Ikaite formation in the recent sediments of the Kara Sea, Arctic Ocean

The article discusses possible mechanisms for the formation of ikaite (CaCO₃·6H₂O) in the sediments of the Kara Sea, Arctic Ocean. We combined our own data with the results of previous studies on calcium carbonate hexahydrates in this region. X-ray diffraction analysis of ikaite revealed its transformation into calcite, providing valuable insight into its mineralogical evolution. Stable isotope analyses suggest that the main process leading to ikaite formation is microbial degradation of organic matter through sulfate reduction, which increases the concentration of dissolved inorganic carbon and phosphate ions in the pore water. Carbon that enters the bicarbonate ion as a result of anaerobic oxidation of methane (AOM) can also contribute to the formation of ikaite in some cases. However, AOM alone does not lead to the release of magnesium or phosphate ions, which are known to inhibit calcite precipitation. Consequently, without the simultaneous microbial decomposition of organic matter or the presence of unstable magnesium-rich minerals, AOM alone is not sufficient for ikaite formation. Our sample analysis shows that the proportion of methane-derived carbon in ikaite does not exceed 62%, while the contribution of organic carbon can be as high as 100%. Furthermore, rare earth element analysis supports the significant role of diagenetic organic matter degradation in sediments where ikaite is present. These findings highlight the complex relationship between the decomposition of organic matter, the oxidation of methane, and the formation of ikaite in the early diagenetic sediments of the Kara Sea.