Ikaite (CaCO3*6H2O) -> ACC (amorphous calcium carbonate) -> calcite transformation and its paleoclimatological implication

Abstract

Ikaite (CaCO3*6H2O) is a cryogenic calcium carbonate phase, which forms below about 5°C. If the temperature increases above 5-7 °C ikaite transforms to calcite. Understanding the transformation process is important to interpret paleoclimatological data from glendonites, i.e., calcite pseudomorphs after ikaite in sediments. Tollefsen et al. (2020) suggested that the transformation occurs via a coupled dissolution–reprecipitation mechanism at the ikaite–calcite interface (1). In contrast, Vickers et al. (2022) proposed a quasi-solid state ikaite to calcite transformation mechanismand suggested that stable isotope data of glendonite can be used for reconstructing paleotemperatures(2). However, in sediments the majority of the ikaite to calcite transformation occurs in diagenetic environments, where ambient solutions interact with the transforming mineral.We synthesized ikaite at 2 °C in alkaline environment in order to study its transformation using organic solvents, vacuum pumping and rapid (1 min) heating from 5 to 30 °C. These experiments indicated the formation of amorphous calcium carbonate (ACC) during the ikaite to calcite transition. We also monitored the ikaite transformation by letting the 2 °C parent solution to reach room temperature (25 °C) within ~5 hours. We observed ACC and calcite formations depending on the alkalinity of the parent solution. Our experiments suggest that the ikaite to calcite transition is a two-step process consisting of the solid-state ikaite → ACC transformation and the ACC → calcite dissolution–reprecipitation mechanisms. During these transitions ikaite lost all its water but preserved its original morphology. We hypothesize that the occurrence of a transient amorphous phase during the ikaite to calcite transition implies the alteration of the isotopic data, similar to what was reported for the ACC to calcite transition (3).We acknowledge the financial support of NKFIH ANN141894 grant.