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

Because strontium and magnesium occur in seawater with nearly constant rati os to calcium, variations in Sr/Ca and Mg/Ca in modern foraminifer shells a re due to the influence of environmental parameters on calcification. We ha ve cultured two species of planktonic foraminifera, Globigerina bulloides a nd Orbulina universa, to establish the influence of temperature, pH, and sa linity. Experimental results indicate that temperature is the primary contr ol on shell Mg/Ca and that shell Mg/Ca increases exponentially by about 8 t o 10% per degrees C. The exponential rise in shell Mg with temperature mine rs the results from inorganic precipitation experiments and suggests at lea st partial thermodynamic control on Mg incorporation. Both seawater pH and salinity are secondary influences on shell Mg/Ca: -6% per 0.1 pH unit incre ase and +4% per salinity unit increase. Shell Sr/Ca responds far more weakl y 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 potentia l of Mg/Ca in G. bulloides as a paleothermometer. The culturing results sug gest that the standard error of Mg paleothermometry is +/-1.1 degrees C, bu t when the secondary effects of salinity and pH are considered the error in creases to +/-1.3 degrees C. Copyright (C) 1999 Elsevier Science Ltd.