III.

Composition of Sediment

The ocean floor is covered by an average of 0.5 km (0.3 mi) of sediment, but the thickness varies up to about 7 km (4.3 mi) in the Argentine Basin in the South Atlantic. Some regions, particularly the central parts of the midocean ridges where new crust is formed, have little, if any, sediment on them. The sediments are studied by dredging and by deep-sea exploration projects such as the Ocean Drilling Program, which obtains core samples of seafloor sediment from all the world’s oceans (see Deep-Sea Exploration).

The sediments are found to consist of rock particles and organic remains; the compositions depend on depth, distance from continents, and local variants such as submarine volcanoes or high biological productivity. Clay minerals, which are formed by the weathering of continental rocks and carried out to sea by rivers and wind, are usually abundant in the deep sea. Thick deposits of such detrital material are often found near mouths of rivers and on continental shelves; fine particles of clay are spread through the ocean and accumulate slowly on the deep-ocean floor. These sediments are stirred up and periodically redistributed by fierce current-generated disturbances that are called benthic storms because they occur in the sparsely populated deep-sea habitat known as the benthic zone (see Marine Life). Also accumulating as sediment in the benthic zone are the calcium carbonate shells of small organisms such as foraminifera and the siliceous shells of marine protozoans (see Diatom; Protozoa).