THE cold seepage has been revealed in Off Hatsushima located at western Sagami Bay, central Japan in1984 by the submersible Shinkai 2000, which is characterized in dense deep-sea biological communitiesdominated by the White Clam Calyptogena soyoae. The clam communities stretch for 7 km along the footof a steep escarpment off Izu Peninsula at water depths of 900-1200 m. The area is located near theconvergence from the northern tip of the Philippine Sea Plate, which is subducting beneath the Eurasianand North American plates. Although numerous studies have become active around this region since1986, most of previous works were focused on biology, geology and physics. The geochemical data areextremely limited. The source of the cold seep fluid is not well known. In order to understand the control THE cold seepage has been revealed in Off Hatsushima located at western Sagami Bay, central Japan in 1984 by the submersible Shinkai 2000, which is characterized in dense deep-sea biological communities by the White Clam Calyptogena soyoae. The clam communities stretch for 7 km along the footof a steep escarpment off Izu Peninsula at water depths of 900-1200 m. The area is located near theconvergence from the northern tip of the Philippine Sea Plate, which is subducting beneath the Eurasianand North American plates. region since1986, most of previous works were focused on biology, geology and physics. The geochemical data areextremely limited. The source of the cold seep fluid is not well known. In order to understand the control