128 A Tribute to Rodney C. Ewing: Celebrating a Half-Century of Transformative Contributions to Geoscience, Mineralogy, and Material Science

Honoring Prof. Rodney C. Ewing (1946–2024), this session celebrates over 50 years of groundbreaking contributions to mineralogy, geoscience, and materials science, from atomic-scale processes to field-scale applications.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 217A

The Effect of Internal Water and Organic Matter on Carbonate Clumped Isotope Reactivity

Paper number: 128-10
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

Carbonate clumped isotopes (Δ47) serve as a powerful proxy for paleoclimate and basin evolution. However, Δ47 values can be reset at elevated temperatures either through solid-state 13C—18O bond reordering or oxygen exchange with internal water sourced from structural water, inclusions, and the organic matrix. Although recent first-principles simulations and heating experiments indicate internal water can facilitate carbonate Δ47 resetting, the relationship between clumped isotope reactivity and internal water remains unclear. Here, we present new data on clumped isotope resetting kinetics in four calcitic materials with distinct contents of internal water and organic matter: optical calcite, speleothem calcite, a Carboniferous brachiopod and a modern brachiopod. Measurements using a high-temperature elemental analyzer (TC/EA)-isotope ratio mass spectrometer (IRMS) system reveal total water contents of 0.01, 0.30, 0.77, and 1.47 wt% for these materials, respectively. The resetting rates obtained for optical calcite and the fossil brachiopod are very similar and match those of previous studies, despite differences in taxa and shell microstructure of the Paleozoic brachiopods of this and the earlier research. The slow and similar resetting rates of optical calcite and the fossil brachiopod shell indicate that resetting in these materials proceeded dominantly through solid-state reordering. Speleothem containing fluid inclusions exhibits resetting rates up to 10 times faster and an activation energy 50% lower than those of optical calcite and fossil brachiopod. With increasing heating temperatures, the resetting rate of speleothem gradually converges to those of optical calcite and the fossil brachiopod shell. Since most of internal water in speleothem might be released at higher temperatures, the solid-state reordering becomes the dominant mechanism of resetting. The modern brachiopod, in which water is most abundant and accessible (high specific surface area), achieves clumped isotope resetting rates at temperatures more than 120 °C lower than those needed for the other three materials. Forward resetting modeling on the modern brachiopod shell shows that its clumped temperature changes over 20 °C when heated at 50 °C for 100 Myr, contrasting with the evidence of clumped temperatures below 45 °C for fossil brachiopod shells. Our experiment and modeling results suggest that clumped isotope reactivity in brachiopod shell declines to a level similar to optical calcite after burial, probably due to shell dehydration and degradation of organic components, especially proteins and amino acids.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414889

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