Formation of authigenic carbonates contributed by sulfate- and metal-driven anaerobic oxidation of methane in the northern Okinawa Trough, East China Sea

Authigenic carbonates of the northern Okinawa Trough have been shown to be contributed by both sulfate-and Fe-driven anaerobic oxidation of methane (SD-AOM and Fe-AOM, respectively); however, the microorganisms involved remain poorly constrained. To better understand the types and roles of microorganisms in specific bio-logical methane oxidation processes, authi-genic carbonates collected from the northern Okinawa Trough at water depths between & SIM;540 m and 700 m were studied using pe-trographic, mineralogical, elemental, and lipid biomarker analyses. The carbonate minerals were primarily aragonite, high -magnesium calcite, low-magnesium calcite, or dolomite. The extremely low & delta;13C values of bulk carbonates (-59.2%0), molecular fos-sils of anaerobic methane oxidizing archaea (ANME; -131%0), and sulfate-reducing bac-teria (SRB; -77%0) indicated that the au-thigenic carbonates formed as products of AOM. Biogenic methane was confirmed to be the main carbon source for all carbonates, whereas bio-degraded oil and hydrother-mal input complemented carbon sources, as revealed by the occurrence of unresolved complex mixture, strongly enriched As and Sb, and their correlations with Fe/Al. Com-bined with enriched & delta;18O values (as high as +5%0), the methane fluids were suggested to derive from the dissociation of gas hydrates. ANME-2 and a mixture of ANME-1 and ANME-2 were identified for the two calcitic carbonates, respectively, as revealed by their specific biomarker patterns. The relatively higher contents of 13C-depeleted isopren-oids and SRB-related fatty acids, strong Mo enrichment, and biological debris in both calcite-and aragonite-dominated carbon-ates reflect precipitation driven by SD-AOM in a shallow sulfate-methane transition. For other seep carbonates, low amounts of 13C-depeleted isoprenoids and the near ab-sence of crocetane suggest dominance of the ANME-1 assemblage. The extremely low amounts and non -13C-depleted iso-/anteiso-C15:0 fatty acids (-27%0) in the dolomite -con-taining carbonates indicate the decoupling of ANME and SRB. These carbonates were most likely formed below or at the bottom of the sulfate-methane transition and included contributions from Fe-AOM, as indicated by the low Mo content, occurrence of dolomite and siderite, and decreased & delta;56Fe values with increasing Fe/Al ratios.