Elucidation of source and flux of methane in marine environments

本研究旨在阐明甲烷在海洋环境中的行为，特别是在海洋沃茨和沉积层孔隙流体中。利用<13><12>本研究中新建立的同位素比值监测气相色谱/质谱系统，精确测定了甲烷的浓度和稳定碳同位素比值（^ C/^ C）。扩张中心海底热液活动区和弧火山（中印度洋海岭、亚丁湾、马里亚纳海槽、冲绳海槽、水予海山等）板块俯冲带（南海海槽、东日本海等）的冷渗区使用研究船和潜水器详细调查了这些地区的情况，以便从这些地区采集液体样本进行甲烷分析。甲烷的碳同位素比使我们能够区分以下三种不同类型的甲烷：岩浆甲烷、有机质热催化分解甲烷和生物甲烷。此外，通过检测微生物过程中甲烷的^ C富集，在孔隙流体和海水中识别出甲烷的氧化分解<13>。还讨论了海底以下的天然气水合物，认为它是从沉积物向海水输送甲烷的一个重要来源。

This study aimed at elucidating the behavior of methane in the oceanic environment, especially in oceanic waters and pore fluids in sedimentary layers. Concentration and stable carbon isotope ratios (^<13>C/^<12>C) of methane were precisely measured by using an isotope-ratio-monitoring gas chromatography/mass spectrometry system newly constructed in this study. Submarine hydrothermally active areas at spreading centers and arc volcanoes (Central Indian Ridge, Gulf of Aden, Mariana Trough, Okinawa Trough, Suiyo Seamount etc.) and cold seepage areas at plate subduction zones (Nankai Trough, Eastern Japan Sea etc.) were investigated in detail using research vessels and submersibles, in order to take fluid samples for methane analysis from these areas. The carbon isotope ratio of methane has enabled us to distinguish the following three different types of methane: magmatic methane, methane from thermocatalytic decomposition of organic matter, and biogenic methane. In addition, oxidative decomposition of methane was recognized in pore fluids as well as in seawater, by detecting the enrichment of ^<13>C of methane to occur during microbiological processes. Gas hydrate below seafloor was also discussed as an important source of methane from the sediment to seawater.