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Experimental Constraints on Clumped Isotope Fractionation During BaCO3 Precipitation

Reliable temperature reconstruction by carbonate clumped isotope (Δ47) thermometry requires isotopic equilibrium during carbonate growth. However, many carbonate minerals grow at high rates and exhibit disequilibrium isotopic states. Carbonate Δ47 disequilibrium arising from kinetic isotope effects (KIEs) specific to carbonate growth still remains unclear, and requires experimental constraints. Here we present a series of rapid witherite (BaCO3) precipitation experiments intended to constrain bulk carbon, oxygen and clumped isotopic fractionation during unidirectional precipitation of BaCO3 from dissolved inorganic carbon solutions under various pH and temperature conditions. We found that rapid BaCO3 growth can lead to lower δ13C, δ18O, and Δ47 values in HCO3−‐dominated solutions, especially at low temperatures. Our experiments provide constraints on kinetic fractionation factors (KFFs) associated with the rapid carbonate growth and their temperature dependence. KFFs for δ13C and δ18O are broadly consistent with previous experimental estimates, although the Δ47 KFF for BaCO3 precipitation from HCO3−‐dominated solution is not consistent with an earlier theoretical estimate, necessitating a re‐evaluation of the current model of the KIE associated with carbonate growth. Our results clearly verify a preference for the CO32− pathway during carbonate precipitation, with important implications for new isotope fractionation models for natural carbonates. The fractionation relationship between Δ47 and δ18O found in this study allows more precise identification of KIEs associated with rapid carbonate growth.