Integrated study of environmental impact of gas hydrate

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The main objective of this program is to make an assessment of the environmental impact of marine gas hydrates through the geological, geophysical, and marine biological approaches and integration of newly obtained data sets. The following are the summary and results of the 3 years program.1. We conducted hydrate-research cruises in the gas hydrate field off Naoetsu by "T/V Umitaka-maru" of Tokyo University of Marine Science and Technology and "Natsushima" and "Kaiyo" of JAMSTEC.2. We identified strong methane seep sites off Naoetsu in Japan Sea. Plumes of methane bubbles stand about 600 meter high on the sea floor of about 900 meter below sea level. The plumes were observed to supply significant amount of methane to shallow waters. The bottom and intermediate waters are very low, -0.2℃, which allows gas bubbles to form gas hydrate in the water masses. This is the main reason why the plumes reach up to shallow waters.3. Submersible dives by "Hyper dolphin" found gas hydrate exposed on the sea floor for the first time around Japan islands. This finding suggests strong methane fluxes in the study area.4. Probably because of low temperatures, benthic communities are relatively simple and monotonous, but characterized by chemosynthetic communities and bacterial mats. Crabs ("Beni-zuwai-gani") are extremely abundant on and near the methane seep sites.5. Large pockmarks, approximately 500 m in diameter, develop on the Umitaka spur where methane seeps and gas hydrate were observed. The pockmarks are inactive, partly buried by mud, implying that extremely violent methane seeps occurred in the past.6. We also conducted gas hydrate cruise off Shimokita peninsula, northwestern Pacific by "Tansei-maru" of JAMSTEC. We could not identify methane seeps or plumes but strong and unusual BSRs seem to indicate development of subsurface gas hydrate and methane generation.