The effect of carbonate chemistry on trace element incorporation in high-Mg calcitic foraminifera

The sodium-to-calcium ratio (Na/Ca) of biogenic CaCO₃ has recently been introduced as a proxy for past seawater Ca²⁺ concentrations ([Ca²⁺_sw]) as demonstrated by a positive correlation between seawater and shell Na/Ca with a minor influence of salinity. In the present study, we investigate the effect of carbonate chemistry on the Na/Ca proxy by conducting a set of experiments in which pH and the concentration of dissolved inorganic carbon (DIC) were independently varied. In addition to Na⁺, the incorporation of Li⁺, Mg²⁺, and Sr²⁺ into the shells of the large benthic high-Mg calcitic foraminifer Operculina ammonoides was assessed by culturing under constant DIC (∼2170 µmol kg⁻¹) with varying pH (7.5–8.4 NBS scale), and under varying DIC (830–2470 µmol kg⁻¹) with constant pH (∼7.9). Foraminiferal growth rate correlates linearly with calcite saturation state (Ω) of the experimental seawater (SW). The lowest pH and DIC experiments were characterized by low population growth rates, and some of these specimens died and their shells partially dissolved.

Na/Ca_shell and Li/Ca_shell in O. ammonoides are positively correlated with SW [CO₃^2–] and Ω, whereas Sr/Ca_shell and Mg/Ca_shell are much less sensitive to these parameters. The relative sensitivity of Na/Ca_shell to Ω in O. ammonoides is ∼ 4 % per Ω unit. However, given that past changes in surface water Ω were probably small relative to changes in [Ca²⁺_sw] the correction for this secondary effect over the Cenozoic is likely to be small. Therefore, we conclude that the sensitivity of O. ammonoides Na/Ca to the carbonate system is unlikely to compromise the use of this proxy to reconstruct past [Ca²⁺_sw]. In the case of the low-Mg planktic and benthic foraminifera, a data compilation exercise indicates that no resolvable carbonate chemistry effect exists on Na/Ca. Thus, the Na/Ca proxy in benthic nummulitid and planktic foraminifera can be utilized for past [Ca²⁺_sw] reconstructions. Furthermore, coupling this information with the distribution coefficients of other elemental and isotopic systems (e.g., Li⁺, Sr²⁺, Mg²⁺, K⁺, B, δ¹¹B) may allow the reconstruction of wider aspects of past ocean chemistry. Finally, comparison of trace and minor element incorporation into low and high-Mg foraminiferal species, coccolithophores, inorganic calcite, and amorphous CaCO₃ (ACC), we propose a modified biomineralization model for hyaline foraminifera centered on SW vacuolization. Foraminiferal data can be explained by a biomineralization process in which high-Mg species utilize a precursor phase (ACC) to produce high-Mg calcite whereas low-Mg species actively remove Mg²⁺ from the site of calcification.