Benthic foraminifer Mg/Ca anomalies in South Atlantic core top sediments and their implications for paleothermometry

We used modern epibenthic foraminifer tests of Cibicidoides mundulus and Planulina wuellerstorfi from South Atlantic core top sediments in order to establish Mg/Ca-temperature relationships for the temperature range from 0 to 15°C. We obtained the following calibrations: Mg/Ca (mmol/mol) = 0.830*exp(0.145*BWT (°C)) for P. wuellerstorfi, and Mg/Ca (mmol/mol) = 0.627*exp(0.143*BWT (°C)) for C. mundulus. However, a number of tests, especially those bathed in North Atlantic Deep Water, revealed higher Mg/Ca ratios than predicted from the calibration. Our data suggest that Δ[CO32-] of bottom water exerts a significant control on ΔMg/Ca (temperature-corrected) of C. mundulus (ΔMg/Ca = 0.017*Δ[CO32-] - 0.14), while ΔMg/Ca of P. wuellerstorfi is more likely to be governed by TCO 2 (ΔMg/ Ca = -0.007*TCO2 + 15). Since both Delta;[CO32-] and TCO2 are closely linked to [CO32-], it is inferred that carbonate ion acts as secondary control, after temperature, on benthic shell Mg/Ca below ∼4°C. A drop in [CO32-] by 25 μmol/kg at 4 km water depth, as suggested for the Last Glacial Maximum, would decrease Mg/Ca by up to 0.4 mmol/mol, which leads to an underestimation of bottom water temperature by ∼3.5°C. Therefore our results indicate that the Mg/Ca thermometer should be used cautiously for benthicforaminifers where changes in the carbonate chemistry are present in the paleoceanographic record. copyright 2008 by the American Geophysical Union.