Clumped Isotope (Δ47) Investigation of Kinetic Isotope Effects: Implications for Biomineralizing Fluids

The interpretation of δ¹⁸O, δ¹³C, and Δ₄₇ isotopic measurements in biogenic calcite and aragonite hinges on whether a mineral precipitate was thought to have formed in isotopic equilibrium with its surrounding fluid. Equilibrium enables paleotemeperature estimates and inferences regarding past carbon cycling. Kinetic isotope effects/vital effects make such interpretations tenuous.

We present a series of experiments on the kinetics of CO₂- H₂O isotopic equilibration. We observe a simple asymptotic approach of δ¹⁸O_CO2 to equilibrium values. However, Δ₄₇ values exhibit systematic excursions from a smooth trajectory towards equilibrium. These excursions help illuminate relative reaction rates for different isotopologues in the interconversion reactions among dissolved inorganic carbon (DIC) species and between CO₂ and H₂O. The use of Δ₄₇ measurements reveals the conditions under which isotopic equilibrium vs. disequilibrium among DIC species prevails. This is relevant to systems with rapid rates of organismal uptake of DIC (e.g., photosynthesis in corals) and release of DIC (e.g., respiration in many genera of calcifiers).