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

Thursday, 18 December 2014
Albert S Colman, Bo He and Gerard Olack, University of Chicago, Chicago, IL, United States
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 δ18OCO2 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).