ABYSSAL

Depths and environments between 3,000 and 6,000 m in the ocean. Although authors differ on precise depth limits, abyssal environments account for about 75 per cent of the oceans. The ocean floor at these depths is typically represented by the abyssal plains, very flat areas (gradient <0.05°) at the base of the continental rise. They are underlain by oceanic crust, the topography of which is subdued by a cover of sediment, largely deposited from turbidity currents and debris flows, but including also pelagic sediment. About 75 true abyssal plains are recognised, ranging in size from small areas of the Mediterranean Sea (e.g. Alboran abyssal plain, 2,600 km2) to large areas of the Southern Ocean (e.g. the Enderby abyssal plain, 3,703,000 km2). They are least common in the Pacific Ocean, where oceanic trenches trap land-derived sediment, and most common in the Southern Ocean, where sediment input is higher. Recent research suggest that deep-ocean ecosystems are affected by conditions at the ocean surface, including climatic change, especially through effects on food supplies. The rapid accumulation of allochthonous sediment in deep water, distance from human populations and scarcity of indigenous life have encouraged their consideration as sites for radioactive waste disposal.

[See alsobathyal, continental margin, hadal, hypsometry, ocean basin]