Sulfur incorporation in (foraminiferal) calcite

Sulfur over calcium ratio (S/Ca) in foraminiferal calcite has been suggested as a potential tool to reconstruct seawater carbonate ion concentration ([CO₃^2-]). The approach of using sulfur incorporation as a proxy for the carbon system was based on benthic foraminiferal controlled growth experiments, which suggested that foraminifera incorporate more sulfur when there is less [CO₃^2-] available in the seawater. With sulfate ([SO₄^2-]) being proposed to be the dominant form in which sulfur is incorporated in the calcium carbonate of the foraminiferal shells, S/Ca would provide an independent parameter for the reconstruction of seawater inorganic carbon chemistry. To further explore the potential of this proxy, we used five planktonic foraminiferal species collected from the field. S/Ca values in planktonic foraminifera collected from core-top sediments that span a large range of growth conditions (temperature, salinity, [CO₃^2-] and [HCO₃^-]) reveal an opposite trend with [CO₃^2-] compared to the results from the benthic foraminifera culture experiments. Moreover, we found an additional effect of incorporation of Mg on S/Ca ratios, or a combined effect on both. Using the ratio of S to Mg overcomes this issue and S/Mg ratios correlate with [CO₃^2-]. Still, these correlations are likely affected by multiple parameters and/or incorporation pathways other than only SO₄^2- as suggested by our inorganic calcite growth experiments. Results of this study suggest a critical evaluation of the use of foraminiferal S/Ca, considering the aqueous species involved during uptake and potentially combining other elements that may share controls.