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 δ18OCARB and Δ47 decreased, while δ13CCARB 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 δ18OCARB 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.