STABLE ISOTOPIC-RATIOS AND ORIGINS OF THE CARBONATES ASSOCIATED WITH COLD SEEPAGE AT THE EASTERN NANKAI TROUGH

Two types of cold-seeps were located at the eastern Nankai Trough: (1) seeps on sediment-covered seafloor of 3800-3600 m in depth at the toe of the accretionary prism, and (2) those between 1900 and 2100 m, situated on the backthrust of the accretionary prism covered with rough outcrops of calcareous sandstones, Fragments of sandstone pipes or chimneys cemented with calcite were recovered from type-1 seeps, whereas in type-2 seeps, massive carbonate-cemented sandstones and silty sandstones were obtained, many of which contain cemented fragments of different lithologies and textures, including dense aggregates of fossil Calyptogena-type clam shells. The carbon isotopic ratios of the carbonates in these sediments range from -36 to -56 parts per thousand PDB. The carbonates formed by mixing between isotopically light carbonate ions derived from biogenic methane carried from depth in advecting fluids and heavy marine sedimentary carbonates. Sharp isotopic gradients in carbon, and sometimes also in oxygen, from the mixing processes are still retained across the walls of the sandstone pipes from type-1 seeps, whereas the isotopic ratios of the samples from type-2 seeps exhibit variation patterns related to lithologies and textures, and confirm the presence of several generations of carbonate cement within a single specimen. These facts and the oxygen isotopic ratios of live and fossil clam shells recovered from the survey areas suggest that the cold-seeps started at least during the last glaciation and have experienced repeated destruction and reformation due to episodic supply of methane-rich fluid of different isotopic ratios and/or temperature.