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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Dec 2018

Research article | 12 Dec 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

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

Inge van Dijk1,2, Christine Barras1, Lennart Jan de Nooijer2, Aurélia Mouret1, Esmee Geerken2, Shai Oron3, and Gert-Jan Reichart1,4 Inge van Dijk et al.
  • 1LPG UMR CNRS 6112, University of Angers, UFR Sciences, 2 bd Lavoisier 49045, Angers CEDEX 01, France
  • 2NIOZ Royal Institute for Sea Research, Department of Ocean Systems (OCS), and Utrecht University, Postbus 59, 1790 AB Den Burg, the Netherlands
  • 3Charney School of Marine Sciences, University of Haifa, Israel
  • 4Utrecht University, Faculty of Geosciences, Budapestlaan 4, 3584 CD Utrecht, the Netherlands

Abstract. 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 SO42− to CO32− in seawater. When comparing species, the amount of SO42− incorporated, and therefore the S/Ca of the shell, increases with increasing magnesium (Mg) content. The uptake of SO42− in foraminiferal calcite is likely coupled to carbon uptake, while the incorporation of Mg is more likely related to Ca uptake since this element substitutes 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 S/Ca is related to Mg/Ca, we analyzed the concentration and within-shell distribution of S/Ca of three benthic species with different shell chemistry: Ammonia tepida, Bulimina marginata and Amphistegina lessonii. Furthermore, we investigated the link between Mg/Ca and S/Ca across species and the potential influence of temperature on foraminiferal S/Ca. We observed that S/Ca is positively correlated with Mg/Ca on microscale within specimens, as well as between and within species. In contrast, when shell Mg/Ca increases with temperature, foraminiferal S/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 the amount of S and Mg that is ultimately incorporated in foraminiferal calcite, these processes do not explain the consistent co-variation between Mg/Ca and S/Ca values. We observe that groups of foraminifera with different calcification pathways, e.g. hyaline versus porcelaneous species, show characteristic values for S/Ca and Mg/Ca, which might be linked to a different calcium and carbon uptake mechanism in porcelaneous and hyaline foraminifera. Whereas Mg incorporation is linked to the Ca-pump, S is linked to carbonate ion concentration via proton pumping. The fact that we observe coupled behavior of S and Mg, within specimens and between species suggests that proton pumping and Ca pumping are intrinsically coupled across scales.

Inge van Dijk et al.
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Status: final response (author comments only)
Status: final response (author comments only)
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Inge van Dijk et al.
Inge van Dijk et al.
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Short summary
Systematics in the incorporation of different elements in shells of marine organisms can be used to test calcification models and hence processes involved in precipitation of calcium carbonates. The observed link between sulfur and magnesium incorporation in shells of foraminifera, unicellar protists, provides insights into the mechanics behind shell formation. The observed patterns imply all species of foraminifera actively take up calcium and carbon in a coupled process.
Systematics in the incorporation of different elements in shells of marine organisms can be used...