Ocean basins receive rock fragments of all sizes that have been either washed or blown off the land. These particles mix with the shells and bones of marine organisms that have sunk to the ocean bottom, and the accumulated mixture forms what we call sediment deposits. In addition to these two sources of sediments deposits, there is occasionally third source. In some locations where few other materials accumulate, chemical reactions in sea water produce particles that coat any hard surface on the bottom.
The fossils in these sediment deposits are important to us because they record Earth’s history of life in them. Comparable sedimentary rocks preserved on land record similar changes, including changes in Earth’s climate over billion of years.
Sediment deposits from the continents cover about one quarter of the ocean floor, with accumulations thickest in marginal ocean basins. Although ocean margins account for only 2% of total ocean-floor surface area, the deposits on these margins contain about one-sixth of all oceanic sediment and are important sources of oil and gas.
Marine sediments occur in a broad range of sizes and types. A scheme to classify marine sediments is by their origin was first proposed in 1891 by Sir John Murray and A.F. Renard after a thorough study of sediments collected during the Challenger Expedition. This scheme separates sediments into four categories by source: terrigenous, biogenous, hydrogenous (also called authigenic), and cosmogenous.
Terrigenous Sediments are the most abundant. As the name implies, terrigenous sediment originates on the continents or on islands near them. The rocks of earth’s crust are made up of minerals, inorganic crystalline materials with specific chemical compositions. The texture of igneous rocks – rocks that crystallize from magma – is by how rapidly they cool. Igneous rocks that cool rapidly, such as the basalt that forms the ocean floor at spreading centers or pours from volcanic vents on land, solidify so quickly that obvious crystals do not have a chance to form. Igneous rocks that cool very slowly, as would occur in masses rising from a subducted lithospheric plate, solidify very slowly and form large crystals. Nearly all terrigenous sediments are derived directly or indirectly from these crystals.
Biogenous Sediments are the next most abundant marine sediment. The siliceous (silicon-containing) and calcareous (calcium carbonate – containing) compounds that make up these sediments of biological origin were originally brought to the ocean in solution by rivers or dissolved in the ocean at mid-ocean ridges. The siliceous and calcareous materials were then extracted from the seawater by the normal activity of tiny plants and animals to build protective shells and skeletons. Some of this sediment derives from larger mollusk shells or from stationary colonial animals such as corals, but most of the organisms that produce biogenous sediments drift free in the water as plankton. After the death of their owners, the hard structures fall slowly to the bottom and accumulate in layers. Biogenous sediments are most abundant where ample nutrients encourage high biological productivity, usually near continental margins. Over millions of years, organic molecules within these sediments can form oil and natural gas. Biogenous sediments cover a larger percentage of the area of the ocean floor than terrigenous sediments, but the terrigenous sediments dominate in total volume.
Deep-ocean sediment containing at least 30% biogenous material is called an ooze (surely one of the most descriptive terms in the marine sciences). Oozes are named after the dominant remanent organism constituting them. The organisms contributing their remains to deep-sea oozes are small, single-celled, drifting, plantlike organism and the single-celled animals that feed on them. The hard shells and skeletal remains of these creatures are of relatively dense glasslike silica or calcium carbonate (limy) substances. When these organisms die, their shells settle slowly toward the bottom, mingle with fine-grained terrigenous sits and clays, and accumulate as ooze. The silica-rich residues give rise to siliceous ooze, the calcium-containing material to