Authigenic carbonate formation influenced by freshwater inputs and methanogenesis in coal-bearing strata offshore Shimokita, Japan (IODP site C0020)

Authigenic carbonate chemistry and mineralogy provide key insights into diagenetic pathways and microbial processes in sedimentary records. We characterize the mineralogy, elemental composition, and isotopic composition of 28 authigenic carbonate nodules recovered from coal-bearing forearc sediments offshore Shimokita Peninsula (Japan) to better understand fluid sources, diagenetic environments, and biogeochemical processes associated with subsiding sediments containing high organic carbon content. The carbonate nodules were collected at Integrated Ocean Drilling Program (IODP) Site C0020 from sediment cores between 1604 and 2460 m below seafloor (mbsf) spanning a transition from terrestrial to marine depositional environments. These carbonates range from <1 cm to 9 cm thick and most commonly occur in sandstones/sands adjacent to the 2 km-deep early Miocene to late Oligocene lignite beds deposited within terrestrial to estuarine sediments. Almost all samples are composed of relatively pure siderite (74-92% FeCO3) with two interlayered siderite and low Mg-calcite samples below 2400 mbsf. High δ13C (−1.2 to +12.8‰ VPDB) signatures indicate the carbonates were precipitated within the zone of methanogenesis, which was likely coupled to the weathering of silicate minerals, with no evidence of any carbonate produced as a result of anaerobic oxidation of methane. Low δ18O values (−7.4 to −1.6‰ VPDB) suggest the influence of meteoric waters during carbonate precipitation. High 87Sr/86Sr values in the carbonates within the main coal-bearing unit relative to the values in pore water suggest mineral precipitation began more shallowly than their current depth and was influenced by meteoric water and/or volcanic material weathering endmembers. Low rare earth element (REE) content and enrichment in heavier REEs suggest carbonate precipitation occurred mainly during early burial. Iron availability for siderite precipitation was likely influenced by microbial reduction of Fe (oxyhydr)oxides. The precipitation of these carbonates likely began within shallow freshwater/brackish aquifers and continued with burial, based on their composition and host strata.