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. Shell chemistry of foraminiferal carbonate proves to beuseful in reconstructing past ocean conditions. A new addition to the proxytoolbox is the ratio of sulfur (S) to calcium (Ca) in foraminiferal shells,reflecting the ratio of SO 4 2 - to CO 3 2 - in seawater. Whencomparing species, the amount of SO 4 2 - incorporated, and thereforethe S∕Ca of the shell, increases with increasing magnesium (Mg) content. Theuptake of SO 4 2 - in foraminiferal calcite is likely connected tocarbon uptake, while the incorporation of Mg is more likely related to Cauptake since this element substitutes for Ca in the crystal lattice. Therelation between S and Mg incorporation in foraminiferal calcite thereforeoffers the opportunity to investigate the timing of processes involved in Caand carbon uptake. To understand how foraminiferal S∕Ca is related to Mg∕Ca ,we analyzed the concentration and within-shell distribution of S∕Ca of threebenthic species with different shell chemistry: Ammonia tepida, Bulimina marginata and Amphistegina lessonii. Furthermore, weinvestigated the link between Mg∕Ca and S∕Ca across species and thepotential influence of temperature on foraminiferal S∕Ca . We observed that S∕Ca is positively correlated with Mg∕Ca on a microscale within specimens, aswell as between and within species. In contrast, when shell Mg∕Ca increaseswith temperature, foraminiferal S∕Ca values remain similar. We evaluate ourfindings in the light of previously proposed biomineralization models andabiological processes involved during calcite precipitation. Although allkinds of processes, including crystal lattice distortion and elementspeciation at the site of calcification, may contribute to changes in eitherthe amount of S or Mg that is ultimately incorporated in foraminiferalcalcite, these processes do not explain the covariation between Mg∕Ca and S∕Ca values within specimens and between species. We observe that groups offoraminifera with different calcification pathways, e.g., hyaline versusporcelaneous species, show characteristic values for S∕Ca and Mg∕Ca , whichmight be linked to a different calcium and carbon uptake mechanism inporcelaneous and hyaline foraminifera. Whereas Mg incorporation might becontrolled by Ca dilution at the site of calcification due to Ca pumping, Sis linked to carbonate ion concentration via proton pumping. The fact thatwe observe a covariation of S and Mg within specimens and between speciessuggests that proton pumping and Ca pumping are intrinsically coupled acrossmultiple scales.

Coupled calcium and inorganic carbon uptake suggested by magnesium and sulfur incorporation in foraminiferal calcite