Different AOM processes driving the formation of carbonate crusts and nodules in the Okinawa Trough

Anaerobic oxidation of methane (AOM) serves as a critical process that consumes methane formed in marine sediments and induces the precipitation of cold seep carbonates. However, the factors influencing sulfate-driven AOM (SD-AOM) and iron-driven AOM (Fe-AOM) during the formation of different types of carbonates remain unclear. In this study, mineralogical and geochemical studies were conducted on cold seep carbonate crusts and nodules in the northern Okinawa Trough (OT). Carbonate crusts exhibit rough surfaces with loose textures, while carbonate nodules are characterized by dense and homogeneous textures. The carbonate minerals in the crust samples are predominantly aragonite, whereas the primary carbonate minerals in the nodule samples are calcite and dolomite, as well as calcite and aragonite. The negative δ¹³C values of all samples indicate that the carbon is mainly sourced from biogenic methane. The δ¹⁸O enrichment suggests that the fluid involved in authigenic carbonate formation could be derived from the dissociation of natural gas hydrates. Mineralogical and rare earth element (REE) features reveal that carbonate crusts formed close to the seafloor under high methane fluxes, whereas carbonate nodules developed at greater depths under lower methane fluxes. The predominance of aragonite and low δ¹⁸O_CAS/δ³⁴S_CAS slope (carbonate-associated sulfate, CAS) indicate that SD-AOM drove the precipitation of carbonate crusts. In contrast, the presence of dolomite and siderite, iron-rich microbial structures, and elevated levels of carbonate-associated Fe collectively support the contribution of Fe-AOM during the formation of carbonate nodules. Our study highlights that the distinct formation conditions of carbonate crusts and nodules govern their differential AOM processes.