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First measurements of methane and its carbon isotope ratio in the Japan Sea (East Sea)

机译：日本海（东海）中甲烷及其碳同位素比的首次测量

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We measured dissolved methane (CH_4) and its carbon isotope ratio (~(13)C/ ~(12)C or δ~(13)C) from surface to bottom seawaters (maximum depth: 3574 m) for the first time in three major offshore areas of the Japan Sea (East Sea): the eastern Japan Basin, Yamato Basin, and Ulleung Basin. Seawater samples were taken during the KT-07-24 cruise of the R/V Tansei Maru in 2007 as part of a Japan-Korea collaborative study. The CH_4 concentration and its δ(13)C_(PDM) were obtained by continuous flow isotope ratio mass spectrometry. While the CH_4 concentrations of surface seawater were 2.6-3.8 nmol kg~(-1), which was 43( ± 22)% supersaturated compared to the atmospheric equilibrium values, those in the subsurface (D = 30-150 m) showed much higher values up to 9.7 nmol kg~(-1) (～300% supersaturation). This subsurface CH_4 had the minimum δ~(13)C value of - 54.3‰ (less than the atmospheric CH_4 value of -47‰), implying a contribution from in situ microbial CH_4 production in some anoxic microenvironments. The CH_4 concentration decreased with depth toward minimum values of-1 nmol kg~(-1) (δ~(13)C: -30 to -40‰) at 1500-2000 m depth. Below a depth of 2000 m, we found a clear difference in CH_4 profiles among the three basins; in the eastern Japan Basin the CH_4 concentration remained almost constant down to the seafloor, while it gradually increased with depth up to >2.0 nmol kg~(-1) in the Yamato and Ulleung basins. This CH_4 increase toward the bottom was accompanied by a decrease of δ~(13)C-CH_4 to less than-50‰, suggesting a microbial CH_4 supply from bottom sediment in the Yamato and Ulleung basins. We observed a local CH_4 plume over a deep channel east of the Yamato Basin, indicating unknown submarine CH_4 seepage with a microbial δ~(13)C value of about -60‰.
机译：我们在三个区域中首次测量了地表至底部海水（最大深度：3574 m）中的溶解甲烷（CH_4）及其碳同位素比（〜（13）C /〜（12）C或δ〜（13）C）日本海（东海）的主要近海地区：日本东部海盆，大和盆地和宇隆盆地。日韩合作研究的一部分是在2007年R / V Tansei Maru的KT-07-24巡游期间抽取了海水样本。通过连续流同位素比质谱法获得CH_4浓度及其δ（13）C_（PDM）。 CH_4的表层海水浓度为2.6-3.8 nmol kg〜（-1），与大气平衡值相比过饱和度为43（±22）％，而地下的D_ = 30-150 m最高值9.7 nmol kg〜（-1）（〜300％过饱和）。该次表层CH_4的最小δ〜（13）C值为-54.3‰（小于大气CH_4的值-47‰），这暗示着在某些缺氧的微环境中原位微生物CH_4的产生。在1500-2000 m深度处，CH_4的浓度随着深度的增加而降低，最小值为-1 nmol kg〜（-1）（δ〜（13）C：-30至-40‰）。在2000 m深度以下，我们发现三个盆地之间的CH_4剖面有明显的差异。在日本东部盆地，CH_4浓度直到海底几乎保持恒定，而在大和盆地和Ulleung盆地中，CH_4的浓度随着深度的增加而逐渐增加，直至> 2.0 nmol kg〜（-1）。 CH_4向底部的增加伴随着δ〜（13）C-CH_4的减少到小于50‰，表明大和盆地和Ulleung盆地底部沉积物中的微生物CH_4供应。我们在大和盆地以东的深水沟中观察到了局部CH_4羽流，表明未知的CH_4潜艇渗流，其微生物δ〜（13）C值为约-60‰。

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