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

Uchikawa, Joji and Chen, Sang and Eiler, John M. and Adkins, Jess F. and Zeebe, Richard E. (2021) Trajectory and timescale of oxygen and clumped isotope equilibration in the dissolved carbonate system under normal and enzymatically-catalyzed conditions at 25 °C. Geochimica et Cosmochimica Acta, 314 . pp. 313-333. ISSN 0016-7037. doi:10.1016/j.gca.2021.08.014.

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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 pH_(NBS) 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.

Item Type:Article
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URLURL TypeDescription
Uchikawa, Joji0000-0002-9700-0888
Chen, Sang0000-0001-8941-0791
Eiler, John M.0000-0001-5768-7593
Adkins, Jess F.0000-0002-3174-5190
Additional Information:© 2021 Elsevier. Received 19 February 2021, Accepted 12 August 2021, Available online 20 August 2021. Nami Kitchen, Uri Ryb, Fuyun Cong and Yuchen Liu are thanked for their help in the lab. We credit the original work of Staudigel and Swart (2018) that inspired us to launch this study. We are grateful to Jim Watkins, Philip Staudigel and an anonymous reviewer for their technical reviews and follow-up discussions with us, which were instrumental in improving this paper. This study was supported by U.S. NSF grants OCE1558699 and OCE2048436 to REZ and JU. SC received financial support from the China Scholarship Council. SOEST Pub. #11384. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
China Scholarship Council11384
Subject Keywords:Clumped isotopes; Oxygen isotopes; Kinetic isotope effects; Carbonic anhydrase; Isotopic equilibration
Record Number:CaltechAUTHORS:20211202-230941959
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Official Citation:Joji Uchikawa, Sang Chen, John M. Eiler, Jess F. Adkins, Richard E. Zeebe, Trajectory and timescale of oxygen and clumped isotope equilibration in the dissolved carbonate system under normal and enzymatically-catalyzed conditions at 25 °C, Geochimica et Cosmochimica Acta, Volume 314, 2021, Pages 313-333, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112184
Deposited By: George Porter
Deposited On:03 Dec 2021 15:55
Last Modified:03 Dec 2021 15:55

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