Double BSR in Nankai Trough as a tool to study stability of subsurface gas hydrates

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1. Double BSR was reported from the eastern Nankai Trough in 1995. This research has conducted extensive analysis of seismic profiles obtained by JNOC methane hydrate project, and has revealed that DBSRs are widely distributed in the eastern Nankai Trough. This implies that this unique phenomena were probably caused by regional geologic events.2. Submersible dives to the area where DBSR were recognized were not so productive during the cruise. However, carbonate nodules which were incidentally collected from the northern extension of DBSR area by fisherman showed very interesting data, indicating that the area experienced strong methane expultion, and strong methane flux were caused by gas hydrate dissociation.3. Variable types of carbonate nodules were collected from active methane seep sites on the Kuroshima Knoll off Ishigaki Island. Geochemical signatures of the nodules indicate that chimney like nodules are all composed of dolomite, carbonate carbon is strongly depleted in 13C and enriched in 18O, suggesting influence of methane and hydrate-derived waters.4. Large number of water and sediment samples have been collected from the Cascadia margin off Vancouver, Canada, where DBSRs are identified below active methane seep sites. Geochemical analysis has revealed that top sediments are characterized by positive anomaly of chloride, seep gases contain significant amount of thermogenic methane, and sulfate concentration of pore waters decrease with depth depending of methane flux.