The sediments in the ocean, which sometimes later become rocks, come from two main sources. Terrigenous sediments originate from weathering on land and biogenic sediments consist predominately of the skeletons of tiny marine organisms. Biogenic sediments can provide answers about the ecology, climate and chemistry of the oceans millions of years ago.
Near the continental margins there is a huge output of material from the land and biogenic remains get swamped. Further away though they predominate, although they build up far more slowly. The highest build up of biogenic sediments happens when the productivity of plankton at the surface is high and relies to a great extent on the skeletons of these microscopic organisms reaching the ocean floor.
Sometimes the material forming planktonic shells or skeletons can dissolve into the water or the entire organism is eaten. Otherwise they fall to the ocean floor in the form of marine snow, where a large number of dead plankton stick together, and very commonly in the faecal pellets of larger organisms. It would take a single organism a very long time to reach the floor, and the chances are high the skeleton would have disintegrated before it got there.
There are two main kinds of biogenic sediments, distinguished by the skeletal material of the prevalent planktonic remains. These are siliceous sediments, consisting mainly of silica skeletons, and carbonate (or calcareous) sediments, consisting of skeletons formed from some form of calcium carbonate.
Siliceous sediments are mainly formed from the shells of diatoms and radiolarians. Diatoms are photosynthesising micro-organisms popularly classified as algae and usually extremely small. They have hard external shells. Radiolarians are generally bigger zooplankton, also with silica skeletons.
Calcareous sediments are mainly made up of the skeletons of foraminiferans, coccolithophores and pteropods. Coccolithophores are another group of algae, covered in tiny calcite plates. Foraminiferans are usually found in the zooplankton, although there are also some benthic (seabed) species. Pteropods are far bigger. These are actually molluscs and some can grow to a centimetre in length. They usually have thin shells of aragonite (another form of calcium carbonate).
Siliceous sediments predominate around a latitude of about 60 degrees south and along the equator. Calcareous sediments are found in deeper stretches of the oceans at mid and low latitudes. In both cases biogenic sediments are found in greater proportions in the southern, rather than the northern hemisphere and this is probably related to there being less land mass, and hence terrigenous sediments, below the equator.
Biogenic sediments form a crucial part of the ocean floor and have been building up for millions of years. Under certain circumstances they can become rocks, for example the famous white cliffs of Dover are made almost entirely from the skeletons of phytoplankton. Biogenic sediments are the subject of much study because of what they can reveal about the earth’s past, and so its possible future, and developments are worth watching with interest.