Mineralogies and Stable Isotopic Compositions of Methane-derived Carbonates from the Northeastern South China Sea

Three authigenic carbonates are retrieved from the seafloor of northeastern South China Sea, showing the morphologies of doughnut, concretion and crust. X-ray results show that carbonates mainly consist of clay minerals, terrigenous constituents (quartz, feldspar), high-Mg calcite, and ankerite. Except the doughnut, the concretion and the crust are characterized by very light δ¹³C values (-32.83‰PDB and -38.01‰PDB, respectively) and positive values of δ¹⁸O (2.19‰PDB and 2.96‰PDB, respectively) in comparison with ‘normal carbonates’. These isotopic compositions of the carbonates suggest that methane participated in the formation of carbonates, due to its anaerobic oxidation. The methane-derived carbonates are the recorders of methane-associated seeping, which is active on the seafloor of northeastern South China Sea in the past.