CHARACTER AND ORIGIN OF AUTHIGENIC CARBONATES IN THE WESTERN CONTINENTAL SLOPE OF THE EAST JAPAN SEA

Authigenic carbonates collected from a sediment core on the western continental slope of East Japan Sea have been synthetically studied on X-ray diffraction (XRD), scanning electron microscope (SEM), geochemistry, and carbon and oxygen isotope. The analysis results on XRD and SEM show that carbonate minerals are mainly composed of authigenic high-Mg calcite (HMC) in grain shape, while authigenic elongate aragonite only appears in one layer. Combining the analysis on geochemical composition, we deduce that the studied carbonates come from fluids enriched in Ca²⁺, Mg²⁺, and HCO₃^-. The moderately depleted ¹³C (-33.85‰~-39.53‰) reflects that carbon should be mainly derived from the anaerobic oxidation of methane, which is also an evidence for gas venting in the local seafloor. The heavy ¹⁸O values (5.28‰~5.31‰) should be closely related with ¹⁸O-rich fluids, resulting from the decomposition of gas hydrate. Therefore, the studied carbonates should precipitate from upward cold seep fluids enriched in methane, indicating the occurrence and dissociation of underlying gas hydrate.