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Published August 1, 2017 | Supplemental Material + Published
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Catalysis and chemical mechanisms of calcite dissolution in seawater


Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO_2 on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve ^(13)C-labeled calcites in natural seawater. We show that the time-evolving enrichment of δ^(13)C in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the ^(13)C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution–precipitation shifts significantly toward a dissolution-dominated mechanism below about Ω= 0.7. Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of CO_2 is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid–solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at Ω= 0.7, which we interpret as the onset of homogeneous etch pit nucleation.

Additional Information

© 2017 National Academy of Sciences. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved June 26, 2017 (received for review March 6, 2017). Published online before print July 18, 2017, doi: 10.1073/pnas.1703604114. We acknowledge Alex Gagnon for helpful discussions on the formulation of the box model, and Yunbin Guan for help with SIMS analysis. We also thank Sijia Dong for discussions in general about carbonate dissolution in seawater. We thank Mathis Hain and one anonymous reviewer, whose careful reading and detailed comments greatly improved this manuscript. Thanks go to National Science Foundation Graduate Research Fellowship Program and the Resnick Institute Graduate Fellowships for supporting A.V.S. and J.N. We acknowledge support from NSF Grants OCE1220600 and OCE1220302. Author contributions: A.V.S., J.F.A., N.E.R., J.E., and W.M.B. designed research; A.V.S., N.E.R., and J.N. performed research; A.V.S., J.F.A., N.E.R., J.N., and W.M.B. analyzed data; and A.V.S., J.F.A., J.N., and W.M.B. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1703604114/-/DCSupplemental.

Attached Files

Published - PNAS-2017-Subhas-8175-80.pdf

Supplemental Material - pnas.1703604114.sapp.pdf


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