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 toevaluate data from calcite precipitation experiments in which the δ18O and Δ63 KIEs were found to exceed the theoretical limits for the CO2 hydration and hydroxylation reactions, and reveals that the excess Δ63 KIEs are due to an increase in the Δ47 of CO2(aq) as it converts to during hydration and hydroxylation reactions. The model can also explain the extreme δ18O KIEs but requires assumptions about the isotopic composition of the source of CO2(aq) in the experiments. The example developed here can be adapted to other situations involving CO2 absorption (e.g., corals, foraminifera, high-pH travertines) or degassing (e.g., speleothems, low-pH travertines, cryogenic carbonates) and/or mixing with other DIC sources.
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