Long-Term ¹³C Uptake by ¹²C-Enriched Calcite

Knowledge of the exchange of carbon isotopes between dissolved inorganic carbon and calcite minerals is of long-standing importance for the interpretation of sedimentary paleoclimate records and 14C transport in the geosphere. To assess the mechanism and rates of carbon isotope exchange, we equilibrated 12C-pure synthetic calcite particles in water, first in a glovebox and then in contact with atmospheric PCO2 and 13C/12C ratio, at two different temperatures. Cavity ring-down infrared spectroscopy δ13C measurements of the solid revealed sustained 13C incorporation for over a period of 500 days (21 °C) and 125 days (50 °C). We developed a quantitative model for recrystallization and isotope exchange, assuming that the interfacial free energy provides a thermodynamic driving force for the growth of larger particles at the expense of smaller ones. However, this Ostwald ripening model did not reproduce the kinetics of 13C uptake and required greater coarsening than observed. Rather, the data were best explained by a mechanism involving surface exchange and solid-phase diffusion of 13C into the particles with an inferred effective diffusion constant at 21 °C of about 10-25 m2/s. Although this work cannot rule out the possibility that structural or chemical aspects of the synthetic particles enabled faster 13C uptake than could be observed in natural systems, this study adds to the body of the recent work, suggesting that fast exchange processes are possible, likely through grain boundaries and other defects.