A Coral Biomineralization Model for Dual Clumped Isotopes

Corals exhibit larger and more variable deviations from equilibrium in stable isotope composition (${\delta }^{13}$C, ${\delta }^{18}$O, ${\Delta }_{47}$ and ${\Delta }_{48}$) than most marine biocalcifiers. The disequilibrium isotope effects complicate paleoclimate applications but offer a window into biocalcification processes. Here, we merge a ${\Delta }_{47}$-${\Delta }_{48}$ isotope model in the ${\text{CaCO}}_3$-DIC-$H_2$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 ${\text{CO}}_2$ 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 ${\delta }^{13}$C-${\delta }^{18}$O and ${\Delta }_{47}$-${\Delta }_{48}$ is achieved where CA increases the ${\text{CO}}_2$ hydration reaction rate by $\sim$2,000x for deep-sea corals and by 1–500x for tropical corals. (c) The ${\Delta }_{47}$-${\Delta }_{48}$ co-variation slope is sensitive to the cellular ${\text{CO}}_2$ flux relative to the seawater DIC flux, with higher cellular ${\text{CO}}_2$ and/or lower seawater throughput favoring a shallower slope. (d) For the most part, the modeled compositions of the calcifying fluids (e.g., pH, [${\text{CO}}_3^{2-}$], [${\text{Ca}}^{2+}$]) 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.