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 siliceous shells of phytoplankton or zooplankton; clay-size siliciclastic sediment; or some mixture of these. Trace amounts of meteoric dust and variable amounts of volcanic ash also occur within pelagic sediments. Based upon the composition of the ooze, there are three main types of pelagic sediments: siliceous oozes, calcareous oozes, and red clays.
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 and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may eventually become sandstone and siltstone.
Phytoplankton are the autotrophic (self-feeding) components of the plankton community and a key part of oceans, seas and freshwater basin ecosystems. The name comes from the Greek words φυτόν (phyton), meaning "plant", and πλαγκτός (planktos), meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, some varieties may be noticeable as colored patches on the water surface due to the presence of chlorophyll within their cells and accessory pigments in some species.
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word zooplankton is derived from the Greek zoon (ζῴον), meaning "animal", and planktos (πλαγκτός), meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some are larger and visible to the naked eye.
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.
A biogenic substance is a product made by or of life forms. The term encompasses constituents, secretions, and metabolites of plants or animals. In context of molecular biology, biogenic substances are referred to as biomolecules.
In case of marine sediments, ooze does not refer to a sediment's consistency, but to its composition, which directly reflects its origin. Ooze is pelagic sediment that consists of at least 30% of microscopic remains of either calcareous or siliceous planktonic debris organisms. The remainder typically consists almost entirely of clay minerals. As a result, the grain size of oozes is often bimodal with a well-defined biogenic silt- to sand-size fraction and siliciclastic clay-size fraction. Oozes can be defined by and classified according to the predominate organism that compose them. For example, there are diatom, coccolith, foraminifera, globigerina, pteropod, and radiolarian oozes. Oozes are also classified and named according to their mineralogy, i.e. calcareous or siliceous oozes. Whatever their composition, all oozes accumulate extremely slowly, at no more than a few centimeters per millennium.
Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz (SiO2), metal oxides (Al2O3, MgO etc.) and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to particle size and geometry as well as water content, and become hard, brittle and non–plastic upon drying or firing. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red.
Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.
Diatoms are a major group of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms number in the trillions: they generate about 20 percent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live, and contribute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half mile deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by east-to-west (easterly) transatlantic winds from the bed of a dried up lake once covering much of the African Sahara.
Calcareous ooze is ooze that is composed of at least 30% of the calcareous microscopic shells—also known as tests—of foraminifera, coccolithophores, and pteropods. This is the most common pelagic sediment by area, covering 48% of the world ocean's floor. This type of ooze accumulates on the ocean floor at depths above the carbonate compensation depth. It accumulates more rapidly than any other pelagic sediment type, with a rate that varies from 0.3–5 cm/1000 yr.
Calcite compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite lags behind the rate of solvation, such that no calcite is preserved. Aragonite compensation depth describes the same behaviour in reference to aragonitic carbonates. Aragonite is more soluble than calcite, so the aragonite compensation depth is generally shallower than the calcite compensation depth.
Siliceous ooze is ooze that is composed of at least 30% of the siliceous microscopic "shells" of plankton, such as diatoms and radiolaria. Siliceous oozes often contain lesser proportions of either sponge spicules, silicoflagellates or both. This type of ooze accumulates on the ocean floor at depths below the carbonate compensation depth. Its distribution is also limited to areas with high biological productivity, such as the polar oceans, and upwelling zones near the equator. The least common type of sediment, it covers only 15% of the ocean floor. It accumulates at a slower rate than calcareous ooze: 0.2–1 cm/1000 yr.
Sponges, the members of the phylum Porifera, are a basal Metazoa (animal) clade as a sister of the Diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. The branch of zoology that studies sponges is known as spongiology.
Dictyochales are a small group of unicellular heterokont algae, found in marine environments.
Red clay, also known as either brown clay or pelagic clay, accumulates in the deepest and most remote areas of the ocean. It covers 38% of the ocean floor and accumulates more slowly than any other sediment type, at only 0.1–0.5 cm/1000 yr. Containing less than 30% biogenic material, it consists of sediment that remains after the dissolution of both calcareous and siliceous biogenic particles while they settled through the water column. These sediments consist of aeolian quartz, clay minerals, volcanic ash, subordinate residue of siliceous microfossils, and authigenic minerals such as zeolites, limonite and manganese oxides. The bulk of red clay consists of eolian dust. Accessory constituents found in red clay include meteorite dust, fish bones and teeth, whale ear bones, and manganese micro-nodules.
Aeolian processes, also spelled eolian or æolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials and are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.
Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.
Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away.
These pelagic sediments are typically bright red to chocolate brown in color. The color results from coatings of iron and manganese oxide on the sediment particles. In the absence of organic carbon, iron and manganese remain in their oxidized states and these clays remain brown after burial. When more deeply buried, brown clay may change into red clay due to the conversion of iron-hydroxides to hematite.
These sediments accumulate on the ocean floor within areas characterized by little planktonic production. The clays which comprise them were transported into the deep ocean in suspension, either in the air over the oceans or in surface waters. Both wind and ocean currents transported these sediments in suspension thousands of kilometers from their terrestrial source. As they were transported, the finer clays may have stayed in suspension for a hundred years or more within the water column before they settled to the ocean bottom. The settling of this clay-size sediment occurred primarily by the formation of clay aggregates by flocculation and by their incorporation into fecal pellets by pelagic organisms.
|Region||Percent of ocean area||Percent of total volume of marine sediments||Average thickness|
|Continental shelves||9%||15%||2.5 km (1.6 mi)|
|Continental slopes||6%||41%||9 km (5.6 mi)|
|Continental rises||6%||31%||8 km (5 mi)|
|Deep-ocean floor||78%||13%||0.6 km (0.4 mi)|
|Sediment type||Source||Examples||Distribution||Percent of all ocean floor area covered|
|Terrigenous||Erosion of land, volcanic eruptions, blown dust||Quartz sand, clays, estuarine mud||Dominant on continental margins, abyssal plains, polar ocean floors||~45%|
|Biogenous||Organic; accumulation of hard parts of some marine organisms||Calcareous and siliceous oozes||Dominant on deep-ocean floor (siliceous ooze below about 5 km)||~55%|
|Hydrogenous (authigenic)||Precipitation of dissolved mineral from water, often by bacteria||Manganese nodules, phosphorite deposits||Present with other, more dominant sediments||1%|
|Cosmogenous||Dust from space, meteorite debris||Tektite spheres, glassy nodules||Mixed in very small proportion with more dominant sediments||1%|
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Flint (a type of chert) is very common as bands parallel to the bedding or as nodules embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).
Chert is a hard, fine-grained sedimentary rock composed of crystals of quartz (silica) that are very small (microcrystalline or cryptocrystalline). Quartz (silica) is the mineral form of silicon dioxide (SiO2). Chert is often of biological origin (organic) but may also occur inorganically as a chemical precipitate or a diagenetic replacement (e.g., petrified wood). Geologists use chert as a generic name for any type of microcrystalline or cryptocrystalline quartz.
Micropaleontology is the branch of paleontology (palaeontology) that studies microfossils, or fossils that require the use of a microscope to see the organism, its morphology and its characteristic details.
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals or clasts are embedded.
The seabed is the bottom of the ocean.
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.
Mudrocks are a class of fine grained siliciclastic sedimentary rocks. The varying types of mudrocks include: siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 0.0625 mm and are too small to study readily in the field. At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including:
Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. For example, microscopic particles of silica called phytoliths can be found in grasses and other plants. Silica is an amorphous metal oxide formed by complex inorganic polymerization processes. This is opposed to the other major biogenic minerals, comprising carbonate and phosphate, which occur in nature as crystalline iono-covalent solids (e.g. salts) whose precipitation is dictated by solubility equilibria. Chemically, bSi is hydrated silica (SiO2·nH2O), which is essential to many plants and animals.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.
Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians. This term is also used for indurated radiolarian oozes and sometimes as a synonym of radiolarian earth. However, radiolarian earth is typically regarded by Earth scientists to be the unconsolidated equivalent of a radiolarite. A radiolarian chert is well-bedded, microcrystalline radiolarite that has a well-developed siliceous cement or groundmass.
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 at least 30% skeletal remains of pelagic microorganisms. Siliceous oozes are largely composed of the silica based skeletons of microscopic marine organisms such as diatoms and radiolarians. 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 oozes, which are made from skeletons of calcium carbonate organisms (i.e. coccolithophores). 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.
In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below. The term was first coined by the explorer William Beebe as he observed it from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic activity and ocean currents. Marine snow can be an important food source for organisms living in the aphotic zone, particularly for organisms which live very deep in the water column.
The Southern Pacific Gyre is part of the Earth’s system of rotating ocean currents, bounded by the Equator to the north, Australia to the west, the Antarctic Circumpolar Current to the south, and South America to the east. The center of the South Pacific Gyre is the oceanic pole of inaccessibility, the site on Earth farthest from any continents and productive ocean regions and is regarded as Earth’s largest oceanic desert. The gyre, as with Earth's other four gyres, contains an area with elevated concentrations of pelagic plastics, chemical sludge, and other debris known as the South Pacific garbage patch.
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. Hemipelagic sediments are deposited on continental shelves and continental rises, and differ from pelagic sediment 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 like feldspar or quartz. Volcanism 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.
The Ruhpolding Formation is a sedimentary formation of the Northern Calcareous Alps deposited during the Upper Jurassic. The open marine radiolarite is very rich in silica.
Pelagic red clay, also known as simply red clay, brown clay or pelagic clay, is a type of pelagic sediment.
Allison Guyot is a tablemount (guyot) in the underwater Mid-Pacific Mountains of the Pacific Ocean. It is a trapezoidal flat mountain rising 1,500 metres above the seafloor to a depth of less than 1,500 m, with a summit platform 35 by 70 kilometres wide. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of their formation were located in the Southern Hemisphere.