Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing

摘要

Because strontium and magnesium occur in seawater with nearly constant ratios to calcium, variations in Sr/Ca and Mg/Ca in modern foraminifer shells are due to the influence of environmental parameters on calcification. We have cultured two species of planktonic foraminifera, Globigerina bulloides and Orbulina universa, to establish the influence of temperature, pH, and salinity. Experimental results indicate that temperature is the primary control on shell Mg/Ca and that shell Mg/Ca increases exponentially by about 8 to 10% per deg C. The exponential rise in shell Mg with temperature mirrors the results from inorganic precipitation experiments and suggests at least partial thermodynamic control on Mg incorporation. Both seawater pH and salinity are secondary influences on shell Mg/Ca: -6% per 0.1 pH unit increase and +4% per salinity unit increase. Shell Sr/Ca responds far more weakly to environmental parameters, and the small range observed in shell Sr/Ca relative to measurement precision of the ICP-MS method used here limits how well controls on shell Sr can be determined. Higher temperature, salinity, and pH all appear to increase shell Sr/Ca, most likely through the kinetic influence of calcification. Our culturing results demonstrate the potential of Mg/Ca in G. bulloides as a paleothermometer. The culturing results suggest that the standard error of Mg paleothermometry is +- 1.1 deg C, but when the secondary effects of salinity and pH are considered the error increases to +-1.3 deg C.