Continental rise

The continental rise is an underwater feature found between the continental slope and the abyssal plain. This feature can be found all around the world, and it represents the final stage in the boundary between continents and the deepest part of the ocean. The environment in the continental rise is quite unique, and many oceanographers study it extensively in the hopes of learning more about the ocean and geologic history.

At the bottom of the continental slope, one will find the continental rise, an underwater hill composed of tons of accumulated sediments.[1] The general slope of the continental rise is between 0.5 degrees and 1.0 degrees. Deposition of sediments at the mouth of submarine canyons may form enormous fan-shaped accumulations called submarine fans. Submarine fans form part of the continental rise. Beyond the continental rise stretches the abyssal plain, an extremely deep and flat area of the sea floor. The abyssal plain hosts many unique life forms which are uniquely adapted to survival in its cold, high pressure, and dark conditions. The flatness of the abyssal plain is interrupted by massive underwater mountain chains near the tectonic boundaries of the Earth's plates. The sediments are mostly sand and pieces of coral or rock.

Oceanic basin
Diagrammatic cross-section of an oceanic basin

References

  1. ^ Burk, C.A.; Drake, C.L. The Geology of Continental Margins. Springer. p. 18.
Abyssal fan

Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater version of alluvial fans and can vary dramatically in size, with widths from several kilometres to several thousands of kilometres The largest is the Bengal Fan, followed by the Indus Fan, but major fans are also found at the outlet of the Amazon, Congo, Mississippi and elsewhere.

Abyssal grenadier

The abyssal grenadier, Coryphaenoides armatus, is an abyssal fish of the genus Coryphaenoides, found in all the world's oceans, at depths between 800 and 4,000 m. Its adult length is 20 to 40 cm, although Fishbase gives lengths up to 1 m. The abyssal grenadier's body is unique in that it contains two dorsal spines and about 124 dorsal soft rays, which are the flexible jointed rays supporting a fin nearest to the back in the spinal column. It has no anal spines, but has 115 anal soft rays along its body. The head and eyes of this fish are very large, while the mouth is very small. The color of the abyssal grenadier is brown apart from the abdomen, which is bluish.Coryphaenoides armatus occurs at the deep-slope, on the upper continental rise between 2,000 m and 4,700 m. Its diet changes as it matures, from benthic invertebrates such as crustaceans and holothuroids when young to mesopelagic and bathypelagic fish, sea urchins and cephalopods when adult. Examination of the stomach contents of specimens collected by trawling below 2,600 m deep in the Hudson Canyon showed that an important proportion of the diet of C. armatus is caught in the deep mesopelagic and bathypelagic regions. Very few ripe females, and no spent individuals have been collected, and this is suggestive of this species being semelparous.

Abyssal plain

An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3,000 metres (9,800 ft) and 6,000 metres (20,000 ft). 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. They are among the flattest, smoothest, and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills).

The creation of the abyssal plain is the result of the spreading of the seafloor (plate tectonics) and the melting of the lower oceanic crust. Magma rises from above the asthenosphere (a layer of the upper mantle), and as this basaltic material reaches the surface at mid-ocean ridges, it forms new oceanic crust, which is constantly pulled sideways by spreading of the seafloor. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons into deeper water. The rest is composed chiefly of pelagic sediments. Metallic nodules are common in some areas of the plains, with varying concentrations of metals, including manganese, iron, nickel, cobalt, and copper.

Owing in part to their vast size, abyssal plains are believed to be major reservoirs of biodiversity. They also exert significant influence upon ocean carbon cycling, dissolution of calcium carbonate, and atmospheric CO2 concentrations over time scales of a hundred to a thousand years. The structure of abyssal ecosystems are strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles. Factors such as climate change, fishing practices, and ocean fertilization have a substantial effect on patterns of primary production in the euphotic zone.Abyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until very recently, none had been studied on a systematic basis. They are poorly preserved in the sedimentary record, because they tend to be consumed by the subduction process.

Argentine Basin

The Argentine Basin is a region of the Atlantic Ocean floor off the east coast of Argentina, between the Mid-Atlantic Ridge to the east and the Scotia Basin to the west. To the north is the Brazil Basin, with the two basins being separated by the Rio Grande Rise. The southern edge of the Argentine Basin is formed by the Malvinas/Falkland Escarpment. The Argentine Basin has an average depth of 5,000 m (16,000 ft) and is characterized by low temperatures. The deepest point is the Argentine Abyssal Plain at the base of the Falkland Escarpment, which reaches a depth of 6,212 m (20,381 ft).The Antarctic Bottom Water current, which dominates the circulation in the abyssal layer of the southwest Atlantic, enters the southwest part of the Argentine Basin, then is deflected northward along the continental rise. It passes into the Brazil Basin through the Vema Channel (39° 30′ W) to the west of the Rio Grande Rise. Here, the current reaches velocities of 20–25 cm/s.

Bellingshausen Plain

Bellingshausen Plain (64°0′S 90°0′W), also known as Bellinghausen Abyssal Plain, is an undersea plain parallel to the continental rise in the Bellingshausen Sea, named for Admiral Fabian Gottlieb von Bellingshausen, commander of the Russian Antarctic Expedition (1818–1821). The name was approved by the Advisory Committee for Undersea Features in April, 1974.

Borchgrevink Canyon

Borchgrevink Canyon (70°15′S 170°15′E) is an undersea canyon on the continental rise east of Iselin Bank in the Ross Sea. It was named in association with Borchgrevink Coast, the name being approved by the Advisory Committee for Undersea Features in June 1988.

Cambrian Stage 10

Stage 10 of the Cambrian is the still unnamed third and final stage of the Furongian series. It follows the Jiangshanian and precedes the Ordovician Tremadocian stage. The proposed lower boundary is the first appearance of the trilobite Lotagnostus americanus around 489.5 million years ago, but other fossils are also being discussed (see below). The upper boundary is defined as the appearance of the conodont Iapetognathus fluctivagus which marks the beginning of the Tremadocian and is radiometrically dated as 485.4 million years ago.

Continental margin

The continental margin 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 is the shallow water area found in proximity to continent. The continental margin consists of three different features: the continental rise, the continental slope, and the continental shelf. Continental margins constitute about 28% of the oceanic area.[1]

The continental shelf is the portion of the continental margin that transitions from the shore out towards to ocean. They are believed to make up 7 percent of the sea floor. The width of continental shelves worldwide varies from a 30 meters to 1500 kilometers. It is generally flat, and ends at the shelf break, where there is a drastic increase in slope angle. The mean slope of continental shelves worldwide is 0° 07’ degrees, and typically steeper closer to the coastline than it is near the shelf break. At the shelf break begins the continental slope, which can be one to five kilometers above the deep-ocean floor. The continental slope often exhibits features called submarine canyons. Submarine canyons often cut into the continental shelves deeply, with near vertical slopes, and continue to cut the morphology to the abyssal plain. The valleys are often V-shaped, and can sometime enlarge onto the continental shelf. At the base of the continental slope, there is a sudden decrease in slope, and the sea floor begins to level out towards the abyssal plain. This portion of the seafloor is called the continental rise, and marks the end of the continental margin.

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 the shelves were exposed during glacial periods and interglacial periods. The shelf surrounding an island is known as an insular shelf.

The continental margin, between the continental shelf and the abyssal plain, comprises a steep continental slope followed by the flatter continental rise. Sediment from the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope, called the continental rise. Extending as far as 500 km (310 mi) from the slope, it consists of thick sediments deposited by turbidity currents from the shelf and slope. The continental rise's gradient is intermediate between the slope and the shelf.

Under the United Nations Convention on the Law of the Sea, the name continental shelf was given a legal definition as the stretch of the seabed adjacent to the shores of a particular country to which it belongs.

Contourite

A contourite is a sedimentary deposit commonly formed on continental rise to lower slope settings, although they may occur anywhere that is below storm wave base. Countourites are produced by thermohaline-induced deepwater bottom currents and may be influenced by wind or tidal forces. The geomorphology of contourite deposits is mainly influenced by the deepwater bottom-current velocity, sediment supply, and seafloor topography.

Demersal fish

Demersal fish live and feed on or near the bottom of seas or lakes (the demersal zone). They occupy the sea floors and lake beds, which usually consist of mud, sand, gravel or rocks. In coastal waters they are found on or near the continental shelf, and in deep waters they are found on or near the continental slope or along the continental rise. They are not generally found in the deepest waters, such as abyssal depths or on the abyssal plain, but they can be found around seamounts and islands. The word demersal comes from the Latin demergere, which means to sink.

Demersal fish are bottom feeders. They can be contrasted with pelagic fish which live and feed away from the bottom in the open water column.

Demersal fish fillets contain little fish oil (one to four percent), whereas pelagic fish can contain up to 30 percent.

Eustatic sea level

The eustatic sea level is the distance from the center of the earth to the sea surface. An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or more mid-oceanic ridges. Conversely, increasing glaciation, decreasing spreading rates or fewer mid-ocean ridges lead to a fall of the eustatic sea level.

Changes in the eustatic sea level lead to changes in accommodation and therefore affect the deposition of sediments in marine environments.

Eustatic (global) sea level refers to the sea level change of the volume of Earth’s oceans. This is not a physical level but instead represents the sea level if all of the water in the oceans were contained in a single basin. Eustatic sea level is not relative to local surfaces, because relative sea level is dependent on many factors including tectonics, continental rise and subsidence. Eustatic sea level follows the ‘bathtub approach’ which describes the ocean as a single bathtub. One can add or remove water and Earth’s oceans will gain or lose water globally. Differences of eustatic sea level are caused by three main factors:

Changes in total ocean water mass, for instance, by ice sheet runoff. When an ice sheet such as Greenland begins to lose its ice mass due to melt, the liquid water is transported to the ocean. According to the ‘bathtub approach’, ice sheet runoff from Greenland will affect eustatic sea level in all areas of the world, whether nearby or distant. Ocean water mass may also shrink in size if the continental ice sheets grow in size, thereby removing liquid water from oceans and converting them to grow ice sheets

Changes in the size of the ocean basin, for instance, by tectonic seafloor spreading or by sedimentation. These slow processes can cause the total volume of the oceanic basin to change.

Density changes of the water, for instance, by thermal expansion. One driver of thermal expansion is a rise in greenhouse gases such as carbon dioxide, methane, and ozone. These gases cause surface temperatures to increase, which in turn increases ocean temperatures. This warming will cause water to experience greater molecular motion, thus increasing the volume a molecule will occupy. A decrease in atmospheric greenhouse gases will cause surface temperatures to drop. Expansion of water may also be caused by changes in ocean salinity. As continental ice accumulates, the ocean water freezes onto land but the salt it carried will mostly remain in the ocean. Thus, as ice sheets increase, ocean salinity also increases (and vice versa). An increase in salinity will increase the density of the ocean basin. Melting of ice sheets and a decrease of ocean salinity will effectively decrease the density of the water. These two effects together are called the steric sea level. The thermal part is called the thermosteric sea level, whereas the salinity part is called the halosteric sea level.

Hudson Canyon

The Hudson Canyon is a submarine canyon that begins from the shallow outlet of the estuary at the mouth of the Hudson River. It extends out over 400 mi (640 km) seaward across the continental shelf finally connecting to the deep ocean basin at a depth of 3 to 4 km below sea level. It begins as a natural channel of several kilometers width, starting as a 20–40 m depression at Hudson Channel southward from Ambrose Light, then carving through a deep notch of about 1 km depth in the shelf break, and running down the continental rise. Tidally associated flows of about 30 cm/s (1.1 km/h) up and down the deeper parts of the canyon have been recorded. As silt, sand and mud are carried down the Hudson River, they flow into the canyon and out into the deep sea.

The Hudson Canyon proper is located about 100 miles (160 km) east of the mouth of the Hudson River off the New Jersey coast. Its walls rise three-quarters of a mile from the canyon floor, making it comparable to the Grand Canyon, whose cliffs are over a mile deep and 270 miles (430 km) long. It is the largest known ocean canyon off the East Coast of the United States, and one of the largest submarine canyons in the world. The canyon is located near the 100 meter isobath on the continental shelf and is 2,200 m (7,200 ft) deep at the base of the continental slope. Over an 80 km (50 mi) distance, the average slope of the canyon floor is 1.5°. At this point the canyon is as much as 12 km (7.5 mi) wide (from east rim to west rim) and as much as 1,100 m (3,600 ft) deep from canyon rim to canyon floor across the continental slope. The floor of the canyon is less than 0.5 km (1,600 ft) wide across the upper part of the slope and broadens to about 0.9 km (3,000 ft) at the base of the slope. The canyon was last exposed during the last Ice Age, over 10,000 years ago, when the sea level was about 400 feet (120 m) lower and the mouth of the Hudson River was near the edge of the continental shelf, about 100 miles (160 km) east of its present site. The river discharged sediment that helped carve the canyon aided by underwater avalanches of mud and sand. Recent maps of the canyon reveal tributaries of an extraordinary underwater drainage network that is strikingly similar to terrestrial rivers. Tidal currents sweep up and down the channel; and on occasion, during big storms, cold ocean water is pushed up the Hudson Canyon to spread out on the shelf. Thus the Hudson Canyon continues to be cut by traveling sediments.The Hudson Canyon is thought to contain residues of pollution and trash from the days when New York City's sewage and garbage was dumped directly into the river. If true, many parts of the trench bottom may be covered in sludge. Scientists plan to use new maps of the canyon to track contaminants from six abandoned dump sites off New York Harbor.

"Hudson Canyon" also designates a location marked by a navigational buoy indicating the seaward end of the vessel traffic separation scheme of the Hudson Canyon–Ambrose lanes which lead into and out of New York Harbor for Atlantic shipping.

Integrated Marine and Coastal Regionalisation of Australia

The Integrated Marine and Coastal Regionalisation of Australia (IMCRA), formerly the Interim Marine and Coastal Regionalisation for Australia, is a biogeographic regionalisation of the oceanic waters of Australia's Exclusive Economic Zone (EEZ). As of 2008, the most recent version is IMCRA Version 4.0.IMCRA actually defines two bioregionalisations: a benthic bioregionalisation, based on biogeography of fish together with geophysical data; and a pelagic bioregionalisation, base on oceanographic characteristics.The benthic bioregionalisation incorporates three separate regionalisations:

A regionalisation of the EEZ into provincial bioregions, based on the biogeography of bottom dwelling fishes. In IMCRA 4.0, 41 provincial bioregions, consisting of 24 provinces and 17 transitions.

A regionalisation of the continental shelf into meso-scale regions based on biological and physical characters, and the distance from the coast. In IMCRA 4.0 there are 60 meso-scale regions.

A regionalisation of the EEZ into 14 geomorphic units, formed by grouping the 1,134 geomorphic units defined by Geoscience Australia.The pelagic bioregionalisation divides the continental shelf into four provincial bioregions based on pelagic fish species biodiversity and richness. Offshore waters are divided into three-dimensional water masses, taking into account water properties, circulation patterns and energetics.

Oates Canyon

Oates Canyon (68°0′S 164°30′E) is an undersea canyon on the continental rise east of Iselin Bank in Antarctica. Its name (approved June 1988 (ACUF 228)) is associated with Oates Coast.

Obduction

Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.Obduction occurs where a fragment of continental crust is caught in a subduction zone with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction often occurs where a small tectonic plate is caught between two larger plates, with the crust (both island arc and oceanic) welding onto an adjacent continent as a new terrane. When two continental plates collide, obduction of the oceanic crust between them is often a part of the resulting orogeny.Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.

Riiser-Larsen Sea

The Riiser-Larsen Sea is one of the marginal seas located in the Southern Ocean off East Antarctica and south of the Indian Ocean. It is delimited Astrid Ridge in the west and the Gunnerus Ridge and the Kainanmaru Bank in the east.

It is bordered by the Lazarev Sea to the west and the Cosmonauts Sea to the east, or between 14°E and 30°E. Its northern border is defined to be the 65th parallel south. The name, proposed by the Soviet Union, was never officially approved by the International Hydrographic Organization (IHO).

To the south of this area lies the Princess Astrid Coast and Princess Ragnhild Coast of Queen Maud Land. In the western part is the Lazarev Ice Shelf, and further east are Erskine Iceport and Godel Iceport, and the former Belgian Roi-Baudouin Station.

Seabed

The seabed (also known as the seafloor, sea floor, or ocean floor) is the bottom of the ocean.

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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