A Coral Biomineralization Model for Dual Clumped Isotopes

Abstract Corals exhibit larger and more variable deviations from equilibrium in stable isotope composition (C, O, and ) than most marine biocalcifiers. The disequilibrium isotope effects complicate paleoclimate applications but offer a window into biocalcification processes. Here, we merge a ‐ isotope model in the ‐DIC‐O system (Watkins & Devriendt, 2022, https://doi.org/10.1029/2021gc010200 ) with a coral biomineralization model (Chen et al., 2018, https://doi.org/10.1016/j.gca.2018.02.032 ) and compare its outputs to recent isotopic measurements. The model simultaneously fits the data from multiple coral species but requires a different set of parameters for deep‐sea corals versus tropical corals. We find that: (a) Deviations from dual clumped isotope equilibrium are due primarily to the hydration reaction, the reversibility of which is modulated by the enzyme carbonic anhydrase (CA), the strength of the biological proton pump, and the kinetics of calcification. (b) Optimal data‐model agreement for both C‐O and ‐ is achieved where CA increases the hydration reaction rate by 2,000x for deep‐sea corals and by 1–500x for tropical corals. (c) The ‐ co‐variation slope is sensitive to the cellular flux relative to the seawater DIC flux, with higher cellular and/or lower seawater throughput favoring a shallower slope. (d) For the most part, the modeled compositions of the calcifying fluids (e.g., pH, [], []) are in good agreement with in‐situ measurements. The data‐model agreement constitutes an important step toward a general quantitative biocalcification model applicable to a wide variety of calcifying organisms.