No discernible effect of Mg2Â + ions on the equilibrium oxygen isotope fractionation in the CO2-H2O system

► We studied the effect of Mg2 + on equilibrium 18O fractionation in the CO2-H2O system. ► Quantitative BaCO3 precipitations from solutions containing Mg2 + ions were employed. ► MgCO30 comprised ~ 36% of DIC in the experiments (as opposed to ~ 4% in seawater). ► The results with and without Mg2 +-additions were not significantly different. ► Thus, Mg2 + ions do not affect equilibrium 18O fractionation in the CO2-H2O system.