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 ( p CO 2 ) 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 (D Cd ) is temperature sensitive (D Cd =0.637 exp 0.15 T ). 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 p CO 2 must be looked for elsewhere.