9:30 AM - 9:45 AM
[MIS16-03] Time-series analysis of pore waters collected by OsmoSampler from the perspective of gas venting strength in shallow gas hydrate field, Japan Sea
Keywords:Gas venting, Shallow gas hydrate, Japan Sea, Long-term fluid sampling system, Interstitial water, OsmoSampler
To clarify how the difference of gas venting density and strength among sites have affected the geochemical environment including shallow gas hydrate system, we have collected interstitial waters at 30 cm below the seafloor for one year using a long-term osmotic fluid sampling system called OsmoSampler at three sites; significant venting site at Torigakubi Spur, venting site at Umitaka Spur and venting-free site at Torimi Guri, and have measured concentrations of dissolved ions and gases with a resolution of ~1 day.
All the major ion concentrations show synchronous increase and decrease repeatedly over periods of 3~5 days at all sites. Spiky changes are also present but appear irregularly. The range of synchronous change and frequency of spiky change are obviously different among sites. The largest synchronous change through one year is observed at Torigakubi Spur, which corresponds to high density and volume of gas venting. The higher density and volume of gas venting induces massive/rapid formation of gas hydrate, which may significantly control ion concentrations around gas hydrate deposits. The changes of methane concentration is also large at Torigakubi Spur, exceeding 10,000 mM, 5,000 times higher than other sites, the interstitial water may contain methane bubbles. There are no obvious correlations among methane concertation, venting density, and volume, this is because gas hydrate growth triggered by enhanced gas venting might have plugged the gas migration paths. We have clarified that the geochemical environments of interstitial water in shallow gas hydrate fields are constrained mainly by the density and volume of gas venting. Further investigations will clarify the relationship between venting strength and interstitial water geochemistry.
This study was conducted under the commission from AIST as a part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry, Japan.