Internal Water Facilitates Thermal Resetting of Clumped Isotopes in Biogenic Aragonite

Biogenic and inorganic calcium carbonates contain considerable amounts of internal water, both as free and organically associated water. The oxygen isotopic compositions (δ¹⁸O) of internal water and hosting carbonate are analyzed for various carbonates before and after heating at 175°C for 90 minutes. During heating, the δ¹⁸O values of internal water significantly increased in biogenic aragonites and speleothem calcite, whereas the δ¹⁸O carbonate values were lowered. Correspondingly, an aragonitic bivalve’s clumped-isotope distribution (Δ₄₇) changed during heating, increasing reconstructed paleotemperatures. In contrast, an inorganic aragonite crystal, containing a comparable amount of internal water, showed no oxygen isotope exchange, and its Δ₄₇ values remained unaltered during heating, implying that there is a link between internal oxygen isotope exchange and Δ₄₇ resetting. This alteration process occurred without any detectable transformation from aragonite to calcite. Our results therefore reveal a mechanism that facilitates oxygen isotope exchange between biogenic aragonite and its internal water, while simultaneously resetting the Δ₄₇ values, without affecting mineralogy. Future studies may therefore apply coupled water-carbonate analyses to scrutinize these kinds of diagenetic alteration processes. It appears that in biogenic aragonites, more carbonate is available for exchange reactions with the internal water reservoir than in inorganic aragonites, a feature that can be attributed to the distribution of organic-associated water and/or high surface area fluid inclusions. This water-aragonite exchange occurs at lower temperatures than those required for solid-state bond reordering at the same timescale, and thus likely has occurred earlier during the burial of biogenic aragonites.