期刊
GEOCHIMICA ET COSMOCHIMICA ACTA
卷 308, 期 -, 页码 384-392出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2021.05.029
关键词
First-principles; Fluorine; Calcium carbonates; F/Ca ratios; Marine chemistry
资金
- Chinese Scholarship Council
- NERC [NE/P012167/1]
The incorporation mechanisms of fluorine into crystalline calcium carbonates differ from that of iodine, with fluorine preferentially incorporating into vaterite and aragonite over calcite. These results support the use of F/Ca as a proxy for ocean pCO(2).
The abundant occurrence of calcium carbonate minerals in marine sediments and their high fluorine content suggests that fluorine is a good candidate for reconstructing pale-oceanographic parameters. However, the potential of fluorine as a paleoproxy had hardly been explored, and fundamental insights into the behaviour of fluorine in biogenic carbonates and marine sediments is required. A first-principles modelling approach is used here to analyse the incorporation mechanisms of fluorine into crystalline calcium carbonates. We compute F incorporation into the CaCO3 lattice via a number of mechanisms, but concentrate on comparison of the energetics of the two easiest substitution mechanisms: replacing one oxygen atom within the carbonate group to form a (CO2F)(-) group as against a substitution involving replacement of the CO3 group by two fluorine ions to form a CaF2 defect. These incorporation mechanisms are fundamentally different from that of iodine into calcium carbonates, where a carbon atom is replaced. Our simulations suggest that the substitution of CO32- by F-2(2-) is the most favoured and that fluorine is preferentially incorporated into the three naturally-occurring polymorphs of calcium carbonate in the order greater than or similar to vaterite aragonite >> calcite. These results explain the previously-reported preponderance of fluorine in aragonite corals, and lend support to the use of F/Ca as a proxy for ocean pCO(2). (C) 2021 Elsevier Ltd. All rights reserved.
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