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

Abstract

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 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 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 a 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 either the amount of S or Mg that is ultimately incorporated in foraminiferal calcite, these processes do not explain the covariation between Mg∕Ca and S∕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 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 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.

Light Impacts Mg Incorporation in the Benthic Foraminifer Amphistegina lessonii

“…The slope in Mg/Ca as a function of %dark_calcite is therefore an increase of about 2.3 mmol/mol Mg/Ca per 10% decrease of light exposure duration during calcification. Translating this to temperature reconstructions (van Dijk et al, 2019), implies an effect of +1.36 • C in A. lessonii per 10% increase in light exposure during calcification. Clearly such changes could be of major importance, especially when investigating temperatures at high latitudes where differences in day lengths are substantial, assuming that high latitude inhabitants respond similarly, to day/night cyclicity as A. lessonii.…”

“…the correlation between elevated Ba/Ca cc and Mg/Ca cc presented in de Nooijer et al (2017a) was first corrected to the correct Mg concentrations based on the experimental temperatures used. The difference in Mg/Ca [mmol/mol] due to a 4 • C temperature offset was calculated using a previously published Mg/Ca-temperature calibration for this species (van Dijk et al, 2019). In contrast, the Ba concentrations were not adjusted, since temperature is reported to have no effect on foraminiferal Ba/Ca cc (Lea and Spero, 1994;Hönisch et al, 2011).…”

“…This effect could be enhanced, for example by migration of planktonic species in the water column, hence affecting symbiont activity Kühl, 2000, 2005). Photosynthesis by these symbionts determines the pH and saturation state in the microenvironment of the foraminifera (Köhler-Rink and Kühl, 2000), as well as the oxygen levels near the foraminifera (Köhler-Rink and Kühl, 2005) which in turn, could affect the chemical speciation of trace and minor elements [e.g., S;(van Dijk et al, 2019)], and hence their uptake rates during calcification.…”

Distribution of chlorine and fluorine in benthic foraminifera

“…Thus, 40 to 47 separate data points (ROIs) per species, grouped per specimen, were extracted from the NanoSIMS images. Subsequently, correlation matrices were calculated for the accumulated ion count ratios in those ROIs using the corrplot package (Wei and Simko, 2017) in R (R Core Team, 2018).…”

Chemical Heterogeneity of Mg, Mn, Na, S, and Sr in Benthic Foraminiferal Calcite

“…Heterogeneity is a trait of biologically precipitated minerals (Allison et al, 2001;Freitas et al, 2009), including the calcium carbonate precipitated by foraminifera. This trace element variability has been explained by so-called vital effects, a term introduced by Urey et al (1951), which covers varying factors and processes, including (i) ontogenetic-related changes or migration of foraminifera in the sediment or water column during their life cycle (e.g., Elderfield et al, 2002;Hintz et al, 2006) and (ii) processes involved in or active during biomineralization (Weiner and Dove, 2003), for instance, pH regulation during chamber formation (Toyofuku et al, 2017), or the presence of organic compounds (Kunioka et al, 2006;Mavromatis et al, 2017).…”