Geological-geophysical studies carried out in the Ocean during recent decades have revealed widespread indications of fluid discharge from the sea floor. We have found that gas hydrate formation in the near sea floor corresponds to the
majority of these fluid discharges. The shallowest water depths in which structure II hydrates have been detected on the seafloor are 480 m in the Caspian Sea and 530 m in the Gulf of Mexico. The shallowest depth of water at which of structure I hydrate has been recovered from the seafloor is 800 m in the Gulf of Mexico (Brooks et al., 1986). At higher latitudes, where the near sea floor temperature of water is lower, gas hydrate can exist on the sea floor at shallower water depths (theoretically as shallow as 300 m), although none have yet been recognised.
All fluid discharges on the sea floor (Fig. 1) are comprised of water, gas and petroleum, or a mixture of them. In physical form, seafloor features associated with these discarges can be characterized as pockmarks, mud volcanoes (MVs), clay diapers; chemosynthetic communities; particular affiliations of authigenic mineralization; sea-bottom accumulation of gas hydrate; and also low-temperature hydrothermal vents and some geophysical features (VAMP's and "Pagoda" structures). The distribution of fluid discharge areas on the sea floor in deep-water parts of the ocean is often closely related to tectonic features. Fluid discharge areas are mostly found at continental slopes and rises, and in closed and border seas. Conditions in these areas are ideal for both the generation of bio- and therm-ogenic gas, and for fluid migration towards the sea floor.
A large majority of the fluid venting sites appear to be the result of concentrated fluid migration through the sea floor. More than half of all known areas having fluid venting are related to mud volcanoes and clay diapirs. By in large, the mud volcanoes, diapirs and pockmarks are related to tectonically active zones (collision and subduction), especially with accretionary complexes. Mud volcanoes and diapirs are less common in areas that have a thick and less disturbed sedimentary cover. The fluid venting in these regions is distributed more locally and is usually associated with basement tectonic troughs, large slides and large-scale fault zones. The discharged fluid flows exert a substantial influence on formation of authigenic minerals (carbonate concretions, crust and build-ups, which exceed in some regions tens of meters in height) in the near-seafloor sediments.
A separate group of fluid vents are associated with low-temperature hydrothermal sources. In these vents, not only are hydrocarbon-rich fluids observed, but carbon dioxide and hydrogen sulphide rich fluids are also found. In the near-bottom deposits near the hydrothermal vents, authigenic minerals of barite, sulfur and silica also occur. Rare, naturally occurring carbon dioxide gas hydrates have been sampled from the sea-bottom in one of the low-temperature hydrothermal vents of the so-called "black smokers" in the Okinawa Trough in the East China Sea (Sakai et al., 1990).
Particular chemosynthetic benthic biological communities, such as Pogonoforah and Vestimentiferah tubeworms, methanotrophic bivalves, bacterial mats and others, can also be used as indirect indications of the bottom fluid venting. Geophysical anomalies, such as reflection seismic "VAMP's", can be considered as substantial indications of fluid discharging. The Bering Sea, where more than 300 anomalies have been recognized over an area of 25,000 square kilometres (Scholl & Cooper, 1978), is probably the best-known region hosting these features.
Most frequently, accumulations of gas hydrates are associated with mud volcanoes (Fig. 2). At present, gas hydrates have been recovered by gravity coring from more than 20 mud volcanoes: Buzdag and Elm in the Caspian Sea; Blake-diapir in the north-western Atlantic; Haakon Mosby in the Norwegian Sea; La-Atalante offshore Barbados; Ginsburg and Bonjardim in the Gulf of Cadiz; Kula, Milano and Amsterdam in the eastern Mediterranean; Moscow University, Tredmar, Kovalevsky, Vassoyevich, Nioz, Kazakov, Odessa etc. in the Black sea. Many more mud volcanoes in potential gas hydrate-bearing areas are known through geophysical investigations: About 40 regions with mud volcanoes and/or clay diapirs are known at the shelf borders, and a total amount of 103 in the deep-water regions of World Ocean (Dimitrov, 2001). It is likely that all active deep-water mud volcanoes are associated to some degree with the development of near sea floor gas hydrates.
Accumulations of gas hydrates associated with the venting of gas and/or gas-bearing water are also widespread, but are not so numerous in comparison with hydrate accumulations related to mud volcanoes. Gas hydrates within the gas venting sites are known in the following areas: in the Sakhalin slope of the Derugin Basin and near Paramushir Island in the Sea of Okhotsk (Fig. 3), in the Gulf of Mexico and offshore Northern California (Fig. 1, also see Kvenvolden,).
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