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 Δ 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 δ18O and Δ 47 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 Δ 47 of inorganic BaCO 3 samples from Uchikawa and Zeebe (2012) (denoted as UZ12), which were quantitatively precipitated from NaHCO 3 solutions at various times over the course of isotopic equilibration at 25 °C and pH NBS of 8.9. Our data show that, although the timescales for δ18O and Δ 47 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), Δ 47 equilibration initially moves away from equilibrium and then changes its course towards equilibrium. This excess Δ 47 disequilibrium is manifested as a characteristic "dip" in the Δ 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 Δ 47 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 Ex change and Clump ing of 13 C and 18 O in DCP (ExClump38 model) to trace the evolution of singly- and doubly-substituted isotopic species (i.e., δ13C, δ18O and Δ 47). The model suggests that the dip in the Δ 47 equilibration trajectory is due largely to kinetic carbon isotope fractionation for hydration and hydroxylation of CO 2. We additionally examined the BaCO 3 samples prepared from NaHCO 3 solutions supplemented with carbonic anhydrase (CA), an enzyme known to facilitate δ18O equilibration in DCP by catalyzing CO 2 hydration (UZ12). These samples revealed that, while CA effectively shortens the time required for Δ 47 equilibrium in DCP, the overall pattern and magnitude of the dip in the Δ 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 Δ 47 paleothermometry. [ABSTRACT FROM AUTHOR]