Most Earth-surface calcites precipitate out of isotopic equilibrium

• Published in: Nature communications Vol. 10; no. 1; p. 429
• Main Authors: Daëron, M., Drysdale, R. N., Peral, M., Huyghe, D., Blamart, D., Coplen, T. B., Lartaud, F., Zanchetta, G.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25-01-2019; Nature Publishing Group; Nature Portfolio
• Subjects: 140/58; 704/106/2738; 704/106/413; 704/172/169/209; Calcite; Earth Sciences; Earth surface; Equilibrium; Fractionation; Geochemistry; Humanities and Social Sciences; Isotope fractionation; Isotopes; Minerals; multidisciplinary; Oxygen; Science; Science (multidisciplinary); Sciences of the Universe; Thermodynamic equilibrium; Geochemistry; Palaeoceanography; Palaeoclimate
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-019-08336-5

Oxygen-isotope thermometry played a critical role in the rise of modern geochemistry and remains extensively used in (bio-)geoscience. Its theoretical foundations rest on the assumption that
18
O/
16
O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium
18
O/
16
O fractionation between calcite and water (
18
α
cc/w
). Here, we constrain the equilibrium relations linking temperature,
18
α
cc/w
, and clumped isotopes (
Δ
47
) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium
18
α
cc/w
values are systematically ~1.5‰ greater than those in biogenic and synthetic calcite traditionally considered to approach oxygen-isotope equilibrium. We further demonstrate that subtle disequilibria also affect
Δ
47
in biogenic calcite. These observations provide evidence that most Earth-surface calcites fail to achieve isotopic equilibrium, highlighting the need to improve our quantitative understanding of non-equilibrium isotope fractionation effects instead of relying on phenomenological calibrations.
Isotopic thermometry of carbonate minerals postulates that their composition reflects thermodynamic equilibrium constants. Here the authors constrain equilibrium relationships between temperature,
18
O/
16
O and clumped isotopes and find that most natural calcites form out of isotopic equilibrium.