Long-Term Monitoring at Hydrothermal Sites of Suiyo Seamount, Izu-Ogasawara Arc, Western Pacific

Many attempts have been made to acquire time-series measurements at Suiyo Seamount, Izu-Bonin Arc, Japan, during Aug. 2001 - Aug. 2002. Among many instruments deployed on the seafloor at a hydrothermal site in the summit caldera of the Suiyo Seamount, we report the results by Medusa/Gemini (deep-sea fluid monitoring system) and SAHF (Stand Alone Heat Flow meter). We deployed Medusa/Geminis at hydrothermal systems in August 2001 and August 2002 for about two weeks. The Medusa/Gemini monitors the flow rates and temperatures of effluent flowing out of cased seafloor boreholes and areas of warm diffusive flow. Fluctuations in the fluid temperatures show strong correlation with seawater temperatures for all Geminis. The fluid velocities show strong positive correlation with those of temperatures in cases of Geminis on top of cased seafloor boreholes; however, they show negative correlation in cases of Geminis on diffusive flow areas. It suggests that the fluid is driven by buoyancy through the cased seafloor borehole. Spectral analyses provide no strong evidence for tidal modulations. We monitored temperatures for about four months using 60-cm geothermal probe (SAHF) in the hydrothermal site. Temperatures showed tidally correlated fluctuations and long-term, a month-long, fluctuations. These fluctuations can be explained by the movement of bottom water induced by tidal pressure variations. This research was funded by Ministry of Education, Science and Technology through Special Coordination Fund "Archaean Park" project.