James Watkins

Most Earth surface carbonates precipitate out of isotopic equilibrium with their host solution, complicating the use of stable isotopes in paleoenvironment reconstructions. Disequilibrium can arise from exchange reactions in the DIC‐H 2 O system as well as during crystal growth reactions in the DIC‐CaCO 3 system. Existing models account for kinetic isotope effects (KIEs) in these systems separately but the models have yet to be combined in a general framework. Here, an open‐system box model is developed for describing disequilibrium carbon, oxygen, and clumped (Δ 47 , Δ 48 , and Δ 49 ) isotope effects in the CaCO 3 ‐DIC‐H 2 O system. The model is used to simulate calcite precipitation experiments in which the fluxes and isotopic compositions of CO 2 and CaCO 3 were constrained. Using a literature compilation of equilibrium and kinetic fractionation factors, modeled δ 18 O and Δ 47 values of calcite are in good agreement with the experimental data covering a wide range in crystal growth rate and solution pH. This relatively straightforward example provides a foundation for adapting the model to other situations involving CO 2 absorption (e.g., corals, foraminifera, and high‐pH travertines) or degassing (e.g., speleothems, low‐pH travertines, and cryogenic carbonates) and/or mixing with other dissolved inorganic carbon sources.