Planktonic foraminiferal Cd/Ca ratios

Records of Cd/Ca in planktonic foraminiferal calcite of Globigerinoides bulloides in cores from the Subantarctic region of the Southern Ocean show large glacial-interglacial variations with lower Cd/Ca (by 0.06-0.10 µmol/mol) at glacial times. Interpretation of these records in terms of lower dissolved phosphate and inferred higher glacial nutrient utilization has significant implications for glacial atmospheric carbon dioxide (pCO2) draw-down. However, box core-top data for G. bulloides in the North Atlantic suggest that the incorporation of Cd into planktonic foraminifera relative to seawater (DCd) is temperature sensitive (DCd=0.637 exp 0.15T). When the Subantarctic planktonic Cd/Ca records are corrected for this temperature dependence, they show little or no glacial-interglacial diferences. If, as seems likely, this observation can be interpreted to indicate a minimal change (< 0.5 µmol/kg) in surface water phosphate concentrations, then the explanation for lowered glacial pCO2 must be looked for elsewhere.南大洋亚南极区域各岩芯中布氏球室虫（Globigerinoides bulloides）浮游有孔虫方解石（planktonic foraminiferal calcite）的镉钙比（Cd/Ca）记录，呈现出显著的冰期-间冰期变化特征：冰期时段的Cd/Ca值较间冰期低0.06~0.10 µmol/mol。基于该记录提出的冰期溶解态磷酸盐（dissolved phosphate）含量更低、营养盐利用（nutrient utilization）程度更高的解释，对冰期大气二氧化碳分压（pCO2）降低的机制研究具有重要启示意义。然而，北大西洋G. bulloides的箱式表层岩芯数据（box core-top data）显示，浮游有孔虫对海水中镉的分配系数（DCd）具有温度敏感性，其关系式为DCd=0.637 exp(0.15T)。若对南大洋亚南极区域的浮游Cd/Ca记录进行该温度效应校正后，其冰期-间冰期差异已微弱甚至消失。若该观测结果可合理推断为表层水磷酸盐浓度（surface water phosphate concentrations）变化极小（<0.5 µmol/kg），那么冰期大气pCO2降低的成因则需从其他方向探寻。