Impact of Seawater Inorganic Carbon Chemistry on Element Incorporation in Foraminiferal Shell Carbonate

Reconstruction of the marine inorganic carbon system relies on proxy signal carriers, such as element/calcium (El/Ca) ratios in foraminiferal shells. Concentrations of boron, lithium, strontium, and sulfur have been shown to vary with carbonate system parameters, but when comparing individual proxy reconstructions based on these elements, they are rarely in complete agreement. This is likely caused by the simultaneous effects of multiple environmental factors on element incorporation. Culture experiments with benthic foraminifera have revealed that the shell's S/Ca reflects the carbon chemistry and can potentially be used as a proxy for seawater [CO32- ${{\text{CO}}_{3}}<^>{2-}$]. Aiming to investigate the application potential of sulfur incorporation for carbonate speciation reconstruction, we present S/Ca ratios in five planktonic foraminiferal species, namely Globigerina bulloides, Globigerinoides ruber albus, Globigerinoides ruber ruber, Trilobatus sacculifer, and Neogloboquadrina incompta from core-top sediments in regions with contrasting [CO32- ${{\text{CO}}_{3}}<^>{2-}$], [HCO3- ${{\text{HCO}}_{3}}<^>{-}$], temperature, and salinity. Analyses of B/Ca and Mg/Ca ratios are included here since these elements have been shown to depend to a certain degree on carbon system parameters (e.g., calcite saturation state and pH, respectively) as well. Moreover, foraminiferal Mg/Ca values covary with S/Ca values and thereby might compromise its proxy application. In contrast to previously published results, this new data set shows a positive correlation between the incorporation of sulfur in the foraminifer's shell and seawater [CO32- ${{\text{CO}}_{3}}<^>{2-}$]. As the incorporation of sulfur and magnesium are positively correlated, S/Mg values of the same foraminifera may be used to improve inorganic carbon system reconstructions. Ions incorporated in the shells of foraminifera, a group of unicellular calcifying organisms, provide crucial insight into past changes in the ocean's carbon chemistry. Inorganic carbon chemistry includes six parameters: [CO2], [HCO3- ${{\text{HCO}}_{3}}<^>{-}$], [CO32- ${{\text{CO}}_{3}}<^>{2-}$], pH, dissolved inorganic carbon concentration, and total alkalinity; and previous studies have shown that the incorporation of sulfur, magnesium, and boron depend on one or more of these parameters. We analyzed sulfur, magnesium, and boron concentrations in planktonic foraminifera from regions with various carbon chemistry conditions to investigate the application potential of these elements in reconstructing parameters of carbon chemistry. Results of this field study indicate that [HCO3- ${{\text{HCO}}_{3}}<^>{-}$] may also have a strong influence on the incorporation of magnesium in addition to the previously reported temperature, salinity, and pH control, and the sulfur concentrations in foraminifera are likely affected by more than one parameter of the carbon chemistry. A significant positive correlation between sulfur and magnesium concentrations hints at shared parameters controlling their incorporations. Therefore, taking the ratio of sulfur and magnesium concentrations may correct the parameters that influence the incorporation of both elements. Here, we suggest investigating the potential of combining multiple elements in future work for better constraining the marine inorganic carbon chemistry. S/Ca in the planktonic foraminifera, Globigerina bulloides, Globigerinoides ruber, Trilobatus sacculifer, and Neogloboquadrina incompta show a positive correlation with [CO32- ${{\text{CO}}_{3}}<^>{2-}$] Seawater [HCO3- ${{\text{HCO}}_{3}}<^>{-}$] may have a stronger influence on the incorporation of magnesium in the foraminifera's shells than salinity A multi-element approach may correct for the influence of multiple controls on the incorporation of elements