Ikaite in the methane anomaly zone on the continental slope of the sea of japan

The results of a study of authigenic carbonate nodules found in bottom sediments on the south-western continental slope of Peter the Great Bay, Sea of Japan, are presented. The concretions were recorded in the aleuropelitic sediments, presumably of late Pleistocene-Holocene age, with abnormally high methane concentrations (up to 13% vol.). X-ray diffraction analysis showed that the inner part of the carbonate nodules consists of calcite, and the outer part consists of ikaite. After a day's stay at room temperature, ikaite was transformed into calcite and vaterite. The isotopic composition of carbon (δ13С) and oxygen (δ18О) of the samples varies from -21.2 ‰ to -20.0 ‰ and from 1.3 to 1.49 ‰ VPDB, respectively. Such δ13С values suggest that organic matter was the main source of carbon during formation of ikaite. Anomalous concentrations of methane in the sediments may indicate the presence of gas hydrates on the studied area, the destruction of which leads to conditions favorable for the formation of ikaite. The study area may be a manifestation of a new gas hydrate-bearing region in the Sea of Japan. Three energy states of carbon atoms were recorded, which is new in the study of authigenic carbonate mineralization.