Sulfur in foraminiferal calcite as a potential proxy for seawater carbonate ion concentration

Sulfur (S) incorporation in foraminiferal shells is hypothesized to change with carbonate ion concentration [CO₃ ²⁻], due to substitution of sulfate for carbonate ions in the calcite crystal lattice. Hence S/Ca values of foraminiferal carbonate shells are expected to reflect sea water carbonate chemistry. To generate a proxy calibration linking the incorporation of S into foraminiferal calcite to carbonate chemistry, we cultured juvenile clones of the larger benthic species Amphistegina gibbosa and Sorites marginalis over a 350–1200 ppm range of pCO₂ values, corresponding to a range in [CO₃ ²⁻] of 93 to 211 μmol/kg. We also investigated the potential effect of salinity on S incorporation by culturing juvenile Amphistegina lessonii over a large salinity gradient (25–45). Results show S/Ca_CALCITE is not impacted by salinity, but increases with increasing pCO₂ (and thus decreasing [CO₃ ²⁻] and pH), indicating S incorporation may be used as a proxy for [CO₃ ²⁻]. Higher S incorporation in high-Mg species S. marginalis suggests a superimposed biomineralization effect on the incorporation of S. Microprobe imaging reveals co-occurring banding of Mg and S in Amphistegina lessonii, which is in line with a strong biological control and might explain higher S incorporation in high Mg species. Provided a species-specific calibration is available, foraminiferal S/Ca values might add a valuable new tool for reconstructing past ocean carbonate chemistry.