Marine Sediment
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Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the
seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...
. These particles have their origins in
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
and
rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...
s and have been
transported ''Transported'' is an Australian convict melodrama film directed by W. J. Lincoln. It is considered a lost film. Plot In England, Jessie Grey is about to marry Leonard Lincoln but the evil Harold Hawk tries to force her to marry him and she w ...
from the land to the sea, mainly by rivers but also by dust carried by wind and by the flow of glaciers into the sea. Additional deposits come from marine organisms and chemical precipitation in seawater, as well as from underwater volcanoes and meteorite debris. Except within a few kilometres of a
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
, where the volcanic rock is still relatively young, most parts of the seafloor are covered in
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
. This material comes from several different sources and is highly variable in composition. Seafloor sediment can range in thickness from a few millimetres to several tens of kilometres. Near the surface seafloor sediment remains unconsolidated, but at depths of hundreds to thousands of metres the sediment becomes
lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
(turned to rock). Rates of sediment accumulation are relatively slow throughout most of the ocean, in many cases taking thousands of years for any significant deposits to form. Sediment transported from the land accumulates the fastest, on the order of one metre or more per thousand years for coarser particles. However, sedimentation rates near the mouths of large rivers with high discharge can be orders of magnitude higher. Biogenous oozes accumulate at a rate of about one centimetre per thousand years, while small clay particles are deposited in the deep ocean at around one millimetre per thousand years. Sediments from the land are deposited on the
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
s by
surface runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...
,
river discharge In hydrology, discharge is the volumetric flow rate of water that is transported through a given cross-sectional area. It includes any suspended solids (e.g. sediment), dissolved chemicals (e.g. CaCO3(aq)), or biologic material (e.g. diatoms) in ad ...
, and other processes.
Turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s can transport this sediment down the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
to the deep ocean floor. The deep ocean floor undergoes its own process of spreading out from the mid-ocean ridge, and then slowly
subduct Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
s accumulated sediment on the deep floor into the molten interior of the earth. In turn, molten material from the interior returns to the surface of the earth in the form of
lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s and emissions from deep sea
hydrothermal vent A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
s, ensuring the process continues indefinitely. The sediments provide habitat for a multitude of
marine life Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. M ...
, particularly of
marine microorganisms Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe) is any microscopic livin ...
. Their fossilized remains contain information about
past climates Paleoclimatology ( British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important t ...
,
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
,
ocean circulation An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
patterns, and the timing of major extinctions.Webb, Paul (2019) ''Introduction to Oceanography''
Chapter 12: Ocean Sediments
page 273–297, Rebus Community. Updated 2020. Modified text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License


Overview

Except within a few kilometres of a
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
, where the volcanic rock is still relatively young, most parts of the
seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...
are covered in sediments. This material comes from several different sources and is highly variable in composition, depending on proximity to a continent, water depth, ocean currents, biological activity, and climate. Seafloor sediments (and
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s) can range in thickness from a few millimetres to several tens of kilometres. Near the surface, the sea-floor sediments remain unconsolidated, but at depths of hundreds to thousands of metres (depending on the type of sediment and other factors) the sediment becomes
lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
.Earle, Steven (2019) ''Physical geology'', second edition, "Sea-Floor Sediments", chapter 18.3. Modified text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
The various sources of seafloor sediment can be summarized as follows: *
Terrigenous sediment In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, they are derived from ''terrestrial'' (as opposed to marine) environments. Consisting of sand, mud, and silt carried to sea by rivers, their com ...
is derived from continental sources transported by rivers, wind, ocean currents, and glaciers. It is dominated by quartz, feldspar, clay minerals, iron oxides, and terrestrial organic matter. * Pelagic carbonate sediment is derived from organisms (e.g., foraminifera) living in the ocean water (at various depths, but mostly near surface) that make their shells (a.k.a. tests) out of carbonate minerals such as calcite. * Pelagic silica sediment is derived from marine organisms (e.g., diatoms and radiolaria) that make their tests out of silica (microcrystalline quartz). *
Volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...
and other volcanic materials are derived from both terrestrial and submarine eruptions. * Iron and
manganese nodule Polymetallic nodules, also called manganese nodules, are mineral concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. As nodules can be found in vast quantities, and contain valuable metals, de ...
s form as direct precipitates from ocean-bottom water. The distributions of some of these materials around the seas are shown in the diagram at the start of this article ↑. Terrigenous sediments predominate near the continents and within inland seas and large lakes. These sediments tend to be relatively coarse, typically containing sand and silt, but in some cases even pebbles and cobbles. Clay settles slowly in nearshore environments, but much of the clay is dispersed far from its source areas by ocean currents. Clay minerals are predominant over wide areas in the deepest parts of the ocean, and most of this clay is terrestrial in origin. Siliceous oozes (derived from radiolaria and diatoms) are common in the south polar region, along the equator in the Pacific, south of the Aleutian Islands, and within large parts of the Indian Ocean. Carbonate oozes are widely distributed in all of the oceans within equatorial and mid-latitude regions. In fact, clay settles everywhere in the oceans, but in areas where silica- and carbonate-producing organisms are prolific, they produce enough silica or carbonate sediment to dominate over clay. Carbonate sediments are derived from a wide range of near-surface pelagic organisms that make their shells out of carbonate. These tiny shells, and the even tinier fragments that form when they break into pieces, settle slowly through the water column, but they don’t necessarily make it to the bottom. While calcite is insoluble in surface water, its solubility increases with depth (and pressure) and at around 4,000 m, the carbonate fragments dissolve. This depth, which varies with latitude and water temperature, is known as the
carbonate compensation depth Carbonate compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite ( calcium carbonate) lags behind the rate of solvation, such that no calcite is preserved. Shells of animals therefore dissolve and carbonate ...
. As a result, carbonate oozes are absent from the deepest parts of the ocean (deeper than 4,000 m), but they are common in shallower areas such as the mid-Atlantic ridge, the East Pacific Rise (west of South America), along the trend of the Hawaiian/Emperor Seamounts (in the northern Pacific), and on the tops of many isolated seamounts.


Texture

Sediment texture can be examined in several ways. The first way is grain size. Sediments can be classified by particle size according to the
Wentworth scale Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refer ...
.
Clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
sediments are the finest with a grain diameter of less than .004 mm and
boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...
s are the largest with grain diameters of 256 mm or larger. tp://ftp.library.noaa.gov/noaa_documents.lib/NMFS/NEMO_Curriculum/Lesson15/Lesson15_Teacher_Guide.pdf Sediments''
NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
''. Accessed 5 April 2021.
Among other things, grain size represents the conditions under which the sediment was deposited. High energy conditions, such as strong currents or waves, usually results in the deposition of only the larger particles as the finer ones will be carried away. Lower energy conditions will allow the smaller particles to settle out and form finer sediments.
Sorting Sorting refers to ordering data in an increasing or decreasing manner according to some linear relationship among the data items. # ordering: arranging items in a sequence ordered by some criterion; # categorizing: grouping items with similar pro ...
is another way to categorize sediment texture. Sorting refers to how uniform the particles are in terms of size. If all of the particles are of a similar size, such as in beach sand, the sediment is well-sorted. If the particles are of very different sizes, the sediment is poorly sorted, such as in
glacial deposit image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
s. A third way to describe marine sediment texture is its maturity, or how long its particles have been transported by water. One way which can indicate maturity is how
round Round or rounds may refer to: Mathematics and science * The contour of a closed curve or surface with no sharp corners, such as an ellipse, circle, rounded rectangle, cant, or sphere * Rounding, the shortening of a number to reduce the number ...
the particles are. The more mature a sediment the rounder the particles will be, as a result of being abraded over time. A high degree of sorting can also indicate maturity, because over time the smaller particles will be washed away, and a given amount of energy will move particles of a similar size over the same distance. Lastly, the older and more mature a sediment the higher the quartz content, at least in sediments derived from rock particles. Quartz is a common mineral in terrestrial rocks, and it is very hard and resistant to abrasion. Over time, particles made from other materials are worn away, leaving only quartz behind. Beach sand is a very mature sediment; it is composed primarily of quartz, and the particles are rounded and of similar size (well-sorted).


Origins

Marine sediments can also classified by their source of origin. There are four types: * Lithogenous sediments, also called
terrigenous sediment In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, they are derived from ''terrestrial'' (as opposed to marine) environments. Consisting of sand, mud, and silt carried to sea by rivers, their com ...
s, are derived from preexisting rock and come from land via rivers, ice, wind and other processes. They are referred to as terrigenous sediments since most comes from the land. * Biogenous sediments are composed of the remains of marine organisms, and come from organisms like plankton when their exoskeletons break down * Hydrogenous sediments come from chemical reactions in the water, and are formed when materials that are dissolved in water precipitate out and form solid particles. * Cosmogenous sediments are derived from extraterrestrial sources, coming from space, filtering in through the atmosphere or carried to Earth on meteorites.


Lithogenous

Lithogenous or
terrigenous sediment In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, they are derived from ''terrestrial'' (as opposed to marine) environments. Consisting of sand, mud, and silt carried to sea by rivers, their com ...
is primarily composed of small fragments of preexisting rocks that have made their way into the ocean. These sediments can contain the entire range of particle sizes, from microscopic clays to large boulders, and they are found almost everywhere on the ocean floor. Lithogenous sediments are created on land through the process of weathering, where rocks and minerals are broken down into smaller particles through the action of wind, rain, water flow, temperature- or ice-induced cracking, and other erosive processes. These small eroded particles are then transported to the oceans through a variety of mechanisms: Streams and rivers: Various forms of runoff deposit large amounts of sediment into the oceans, mostly in the form of finer-grained particles. About 90% of the lithogenous sediment in the oceans is thought to have come from river discharge, particularly from Asia. Most of this sediment, especially the larger particles, will be deposited and remain fairly close to the coastline, however, smaller clay particles may remain suspended in the water column for long periods of time and may be transported great distances from the source. Wind: Windborne (aeolian) transport can take small particles of sand and dust and move them thousands of kilometres from the source. These small particles can fall into the ocean when the wind dies down, or can serve as the nuclei around which raindrops or snowflakes form. Aeolian transport is particularly important near desert areas.
Glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
s and
ice rafting Ice rafting is the transport of various materials by ice. Various objects deposited on ice may eventually become embedded in the ice. When the ice melts after a certain amount of drifting, these objects are deposited onto the bottom of the wate ...
: As glaciers grind their way over land, they pick up lots of soil and rock particles, including very large boulders, that get carried by the ice. When the glacier meets the ocean and begins to break apart or melt, these particles get deposited. Most of the deposition will happen close to where the glacier meets the water, but a small amount of material is also transported longer distances by rafting, where larger pieces of ice drift far from the glacier before releasing their sediment. Gravity: Landslides, mudslides, avalanches, and other gravity-driven events can deposit large amounts of material into the ocean when they happen close to shore. Waves: Wave action along a coastline will erode rocks and will pull loose particles from beaches and shorelines into the water. Volcanoes: Volcanic eruptions emit vast amounts of ash and other debris into the atmosphere, where it can then be transported by wind to eventually get deposited in the oceans.
Gastrolith A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In othe ...
s: Another, relatively minor, means of transporting lithogenous sediment to the ocean are gastroliths. Gastrolith means "stomach stone". Many animals, including seabirds, pinnipeds, and some crocodiles deliberately swallow stones and regurgitate them latter. Stones swallowed on land can be regurgitated at sea. The stones can help grind food in the stomach or act as ballast regulating buoyancy. Mostly these processes deposit lithogenous sediment close to shore. Sediment particles can then be transported farther by waves and currents, and may eventually escape the continental shelf and reach the deep ocean floor. ; Composition Lithogenous sediments usually reflect the composition of whatever materials they were derived from, so they are dominated by the major minerals that make up most terrestrial rock. This includes quartz, feldspar, clay minerals, iron oxides, and terrestrial organic matter.
Quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
(silicon dioxide, the main component of glass) is one of the most common minerals found in nearly all rocks, and it is very resistant to abrasion, so it is a dominant component of lithogenous sediments, including sand.


Biogenous

Biogenous sediment Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceou ...
s come from the remains of living organisms that settle out as sediment when the organisms die. It is the "hard parts" of the organisms that contribute to the sediments; things like shells, teeth or skeletal elements, as these parts are usually mineralized and are more resistant to decomposition than the fleshy "soft parts" that rapidly deteriorate after death. Macroscopic sediments contain large remains, such as skeletons, teeth, or shells of larger organisms. This type of sediment is fairly rare over most of the ocean, as large organisms do not die in enough of a concentrated abundance to allow these remains to accumulate. One exception is around
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...
s; here there is a great abundance of organisms that leave behind their remains, in particular the fragments of the stony skeletons of corals that make up a large percentage of tropical sand. Microscopic sediment consists of the hard parts of microscopic organisms, particularly their shells, or
tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
. Although very small, these organisms are highly abundant and as they die by the billions every day their tests sink to the bottom to create biogenous sediments. Sediments composed of microscopic tests are far more abundant than sediments from macroscopic particles, and because of their small size they create fine-grained, mushy sediment layers. If the sediment layer consists of at least 30% microscopic biogenous material, it is classified as a biogenous ooze. The remainder of the sediment is often made up of clay. The primary sources of microscopic biogenous sediments are unicellular algaes and protozoans (single-celled amoeba-like creatures) that secrete tests of either calcium carbonate (CaCO3) or
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(SiO2). Silica tests come from two main groups, the
diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s (algae) and the
radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...
s (
protozoan Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
s). Diatoms are particularly important members of the phytoplankton, functioning as small, drifting algal photosynthesizers. A diatom consists of a single algal cell surrounded by an elaborate silica shell that it secretes for itself. Diatoms come in a range of shapes, from elongated, pennate forms, to round, or centric shapes that often have two halves, like a Petri dish. In areas where diatoms are abundant, the underlying sediment is rich in silica diatom tests, and is called
diatomaceous earth Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3  μm to le ...
. Radiolarians are planktonic protozoans (making them part of the zooplankton), that like diatoms, secrete a silica test. The test surrounds the cell and can include an array of small openings through which the radiolarian can extend an amoeba-like "arm" or pseudopod. Radiolarian tests often display a number of rays protruding from their shells which aid in buoyancy. Oozes that are dominated by diatom or radiolarian tests are called
siliceous ooze Siliceous ooze is a type of biogenic pelagic sediment located on the deep ocean floor. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain ...
s. Like the siliceous sediments, the calcium carbonate, or calcareous sediments are also produced from the tests of microscopic algae and protozoans; in this case the coccolithophores and foraminiferans. Coccolithophores are single-celled planktonic algae about 100 times smaller than diatoms. Their tests are composed of a number of interlocking CaCO3 plates (coccoliths) that form a sphere surrounding the cell. When coccolithophores die the individual plates sink out and form an ooze. Over time, the coccolithophore ooze lithifies to becomes chalk. The White Cliffs of Dover in England are composed of coccolithophore-rich ooze that turned into chalk deposits. Foraminiferans (also referred to as ''forams'') are protozoans whose tests are often chambered, similar to the shells of snails. As the organism grows, is secretes new, larger chambers in which to reside. Most foraminiferans are benthic, living on or in the sediment, but there are some planktonic species living higher in the water column. When coccolithophores and foraminiferans die, they form
calcareous ooze Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...
s. Older calcareous sediment layers contain the remains of another type of organism, the
discoaster ''Discoaster'' is a genus of extinct star-shaped marine algae, with calcareous exoskeletons of between 5-40 μm across that are abundant as nanofossils in tropical deep-ocean deposits of Neogene age. ''Discoaster'' belongs to the haptophyt ...
s; single-celled algae related to the coccolithophores that also produced calcium carbonate tests. Discoaster tests were star-shaped, and reached sizes of 5-40 µm across. Discoasters went extinct approximately 2 million years ago, but their tests remain in deep, tropical sediments that predate their extinction. Because of their small size, these tests sink very slowly; a single microscopic test may take about 10–50 years to sink to the bottom! Given that slow descent, a current of only 1 cm/sec could carry the test as much as 15,000 km away from its point of origin before it reaches the bottom. Despite this, the sediments in a particular location are well-matched to the types of organisms and degree of productivity that occurs in the water overhead. This means the sediment particles must be sinking to the bottom at a much faster rate, so they accumulate below their point of origin before the currents can disperse them. Most of the tests do not sink as individual particles; about 99% of them are first consumed by some other organism, and are then aggregated and expelled as large
fecal pellet Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
s, which sink much more quickly and reach the ocean floor in only 10–15 days. This does not give the particles as much time to disperse, and the sediment below will reflect the production occurring near the surface. The increased rate of sinking through this mechanism has been called the "fecal express".


Hydrogenous

Seawater contains many different dissolved substances. Occasionally chemical reactions occur that cause these substances to precipitate out as solid particles, which then accumulate as hydrogenous sediment. These reactions are usually triggered by a change in conditions, such as a change in temperature, pressure, or pH, which reduces the amount of a substance that can remain in a dissolved state. There is not a lot of hydrogenous sediment in the ocean compared to lithogenous or biogenous sediments, but there are some interesting forms. In
hydrothermal vent A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
s seawater percolates into the seafloor where it becomes superheated by magma before being expelled by the vent. This superheated water contains many dissolved substances, and when it encounters the cold seawater after leaving the vent, these particles precipitate out, mostly as metal sulfides. These particles make up the "smoke" that flows from a vent, and may eventually settle on the bottom as hydrogenous sediment. Hydrothermal vents are distributed along the Earth's plate boundaries, although they may also be found at intra-plate locations such as hotspot volcanoes. Currently there are about 500 known active submarine hydrothermal vent fields, about half visually observed at the seafloor and the other half suspected from water column indicators and/or seafloor deposits.
Manganese nodule Polymetallic nodules, also called manganese nodules, are mineral concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. As nodules can be found in vast quantities, and contain valuable metals, de ...
s are rounded lumps of
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
and other metals that form on the seafloor, generally ranging between 3–10 cm in diameter, although they may sometimes reach up to 30 cm. The nodules form in a manner similar to pearls; there is a central object around which concentric layers are slowly deposited, causing the nodule to grow over time. The composition of the nodules can vary somewhat depending on their location and the conditions of their formation, but they are usually dominated by manganese- and iron oxides. They may also contain smaller amounts of other metals such as copper, nickel and cobalt. The precipitation of manganese nodules is one of the slowest geological processes known; they grow on the order of a few millimetres per million years. For that reason, they only form in areas where there are low rates of lithogenous or biogenous sediment accumulation, because any other sediment deposition would quickly cover the nodules and prevent further nodule growth. Therefore, manganese nodules are usually limited to areas in the central ocean, far from significant lithogenous or biogenous inputs, where they can sometimes accumulate in large numbers on the seafloor (Figure 12.4.2 right). Because the nodules contain a number of commercially valuable metals, there has been significant interest in mining the nodules over the last several decades, although most of the efforts have thus far remained at the exploratory stage. A number of factors have prevented large-scale extraction of nodules, including the high costs of
deep sea mining Deep sea mining is a growing subfield of experimental seabed mining that involves the retrieval of minerals and deposits from the ocean floor found at depths of or greater. As of 2021, the majority of marine mining efforts are limited to shal ...
operations, political issues over mining rights, and environmental concerns surrounding the extraction of these non-renewable resources.
Evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...
s are hydrogenous sediments that form when seawater evaporates, leaving the dissolved materials to precipitate into solids, particularly
halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...
(salt, NaCl). In fact, the evaporation of seawater is the oldest form of salt production for human use, and is still carried out today. Large deposits of halite evaporites exist in a number of places, including under the Mediterranean Sea. Beginning around 6 million years ago, tectonic processes closed off the Mediterranean Sea from the Atlantic, and the warm climate evaporated so much water that the Mediterranean was almost completely dried out, leaving large deposits of salt in its place (an event known as the
Messinian Salinity Crisis The Messinian salinity crisis (MSC), also referred to as the Messinian event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation (dr ...
). Eventually the Mediterranean re-flooded about 5.3 million years ago, and the halite deposits were covered by other sediments, but they still remain beneath the seafloor.
Oolite Oolite or oölite (''egg stone'') is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Ancient Greek word for egg (ᾠόν). Strictly, oolites consist of ooids of diameter 0.25–2 ...
s are small, rounded grains formed from concentric layers of precipitation of material around a suspended particle. They are usually composed of calcium carbonate, but they may also from phosphates and other materials. Accumulation of oolites results in oolitic sand, which is found in its greatest abundance in the Bahamas.
Methane hydrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...
s are another type of hydrogenous deposit with a potential industrial application. All terrestrial erosion products include a small proportion of organic matter derived mostly from terrestrial plants. Tiny fragments of this material plus other organic matter from marine plants and animals accumulate in terrigenous sediments, especially within a few hundred kilometres of shore. As the sediments pile up, the deeper parts start to warm up (from geothermal heat), and bacteria get to work breaking down the contained organic matter. Because this is happening in the absence of oxygen (a.k.a. anaerobic conditions), the by-product of this metabolism is the gas methane (CH4). Methane released by the bacteria slowly bubbles upward through the sediment toward the seafloor. At water depths of 500 m to 1,000 m, and at the low temperatures typical of the seafloor (close to 4 °C), water and methane combine to create a substance known as methane hydrate. Within a few metres to hundreds of metres of the seafloor, the temperature is low enough for methane hydrate to be stable and hydrates accumulate within the sediment. Methane hydrate is flammable because when it is heated, the methane is released as a gas. The methane within seafloor sediments represents an enormous reservoir of fossil fuel energy. Although energy corporations and governments are anxious to develop ways to produce and sell this methane, anyone that understands the climate-change implications of its extraction and use can see that this would be folly.


Cosmogenous

Cosmogenous sediment is derived from extraterrestrial sources, and comes in two primary forms; microscopic spherules and larger meteor debris.
Spherule A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ...
s are composed mostly of silica or iron and nickel, and are thought to be ejected as meteors burn up after entering the atmosphere. Meteor debris comes from collisions of meteorites with Earth. These high impact collisions eject particles into the atmosphere that eventually settle back down to Earth and contribute to the sediments. Like spherules, meteor debris is mostly silica or iron and nickel. One form of debris from these collisions are
tektite Tektites (from grc, τηκτός , meaning 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz ...
s, which are small droplets of glass. They are likely composed of terrestrial silica that was ejected and melted during a meteorite impact, which then solidified as it cooled upon returning to the surface. Cosmogenous sediment is fairly rare in the ocean and it does not usually accumulate in large deposits. However, it is constantly being added to through space dust that continuously rains down on Earth. About 90% of incoming cosmogenous debris is vaporized as it enters the atmosphere, but it is estimated that 5 to 300 tons of space dust land on the Earth's surface each day.


Composition


Siliceous ooze

Siliceous ooze Siliceous ooze is a type of biogenic pelagic sediment located on the deep ocean floor. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain ...
is a type of biogenic
pelagic sediment Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceou ...
located on the
deep Deep or The Deep may refer to: Places United States * Deep Creek (Appomattox River tributary), Virginia * Deep Creek (Great Salt Lake), Idaho and Utah * Deep Creek (Mahantango Creek tributary), Pennsylvania * Deep Creek (Mojave River tributary), ...
ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain at least 30% skeletal remains of pelagic microorganisms. Siliceous oozes are largely composed of the silica based skeletons of microscopic marine organisms such as
diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s and
radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...
s. Other components of siliceous oozes near continental margins may include terrestrially derived silica particles and sponge spicules. Siliceous oozes are composed of skeletons made from opal silica Si(O2), as opposed to
calcareous ooze Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...
s, which are made from skeletons of calcium carbonate organisms (i.e.
coccolithophore Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdo ...
s). Silica (Si) is a bioessential element and is efficiently recycled in the marine environment through the silica cycle. Distance from land masses, water depth and ocean fertility are all factors that affect the opal silica content in seawater and the presence of siliceous oozes. File:Diatomaceous Earth BrightField.jpg,
Diatomaceous earth Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3  μm to le ...
is a soft,
siliceous Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
,
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
made up of microfossils in the form of the
frustule A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on ...
s (shells) of single cell
diatoms A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...

(click 3X to fully magnify)


Calcareous ooze

The term ''calcareous'' can be applied to a fossil, sediment, or sedimentary rock which is formed from, or contains a high proportion of, calcium carbonate in the form of
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
or
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...
. Calcareous sediments (
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
) are usually deposited in shallow water near land, since the carbonate is precipitated by marine organisms that need land-derived nutrients. Generally speaking, the farther from land sediments fall, the less calcareous they are. Some areas can have interbedded calcareous sediments due to storms, or changes in ocean currents.
Calcareous ooze Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...
is a form of calcium carbonate derived from planktonic organisms that accumulates on the sea floor. This can only occur if the ocean is shallower than the
carbonate compensation depth Carbonate compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite ( calcium carbonate) lags behind the rate of solvation, such that no calcite is preserved. Shells of animals therefore dissolve and carbonate ...
. Below this depth, calcium carbonate begins to dissolve in the ocean, and only non-calcareous sediments are stable, such as
siliceous ooze Siliceous ooze is a type of biogenic pelagic sediment located on the deep ocean floor. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain ...
or
pelagic red clay Pelagic red clay, also known as simply red clay, brown clay or pelagic clay, is a type of pelagic sediment.Rothwell, R.G., (2005) ''Deep Ocean Pelagic Oozes'', Vol. 5. of Selley, Richard C., L. Robin McCocks, and Ian R. Plimer, Encyclopedia of Ge ...
. File:Nanoplankton-fossil-sediment hg.jpg, File:PSM V44 D483 Globigerina ooze.jpg, Illustration of a ''
Globigerina ''Globigerina'' () is a genus of planktonic Foraminifera, in the order of Rotaliida.Glo ...
'' ooze File:FMIB 47660 Shells from Globigerina Ooze.jpeg, Shells (
tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
), usually made of calcium carbonate, from a
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...
l ooze on the deep ocean floor


Lithified sediments

File:Coober Pedy Opal Doublet.jpg, Opal can contain protist microfossils of diatoms, radiolarians, silicoflagellates and ebridians Haq B.U. and Boersma A. (Eds.) (1998
''Introduction to Marine Micropaleontology''
Elsevier.
File:MarmoCipollino FustoBasMassenzioRoma.jpg, Marble can contain protist microfossils of foraminiferans, coccolithophores, calcareous nannoplankton and algae,
ostracode Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...
s, pteropods, calpionellids and
bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...
File:Carbonate-Silicate Cycle (Carbon Cycle focus).jpg, Carbonate-silicate cycle


Distribution

File:Continentalmargin.jpg Where and how sediments accumulate will depend on the amount of material coming from a source, the distance from the source, the amount of time that sediment has had to accumulate, how well the sediments are preserved, and the amounts of other types of sediments that are also being added to the system. Rates of sediment accumulation are relatively slow throughout most of the ocean, in many cases taking thousands of years for any significant deposits to form. Lithogenous sediment accumulates the fastest, on the order of one metre or more per thousand years for coarser particles. However, sedimentation rates near the mouths of large rivers with high discharge can be orders of magnitude higher. Biogenous oozes accumulate at a rate of about 1 cm per thousand years, while small
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
particles are deposited in the deep ocean at around one millimetre per thousand years. As described above, manganese nodules have an incredibly slow rate of accumulation, gaining 0.001 millimetres per thousand years. File:Mid-ocean ridge cut away view.png Marine sediments are thickest near the
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
s where they can be over 10 km thick. This is because the crust near passive continental margins is often very old, allowing for a long period of accumulation, and because there is a large amount of terrigenous sediment input coming from the continents. Near
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
systems where new
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...
is being formed, sediments are thinner, as they have had less time to accumulate on the younger crust. As distance increases from a ridge spreading center the sediments get progressively thicker, increasing by approximately 100–200 m of sediment for every 1000 km distance from the ridge axis. With a seafloor spreading rate of about 20–40 km/million years, this represents a sediment accumulation rate of approximately 100–200 m every 25–50 million years. The diagram at the start of this article ↑ shows the distribution of the major types of sediment on the ocean floor. Cosmogenous sediments could potentially end up in any part of the ocean, but they accumulate in such small abundances that they are overwhelmed by other sediment types and thus are not dominant in any location. Similarly, hydrogenous sediments can have high concentrations in specific locations, but these regions are very small on a global scale. So cosmogenous and hydrogenous sediments can mostly be ignored in the discussion of global sediment patterns. Coarse lithogenous/terrigenous sediments are dominant near the continental margins as
land runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when the ...
,
river discharge In hydrology, discharge is the volumetric flow rate of water that is transported through a given cross-sectional area. It includes any suspended solids (e.g. sediment), dissolved chemicals (e.g. CaCO3(aq)), or biologic material (e.g. diatoms) in ad ...
, and other processes deposit vast amounts of these materials on the
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
. Much of this sediment remains on or near the shelf, while
turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can ...
currents can transport material down the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
to the deep ocean floor (
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. T ...
). Lithogenous sediment is also common at the poles where thick ice cover can limit
primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
, and glacial breakup deposits sediments along the ice edge. Coarse lithogenous sediments are less common in the central ocean, as these areas are too far from the sources for these sediments to accumulate. Very small clay particles are the exception, and as described below, they can accumulate in areas that other lithogenous sediment will not reach. The distribution of biogenous sediments depends on their rates of production, dissolution, and dilution by other sediments. Coastal areas display very high primary production, so abundant biogenous deposits might be expected in these regions. However, sediment must be >30% biogenous to be considered a biogenous ooze, and even in productive coastal areas there is so much lithogenous input that it swamps the biogenous materials, and that 30% threshold is not reached. So coastal areas remain dominated by lithogenous sediment, and biogenous sediments will be more abundant in pelagic environments where there is little lithogenous input. In order for biogenous sediments to accumulate their rate of production must be greater than the rate at which the
tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
dissolve. Silica is undersaturated throughout the ocean and will dissolve in seawater, but it dissolves more readily in warmer water and lower pressures; that is, it dissolves faster near the surface than in deep water. Silica sediments will therefore only accumulate in cooler regions of high productivity where they accumulate faster than they dissolve. This includes
upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...
regions near the equator and at high latitudes where there are abundant nutrients and cooler water. Oozes formed near the equatorial regions are usually dominated by radiolarians, while diatoms are more common in the polar oozes. Once the silica tests have settled on the bottom and are covered by subsequent layers, they are no longer subject to dissolution and the sediment will accumulate. Approximately 15% of the seafloor is covered by siliceous oozes. Biogenous calcium carbonate sediments also require production to exceed dissolution for sediments to accumulate, but the processes involved are a little different than for silica. Calcium carbonate dissolves more readily in more acidic water. Cold seawater contains more dissolved CO2 and is slightly more acidic than warmer water. So calcium carbonate tests are more likely to dissolve in colder, deeper, polar water than in warmer, tropical, surface water. At the poles the water is uniformly cold, so calcium carbonate readily dissolves at all depths, and carbonate sediments do not accumulate. In temperate and tropical regions calcium carbonate dissolves more readily as it sinks into deeper water. The depth at which calcium carbonate dissolves as fast as it accumulates is called the calcium carbonate compensation depth or calcite compensation depth, or simply the CCD. The
lysocline The lysocline is the depth in the ocean dependent upon the carbonate compensation depth (CCD), usually around 3.5 km, below which the rate of dissolution of calcite increases dramatically because of a pressure effect. While the lysocline is the u ...
represents the depths where the rate of calcium carbonate dissolution increases dramatically (similar to the
thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
and
halocline In oceanography, a halocline (from Greek ''hals'', ''halos'' 'salt' and ''klinein'' 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temp ...
). At depths shallower than the CCD carbonate accumulation will exceed the rate of dissolution, and carbonate sediments will be deposited. In areas deeper than the CCD, the rate of dissolution will exceed production, and no carbonate sediments can accumulate (see diagram at right). The CCD is usually found at depths of 4 – 4.5 km, although it is much shallower at the poles where the surface water is cold. Thus calcareous oozes will mostly be found in tropical or temperate waters less than about 4 km deep, such as along the mid-ocean ridge systems and atop
seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...
s and
plateaus In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides ha ...
. The CCD is deeper in the Atlantic than in the Pacific since the Pacific contains more CO2, making the water more acidic and calcium carbonate more soluble. This, along with the fact that the Pacific is deeper, means that the Atlantic contains more calcareous sediment than the Pacific. All told, about 48% of the seafloor is dominated by calcareous oozes. Much of the rest of the deep ocean floor (about 38%) is dominated by abyssal clays. This is not so much a result of an abundance of clay formation, but rather the lack of any other types of sediment input. The clay particles are mostly of terrestrial origin, but because they are so small they are easily dispersed by wind and currents, and can reach areas inaccessible to other sediment types. Clays dominate in the central North Pacific, for example. This area is too far from land for coarse lithogenous sediment to reach, it is not productive enough for biogenous tests to accumulate, and it is too deep for calcareous materials to reach the bottom before dissolving. Because clay particles accumulate so slowly, the clay-dominated deep ocean floor is often home to hydrogenous sediments like manganese nodules. If any other type of sediment was produced here it would accumulate much more quickly and would bury the nodules before they had a chance to grow.


Coastal sediments

Shallow water marine environment Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this ...
s are found in areas between the
shore A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...
and deeper water, such as a
reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock out ...
wall or a shelf break. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils. The
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
itself is often composed of
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
, which forms readily in shallow, warm calm waters. The shallow marine environments are not exclusively composed of
siliciclastic Siliciclastic (or ''siliclastic'') rocks are clastic noncarbonate sedimentary rocks that are composed primarily of silicate minerals, such as quartz or clay minerals. Siliciclasic rock types include mudrock, sandstone, and conglomerate Conglomera ...
or
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
aceous sediments. While they cannot always coexist, it is possible to have a shallow marine environment composed solely of carbonaceous sediment or one that is composed completely of siliciclastic sediment. Shallow water marine sediment is made up of larger grain sizes because smaller grains have been washed out to deeper water. Within sedimentary rocks composed of carbonaceous sediment, there may also be
evaporite An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...
minerals. The most common evaporite minerals found within modern and ancient deposits are gypsum, anhydrite, and halite; they can occur as crystalline layers, isolated crystals or clusters of crystals. In terms of geologic time, it is said that most Phanerozoic sedimentary rock was deposited in shallow marine environments as about 75% of the sedimentary carapace is made up of shallow marine sediments; it is then assumed that Precambrian sedimentary rocks were too, deposited in shallow marine waters, unless it is specifically identified otherwise. This trend is seen in the North American and Caribbean region. Also, as a result of supercontinent breakup and other shifting tectonic plate processes, shallow marine sediment displays large variations in terms of quantity in the geologic time.


Bioturbation

Bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
is the reworking of sediment by animals or plants. These include burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a primary driver of
biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
. The formal study of bioturbation began in the 1800s by
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
experimenting in his garden. The disruption of aquatic sediments and terrestrial soils through bioturbating activities provides significant
ecosystem services Ecosystem services are the many and varied benefits to humans provided by the natural environment and healthy ecosystems. Such ecosystems include, for example, agroecosystems, forest ecosystem, grassland ecosystems, and aquatic ecosystems. Th ...
. These include the alteration of
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s in aquatic sediment and overlying water, shelter to other species in the form of burrows in terrestrial and water ecosystems, and soil production on land.Shaler, N. S., 1891, The origin and nature of soils, in Powell, J. W., ed., USGS 12th Annual report 1890-1891: Washington, D.C., Government Printing Office, p. 213-45. Bioturbators are
ecosystem engineer An ecosystem engineer is any species that creates, significantly modifies, maintains or destroys a habitat. These organisms can have a large impact on species richness and landscape-level heterogeneity of an area. As a result, ecosystem enginee ...
s because they alter resource availability to other species through the physical changes they make to their environments. This type of ecosystem change affects the evolution of cohabitating species and the environment, which is evident in
trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...
s left in marine and terrestrial sediments. Other bioturbation effects include altering the texture of sediments (
diagenesis Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
),
bioirrigation Bioirrigation refers to the process of benthos, benthic organisms flushing their burrows with overlying water. The exchange of dissolved substances between the porewater and overlying seawater that results is an important process in the context o ...
, and displacement of microorganisms and non-living particles. Bioturbation is sometimes confused with the process of
bioirrigation Bioirrigation refers to the process of benthos, benthic organisms flushing their burrows with overlying water. The exchange of dissolved substances between the porewater and overlying seawater that results is an important process in the context o ...
, however these processes differ in what they are mixing; bioirrigation refers to the mixing of water and solutes in sediments and is an effect of bioturbation
Walrus The walrus (''Odobenus rosmarus'') is a large pinniped, flippered marine mammal with a discontinuous distribution about the North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere. The walrus is the only living species in ...
es and
salmon Salmon () is the common name for several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
are examples of large bioturbators.Humphreys, G. S., and Mitchell, P. B., 1983, A preliminary assessment of the role of bioturbation and rainwash on sandstone hillslopes in the Sydney Basin, in Australian and New Zealand Geomorphology Group, p. 66-80. Although the activities of these large macrofaunal bioturbators are more conspicuous, the dominant bioturbators are small invertebrates, such as
polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...
s, ghost shrimp and mud shrimp. The activities of these small invertebrates, which include burrowing and ingestion and defecation of sediment grains, contribute to mixing and the alteration of sediment structure.


Bioirrigation

Bioirrigation Bioirrigation refers to the process of benthos, benthic organisms flushing their burrows with overlying water. The exchange of dissolved substances between the porewater and overlying seawater that results is an important process in the context o ...
is the process of
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s flushing their
burrow An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of sh ...
s with overlying
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
. The exchange of dissolved substances between the
porewater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...
and overlying seawater that results is an important process in the context of the biogeochemistry of the oceans.
Coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
al aquatic environments often have organisms that destabilize
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
. They change the physical state of the sediment. Thus improving the conditions for other organisms and themselves. These organisms often also cause
Bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
, which is commonly used interchangeably or in reference with bioirrigation. Bioirrigation works as two different processes. These processes are known as
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...
reworking and
ventilation Ventilation may refer to: * Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation ** Mechanical ventilation, in medicine, using artificial methods to assist breathing *** Ventilator, a m ...
, which is the work of benthic macro-
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s (usually ones that burrow). This particle reworking and ventilation is caused by the organisms when they feed (faunal feeding),
defecate Defecation (or defaecation) follows digestion, and is a necessary process by which organisms eliminate a solid, semisolid, or liquid waste material known as feces from the digestive tract via the anus. The act has a variety of names ranging fro ...
, burrow, and respire. Bioirrigation is responsible for a large amount of
oxidative Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
transport and has a large impact on
biogeochemical cycle A biogeochemical cycle (or more generally a cycle of matter) is the pathway by which a chemical substance cycles (is turned over or moves through) the biotic and the abiotic compartments of Earth. The biotic compartment is the biosphere and the ...
s.


Pelagic sediments

File:Ocean hemipelagic and pelagic processes.webp, Sediment supply from terrigenous and biological sources
as well as its dispersion and settling through the water column
Pelagic sediment Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceou ...
s, or ''pelagite'', are fine-grained
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s that accumulate as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceous shells of
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Ph ...
or
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
;
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
-size
siliciclastic Siliciclastic (or ''siliclastic'') rocks are clastic noncarbonate sedimentary rocks that are composed primarily of silicate minerals, such as quartz or clay minerals. Siliciclasic rock types include mudrock, sandstone, and conglomerate Conglomera ...
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
; or some mixture of these. Trace amounts of meteoric dust and variable amounts of
volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...
also occur within pelagic sediments. Based upon the composition of the ooze, there are three main types of pelagic sediments:
siliceous ooze Siliceous ooze is a type of biogenic pelagic sediment located on the deep ocean floor. Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain ...
s, calcareous oozes, and red clays.Rothwell, R.G., (2005) ''Deep Ocean Pelagic Oozes'', Vol. 5. of Selley, Richard C., L. Robin McCocks, and Ian R. Plimer, Encyclopedia of Geology, Oxford: Elsevier Limited. HüNeke, H., and T. Mulder (2011) ''Deep-Sea Sediments''. Developments in Sedimentology, vol. 63. Elsiever, New York. 849 pp. An extensive body of work on deep-water processes and sediments has been built over the past 150 years since the voyage of HMS Challenger (1872–1876), during which the first systematic study of seafloor sediments was made. For many decades since that pioneering expedition, and through the first half of the twentieth century, the deep sea was considered entirely pelagic in nature. Modified text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
The composition of pelagic sediments is controlled by three main factors. The first factor is the distance from major landmasses, which affects their dilution by terrigenous, or land-derived, sediment. The second factor is water depth, which affects the preservation of both siliceous and calcareous biogenic particles as they settle to the ocean bottom. The final factor is ocean fertility, which controls the amount of biogenic particles produced in surface waters.


Turbidites

File:NOAA Turbidity Current Diagram.jpg, Continental margins can experience slope failures triggered by earthquakes or other geological disturbances. These can result in
turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s as turbid water dense with suspended sediment rushes down the slope. Chaotic motion within the sediment flow can sustain the turbidity current, and once it reaches the deep
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. T ...
it can flow for hundreds of kilometres.
Turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites wer ...
s are the geologic deposits of a
turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
, which is a type of amalgamation of fluidal and
sediment gravity flow upright=1.25, This turbidite from the Devonian Becke-Oese Sandstone">Devonian.html" ;"title="turbidite from the Devonian">turbidite from the Devonian Becke-Oese Sandstone of Germany is an example of a deposit from a sediment gravity flow. Note th ...
responsible for distributing vast amounts of clastic
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
into the
deep ocean The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...
. Turbidites are deposited in the deep ocean troughs below the continental shelf, or similar structures in deep lakes, by underwater avalanches which slide down the steep slopes of the continental shelf edge. When the material comes to rest in the ocean trough, it is the sand and other coarse material which settles first followed by mud and eventually the very fine particulate matter. This sequence of deposition creates the
Bouma sequence 300px, thumbnail, Turbidite from the Devonian-age Becke-Oese Sandstone">Devonian.html" ;"title="Turbidite from the Devonian">Turbidite from the Devonian-age Becke-Oese Sandstone, Germany showing a complete Bouma sequence. The Bouma Sequence (afte ...
s that characterize these rocks. Turbidites were first recognised in the 1950s and the first
facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
model was developed by Bouma in 1962. Since that time, turbidites have been one of the better known and most intensively studied deep-water sediment facies. They are now very well known from sediment cores recovered from modern deep-water systems, subsurface (hydrocarbon) boreholes and ancient outcrops now exposed on land. Each new study of a particular turbidite system reveals specific deposit characteristics and facies for that system. The most commonly observed facies have been variously synthesised into a range of facies schemes.


Contourites

File:Ocean bottom (contour) current.webp, Identifying the current core, eddies and strands within a deep-water mass A contourite is a sedimentary deposit commonly formed on
continental rise The continental rise is a low-relief zone of accumulated sediments that lies between the continental slope and the abyssal plain. It is a major part of the continental margin, covering around 10% of the ocean floor. Formation This geologic str ...
to lower slope settings, although they may occur anywhere that is below storm
wave base The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. At water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion a ...
. Countourites are produced by
thermohaline Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to tempe ...
-induced deepwater bottom currents and may be influenced by wind or
tidal Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...
forces.Rebesco, M. & Camerlenghi, A. 2008. Contourites, Elsevier Science, 688pp.
The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography. Contourites were first identified in the early 1960s by
Bruce Heezen Bruce Charles Heezen (; April 11, 1924 – June 21, 1977) was an American geologist. He worked with oceanographic cartographer Marie Tharp at Columbia University to map the Mid-Atlantic Ridge in the 1950s. Biography Heezen was born in Vinton, Io ...
and co-workers at
Woods Hole Oceanographic Institute The Woods Hole Oceanographic Institution (WHOI, acronym pronounced ) is a private, nonprofit research and higher education facility dedicated to the study of marine science and engineering. Established in 1930 in Woods Hole, Massachusetts, it ...
. Their now seminal paper demonstrated the very significant effects of contour-following bottom currents in shaping sedimentation on the deep continental rise off eastern North America. The deposits of these semi-permanent alongslope currents soon became known as contourites, and the demarcation of slope-parallel, elongate and mounded sediment bodies made up largely of contourites became known as contourite drifts.


Hemipelagic

Hemipelagic sediment Hemipelagic sediment, or hemipelagite, is a type of marine sediment that consists of clay and silt-sized grains that are terrigenous and some biogenic material derived from the landmass nearest the deposits or from organisms living in the water. ...
s, or ''hemipelagite'', are a type of marine sediments that consists of clay and silt-sized grains that are
terrigenous In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, they are derived from ''terrestrial'' (as opposed to marine) environments. Consisting of sand, mud, and silt carried to sea by rivers, their ...
and some biogenic material derived from the landmass nearest the deposits or from organisms living in the water. Hemipelagic sediments are deposited on continental shelves and
continental rise The continental rise is a low-relief zone of accumulated sediments that lies between the continental slope and the abyssal plain. It is a major part of the continental margin, covering around 10% of the ocean floor. Formation This geologic str ...
s, and differ from
pelagic sediment Pelagic sediment or pelagite is a fine-grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. These particles consist primarily of either the microscopic, calcareous or siliceou ...
compositionally. Pelagic sediment is composed of primarily biogenic material from organisms living in the water column or on the seafloor and contains little to no terrigenous material. Terrigenous material includes minerals from the
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
like
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
or
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
.
Volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...
on land, wind blown sediments as well as particulates discharged from rivers can contribute to Hemipelagic deposits. These deposits can be used to qualify climatic changes and identify changes in sediment provenances. ---- File:Ocean medium-grained turbidite family.webp, Medium-grained turbidite family
The ideal Bouma facies model showing the complete sequence of divisions A–E, while F is a typical partial sequence found in nature. File:Ocean sandy contourite family.webp, Sandy contourite family
for muddy sands, fine-to-medium sands
and medium-to-coarse sands File:Ocean hemipelagite facies models.webp, Hemipelagite facies models
Standard model showing simple cyclicity between clay-rich and biogenic-rich parts. Variations depend on component inputs.
----


Ecology

''
Benthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.community A community is a social unit (a group of living things) with commonality such as place, norms, religion, values, customs, or identity. Communities may share a sense of place situated in a given geographical area (e.g. a country, village, tow ...
of
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s that live on, in, or near the
seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...
, also known as the
benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
. * '' Hyperbenthos'' (or hyperbenthic organisms), prefix , live just above the sediment. * ''
Epibenthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.Endobenthos'' (or endobenthic organisms), prefix , live buried, or burrowing in the sediment, often in the oxygenated top layer.


Microbenthos

Marine microbenthos are microorganisms that live in the
benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
of the ocean – that live near or on the seafloor, or within or on surface seafloor sediments. The word ''benthos'' comes from Greek, meaning "depth of the sea". Microbenthos are found everywhere on or about the seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In shallow waters,
seagrass meadow A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and ...
s,
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...
s and
kelp forest Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Ea ...
s provide particularly rich habitats. In photic zones benthic diatoms dominate as photosynthetic organisms. In
intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...
s changing
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...
s strongly control opportunities for microbenthos. File:Elphidium-incertum hg.jpg, '' Elphidium'' a widespread abundant genus of benthic forams File:FMIB 50025 Textilaria.jpeg, ''
Heterohelix Heterohelix is an extinct genus of foraminifera belonging to the family Heterohelicidae of the superfamily Heterohelicoidea and the suborder Globigerinina The Globigerinina is a suborder of foraminiferans that are found as marine plankton. Th ...
'', an extinct genus of benthic forams
File:Gastrotrich.jpg, Darkfield photo of a
gastrotrich The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, acoelomate animals, and are widely distributed and abundant in freshwater and marine environmen ...
, 0.06-3.0 mm long, a worm-like animal living between sediment particles File:Pliciloricus enigmatus.jpg, Armoured ''
Pliciloricus enigmaticus '' Pliciloricus enigmaticus'' is a marine Loriciferan species of genus ''Pliciloricus'' first described by Higgins & Kristensen 1986. Description ''Pliciloricus enigmaticus'' is a marine species in which adults measure between 160–268&nbs ...
'', about 0.2 mm long, live in spaces between marine gravel
Diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s form a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year,The Air You're Breathing? A Diatom Made That
/ref> take in over 6.7 billion metric tons of
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
each year from the waters in which they live, and contribute nearly half of the organic material found in the oceans. File:Diatoms (248 05) Various diatoms.jpg,
Diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s are one of the most common types of phytoplankton File:Diatom Helipelta metil.jpg, Their protective shells (frustles) are made of silicon File:Diatom - Triceratium favus.jpg File:Diatom2.jpg, They come in many shapes and sizes
Coccolithophore Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdo ...
s are minute unicellular photosynthetic protists with two
flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...
for locomotion. Most of them are protected by a shell covered with ornate circular plates or scales called
coccolith Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as '' Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere' ...
s. The coccoliths are made from calcium carbonate. The term coccolithophore derives from the Greek for a ''seed carrying stone'', referring to their small size and the coccolith stones they carry. Under the right conditions they bloom, like other phytoplankton, and can turn the ocean milky white. File:Coccolithus pelagicus.jpg File:JRYSEM-247-05-azurapl.jpg,
Radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...
s are unicellular predatory
protists A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...
encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the
ocean sediment Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mainly ...
. These remains, as
microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...
, provide valuable information about past oceanic conditions.Wassilieff, Maggy (2006
"Plankton - Animal plankton"
''Te Ara - the Encyclopedia of New Zealand''. Accessed: 2 November 2019.
File:Mikrofoto.de-Radiolarien 6.jpg, Like diatoms, radiolarians come in many shapes File:Theocotylissa ficus Ehrenberg - Radiolarian (34638920262).jpg, Also like diatoms, radiolarian shells are usually made of silicate File:Acantharian radiolarian Xiphacantha (Haeckel).jpg, However
acantharian The Acantharea (Acantharia) are a group of radiolarian protozoa, distinguished mainly by their strontium sulfate skeletons. Acantharians are heterotrophic marine microplankton that range in size from about 200 microns in diameter up to several ...
radiolarians have shells made from
strontium sulfate Strontium sulfate (SrSO4) is the sulfuric acid, sulfate salt of strontium. It is a white crystalline powder and occurs in nature as the mineral Celestine (mineral), celestine. It is poorly soluble in water to the extent of 1 part in 8,800. It is mo ...
crystals File:Spherical radiolarian 2.jpg, Cutaway schematic diagram of a spherical radiolarian shell
Like radiolarians,
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...
ns (''forams'' for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called
tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) of silica. Most forams are
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
, but about 40 species are
planktic Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
. They are widely researched with well established
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
s which allow scientists to infer a lot about past environments and climates. Both foraminifera and diatoms have
planktonic Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
and
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
forms, that is, they can drift in the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
or live on sediment at the bottom of the ocean. Either way, their shells end up on the seafloor after they die. These shells are widely used as climate proxies. The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stable
oxygen isotope There are three known stable isotopes of oxygen (8O): , , and . Radioactive isotopes ranging from to have also been characterized, all short-lived. The longest-lived radioisotope is with a half-life of , while the shortest-lived isotope is ...
s in the shells, since lighter isotopes evaporate more readily in warmer water leaving the heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water. The sudden
extinction event An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...
which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever.Westerhold, T., Marwan, N., Drury, A.J., Liebrand, D., Agnini, C., Anagnostou, E., Barnet, J.S., Bohaty, S.M., Vleeschouwer, D., Florindo, F. and Frederichs, T. (2020
"An astronomically dated record of Earth’s climate and its predictability over the last 66 Million Years"
''Science'', 369(6509): 1383–1387. .
Some endoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years. These are slowly metabolizing and not in a dormant state. Some Actinomycetota found in Siberia are estimated to be half a million years old.


Sediment cores

The diagram on the right shows an example of a sediment core. The sample was retrieved from the Upernavik Fjord circa 2018. Grain-size measurements were made, and the top 50 cm was dated with the :de:210Pb-Datierung, 210Pb method.


Carbon processing

Thinking about ocean carbon and carbon sequestration has shifted in recent years from a structurally-based chemical reactivity viewpoint toward a view that includes the role of the ecosystem in organic carbon degradation rates. This shift in view towards organic carbon and ecosystem involvement includes aspects of the "molecular revolution" in biology, discoveries on the limits of life, advances in quantitative modelling, paleo studies of Ocean carbon cycle, ocean carbon cycling, novel analytical techniques, and interdisciplinary efforts. In 2020, LaRowe et al. outlined a broad view of this issue that is spread across multiple scientific disciplines related to marine sediments and global carbon cycling.


Evolutionary history

To begin with, the Earth was molten due to extreme volcanism and frequent collisions with other bodies. Eventually, the outer layer of the planet cooled to form a solid crust (geology), crust and water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as a result of the impact of a planetoid with the Earth. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered from comets, Origin of water on Earth, produced the oceans. By the start of the Archean, about four billion years ago, rocks were often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments and banded iron formations. Greenstone belts are typical Archean formations, consisting of alternating high- and low-grade metamorphic rocks. High-grade rocks were derived from volcanic island arcs, while low-grade metamorphic rocks represented deep-sea sediments eroded from the neighboring island rocks and deposited in a forearc, forearc basin. The earliest-known supercontinent Rodinia assembled about one billion years ago, and began to break apart after about 250 million years during the latter part of the Proterozoic. The Paleozoic, (Ma), started shortly after the breakup of Pannotia and at the end of a global ice age. Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the era the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area. During the Silurian, which started 444 Ma, Gondwana continued a slow southward drift to high southern latitudes. The melting of ice caps and glaciers contributed to a rise in sea levels, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. During the Triassic deep-ocean sediments were laid down and subsequently disappeared through the subduction of oceanic plates, so very little is known of the Triassic open ocean. The supercontinent Pangaea rifted during the Triassic – especially late in the period – but had not yet separated. The first non-marine sediments in the rift that marks the initial break-up of Pangea are of Late Triassic age. Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare; despite their prominence in Western Europe where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west. Thus Triassic stratigraphy is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria (crustacean), ''Estheria'' crustaceans and terrestrial vertebrates. Patterns or traces of bioturbation are preserved in Lithification, lithified rock. The study of such patterns is called ichnology, or the study of "trace fossils", which, in the case of bioturbators, are fossils left behind by digging or burrowing animals. This can be compared to the footprint left behind by these animals. In some cases bioturbation is so pervasive that it completely obliterates sedimentary structures, such as laminated layers or cross-bedding. Thus, it affects the disciplines of sedimentology and stratigraphy within geology. The study of bioturbator ichnofabrics uses the depth of the fossils, the cross-cutting of fossils, and the sharpness (or how well defined) of the fossil to assess the activity that occurred in old sediments. Typically the deeper the fossil, the better preserved and well defined the specimen. Important
trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...
s from bioturbation have been found in marine sediments from tidal, coastal and deep sea sediments. In addition sand dune, or Eolian dust, Eolian, sediments are important for preserving a wide variety of fossils. Evidence of bioturbation has been found in deep-sea sediment cores including into long records, although the act extracting the core can disturb the signs of bioturbation, especially at shallower depths. Arthropods, in particular are important to the geologic record of bioturbation of Eolian sediments. Dune records show traces of burrowing animals as far back as the lower Mesozoic, 250 Ma, although bioturbation in other sediments has been seen as far back as 550 Ma.


Research history

The first major study of deep-ocean sediments occurred between 1872 and 1876 with the Challenger expedition, HMS ''Challenger'' expedition, which travelled nearly 70,000 nautical miles sampling seawater and marine sediments. The scientific goals of the expedition were to take physical measurements of the seawater at various depths, as well as taking samples so the chemical composition could be determined, along with any particulate matter or marine organisms that were present. This included taking samples and analysing sediments from the deep ocean floor. Before the ''Challenger'' voyage, oceanography had been mainly speculative. As the first true oceanographic cruise, the ''Challenger'' expedition laid the groundwork for an entire academic and research discipline. Earlier theories of continental drift proposed that continents in motion "plowed" through the fixed and immovable seafloor. Later in the 1960s the idea that the seafloor itself moves and also carries the continents with it as it spreads from a central rift axis was proposed by Harry Hammond Hess, Harold Hess and Robert S. Dietz, Robert Dietz. The phenomenon is known today as
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
. In locations where two plates move apart, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading. In 1968, the oceanographic research vessel ''Glomar Challenger'' was launched and embarked on a 15-year-long program, the Deep Sea Drilling Program. This program provided crucial data that supported the seafloor spreading hypothesis by collecting rock samples that confirmed that the farther from the
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
, the older the rock was.


See also

*
Bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
* Depositional environment * Cosmic dust ** Interplanetary dust cloud, Interplanetary dust * Deep biosphere * Marine clay * Microbially induced sedimentary structure * Oolitic aragonite sand * Organic-rich sedimentary rocks * Paleolimnology#Paleoclimate proxies * Redox gradient * Seafloor depth versus age * Sediment-water interface * Sedimentary rock * Sediment transport ** Coastal sediment transport ** Coastal sediment supply


References


Sources

* {{wikibooks, Historical Geology Marine geology Oceanography Sediments Sedimentary rocks