Paleoclimate proxies based on isotopic fractionation in biogenic calcium carbonate minerals constrain paleotemperatures and aid in assessments of the current rate of anthropogenic global warming. Corals and some other calcifiers tend to display δ13C and δ18O at isotopic disequilibrium with surrounding waters (so-called “vital effects”), introducing potential errors to paleoproxies . Some research attributes vital effects to slow isotopic exchange during the hydration reaction between CO2 and H2O to form H2CO3 in the semipermeable membrane-bound calcifying space (the “kinetic” vital effect model). We present atomistic, quantum mechanical simulations to elucidate the potential contribution of kinetic vital effects to non-equilibrium C and O isotope fractionation and clumped isotope fractionation in the skeletons of calcifying organisms. The model utilizes density functional theory and transition state theory to evaluate isotopologue-specific reaction rates for CO2 hydration and hydroxylation. Model accuracy is tested against experimental, non-isotopologue-specific reaction rates and against equilibrium fractionation factors using both forward and reverse reactions. More accurate results are obtained when using more H2O molecules in the hydration shell surrounding CO2, highlighting the importance of accurately modeling the local chemical environment around the reactive species. These ab initio models will show whether or not the kinetics of hydration and hydroxylation reactions can account for vital effects in calcifiers, or whether other models are necessary to account for non-equilibrium fractionation.

CO2 hydration/hydroxylation and the origin of carbonate kinetic isotope effects

251st National Meeting (2016)

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