Variation of benthic foraminiferal assemblage and carbon and oxygen isotopic composition for the last 28,000 years in the methane seep area of Joetsu, eastern margin of Japan Sea

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A number of piston cores have been recovered from the active methane seep area off Joetsu, eastern margin of Japan Sea. The sediment cores, approximately 7.0 m long, are typically composed of intensively bioturbated unit, thinly laminated unit, and bioturbated unit, in descending order. Carbon 14 age determination has revealed that the recovered sediments span the last 28,000 years, and the middle laminated unit corresponds to the low stand stage of the Last Glacial Maximum (LGM). Detailed foraminiferal study has revealed that (1) the abundance of benthic foraminifers dropped in the laminated unit whereas the planktonic ones increased for the interval, and (2) Stainforthia rotundata, which is characteristic in methane dominated environment, occurs only in the lower part of the laminated unit. Carbon isotopic composition of the benthic foraminifers shows remarkable negative shift from -1.5‰ to -3.0‰, for approximately 24,000 to 20,000 BP. These lines of evidence strongly suggest that methane seeps in the study area were enhanced for the period of about 24,000 to 20,000BP in LGM.
日本堆積学会の論文