Sediment–pore water system associated with native sulfur formation at jade hydrothermal field in okinawa trough

Abstract

The subsurface sediment–pore water system in the Jade hydrothermal field was investigated along with pore water chemistry and X-ray absorption fine structure analysis for sediment samples. Sediments were collected by coring near the TBS vent (Core 1186MBL) and from near the Biwako vent (Core 1188MB), which are active hydrothermal vents with high (320 °C) and low (90°C) temperature fluids, respectively. Core 1186MBL is characterized by occurrences of chimney fragments in the shallow part (2–9 cmbsf) and native sulfur in the deep part (6–19 cmbsf). The results of chemical analyses of the pore water suggest a seawater recharge from the seafloor into Core 1186MBL. This hydraulic characteristic of the pore water, which is commonly observed near active submarine hydrothermal vents, leads to oxidation of sulfide minerals in chimney fragments in the shallow part by the oxic recharged seawater. The resulting acidic and suboxic pore water is transported downward, and can form native sulfur in the deeper part of Core 1186MBL. Core 1188MB shows a wide distribution of native sulfur, and its pore water chemistry indicates anaerobic oxidation of methane below 8 cmbsf and a mixing of seawater and hydrothermal fluid below 10 cmbsf. The mixing of acidic and anoxic hydrothermal fluid and seawater allows the precipitation of native sulfur in Core 1188MB, and the uniform circumneutral pH condition despite the input of acidic hydrothermal fluid. The native sulfur deposits in the arc–back-arc hydrothermal fields provide important geochemical information that is useful to understand the subsurface sediment–pore water system involving hydrothermal fluids.