SR/CA OF PLANKTIC FORAMINIFERAL CALCITE VARIES WITH PH AND DIC IN CULTURE EXPERIMENTS

Experiments with live foraminifera enable us to quantify environmental controls on the trace element composition of foramininferal calcite. In turn, these culture calibrations provide a means of determining past surface ocean conditions. Here we have assembled the results of culture experiments for three species of symbiont-bearing planktic foraminifera: Globigerinoides ruber, Globigerinoides sacculifer, and Orbulina universa, to evaluate Sr/Ca responses to temperature, salinity, pH, carbonate ion, and dissolved inorganic carbon (DIC) growth conditions. Temperature and salinity do not significantly affect Sr/Ca in any of the species studied. In G. sacculifer and O. universa, additional experiments reveal that the response of Sr/Ca to [CO₃^2-] is ~5-10 times higher when [CO₃^2-] is raised by increasing DIC (and holding pH constant) than when it is raised by increasing pH (and holding DIC constant). We speculate that the difference between our DIC and pH experiments might reflect a [CO₃^2-]/[Ca²⁺]-driven rate influence on Sr incorporation. For expected glacial-interglacial changes in the carbonate system, Sr/Ca changes predicted from these calibrations are too small to reliably detect, but for larger shifts in the carbon cycle (e.g., earlier in the Cenozoic) this parameter may be useful for constraining changes in the carbonate system. When combined with other trace element data from the same experiments, our results suggest that a combination of kinetic and biological influences dominate trace element partitioning in foraminiferal calcite.