Temperature and [CO3 ] Effects on Mg/Ca and Sr/Ca in the Benthic Foraminifera Species of Hoeglundina Elegans and Cibicidoides

Current core-top calibrations suggest that the temperature sensitivity of Mg/Ca in Cibicidoides sp. tests is essentially identical with the 10% increase in Mg/Ca per 1°C observed in many planktonic species. However, there is also evidence suggesting that non temperature-related effects may exert additional control on Mg/Ca in foraminiferal calcite. Here we use core-top samples from Atlantic (Little Bahama Banks, LBB; Arctic) and Pacific (Hawaii, and Indonesia) depth transects to assess the influence of bottom water temperature (BWT) and carbonate saturation levels on metal/Ca ratios in the aragonitic benthic foraminifer Hoeglundina elegans and calcitic Cibicidoides sp. We find that in situ CO3 ion concentrations play an important role in determining the Mg/Ca ratios in Cibicidoides sp. and Mg/Ca and Sr/Ca in H. elegans. The CO3 ion effect is limited, however, only to relatively low saturation levels, whereas at over-saturated levels, temperature exert the dominant effect on these metal ratios. Because we find no significant difference between results obtained on ``live'' and ``dead'' specimens, we propose that these differences are due to primary effects on metal uptake during test formations and not because of dissolution. Based on the new results we reappraise the Mg/Ca-temperature relationship of Cibicidoides sp. and propose a new calibration. We also propose that Sr/Ca and Mg/Ca in H. elegans tests can be used to reconstruct thermocline temperatures. Sr/Ca-thermometry in H. elegans may be especially useful in cases where the use of Mg/Ca thermometry in calcitic foraminfera has been compromised by contamination issues. In both cases, temperature estimates in low carbonate saturation levels may be grossly inaccurate.