The mechanisms and stable isotope effects of transforming hydrated carbonate into calcite pseudomorphs

Ikaite (CaCO₃·6H₂O) and monohydrocalcite (CaCO₃·H₂O; MHC) are hydrated carbonates that form at frigid (<9 °C) temperatures. During gradual heating and dehydration, the more thermodynamically stable anhydrous calcite replaces and pseudomorphs ikaite and MHC. Previously, ikaite pseudomorphs have been identified in the sedimentary record by characteristic replacive macro- and microtextures and interpreted as evidence for near-freezing marine paleotemperatures. Prior to this study, we lacked an understanding of isotopic exchange during mineral dehydration necessary to interpret isotopic compositions of such fabrics. Specifically, do the stable isotopic compositions of ikaite pseudomorphs preserve the primary environmental signal, or are they altered during mineral transformation? Through heating experiments of MHC from Ikka Fjord, we find that δ¹⁸O_CARB and Δ₄₇ decreased, while δ¹³C_CARB remained nearly unchanged during progressive dehydration. An oxygen isotopic exchange model fitted to experimental data suggests that the isotopic changes reflected partial re-equilibration of δ¹⁸O_CARB and Δ₄₇ towards the new diagenetic conditions due to oxygen equilibrium exchange between CO₃²⁻ and H₂O within the MHC lattice. However, this process never reaches full equilibrium, an effect we argue reflects the fact that structural H₂O escapes the solid carbonate structure faster than isotopic exchange can reach equilibrium. In addition, labelled water experiments demonstrated that oxygen isotopic exchange also occurs with secondary external waters during dehydration. We apply this new framework to interpret the isotopic compositions of Pleistocene ikaite pseudomorphs from Mono Lake (CA, USA) high-stand deposits, which have undergone subaerial dehydration. Specifically, dehydration diagenetic overprinting of Δ₄₇ similar to the results of our controlled heating experiments can explain the warm temperatures, 6–26 °C, recorded by ikaite pseudomorphs and the higher δ¹⁸O_CARB recorded by ikaite pseudomorph compared to other Pleistocene Mono Lake calcite tufas. We show that Mono Lake ikaite pseudomorph tufa isotopic data can be explained in a model scenario where approximately coeval precursor ikaite and other calcite tufas in Pleistocene Mono Lake formed from similar fluid compositions but at different water temperatures, likely related to changing seasons. In summary, we show that the isotopic composition of ikaite and MHC pseudomorphs can be used for paleoclimate reconstruction of water temperature, δ¹⁸O_CARB, δ¹⁸O_fluid, and δ¹³C_CARB when considering dehydration diagenetic effects.