Catalogus

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 CO_2(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 CO_2(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.