Oxygen isotopic re-equilibration during transformation of ikaite to calcite via amorphous calcium carbonate

Glendonites are calcite pseudomorphs of ikaite (CaCO 3 ·6H 2 O), a mineral that typically forms below 4 °C. Ikaite is found in marine clastic sediments, caves and lacustrine sediments, and its presence is correlated with cold paleo-environments. However, it is uncertain whether the isotopic composition of glendonite preserves the low-temperature paleoclimate conditions. Here we study the ikaite to calcite transition by following the evolution of solutions containing ikaite, prepared at pH 12.1, 10.3 and 9.1, and filtered materials exposed to air. Ikaite transformation starts with the formation of 100–200 nm-sized amorphous calcium carbonate (ACC) on the surface of ikaite grains, which transforms to carbonates dominated by calcite. In Mg-free solutions, vaterite also forms, whereas low concentrations of Mg (e.g., 26 mg/l) favor aragonite formation. Our data suggest that synthetic ikaites are formed with strong 16 O-enrichment relative to other carbonate species due to kinetic fractionation. The ikaite-derived carbonates formed in contact with ambient air show a minor δ 13 C change (−3.0 ± 0.8 ‰) relative to ikaite (−4.2 ± 0.3 ‰), but their oxygen isotope compositions remain constant. In contrast, transformation in solutions results in significant oxygen isotopic changes (from 17.7 to 20.5 ‰). Calcite-water oxygen isotopic equilibrium is approached within one week at 8 °C, and one day at 15 °C. Our experiments suggest that glendonite calcites formed by ikaite dehydration during early diagenesis undergo oxygen isotope exchange with pore waters and can record temperature and water composition conditions similar to those of the ikaite formation. • Ikaite transformation in solution and in air occurs via amorphous calcium carbonate. • pH, temperature and Mg-content influence the structure of ikaite-derived carbonate. • δ 18 O values of ikaite and the transformation products formed in air are similar. • Transformation results in strong oxygen exchange with the aqueous phase. • Oxygen isotopic re-equilibration occurs during diagenesis of glendonites.