Distribution and speciation of sulfur in foraminiferal calcite

The sulfur to calcium ratio (S/Ca) of foraminiferal calcite is currently being investigated as an indicator (proxy) of past seawater carbonate ion concentration ([CO32-]), one of the most coveted palaeo-oceanographic parameters needed to reconstruct past atmospheric CO2. Uncertainty surrounding the pathway of uptake, transport and precipitation of sulfur from seawater to foraminiferal calcite impedes the application of this proxy. We propose to investigate the micro-distribution and speciation of sulfur, and its link to Mg, to further the development of this proxy relation. Using μXRF and µXANES analysis on both hyaline and porcelaneous species of foraminifera cultured at different seawater [CO32-] conditions will 1) confirm, for the first time, if sulfur is indeed present as SO42- in foraminiferal shells and reveal how uptake of sulfur changes for different seawater [CO32-], and 2) increase our understanding of anion uptake in both hyaline and porcelaneous calcification pathways.