- 公益社団法人 東京地学協会
- 地学雑誌 (ISSN:0022135X)
- vol.118, no.1, pp.7-42, 2009-02-25 (Released:2010-04-05)
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Gas hydrate, an ice-like solid compound composed of methane and water molecules, was “re-discovered” from ocean sediments in the mid-20th century, while it had been known as a chemical material to chemists and chemical engineers even in the early 19th century. Since the re-discovery of natural gas hydrate it has been attracting growing interest among geoscientists from the viewpoint of potential natural gas resources, possible impact on global environmental changes, and trigger of geo-hazards such as landslides and coastal erosion. The development of gas hydrate science has been marked by a rapid increase of studies in publications from 1991 to 1999, reflecting ODP expeditions to the mid-America Trench and Blake Ridge, where deep corings recovered solid gas hydrate samples. The number of papers in international journals has increased to 500 to 600 annually in the last few years. Recent development of marine geology and geophysics, in particular of the Ocean Drilling Program (ODP), has dramatically increased our knowledge of gas hydrate and related phenomena. Bottom simulating reflector (BSR) on seismic profiles corresponds to the base of the gas hydrate zone in sediments, and is considered to be a useful tool to identify the distribution of marine gas hydrates. The base of gas hydrate stability (BGHS) is determined from P-T conditions of sediments and water depth, and BSR is expected to occur at the depth of BGHS. However, BSR is not always consistent with BGHS; and, in some cases, even two BSRs are identified at around the depth of BGHS. These observations seem to imply that marine gas hydrate is not necessarily stable at the present position but represents ephemeral and transient conditions. Integrated research activities of scientific projects and industry exploration efforts have identified two types of gas hydrate in marine sediments. These are deep-seated, stratigraphic-type deposits and shallow/structural accumulation. Japan's long-term exploration project led by Ministry of Economy, Trade and Industry (METI) has been targeting the stratigraphic type in the Nankai Trough, where 40 tcf of methane has been estimated to occur as concentrated gas hydrate deposits. Shallow accumulations are usually associated with gas chimney structures, and are common throughout the marginal seas of the western Pacific. Massive accumulation of the shallow type seems to be promising for gas production from gas hydrate as well. Sudden and major changes to the earth's environment and mass extinctions are characterized by sharp negative excursions of carbon isotopic composition. Massive dissociation of C-13 depleted gas hydrate with δ13C of -40 to -100‰, is believed to have caused such global changes. The Paleocene-Eocene boundary event (PETM event) is the best-explained case of gas hydrate-induced biotic overturn. However, serious problems have recently emerged from considerations of thermal propagation through sediments. A sudden increase of methane concentration at the Last Glacial Maximum has also been considered to result from gas hydrate dissociation, but the response of gas hydrate was not so simple during the Quaternary, when low sea level during glacial periods possibly de-stabilized subsurface gas hydrate, unlike the PETM of ice-free ocean.