Formation of authigenic carbonates at a methane seep site in the middle Okinawa Trough, East China Sea

Abstract

The formation of authigenic carbonates at cold seep is closely related to underlying methane seepage and anaerobic oxidation of methane (AOM), and these processes can be reflected in the geochemical characteristics of pore water in the sediments. In terms of petrological, mineralogical and geochemical analyses of research materials, including authigenic carbonates and pore water samples from the middle Okinawa Trough, East China Sea,these results show that authigenic carbonate minerals are mainly composed of high-magnesium calcite, low-magnesium calcite, dolomite, and iron dolomite and exhibit extremely negative carbon isotopic compositions (δ13C = −52.30‰ to −31.10‰ V-PDB) and positive oxygen isotopic compositions (δ18O = 3.36‰ to 7.27‰ V-PDB). Geochemical analyses of pore water suggest linear sulfate gradients and steep increase of methane and total alkalinity concentrations with depth, which indicate sulfate-induced AOM activities. Based on the sulfate concentration reduction, carbon isotope characteristics of dissolved inorganic carbon (DIC) and increased methane concentration, the sulfate-methane interface (SMI) is considered to be shallow. These data, combined with the variations in ion concentrations in pore water, suggest that the leakage of biogenic methane and intense AOM resulted in the formation of abundant authigenic carbonates, which inherited light carbon (12C)-enriched isotopic signature of methane in the shallow sediments. Combined geochemical characteristics of pore water and authigenic carbonates, intense methane seep activities and associated AOM can be inferred both in present-day and ancient geological records on the western slope of the middle Okinawa Trough.