Planktonic foraminiferal Cd/Ca: Paleonutrients or paleotemperature?

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−1) 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−1) in surface water phosphate concentrations, then the explanation for lowered glacial pCO2 must be looked for elsewhere.