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

Shell chemistry of foraminiferal carbonate proves to be useful in reconstructing past ocean conditions. A new addition to the proxy toolbox is the ratio of sulfur (S) to calcium (Ca) in foraminiferal shells, reflecting the ratio of inline-formula $M1inlinescrollmathmlchem{\mathrm{normal SO}}_{normal 4}^{normal 2-}$ 29pt17ptsvg-formulamathimg43c7521f547bc9d1c731092871dcc89b bg-16-2115-2019-ie00001.svg29pt17ptbg-16-2115-2019-ie00001.png to inline-formula $M2inlinescrollmathmlchem{\mathrm{normal CO}}_{normal 3}^{normal 2-}$ 30pt17ptsvg-formulamathimg67af9c17fdce8d9e433e2d9227decdd9 bg-16-2115-2019-ie00002.svg30pt17ptbg-16-2115-2019-ie00002.png in seawater. When comparing species, the amount of inline-formula $M3inlinescrollmathmlchem{\mathrm{normal SO}}_{normal 4}^{normal 2-}$ 29pt17ptsvg-formulamathimg6060a0eb6022af681aa55d19b3180df9 bg-16-2115-2019-ie00003.svg29pt17ptbg-16-2115-2019-ie00003.png incorporated, and therefore the inline-formulaS∕Ca of the shell, increases with increasing magnesium (Mg) content. The uptake of inline-formula $M5inlinescrollmathmlchem{\mathrm{normal SO}}_{normal 4}^{normal 2-}$ 29pt17ptsvg-formulamathimg28cd4f8c12cf9ef751545712573a522a bg-16-2115-2019-ie00004.svg29pt17ptbg-16-2115-2019-ie00004.png in foraminiferal calcite is likely connected to carbon uptake, while the incorporation of Mg is more likely related to Ca uptake since this element substitutes for Ca in the crystal lattice. The relation between S and Mg incorporation in foraminiferal calcite therefore offers the opportunity to investigate the timing of processes involved in Ca and carbon uptake. To understand how foraminiferal inline-formulaS∕Ca is related to inline-formulaMg∕Ca, we analyzed the concentration and within-shell distribution of inline-formulaS∕Ca of three benthic species with different shell chemistry: Ammonia tepida, Bulimina marginata and Amphistegina lessonii. Furthermore, we investigated the link between inline-formulaMg∕Ca and inline-formulaS∕Ca across species and the potential influence of temperature on foraminiferal inline-formulaS∕Ca. We observed that inline-formulaS∕Ca is positively correlated with inline-formulaMg∕Ca on a microscale within specimens, as well as between and within species. In contrast, when shell inline-formulaMg∕Ca increases with temperature, foraminiferal inline-formulaS∕Ca values remain similar. We evaluate our findings in the light of previously proposed biomineralization models and abiological processes involved during calcite precipitation. Although all kinds of processes, including crystal lattice distortion and element speciation at the site of calcification, may contribute to changes in either the amount of S or Mg that is ultimately incorporated in foraminiferal calcite, these processes do not explain the covariation between inline-formulaMg∕Ca and inline-formulaS∕Ca values within specimens and between species. We observe that groups of foraminifera with different calcification pathways, e.g., hyaline versus porcelaneous species, show characteristic values for inline-formulaS∕Ca and inline-formulaMg∕Ca, which might be linked to a different calcium and carbon uptake mechanism in porcelaneous and hyaline foraminifera. Whereas Mg incorporation might be controlled by Ca dilution at the site of calcification due to Ca pumping, S is linked to carbonate ion concentration via proton pumping. The fact that we observe a covariation of S and Mg within specimens and between species suggests that proton pumping and Ca pumping are intrinsically coupled across multiple scales.

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van Dijk, Inge / Barras, Christine / de Nooijer, Lennart Jan / et al: Coupled calcium and inorganic carbon uptake suggested by magnesium and sulfur incorporation in foraminiferal calcite. 2019. Copernicus Publications.

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