Biogenic sediments are the skeletal remains of either plant or animal marine species. The hard skeletal structures of marine organisms are important constituents of the deep ocean sediment and many layers of the deep ocean floor are almost entirely composed of calcareous and siliceous skeletal remains. While a great number of plant and animal marine species contribute to the accumulation of marine sediments, a limited group of marine organisms contribute significantly to the deposition of calcareous and siliceous oozes on the sea floor. Biogenic sediment from the skeletal remains of marine organisms covers approximately 65% of the ocean floor. The rest is covered by terrigemous material.
The main sources of sediment in the deep ocean floor stem from planktonic marine organisms that live in the upper surface waters. The calcareous and siliceous types of skeletal structures may be divided into two distinct groups of plants and animals. Those that produce calcareous ooze include organisms, such as foraminifera, coccolithophores and pteropods. Calcareous sediment is the most common type of sediment by area and covers approximately 50% of the planet´s ocean floor. Pelagic marine creatures also contribute to the deposition of sediment in the ocean floor, although not at the same rate as planktonic organisms do.
Most calcareous oozes are produced by two different groups of organisms. Single-celled animals (foraminifera) and single-celled plants (coccolithophores). Calcareous structures of animal origin are more abundant than those of plants. Foraminifera are the most abundant and large quantities of foraminifera (globigerina ooze) descend to the ocean bottoms every year. Coccolithophores are the second contributors of calcareous ooze into the ocean floor. They are found in large numbers in the euphotic zone and sink to the ocean bottom either individually or in the fecal pellets of plankton-feeding marine animals. Benthic animals, including worms, corals, brachiopods, mollusks, echinoderms, arthropods and vertebrates, contribute to the sediments in the ocean floor.
Siliceous sediments are not as abundant as calcareous structures, but in certain ocean regions, they occur in specific amounts that the depositions are recognized by their type. Diatoms are microscopic silica-secreting plants, that are found in the euphotic zone, either as individually drifting planktonic organisms or in shallow water as benthic organisms. They´re more abundant in high latitudes and large sediment depositions on the ocean floor are made of diatoms. Radiolarians are a large group of marine protists, which also construct Opaline silica skeletons, and are distantly related to foraminifera. Radiolarian is most abundant at lower latitudes, where their skeletal remains cover large portions of the deep ocean floor as radiolarian ooze.
The main contribution of marine organisms is not only the calcareous structures that are deposited as sediment in the deep ocean floor, but also the decomposing organic matter which is continuously falling from above as marine snow. The organic matter may contain hydrogen, carbon, nitrogen, oxygen, phosphorus, sulphur and many more elements. On the ocean floor, benthic marine animals depend on the supply of marine snow for their subsistence. Bacteria and bottom-living animals decompose much of this material and return the constituent elements in solution. Marine sediments may contain from none to about 20% of organic matter.
Benthic marine animals, especially those thriving in shallow waters, contribute with significant amounts of calcareous sediments. This is typically true in lower latitudes of the world and in regions where the supply of terrigenous material is scarce. In such cases, the sedimentary material may come from calcareous organisms, as well as the remains of many other kinds of marine animals living on the ocean floor. Although much of the organic matter in the oceans is of marine origin, in certain regions, certain proportions of sediment deposits may be of terrigenous origin.