Most Earth-surface calcites precipitate out of isotopic equilibrium

Nature Communications
By: , and 

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Abstract

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 18O/16O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium 18O/16O fractionation between calcite and water (18αcc/w). Here, we constrain the equilibrium relations linking temperature, 18αcc/w, and clumped isotopes (Δ47) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium 18αcc/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 Δ47 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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Most Earth-surface calcites precipitate out of isotopic equilibrium
Series title Nature Communications
DOI 10.1038/s41467-019-08336-5
Volume 10
Issue 1
Year Published 2019
Language English
Publisher Nature
Contributing office(s) WMA - Earth System Processes Division
Description Article number: 429; 7 p.
First page 1
Last page 7