Integrated study of environmental impact of gas hydrate

该项目的主要目标是通过地质、地球物理和海洋生物方法以及整合新获得的数据集，对海洋天然气水合物的环境影响进行评估。以下是3年课程的总结和成果。我们由东京海洋科技大学的“T/V Umitaka-maru”和JAMSTEC.2的“Natsushima”和“Kaiyo”在直越附近的天然气水合物领域进行了水合物研究巡航。我们在日本海直越附近发现了强烈的甲烷渗漏点。在海平面以下约900米的海底，甲烷气泡柱高达600米。据观察，这些羽流向浅水提供了大量的甲烷。底部和中间水温度很低，-0.2℃，使得气泡在水团中形成天然气水合物。这就是羽流到达浅水区的主要原因。“超级海豚”号潜水器首次在日本岛屿附近的海底发现了天然气水合物。这一发现表明研究区存在很强的甲烷通量。可能由于温度较低，底栖生物群落相对简单和单调，但以化学合成群落和细菌席为主。螃蟹（“Beni-zuwai-gani”）在甲烷渗漏点及其附近非常丰富。在Umitaka断层上发育了直径约500米的大麻坑，在那里观察到甲烷渗漏和天然气水合物。麻子是不活动的，部分被泥浆掩埋，这意味着过去发生过极其剧烈的甲烷渗漏。在西北太平洋下田半岛附近，我们还乘坐了JAMSTEC的“Tansei-maru”号进行了天然气水合物巡航。虽然没有发现甲烷渗漏或羽状物，但强烈且异常的bsr似乎表明地下天然气水合物的发育和甲烷的生成。

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.