Anoxic zone

Article By:

Emerson, Steven R. Department of Oceanography, University of Washington, Seattle, Washington.

Thomas, Helmuth Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada.

Last reviewed:August 2021

DOI:https://doi.org/10.1036/1097-8542.037400

An oxygen-depleted region in a marine environment. The dynamic steady state between oxygen supply and consumption determines the oceanic oxygen concentration. In regions where the rate of consumption equals or exceeds the rate of supply, seawater becomes devoid of oxygen and thus anoxic. In the open ocean, the only large regions that approach anoxic conditions are in the equatorial Pacific between 50 and 1000 m (165 and 3300 ft) depth and in the northern Arabian Sea and Bay of Bengal in the Indian Ocean between 100 and 1000 m (330 and 3300 ft) depth. The Pacific oxygen-depleted 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 micromoles per liter (atmospheric saturation is 200–300 μmol/L). 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. See also: Marine sediments

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