Long-term, continuous OsmoSampler results for interstitial waters from an active gas venting site at a shallow gas hydrate field, Umitaka Spur, eastern margin of the Japan Sea

Abstract Highly active gas venting and outcrops of gas hydrates were recently found at the Umitaka Spur, on the eastern margin of the Japan Sea, through seafloor observations using a remotely operated vehicle (ROV). This was followed by sampling by using conventional coring techniques. In this study, we deployed a long-term osmotic fluid sampling system (OsmoSampler) to record geochemical changes in the shallow sedimentary environment over time. The OsmoSampler collected interstitial water at 30 cm below seafloor (cmbsf), providing us with a continuous daily record of the concentrations of SO 4 2− , Cl − , CH 4 , and C 2 H 6 dissolved in interstitial waters over one year. General mean concentrations of SO 4 2− and Cl − repeatedly showed synchronous increases and decreases over both long-term (i.e. dozens of days) and short-term (i.e. 3–5 days) periods. Such fluctuations relative to background seawater concentrations were likely caused by saline and fresh water generation due to gas hydrate formation and dissolution. The CH 4 concentration was significantly higher (>1 mM) during large fluctuations in SO 4 2− and Cl − over the first 3 months, which was attributable to gas-venting activity. This high venting activity of CH 4 could have promoted the rapid, large-scale formation of gas hydrate. The CH 4 concentration decreased gradually after the first 3 months, and was accompanied by a period of relatively low SO 4 2− and Cl − concentrations due to the predominance of gas hydrate dissolution around the sampling site. In later months, the C 2 H 6 concentration approached ∼1 μM and was accompanied by occasional, positive CH 4 spikes, reflecting the release of trapped gases associated with the dissolution of hydrates. These processes are essentially controlled by the fluctuations in gas concentrations along the gas migration path, and are well-characterized by shallow interstitial water geochemistry.