Contribution of triple oxygen isotopes measurement by Cavity Ring Down spectroscopy and Clumped isotopes to the understanding of kinetics effect in cold water corals.

Kinetic isotopes effect (KIEs) describes a very common phenomenon related to change in chemical reaction rate due isotopic substitution. If in biological sciences, KIEs has received a lot of interest with the aim at understanding reaction mechanisms, their control on the isotopic composition of biogenic carbonate has long been overlooked. However, the initial assumption that isotopic fractionation primarily reflects a thermodynamic equilibrium process in the H₂O-DIC-CaCO₃ system is challenged by a growing number of observations. Not accounting for these disequilibrium effects leads to inaccurate estimates of carbonate growing temperature. In this scientific context, Triple oxygen isotopes systematic can help constraining kinetics isotopes fractionation associated with metabolic reactions implicated in biocarbonates formation. We thus took advantage from recent development in spectroscopic technique (VCOF-CRDS) to measured O¹⁷isotopic anomalies in CO₂produced by carbonate acid reaction [1]. These samples were also analyzed for their δ¹³C, δ¹⁸O and Δ₄₇ composition using a more classical mass spectroscopy technic. In this contribution we investigated cold-water corals (CWC) known to display strong isotopic disequilibria. For this 1^st application, we selected four modern CWC species for which calcification conditions (T, S, pH, δ¹⁸O_water, Δ¹⁷O_water and δ¹³C_DIC) are independently constrained. The measured isotopic signatures were compared to their respective expected values based on environmental constrains, assuming “pseudo-equilibrium” carbonate precipitation. In particular, corals Δ¹⁷O signatures were compared to the newly established equilibrium for O¹⁷ fractionation between calcite-water based on slow growing carbonates from Laghetto Basso and Devils Hole, measured using the same VCOF-CRDS technic [2]. We finally compared our experimental data with theoretical predictions for KIEs on DIC isotopic composition [3]. Interestingly, the correlation slope among Δ₄₇ - Δ¹⁷O disequilibrium differ from the previous one derived from dual clumped (Δ₄₇ - Δ₄₈) isotopic measurements of the same species [4]. This founding suggesting that other biological parameter(s) should be taken into account to resolve CWC isotopic disequilibria.

[1] Chaillot. J., Daëron. M., Casado, M., Landais. A., Pesnin. M., Clauzel. T., Kassi. S. (in prep) Triple oxygen analyses of carbon dioxide, water and carbonates using VCOF-CRDS.

[2] Clauzel, T., Chaillot, J., Pesnin, M., Jautzy, J., Kessy, S., Daëron, M. (in prep) Advancing triple oxygen isotope analysis of carbonate and water using V-shaped Cavity Optical Feedback Cavity Ring-Down Spectroscopy (VCOF-CRDS): Calibration and implications for paleoclimate reconstruction.

[3] Guo. W. (2020). Kinetic clumped isotope fractionation in the DIC-H₂O-CO₂ system: Patterns, controls, and implications. Geochimica et Cosmochimica Acta, 268, 230-257.

[4] Davies. A. J., Guo. W., Bernecker. M., Tagliavento. M., Raddatz. J., Gischler. E., Floter. S., Fiebig. J. (2022). Dual clumped isotope thermometry of coral carbonate. Geochimica et Cosmochimica Acta, 338, 66-78.