New Caledonian Large Benthic Foraminiferal S/Ca signatures as sulfate seawater proxies: Results from controlled growth experiments

Sulfate (SO₄^2-) is the second most abundant anion in seawater. Nevertheless, its long-term accumulation and variability throughout Earth's history remains largely uncertain. Considering the preservation potential of benthic foraminifera in the fossil record, geochemical signatures of large tropical benthic foraminifera (LBF) are an important tool for paleoclimatic proxies. Considering their significant deposit of high-magnesium calcite within fragile tropical ecosystems largely threatened by climate change, an improved understanding of both their calcification processes and geochemical signatures in the face of climate change environmental variables, is of main interest, particularly as LBF have not been as intensively studied as planktonic - and smaller benthic foraminifera. The study of Marginopora sp., an ubiquitous LBF in tropical regions of the Pacific Ocean, provides a unique opportunity to reconstruct changes in environmental parameters in seawater over time. Cultures of Marginopora sp. from New Caledonian environments were performed with modern and decreased seawater pH values and modern and increased [SO₄^2-]_sw to calibrate impact of seawater sulfate concentration on both S/Ca concentrations and δ³⁴S composition of the S incorporated in the shells of Marginopora sp.. Here, we present S/Ca data and other Element/Calcium ratios derived from LA-Q-ICP-MS (NWR193UC & Thermo Fisher Scientific iCAP-Q) and solution SF-ICP-MS (Thermo Fisher Scientific Element-2) analyses, as well as sulfur isotope data acquired from Isotope Ratio Mass Spectromety (IRMS with Elementar vario EL cube). As already described in previous studies looking at the geochemical signatures of small benthic foraminifera, S/Ca ratios within Marginopora sp. test, increased with increasing seawater S/Ca concentration. However, and contrarily to what was reported previously, the Mg/Ca content of Marginopora was observed to decrease with increasing S/Ca calcite content, highlighting potential differences in calcification disruption between low-Mg and high-Mg calcitic foraminifers when exposed to an increase in seawater sulfate concentrations.