Abstract IPOD/DSDP Leg 50 collected 18 carbonate nodules drilling the inner trench slope of the Japan Trench. They found the nodules in the sedimentary sequence deeper than 180 m where abundant methane occurs. The carbonate minerals in the nodules were comprised of magnesian calcite, dolomite, manganese calcite, and rhodochrosite. Petrographic evidence and oxygen and carbon isotopic compositions show that these carbonate nodules were authigenically formed at or near the position where they were found. Carbon isotopic compositions of many of these nodules reveal that the carbon in carbonate nodules may have been derived from carbon dioxide evolved during the sulfate reduction by microbiological activities.