オホーツク海のメタンハイドレートとプルーム
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概要
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Methane hydrates exist beneath the sea bottom near cold seeps NE off the Sakhalin in theSea of Okhotsk. Multidisciplinary field operations were performed at a study area （approximately16 × 20 km2）to investigate seepage characteristics and understand gas hydrate formationmechanisms. A continuous profiling survey was conducted to obtain a distribution map ofseepage structures on the floor by using a deep-tow, side-scan-sonar equipment. The distributionmap reveals that the dense area of seepage structures coincides with a sea-floor area of deformedsediments caused possibly by repeated sediment slumping and debris flows in the past. We speculatethat this deformation may have created shallow faults that are suitable to conduits for themigration and discharge of gas and fluid. Three seepage structures were selected to study about their fluid-seep conditions around thesea floor level. Hieroglyph seepage structure is located at the northern end of the dense area ofthe structures. Kitami and Chaos structures are located about 2 and 7 km respectively apartfrom the Hieroglyph structure within the dense area. Large plumes on echograms and highermethane contents in the water column confirm gas seepage activities at the three structures.There observed at least two and four plumes at the Hieroglyph and Chaos structures, respectively.Each gas chimney image in seismic reflection profiles was traced to connect each BSR andseepage structure. Both pull-up and disturbed structures of BSR around the gas chimney imageswere interpreted as to be indications of significant heat flows caused by ascending fluid atboth Kitami and Chaos structures. On the other hand, almost no pull-up/disturbance of BSR was observed at the Hieroglyph structure, suggesting little water seepage. The seep activity may vary with time off the Sakhalin. The Hieroglyph structure is locatedat the edge of a dense area of the seepage structures. It might serve as an indicator for the longtermactivity of the fluid seepage system off the Sakhalin.
- 東京地学協会の論文
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