Changes in calcium ion concentration as the common driver for Na, K, S, and B incorporation during inorganic calcite precipitation in Mg-free artificial seawater

Calcite is known to incorporate a range of non-constituent ions during its precipitation from aqueous solutions. Their concentrations (measured as E/Ca ratios, where E denotes the elemental forms of non-constituent ions) in calcite formed in seawater can serve as useful tools for paleoceanographic studies. But this requires concrete understanding of the incorporation patterns and their dependence to environmental factors at the time of mineral precipitation. Here, we present Na/Ca, K/Ca, S/Ca, and B/Ca ratios of inorganic calcite samples generated in laboratory experiments using Mg-free artificial seawater with systematic manipulations of pH, [DIC], and [Ca²⁺]. The three parameters were varied both individually (the pH, DIC, and Ca experimental series) and in tandem (the pH-Ca and DIC-Ca series) to form calcites under variable versus near-constant precipitation rates (denoted as R). All measured E/Ca ratios showed a robust positive linear dependence to changes in [Ca²⁺] in the Ca, pH-Ca, and DIC-Ca series, irrespective of changes in R. While K/Ca and S/Ca ratios changed almost exclusively with [Ca²⁺], Na/Ca and B/Ca ratios showed an additionally strong increase with increasing pH and a more moderate increase with rising [DIC], when R changed accordingly in the pH and DIC series. While R-driven kinetic effects and/or formation of certain cation–anion pairs may be important for the elemental uptake in calcite under some circumstances, these mechanisms or processes cannot fully account for the observed trends in every experimental series for all E/Ca ratios considered here. We propose that the observed E/Ca trends can be comprehensively explained by simultaneously considering the nonequivalent influence of changes in solution [Ca²⁺] and [CO₃²⁻] on step-specific kink formation dynamics and the size difference between the respective non-constituent ions (K⁺, Na⁺, SO₄²⁻, and B(OH)₄⁻ and B(OH)₃) relative to Ca²⁺ and CO₃²⁻ that constitute the calcite lattice.