Fresh water seepage and pore water recycling on the seafloor: Sagami Trough subduction zone, Japan

We collected 19 samples of bottom sea water (including 8 samples of shimmering fluid), using the Japanese submersible Shinkai 2000, in the bathyal Calyptogena community area (the Hatsushima area) along Sagami Trough, at the northern end of the Philippine Sea plate, in order to clarify the source of fluid seepage. Shimmering fluid samples showed a maximum temperature of 11.6°C, which is about 9°C higher than the ambient sea water. The fluid chemistry of these samples is summarized as follows: (1) alkalinity, H₂S, SiO₂, NH₄, ΣCO₂ and CH₄ show enrichment and Cl, SO₄, K, Na, Mg and Ca show depletion compared with ambient sea water; (2) Cl depletion is about 20%; and (3) the content and ³He/⁴He ratio of dissolved helium show little anomaly compared with ambient sea water. The observed chemical composition and temperature of the fluid is well explained by mixing of sea water, pore water and land-derived groundwater with a temperature of about 40°C and a seeping flux of about 400 m³ per day. A preliminary estimation of the global chemical flux implies that groundwater discharge to the ocean (including pore water recycling) is an important factor in controlling ocean chemistry.