<P> Chemosynthetic communities thrive where cold fluids seep out of the forearc . Cold seep communities have been discovered in inner trench slopes down to depths of 7000 m in the western Pacific, especially around Japan, in the Eastern Pacific along North, Central and South America coasts from the Aleutian to the Peru--Chile trenches, on the Barbados prism, in the Mediterranean, and in the Indian Ocean along the Makran and Sunda convergent margins . These communities receive much less attention than the chemosynthetic communities associated with hydrothermal vents . Chemosynthetic communities are located in a variety of geological settings: above over-pressured sediments in accretionary prisms where fluids are expelled through mud volcanoes or ridges (Barbados, Nankai and Cascadia); along active erosive margins with faults; and along escarpments caused by debris slides (Japan trench, Peruvian margin). Surface seeps may be linked to massive hydrate deposits and destabilization (e.g. Cascadia margin). High concentrations of methane and sulfide in the fluids escaping from the seafloor are the principal energy sources for chemosynthesis . </P> <P> There are several factors that control the depth of trenches . The most important control is the supply of sediment, which fills the trench so that there is no bathymetric expression . It is therefore not surprising that the deepest trenches (deeper than 8,000 m (26,000 ft)) are all nonaccretionary . In contrast, all trenches with growing accretionary prisms are shallower than 8,000 m (26,000 ft). A second order control on trench depth is the age of the lithosphere at the time of subduction . Because oceanic lithosphere cools and thickens as it ages, it subsides . The older the seafloor, the deeper it lies, and this determines the minimum depth from which the seafloor begins to descend . This obvious correlation can be removed by looking at the relative depth, the difference between regional seafloor depth and maximum trench depth . Relative depth may be controlled by the age of the lithosphere at the trench, the convergence rate, and the dip of the subducted slab at intermediate depths . Finally, narrow slabs can sink and roll back more rapidly than broad plates, because it is easier for underlying asthenosphere to flow around the edges of the sinking plate . Such slabs may have steep dips at relatively shallow depths and so may be associated with unusually deep trenches, such as the Challenger Deep . </P> <Table> <Tr> <Th> Trench </Th> <Th> Ocean </Th> <Th> Maximum Depth </Th> </Tr> <Tr> <Td> Mariana Trench </Td> <Td> Pacific Ocean </Td> <Td> 11,034 m (36,201 ft) </Td> </Tr> <Tr> <Td> Tonga Trench </Td> <Td> Pacific Ocean </Td> <Td> 10,882 m (35,702 ft) </Td> </Tr> <Tr> <Td> Philippine Trench </Td> <Td> Pacific Ocean </Td> <Td> 10,545 m (34,596 ft) </Td> </Tr> <Tr> <Td> Kuril--Kamchatka Trench </Td> <Td> Pacific Ocean </Td> <Td> 10,542 m (34,587 ft) </Td> </Tr> <Tr> <Td> Kermadec Trench </Td> <Td> Pacific Ocean </Td> <Td> 10,047 m (32,963 ft) </Td> </Tr> <Tr> <Td> Izu - Bonin Trench (Izu - Ogasawara Trench) </Td> <Td> Pacific Ocean </Td> <Td> 9,810 m (32,190 ft) </Td> </Tr> <Tr> <Td> Japan Trench </Td> <Td> Pacific Ocean </Td> <Td> 10,375 m (34,039 ft) </Td> </Tr> <Tr> <Td> Puerto Rico Trench </Td> <Td> Atlantic Ocean </Td> <Td> 8,800 m (28,900 ft) </Td> </Tr> <Tr> <Td> South Sandwich Trench </Td> <Td> Atlantic Ocean </Td> <Td> 8,428 m (27,651 ft) </Td> </Tr> <Tr> <Td> Peru--Chile Trench or Atacama Trench </Td> <Td> Pacific Ocean </Td> <Td> 8,065 m (26,460 ft) </Td> </Tr> </Table> <Tr> <Th> Trench </Th> <Th> Ocean </Th> <Th> Maximum Depth </Th> </Tr>

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