Boric acid and borate incorporation in inorganic calcite inferred from B/Ca, boron isotopes and surface kinetic modeling

Farmer, Jesse R and Branson, Oscar and Uchikawa, Joji and Penman, Donald E. and Hönisch, Bärbel and Zeebe, Richard E. (2019) Boric acid and borate incorporation in inorganic calcite inferred from B/Ca, boron isotopes and surface kinetic modeling. Geochimica Et Cosmochimica Acta, 244. pp. 229-247. ISSN 0016-7037 DOI https://doi.org/10.1016/j.gca.2018.10.008

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Abstract

The boron concentration (B/Ca ratio) and isotopic composition (δ11B) of biogenic calcite are widely applied to reconstruct past changes in seawater carbonate chemistry. Knowledge of B incorporation pathways into calcite is critical for these applications and for improving the theoretical basis of B proxies. While the canonical interpretation of δ11B holds that B in calcite predominantly derives from dissolved borate anion in seawater, recent studies of the B content, coordination, and isotopic composition in calcite suggest more complex B incorporation pathways. To provide new insights into these pathways, here we present δ11B of inorganic calcite precipitated from saline solutions of varying pH, calcium and dissolved inorganic carbon (DIC) concentration for which B/Ca data were previously reported by Uchikawa et al. (2015). Results show that calcite δ11B significantly increases with increasing pH and decreases with increasing [Ca2+] and [DIC]. In combination, these experiments show that the difference in δ11B between solid calcite and aqueous borate linearly decreases with increasing calcite precipitation rate. To interpret these data, we present the first application of surface kinetic modeling (SKM) to boron incorporation. The SKM can simultaneously explain rate-dependent B/Ca and δ11B patterns observed in our and previously published inorganic calcite precipitation experiments when both aqueous borate and boric acid contribute to boron in inorganic calcite. If the B incorporation mechanism shown here for inorganic calcite is applicable to biogenic calcite, precipitation rate variations could modify δ11B patterns by changing the contributions of aqueous boric acid and borate to boron in calcite. However, better knowledge of biogenic calcite precipitation mechanisms and rates is needed to assess the importance of this effect for applications of B proxies in biogenic carbonates.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: Geochimica Et Cosmochimica Acta
Volume: 244
Page Range: pp. 229-247
Identification Number: https://doi.org/10.1016/j.gca.2018.10.008
Depositing User: Sarah Humbert
Date Deposited: 13 Dec 2019 17:00
Last Modified: 13 Dec 2019 17:00
URI: http://eprints.esc.cam.ac.uk/id/eprint/4595

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