A Combined Model for Kinetic Clumped Isotope Effects in the CaCO3‐DIC‐H2O System

Abstract 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‐H2O system as well as during crystal growth reactions in the DIC‐CaCO3 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 (Δ47, Δ48, and Δ49) isotope effects in the CaCO3‐DIC‐H2O system. The model is used to simulate calcite precipitation experiments in which the fluxes and isotopic compositions of CO2 and CaCO3 were constrained. Using a literature compilation of equilibrium and kinetic fractionation factors, modeled δ18O and Δ47 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 CO2 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.