Pelagic Sediments

Pelagic sediment comprises fine-grained material composed of lithogenous, biogenous, and cosmogenous particles that have been deposited in the ocean floor away from the neritic zone. Pelagic sediment is composed of calcareous and siliceous shells from marine organisms, fine-grained clay material, volcanic ash and trace amounts of meteor debris. This material has been deposited, at slow depositional rates, in the ocean floor by wind, ice glaciers, rivers, and the skeletal remains of surface dwelling microorganisms.

Deep ocean environment

The marine environment comprises the continental shelf, continental slope, continental rise and the deep ocean floor. Much of the sediment derived from the continents passes through the continental shelf and marine canyons moved by benthic turbidity currents, eventually ending in the continental slope and continental rise. Turbidity currents carry sediments beyond the continental shelf, which accumulate as submarine fans. Beyond the continental shelf, the deep ocean floor is nearly covered by fine-grained deposits of pelagic clay and pelagic sediment (ooze). Ooze consists of 30% microscopic remains of either siliceous or calcareous planktonic marine organisms. The remainder percentage consists of clay sediments.

Sources of pelagic sediment

The main sources of pelagic sediment derive from ice glaciers, wind, volcanic ash and rivers. Sediments are deposited in the sea floor by ice glaciers only at high latitudes. Almost 95% of this type of sediment is deposited off the coast of the Antarctic continent. Wind-blown dust is distributed throughout the globe, although most of it originated at the tropics, where trade winds carry it from continental desert regions. Much volcanic ash is deposited not far from its source, although large volcanic eruptions can eject large quantities of ash and dust which is dispersed by high altitude blowing winds. Fine-grained material remains in suspension in rivers, and is carried to the open ocean, along with dust from the continents.

Sediments on the deep ocean

The principal contributors to the deep ocean floor pelagic sediment are the planktonic marine organisms that thrive in the surface of the oceans. Pelagic sediment from calcareous ooze, including microscopic skeletons of foraminifera, pteropods and coccolithophores, is the most common type of pelagic sediment by area, covering a little less than 50% of the world´s deep ocean. Siliceous ooze, which is composed of microscopic remains of radiolarians and diatoms, cover only 15% of the ocean floor. Red (brown) pelagic clay is deposited at slower rates than the other types of pelagic sediment. Pelagic clay covers approximately 38% of the ocean floor. The deep ocean floor is covered by manganese nodules that represent potential ore reserves.

Red clay is red to brown in color due to iron and manganese coatings on the sedimentary particles. Red clay is deposited in areas of the ocean floor where surface production of planktonic species is scarce. Sediment clays are usually transported in suspension either by air or within surface ocean currents, often taking hundreds of years before being deposited in the ocean floor. Pelagic oozes may drift along the ocean column for hundreds of kilometer, slowly descending to the ocean floor. Pelagic sediment deposition is a very slow process, and the rates of sedimentation vary from fractions of cm to a few cm every one thousand years, with some sediments deposited at slower rates than others.

Pelagic sediment is distributed around the world based on the type of sediment, thus, red clay and calcareous ooze are the main types of deposits. Diatom oozes are found along a continuous belt around Antarctica and the north pacific. Radiolarian ooze is limited to an equatorial band in the Pacific Ocean. Pteropod ooze is found in great amounts in the Atlantic Ocean. Calcareous sediment deposits abound in the Atlantic and Indian oceans. In the Pacific Ocean, red clay is predominant. The Pacific Ocean contains the largest percentage of the three types of pelagic sediments. Radiolarian ooze and red clay abound at depths up to 4,000 meters (1,312 ft.), whereas diatom oozes and calcareous sediments are restricted to lower ocean depths.