Anomalies of bottom CH₄ and trace metal concentrations associated with high heat flow at the Calyptogena community off Hatsu-shima Island, Sagami Bay, Japan: A preliminary report of Tansei Maru KT-88-1 cruise Leg-1

Geochemical studies of bottom waters and sediments, including deep sea camera surveys and heat flow measurements, were conducted in the western Sagami Bay, central Japan, where dense clam (Calyptogena soyoae) communities had been previously located by submersible observations. The bottom waters above the communities contained an order of magnitude higher CH₄(360-4200 nl kg^-1) than those at nearby stations a few kilometers away from the community site (- 130 nl kg’¹), implying significant fluid venting rich in CH₄ from the bottom at the very community site. The mean life of the seepage CH₄ in seawater was estimated to be several days by using a simple vertical diffusion model. Weak correlations were observed between the concentrations of CH₄ and trace metals such as Fe, Mn, AI, and Zn. It was found that the community site is characterized by anomalously high heat flow up to 1680 mW m^-2, suggesting volcanic heat sources below the site.