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Anoxic zones

Oxygen-depleted regions in marine environments. The dynamic steady state between oxygen supply and consumption determines the oxygen concentration. In regions where the rate of consumption equals the rate of supply, seawater becomes devoid of oxygen and thus anoxic. In the open ocean, the only large regions which approach anoxic conditions are between 165 and 3300 ft (50 and 1000 m) deep in the equatorial Pacific and between 330 and 3300 ft (100 and 1000 m) in the northern Arabian Sea and the Bay of Bengal in the Indian Ocean. The Pacific region consists of vast tongues extending from Central America and Peru nearly to the middle of the ocean in some places. In parts of this zone, oxygen concentrations become very low, 15 μmol/liter (atmospheric saturation is 200–300 μmol/liter). Pore waters of marine sediments are sometimes anoxic a short distance below the sediment-water interface. The degree of oxygen consumption in sediment pore waters depends upon the amount of organic matter reaching the sediments and the rate of bioturbation (mixing of the surface sediment by benthic animals). In shallow regions (continental shelf and slope), pore waters are anoxic immediately below the sediment-water interface; in relatively rapid sedimentation-rate areas of the deep sea, the pore waters are usually anoxic within a few centimeters of the interface; and in pore waters of slowly accumulating deep-sea sediments, oxygen may never become totally depleted. Marine sediments

Restricted basins (areas where water becomes temporarily trapped) are often either permanently or intermittently anoxic. Classic examples are the Black Sea, the Carioca Trench off the coast of Venezuela, and fiords which occupy the Norwegian and British Columbia coasts. Lakes which receive a large amount of nutrient inflow (either from natural or human-produced sources) are often anoxic during the period of summer stratification. Black sea Fiord

The chemistry of many elements dissolved in seawater (particularly the trace elements) is vastly changed by the presence or absence of oxygen. Since large areas of the ocean water mass are in contact with oxygen-depleted pore waters, the potential exists for anoxic conditions to have a marked effect on the chemistry of the sea. Seawater Seawater fertility

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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