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Ca-paleothermometry: a revised core-top calibration
Geochimica et Cosmochimica Acta, Vol. 66, No. 19, pp. 3375–3387, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 ⫹ .00
Pergamon
PII S0016-7037(02)00941-9
Benthic foraminiferal Mg/Ca-paleothermometry: A revised core-top calibration CAROLINE H. LEAR,1,* YAIR ROSENTHAL,1 and NIALL SLOWEY2 1
Institute for Marine and Coastal Sciences, and Department of Geology, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901, USA 2 Department of Oceanography, Texas A&M University, College Station, TX 77843, USA (Received October 15, 2001; accepted in revised form April 24, 2002)
Abstract—Core-top samples from different ocean basins have been analyzed to refine our current understanding of the sensitivity of benthic foraminiferal calcite magnesium/calcium (Mg/Ca) to bottom water temperatures (BWT). Benthic foraminifera collected from Hawaii, Little Bahama Bank, Sea of Okhotsk, Gulf of California, NE Atlantic, Ceara Rise, Sierra Leone Rise, the Ontong Java Plateau, and the Southern Ocean covering a temperature range of 0.8 to 18°C were used to revise the Cibicidoides Mg/Ca-temperature calibration. The Mg/Ca–BWT relationship of three common Cibicidoides species is described by an exponential equation: Mg/Ca ⫽ 0.867 ⫾ 0.049 exp (0.109 ⫾ 0.007 ⫻ BWT) (stated errors are 95% CI). The temperature sensitivity is very similar to a previously published calibration. However, the revised calibration has a significantly different preexponential constant, resulting in different predicted absolute temperatures. We attribute this difference in the preexponential constant to an analytical issue of accuracy. Some genera, notably Uvigerina, show apparently lower temperature sensitivity than others, suggesting that the use of constant offsets to account for vital effects in Mg/Ca may not be appropriate. Downcore Mg/Ca reproducibility, as determined on replicate foraminiferal samples, is typically better than 0.1 mmol mol⫺1 (2 S.E.). Thus, considering the errors associated with the Cibicidoides calibration and the downcore reproducibility, BWT may be estimated to within ⫾1°C. Application of the revised core-top Mg/Ca–BWT data to Cenozoic foraminiferal Mg/Ca suggests that seawater Mg/Ca was not more than 35% lower than today in the ice-free ocean at 50 Ma. Copyright © 2002 Elsevier Science Ltd based on a study of surface sediments intersecting the subtropical thermocline has been constructed (Rosenthal et al., 1997). The Rosenthal et al. (1997) calibration was based on a 300 to 1600 m depth transect from the Little Bahama Bank, covering a temperature range of 4 to 18°C. Support for calcite Mg/Ca as a paleotemperature proxy is found in laboratory experiments (Katz, 1973; Burton and Walter, 1991; Hartley and Mucci, 1996) and empirical and culture studies of marine biogenic calcites such as ostracodes (Dwyer et al., 1995), corals (Mitsuguchi et al., 1996), and foraminifera (Nu¨rnberg et al., 1996; Lea et al., 1999; Toyofuku et al., 2000). The temperature dependence of foraminiferal Mg/Ca probably involves a temperature effect on both the inorganic distribution coefficient, and also the physiologic processes that affect the uptake of Mg into the calcite test (Rosenthal et al., 1997). The published calibration has been used in both Quaternary and Cenozoic studies of climate change (Lear et al., 2000; Billups and Schrag, in press; Martin et al., in press), but is in need of refinement for several reasons. (1) The current calibration contains no Mg/Ca data for BWT less than 4°C, which reflects most of the modern deep ocean. (2) The calibration is based on only one benthic foraminiferal species from one locality. (3) There is significant scatter at the high temperature end of the published calibration, and it is not clear whether this is expected in all foraminifera living at these temperatures, or if it is caused by a process specific to the Little Bahama Bank. Reducing this scatter would improve the accuracy of temperature estimates made using the calibration. (4) The foraminifera analyzed for the published calibration were not prepared using the rigorous cleaning technique now commonly used by most laboratories (Boyle and Keigwin, 1985/86). (5) Subsequent analyses of core-top benthic foraminiferal Mg/Ca are not in agreement with the
1. INTRODUCTION
Evaluation of past climate change is dependent on proxy records for physical and chemical conditions of Earth’s past. Many different proxies for marine and continental temperatures exist, including approaches as diverse as coral strontium/calcium thermometry, groundwater noble gas measurements, and leaf margin analysis (Wolfe, 1979; Beck et al., 1992; Stute et al., 1995). No single proxy used in isolation is ideal; this observation is exemplified by the uncertainties associated with perhaps the most reliable and established quantitative proxy for past global temperature change, the oxygen isotopic composition of benthic foraminifera (Shackleton and Kennett, 1975; Miller et al., 1987). The main uncertainty associated with interpreting such records stems from the fact that the oxygen isotopic composition of foraminiferal calcite is dependent on both the temperature and also the oxygen isotopic composition of the seawater in which the foraminifera lived. The oxygen isotopic composition of seawater is largely determined by global ice volume, because the lighter isotope of oxygen is preferentially stored in ice sheets, although additional geographical variations in the oxygen isotopic composition of seawater associated with salinity variations also exist. A salinity-independent quantitative paleothermometer would therefore not only have the advantage of determining past ocean temperatures, but also global and local variations in seawater salinity. The magnesium/calcium (Mg/Ca) of benthic foraminiferal calcite has been proposed as such a proxy for bottom water temperatures (BWT), and an empirical temperature calibration
* Author to whom correspondence ([email protected]).
should
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C. H. Lear, Y. Rosenthal, and N. Slowey Table 1. Core-top locations. Latitude
Longitude
Depth (m)
BWT (°C)a
26.23°N 26.23°N 26.23°N 26.24°N 26.24°N 26.23°N 26.23°N 26.16°N 26.14°N 26.12°N
Bahamas 77.65°W 77.66°W 77.66°W 77.68°W 77.69°W 77.69°W 77.70°W 77.72°W 77.74°W 77.75°W
301 433 529 580 668 735 830 1243 1312 1585
18.38 16.55 14.20 13.31 11.49 9.86 8.20 4.50 4.35 4.05
BC BC BC BC BC BC BC BC BC BC
MC047 MC032 MC046 MC017 MC039 MC028 MC037 MC139 MC029 MC138 MC137 MC042 MC135
20.78°N 20.92°N 20.80°N 20.81°N 20.88°N 20.81°N 20.92°N 20.88°N 20.84°N 20.87°N 20.77°N 20.77°N 20.72°N
Hawaii 157.10°W 157.10°W 157.05°W 157.07°W 159.15°W 157.12°W 157.20°W 157.22°W 157.20°W 157.33°W 157.32°W 157.32°W 157.32°W
194 320 402 550 715 797 931 1095 1175 1775 2578 2583 2682
17.26 12.00 8.74 6.07 5.02 4.60 4.28 3.93 3.76 2.36 1.76 1.76 1.75
MC MC MC MC MC MC MC MC MC MC MC MC MC
BC30 BC32 BC34–91 GGC27 GGC26 GGC15
52.05°N 53.53°N 49.13°N 49.60°N 49.28°N 48.60°N
Okhotsk Sea 147.52°E 144.55°E 150.33°E 150.17°E 150.32°E 150.42°E
1310 1000 1227 995 1112 1980
2.20 2.31 2.27 2.28 2.3 1.97
BC BC BC GGC GGC GGC
26.00°N 26.82°N 30.57°N 30.68°N 26.52°N 27.90°N 27.93°N 26.08°N 27.62°N 30.98°N
Gulf of California 110.67°W 110.22°W 114.10°W 114.12°W 110.50°W 111.65°W 111.80°W 110.83°W 111.93°W 114.17°W
1295 527 154 190 1330 655 552 1020 1442 210
NBP9802 MC5 NBP9802 MC7 NBP9802 MC8
63.17°S 60.28°S 58.69°S
Southern Ocean 169.85°W 170.02°W 169.98°W
2861 3995 4324
1.2 0.84 0.84
MC MC MC
NEAP 13B NEAP 19B
58.93°N 52.77°N
NE Atlantic 24.40°W 30.33°W
2546 3283
3.3 2.7
BC BC
Core OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2
BC79 BC77 BC76 BC48 BC52 BC69 BC51 BC57 BC60 BC61
MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 Nes25-1 Nes25-1 Nes25-1 Nes25-1 Nes25-1 Nes25-1 AII AII AII AII AII AII AII AII AII AII
125-8 125-8 125-8 125-8 125-8 125-8 125-8 125-8 125-8 125-8
GGC16 BC6 GGC73 GGC71 GGC13 BC43 GGC47 GGC24 GGC53 GGC69
3.5 8.1 14 13 3.0 7.0 8.0 4.5 3.0 13.0
Core typeb
GGC BC GGC GGC GGC BC GGC GGC GGC GGC
a Source of temperatures: Little Bahama Bank: Rosenthal et al., 1997; Hawaii: shipboard CDT (conductivity, density, temperature) data; Sea of Okhotsk: Keigwin, 1998; Gulf of California: shipboard CTD data; Southern Ocean: estimated from GEOSECS station 290; NE Atlantic: estimated from GEOSECS station 25. b BC ⫽ box core; MC ⫽ multicore; GGC ⫽ giant gravity core.
original calibration (Martin et al in press). For these reasons we have refined the existing calibration by studying numerous species from different ocean basins, covering a wider temperature range, and have prepared all samples using a rigorous cleaning technique, which limits contamination from clays, organic matter, and metal oxides.
2. METHODS 2.1. Sampling Strategy Samples were obtained from six different geographical areas: the Little Bahama Bank, Hawaiian Islands, Sea of Okhotsk, Gulf of California, the Southern Ocean, and the NE Atlantic (Table 1). The core-
Benthic foraminiferal Mg/Ca-temperature calibration
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2.2. Analytical Techniques
Fig. 1. Effect of solution Ca concentration on measured Mg/Ca of a standard. True standard Mg/Ca of 5.34 mmol mol⫺1 is shown by horizontal line. Shaded area represents typical sample Ca concentrations. The external standard used to correct for instrumental mass discrimination has a Ca concentration of 4 mmol L⫺1. Vertical black bar represents long-term analytical precision (⫾1 standard deviation) determined from two consistency standards with Mg/Ca of 2.40 mmol mol⫺1 and 6.10 mmol mol⫺1.
tops cover a range in BWT of 0.8 to 18°C and a water depth range of 194 to 4324 m. Sediments were disaggregated by shaking overnight in deionized water and wet-sieved through 60-m mesh. The Hawaii samples were Rose Bengal stained immediately after collecting the sediments to separate live and dead individuals (Corliss and Emerson, 1990). Samples were dry-sieved to obtain typically between 6 and 20 benthic foraminifera, including Cibicidoides, Melonis, Uvigerina, Planulina, and Gyroidinoides genera from ⬎250 m size fraction. The species we refer to as Cibicidoides wuellerstorfi is also called Planulina wuellerstorfi in the literature. The foraminiferal tests were cleaned using a protocol to remove clays, organic matter, and metal oxides (Boyle and Keigwin, 1985/86). The foraminifera were gradually dissolved in trace metal clean 0.065N HNO3 (Seastar, Vancouver, BC, Canada), and 100 L of this solution was diluted with 300 L trace metal clean 0.5N HNO3 to obtain a Ca concentration of 3 ⫾ 1 mmol L⫺1. More details of this method are available from the author upon request.
Samples were analyzed using a Finnigan MAT (Bremen, Germany) Element Sector Field Inductively Coupled Plasma Mass Spectrometer (ICP-MS) operated in low resolution (m/⌬m ⫽ 300) following the method outlined in Rosenthal et al. (1999). However, since the publication of the earlier work there have been some changes in the operational mode of the instrument. Foremost is the addition of a guard electrode between the RF coil and the torch (i.e., the plasma) (Appelblad et al., 2000). The addition of a guard electrode significantly improved the instrument sensitivity, though not equally throughout the entire mass range. However, it also had other consequences. Notable is the significant increase in the magnitude of matrix-induced mass discrimination and its effect on Mg/Ca determination. In our previous study, conducted before installation of the guard electrode, we have found that Mg/Ca changes by ⬃0.003 mmol mol⫺1 for a 1 mmol L⫺1 change in the Ca concentration of the solution (Rosenthal et al., 1999). In this study we have used the guard electrode to obtain a greater sensitivity for Cd/Ca analysis, which has led to substantially greater dependence of Mg/Ca mass discrimination on the solution’s Ca content. Repeated analysis of five standard solutions with the same Mg/Ca ratio but varying Ca concentrations shows that the ratio of measured Mg/Ca to true Mg/Ca decreases by 6% over our typical sample Ca concentration range of between 0.5 and 4 mmol L⫺1 (Fig. 1). The reasons for increased matrix-induced mass discrimination when operating with a guard electrode are not fully understood. In general, mass-dependent discrimination is related to the preferential transmission of heavier ions, which have higher momentum while travelling through the plasma. The matrix effect results in a mass bias that causes deviations of measured elemental ratios from true ratios. The increased sensitivity obtained with guard electrode reflects increased ion density in the plasma, which in turn leads to greater discrimination between the heavier and more abundant Ca ions and the lighter and less abundant Mg ions. A more detailed description of the effect of the guard electrode on this analytical technique will be presented elsewhere. Direct determination of element ratios from intensity ratios therefore requires control of the sample Ca concentration. Below ⬃0.5 mmol L⫺1 Ca concentration, the deviation of measured Mg/Ca from true Mg/Ca is highly nonlinear (Fig. 1). In such cases, a low Ca concentration standard closely matching the sample matrix is required for the most precise analyses in this concentration range. Our analyses over the past 6 months suggest that the magnitude of the matrix effect on Mg/Ca measurements is stable within and between instrument runs. Furthermore, in contrast to inductively coupled plasma-atomic emission spectroscopy (ICP-AES) methods (de Villiers et al., 2002), we have found
Table 2. Mg/Ca (mmol mol⫺1) of benthic foraminiferal calcite from Little Bahama Bank core-top samples collected during cruise OC205-2. Speciesa Core BC79 BC79 BC79 BC79 BC77 BC77 BC77 BC76 BC48 BC52 BC69 BC51 BC57 BC60 BC61
O. umb
C. pac
2.30 2.26
7.18 9.44 5.53 8.63 6.64 4.03 4.43 3.60 4.17 2.86 2.73 2.81
3.43 2.04
1.50
C. wue
P. bull
B.cf mex
B.cf acul
Uvi spp.
2.63
2.08 2.45 3.33 2.10 1.77
3.25 3.21
1.96 1.65 1.41
M. barl
G.cf med
C. rob
5.65 6.01
6.97
10.8
4.29 5.46
6.30
4.01 3.29 3.86 3.09 2.56 2.12 2.42
G.cf sold
5.28 6.27 5.31 4.51 3.65 3.73
4.51 3.62 3.46 3.73
C. kul
Sip sp.
5.81 6.25 6.50 5.89 5.50 5.18
5.77 5.54 5.39 4.96 4.76 2.80 2.72 2.80
a O.umb ⫽ Oridorsalis umbonatus; C.pac ⫽ Cibicidoides pachyderma; C.wue ⫽ Cibicidoides wuellerstorfi; P.bull ⫽ Pullenia bulloides; B.cf mex ⫽ Bulimina cf. mexicana; B.cf acul ⫽ Bulimina cf. Aculeata; Uvi spp. ⫽ Uvigerina spp; M.barl ⫽ Melonis barleeanum; G.cf sold ⫽ Gyroidinoides cf soldanii; G. cf med ⫽ Gyroidinoides cf. mediceus; C.kul ⫽ Cibicidoides kullenbergi; C.rob ⫽ Cibicidoides robertsonianus; Sip.sp ⫽ Siphonina sp.
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Fig. 2. Direct comparison of benthic foraminiferal Mg/Ca data measured by ICP-MS (y-axis) with those measured by FAAS (Rosenthal et al., 1997) (x-axis) from the same Little Bahama Bank core-tops. Where more than one sample was analyzed from a single box core, we have plotted the mean and ⫾ 1 standard deviation of all analyses.
that this relationship is independent of Mg concentration. The corrections are typically ⬍0.1 mmol mol⫺1 Mg/Ca. Instrument precision was determined by repeated analysis of three consistency standards covering a range of 1 to 6 mmol mol⫺1 over the course of this study. The precision of the consistency standard with Mg/Ca of 1.10 mmol mol⫺1 was ⫾3.7% (r.s.d.); the precisions of the consistency standards with Mg/Ca of 2.40 mmol mol⫺1 and 6.10 mmol mol⫺1 were ⫾1.5% and ⫾1.6% respectively. The unpublished data from the two NE Atlantic cores were prepared and analyzed separately by simultaneous ICP-AES at Cambridge University. It has since been demonstrated that the Ca matrix effect of this instrumental method varies with solution Mg/Ca. A method has been developed which accounts for this matrix effect by calibrating the instrument using raw intensity ratios rather than elemental intensities (de Villiers et al., 2002). Although this new method was not employed on the data presented here, two factors imply that this matrix effect does not seriously compromise the accuracy of the data presented here. The first is that the batch runs of benthic foraminiferal Mg/Ca covered a relatively narrow range of Mg/Ca, e.g., 1.3 to 2.7 mmol mol⫺1 for this study, compared with 0.5 to 18 mmol mol⫺1 (Fig. 4a in de Villiers et al., 2002). Secondly, for each batch run, pairs of lines were chosen which optimized the accuracy of a standard solution with similar Mg/Ca as the samples. 2.3. Interlaboratory Comparison A consistent offset between two of the core-top benthic foraminiferal Mg/Ca data sets used here to revise the temperature calibration has been documented (Martin et al in press). Martin et al. (in press) compiled data measured by flame atomic absorption spectrophotometry (FAAS) from Rosenthal et al. (1997) with unpublished FAAS analyses also made by Y. Rosenthal at MIT. Martin et al. (in press) note that these data have consistently higher Mg/Ca than samples analyzed by ICP-MS at the University of California, Santa Barbara (UCSB). The FAAS data represent samples cleaned using ultrasonication plus acid
Fig. 3. Depth transect of G. ruber (open circles and crosses) and C. pachyderma (closed circles) Mg/Ca from surface sediments of the Little Bahama Bank.
leaches in addition to samples cleaned using the full Boyle and Keigwin (1985/86) method. All the ICP-MS data represent samples that were cleaned using methods set out in Boyle and Keigwin (1985/86). Interestingly, Ceara Rise (Atlantic) core-top benthic foraminiferal Mg/Ca analyzed by ICP-MS (Martin et al in press) are also lower (by a factor of ⬃0.63) than those analyzed by FAAS (Russell et al., 1994). Data presented here (analyzed by ICP-MS at Rutgers University and by ICP-AES at Cambridge University) are in good agreement with the Martin et al. (in press) data analyzed by ICP-MS at UCSB. The offset between ICP-MS and FAAS data is observed for all species analyzed from both warm (e.g., Little Bahama Bank) and cold (e.g., Ceara Rise) core-tops regardless of whether the full or shortened cleaning method was applied. This discrepancy reflects an issue of accuracy rather than precision, most likely caused by an instrumental bias, although the exact nature of this bias has not been determined. It is noteworthy that the different data sets do not show such an offset for Sr/Ca. To quantify the offset in Mg/Ca between the FAAS and the ICP data sets, we analyzed benthic foraminifera from the same core-tops as Rosenthal et al. (1997). Rosenthal et al. (1997) used a two-point linear calibration fit on the FAAS. Therefore, the accuracy error of all samples may be expressed as a single factor F, where (F ⫽ true Mg/Ca/measured Mg/Ca). Plotting our new benthic foraminiferal Mg/Ca against the original data for the same species from the same Little Bahama Bank core-tops, we find that F ⫽ 0.73 (Fig. 2). We use F (⫽ 0.73) to correct
Benthic foraminiferal Mg/Ca-temperature calibration
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Table 3. Mg/Ca (mmol mol⫺1) of benthic foraminiferal calcite from core-tops collected from Hawaii Islands, cruise MW98-13. Speciesa Core MC47 MC32 MC46 MC46 MC17 MC39 MC28 MC37 MC37 MC139 MC29 MC138 MC138 MC138 MC138 MC137 MC137 MC42 MC42 MC135 MC135
O. umb
C. pach
C. wue
4.18us 3.44us 2.02us
5.85us
1.38us
C. comp
Plan sp.
Uvi groo
Uvi spiky
Uvi smoo
Gyroi spp.
1.64us 3.14us 2.13us
2.06us 1.51s
1.68us
1.68s
2.01us
1.53us 1.41us 1.36s 1.16s
1.48us 0.98s 0.92s 1.84us 1.00us 0.99us 1.03s 1.05us 0.95s 0.96us
1.27us
1.47s 1.50us
1.43us
2.74s 3.03s
1.29us 1.11us
a O.umb ⫽ Oridorsalis umbonatus; C.pach ⫽ Cibicidoides pachyderma; C.wue ⫽ Cibicidoides wuellerstorfi; C.comp ⫽ Cibicidoides compressus; Plan sp. ⫽ Planulina sp.; Uvi groo ⫽ a grooved Uvigerina species; Uvi spiky ⫽ a spiky Uvigerina species; Uvi smoo ⫽ a smooth Uvigerina species; Gyroi spp. ⫽ Gyroidinoides spp. Superscripts s and us represent samples composed of stained and unstained (Rose Bengal) individuals respectively.
all the FAAS data as all were analyzed using the same method and standards. 2.4. Scatter within the Little Bahama Bank Samples Benthic foraminiferal Mg/Ca from the shallow sites of the Little Bahama Bank are very scattered (Rosenthal et al., 1997). To determine whether this scatter is a result of (1) downslope transport of tests, (2) contamination by high-Mg calcite, or (3) natural variability, we analyzed planktonic foraminifera (white and pink Globigerinoides ruber) from the same Little Bahama Bank depth transect. About 30 planktonic foraminifera per sample were picked from the same size fraction (250 to 355 m), and prepared and analyzed (by ICP-MS) using the same protocol as the benthic foraminifera. All but one of the planktonic foraminifera samples show constant Mg/Ca with water depth and display little scatter in Mg/Ca (Fig. 3). One sample of white G. ruber from the shallowest site has anomalously high Mg/Ca, probably a result of contamination by secondary precipitation of high-Mg calcite, a common feature of Little Bahama Bank sediments. We consider it likely that at least part of the scatter in the Little Bahama Bank benthic foraminiferal Mg/Ca is caused by some combination of downslope transport and/or high-Mg calcite contamination. Therefore in compiling data for the temperature calibration we have rejected three samples from the shallow sites of the Little Bahama Bank that have Mg/Ca in the upper 5 percentile of the standard error. 3. RESULTS
Benthic foraminiferal Mg/Ca from the study sites show a general decrease with increasing water depth, consistent with the postulated temperature control on calcite Mg/Ca (Figs. 4 – 6 and Tables 2– 4). No single species analyzed in this study covers the full bathymetric range, making it difficult to compare precisely the temperature sensitivities of different species. However, many species were found at more than one site, and
Oridorsalis umbonatus was analyzed from every region. Fig. 4 depicts Mg/Ca of eight benthic foraminiferal species from a water depth transect on the Hawaiian margin. Cibicidoides and Planulina species appear to have similar temperature sensitivities. One C. pachyderma sample has a relatively high Mg/Ca at around 1750 m water depth. This anomalous value could represent individuals transported downslope, as C. pachyderma primarily inhabits the upper bathyal region (200 to 600 m water depth) although it has been documented in abyssal sediments (van Morkhoven et al., 1986). Living (stained) and dead (unstained) foraminifera from the Hawaii samples were analyzed separately (Table 3), and no consistent differences in calcite Mg/Ca were found between the two groups. Eleven benthic foraminiferal species from the Little Bahama Bank core-tops are shown in Fig. 5. The C. pachyderma species in Fig. 5 is equivalent to C. floridanus of Rosenthal et al. (1997). The considerable scatter in the previously published Mg/Ca data of C. pachyderma from shallow (⬍600 m) water depths of the Little Bahama Bank is also seen in these new data, suggesting that the scatter was not caused by the shorter cleaning technique. Fig. 6 depicts Mg/Ca of seven species of benthic foraminifera covering a depth transect from the Gulf of California, five species of benthic foraminifera from the Okhotsk Sea, and five species from two box cores collected in the North Atlantic. Six benthic foraminiferal species were analyzed from multicore core-tops from the Southern Ocean to provide data for the cold end of the temperature calibration. Short (0 –20 cm) downcore records for these three sites suggest that the core-tops are well bioturbated and that downcore Mg/Ca is relatively constant with depth (Fig. 7; Table 5). Foraminifera from the upper 5 cm of these cores were used in the calibration compilation.
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Table 4. Mg/Ca (mmol mol⫺1) of benthic foraminiferal calcite from core-tops collected from the Gulf of California, the Sea of Okhotsk and the NE Atlantic. Speciesa Core
O. umb
GGC16 BC6
1.22
GGC73 GGC71 GGC13 BC43 GGC47 GGC47 GGC24 GGC24 GGC53 GGC53 GGC69
B. mex
P. bull
C. wue
Epis sp.
P. arim
Plan spp.
M. park
M. pom
M. bar
Gulf of California 1.00 2.67 2.37 1.15
1.05 1.74 1.43 0.92
1.22 1.24
1.29
1.39 2.60
Uvi spp.
Gyr spp.
1.14 12.4 * 2.34 1.46 1.46 1.40 1.29 1.49 0.77 0.82 0.92 0.98 2.85
1.50
1.60
Sea of Okhotsk BC30 BC32 BC32 BC32 BC32 BC3491 GGC27 GGC27 GGC27 GGC27 GGC27 GGC26 GGC26 GGC26 GGC26 GGC15 GGC15 GGC15
1.09
13B 13B 13B 19B 19B
1.84
0.91
1.28 1.33
0.75 0.84 0.86 0.99 0.86
1.42
0.68 0.83 0.89 1.10 1.04 0.81 0.85 0.87 0.83 1.42 1.16 1.00
1.80
NE Atlantic 2.74 2.32
1.91 1.84
1.62 1.69 1.55 1.30
1.46 1.48 1.45
1.69
Asterisk denotes rejected data. a O.umb ⫽ Oridorsalis umbonatus; B.mex ⫽ Bulimina mexicana; P.bull ⫽ Pullenia bulloides; C.wue ⫽ Cibicidoides wuellerstorfi; Epis sp. ⫽ Epistominella sp.; P.arim ⫽ Planulina ariminensis; Plan spp. ⫽ Planulina spp. M.park ⫽ Melonis parkerae; M.pom ⫽ Melonis pompilioides, M.bar ⫽ Melonis barleeanum; Uvi spp. ⫽ Uvigerina spp.; Gyr spp. ⫽ Gyroidinoides spp.
4. DISCUSSION
4.1. Compilation of Core-top Data There are several possible approaches to fitting a curve through the compiled data and determining a Mg/Ca-temperature calibration. Thermodynamic considerations suggest that the Mg/Ca-temperature relationship is best described by an exponential fit (Rosenthal et al., 1997). In addition, Mg/Catemperature relationships determined for planktonic foraminifera are exponential in form (Nu¨ rnberg et al., 1996; Lea et al., 1999; Mashiotta et al., 1999; Elderfield and Ganssen, 2000), although we find no statistical evidence to favor an exponential fit over a linear fit in this study.
Exponential fits through the uncorrected FAAS data and the ICP-MS data have similar exponential constants, yet the preexponential constants are different by a factor of 0.77 (Fig. 8a). This factor is similar to that obtained by direct comparison of benthic foraminiferal Mg/Ca data measured by FAAS and ICP-MS from the same Little Bahama Bank box cores (Fig. 2), supporting our view that the difference between the two data sets is an issue of analytical bias. As argued in section 2.4, we believe that the three high Mg/Ca ratios from Little Bahama Bank are contaminated with high-Mg calcite and/or have been transported downslope. The other sample rejected is also outside the 95% confidence limit of the standard error. To determine the most robust Mg/Ca-temperature calibration, we have
Benthic foraminiferal Mg/Ca-temperature calibration
Fig. 4. Mg/Ca of benthic foraminifera collected from Hawaii multicore core-tops plotted against water depth. The form of the temperature profile is shown in gray.
Fig. 5. Benthic foraminiferal Mg/Ca from core-tops collected on the Little Bahama Bank plotted against water depth. The form of the temperature profile is shown in gray.
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C. H. Lear, Y. Rosenthal, and N. Slowey
Fig. 6. Benthic foraminiferal Mg/Ca plotted against water depth for (a) Gulf of California, (b) Sea of Okhotsk, (c) NE Atlantic. The form of the temperature profile is shown in gray.
compiled all FAAS (corrected as described in section 2.3) and ICP-MS core-top data (Rosenthal et al., 1997; Martin et al., in press; this study) for several Cibicidoides species (Fig. 8b). This revised Mg/Ca-temperature calibration is based on C. wuellerstorfi, C. pachyderma, C. compressus, and a wuellerstorfi-like Cibicidoides. Data from C. kullenbergi and C. robertsonianus were not used for this calibration. This compilation represents samples from Little Bahama Bank, Hawaii, Southern Ocean, NE Atlantic, Ceara Rise, Sierra Leone Rise, and the Ontong Java Plateau. Fitting an exponential curve through these data results in the equation: Mg/Ca ⫽ 0.867 ⫾ 0.049 exp (0.109 ⫾ 0.007 ⫻ BWT), R2 ⫽ 0.94
(1)
The errors in Eqn. 1 represent the 95% confidence interval for the exponential and preexponential constants. The standard error of the regression is 1.7°C. However, because downcore records do not display similar scatter (Fig. 7; Martin et al in press), the scatter in the core-top data likely reflects the nature of the sampling for the calibration. An alternative approach to fitting a calibration curve is to assume that only the lowest Mg/Ca from the Little Bahama Bank sites are reliable, and that higher ratios are falling on a mixing line between unaltered and
reworked or contaminated samples. An exponential curve fit through the compiled data, using only the lowest measured Mg/Ca from the Little Bahama Bank sites of 13°C and warmer, yields an equation that is consistent within error of Eqn. 1.. 4.2. Species Effects Melonis spp., Oridorsalis umbonatus, and most Planulina spp. appear to have a similar Mg/Ca–BWT relationship as the Cibicidoides calibration described above (Fig. 9a). However, other species have different temperature responses (Fig. 9b), and these are summarized in Table 6. Considering the large genetic variability within benthic foraminifera, such differences are perhaps not surprising, yet the causes of such “vital effects” apparently superimposed on the temperature control of test Mg/Ca are unclear. One possibility is that the foraminiferal microhabitats are partly responsible. Variations in pore-water chemistry within the upper few centimeters of the sediment column have been shown to cause differences in trace metal compositions between epifaunal and infaunal benthic foraminiferal calcite (Tachikawa and Elderfield, in review). This mechanism would result in a set of calibration equations with similar temperature sensitivities (A, Table 6) but with preex-
Benthic foraminiferal Mg/Ca-temperature calibration
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Fig. 7. Downcore records of benthic foraminferal Mg/Ca from three multicores taken from the Southern Ocean. The 2 S.E. of Mg/Ca of Oridorsalis umbonatus, Nuttallides umbonifera, and Melonis pompilioides from MC7 and MC8 is less than or equal to ⫾0.1 mmol/mol.
ponential constants (B, Table 6) being a function of microhabitat. This is not supported by our data which suggest that Uvigerina spp. and Planulina ariminensis (infaunal and epifaunal species respectively) both have apparently lower temperature sensitivity (i.e., % change in Mg/Ca per degree Celsius) to the majority of the other species studied here. In addition, the preexponential constants show a significant variation within the epifaunal species. Another possibility is that the differences in the Mg/Ca-temperature responses between species result from differences in the architecture of the calcite test. Coprecipitation experiments have shown that the partition coefficient of divalent cations into calcite is strongly affected by the relative proportions of vicinal faces on growth spirals, which in turn are affected by crystal size and shape (Reeder, 1996). Alternatively, the species offsets could result from differences in the amount of fractionation of trace metals in internal calcification pools (Elderfield et al., 1996) or different growth rates, perhaps caused by varying metabolic rates between species. It is difficult to test the last three possibilities, but it seems clear that whatever is responsible for the differences between species, temperature remains an overriding control on foraminiferal Mg/Ca. 4.3. Comparison of Mg/Ca-Temperature Calibrations Mg/Ca-temperature calibrations for planktonic foraminifera (Nu¨ rnberg et al., 1996; Lea et al., 1999; Mashiotta et al., 1999; Elderfield and Ganssen, 2000) are shown in Table 6 for com-
parison with the benthic foraminiferal calibrations presented here. The temperature sensitivity is similar for both benthic and planktonic foraminifera. Lear et al. (2000) estimated the preexponential constant (B) for their primary benthic foraminiferal species O. umbonatus to calculate Cenozoic BWT. To do this, they assumed that the exponential constant (A) in the Rosenthal et al. (1997) calibration was robust, and calculated B using temperature estimated from ␦18O data from the ice-free early Paleogene. Using a modeled output of seawater Mg/Ca at 50 Ma (Wilkinson and Algeo, 1989) gave B ⫽ 1.06. This estimate is similar to the preexponential constant determined here from core-top samples (1.01; Table 6). Although the biased nature of the FAAS data used for the Rosenthal et al. (1997) calibration had not been realized, this procedure essentially eliminated the effect of the analytical bias on the calculated Cenozoic BWT. 4.4. Constraints on Low-temperature (0 –5 °C) Reconstructions The revised Cibicidoides calibration (Fig. 8b) predicts an increase in Mg/Ca of 0.1 mmol mol⫺1 from 0 to 1°C. This difference is larger than analytical precision, although the scatter in the calibration data suggests that the signal would be lost in the natural variability of samples (Fig. 8). However, paired Mg/Ca and ␦18O downcore records for the Quaternary reveal variations in benthic foraminiferal Mg/Ca on the order of 0.1 mmol mol⫺1 apparently caused by variations in BWT (Martin et al in press). A similar precision is also estimated from
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C. H. Lear, Y. Rosenthal, and N. Slowey
⌬CO32⫺, pressure, and salinity. The scatter in the calibration data is small enough to suggest that temperature is the overriding control on benthic foraminiferal Mg/Ca. However, water depth transects from Ceara Rise (equatorial Atlantic; 3270 to 4670 m, 2.6 to 1.5°C) and the Ontong Java Plateau (equatorial Pacific; 1615 to 2960 m, 2.8 to 1.6°C) show a steep trend of decreasing Mg/Ca with water depth (Russell et al., 1994; Martin et al., in press). This trend is steeper than that predicted by the temperature calibration. Similar water depth trends in ⌬␦13C, Cd/Ca, Ba/Ca, and Sr/Ca from the Ontong Java Plateau have previously been documented and interpreted in terms of dissolution or undersaturation effects (McCorkle et al., 1995; Elderfield et al., 1996). Published benthic foraminiferal ␦18O from the Ontong Java Plateau (McCorkle et al., 1995) and unpublished ␦18O from the Ceara Rise (McCorkle, personal communication) do not show any trend with water depth, arguing against the up-mixing of glacial foraminifera as the cause of the Mg/Ca–water depth trend. The steep trend is from the cold end of the calibration (⬍3°C) where the exponential fit predicts small changes in benthic foraminiferal Mg/Ca. The effects of any additional processes other than temperature on benthic foraminiferal Mg/Ca would be relatively pronounced in benthic foraminifera living at lower temperatures. For this reason, we have also put an exponential fit through the data covering just the ⬎5°C data, and this results in a Mg/Ca–BWT relationship: Mg/Ca ⫽ 1.009 ⫾ 0.187 exp (0.0972 ⫾ 0.132 ⫻ BWT) R2 ⫽ 0.84 Fig. 8. Benthic foraminiferal Mg/Ca vs. temperature. (a) Uncorrected FAAS data (open symbols and dashed curve fit) and ICP-AES plus ICP-MS data (crosses and closed symbols and solid curve fit). The difference in the preexponential constants between the two curve fits results from analytical bias. (b) Cibicidoides spp. Mg/Ca-temperature calibration. Compilation of corrected FAAS, ICP-AES, and ICP-MS data for C. wuellerstorfi, C. pachyderma, C. compressus, and a wuellerstorfi-like Cibicidoides. Triangles represent rejected data lying in the upper 5 percentile of the standard error.
benthic foraminiferal Mg/Ca in the top 15 cm of three cores collected in the Southern Ocean (Fig. 7). The sediments were collected by multicorers so the mixed layer is certainly present. The constancy of benthic foraminiferal Mg/Ca with depth in the sediment suggests that the top 15 cm is relatively homogeneous, presumably resulting from bioturbation. Five or more samples of three species (Oridorsalis umbonatus, Nuttallides umbonifera, and Melonis pompilioides) were analyzed from the top 15 cm of cores MC7 and MC8. The 2 S.E. of these analyses is less than or equal to ⫾0.1 mmol mol⫺1. Therefore, it seems likely that processes superimposed on the temperature control are responsible for some of the scatter in the Mg/Ca calibration data. Considering that the calibration samples were collected from a variety of ocean basins, there are many possible factors in addition to temperature which could potentially affect the Mg/Ca of these samples. These factors include vital effects (e.g., different Cibicidoides species), kinetic effects (e.g., varying growth rates caused by environmental conditions), varying rates of bioturbation and up-mixing of glacial foraminiferal tests, downslope transport of core-top tests, dissolution,
(2)
The exponential constant in Eqn. 2 is significantly lower than that obtained using all the core-top data (Eqn. 1). The standard error of the regression is 1.3°C, smaller than obtained for the entire data set. 4.5. Implications for Cenozoic Seawater Mg/Ca Oridorsalis umbonatus is an extant species that is found in deep-sea sediments spanning the entire Cenozoic. Comparison of O. umbonatus Mg/Ca from a variety of modern oceanographic settings with fossil O. umbonatus Mg/Ca gives some constraint on the extent to which seawater Mg/Ca may have varied through the Cenozoic. Variations in seawater Mg/Ca are caused by changes in the rates of submarine hydrothermal activity, dolomitization, carbonate production, and river fluxes (Drever et al., 1988). These processes have consequent effects on the global carbon cycle (through volcanism, chemical weathering and carbonate production) and climate change (Berner et al., 1983). It has also been proposed that variations in seawater Mg/Ca in the geologic past have forced global oscillations between calcite and aragonite seas, in turn affecting the dominance of marine calcifying organisms (Stanley and Hardie, 1998). Here we use our new Mg/Ca-temperature relationship for O. umbonatus, an extant species for which we have Mg/Ca data from the ice-free Paleogene (Lear et al., 2000) to give a first-order estimate of how seawater Mg/Ca may have varied since 50 Ma. We do this by solving Eqn. 3: Mg/Caforam ⫽
Mg/CaSW-t ⫻ B exp (A ⫻ BWT) Mg/CaSW-0
(3)
where Mg/Caforam is Mg/Ca of O. umbonatus at 49 Ma (Lear et
Benthic foraminiferal Mg/Ca-temperature calibration
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Table 5. Downcore benthic foraminiferal Mg/Ca (mmol mol⫺1) from three Southern Ocean cores. Speciesa Depth (cm) 0–1 1–2 2–3
M.pomp
Gyroid
0.94 0.87 0.74
1.35 1.28 1.37
Cib sp. STA 5 1.03 1.21 1.50
P.bull
O.umb
1.26 1.43
1.14 1.03
N.umb
STA 7 0–1 1–2 2–3 4–5 6–7 14–15 16–17
0.95 0.78 0.76
0.90 1.05
1.03 0.84
1.22 1.10
0.72 0.77 0.75
1.12
0.59
1.30 1.34
0.53 2.31
1.51
1.03 1.33 1.03 0.90 0.95 0.93 0.98
0.30 0.47 0.51 0.30 0.31 0.37 0.57
0.92 0.86 0.88 1.06 0.82 0.86 0.91
0.52 0.29 0.28 0.27 0.32 0.32 0.28
STA 8 0–1 1–2 2–3 2–3 3–4 4–5 9–10
1.03 1.08 1.84
1.51 0.67 0.72 1.04
a M.pomp ⫽ Melonis pompilioides; Gyroid ⫽ Gyroidinoides spp.; Cib sp. ⫽ Cibicidoides sp.; P.bull ⫽ Pullenia bulloides; O.umb ⫽ Oridorsalis umbonatus; N.umb ⫽ Nuttallides umbonifera.
al., 2000) and Mg/CaSW is seawater Mg/Ca at 49 Ma and at the present day (subscript t and 0, respectively), and the constants A and B are as defined in Table 6. We use a BWT of 12.4°C from oxygen isotopes assuming an ice-free world at 49 Ma and assume modern seawater Mg/Ca ⫽ 5.2 mol mol⫺1. Because our O. umbonatus calibration is not very well constrained from our core-top data (R2 ⫽ 0.40; Table 6), we use a range of values for A and B in calculating seawater Mg/Ca at 49 Ma (Table 7). However, even considering uncertainties in the exact values of A and B, we consider it unlikely that seawater Mg/Ca at 50 Ma was less than two-thirds of today’s value. This estimate is in good agreement with the modeled estimates of Wilkinson and Algeo (1989) but is at odds with the Stanley and Hardie (1998) model which predicts seawater Mg/Ca at 49 Ma to have been below their hypothesized calcite–aragonite sea threshold of 2 mol mol⫺1. Support for our estimate is found in the chemistry of fluid inclusions of marine halites. Assuming reasonable estimates of seawater Ca concentration (10 to 15 mmol kg⫺1) and Mg concentrations derived from halite inclusions (Zimmermann et al., 2000), Lowenstein et al. (2001) estimate early Cenozoic seawater Mg/Ca to have been in the range of 2.5 to 3.7 mol mol⫺1. Variations in seawater Mg/Ca clearly remain one of the largest uncertainties in using Mg paleothermometry to determine BWT over long (tens of m.y.) timescales. However, Mg and Ca are conservative elements with long residence times (⬃10 m.y. and ⬃1 m.y. respectively) in the oceans, so that Mg paleothermometry remains a very useful tool for looking at relative temperature changes in BWT throughout the Cenozoic. The Mg/Ca of seawater, or the preexponential constant of the temperature calibration is not required to calculate the magnitude of short-term temperature shifts using Mg paleothermometry.
Fig. 9. (a) Oridorsalis umbonatus, Melonis spp., and Planulina spp. (excluding ariminensis) Mg/Ca vs. temperature. The Cibicidoides spp. calibration is shown as a solid line. (b) Planulina ariminensis and Uvigerina spp. Mg/Ca vs. temperature. The Cibicidoides spp. calibration is shown as a solid line.
3386
C. H. Lear, Y. Rosenthal, and N. Slowey Table 6. Foraminiferal Mg/Ca-temperature calibrations. Calibration equationsa n
Temperature range (°C)
B
A
R2
101
0.8–18.4
0.867
0.109
0.94
10 7 58 42 23
3.0–14.5 2.3–12.0 1.8–18.4 0.8–18.4 0.8–9.9
0.911 0.788 0.924 0.982 1.008
0.062 0.119 0.061 0.101 0.114
0.69 0.96 0.69 0.84 0.40
1.36 1.06
0.10 0.10
1.36 0.53 0.474
0.085 0.10 0.107
0.52
0.10
Species Benthic foraminifera (this study) Cibicidoides including wuellerstorfi, pachyderma, and compressus P. ariminensis Planulina spp. Uvigerina spp. M. barleeanum and M. pompilioides O. umbonatus Benthic foraminifera (previous studies) Cibicidoides (Rosenthal et al., 1997)b O. umbonatus (Lear et al., 2000) Planktonic foraminifera Culture O. universa (Lea et al., 1999) Culture G. bulloides (Lea et al., 1999) Culture and core-tops G. bulloides (Mashiotta et al., 1999) (Mashiotta et al., 1999) Core-tops (Elderfield and Ganssen, 2000)
a The Mg/Ca-temperature relationship for benthic foraminifera may be expressed in the form of a general equation Mg/Ca ⫽ B exp (A ⫻ BWT). R2 is the correlation coefficient. The exponential constant B reflects the temperature sensitivity and is similar for most benthic and planktonic foraminifera. n represents the number of samples used in determining each equation. b This calibration (Rosenthal et al., 1997) appears to be biased by an analytical accuracy issue (see text).
5. CONCLUSIONS
The revised Mg/Ca–BWT calibration based on common Cibicidoides species is described by the exponential equation Mg/Ca ⫽ 0.867 ⫾ 0.049 exp (0.109 ⫾ 0.007 ⫻ BWT). The exponential constant defines the temperature sensitivity of benthic foraminiferal Mg/Ca and is crucial for reconstructing the amplitudes of past BWT variations. Both the exponential and the preexponential constants vary between species. Considering the errors associated with temperature prediction using
Table 7. Estimates of seawater Mg/Ca at 49 Ma. Seawater Mg/Caa A ⫽ 0.11
A ⫽ 0.10
B ⫽ 0.9
4.1
4.6
B ⫽ 1.0
3.7
4.1
B ⫽ 1.1
3.3
3.8
Halite inclusions (Zimmermann, 2000; Lowenstein et al., 2001). Wilkinson and Algeo (1989) Stanley and Hardie (1998)
2.5–3.7 3.9 1.6
a Seawater Mg/Ca (mol mol⫺1), calculated from Eqn. 3 using Mg/Ca of O. umbonatus at 49 Ma ⫽ 2.78 mmol mol⫺1 and temperature ⫽ 12.4°C (Lear et al., 2000). We use a range of values for the Mg/Catemperature calibration constants A and B (Table 6) to give an indication of the error associated with this method. Seawater Mg/Ca was estimated from the magnesium content of halite fluid inclusions; two computer models are shown for comparison.
the Cibicidoides calibration (a combined exponent and preexponent error of 8.6%) and the downcore Mg/Ca reproducibility (2 S.E. ⱕ ⫾0.1 mmol mol⫺1), we suggest that Mg-paleothermometry has the potential to resolve BWT to better than ⫾1°C. The uncertainty in estimating deep water temperature (BWT ⬍3°C) is dominated by the natural variability in downcore foraminiferal Mg/Ca. Therefore, at low temperatures the uncertainty may be reduced by improving the standard errors associated with the downcore record (i.e., increasing the number of replicates). The primary uncertainty in estimating thermocline temperatures (⬎6°C) is, however, determined to a large extent by the scatter in the calibration rather than downcore reproducibility. This scatter is a result of compiling Mg/Ca data of different Cibicidoides species from core-tops of different quality. Therefore, the error in comparing relative, downcore temperature variations should be significantly better than ⫾1°C. While the value of 0.109 (⫾0.007) appears to be a robust estimate of the temperature dependency of Mg uptake by benthic foraminiferal calcite, a question remains regarding the temperature sensitivity below 5°C. Additional processes superimposed on temperature may cause deviations of benthic foraminiferal Mg/Ca from the calibration curve; these would create larger temperature errors at lower temperatures. Changes in seawater Mg/Ca remain one of the largest uncertainties in using Mg paleothermometry to determine BWT over long (tens of m.y.) timescales. Comparison of calibration data with benthic foraminiferal Mg/Ca and ␦18O from the ice-free early Cenozoic indicates that seawater Mg/Ca was not more than 35% lower than today at 49 Ma.
Benthic foraminiferal Mg/Ca-temperature calibration Acknowledgments—We thank Lloyd Keigwin for provision of samples (and some picked foraminifera) from the Sea of Okhotsk and the Gulf of California, Adina Paytan for samples from the Southern Ocean, Paul Field for help with analyses, and Pam Martin and Dan McCorkle for useful discussions. This manuscript was greatly improved by reviews from Peggy Delaney, David Lea, and two anonymous reviewers. This work was supported by a post-doctoral fellowship at IMCS, Rutgers University (CHL) and NSF grants 9810717 and 9819675 (Y.R.) and OCE9712763 (N.S.). Associate editor: D. W. Lea REFERENCES Appelblad P. K., Rodushkin I., and Baxter D. C. (2000) The use of Pt guard electrode in inductively coupled plasma sector field mass spectrometry: Advantages and limitations. J. Anal. At. Spectrom. 15, 359 –364. Beck W. J., Edwards L. R., Ito E., Taylor F. W., Recy J., Rougerie F., Joannot P., and Henin C. (1992) Sea-surface temperature from coral skeletal strontium/calcium ratios. Science 257, 644 – 647. Berner R. A., Lasaga A. C., and Garrels R. M. (1983) The carbonatesilicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Am. J. Sci. 283, 641– 683. Billups K. and Schrag D. P. (2002) Paleotemperatures and ice-volume of the past 27 myr revisited with paired Mg/Ca and stable isotope measurements on benthic foraminifera. Paleoceanography 17 10.1029/2000 PA 000567. Boyle E. A. and Keigwin L. D. (1985/86) Comparison of Atlantic and Pacific paleochemical records for the last 250,000 years: Changes in deep ocean circulation and chemical inventories. Earth Planet. Sci. Lett. 76, 135–150. Burton E. A. and Walter L. M. (1991) The effects of pCO2 and temperature on magnesium incorporation in calcite in seawater and MgCl2-CaCl2 solutions. Geochim. Cosmochim. Acta. 55, 775–785. Corliss B. H. and Emerson S. (1990) Distribution of rose-bengal stained deep-sea benthic foraminifera from the Nova Scotian continental margin and Gulf of Maine. Deep Sea Res. 37, 381– 400. deVilliers S., Greaves, M., Elderfield H. (2002) An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP-AES. Geochem. Geophys. Geosyst. 3, paper number 10.1029/2001GC000169. Drever J. I., Li Y.-H., and Maynard J. B. (1988) Geochemical cycles: The continental crust and the oceans. In Chemical Cycles in the Evolution of the Earth (ed. C. B. Gregor), pp. 17–53. Wiley. Dwyer G. S., Cronin T. M., Baker P. A., Raymo M. E., Buzas J. S., and Correge T. (1995) North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347–1351. Elderfield H., Bertram C. J., and Erez J. (1996) A biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate. Earth Planet. Sci. Lett. 142, 409 – 423. Elderfield H. and Ganssen G. (2000) Past temperature and ␦18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios. Nature 405, 442– 445. Hartley G. and Mucci A. (1996) The influence of pCO2 on the partitioning of magnesium in calcite overgrowths precipitated from artificial seawater at 25° and 1atm total pressure. Geochim. Cosmochim. Acta 60, 315–324. Katz A. (1973) The interaction of magnesium with calcite during crystal growth at 25–90°C and one atmosphere. Geochim. Cosmochim. Acta 37, 1563–1586. Keigwin L. D. (1998) Glacial-age hydrography of the far northwest Pacific Ocean. Paleoceanography 13, 323–339. Lea D. W., Mashiotta T. A., and Spero H. (1999) Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing. Geochim. Cosmochim. Acta 63, 2369 –2379. Lear C. H., Elderfield H., and Wilson P. A. (2000) Cenozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite. Science 287, 269 –272.
3387
Lowenstein T. K., Timofeeff M. N., Brennan S. T., Hardie L. A., and Demicco R. V. (2001) Oscillations in Phanerozoic seawater chemistry: Evidence from fluid inclusions. Science 294, 1086 –1088. Martin P. A., Lea D. W., Rosenthal Y., Shackleton N. J., Sarnthein M., and Papenfuss T. (2002) Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca. Earth Planet. Sci. Lett. 198, 193–209. Mashiotta T. A., Lea D. W., and Spero H. J. (1999) Glacial-interglacial changes in Subantarctic sea surface temperature and ␦18O-water using foraminiferal Mg. Earth Planet. Sci. Lett. 170, 417– 432. McCorkle D. C., Martin P. A., Lea D. W., and Klinkhammer G. P. (1995) Evidence of a dissolution effect on benthic foraminiferal shell chemistry: Delta-C-13, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau. Paleoceanography 170, 417– 432. Miller K. G., Fairbanks R. G., and Mountain G. S. (1987) Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion. Paleoceanography 2, 1–19. Mitsuguchi T., Matsumoto E., Abe O., Uchida T., and Isdale P. J. (1996) Mg/Ca thermometry in coral skeletons. Science 274, 961– 963. Nu¨ rnberg D., Bijma J., and Hemleben C. (1996) Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperature. Geochim. Cosmochim. Acta 60, 803– 814. Reeder R. J. (1996) Interaction of divalent cobalt, zinc, cadmium, and barium with the calcite surface during layer growth. Geochim. Cosmochim. Acta 60, 1543–1552. Rosenthal Y., Boyle E. A., and Slowey N. (1997) Environmental controls on the incorporation of Mg, Sr, F and Cd into benthic foraminiferal shells from Little Bahama Bank: Prospects for thermocline paleoceanography. Geochim. Cosmochim. Acta 61, 3633– 3643. Rosenthal Y., Field F., and Sherrell R. M. (1999) Precise determination of element/calcium ratios in calcareous samples using sector field inductively coupled plasma mass spectrometry. Analyt. Chem. 71, 3248 –3253. Russell A. D., Emerson S., Mix A. C., and Peterson L. C. (1994) The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content. Paleoceanography 11, 649 – 663. Shackleton N. J. and Kennett J. P. (1975) Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: Oxygen and carbon isotope analyses in DSDP Sites 277, 279 and 281. Initial Reports of the Deep Sea Drilling Project 29, 743–755. Stanley S. M. and Hardie L. A. (1998) Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeog. Palaeoclimatol. Palaeoecol. 144, 3–19. Stute M., Forster M., Frischkorn H., Serejo A., Clark J. F., Schlosser P., Broecker W. S., and Bonani G. (1995) Cooling of tropical Brazil (5°C) during the last glacial maximum. Science 269, 379 –382. Tachikawa K. and Elderfield H. Microhabitat effects on Cd/Ca and␦13C of benthic foraminifera Earth Planet. Sci. Lett. (in review). Toyofuku T., Kitazato H., Kawahata H., Tsuchiya M., and Nohara M. (2000) Evaluation of Mg/Ca thermometry in foraminifera: Comparison of experimental results and measurements in nature. Paleoceanography 15, 456 – 464. van Morkhoven F. P. C. M., Berggren W. A., and Edwards A. S. (1986) Cenozoic cosmopolitan deep-water benthic foraminifera. Bull. Centres Rech. Explor.–Prod. Elf-Aquitaine, Mem. 11, 68 –70. Wilkinson B. H. and Algeo T. J. (1989) Sedimentary carbonate record of calcium-magnesium cycling. Am. J. Sci. 289, 1158 –1194. Wolfe J. A. (1979) Temperature parameters of humid to mesic forests of eastern Asia and relation to forests of other regions of the northern hemisphere and Australasia. U.S. Geological Survey Professional Paper 1106, 1–37. Zimmermann H. (2000) Tertiary seawater chemistry: Implications from primary fluid inclusions in marine halite. Am. J. Sci. 300, 723–767.
Pergamon
PII S0016-7037(02)00941-9
Benthic foraminiferal Mg/Ca-paleothermometry: A revised core-top calibration CAROLINE H. LEAR,1,* YAIR ROSENTHAL,1 and NIALL SLOWEY2 1
Institute for Marine and Coastal Sciences, and Department of Geology, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901, USA 2 Department of Oceanography, Texas A&M University, College Station, TX 77843, USA (Received October 15, 2001; accepted in revised form April 24, 2002)
Abstract—Core-top samples from different ocean basins have been analyzed to refine our current understanding of the sensitivity of benthic foraminiferal calcite magnesium/calcium (Mg/Ca) to bottom water temperatures (BWT). Benthic foraminifera collected from Hawaii, Little Bahama Bank, Sea of Okhotsk, Gulf of California, NE Atlantic, Ceara Rise, Sierra Leone Rise, the Ontong Java Plateau, and the Southern Ocean covering a temperature range of 0.8 to 18°C were used to revise the Cibicidoides Mg/Ca-temperature calibration. The Mg/Ca–BWT relationship of three common Cibicidoides species is described by an exponential equation: Mg/Ca ⫽ 0.867 ⫾ 0.049 exp (0.109 ⫾ 0.007 ⫻ BWT) (stated errors are 95% CI). The temperature sensitivity is very similar to a previously published calibration. However, the revised calibration has a significantly different preexponential constant, resulting in different predicted absolute temperatures. We attribute this difference in the preexponential constant to an analytical issue of accuracy. Some genera, notably Uvigerina, show apparently lower temperature sensitivity than others, suggesting that the use of constant offsets to account for vital effects in Mg/Ca may not be appropriate. Downcore Mg/Ca reproducibility, as determined on replicate foraminiferal samples, is typically better than 0.1 mmol mol⫺1 (2 S.E.). Thus, considering the errors associated with the Cibicidoides calibration and the downcore reproducibility, BWT may be estimated to within ⫾1°C. Application of the revised core-top Mg/Ca–BWT data to Cenozoic foraminiferal Mg/Ca suggests that seawater Mg/Ca was not more than 35% lower than today in the ice-free ocean at 50 Ma. Copyright © 2002 Elsevier Science Ltd based on a study of surface sediments intersecting the subtropical thermocline has been constructed (Rosenthal et al., 1997). The Rosenthal et al. (1997) calibration was based on a 300 to 1600 m depth transect from the Little Bahama Bank, covering a temperature range of 4 to 18°C. Support for calcite Mg/Ca as a paleotemperature proxy is found in laboratory experiments (Katz, 1973; Burton and Walter, 1991; Hartley and Mucci, 1996) and empirical and culture studies of marine biogenic calcites such as ostracodes (Dwyer et al., 1995), corals (Mitsuguchi et al., 1996), and foraminifera (Nu¨rnberg et al., 1996; Lea et al., 1999; Toyofuku et al., 2000). The temperature dependence of foraminiferal Mg/Ca probably involves a temperature effect on both the inorganic distribution coefficient, and also the physiologic processes that affect the uptake of Mg into the calcite test (Rosenthal et al., 1997). The published calibration has been used in both Quaternary and Cenozoic studies of climate change (Lear et al., 2000; Billups and Schrag, in press; Martin et al., in press), but is in need of refinement for several reasons. (1) The current calibration contains no Mg/Ca data for BWT less than 4°C, which reflects most of the modern deep ocean. (2) The calibration is based on only one benthic foraminiferal species from one locality. (3) There is significant scatter at the high temperature end of the published calibration, and it is not clear whether this is expected in all foraminifera living at these temperatures, or if it is caused by a process specific to the Little Bahama Bank. Reducing this scatter would improve the accuracy of temperature estimates made using the calibration. (4) The foraminifera analyzed for the published calibration were not prepared using the rigorous cleaning technique now commonly used by most laboratories (Boyle and Keigwin, 1985/86). (5) Subsequent analyses of core-top benthic foraminiferal Mg/Ca are not in agreement with the
1. INTRODUCTION
Evaluation of past climate change is dependent on proxy records for physical and chemical conditions of Earth’s past. Many different proxies for marine and continental temperatures exist, including approaches as diverse as coral strontium/calcium thermometry, groundwater noble gas measurements, and leaf margin analysis (Wolfe, 1979; Beck et al., 1992; Stute et al., 1995). No single proxy used in isolation is ideal; this observation is exemplified by the uncertainties associated with perhaps the most reliable and established quantitative proxy for past global temperature change, the oxygen isotopic composition of benthic foraminifera (Shackleton and Kennett, 1975; Miller et al., 1987). The main uncertainty associated with interpreting such records stems from the fact that the oxygen isotopic composition of foraminiferal calcite is dependent on both the temperature and also the oxygen isotopic composition of the seawater in which the foraminifera lived. The oxygen isotopic composition of seawater is largely determined by global ice volume, because the lighter isotope of oxygen is preferentially stored in ice sheets, although additional geographical variations in the oxygen isotopic composition of seawater associated with salinity variations also exist. A salinity-independent quantitative paleothermometer would therefore not only have the advantage of determining past ocean temperatures, but also global and local variations in seawater salinity. The magnesium/calcium (Mg/Ca) of benthic foraminiferal calcite has been proposed as such a proxy for bottom water temperatures (BWT), and an empirical temperature calibration
* Author to whom correspondence ([email protected]).
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C. H. Lear, Y. Rosenthal, and N. Slowey Table 1. Core-top locations. Latitude
Longitude
Depth (m)
BWT (°C)a
26.23°N 26.23°N 26.23°N 26.24°N 26.24°N 26.23°N 26.23°N 26.16°N 26.14°N 26.12°N
Bahamas 77.65°W 77.66°W 77.66°W 77.68°W 77.69°W 77.69°W 77.70°W 77.72°W 77.74°W 77.75°W
301 433 529 580 668 735 830 1243 1312 1585
18.38 16.55 14.20 13.31 11.49 9.86 8.20 4.50 4.35 4.05
BC BC BC BC BC BC BC BC BC BC
MC047 MC032 MC046 MC017 MC039 MC028 MC037 MC139 MC029 MC138 MC137 MC042 MC135
20.78°N 20.92°N 20.80°N 20.81°N 20.88°N 20.81°N 20.92°N 20.88°N 20.84°N 20.87°N 20.77°N 20.77°N 20.72°N
Hawaii 157.10°W 157.10°W 157.05°W 157.07°W 159.15°W 157.12°W 157.20°W 157.22°W 157.20°W 157.33°W 157.32°W 157.32°W 157.32°W
194 320 402 550 715 797 931 1095 1175 1775 2578 2583 2682
17.26 12.00 8.74 6.07 5.02 4.60 4.28 3.93 3.76 2.36 1.76 1.76 1.75
MC MC MC MC MC MC MC MC MC MC MC MC MC
BC30 BC32 BC34–91 GGC27 GGC26 GGC15
52.05°N 53.53°N 49.13°N 49.60°N 49.28°N 48.60°N
Okhotsk Sea 147.52°E 144.55°E 150.33°E 150.17°E 150.32°E 150.42°E
1310 1000 1227 995 1112 1980
2.20 2.31 2.27 2.28 2.3 1.97
BC BC BC GGC GGC GGC
26.00°N 26.82°N 30.57°N 30.68°N 26.52°N 27.90°N 27.93°N 26.08°N 27.62°N 30.98°N
Gulf of California 110.67°W 110.22°W 114.10°W 114.12°W 110.50°W 111.65°W 111.80°W 110.83°W 111.93°W 114.17°W
1295 527 154 190 1330 655 552 1020 1442 210
NBP9802 MC5 NBP9802 MC7 NBP9802 MC8
63.17°S 60.28°S 58.69°S
Southern Ocean 169.85°W 170.02°W 169.98°W
2861 3995 4324
1.2 0.84 0.84
MC MC MC
NEAP 13B NEAP 19B
58.93°N 52.77°N
NE Atlantic 24.40°W 30.33°W
2546 3283
3.3 2.7
BC BC
Core OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2 OC205-2
BC79 BC77 BC76 BC48 BC52 BC69 BC51 BC57 BC60 BC61
MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 MW98-13 Nes25-1 Nes25-1 Nes25-1 Nes25-1 Nes25-1 Nes25-1 AII AII AII AII AII AII AII AII AII AII
125-8 125-8 125-8 125-8 125-8 125-8 125-8 125-8 125-8 125-8
GGC16 BC6 GGC73 GGC71 GGC13 BC43 GGC47 GGC24 GGC53 GGC69
3.5 8.1 14 13 3.0 7.0 8.0 4.5 3.0 13.0
Core typeb
GGC BC GGC GGC GGC BC GGC GGC GGC GGC
a Source of temperatures: Little Bahama Bank: Rosenthal et al., 1997; Hawaii: shipboard CDT (conductivity, density, temperature) data; Sea of Okhotsk: Keigwin, 1998; Gulf of California: shipboard CTD data; Southern Ocean: estimated from GEOSECS station 290; NE Atlantic: estimated from GEOSECS station 25. b BC ⫽ box core; MC ⫽ multicore; GGC ⫽ giant gravity core.
original calibration (Martin et al in press). For these reasons we have refined the existing calibration by studying numerous species from different ocean basins, covering a wider temperature range, and have prepared all samples using a rigorous cleaning technique, which limits contamination from clays, organic matter, and metal oxides.
2. METHODS 2.1. Sampling Strategy Samples were obtained from six different geographical areas: the Little Bahama Bank, Hawaiian Islands, Sea of Okhotsk, Gulf of California, the Southern Ocean, and the NE Atlantic (Table 1). The core-
Benthic foraminiferal Mg/Ca-temperature calibration
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2.2. Analytical Techniques
Fig. 1. Effect of solution Ca concentration on measured Mg/Ca of a standard. True standard Mg/Ca of 5.34 mmol mol⫺1 is shown by horizontal line. Shaded area represents typical sample Ca concentrations. The external standard used to correct for instrumental mass discrimination has a Ca concentration of 4 mmol L⫺1. Vertical black bar represents long-term analytical precision (⫾1 standard deviation) determined from two consistency standards with Mg/Ca of 2.40 mmol mol⫺1 and 6.10 mmol mol⫺1.
tops cover a range in BWT of 0.8 to 18°C and a water depth range of 194 to 4324 m. Sediments were disaggregated by shaking overnight in deionized water and wet-sieved through 60-m mesh. The Hawaii samples were Rose Bengal stained immediately after collecting the sediments to separate live and dead individuals (Corliss and Emerson, 1990). Samples were dry-sieved to obtain typically between 6 and 20 benthic foraminifera, including Cibicidoides, Melonis, Uvigerina, Planulina, and Gyroidinoides genera from ⬎250 m size fraction. The species we refer to as Cibicidoides wuellerstorfi is also called Planulina wuellerstorfi in the literature. The foraminiferal tests were cleaned using a protocol to remove clays, organic matter, and metal oxides (Boyle and Keigwin, 1985/86). The foraminifera were gradually dissolved in trace metal clean 0.065N HNO3 (Seastar, Vancouver, BC, Canada), and 100 L of this solution was diluted with 300 L trace metal clean 0.5N HNO3 to obtain a Ca concentration of 3 ⫾ 1 mmol L⫺1. More details of this method are available from the author upon request.
Samples were analyzed using a Finnigan MAT (Bremen, Germany) Element Sector Field Inductively Coupled Plasma Mass Spectrometer (ICP-MS) operated in low resolution (m/⌬m ⫽ 300) following the method outlined in Rosenthal et al. (1999). However, since the publication of the earlier work there have been some changes in the operational mode of the instrument. Foremost is the addition of a guard electrode between the RF coil and the torch (i.e., the plasma) (Appelblad et al., 2000). The addition of a guard electrode significantly improved the instrument sensitivity, though not equally throughout the entire mass range. However, it also had other consequences. Notable is the significant increase in the magnitude of matrix-induced mass discrimination and its effect on Mg/Ca determination. In our previous study, conducted before installation of the guard electrode, we have found that Mg/Ca changes by ⬃0.003 mmol mol⫺1 for a 1 mmol L⫺1 change in the Ca concentration of the solution (Rosenthal et al., 1999). In this study we have used the guard electrode to obtain a greater sensitivity for Cd/Ca analysis, which has led to substantially greater dependence of Mg/Ca mass discrimination on the solution’s Ca content. Repeated analysis of five standard solutions with the same Mg/Ca ratio but varying Ca concentrations shows that the ratio of measured Mg/Ca to true Mg/Ca decreases by 6% over our typical sample Ca concentration range of between 0.5 and 4 mmol L⫺1 (Fig. 1). The reasons for increased matrix-induced mass discrimination when operating with a guard electrode are not fully understood. In general, mass-dependent discrimination is related to the preferential transmission of heavier ions, which have higher momentum while travelling through the plasma. The matrix effect results in a mass bias that causes deviations of measured elemental ratios from true ratios. The increased sensitivity obtained with guard electrode reflects increased ion density in the plasma, which in turn leads to greater discrimination between the heavier and more abundant Ca ions and the lighter and less abundant Mg ions. A more detailed description of the effect of the guard electrode on this analytical technique will be presented elsewhere. Direct determination of element ratios from intensity ratios therefore requires control of the sample Ca concentration. Below ⬃0.5 mmol L⫺1 Ca concentration, the deviation of measured Mg/Ca from true Mg/Ca is highly nonlinear (Fig. 1). In such cases, a low Ca concentration standard closely matching the sample matrix is required for the most precise analyses in this concentration range. Our analyses over the past 6 months suggest that the magnitude of the matrix effect on Mg/Ca measurements is stable within and between instrument runs. Furthermore, in contrast to inductively coupled plasma-atomic emission spectroscopy (ICP-AES) methods (de Villiers et al., 2002), we have found
Table 2. Mg/Ca (mmol mol⫺1) of benthic foraminiferal calcite from Little Bahama Bank core-top samples collected during cruise OC205-2. Speciesa Core BC79 BC79 BC79 BC79 BC77 BC77 BC77 BC76 BC48 BC52 BC69 BC51 BC57 BC60 BC61
O. umb
C. pac
2.30 2.26
7.18 9.44 5.53 8.63 6.64 4.03 4.43 3.60 4.17 2.86 2.73 2.81
3.43 2.04
1.50
C. wue
P. bull
B.cf mex
B.cf acul
Uvi spp.
2.63
2.08 2.45 3.33 2.10 1.77
3.25 3.21
1.96 1.65 1.41
M. barl
G.cf med
C. rob
5.65 6.01
6.97
10.8
4.29 5.46
6.30
4.01 3.29 3.86 3.09 2.56 2.12 2.42
G.cf sold
5.28 6.27 5.31 4.51 3.65 3.73
4.51 3.62 3.46 3.73
C. kul
Sip sp.
5.81 6.25 6.50 5.89 5.50 5.18
5.77 5.54 5.39 4.96 4.76 2.80 2.72 2.80
a O.umb ⫽ Oridorsalis umbonatus; C.pac ⫽ Cibicidoides pachyderma; C.wue ⫽ Cibicidoides wuellerstorfi; P.bull ⫽ Pullenia bulloides; B.cf mex ⫽ Bulimina cf. mexicana; B.cf acul ⫽ Bulimina cf. Aculeata; Uvi spp. ⫽ Uvigerina spp; M.barl ⫽ Melonis barleeanum; G.cf sold ⫽ Gyroidinoides cf soldanii; G. cf med ⫽ Gyroidinoides cf. mediceus; C.kul ⫽ Cibicidoides kullenbergi; C.rob ⫽ Cibicidoides robertsonianus; Sip.sp ⫽ Siphonina sp.
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Fig. 2. Direct comparison of benthic foraminiferal Mg/Ca data measured by ICP-MS (y-axis) with those measured by FAAS (Rosenthal et al., 1997) (x-axis) from the same Little Bahama Bank core-tops. Where more than one sample was analyzed from a single box core, we have plotted the mean and ⫾ 1 standard deviation of all analyses.
that this relationship is independent of Mg concentration. The corrections are typically ⬍0.1 mmol mol⫺1 Mg/Ca. Instrument precision was determined by repeated analysis of three consistency standards covering a range of 1 to 6 mmol mol⫺1 over the course of this study. The precision of the consistency standard with Mg/Ca of 1.10 mmol mol⫺1 was ⫾3.7% (r.s.d.); the precisions of the consistency standards with Mg/Ca of 2.40 mmol mol⫺1 and 6.10 mmol mol⫺1 were ⫾1.5% and ⫾1.6% respectively. The unpublished data from the two NE Atlantic cores were prepared and analyzed separately by simultaneous ICP-AES at Cambridge University. It has since been demonstrated that the Ca matrix effect of this instrumental method varies with solution Mg/Ca. A method has been developed which accounts for this matrix effect by calibrating the instrument using raw intensity ratios rather than elemental intensities (de Villiers et al., 2002). Although this new method was not employed on the data presented here, two factors imply that this matrix effect does not seriously compromise the accuracy of the data presented here. The first is that the batch runs of benthic foraminiferal Mg/Ca covered a relatively narrow range of Mg/Ca, e.g., 1.3 to 2.7 mmol mol⫺1 for this study, compared with 0.5 to 18 mmol mol⫺1 (Fig. 4a in de Villiers et al., 2002). Secondly, for each batch run, pairs of lines were chosen which optimized the accuracy of a standard solution with similar Mg/Ca as the samples. 2.3. Interlaboratory Comparison A consistent offset between two of the core-top benthic foraminiferal Mg/Ca data sets used here to revise the temperature calibration has been documented (Martin et al in press). Martin et al. (in press) compiled data measured by flame atomic absorption spectrophotometry (FAAS) from Rosenthal et al. (1997) with unpublished FAAS analyses also made by Y. Rosenthal at MIT. Martin et al. (in press) note that these data have consistently higher Mg/Ca than samples analyzed by ICP-MS at the University of California, Santa Barbara (UCSB). The FAAS data represent samples cleaned using ultrasonication plus acid
Fig. 3. Depth transect of G. ruber (open circles and crosses) and C. pachyderma (closed circles) Mg/Ca from surface sediments of the Little Bahama Bank.
leaches in addition to samples cleaned using the full Boyle and Keigwin (1985/86) method. All the ICP-MS data represent samples that were cleaned using methods set out in Boyle and Keigwin (1985/86). Interestingly, Ceara Rise (Atlantic) core-top benthic foraminiferal Mg/Ca analyzed by ICP-MS (Martin et al in press) are also lower (by a factor of ⬃0.63) than those analyzed by FAAS (Russell et al., 1994). Data presented here (analyzed by ICP-MS at Rutgers University and by ICP-AES at Cambridge University) are in good agreement with the Martin et al. (in press) data analyzed by ICP-MS at UCSB. The offset between ICP-MS and FAAS data is observed for all species analyzed from both warm (e.g., Little Bahama Bank) and cold (e.g., Ceara Rise) core-tops regardless of whether the full or shortened cleaning method was applied. This discrepancy reflects an issue of accuracy rather than precision, most likely caused by an instrumental bias, although the exact nature of this bias has not been determined. It is noteworthy that the different data sets do not show such an offset for Sr/Ca. To quantify the offset in Mg/Ca between the FAAS and the ICP data sets, we analyzed benthic foraminifera from the same core-tops as Rosenthal et al. (1997). Rosenthal et al. (1997) used a two-point linear calibration fit on the FAAS. Therefore, the accuracy error of all samples may be expressed as a single factor F, where (F ⫽ true Mg/Ca/measured Mg/Ca). Plotting our new benthic foraminiferal Mg/Ca against the original data for the same species from the same Little Bahama Bank core-tops, we find that F ⫽ 0.73 (Fig. 2). We use F (⫽ 0.73) to correct
Benthic foraminiferal Mg/Ca-temperature calibration
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Table 3. Mg/Ca (mmol mol⫺1) of benthic foraminiferal calcite from core-tops collected from Hawaii Islands, cruise MW98-13. Speciesa Core MC47 MC32 MC46 MC46 MC17 MC39 MC28 MC37 MC37 MC139 MC29 MC138 MC138 MC138 MC138 MC137 MC137 MC42 MC42 MC135 MC135
O. umb
C. pach
C. wue
4.18us 3.44us 2.02us
5.85us
1.38us
C. comp
Plan sp.
Uvi groo
Uvi spiky
Uvi smoo
Gyroi spp.
1.64us 3.14us 2.13us
2.06us 1.51s
1.68us
1.68s
2.01us
1.53us 1.41us 1.36s 1.16s
1.48us 0.98s 0.92s 1.84us 1.00us 0.99us 1.03s 1.05us 0.95s 0.96us
1.27us
1.47s 1.50us
1.43us
2.74s 3.03s
1.29us 1.11us
a O.umb ⫽ Oridorsalis umbonatus; C.pach ⫽ Cibicidoides pachyderma; C.wue ⫽ Cibicidoides wuellerstorfi; C.comp ⫽ Cibicidoides compressus; Plan sp. ⫽ Planulina sp.; Uvi groo ⫽ a grooved Uvigerina species; Uvi spiky ⫽ a spiky Uvigerina species; Uvi smoo ⫽ a smooth Uvigerina species; Gyroi spp. ⫽ Gyroidinoides spp. Superscripts s and us represent samples composed of stained and unstained (Rose Bengal) individuals respectively.
all the FAAS data as all were analyzed using the same method and standards. 2.4. Scatter within the Little Bahama Bank Samples Benthic foraminiferal Mg/Ca from the shallow sites of the Little Bahama Bank are very scattered (Rosenthal et al., 1997). To determine whether this scatter is a result of (1) downslope transport of tests, (2) contamination by high-Mg calcite, or (3) natural variability, we analyzed planktonic foraminifera (white and pink Globigerinoides ruber) from the same Little Bahama Bank depth transect. About 30 planktonic foraminifera per sample were picked from the same size fraction (250 to 355 m), and prepared and analyzed (by ICP-MS) using the same protocol as the benthic foraminifera. All but one of the planktonic foraminifera samples show constant Mg/Ca with water depth and display little scatter in Mg/Ca (Fig. 3). One sample of white G. ruber from the shallowest site has anomalously high Mg/Ca, probably a result of contamination by secondary precipitation of high-Mg calcite, a common feature of Little Bahama Bank sediments. We consider it likely that at least part of the scatter in the Little Bahama Bank benthic foraminiferal Mg/Ca is caused by some combination of downslope transport and/or high-Mg calcite contamination. Therefore in compiling data for the temperature calibration we have rejected three samples from the shallow sites of the Little Bahama Bank that have Mg/Ca in the upper 5 percentile of the standard error. 3. RESULTS
Benthic foraminiferal Mg/Ca from the study sites show a general decrease with increasing water depth, consistent with the postulated temperature control on calcite Mg/Ca (Figs. 4 – 6 and Tables 2– 4). No single species analyzed in this study covers the full bathymetric range, making it difficult to compare precisely the temperature sensitivities of different species. However, many species were found at more than one site, and
Oridorsalis umbonatus was analyzed from every region. Fig. 4 depicts Mg/Ca of eight benthic foraminiferal species from a water depth transect on the Hawaiian margin. Cibicidoides and Planulina species appear to have similar temperature sensitivities. One C. pachyderma sample has a relatively high Mg/Ca at around 1750 m water depth. This anomalous value could represent individuals transported downslope, as C. pachyderma primarily inhabits the upper bathyal region (200 to 600 m water depth) although it has been documented in abyssal sediments (van Morkhoven et al., 1986). Living (stained) and dead (unstained) foraminifera from the Hawaii samples were analyzed separately (Table 3), and no consistent differences in calcite Mg/Ca were found between the two groups. Eleven benthic foraminiferal species from the Little Bahama Bank core-tops are shown in Fig. 5. The C. pachyderma species in Fig. 5 is equivalent to C. floridanus of Rosenthal et al. (1997). The considerable scatter in the previously published Mg/Ca data of C. pachyderma from shallow (⬍600 m) water depths of the Little Bahama Bank is also seen in these new data, suggesting that the scatter was not caused by the shorter cleaning technique. Fig. 6 depicts Mg/Ca of seven species of benthic foraminifera covering a depth transect from the Gulf of California, five species of benthic foraminifera from the Okhotsk Sea, and five species from two box cores collected in the North Atlantic. Six benthic foraminiferal species were analyzed from multicore core-tops from the Southern Ocean to provide data for the cold end of the temperature calibration. Short (0 –20 cm) downcore records for these three sites suggest that the core-tops are well bioturbated and that downcore Mg/Ca is relatively constant with depth (Fig. 7; Table 5). Foraminifera from the upper 5 cm of these cores were used in the calibration compilation.
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Table 4. Mg/Ca (mmol mol⫺1) of benthic foraminiferal calcite from core-tops collected from the Gulf of California, the Sea of Okhotsk and the NE Atlantic. Speciesa Core
O. umb
GGC16 BC6
1.22
GGC73 GGC71 GGC13 BC43 GGC47 GGC47 GGC24 GGC24 GGC53 GGC53 GGC69
B. mex
P. bull
C. wue
Epis sp.
P. arim
Plan spp.
M. park
M. pom
M. bar
Gulf of California 1.00 2.67 2.37 1.15
1.05 1.74 1.43 0.92
1.22 1.24
1.29
1.39 2.60
Uvi spp.
Gyr spp.
1.14 12.4 * 2.34 1.46 1.46 1.40 1.29 1.49 0.77 0.82 0.92 0.98 2.85
1.50
1.60
Sea of Okhotsk BC30 BC32 BC32 BC32 BC32 BC3491 GGC27 GGC27 GGC27 GGC27 GGC27 GGC26 GGC26 GGC26 GGC26 GGC15 GGC15 GGC15
1.09
13B 13B 13B 19B 19B
1.84
0.91
1.28 1.33
0.75 0.84 0.86 0.99 0.86
1.42
0.68 0.83 0.89 1.10 1.04 0.81 0.85 0.87 0.83 1.42 1.16 1.00
1.80
NE Atlantic 2.74 2.32
1.91 1.84
1.62 1.69 1.55 1.30
1.46 1.48 1.45
1.69
Asterisk denotes rejected data. a O.umb ⫽ Oridorsalis umbonatus; B.mex ⫽ Bulimina mexicana; P.bull ⫽ Pullenia bulloides; C.wue ⫽ Cibicidoides wuellerstorfi; Epis sp. ⫽ Epistominella sp.; P.arim ⫽ Planulina ariminensis; Plan spp. ⫽ Planulina spp. M.park ⫽ Melonis parkerae; M.pom ⫽ Melonis pompilioides, M.bar ⫽ Melonis barleeanum; Uvi spp. ⫽ Uvigerina spp.; Gyr spp. ⫽ Gyroidinoides spp.
4. DISCUSSION
4.1. Compilation of Core-top Data There are several possible approaches to fitting a curve through the compiled data and determining a Mg/Ca-temperature calibration. Thermodynamic considerations suggest that the Mg/Ca-temperature relationship is best described by an exponential fit (Rosenthal et al., 1997). In addition, Mg/Catemperature relationships determined for planktonic foraminifera are exponential in form (Nu¨ rnberg et al., 1996; Lea et al., 1999; Mashiotta et al., 1999; Elderfield and Ganssen, 2000), although we find no statistical evidence to favor an exponential fit over a linear fit in this study.
Exponential fits through the uncorrected FAAS data and the ICP-MS data have similar exponential constants, yet the preexponential constants are different by a factor of 0.77 (Fig. 8a). This factor is similar to that obtained by direct comparison of benthic foraminiferal Mg/Ca data measured by FAAS and ICP-MS from the same Little Bahama Bank box cores (Fig. 2), supporting our view that the difference between the two data sets is an issue of analytical bias. As argued in section 2.4, we believe that the three high Mg/Ca ratios from Little Bahama Bank are contaminated with high-Mg calcite and/or have been transported downslope. The other sample rejected is also outside the 95% confidence limit of the standard error. To determine the most robust Mg/Ca-temperature calibration, we have
Benthic foraminiferal Mg/Ca-temperature calibration
Fig. 4. Mg/Ca of benthic foraminifera collected from Hawaii multicore core-tops plotted against water depth. The form of the temperature profile is shown in gray.
Fig. 5. Benthic foraminiferal Mg/Ca from core-tops collected on the Little Bahama Bank plotted against water depth. The form of the temperature profile is shown in gray.
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C. H. Lear, Y. Rosenthal, and N. Slowey
Fig. 6. Benthic foraminiferal Mg/Ca plotted against water depth for (a) Gulf of California, (b) Sea of Okhotsk, (c) NE Atlantic. The form of the temperature profile is shown in gray.
compiled all FAAS (corrected as described in section 2.3) and ICP-MS core-top data (Rosenthal et al., 1997; Martin et al., in press; this study) for several Cibicidoides species (Fig. 8b). This revised Mg/Ca-temperature calibration is based on C. wuellerstorfi, C. pachyderma, C. compressus, and a wuellerstorfi-like Cibicidoides. Data from C. kullenbergi and C. robertsonianus were not used for this calibration. This compilation represents samples from Little Bahama Bank, Hawaii, Southern Ocean, NE Atlantic, Ceara Rise, Sierra Leone Rise, and the Ontong Java Plateau. Fitting an exponential curve through these data results in the equation: Mg/Ca ⫽ 0.867 ⫾ 0.049 exp (0.109 ⫾ 0.007 ⫻ BWT), R2 ⫽ 0.94
(1)
The errors in Eqn. 1 represent the 95% confidence interval for the exponential and preexponential constants. The standard error of the regression is 1.7°C. However, because downcore records do not display similar scatter (Fig. 7; Martin et al in press), the scatter in the core-top data likely reflects the nature of the sampling for the calibration. An alternative approach to fitting a calibration curve is to assume that only the lowest Mg/Ca from the Little Bahama Bank sites are reliable, and that higher ratios are falling on a mixing line between unaltered and
reworked or contaminated samples. An exponential curve fit through the compiled data, using only the lowest measured Mg/Ca from the Little Bahama Bank sites of 13°C and warmer, yields an equation that is consistent within error of Eqn. 1.. 4.2. Species Effects Melonis spp., Oridorsalis umbonatus, and most Planulina spp. appear to have a similar Mg/Ca–BWT relationship as the Cibicidoides calibration described above (Fig. 9a). However, other species have different temperature responses (Fig. 9b), and these are summarized in Table 6. Considering the large genetic variability within benthic foraminifera, such differences are perhaps not surprising, yet the causes of such “vital effects” apparently superimposed on the temperature control of test Mg/Ca are unclear. One possibility is that the foraminiferal microhabitats are partly responsible. Variations in pore-water chemistry within the upper few centimeters of the sediment column have been shown to cause differences in trace metal compositions between epifaunal and infaunal benthic foraminiferal calcite (Tachikawa and Elderfield, in review). This mechanism would result in a set of calibration equations with similar temperature sensitivities (A, Table 6) but with preex-
Benthic foraminiferal Mg/Ca-temperature calibration
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Fig. 7. Downcore records of benthic foraminferal Mg/Ca from three multicores taken from the Southern Ocean. The 2 S.E. of Mg/Ca of Oridorsalis umbonatus, Nuttallides umbonifera, and Melonis pompilioides from MC7 and MC8 is less than or equal to ⫾0.1 mmol/mol.
ponential constants (B, Table 6) being a function of microhabitat. This is not supported by our data which suggest that Uvigerina spp. and Planulina ariminensis (infaunal and epifaunal species respectively) both have apparently lower temperature sensitivity (i.e., % change in Mg/Ca per degree Celsius) to the majority of the other species studied here. In addition, the preexponential constants show a significant variation within the epifaunal species. Another possibility is that the differences in the Mg/Ca-temperature responses between species result from differences in the architecture of the calcite test. Coprecipitation experiments have shown that the partition coefficient of divalent cations into calcite is strongly affected by the relative proportions of vicinal faces on growth spirals, which in turn are affected by crystal size and shape (Reeder, 1996). Alternatively, the species offsets could result from differences in the amount of fractionation of trace metals in internal calcification pools (Elderfield et al., 1996) or different growth rates, perhaps caused by varying metabolic rates between species. It is difficult to test the last three possibilities, but it seems clear that whatever is responsible for the differences between species, temperature remains an overriding control on foraminiferal Mg/Ca. 4.3. Comparison of Mg/Ca-Temperature Calibrations Mg/Ca-temperature calibrations for planktonic foraminifera (Nu¨ rnberg et al., 1996; Lea et al., 1999; Mashiotta et al., 1999; Elderfield and Ganssen, 2000) are shown in Table 6 for com-
parison with the benthic foraminiferal calibrations presented here. The temperature sensitivity is similar for both benthic and planktonic foraminifera. Lear et al. (2000) estimated the preexponential constant (B) for their primary benthic foraminiferal species O. umbonatus to calculate Cenozoic BWT. To do this, they assumed that the exponential constant (A) in the Rosenthal et al. (1997) calibration was robust, and calculated B using temperature estimated from ␦18O data from the ice-free early Paleogene. Using a modeled output of seawater Mg/Ca at 50 Ma (Wilkinson and Algeo, 1989) gave B ⫽ 1.06. This estimate is similar to the preexponential constant determined here from core-top samples (1.01; Table 6). Although the biased nature of the FAAS data used for the Rosenthal et al. (1997) calibration had not been realized, this procedure essentially eliminated the effect of the analytical bias on the calculated Cenozoic BWT. 4.4. Constraints on Low-temperature (0 –5 °C) Reconstructions The revised Cibicidoides calibration (Fig. 8b) predicts an increase in Mg/Ca of 0.1 mmol mol⫺1 from 0 to 1°C. This difference is larger than analytical precision, although the scatter in the calibration data suggests that the signal would be lost in the natural variability of samples (Fig. 8). However, paired Mg/Ca and ␦18O downcore records for the Quaternary reveal variations in benthic foraminiferal Mg/Ca on the order of 0.1 mmol mol⫺1 apparently caused by variations in BWT (Martin et al in press). A similar precision is also estimated from
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C. H. Lear, Y. Rosenthal, and N. Slowey
⌬CO32⫺, pressure, and salinity. The scatter in the calibration data is small enough to suggest that temperature is the overriding control on benthic foraminiferal Mg/Ca. However, water depth transects from Ceara Rise (equatorial Atlantic; 3270 to 4670 m, 2.6 to 1.5°C) and the Ontong Java Plateau (equatorial Pacific; 1615 to 2960 m, 2.8 to 1.6°C) show a steep trend of decreasing Mg/Ca with water depth (Russell et al., 1994; Martin et al., in press). This trend is steeper than that predicted by the temperature calibration. Similar water depth trends in ⌬␦13C, Cd/Ca, Ba/Ca, and Sr/Ca from the Ontong Java Plateau have previously been documented and interpreted in terms of dissolution or undersaturation effects (McCorkle et al., 1995; Elderfield et al., 1996). Published benthic foraminiferal ␦18O from the Ontong Java Plateau (McCorkle et al., 1995) and unpublished ␦18O from the Ceara Rise (McCorkle, personal communication) do not show any trend with water depth, arguing against the up-mixing of glacial foraminifera as the cause of the Mg/Ca–water depth trend. The steep trend is from the cold end of the calibration (⬍3°C) where the exponential fit predicts small changes in benthic foraminiferal Mg/Ca. The effects of any additional processes other than temperature on benthic foraminiferal Mg/Ca would be relatively pronounced in benthic foraminifera living at lower temperatures. For this reason, we have also put an exponential fit through the data covering just the ⬎5°C data, and this results in a Mg/Ca–BWT relationship: Mg/Ca ⫽ 1.009 ⫾ 0.187 exp (0.0972 ⫾ 0.132 ⫻ BWT) R2 ⫽ 0.84 Fig. 8. Benthic foraminiferal Mg/Ca vs. temperature. (a) Uncorrected FAAS data (open symbols and dashed curve fit) and ICP-AES plus ICP-MS data (crosses and closed symbols and solid curve fit). The difference in the preexponential constants between the two curve fits results from analytical bias. (b) Cibicidoides spp. Mg/Ca-temperature calibration. Compilation of corrected FAAS, ICP-AES, and ICP-MS data for C. wuellerstorfi, C. pachyderma, C. compressus, and a wuellerstorfi-like Cibicidoides. Triangles represent rejected data lying in the upper 5 percentile of the standard error.
benthic foraminiferal Mg/Ca in the top 15 cm of three cores collected in the Southern Ocean (Fig. 7). The sediments were collected by multicorers so the mixed layer is certainly present. The constancy of benthic foraminiferal Mg/Ca with depth in the sediment suggests that the top 15 cm is relatively homogeneous, presumably resulting from bioturbation. Five or more samples of three species (Oridorsalis umbonatus, Nuttallides umbonifera, and Melonis pompilioides) were analyzed from the top 15 cm of cores MC7 and MC8. The 2 S.E. of these analyses is less than or equal to ⫾0.1 mmol mol⫺1. Therefore, it seems likely that processes superimposed on the temperature control are responsible for some of the scatter in the Mg/Ca calibration data. Considering that the calibration samples were collected from a variety of ocean basins, there are many possible factors in addition to temperature which could potentially affect the Mg/Ca of these samples. These factors include vital effects (e.g., different Cibicidoides species), kinetic effects (e.g., varying growth rates caused by environmental conditions), varying rates of bioturbation and up-mixing of glacial foraminiferal tests, downslope transport of core-top tests, dissolution,
(2)
The exponential constant in Eqn. 2 is significantly lower than that obtained using all the core-top data (Eqn. 1). The standard error of the regression is 1.3°C, smaller than obtained for the entire data set. 4.5. Implications for Cenozoic Seawater Mg/Ca Oridorsalis umbonatus is an extant species that is found in deep-sea sediments spanning the entire Cenozoic. Comparison of O. umbonatus Mg/Ca from a variety of modern oceanographic settings with fossil O. umbonatus Mg/Ca gives some constraint on the extent to which seawater Mg/Ca may have varied through the Cenozoic. Variations in seawater Mg/Ca are caused by changes in the rates of submarine hydrothermal activity, dolomitization, carbonate production, and river fluxes (Drever et al., 1988). These processes have consequent effects on the global carbon cycle (through volcanism, chemical weathering and carbonate production) and climate change (Berner et al., 1983). It has also been proposed that variations in seawater Mg/Ca in the geologic past have forced global oscillations between calcite and aragonite seas, in turn affecting the dominance of marine calcifying organisms (Stanley and Hardie, 1998). Here we use our new Mg/Ca-temperature relationship for O. umbonatus, an extant species for which we have Mg/Ca data from the ice-free Paleogene (Lear et al., 2000) to give a first-order estimate of how seawater Mg/Ca may have varied since 50 Ma. We do this by solving Eqn. 3: Mg/Caforam ⫽
Mg/CaSW-t ⫻ B exp (A ⫻ BWT) Mg/CaSW-0
(3)
where Mg/Caforam is Mg/Ca of O. umbonatus at 49 Ma (Lear et
Benthic foraminiferal Mg/Ca-temperature calibration
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Table 5. Downcore benthic foraminiferal Mg/Ca (mmol mol⫺1) from three Southern Ocean cores. Speciesa Depth (cm) 0–1 1–2 2–3
M.pomp
Gyroid
0.94 0.87 0.74
1.35 1.28 1.37
Cib sp. STA 5 1.03 1.21 1.50
P.bull
O.umb
1.26 1.43
1.14 1.03
N.umb
STA 7 0–1 1–2 2–3 4–5 6–7 14–15 16–17
0.95 0.78 0.76
0.90 1.05
1.03 0.84
1.22 1.10
0.72 0.77 0.75
1.12
0.59
1.30 1.34
0.53 2.31
1.51
1.03 1.33 1.03 0.90 0.95 0.93 0.98
0.30 0.47 0.51 0.30 0.31 0.37 0.57
0.92 0.86 0.88 1.06 0.82 0.86 0.91
0.52 0.29 0.28 0.27 0.32 0.32 0.28
STA 8 0–1 1–2 2–3 2–3 3–4 4–5 9–10
1.03 1.08 1.84
1.51 0.67 0.72 1.04
a M.pomp ⫽ Melonis pompilioides; Gyroid ⫽ Gyroidinoides spp.; Cib sp. ⫽ Cibicidoides sp.; P.bull ⫽ Pullenia bulloides; O.umb ⫽ Oridorsalis umbonatus; N.umb ⫽ Nuttallides umbonifera.
al., 2000) and Mg/CaSW is seawater Mg/Ca at 49 Ma and at the present day (subscript t and 0, respectively), and the constants A and B are as defined in Table 6. We use a BWT of 12.4°C from oxygen isotopes assuming an ice-free world at 49 Ma and assume modern seawater Mg/Ca ⫽ 5.2 mol mol⫺1. Because our O. umbonatus calibration is not very well constrained from our core-top data (R2 ⫽ 0.40; Table 6), we use a range of values for A and B in calculating seawater Mg/Ca at 49 Ma (Table 7). However, even considering uncertainties in the exact values of A and B, we consider it unlikely that seawater Mg/Ca at 50 Ma was less than two-thirds of today’s value. This estimate is in good agreement with the modeled estimates of Wilkinson and Algeo (1989) but is at odds with the Stanley and Hardie (1998) model which predicts seawater Mg/Ca at 49 Ma to have been below their hypothesized calcite–aragonite sea threshold of 2 mol mol⫺1. Support for our estimate is found in the chemistry of fluid inclusions of marine halites. Assuming reasonable estimates of seawater Ca concentration (10 to 15 mmol kg⫺1) and Mg concentrations derived from halite inclusions (Zimmermann et al., 2000), Lowenstein et al. (2001) estimate early Cenozoic seawater Mg/Ca to have been in the range of 2.5 to 3.7 mol mol⫺1. Variations in seawater Mg/Ca clearly remain one of the largest uncertainties in using Mg paleothermometry to determine BWT over long (tens of m.y.) timescales. However, Mg and Ca are conservative elements with long residence times (⬃10 m.y. and ⬃1 m.y. respectively) in the oceans, so that Mg paleothermometry remains a very useful tool for looking at relative temperature changes in BWT throughout the Cenozoic. The Mg/Ca of seawater, or the preexponential constant of the temperature calibration is not required to calculate the magnitude of short-term temperature shifts using Mg paleothermometry.
Fig. 9. (a) Oridorsalis umbonatus, Melonis spp., and Planulina spp. (excluding ariminensis) Mg/Ca vs. temperature. The Cibicidoides spp. calibration is shown as a solid line. (b) Planulina ariminensis and Uvigerina spp. Mg/Ca vs. temperature. The Cibicidoides spp. calibration is shown as a solid line.
3386
C. H. Lear, Y. Rosenthal, and N. Slowey Table 6. Foraminiferal Mg/Ca-temperature calibrations. Calibration equationsa n
Temperature range (°C)
B
A
R2
101
0.8–18.4
0.867
0.109
0.94
10 7 58 42 23
3.0–14.5 2.3–12.0 1.8–18.4 0.8–18.4 0.8–9.9
0.911 0.788 0.924 0.982 1.008
0.062 0.119 0.061 0.101 0.114
0.69 0.96 0.69 0.84 0.40
1.36 1.06
0.10 0.10
1.36 0.53 0.474
0.085 0.10 0.107
0.52
0.10
Species Benthic foraminifera (this study) Cibicidoides including wuellerstorfi, pachyderma, and compressus P. ariminensis Planulina spp. Uvigerina spp. M. barleeanum and M. pompilioides O. umbonatus Benthic foraminifera (previous studies) Cibicidoides (Rosenthal et al., 1997)b O. umbonatus (Lear et al., 2000) Planktonic foraminifera Culture O. universa (Lea et al., 1999) Culture G. bulloides (Lea et al., 1999) Culture and core-tops G. bulloides (Mashiotta et al., 1999) (Mashiotta et al., 1999) Core-tops (Elderfield and Ganssen, 2000)
a The Mg/Ca-temperature relationship for benthic foraminifera may be expressed in the form of a general equation Mg/Ca ⫽ B exp (A ⫻ BWT). R2 is the correlation coefficient. The exponential constant B reflects the temperature sensitivity and is similar for most benthic and planktonic foraminifera. n represents the number of samples used in determining each equation. b This calibration (Rosenthal et al., 1997) appears to be biased by an analytical accuracy issue (see text).
5. CONCLUSIONS
The revised Mg/Ca–BWT calibration based on common Cibicidoides species is described by the exponential equation Mg/Ca ⫽ 0.867 ⫾ 0.049 exp (0.109 ⫾ 0.007 ⫻ BWT). The exponential constant defines the temperature sensitivity of benthic foraminiferal Mg/Ca and is crucial for reconstructing the amplitudes of past BWT variations. Both the exponential and the preexponential constants vary between species. Considering the errors associated with temperature prediction using
Table 7. Estimates of seawater Mg/Ca at 49 Ma. Seawater Mg/Caa A ⫽ 0.11
A ⫽ 0.10
B ⫽ 0.9
4.1
4.6
B ⫽ 1.0
3.7
4.1
B ⫽ 1.1
3.3
3.8
Halite inclusions (Zimmermann, 2000; Lowenstein et al., 2001). Wilkinson and Algeo (1989) Stanley and Hardie (1998)
2.5–3.7 3.9 1.6
a Seawater Mg/Ca (mol mol⫺1), calculated from Eqn. 3 using Mg/Ca of O. umbonatus at 49 Ma ⫽ 2.78 mmol mol⫺1 and temperature ⫽ 12.4°C (Lear et al., 2000). We use a range of values for the Mg/Catemperature calibration constants A and B (Table 6) to give an indication of the error associated with this method. Seawater Mg/Ca was estimated from the magnesium content of halite fluid inclusions; two computer models are shown for comparison.
the Cibicidoides calibration (a combined exponent and preexponent error of 8.6%) and the downcore Mg/Ca reproducibility (2 S.E. ⱕ ⫾0.1 mmol mol⫺1), we suggest that Mg-paleothermometry has the potential to resolve BWT to better than ⫾1°C. The uncertainty in estimating deep water temperature (BWT ⬍3°C) is dominated by the natural variability in downcore foraminiferal Mg/Ca. Therefore, at low temperatures the uncertainty may be reduced by improving the standard errors associated with the downcore record (i.e., increasing the number of replicates). The primary uncertainty in estimating thermocline temperatures (⬎6°C) is, however, determined to a large extent by the scatter in the calibration rather than downcore reproducibility. This scatter is a result of compiling Mg/Ca data of different Cibicidoides species from core-tops of different quality. Therefore, the error in comparing relative, downcore temperature variations should be significantly better than ⫾1°C. While the value of 0.109 (⫾0.007) appears to be a robust estimate of the temperature dependency of Mg uptake by benthic foraminiferal calcite, a question remains regarding the temperature sensitivity below 5°C. Additional processes superimposed on temperature may cause deviations of benthic foraminiferal Mg/Ca from the calibration curve; these would create larger temperature errors at lower temperatures. Changes in seawater Mg/Ca remain one of the largest uncertainties in using Mg paleothermometry to determine BWT over long (tens of m.y.) timescales. Comparison of calibration data with benthic foraminiferal Mg/Ca and ␦18O from the ice-free early Cenozoic indicates that seawater Mg/Ca was not more than 35% lower than today at 49 Ma.
Benthic foraminiferal Mg/Ca-temperature calibration Acknowledgments—We thank Lloyd Keigwin for provision of samples (and some picked foraminifera) from the Sea of Okhotsk and the Gulf of California, Adina Paytan for samples from the Southern Ocean, Paul Field for help with analyses, and Pam Martin and Dan McCorkle for useful discussions. This manuscript was greatly improved by reviews from Peggy Delaney, David Lea, and two anonymous reviewers. This work was supported by a post-doctoral fellowship at IMCS, Rutgers University (CHL) and NSF grants 9810717 and 9819675 (Y.R.) and OCE9712763 (N.S.). Associate editor: D. W. Lea REFERENCES Appelblad P. K., Rodushkin I., and Baxter D. C. (2000) The use of Pt guard electrode in inductively coupled plasma sector field mass spectrometry: Advantages and limitations. J. Anal. At. Spectrom. 15, 359 –364. Beck W. J., Edwards L. R., Ito E., Taylor F. W., Recy J., Rougerie F., Joannot P., and Henin C. (1992) Sea-surface temperature from coral skeletal strontium/calcium ratios. Science 257, 644 – 647. Berner R. A., Lasaga A. C., and Garrels R. M. (1983) The carbonatesilicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Am. J. Sci. 283, 641– 683. Billups K. and Schrag D. P. (2002) Paleotemperatures and ice-volume of the past 27 myr revisited with paired Mg/Ca and stable isotope measurements on benthic foraminifera. Paleoceanography 17 10.1029/2000 PA 000567. Boyle E. A. and Keigwin L. D. (1985/86) Comparison of Atlantic and Pacific paleochemical records for the last 250,000 years: Changes in deep ocean circulation and chemical inventories. Earth Planet. Sci. Lett. 76, 135–150. Burton E. A. and Walter L. M. (1991) The effects of pCO2 and temperature on magnesium incorporation in calcite in seawater and MgCl2-CaCl2 solutions. Geochim. Cosmochim. Acta. 55, 775–785. Corliss B. H. and Emerson S. (1990) Distribution of rose-bengal stained deep-sea benthic foraminifera from the Nova Scotian continental margin and Gulf of Maine. Deep Sea Res. 37, 381– 400. deVilliers S., Greaves, M., Elderfield H. (2002) An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP-AES. Geochem. Geophys. Geosyst. 3, paper number 10.1029/2001GC000169. Drever J. I., Li Y.-H., and Maynard J. B. (1988) Geochemical cycles: The continental crust and the oceans. In Chemical Cycles in the Evolution of the Earth (ed. C. B. Gregor), pp. 17–53. Wiley. Dwyer G. S., Cronin T. M., Baker P. A., Raymo M. E., Buzas J. S., and Correge T. (1995) North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347–1351. Elderfield H., Bertram C. J., and Erez J. (1996) A biomineralization model for the incorporation of trace elements into foraminiferal calcium carbonate. Earth Planet. Sci. Lett. 142, 409 – 423. Elderfield H. and Ganssen G. (2000) Past temperature and ␦18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios. Nature 405, 442– 445. Hartley G. and Mucci A. (1996) The influence of pCO2 on the partitioning of magnesium in calcite overgrowths precipitated from artificial seawater at 25° and 1atm total pressure. Geochim. Cosmochim. Acta 60, 315–324. Katz A. (1973) The interaction of magnesium with calcite during crystal growth at 25–90°C and one atmosphere. Geochim. Cosmochim. Acta 37, 1563–1586. Keigwin L. D. (1998) Glacial-age hydrography of the far northwest Pacific Ocean. Paleoceanography 13, 323–339. Lea D. W., Mashiotta T. A., and Spero H. (1999) Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing. Geochim. Cosmochim. Acta 63, 2369 –2379. Lear C. H., Elderfield H., and Wilson P. A. (2000) Cenozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite. Science 287, 269 –272.
3387
Lowenstein T. K., Timofeeff M. N., Brennan S. T., Hardie L. A., and Demicco R. V. (2001) Oscillations in Phanerozoic seawater chemistry: Evidence from fluid inclusions. Science 294, 1086 –1088. Martin P. A., Lea D. W., Rosenthal Y., Shackleton N. J., Sarnthein M., and Papenfuss T. (2002) Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca. Earth Planet. Sci. Lett. 198, 193–209. Mashiotta T. A., Lea D. W., and Spero H. J. (1999) Glacial-interglacial changes in Subantarctic sea surface temperature and ␦18O-water using foraminiferal Mg. Earth Planet. Sci. Lett. 170, 417– 432. McCorkle D. C., Martin P. A., Lea D. W., and Klinkhammer G. P. (1995) Evidence of a dissolution effect on benthic foraminiferal shell chemistry: Delta-C-13, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau. Paleoceanography 170, 417– 432. Miller K. G., Fairbanks R. G., and Mountain G. S. (1987) Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion. Paleoceanography 2, 1–19. Mitsuguchi T., Matsumoto E., Abe O., Uchida T., and Isdale P. J. (1996) Mg/Ca thermometry in coral skeletons. Science 274, 961– 963. Nu¨ rnberg D., Bijma J., and Hemleben C. (1996) Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperature. Geochim. Cosmochim. Acta 60, 803– 814. Reeder R. J. (1996) Interaction of divalent cobalt, zinc, cadmium, and barium with the calcite surface during layer growth. Geochim. Cosmochim. Acta 60, 1543–1552. Rosenthal Y., Boyle E. A., and Slowey N. (1997) Environmental controls on the incorporation of Mg, Sr, F and Cd into benthic foraminiferal shells from Little Bahama Bank: Prospects for thermocline paleoceanography. Geochim. Cosmochim. Acta 61, 3633– 3643. Rosenthal Y., Field F., and Sherrell R. M. (1999) Precise determination of element/calcium ratios in calcareous samples using sector field inductively coupled plasma mass spectrometry. Analyt. Chem. 71, 3248 –3253. Russell A. D., Emerson S., Mix A. C., and Peterson L. C. (1994) The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content. Paleoceanography 11, 649 – 663. Shackleton N. J. and Kennett J. P. (1975) Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: Oxygen and carbon isotope analyses in DSDP Sites 277, 279 and 281. Initial Reports of the Deep Sea Drilling Project 29, 743–755. Stanley S. M. and Hardie L. A. (1998) Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeog. Palaeoclimatol. Palaeoecol. 144, 3–19. Stute M., Forster M., Frischkorn H., Serejo A., Clark J. F., Schlosser P., Broecker W. S., and Bonani G. (1995) Cooling of tropical Brazil (5°C) during the last glacial maximum. Science 269, 379 –382. Tachikawa K. and Elderfield H. Microhabitat effects on Cd/Ca and␦13C of benthic foraminifera Earth Planet. Sci. Lett. (in review). Toyofuku T., Kitazato H., Kawahata H., Tsuchiya M., and Nohara M. (2000) Evaluation of Mg/Ca thermometry in foraminifera: Comparison of experimental results and measurements in nature. Paleoceanography 15, 456 – 464. van Morkhoven F. P. C. M., Berggren W. A., and Edwards A. S. (1986) Cenozoic cosmopolitan deep-water benthic foraminifera. Bull. Centres Rech. Explor.–Prod. Elf-Aquitaine, Mem. 11, 68 –70. Wilkinson B. H. and Algeo T. J. (1989) Sedimentary carbonate record of calcium-magnesium cycling. Am. J. Sci. 289, 1158 –1194. Wolfe J. A. (1979) Temperature parameters of humid to mesic forests of eastern Asia and relation to forests of other regions of the northern hemisphere and Australasia. U.S. Geological Survey Professional Paper 1106, 1–37. Zimmermann H. (2000) Tertiary seawater chemistry: Implications from primary fluid inclusions in marine halite. Am. J. Sci. 300, 723–767.