The influence of Ca:CO3 stoichiometry on Ca isotope fractionation: Implications for process-based models of calcite growth

Abstract

The solution stoichiometry dependence of calcium isotope fractionation during calcite precipitation was investigated as a direct test of the conceptual model of calcium isotope discrimination driven by Ca exchange at surface sites during growth. Classical ion-by-ion models of calcite growth predict a strong solution stoichiometry influence on Δ44/40Cacalcite-fluid: In low Ca2+:CO32–solutions,Δ44/40Cais predicted to approach a kinetic limit (∼−2 to −4‰), while in high Ca2+:CO32–solutions, exchange at dominantly Ca-occupied kink sites drives Δ44/40Catowards the equilibrium fractionation (near 0‰). To test thisprediction,a series ofseeded and unseeded constant composition calcite growth experiments were performed in which all aspects of solution chemistry were held constant and the Ca2+:CO32– activity ratio was varied. Experiments were performed at pH 8.5, ionic strength 0.1 M (adjusted with KCl), and calcite saturation index (SI = log10({Ca2+}{CO32–}/Ksp)) of either 0.5 or 0.8.Calcium isotope fractionation is found to be weakly stoichiometry dependent. The expected trend of larger magnitude fractionations at lower Ca2+:CO32– is observed, but the magnitude of change in Δ44/40Ca over the solution stoichiometries studied (Ca2+:CO32– = 1–250) is only ∼ 0.4‰. Similar trends in Δ44/40Ca with Ca2+:CO32– are observed at SI = 0.5 and 0.8, with smaller magnitude fractionations at lower supersaturation. This yields an inverse correlation between Δ44/40Ca and growth rate, confirming the Δ44/40Ca-rate relationship for inorganic calcite growth observed by Tang et al. (2008). The ion-by-ion model framework captures measured Δ44/40Ca only when a surface complexation model is incorporated, highlighting the role of surface speciation in dictating Ca attachment/detachment dynamics. The model captures observed trends with Ca2+:CO32– using best-fit kinetic and equilibrium fractionations consistent with end-members observed in natural systems (αkinetic ∼ 0.9978, αeq ∼ 0.9998). This result implies a total possible range in Δ44/40Ca of 2‰ and suggests that for most carbonate precipitating environments, solution supersaturation will be a stronger determinant of Δ44/40Ca than stoichiometry. The demonstrated importance of surface speciation, however, implies a strong pH influence on Δ44/40Ca, independent of its influence on carbonate ion activity, that requires further investigation.The results of this study provide strong evidence supporting the model of kink-exchange driven Ca isotope fractionation and suggest that calcite grows by a dominantly classical mechanism over the solution conditions investigated. Model predictions regarding the relationship between Δ44/40Ca and growth inhibition in the presence of impurity ions lay the foundation for the use of Ca isotopes as molecular tracers of carbonate crystal growth pathways.