RECONSTRUCTION OF PALEO-SEEP ECOSYSTEM IN THE LATE CRETACEOUS FORE-ARC BASIN OF NORTHWESTERN PACIFIC USING CARBON AND OXYGEN ISOTOPE RATIO AND MOLECULAR MARKERS

Recognition of microbial diversity and activities is very important to reconstruct chemosynthesis based ecosystem. However, it is difficult in the fossil record. In this study we analyzed biological markers and carbon and oxygen stable isotope ratios of carbonate mounds in the Upper Cretaceous (Late Santonian to Early Campanian) fore-arc basin deposits (Yezo Super Group) in Hokkaido, northern part of Japan. The carbonate mounds are surrounded by muddy deposit and yield abundant remains of tube worms, bivalves, gastropods and brachiopods. The carbon isotope ratios of the carbonate samples analysed range from -39 to -49 per mil vs PDB. The results of biomarker analysis show the abundance of PME (pentamethyleicosane) and crocetane in them. Carbon isotope ratios of these hydrocarbons are below -100 per mil(PDB). These organic compounds are regarded as biological markers of Archaea, particularly anaerobic methane oxidation archaea (AMOA). Presence of the biological markers of AMOA and extremely negative values of carbon isotope ratios in the carbonate mounds strongly suggests an existence of abundant methane. In conclusions, we reveal a part of paleo-ecosystem associating a fossil chemosynthetic community.