The Composition and Distribution of Ocean Sediments (II)

Biogenous Sediments

Biogenous sediments are composed of the shells and debris of organisms that live in the surface waters of the ocean and settle to the seafloor when those organisms die. The most important contributors to biogenous sediment are the single-celled autotrophic and heterotrophic plankton, such as diatoms, coccolithophores, radiolaria, and foraminifera. These organisms produce shells made of either silica (siliceous oozes) or calcium carbonate (calcareous oozes).

Hydrogenous Sediments

Hydrogenous sediments are minerals that are precipitated directly from the ions dissolved in seawater. This can occur in shallow, evaporative environments, such as inland seas, where minerals like halite (table salt), gypsum, and calcite can precipitate. Hydrogenous sediments can also form in deep ocean environments, where dissolved phosphates and manganese become supersaturated and precipitate as nodules.

Cosmogenous Sediments

Cosmogenous sediments are the rarest type of ocean sediment, consisting of meteorites and tektites (glass nodules formed from the melting of Earth's crust after a meteorite impact). These extraterrestrial materials are found in greater abundance in the ocean than on land due to the slow sedimentation rates in the deep ocean, which allow them to accumulate more easily.

The Dominance of Lithogenous Sediments

While biogenous sediments cover the majority of the seafloor surface, lithogenous sediments make up the largest percentage by volume of marine sediments. This is because lithogenous sediments, particularly the fine-grained muds, are able to be transported long distances by rivers and winds and accumulate in the deep ocean basins.

However, the thickest accumulations of sediment are found in the continental margin environments, where lithogenous sediments can pile up to depths of up to 9 kilometers. This is due to the much higher sedimentation rates in these areas, which can range from 0.5 to 800 meters per 1,000 years, compared to the slow sedimentation rates of 1 centimeter per 1,000 years in the deep ocean basins.