Anaerobic oxidation of methane-associated carbon, nitrogen, and sulfur isotopic signatures in sediments of methane hydrate-bearing areas in northeastern Japan Sea

Understanding of the biogeochemical carbon, nitrogen, and sulfur cycles in the methane hydrate-bearing areas is important for assessing the environmental impact of methane hydrate development. Among these areas in the northeastern Japan Sea, Sakata Knoll has been well characterized by the occurrence of anaerobic oxidation of methane (AOM). However, only the limited information is available for other distinctive sites, such as Umitaka Spur. This study evaluated the geochemical data including carbon, nitrogen, and sulfur isotopic compositions and microbial community structure in and around microbial mat-covered sediments associated with methane seepage at Umitaka Spur. The rapid decrease of the interstitial water sulfate and the dominance of anaerobic methanotrophic archaea (up to 54.6% of the total prokaryote) highlighted the AOM operation in the sulfate-methane transition zone (SMTZ) positioned at 2-6 and 16-20 cm depths inside and outside the microbial mats, respectively. The carbon and sulfur isotopic compositions of the Umitaka and Sakata sediments demonstrated the presence of AOM-derived authigenic carbonates and sulfide compounds. The organic carbon contents derived from the AOM-related biomass were estimated to be 0.04-0.52 and 0.02-0.21 wt.% inside and outside the microbial mats, respectively, using the isotope mixing model. The sedimentary nitrogen isotopic composition inside the microbial mats showed the negative anomalies, suggesting nitrogen uptake by the AOM-related biomass. The estimated shallow SMTZ and extensive AOM-related biomass suggest the linkage to high methane fluxes at Umitaka Spur. Consequently, this study provides the comprehensive overview of the AOM-associated element cycles in the northeastern Japan Sea.