4 0.99
0.8–9.9 − 1–18
C. wuellerstorfi C. wuellerstorfi C. wuellerstorfi
0.652 0.64 0.953
0.28 0.255 0.206
0.58 0.81 0.49
1.7–4.4 1.8–3 0.4–2.8
C. wuellerstorfi C. mundulus O. umbonatus O. umbonatus (b 5 °C data only) P. murrhina
0.781 1 0.99 0.79
0.23 0.277 0.252 0.252
0.91 0.54 0.67 0.72
0.95–3.8 0.9–2.9 0.85–3.8 0.84–4.35
3.1
0.569
0.79
0.9–2.9
C. pachyderma C. mundulus
1.2 0.88
0.116 0.11
0.73 0.96
5.8–18.6 1.2–11.8
C. wuellerstorfi C. wuellerstorfi C. wuellerstorfi
0.378 0.448 0.848
0.397 0.309 0.307
0.63 0.81 0.45
1.7–4.4 1.8–3 0.4–2.8
C. wuellerstorfi C. mundulus O. umbonatus O. umbonatus (b 5 °oC data only) P. murrhina
0.67 0.798 0.77 0.57
0.295 0.496 0.449 0.41
0.9 0.54 0.67 0.65
0.95–3.8 0.9–2.9 0.85–3.8 0.84–4.35
0.69
0.9–2.9
Species
B
C. pachyderma, C. wuellerstorfi C. pachyderma, C. wuellerstorfi, C. compressus, other C. spp. O. umbonatus Cibicidoides spp.
0.85
A
Linear equations Mg/Ca = (A ⁎ BWT) + B Published equations Marchitto et al. (2007) Elderfield et al. (2006) Individual species only Lear et al. (2002) Martin et al. (2002) Russell et al. (1994) and Rosenthal (after Martin et al., 2002) This study This study This study Lear et al. (2002) This study
6.31
−2.04
Coefficients, R2, and temperature range of all Mg/Ca-temperature calibrations from this study and other published Mg/Ca data from core-top samples.
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
527
Fig. 5. Mg/Ca ratios of C. wuellerstorfi from core-top samples vs. bottom water temperature for four different data sets: A) This study, B) Lear et al. (2002), C) Martin et al. (2002), D) Russell et al. (1994) and Rosenthal (after Martin et al., 2002). Both linear and exponential curves have been fit to each data set. Dashed lines bracket the 95% confidence interval about each regression. E) A comparison of the exponential equations for each of the data sets. F) A comparison of the linear equations for each of the data sets. For E and F, the Elderfield et al. (2006) equation for Cibicidoides species the Marchitto et al. (2007) equation for C. pachyderma are shown for comparison.
4.1. Comparison of C. wuellerstorfi calibrations As expected, the Mg/Ca ratios of C. wuellerstorfi increase with increasing temperature for each of the four previously published data sets (Fig. 5). Each sample set has been fit with an exponential curve, the expected relationship between temperature and Mg/Ca based on equilibrium thermodynamics and the van't Hoff equation (Lea et al., 1999). In addition, a least squares fit linear regression has been determined for each data set. In all four cases, the linear fit describes the relationship between the Mg/Ca ratio of the benthic foraminiferal calcite and temperature nearly as well, and in some cases better than the exponential fit (Fig. 5; Table 1). The y-intercepts for the various equations differ by 0.5 to 1.0mmol/mol (Fig. 5E and F; Table 1). This may be due to analytical differences (e.g. ICP-MS vs. ICP-AES) and/or differences in the cleaning protocols. However, the slopes of the four exponential equations suggest a very similar response of the Mg/Ca of C. wuellerstorfi to temperature within the range of 1–5 °C. The exponential fits suggest an increase of ~ 20–28% in the Mg/Ca ratio of C. wuellerstorfi per °C (Table 1). This rate of change is considerably higher than the previously published values of an ~ 11% increase in Mg/Ca per °C for combined Cibicidoides species over a temperature range of − 1 to 18 °C (Lear et al., 2002; Martin et al., 2002, Elderfield et al., 2006; Table 1) and a ~ 4.2% increase in Mg/Ca per °C for C. pachyderma over the temperature range of 5.8 to 18.6 °C (Marchitto et al., 2007; Fig. 5E and F, and Table 1).
The linear equations suggest an even higher dependence of the Mg/Ca ratio on temperature, ranging from ~ 0.295–0.397 mmol/mol per °C. 4.2. Comparison of O. umbonatus calibrations We compare our core-top Mg/Ca measurements of O. umbonatus (Fig. 3C) to previously published data for this species from Lear et al. (2002) for the b5 °C temperature range, as described in Appendix A (Fig. 6A and B; Table 1). The linear relationships suggest an increase in Mg/Ca of 0.415 and 0.449mmol/mol per °C for our data and the data from Lear et al. (2002) respectively, with these values being indistinguishable from each other at the 95% confidence interval. The exponential equations indicate a 25.2% increase in Mg/Ca per °C for both data sets (Figs. 6C and D; Table 1). However, this change in Mg/Ca per °C estimated from these two equations is considerably larger than the ~ 11.4% change per °C reported by Lear et al. (2002) for the temperature range of 0–10 °C (Fig. 6C; Table 1). 5. Discussion: comparison of Cibicidoides spp. calibrations Recent core-top studies have shown that the Mg/Ca-temperature relationship for benthic foraminiferal calcite is not necessarily exponential (Marchitto et al., 2007; Elderfield et al., 2006) and that a linear regression describes most data sets as well as the exponential
528
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
Fig. 6. A) Exponential Mg/Ca-temperature equation for O. umbonatus using the Lear et al. (2002) data less than 5 °C. B) Linear Mg/Ca-temperature equation for O. umbonatus using the Lear et al. (2002) data less than 5 °C. Dashed lines bracket the 95% confidence interval about each regression. C) and D) Comparison of the exponential and linear equations for O. umbonatus b5 °C data from this study and the Lear et al. (2002) study compared to the published O. umbonatus equation of Lear et al. (2002).
relationship predicted from thermodynamics. Three previously published C. wuellerstorfi data sets (add references for the three data sets) for samples with bottom water temperatures of b5 °C support this conclusion in that we find no significant difference between a linear and exponential fit for any of the data considered in this study (Fig. 5; Table 1). In all cases, the exponential equations based on samples with bottom water temperatures ">b5 °C predict an increase in Mg/Ca of C. wuellerstorfi of N20% per °C (Fig. 5; Table 1). This temperaturedependent change is much larger than that for previously published equations based on multiple species of Cibicidoides for significantly larger temperature ranges (− 1to 18 °C), (Lear et al., 2002; Martin et al., 2002) and single Cibicidoides species calibrations at higher temperature ranges (Rosenthal et al., 1997; Marchitto et al., 2007). These differences may be due to species specific “vital” effects in the uptake of Mg, even for species in the same genus. To investigate this possibility, we also measured the Mg/Ca of another Cibicidoides species, C. mundulus, from core-top samples and compare these results to our C. wuellerstorfi data. Although Mg uptake by C. mundulus does exhibit slightly higher temperature dependence than for C. wuellerstorfi, it is within the range of exponential slopes exhibited for all four of the C. wuellerstorfi equations (Table 1). The measured Mg/Ca values for C. mundulus are consistently higher than those measured for C. wuellerstorfi for a given temperature (Fig. 4), which is consistent with previous observations (Elderfield et al., 2006). The published Cibicidoides equations based on multiple species (Lear et al., 2002; Martin et al., 2002; Elderfield et al., 2006) are dominated at high temperatures by data for C. pachyderma. This species lives primarily in waters shallower than 2500m and at temperatures above 5 °C (Marchitto et al., 2007). These temperatures are warmer than those typically associated with C. wuellerstorfi (Van Morkhoven et al., 1986). Furthermore, a recently published Mg/Ca-temperature equation for C. pachyderma differs from that of the combined Cibicidoides species data
sets, with the increase in Mg/Ca of C. pachyderma being ~ 4.2% per °C (Marchitto et al., 2007), in contrast to ~ 10% increase in Mg/Ca per °C for the multi-species equations (Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006). In order to further explore differences in the Mg/Ca: temperature relationship for different species of Cibicidoides we compare the linear equation for C. wuellerstorfi from this study
Fig. 7. Comparison of the linear C. wuellerstorfi equation from this study (red) vs. the published linear equation of Marchitto et al. (2007) for C. pachyderma (black), where the solid black line represents the actual calibration temperature range and the dashed line represents an extrapolation of the linear relationship to low temperatures. Blue data points represent C. wuellerstorfi Mg/Ca data from the Sulu Sea, which were not included in the C. wuellerstorfi calibration data set from this study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
with the linear C. pachyderma equation based on samples from the Florida Straights (Marchitto et al., 2007; Fig. 7). The equations for these two species are significantly different, with C. wuellerstorfi exhibiting a much greater change in Mg/Ca per °C than C. pachyderma. However, the C. wuellerstorfi data points above 3 °C could just as reasonably be described by the C. pachyderma equation as the b4 °C equation for C. wuellerstorfi (Fig. 7). To examine this issue further, we measured Mg/Ca on C. wuellerstorfi from five coretop samples from the Sulu Sea with bottom water temperatures of 10.5–11 °C (Fig. 7). These samples are unusual in that C. wuellerstorfi rarely inhabit waters above 5 °C. The Mg/Ca values for these five samples (2.25–2.75mmol/mol) fall close to or on the line describing the C. pachyderma Mg/Ca-temperature relationship and do not exhibit the higher Mg/Ca values that would be predicted (3.77– 3.92mmol/mol) by our low temperature C. wuellerstorfi equation. This suggests that the different C. wuellerstorfi and C. pachyderma Mg/Ca-temperature relationships are not species specific, but due to some other factor. This does not imply that the two species exhibit identical responses to temperature, but rather that the large difference observed in the two Mg/Ca-temperature relationships cannot be attributed simply to interspecies differences in Mg uptake. It has been suggested that the strong decrease in Mg/Ca in C. wuellerstorfi at low temperatures is the result of an enhanced change in carbonate ion concentration per °C in the low temperature range (Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006; Marchitto et al., 2007; Rosenthal et al., 2006). The decrease in ΔCO2− 3 per °C steepens tenfold at temperatures below 3 °C and it is at ~ 3 °C where there is an apparent change in the C. wuellerstorfi Mg/Ca-temperature relationship (Elderfield et al., 2006). Elderfield et al. (2006) used two effect on Mg/Ca in the b3 °C approaches to isolate the ΔCO2− 3 temperature range. One method used a multi-species Cibicidoides equation (Mg/Ca = 0.90e (0.11⁎BWT)) to approximate the true temperature-Mg/Ca relationship and then subtracted the predicted Mg/Ca for the observed temperatures from the measured Mg/Ca of C. wuellerstorfi for each sample. This residual Mg/Ca (Mg/Ca CO3) was attributed to the carbonate ion saturation effect and when plotted versus the ΔCO2− 3 of ambient waters yields the equation: Mg=CaCO3 ¼ 0:00864ΔCO2− 3 −0:15
ð9Þ
Elderfield et al., 2006 In order to test this relationship, we carried out a similar exercise with our C. wuellerstorfi data from b3 °C. Because the Mg/Ca: temperature
529
relationship for C. wuellerstorfi at high temperatures (i.e. Sulu Sea data) is similar to the C. pachyderma equation of Marchitto et al. (2007; Fig. 7), we use this equation (Mg/Ca = 0.116T + 1.20) as an approximation for the first-order response of C. wuellerstorfi Mg/Ca to temperature. We then calculate the difference between our measured Mg/Ca values and the Mg/Ca predicted by this relationship to estimate the Mg/Ca CO3. Plotting Mg/Ca CO3 versus the ΔCO2− 3 at our sample locations yields the following equation: Mg=CaCO3 ¼ 0:0083F0:0024ΔCO2− 3 −0:28 0:028
ð10Þ
This equation has a sensitivity that is identical to that determined by Elderfield et al. (2006; Fig. 8), suggesting a relatively uniform response of Mg/Ca to the carbonate ion saturation of waters b3 °C. Both O. umbonatus and C. mundulus Mg/Ca data for temperatures b3 °C also show a departure (larger decrease in Mg/Ca per °C) from the calibration curves established for these species over a larger temperature range (Figs. 4 and 6, Table 1), further supporting a more observable carbonate ion saturation effect in waters b3 °C. 6. Conclusions We have produced new Mg/Ca measurements on C. wuellerstorfi from core-top samples from five locations in the Atlantic, Pacific, and Indian Oceans, spanning a depth range of 1095–4548m and a bottom water temperature range of 0.8–3.8 °C. A linear regression of the data yields a relationship between the Mg/Ca of C. wuellerstorfi and bottom water temperature of: Mg=Ca ¼ ð0:295F0:0184T Þ þ 0:67F0:04
R2 ¼ 0:90
This is similar to equations derived using previously published results for this species at low temperatures (b4 °C), but is a much larger decrease in Mg/Ca per °C than suggested by either multi-species Cibicidoides equations or single species equations calibrated over larger temperature ranges (Rosenthal et al., 1997; Lear et al., 2002; Martin et al., 2002; Marchitto et al., 2007; Elderfield et al., 2006). This apparent “greater temperature dependence” is most likely due to a combined effect of carbonate ion concentration and temperature on Mg uptake by benthic foraminifera that changes significantly at low (b3 °C) temperatures (Elderfield et al., 2006; Rosenthal et al., 2006). A preliminary quantification of the carbonate ion saturation effect on Mg/Ca ratios of C. wuellerstorfi yields the relationship: Mg=CaCO3 ¼ 0:00834ΔCO2− 3 −0:28
R2 ¼ 0:51
This equation is identical to the relationship proposed recently by Elderfield et al. (2006). This increased slope of the Mg/Catemperature relationship at low temperatures is also observed for O. umbonatus and C. mundulus, with the threshold being at ~ 3 °C. C. mundulus yield higher Mg/Ca ratios than C. wuellerstorfi over the same temperature range, indicating interspecies differences in the uptake of Mg. Acknowledgments We would like to thank T. Marchitto for useful discussions, as well as E. Tappa, M. Luc, and R. McCroan for laboratory assistance. We thank S. Carey from the University of Rhode Island for providing the samples from site TR163-14. Funding for the curation of TR163-14 is provided by NSF grant OCE-0002226. Fig. 8. Mg/CaCO3 vs. ΔCO3 for the C. wuellerstorfi calibration data set from this study (black). Mg/CaCO3 = Mg/Cameasured Cameasured − (0.116 ⁎ T + 1.2), the Mg/Ca-temperature relationship defined by Marchitto et al. (2007) for C. pachyderma. The relationship between Mg/CaCO3 and ΔCO3 for C. wuellerstorfi proposed by Elderfield et al. (2006) (red) is shown for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2008.05.023; Boyle and Keigwin, 1985.
530
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
References Adkins, J.F., Schrag, D.P., 2001. Pore fluid constraints on deep ocean temperature and salinity during the Last Glacial Maximum. Geophys. Res. Lett. 28 (5), 771–774. Boyle, E.A., 1985. Manganese carbonate overgrowths on foraminifera tests. Geochim. Cosmochim. Acta 47, 1815–1819. Boyle, E.A., Keigwin, L.D., 1985. Comparison of Atlantic and Pacific paleochemical records for the last 215,000years: changes in deep ocean circulation and chemical inventories. Earth Planet. Sci. Lett. 76, 135–150. Chappell, J., Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature 324, 137–140. Elderfield, H., Anand, Yu, J., Kiefer, T., Nyland, B., 2006. Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis. Earth Planet. Sci. Lett. 250, 633–649. Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea-level record: influence of glacial melting rates on the Younger Dryas event and deep ocean circulation. Nature 342, 637–642. Labeyrie, L., Duplessy, J.C., Blanc, P.L., 1987. Variations in mode of formation and temperature of oceanic deep waters over the past 125, 000years. Nature 327, 477–482. Lea, D.W., Mashiotta, T.A., Spero, H.J., 1999. Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing. Geochim. Cosmochim. Acta 63 (16), 2369–2379. Lear, C.H., Rosenthal, Y., Slowey, N., 2002. Benthic foraminiferal Mg/Ca paleothermometry: a revised core-top calibration. Geochim. Cosmochim. Acta 66 (19), 3375–3387. Lewis, E., Wallace, D., 1998. Program developed for CO2 system calculations, Rep. ORNL/ CDIAC-105, Carbon Dioxide Inf. Anal. Cent., Oak Ridge Natl. Lab., U.S. Dept. of Energy, Oak Ridge Tenn. (Available at http://cdiac.esd.ornl.gov/oceans/co2rprt.html). Marchitto, T.M., Bryan, S.P., Curry, W.B., McCorkle, D.C., 2007. Mg/Ca temperature calibration for the benthic foraminifer Cibicidoides pachyderma. Paleoceanography 22. Martin, P.A., Lea, D.W., 2002. A simple evaluation of cleaning procedures on fossil benthic foraminiferal Mg/Ca. Geochem. Geophys. Geosyst. 3 (10), 8401.
Martin, P.A., Lea, D.W., Rosenthal, Y., Shackleton, N.J., Sarnthein, M., Papenfuss, T., 2002. Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca. Earth Planet. Sci. Lett. 222, 275–283. McConnell, M.C., Thunell, R.C., 2005. Calibration of the planktonic foraminiferal Mg/Ca paleothermometer: Sediment trap results from the Guaymas Basin, Gulf of California. Paleoceanography 20. Rathburn, A.E., De Deckker, P., 1997. Magnesium and strontium compositions of recent benthic foraminifera from the Coral Sea, Australia and Prydz Bay, Antarctica. Mar. Micropaleontol. 32, 231–248. Rosenthal, Y., Boyle, E.A., Slowey, N., 1997. Temperature control on the incorporation of magnesium, strontium, fluorine, and cadmium into benthic foraminiferal shells from Little Bahamas Bank: Prospects for thermocline paleoceanography. Geochim. Cosmochim. Acta 61 (17), 3633–3643. Rosenthal, Y., Lear, C.H., Oppo, D.W., Linsley, Braddock, 2006. Temperature and carbonate ion effects on Mg/Ca and Sr/Ca ratios in benthic foraminifera: Aragonitic species Hoeglundina elegans. Paleoceanography 21. Russell, A.D., Emerson, S., Nelson, B.K., Erez, J., Lea, D.W., 1994. Uranium in foraminiferal calcite as a recorder of seawater uranium concentrations. Geochim. Cosmochim. Acta 58, 671–681. Schrag, D.P., Hampt, G., Murray, D.W., 1996. Pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean. Science 272, 1930–1932. Schrag, D.P., Adkins, J.F., McIntyre, K., Alexander, J.L., Hodell, D.A., Charles, C.D., McManus, J.F., 2002. The oxygen isotopic composition of seawater during the Last Glacial Maximum. Quat. Sci. Rev. 21, 331–342. Shackleton, N.J., 1967. Oxygen isotope analysis and Pleistocene temperature reassessed. Nature 215, 15–17. Shackleton, N.J., 1987. Oxygen isotopes, ice volume, and sea level. Quat. Sci. Rev. 6, 183–190. Skinner, L.C., Shackleton, N.J., Elderfield, H., 2003. Millennial-scale variability of deep water temperature and d18Odw indicating deep-water source variations in the Northeast Atlantic, 0–34cal ka BP. Geochem. Geophys. Geosyst. 4, 1–17. Van Morkhoven, F.P.C.M., Berggren, W.A., Edwards, A.S., 1986. Cenozoic Cosmopolitan Deep-Water Benthic Foraminifera. Bull. Centres Rech. Explor. -Prod. Elf-Aquitaine, Mem. 11, 48–50 Pau.
Contents lists available at ScienceDirect
Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
The Mg/Ca-temperature relationship of benthic foraminiferal calcite: New core-top calibrations in the b 4 °C temperature range Stephanie L. Healey a,⁎, Robert C. Thunell a, Bruce H. Corliss b a b
Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA Department of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina 27706, USA
A R T I C L E
I N F O
Article history: Received 21 April 2008 Accepted 15 May 2008 Available online 7 July 2008 Editor: M.L. Delaney Keywords: paleoceanography Mg/Ca paleothermometry benthic foraminifera carbonate ion
A B S T R A C T Calcite magnesium/calcium ratios were determined for four species of benthic foraminifera (Cibicidoides wuellerstorfi, Cibicidoides mundulus, Oridorsalis umbonatus and Pyrgo murrhina) from core-top samples collected from the Atlantic, Indian, and Pacific Oceans where modern bottom water temperatures are less than 4 °C. Mg/Ca of all four species increase with increasing temperature, and the Mg/Ca:temperature relationship does not appear to be necessarily exponential. Mg/Ca of C. mundulus is consistently higher than C. wuellerstorfi, implying differences in the uptake of Mg by different species within this genus. The observed Mg/Ca for C. wuellerstorfi at temperatures less than 3 °C is lower than those predicted from published Mg/Ca: temperature relationships derived for Cibicidoides spp. at higher temperatures. Previous studies have attributed this change in the Mg/Ca-temperature relationship of C. wuellerstorfi to the effect of decreased carbonate ion saturation (Δ[CO 2− 3 ]) on Mg/Ca at low temperatures. A relationship between the Δ[CO2− 3 ] and Mg/Ca of 0.0083 ± 0.002 (mmol/mol)/(μmol/kg) has been established for C. wuellerstorfi from this study. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ocean circulation and temperature, along with continental ice volume are important elements of the global climate system and reconstructing past changes in these parameters are basic objectives of paleoclimatology. The change in δ18O composition of seawater due to the melting of continental ice sheets since the last glacial maximum (LGM) has been estimated using fossil coral reef terraces (Fairbanks, 1989) and sediment pore-fluid chemistry (Adkins and Schrag, 2001; Schrag et al., 2002, 1996). Although δ18O changes in benthic foraminiferal calcite in deep-sea cores have been used as a firstorder approximation of Late Pleistocene glacial–interglacial sea-level changes (e.g. Shackleton, 1967), most of these records also contain glacial–interglacial variability of a higher amplitude than what is expected based on independent sea-level estimates (e.g. Labeyrie et al., 1987; Martin et al., 2002; Skinner et al., 2003), suggesting that bottom water temperatures also vary significantly on these timescales. For example, it has been estimated that bottom waters were about 1.5–3 °C colder in the Pacific (Chappell and Shackleton, 1986; Shackleton, 1987; Martin et al., 2002) and 3–4 °C colder in the Atlantic (Schrag et al., 2002; Adkins and Schrag, 2001; Martin et al., 2002) during glacial times than today.
⁎ Corresponding author. E-mail address: [email protected] (S.L. Healey). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.05.023
The oxygen isotope composition (δ18O) of benthic foraminiferal calcite is a function of the temperature and the δ18O composition of sea water at the time of calcification. Likewise, the δ18O composition of sea water is a function of global ice volume and local variations in salinity. Thus, benthic foraminiferal δ18O measurements alone cannot provide quantitative estimates of past changes in any of these parameters. However, with the development of Mg/Ca paleothermometry, it is now possible to use benthic foraminiferal Mg/Ca to directly estimate bottom water temperatures (Rosenthal et al., 1997; Rathburn and De Deckker, 1997; Martin et al., 2002; Lear et al., 2002). Thus, paired Mg/Ca and δ18O measurements of benthic foraminifera offer the potential to identify how much of the total δ18O change of benthic foraminiferal calcite is due to changes in the isotopic composition of seawater. The majority of paleoclimate records utilizing benthic foraminiferal δ18O and/or Mg/Ca are based on sediment cores retrieved from water depths greater than 2000m. At present, over 95% of the global ocean deeper than 2000m has bottom water temperatures that are b5 °C (Fig. 1), and it is reasonable to assume that in most regions temperatures below this depth were colder during glacial periods. Thus, it is important that the relationship between temperature and the Mg/Ca of benthic foraminiferal calcite be well constrained at the low end of the temperature spectrum. Existing benthic foraminiferal Mg/Ca-temperature calibrations are based on data sets spanning bottom water temperatures of 0–18 °C and combine different species of benthic foraminifera, including species that live primarily in waters warmer than
524
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
Fig. 1. World map showing the average annual bottom water temperature at 2000 meters depth (World Ocean Atlas, 1998). The locations of all core-top samples referenced and/or presented in this study are also shown as red symbols: this study (open squares), Lear et al., 2002 (solid squares); Martin et al., 2002 (open circles); Russell et al., 1994 and Rosenthal et al., 1997, after Martin et al., 2002 (solid circles), Marchitto et al., 2007 (open diamond).
5 °C (Lear et al., 2002; Martin et al., 2002; Rosenthal et al., 1997; Elderfield et al., 2006). Martin et al. (2002) were the first to recognize that the Mg/Ca-temperature sensitivity in benthic foraminifera is higher
at cold temperatures (b3 °C) relative to that at warmer temperatures. Similarly, work on planktonic foraminifera (McConnell and Thunell, 2005) has shown that trace metal paleotemperature equations can vary
2 2− 2− Fig. 2. Calculated carbonate ion concentration (black), and ΔCO2− 3 (blue) vs. depth, where ΔCO3 = [CO3 ] sample − [CO3 ]calcite saturation, at the five GEOSECS stations nearest to the locations of core-top samples used in this study. Dashed lines represent the depths of core-top samples from each location. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
based on the temperature range used in the calibration. Most recently, Elderfield et al. (2006) have argued that the greater response of benthic foraminiferal Mg/Ca to temperature in the b3 °C temperature range is attributable to the effect of low carbonate ion saturation (ΔCO3) values on the uptake of Mg at low temperatures (Elderfield et al., 2006). The goal of this study is to examine the Mg/Ca-bottom water temperature relationship of individual benthic foraminifera species in the b4 °C temperature range, and further assess the potential of this proxy as a paleothermometer. The Mg/Ca of four different benthic foraminiferal species were analyzed from the Atlantic, Pacific, and Indian Oceans coretop samples from sites with bottom waters b4 °C. These results were compared to similar data from previous studies in the same temperature range. 2. Methods Core-top samples used for Mg/Ca-determinations in this study were obtained from five locations in the Southeast Indian Ocean, South China Sea, North Atlantic, Equatorial Atlantic, and the South Atlantic (Fig. 1, Table A1 in Appendix). The core-top samples cover a range of water depths from 1095–4548m and a bottom water temperature (BWT) range of 0.8–3.8 °C. Carbonate ion concentration has been estimated for each core location using the nearest GEOSECS station (Lewis and Wallace, 1998), and ΔCO3 has been calculated as: 2− ΔCO3 = [CO2− 3 ] sample location − [CO3 ] calcite saturation (Fig. 2). Sediment samples were disaggregated in buffered water and wetsieved through a 63μm mesh. Samples were dry sieved through a 250μm mesh to obtain typically between 5 and 30 benthic foraminiferal specimens. Species used in this study include Cibicidoides wuellerstorfi, Cibicidoides mundulus, Oridorsalis umbonatus, and
525
Pyrgo murrhina. The foraminiferal shells were cleaned following the rigorous procedure outlined in Martin and Lea (2002), without the alkaline chelation step. Cleaned samples were dissolved in 300–500μL 0.5N HNO3 and Mg/Ca ratios were measured using a Jobin Yvon Ultima C inductively coupled plasma atomic emission spectrometer (ICPAES). Duplicate analyses of a standard solution yielded an analytical precision of 0.4%. Duplicate analyses of samples resulted in an average precision of ~ 1.4%. 3. Results: benthic foraminiferal Mg/Ca-temperature calibrations 3.1. C. wuellerstorfi The new Mg/Ca results for C. wuellerstorfi from this study are shown in Fig. 3A. Exponential and linear regressions of this data yield the following equations: Mg=Ca ¼ 0:781F0:023 expð0:23F0:0134BWTÞ Mg=Ca ¼ ð0:295F0:0184BWTÞ þ 0:67F0:04
R2 ¼ 0:91 R2 ¼ 0:90
ð1Þ ð2Þ
As observed in previous studies (Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006), our data for the temperature range of b4 °C yield Mg/Ca ratios which exhibit a stronger dependence on temperature than the relationship established over a larger temperature range (0–18 °C; Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006; Fig. 3A). For this species, a linear fit describes the Mg/Catemperature relationship as well as an exponential equation (Fig. 3A). The standard errors of the linear and exponential regressions derived from this new data are ± 0.20 and ± 0.18 °C respectively.
Fig. 3. Mg/Ca ratios of A) C. wuellerstorfi, B) C. mundulus, C) O. umbonatus, and D) P. murrhina from core-top samples vs. bottom water temperature from this study. Both linear and exponential curves have been fit to each data set. Dashed lines bracket the 95% confidence interval about each regression.
526
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
set and the small temperature range (1–3 °C), further analyses will be necessary to adequately constrain the Mg/Ca-temperature relationship for this species. 4. Results: comparison with previous benthic foraminiferal Mg/Ca-temperature calibrations In this section, we briefly review previous studies of Lear et al. (2002), and Martin et al. (2002), which have examined the relationship between temperature and Mg/Ca in various species of benthic foraminifera (Figs. 5 and 6). Because the objective of our study is to examine the nature of this relationship at low temperatures, we specifically look at the data from these previous studies for samples that have present day bottom water temperatures of b5 °C. For each of these previous studies, we generate new Mg/Ca-temperature equations using this low temperature sample subset. The locations of the core-top samples used in these studies are shown in Fig.1. A detailed description of the sample selection for these data subsets from previous studies is available in Appendix A. Fig. 4. Comparison of the linear C. mundulus Mg/Ca-temperature equation from this study (red), Elderfield et al. (2006) (black) and the linear C. wuellerstorfi regression and data from this study (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Mg/Ca-temperature calibrations Exponential equations
3.2. C. mundulus
Mg/Ca = BB ⁎ e(A ⁎ BWT)
Mg/Ca of C. mundulus was measured in 21 samples and shows the expected increase with increasing temperature (Fig. 3B). The exponential and linear equations determined from this data are as follows: Mg=Ca ¼ 1:00F0:13 expð0:277F0:0594BWTÞ
R2 ¼ 0:54 R2 ¼ 0:54
Mg=Ca ¼ ð0:496F0:104BWTÞ þ 0:798F0:227
ð3Þ
Study
Published equations Martin et al. (2002) Lear et al. (2002)
ð4Þ
The exponential fit indicates an increase in Mg/Ca of ~ 27% per °C (Fig. 3B), while the linear fit to the same data suggests a dependence of the Mg/Ca ratio on temperature of ~ 0.496mmol/mol per °C (Fig. 3B). In both cases, the C. mundulus Mg/Ca change per °C is larger than the Mg/Ca change in C. wuellerstorfi from this study (Figs. 3A and B, 4, Table 1). However, the slopes of the exponential equations for C. wuellerstorfi and C. mundulus are not significantly different (Figs. 3A and B, 4, Table 1). 3.3. Other benthic foraminiferal species
Lear et al. (2002) Elderfield et al. (2006) Individual species only Lear et al. (2002) Martin et al. (2002) Russell et al. (1994) and Rosenthal (after Martin et al. 2002) This study This study Lear et al. (2002)
We also measured Mg/Ca ratios in the benthic foraminifera O. umbonatus and P. murrhina (Figs. 3C and D). The Mg/Ca:temperature relationship for O. umbonatus from this study is equally well described by the following exponential and linear regressions (Fig. 3C): Mg=Ca ¼ 0:988F0:08 expð0:252F0:0364BWTÞ
R2 ¼ 0:69
Mg=Ca ¼ ð0:449F0:0664BWTÞ þ 0:773F0:151
R2 ¼ 0:72
ð5Þ ð6Þ
P. murrhina exhibits much higher Mg/Ca ratios than any other species analyzed (Fig. 3D), which is consistent with previous results for this species (Boyle, 1985). The exponential and linear equations derived for this species are as follows: Mg=Ca ¼ 3:095F0:608 expð0:569F0:0824BWTÞ Mg=Ca ¼ ð6:312F1:1694BWTÞ−2:038F2:78
R2 ¼ 0:79 R2 ¼ 0:69
ð7Þ ð8Þ
Like other species of benthic foraminifera, the Mg/Ca ratio of P. murrhina increases with increasing temperature, indicating its potential as a paleothermometer. However, given the limited sample
This study
R2
Temperature range (°C)
0.11
0.95
− 1–18
0.867
0.109
0.94
0.8–18
1.008 0.9
0.114 0.11
0.4 0.99
0.8–9.9 − 1–18
C. wuellerstorfi C. wuellerstorfi C. wuellerstorfi
0.652 0.64 0.953
0.28 0.255 0.206
0.58 0.81 0.49
1.7–4.4 1.8–3 0.4–2.8
C. wuellerstorfi C. mundulus O. umbonatus O. umbonatus (b 5 °C data only) P. murrhina
0.781 1 0.99 0.79
0.23 0.277 0.252 0.252
0.91 0.54 0.67 0.72
0.95–3.8 0.9–2.9 0.85–3.8 0.84–4.35
3.1
0.569
0.79
0.9–2.9
C. pachyderma C. mundulus
1.2 0.88
0.116 0.11
0.73 0.96
5.8–18.6 1.2–11.8
C. wuellerstorfi C. wuellerstorfi C. wuellerstorfi
0.378 0.448 0.848
0.397 0.309 0.307
0.63 0.81 0.45
1.7–4.4 1.8–3 0.4–2.8
C. wuellerstorfi C. mundulus O. umbonatus O. umbonatus (b 5 °oC data only) P. murrhina
0.67 0.798 0.77 0.57
0.295 0.496 0.449 0.41
0.9 0.54 0.67 0.65
0.95–3.8 0.9–2.9 0.85–3.8 0.84–4.35
0.69
0.9–2.9
Species
B
C. pachyderma, C. wuellerstorfi C. pachyderma, C. wuellerstorfi, C. compressus, other C. spp. O. umbonatus Cibicidoides spp.
0.85
A
Linear equations Mg/Ca = (A ⁎ BWT) + B Published equations Marchitto et al. (2007) Elderfield et al. (2006) Individual species only Lear et al. (2002) Martin et al. (2002) Russell et al. (1994) and Rosenthal (after Martin et al., 2002) This study This study This study Lear et al. (2002) This study
6.31
−2.04
Coefficients, R2, and temperature range of all Mg/Ca-temperature calibrations from this study and other published Mg/Ca data from core-top samples.
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
527
Fig. 5. Mg/Ca ratios of C. wuellerstorfi from core-top samples vs. bottom water temperature for four different data sets: A) This study, B) Lear et al. (2002), C) Martin et al. (2002), D) Russell et al. (1994) and Rosenthal (after Martin et al., 2002). Both linear and exponential curves have been fit to each data set. Dashed lines bracket the 95% confidence interval about each regression. E) A comparison of the exponential equations for each of the data sets. F) A comparison of the linear equations for each of the data sets. For E and F, the Elderfield et al. (2006) equation for Cibicidoides species the Marchitto et al. (2007) equation for C. pachyderma are shown for comparison.
4.1. Comparison of C. wuellerstorfi calibrations As expected, the Mg/Ca ratios of C. wuellerstorfi increase with increasing temperature for each of the four previously published data sets (Fig. 5). Each sample set has been fit with an exponential curve, the expected relationship between temperature and Mg/Ca based on equilibrium thermodynamics and the van't Hoff equation (Lea et al., 1999). In addition, a least squares fit linear regression has been determined for each data set. In all four cases, the linear fit describes the relationship between the Mg/Ca ratio of the benthic foraminiferal calcite and temperature nearly as well, and in some cases better than the exponential fit (Fig. 5; Table 1). The y-intercepts for the various equations differ by 0.5 to 1.0mmol/mol (Fig. 5E and F; Table 1). This may be due to analytical differences (e.g. ICP-MS vs. ICP-AES) and/or differences in the cleaning protocols. However, the slopes of the four exponential equations suggest a very similar response of the Mg/Ca of C. wuellerstorfi to temperature within the range of 1–5 °C. The exponential fits suggest an increase of ~ 20–28% in the Mg/Ca ratio of C. wuellerstorfi per °C (Table 1). This rate of change is considerably higher than the previously published values of an ~ 11% increase in Mg/Ca per °C for combined Cibicidoides species over a temperature range of − 1 to 18 °C (Lear et al., 2002; Martin et al., 2002, Elderfield et al., 2006; Table 1) and a ~ 4.2% increase in Mg/Ca per °C for C. pachyderma over the temperature range of 5.8 to 18.6 °C (Marchitto et al., 2007; Fig. 5E and F, and Table 1).
The linear equations suggest an even higher dependence of the Mg/Ca ratio on temperature, ranging from ~ 0.295–0.397 mmol/mol per °C. 4.2. Comparison of O. umbonatus calibrations We compare our core-top Mg/Ca measurements of O. umbonatus (Fig. 3C) to previously published data for this species from Lear et al. (2002) for the b5 °C temperature range, as described in Appendix A (Fig. 6A and B; Table 1). The linear relationships suggest an increase in Mg/Ca of 0.415 and 0.449mmol/mol per °C for our data and the data from Lear et al. (2002) respectively, with these values being indistinguishable from each other at the 95% confidence interval. The exponential equations indicate a 25.2% increase in Mg/Ca per °C for both data sets (Figs. 6C and D; Table 1). However, this change in Mg/Ca per °C estimated from these two equations is considerably larger than the ~ 11.4% change per °C reported by Lear et al. (2002) for the temperature range of 0–10 °C (Fig. 6C; Table 1). 5. Discussion: comparison of Cibicidoides spp. calibrations Recent core-top studies have shown that the Mg/Ca-temperature relationship for benthic foraminiferal calcite is not necessarily exponential (Marchitto et al., 2007; Elderfield et al., 2006) and that a linear regression describes most data sets as well as the exponential
528
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
Fig. 6. A) Exponential Mg/Ca-temperature equation for O. umbonatus using the Lear et al. (2002) data less than 5 °C. B) Linear Mg/Ca-temperature equation for O. umbonatus using the Lear et al. (2002) data less than 5 °C. Dashed lines bracket the 95% confidence interval about each regression. C) and D) Comparison of the exponential and linear equations for O. umbonatus b5 °C data from this study and the Lear et al. (2002) study compared to the published O. umbonatus equation of Lear et al. (2002).
relationship predicted from thermodynamics. Three previously published C. wuellerstorfi data sets (add references for the three data sets) for samples with bottom water temperatures of b5 °C support this conclusion in that we find no significant difference between a linear and exponential fit for any of the data considered in this study (Fig. 5; Table 1). In all cases, the exponential equations based on samples with bottom water temperatures ">b5 °C predict an increase in Mg/Ca of C. wuellerstorfi of N20% per °C (Fig. 5; Table 1). This temperaturedependent change is much larger than that for previously published equations based on multiple species of Cibicidoides for significantly larger temperature ranges (− 1to 18 °C), (Lear et al., 2002; Martin et al., 2002) and single Cibicidoides species calibrations at higher temperature ranges (Rosenthal et al., 1997; Marchitto et al., 2007). These differences may be due to species specific “vital” effects in the uptake of Mg, even for species in the same genus. To investigate this possibility, we also measured the Mg/Ca of another Cibicidoides species, C. mundulus, from core-top samples and compare these results to our C. wuellerstorfi data. Although Mg uptake by C. mundulus does exhibit slightly higher temperature dependence than for C. wuellerstorfi, it is within the range of exponential slopes exhibited for all four of the C. wuellerstorfi equations (Table 1). The measured Mg/Ca values for C. mundulus are consistently higher than those measured for C. wuellerstorfi for a given temperature (Fig. 4), which is consistent with previous observations (Elderfield et al., 2006). The published Cibicidoides equations based on multiple species (Lear et al., 2002; Martin et al., 2002; Elderfield et al., 2006) are dominated at high temperatures by data for C. pachyderma. This species lives primarily in waters shallower than 2500m and at temperatures above 5 °C (Marchitto et al., 2007). These temperatures are warmer than those typically associated with C. wuellerstorfi (Van Morkhoven et al., 1986). Furthermore, a recently published Mg/Ca-temperature equation for C. pachyderma differs from that of the combined Cibicidoides species data
sets, with the increase in Mg/Ca of C. pachyderma being ~ 4.2% per °C (Marchitto et al., 2007), in contrast to ~ 10% increase in Mg/Ca per °C for the multi-species equations (Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006). In order to further explore differences in the Mg/Ca: temperature relationship for different species of Cibicidoides we compare the linear equation for C. wuellerstorfi from this study
Fig. 7. Comparison of the linear C. wuellerstorfi equation from this study (red) vs. the published linear equation of Marchitto et al. (2007) for C. pachyderma (black), where the solid black line represents the actual calibration temperature range and the dashed line represents an extrapolation of the linear relationship to low temperatures. Blue data points represent C. wuellerstorfi Mg/Ca data from the Sulu Sea, which were not included in the C. wuellerstorfi calibration data set from this study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
with the linear C. pachyderma equation based on samples from the Florida Straights (Marchitto et al., 2007; Fig. 7). The equations for these two species are significantly different, with C. wuellerstorfi exhibiting a much greater change in Mg/Ca per °C than C. pachyderma. However, the C. wuellerstorfi data points above 3 °C could just as reasonably be described by the C. pachyderma equation as the b4 °C equation for C. wuellerstorfi (Fig. 7). To examine this issue further, we measured Mg/Ca on C. wuellerstorfi from five coretop samples from the Sulu Sea with bottom water temperatures of 10.5–11 °C (Fig. 7). These samples are unusual in that C. wuellerstorfi rarely inhabit waters above 5 °C. The Mg/Ca values for these five samples (2.25–2.75mmol/mol) fall close to or on the line describing the C. pachyderma Mg/Ca-temperature relationship and do not exhibit the higher Mg/Ca values that would be predicted (3.77– 3.92mmol/mol) by our low temperature C. wuellerstorfi equation. This suggests that the different C. wuellerstorfi and C. pachyderma Mg/Ca-temperature relationships are not species specific, but due to some other factor. This does not imply that the two species exhibit identical responses to temperature, but rather that the large difference observed in the two Mg/Ca-temperature relationships cannot be attributed simply to interspecies differences in Mg uptake. It has been suggested that the strong decrease in Mg/Ca in C. wuellerstorfi at low temperatures is the result of an enhanced change in carbonate ion concentration per °C in the low temperature range (Martin et al., 2002; Lear et al., 2002; Elderfield et al., 2006; Marchitto et al., 2007; Rosenthal et al., 2006). The decrease in ΔCO2− 3 per °C steepens tenfold at temperatures below 3 °C and it is at ~ 3 °C where there is an apparent change in the C. wuellerstorfi Mg/Ca-temperature relationship (Elderfield et al., 2006). Elderfield et al. (2006) used two effect on Mg/Ca in the b3 °C approaches to isolate the ΔCO2− 3 temperature range. One method used a multi-species Cibicidoides equation (Mg/Ca = 0.90e (0.11⁎BWT)) to approximate the true temperature-Mg/Ca relationship and then subtracted the predicted Mg/Ca for the observed temperatures from the measured Mg/Ca of C. wuellerstorfi for each sample. This residual Mg/Ca (Mg/Ca CO3) was attributed to the carbonate ion saturation effect and when plotted versus the ΔCO2− 3 of ambient waters yields the equation: Mg=CaCO3 ¼ 0:00864ΔCO2− 3 −0:15
ð9Þ
Elderfield et al., 2006 In order to test this relationship, we carried out a similar exercise with our C. wuellerstorfi data from b3 °C. Because the Mg/Ca: temperature
529
relationship for C. wuellerstorfi at high temperatures (i.e. Sulu Sea data) is similar to the C. pachyderma equation of Marchitto et al. (2007; Fig. 7), we use this equation (Mg/Ca = 0.116T + 1.20) as an approximation for the first-order response of C. wuellerstorfi Mg/Ca to temperature. We then calculate the difference between our measured Mg/Ca values and the Mg/Ca predicted by this relationship to estimate the Mg/Ca CO3. Plotting Mg/Ca CO3 versus the ΔCO2− 3 at our sample locations yields the following equation: Mg=CaCO3 ¼ 0:0083F0:0024ΔCO2− 3 −0:28 0:028
ð10Þ
This equation has a sensitivity that is identical to that determined by Elderfield et al. (2006; Fig. 8), suggesting a relatively uniform response of Mg/Ca to the carbonate ion saturation of waters b3 °C. Both O. umbonatus and C. mundulus Mg/Ca data for temperatures b3 °C also show a departure (larger decrease in Mg/Ca per °C) from the calibration curves established for these species over a larger temperature range (Figs. 4 and 6, Table 1), further supporting a more observable carbonate ion saturation effect in waters b3 °C. 6. Conclusions We have produced new Mg/Ca measurements on C. wuellerstorfi from core-top samples from five locations in the Atlantic, Pacific, and Indian Oceans, spanning a depth range of 1095–4548m and a bottom water temperature range of 0.8–3.8 °C. A linear regression of the data yields a relationship between the Mg/Ca of C. wuellerstorfi and bottom water temperature of: Mg=Ca ¼ ð0:295F0:0184T Þ þ 0:67F0:04
R2 ¼ 0:90
This is similar to equations derived using previously published results for this species at low temperatures (b4 °C), but is a much larger decrease in Mg/Ca per °C than suggested by either multi-species Cibicidoides equations or single species equations calibrated over larger temperature ranges (Rosenthal et al., 1997; Lear et al., 2002; Martin et al., 2002; Marchitto et al., 2007; Elderfield et al., 2006). This apparent “greater temperature dependence” is most likely due to a combined effect of carbonate ion concentration and temperature on Mg uptake by benthic foraminifera that changes significantly at low (b3 °C) temperatures (Elderfield et al., 2006; Rosenthal et al., 2006). A preliminary quantification of the carbonate ion saturation effect on Mg/Ca ratios of C. wuellerstorfi yields the relationship: Mg=CaCO3 ¼ 0:00834ΔCO2− 3 −0:28
R2 ¼ 0:51
This equation is identical to the relationship proposed recently by Elderfield et al. (2006). This increased slope of the Mg/Catemperature relationship at low temperatures is also observed for O. umbonatus and C. mundulus, with the threshold being at ~ 3 °C. C. mundulus yield higher Mg/Ca ratios than C. wuellerstorfi over the same temperature range, indicating interspecies differences in the uptake of Mg. Acknowledgments We would like to thank T. Marchitto for useful discussions, as well as E. Tappa, M. Luc, and R. McCroan for laboratory assistance. We thank S. Carey from the University of Rhode Island for providing the samples from site TR163-14. Funding for the curation of TR163-14 is provided by NSF grant OCE-0002226. Fig. 8. Mg/CaCO3 vs. ΔCO3 for the C. wuellerstorfi calibration data set from this study (black). Mg/CaCO3 = Mg/Cameasured Cameasured − (0.116 ⁎ T + 1.2), the Mg/Ca-temperature relationship defined by Marchitto et al. (2007) for C. pachyderma. The relationship between Mg/CaCO3 and ΔCO3 for C. wuellerstorfi proposed by Elderfield et al. (2006) (red) is shown for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2008.05.023; Boyle and Keigwin, 1985.
530
S.L. Healey et al. / Earth and Planetary Science Letters 272 (2008) 523–530
References Adkins, J.F., Schrag, D.P., 2001. Pore fluid constraints on deep ocean temperature and salinity during the Last Glacial Maximum. Geophys. Res. Lett. 28 (5), 771–774. Boyle, E.A., 1985. Manganese carbonate overgrowths on foraminifera tests. Geochim. Cosmochim. Acta 47, 1815–1819. Boyle, E.A., Keigwin, L.D., 1985. Comparison of Atlantic and Pacific paleochemical records for the last 215,000years: changes in deep ocean circulation and chemical inventories. Earth Planet. Sci. Lett. 76, 135–150. Chappell, J., Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature 324, 137–140. Elderfield, H., Anand, Yu, J., Kiefer, T., Nyland, B., 2006. Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis. Earth Planet. Sci. Lett. 250, 633–649. Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea-level record: influence of glacial melting rates on the Younger Dryas event and deep ocean circulation. Nature 342, 637–642. Labeyrie, L., Duplessy, J.C., Blanc, P.L., 1987. Variations in mode of formation and temperature of oceanic deep waters over the past 125, 000years. Nature 327, 477–482. Lea, D.W., Mashiotta, T.A., Spero, H.J., 1999. Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing. Geochim. Cosmochim. Acta 63 (16), 2369–2379. Lear, C.H., Rosenthal, Y., Slowey, N., 2002. Benthic foraminiferal Mg/Ca paleothermometry: a revised core-top calibration. Geochim. Cosmochim. Acta 66 (19), 3375–3387. Lewis, E., Wallace, D., 1998. Program developed for CO2 system calculations, Rep. ORNL/ CDIAC-105, Carbon Dioxide Inf. Anal. Cent., Oak Ridge Natl. Lab., U.S. Dept. of Energy, Oak Ridge Tenn. (Available at http://cdiac.esd.ornl.gov/oceans/co2rprt.html). Marchitto, T.M., Bryan, S.P., Curry, W.B., McCorkle, D.C., 2007. Mg/Ca temperature calibration for the benthic foraminifer Cibicidoides pachyderma. Paleoceanography 22. Martin, P.A., Lea, D.W., 2002. A simple evaluation of cleaning procedures on fossil benthic foraminiferal Mg/Ca. Geochem. Geophys. Geosyst. 3 (10), 8401.
Martin, P.A., Lea, D.W., Rosenthal, Y., Shackleton, N.J., Sarnthein, M., Papenfuss, T., 2002. Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca. Earth Planet. Sci. Lett. 222, 275–283. McConnell, M.C., Thunell, R.C., 2005. Calibration of the planktonic foraminiferal Mg/Ca paleothermometer: Sediment trap results from the Guaymas Basin, Gulf of California. Paleoceanography 20. Rathburn, A.E., De Deckker, P., 1997. Magnesium and strontium compositions of recent benthic foraminifera from the Coral Sea, Australia and Prydz Bay, Antarctica. Mar. Micropaleontol. 32, 231–248. Rosenthal, Y., Boyle, E.A., Slowey, N., 1997. Temperature control on the incorporation of magnesium, strontium, fluorine, and cadmium into benthic foraminiferal shells from Little Bahamas Bank: Prospects for thermocline paleoceanography. Geochim. Cosmochim. Acta 61 (17), 3633–3643. Rosenthal, Y., Lear, C.H., Oppo, D.W., Linsley, Braddock, 2006. Temperature and carbonate ion effects on Mg/Ca and Sr/Ca ratios in benthic foraminifera: Aragonitic species Hoeglundina elegans. Paleoceanography 21. Russell, A.D., Emerson, S., Nelson, B.K., Erez, J., Lea, D.W., 1994. Uranium in foraminiferal calcite as a recorder of seawater uranium concentrations. Geochim. Cosmochim. Acta 58, 671–681. Schrag, D.P., Hampt, G., Murray, D.W., 1996. Pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean. Science 272, 1930–1932. Schrag, D.P., Adkins, J.F., McIntyre, K., Alexander, J.L., Hodell, D.A., Charles, C.D., McManus, J.F., 2002. The oxygen isotopic composition of seawater during the Last Glacial Maximum. Quat. Sci. Rev. 21, 331–342. Shackleton, N.J., 1967. Oxygen isotope analysis and Pleistocene temperature reassessed. Nature 215, 15–17. Shackleton, N.J., 1987. Oxygen isotopes, ice volume, and sea level. Quat. Sci. Rev. 6, 183–190. Skinner, L.C., Shackleton, N.J., Elderfield, H., 2003. Millennial-scale variability of deep water temperature and d18Odw indicating deep-water source variations in the Northeast Atlantic, 0–34cal ka BP. Geochem. Geophys. Geosyst. 4, 1–17. Van Morkhoven, F.P.C.M., Berggren, W.A., Edwards, A.S., 1986. Cenozoic Cosmopolitan Deep-Water Benthic Foraminifera. Bull. Centres Rech. Explor. -Prod. Elf-Aquitaine, Mem. 11, 48–50 Pau.