FRESH-WATER SEEPAGE AND PORE-WATER RECYCLING ON THE SEA-FLOOR - SAGAMI TROUGH SUBDUCTION ZONE, JAPAN

We collected 19 samples of bottom sea water (including 8 samples of sh immering fluid), using the Japanese submersible Shinkni 2000, in the b athyal Calyptogena community area (the Hatsushima area) along Sagami T rough, at the northern end of the Philippine Sea plate, in order to cl arify the source of fluid seepage, Shimmering fluid samples showed a m aximum temperature of 11.6 degrees C, which is about 9 degrees C highe r than the ambient sea water. The fluid chemistry of these samples is summarized as follows: (1) alkalinity, H2S, SiO2, NH4, Sigma CO2 and C H4 show enrichment and Cl, SO4, K, Na, Mg and Ca show depletion compar ed with ambient sea water; (2) Cl depletion is about 20%; and (3) the content and He-3/He-4 ratio of dissolved helium show little anomaly co mpared with ambient sea water. The observed chemical composition and t emperature of the fluid is well explained by mixing of sea water, pore water and land-derived groundwater with a temperature of about 40 deg rees C and a seeping flux of about 400 m(3) per day. A preliminary est imation of the global chemical flux implies that groundwater discharge to the ocean (including pore water recycling) is an important factor in controlling ocean chemistry.