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

1. 1995年在南海海槽东部发现了双BSR。本研究对JNOC甲烷水合物项目获得的地震剖面进行了广泛的分析，发现DBSR在南开海槽东部广泛分布。这意味着这种独特的现象可能是由区域地质事件引起的。在巡航期间，潜水到DBSR被确认的区域并不是那么富有成效。然而，渔民在DBSR地区北方延伸区偶然采集的碳酸盐结核显示了非常有趣的数据，表明该地区经历了强烈的甲烷排出，强烈的甲烷通量是由天然气水合物分解引起的.从石垣岛外黑岛穹丘的活跃甲烷渗漏点收集了各种类型的碳酸盐结核。结核的地球化学特征表明，烟囱状结核均由白云岩组成，碳酸盐碳强烈亏损13C而富集18O，表明受甲烷和水合物水的影响.从加拿大温哥华外的卡斯卡迪亚边缘采集了大量的水和沉积物样品，在那里，在活跃的甲烷渗漏点下面识别出DBSR。地球化学分析表明，表层沉积物具有氯离子正异常的特征，渗漏气体中含有大量的热成因甲烷，孔隙沃茨中硫酸盐浓度随甲烷通量的增加而降低。

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.