No discernible effect of Mg2 + ions on the equilibrium oxygen isotope fractionation in the CO2–H2O system

Abstract

Equilibrium oxygen isotope fractionation factors (αCO2g–H2O, αHCO3–H2O and αCO3–H2O) are fundamental geochemical parameters that characterize 18O partitioning in the CO2–H2O system. These constants were established in laboratory experiments using deionized H2O (e.g., Beck et al. (2005) and references therein). The applicability of these constants in environmental waters, including natural seawater, appears questionable due to potentially strong ionic interactions in such aqueous media. For instance, considerable portions of carbonate ions in seawater exist as cation–CO32− ion complexes such as MgCO30. In this study, quantitative BaCO3 precipitation experiments were performed to examine the effect of Mg2+ concentrations on the oxygen isotope equilibrium between dissolved inorganic carbon (DIC) species and H2O. Our results from Mg2+-free control experiments in which BaCO3 samples were precipitated from simple NaHCO3 solutions were in good agreement with empirical results from three independent studies and with theoretical calculations. BaCO3 precipitations from solutions with Mg2+ concentrations higher than 2.5mM caused intolerable quantities of Mg(OH)2 co-precipitation, which interfered with δ18O measurements. Within the limit of 2.5mM of [Mg2+], the MgCO30 abundance in the total carbonate ions ([CO32−]T) and [DIC] was varied over approximately 0 to 40% and 0 to 36%, respectively, by manipulating solution chemistry. Despite such chemical treatment, there was no effect of Mg2+-addition on αBaCO3–H2O. These results suggest that the presence of Mg2+ in solutions has a negligible effect on the oxygen isotope equilibrium in the CO2–H2O system. In seawater, Mg2+ is the most important cation that forms complexes with CO32−. Hence, if our results also hold at higher [Mg2+] and higher ionic strength, they imply that the applicability of freshwater-based equilibrium fractionation factors is not compromised by ionic interactions in seawater.