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 18O isotopes and 13C-18O isotopic “clumps” (measured as δ18O and Δ47, 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 δ18O and Δ47 equilibrium. For this, a complete understanding of both equilibrium and kinetics of isotopic partitioning and 13C-18O clumping in DCP is crucial. To this end, we analyzed Δ47 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 °C and pHNBS of 8.9. Our data show that, although the timescales for δ18O and Δ47 equilibrium in DCP are relatively similar, their equilibration trajectories are markedly different. As opposed to a simple unidirectional and asymptotic approach toward δ18O equilibrium (first-order kinetics), Δ47 equilibration initially moves away from equilibrium and then changes its course towards equilibrium. This excess Δ47 disequilibrium is manifested as a characteristic “dip” in the Δ47 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 Δ47 equilibration can be understood as a result of the difference in the exchange rate for oxygen isotopes bound to 12C versus 13C, or an isotope effect of ~25‰. We also developed an independent model for the Exchange and Clumping of 13C and 18O in DCP (ExClump38 model) to trace the evolution of singly- and doubly-substituted isotopic species (i.e., δ13C, δ18O and Δ47). The model suggests that the dip in the Δ47 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 δ18O equilibration in DCP by catalyzing CO2 hydration (UZ12). These samples revealed that, while CA effectively shortens the time required for Δ47 equilibrium in DCP, the overall pattern and magnitude of the dip in the Δ47 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 Δ47 paleothermometry.