Magnesium Isotopic Composition of Tests of Large Benthic Foraminifers: Implications for Biomineralization

Abstract

The Mg isotopic composition in biogenic CaCO3 is expected to reflect oceanic Mg cycles, yet its practical application for paleoenvironmental reconstruction is complicated by the influences of seawater temperature and biologically mediated isotope fractionation. In this study, we investigated the relationship between water temperature, growth rate, and the Mg isotope ratio (δ26Mg) of the biogenic high‐Mg calcite that forms the tests of large benthic foraminifers. We cultured asexually reproduced large benthic foraminifers (Amphisorus kudakajimensis and Calcarina gaudichaudii) at 21–30 °C. We measured Mg/Ca ratios, chemical Mg speciation, and δ26Mg for both species. Mg/Ca ratios showed significant temperature dependence for both species, while the δ26Mg values for different populations of C. gaudichaudii exhibited increasing discrimination of heavy isotope with increasing temperature. X‐ray absorption of near‐edge structure spectra of the foraminiferal tests showed Mg speciation to be consistent with that of inorganic calcite, indicating ideal substitution of Mg for Ca in the crystal lattice. The δ26Mg values for both species were close to those of inorganic calcite and other marine organisms that precipitate high‐Mg calcite. These results suggest that precipitation of high‐Mg calcite tests for both species is similar to that of inorganic calcite and that Mg isotope fractionation for both species is only slightly influenced by individual physiological processes related to growth temperature. The ∆26Mgcarb‐sol values of multiple field samples fell within the ∆26Mgcarb‐sol range for inorganic calcite, providing preliminary evidence that the effects of intraspecific differences in δ26Mg values can be averaged out by careful choice of samples for analysis.

Temperature Impact on Magnesium Isotope Fractionation in Cultured Foraminifera

“…Well established estimates of carbonate precipitation rates would therefore be a valuable addition to take into account and could allow inter-taxa comparisons as well as deepen our understanding of Mg isotope incorporation, such data is unfortunately very difficult to obtain. The largest differences in fractionation are most likely (Maeda et al, 2019;Pogge von Strandmann, 2008;Wombacher et al, 2011;Yoshimura et al, 2011) and this study, showing different responses between groups of organisms. In addition to species specific relationships in benthic foraminifera (see Figure 1), a statistically significant relationship (p-values < 0.05) can be observed for coccoliths (δ 26 Mg −0.23*T+ 4.66, R 2 0.98), and sponges (δ 26 Mg 0.07*T − 4.0, R 2 0.34), while other groups such as corals show no clear trend and the δ 26 Mg values in abiogenic carbonates rise with temperature (Li et al, 2012).…”

“…Results from previous studies combined with ours show that foraminiferal δ 26 Mg values range between −5.5 and −1‰ (DSM-3; Chang et al, 2004;Maeda et al, 2019;Pogge von Strandmann, 2008;Wombacher et al, 2011;Yoshimura et al, 2011;Young and Galy, 2004). To investigate the relation between δ 26 Mg and Mg content, we plotted data from our study as well as previous studies, of which both these parameters were known or calculated through well-established calibrations, in Figure 4.…”

“…If the amount and isotopic composition of Mg incorporated both are determined by the same process within the biomineralization mechanism, the δ 26 Mg will also reflect temperature and hence may be used as a palaeoceanographic temperature proxy. First results show a negative relation between culture temperature and foraminiferal shell δ 26 Mg (Maeda et al, 2019), which is opposite to the relationship observed in inorganically precipitated calcite (Galy et al, 2002;Li et al, 2012), though earlier studies showed no significant impact of temperature on foraminiferal δ 26 Mg (Pogge von Strandmann, 2008;Wombacher et al, 2011), or inorganically precipitated calcite (Saulnier et al, 2012). If foraminiferal δ 26 Mg is independent of temperature but at the same time correlated to the average Mg/Ca of a species, it may constrain Mg/Ca-based temperature reconstructions based on species for which no modern-day calibration is available.…”

“…Previous measurements on foraminiferal δ 26 Mg include mainly small planktonic and benthic hyaline species as well as porcelaneous species (Chang et al, 2004;Pogge von Strandmann, 2008;Wombacher et al, 2011;Maeda et al, 2019). Their observations suggest that foraminiferal species with low Mg incorporation also have a relatively depleted Mg isotopic composition compared to other (biogenic) carbonates.…”

“…For this purpose, especially larger benthic foraminifera are of interest, since this group includes species which have hyaline shells but intermediate to high Mg content. This group has only been studied once before (Maeda et al, 2019) but could provide vital insights into the hyaline calcification pathway.…”