Magnesium/Calcium ratios in benthic foraminifera from surface sediments@en

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 d[CO3 2-] of bottom water exerts a significant control on dMg/Ca (temperature-corrected) of C. mundulus (dMg/Ca = 0.017*d[CO3 2-] -0.14), while dMg/Ca of P. wuellerstorfi is more likely to be governed by TCO2 (dMg/Ca = -0.007*TCO2 + 15). Since both d[CO3 2-] and TCO2 are closely linked to [CO3 2-], it is inferred that carbonate ion acts as secondary control, after temperature, on benthic shell Mg/Ca below -4°C. A drop in [CO3 2-] by 25 ?mol/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 benthic foraminifers where changes in the carbonate chemistry are present in the paleoceanographic record.

本研究采用采自南大西洋表层沉积物中的现代底栖有孔虫壳体（epibenthic foraminifer tests），即小圆盘卷虫（Cibicidoides mundulus）与伍尔斯特扁平虫（Planulina wuellerstorfi），以建立0~15℃温度区间内的Mg/Ca-温度校准关系。我们得到如下校准方程：对于伍尔斯特扁平虫（P. wuellerstorfi），Mg/Ca（mmol/mol）= 0.830×exp(0.145×BWT（℃，Bottom Water Temperature，底水温度）)；对于小圆盘卷虫（C. mundulus），Mg/Ca（mmol/mol）= 0.627×exp(0.143×BWT（℃）)。然而，部分壳体（尤其是暴露于北大西洋深层水（North Atlantic Deep Water）的壳体）的Mg/Ca比值高于校准方程的预测值。我们的数据表明，小圆盘卷虫经温度校正的Mg/Ca比值（dMg/Ca，temperature-corrected Mg/Ca）主要受底水碳酸根离子浓度（d[CO₃²⁻]）的显著调控，其关系式为：dMg/Ca = 0.017×d[CO₃²⁻] - 0.14；而伍尔斯特扁平虫的dMg/Ca则更易受总二氧化碳浓度（TCO2，Total Carbon Dioxide）调控，关系式为：dMg/Ca = -0.007×TCO2 + 15。由于[CO₃²⁻]与TCO2密切相关，据此可推断，在水温低于-4℃的环境中，碳酸根离子是继温度之后，调控底栖有孔虫壳体Mg/Ca比值的第二大因子。据末次冰盛期（Last Glacial Maximum）的相关研究，在4km水深处，若[CO₃²⁻]降低25 μmol/kg，将使Mg/Ca比值最高下降0.4 mmol/mol，进而导致底水温度估算值偏低约3.5℃。因此，本研究结果提示，在古海洋记录中存在碳酸盐化学变化的研究场景下，使用Mg/Ca温度计重建底栖有孔虫古温度时需谨慎行事。