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

• Main Authors: I. van Dijk, C. Barras, L. J. de Nooijer, A. Mouret, E. Geerken, S. Oron, G.-J. Reichart
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-05-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/2115/2019/bg-16-2115-2019.pdf
• ISSN: 1726-4170; 1726-4189

<p>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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="43c7521f547bc9d1c731092871dcc89b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2115-2019-ie00001.svg" width="29pt" height="17pt" src="bg-16-2115-2019-ie00001.png"/></svg:svg></span></span> to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">CO</mi><mn mathvariant="normal">3</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="67af9c17fdce8d9e433e2d9227decdd9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2115-2019-ie00002.svg" width="30pt" height="17pt" src="bg-16-2115-2019-ie00002.png"/></svg:svg></span></span> in seawater. When comparing species, the amount of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="6060a0eb6022af681aa55d19b3180df9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2115-2019-ie00003.svg" width="29pt" height="17pt" src="bg-16-2115-2019-ie00003.png"/></svg:svg></span></span> incorporated, and therefore the <span class="inline-formula">S∕Ca</span> of the shell, increases with increasing magnesium (Mg) content. The uptake of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="28cd4f8c12cf9ef751545712573a522a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-2115-2019-ie00004.svg" width="29pt" height="17pt" src="bg-16-2115-2019-ie00004.png"/></svg:svg></span></span> 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 <span class="inline-formula">S∕Ca</span> is related to <span class="inline-formula">Mg∕Ca</span>, we analyzed the concentration and within-shell distribution of <span class="inline-formula">S∕Ca</span> of three benthic species with different shell chemistry: <i>Ammonia tepida</i>, <i>Bulimina marginata</i> and <i>Amphistegina lessonii</i>. Furthermore, we investigated the link between <span class="inline-formula">Mg∕Ca</span> and <span class="inline-formula">S∕Ca</span> across species and the potential influence of temperature on foraminiferal <span class="inline-formula">S∕Ca</span>. We observed that <span class="inline-formula">S∕Ca</span> is positively correlated with <span class="inline-formula">Mg∕Ca</span> on a microscale within specimens, as well as between and within species. In contrast, when shell <span class="inline-formula">Mg∕Ca</span> increases with temperature, foraminiferal <span class="inline-formula">S∕Ca</span> 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 <span class="inline-formula">Mg∕Ca</span> and <span class="inline-formula">S∕Ca</span> 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 <span class="inline-formula">S∕Ca</span> and <span class="inline-formula">Mg∕Ca</span>, 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.</p>