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 mu mol 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 (pCO(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 0.15T). When the Subantarctic planktonic Cd/Ca records are corrected for this temperature dependence, they show little or no glacial-interglacial differences. If, as seems likely, this observation can be interpreted to indicate a minimal change (< 0.5 mu mol kg(-1)) in surface water phosphate concentrations, then the explanation for lowered glacial pCO(2) must be looked for elsewhere.