Trajectory and timescale of oxygen and clumped isotope equilibration in the dissolved carbonate system under normal and enzymatically-catalyzed conditions at 25 °C

The abundance of O-18 isotopes and C-13-O-18 isotopic clumps (measured as delta O-18 and Delta(47), respectively) in carbonate minerals have been used to infer mineral formation temperatures. An inherent requirement or assumption for these paleothermometers is mineral formation in isotopic equilibrium. Yet, apparent disequilibrium is not uncommon in biogenic and abiogenic carbonates formed in nature and in synthetic carbonates prepared under laboratory settings, as the dissolved carbonate pool (DCP) from which minerals precipitate is often out of delta O-18 and Delta(47) equilibrium. For this, a complete understanding of both equilibrium and kinetics of isotopic partitioning and C-13-O-18 clumping in DCP is crucial. To this end, we analyzed Delta(47) of inorganic BaCO3 samples from Uchikawa and Zeebe (2012) (denoted as UZ12), which were quantitatively precipitated from NaHCO3 solutions at various times over the course of isotopic equilibration at 25 degrees C and pHNBS of 8.9. Our data show that, although the timescales for delta O-18 and Delta(47) equilibrium in DCP are relatively similar, their equilibration trajectories are markedly different. As opposed to a simple unidirectional and asymptotic approach toward delta(18) O equilibrium (first-order kinetics), Delta(47) equilibration initially moves away from equilibrium and then changes its course towards equilibrium. This excess Delta(47) disequilibrium is manifested as a characteristic dip in the Delta(47) equilibration trajectory, a feature consistent with an earlier study by Staudigel and Swart (2018) (denoted as SS18). From the numerical model of SS18, the non-first-order kinetics for Delta(47) equilibration can be understood as a result of the difference in the exchange rate for oxygen isotopes bound to C-12 versus C-13, or an isotope effect of similar to 25%. We also developed an independent model for the Exchange and Clumping of C-13 and O-18 in DCP (ExClump38 model) to trace the evolution of singly- and doubly-substituted isotopic species (i.e., delta C-13, delta O-18 and Delta(47)). The model suggests that the dip in the Delta(47) equilibration trajectory is due largely to kinetic carbon isotope fractionation for hydration and hydroxylation of CO2. We additionally examined the BaCO3 samples prepared from NaHCO3 solutions supplemented with carbonic anhydrase (CA), an enzyme known to facilitate delta O-18 equilibration in DCP by catalyzing CO2 hydration (UZ12). These samples revealed that, while CA effectively shortens the time required for Delta(47) equilibrium in DCP, the overall pattern and magnitude of the dip in the Delta(47) equilibration trajectory remain unchanged. This suggests no additional isotope effects due to the CA enzyme within the tested CA concentrations. With the ExClump38 model, we test various physicochemical scenarios for the timescales and trajectories of isotopic equilibration in DCP and discuss their implications for the Delta(47) paleothermometry. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the abundance of O-18 isotopes and C-13-O-18 isotopic clumps in carbonate minerals for inferring mineral formation temperatures. It reveals that isotopic equilibrium during mineral formation is not always achieved. Analyzing inorganic BaCO3 samples precipitated at different times, it was found that while the timescales for delta O-18 and Delta(47) equilibrium are similar, the equilibration trajectories are notably different, possibly due to differences in exchange rates for oxygen isotopes bound to C-12 versus C-13.