Mg/Ca variability of the planktonic foraminifera G. ruber s.s. and N. dutertrei from shallow and deep cores determined by electron microprobe image mapping

The Mg/Ca ratio of planktonic foraminifera is widely used for reconstructing sea surface and thermocline temperatures. Dissolution, driven by changes in the bottom water calcite saturation state (a function of water mass chemistry and depth/pressure), lowers the bulk Mg/Ca ratio which results in an implied cooler reconstructed temperature. Here we employ electron microprobe analysis (EMPA) to characterize how dissolution alters shell Mg/Ca variability. Using EMPA we have generated Mg/Ca image maps of two species widely used in paleoceanographic reconstructions: Globogerinoides ruber sensu stricto (G. ruber s.s.) and Neogloboquadrina dutertrei (N. dutertrei). We first characterize the Mg/Ca ratio in the better-preserved shells from a shallow core (2.8 km) and compare those characteristics to the samples from a deep core (4.0 km) obtained from the same locality (Ceara Rise, western equatorial Atlantic). Though preliminary, results suggest EMPA Mg/Ca ratio mapping can reveal characteristics of well preserved shells that can be used to assess changes in the Mg/Ca ratio that occur with dissolution.