OCEANS
A hydrosphere (from Greek - hydor "water" and - sphaira "sphere") in physical geography describes the combined mass of water found on under and over the surface of a planet a planet.. ,
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The total mass of the Earth's hydrosphere is about 1.4 × 10 tonnes which is about 0.023% of the Earth's total mass. About 20 × 1012 tonnes of this is in the Earth's atmosphere (the volume of one tonne of water is approximately 1 cubic metre). Approximately 71% of the Earth Earth's 's surface an area of some 361 million million square kilometres (139.5 million million square miles) miles) is covered by ocean ocean.. The average salinity of the Earth's oceans is about 35 grams of salt of salt per kilogram of sea water [1] (35 Å). ,
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An ocean is a major body of saline of saline water, water, and a principal component of the hydrosphere hydrosphere.. Approximately 14
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71% of the Earth Earth's 's surface (~3.61 X 10 m ) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas seas.. Though generally described as several 'separate' oceans, these waters comprise one global, [3][4]
interconnected body of salt water sometimes referred to as the World Ocean or global ocean.
This
concept of a continuous body of water with relatively relatively free interchange among its parts is of fundamental importance tooceanography tooceanography..
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The major oceanic divisions are defined in part by the continents continents,, various archipelagos archipelagos,, and other criteria. These divisions are (in descending order of size):
Pacific Ocean, Ocean, which separates Asia separates Asia and Australia and Australia from the Americas the Americas
Atlantic Ocean, Ocean , which separates the Am ericas from Eurasia and Africa and Africa
Indian Ocean, Ocean, which washes upon southern Asia southern Asia and separates Africa and Australia
Southern Ocean, Ocean, which, unlike other oceans, has no landmass separating it from other oceans and i s therefore sometimes subsumed as the southern portions of the Pacific, Atlantic, and Indian Oceans, which encircles Antarctica encircles Antarctica and covers much of the Antarctic the Antarctic
Arctic Ocean, Ocean , sometimes considered a sea of the Atlantic, which covers much of the Arctic the Arctic and washes upon northern North America and Eurasia
The Pacific and Atlantic may be further subdivided by the equator equator into into northern and southern portions. Smaller regions of the oceans are called seas seas,, gulfs gulfs,, bays bays,,straits and other names. Geologically, an ocean is an area of oceanic of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle mantle..Continental crust is thicker but less dense. From [citation needed ]
this perspective, the earth has three oceans: the World Ocean, the Caspian Sea Sea.. The latter two were formed by the collision of Cimmeria Sea of Cimmeria with Laurasia Laurasia..
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Oceanography (compound of the Greek words meaning "ocean "ocean"" and meaning "to write"), also called oceanology or marine or marine science, science, is the branch of Earth of Earth science that studies the ocean.
Pic/ map showing oceans of the World
Water. As the cosmic dust collapsed billions of years ago to form the planet Earth water was probably locked into rock-forming compounds. These compounds (hydrated silicates) silicates) would wou ld have slowly released the trapped water during the first billion years or so of Earth history and formed the primordial ocean. The duration and time of initiation of this process are not exactly known because rocks containing the record of that time span have been destroyed in the succeeding 3 billion years. Water would not have been released at the earliest stage of the Earth's development however because a molecule of water is lighter than a molecule of any o f the lighter elements or "volatiles " such as neon that would have escaped into space during the intense heat that accompanied acco mpanied the formation of the planet. On the other hand the origin of the primordial ocean must have occurred o ccurred during the first billion billion years because some of the t he earliest rocks found on Earth show evidence of deposition in a large body of water. The
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The time required to accumulate the volume of water in the present oceans is unknown. The water was released during the coo ling of the Earth and attained early after its initiation a volume not drastically different from that of the modern oceans. That the volume vo lume of water in the oceans has not changed drastically during the last few hundred million million years is inferred from evidence indicating that the interiors of the continental land masses have never been inundated by deep oceans. Any incursions of seawater upon the continents cont inents that did occur were in response to tectonic changes (that is changes due to deformations of the Earth's crust) rather than changes in the volume of water in the oceans. ,
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The most recent changes in ocean volume have accompanied the ice ages during the last two to three million years. The northern polar icecap has expanded and contracted with great regularity and the volume of the oceans has fluctuated correspondingly correspo ndingly as water is alternately locked into
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or released from the ice cap. These fluctuations however have accounted for changes in sea level of no more than 200 m (660 ft). ,
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Basins.
Hydrologically, an
oceanic basin may be anywhere on Earth that is covered byseawater byseawater , but geologically ocean basins are large geologic basins that are below sea level. level. Geologically, oceanic basins may be actively changing size or m ay be inactive, depending on whether there is a moving plate tectonic boundary associated with it. The elements of an active - and growing - oceanic basin include an elevated mid-ocean ridge, ridge, flanking abyssal hills leading down to abyssal plains. plains. The elements of an active oceanic basin often include the oceanic trench associated with a subduction zone. zone. Much of our
knowledge about the topographic features that exist here are derived from the following technologies: seismic surveying; echo sounder; side-scan sonar; and the measurement of the height of sea surfaces by satellites.
Oceanographers divide the ocean into regions depending on physical and biological conditions of of these areas. The pelagic zone includes all open ocean regions, and can be divided into further regions categorized by depth and light abundance. The photic zonecovers zonecovers the oceans from surface level to 200 metres down. This is the region where photosynthesis can occur and therefore is the most biodiverse. Since plants require photosynthesis, life found deeper than this must either rely on material sinking from above (see marine snow) snow) or find another energy source; hydrothermal vents are the primary option in what is known as the aphotic zone (depths exceeding 200 m). The pelagic part of the photic
zone is known as the epipelagic epipelagic.. The pelagic part of the aphotic zone can be further divided into regions that succeed each other vertically according to temperature. The mesopelagic is the uppermost region. Its lowermost boundary is at a thermocline of 12 °C (54 °F), which, in the tropics generally lies at 700±1,000 metres (2,300±3,300 ft). Next is the bathypelagic lying between 10-4 °C (43 °F), typically between 700±1,000 metres (2,300±3,300 ft) and 2,000±4,000 metres (6,600±13,000 ft) Lying along the top of the abyssal plain is the abyssalpelagic abyssalpelagic,, whose lower boundary lies at about 6,000 metres (20,000 ft). The last zone includes the deep trenches, and is known as the hadalpelagic hadalpelagic.. This lies between 6,000±11,000 metres (20,000±36,000 ft) and is the deepest oceanic zone. Along with pelagic aphotic zones there are also benthic aphotic zones. These correspond to the three deepest zones of the deep-sea deep-sea.. The bathyal zone covers the continental slope down to about 4,000 metres (13,000 ft). The abyssal zone covers the abyssal plai ns between 4,000 and 6,000 m. Lastly, the hadal zone corresponds to the hadalpelagic zone which is found in the oceanic trenches. The pelagic zone can also be split into two subregions, the neritic zone and theoceanic theoceanic zone. zone. The neritic encompasses the water mass directly above the continental shelves, shelves, while the oceanic zone includes all the completely open water. In contrast, the littoral zone covers the region between low and high tide and represents the transitional area between marine and terrestrial conditions. It is also known as the intertidal zone because it is the area where tide level affects the conditions of the region.
Physiography
of the Seafloor The seafloor is shaped into a host of volcanic The shape of o f the seafloor in turn affects the origin and distribution of some of the great o ceanic circulation systems. These systems play a role in the distribution of the oceanic nutrients that control biogenic biogenic productivity pro ductivity in the oceans. ,
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and Trenches. the mid-oceanic ridge system that through branches extends across all the major ocean basins. Typically T ypically the central axis of this system is marked by a steepwalled valley usually about 40 km (25 mi) wide and 2 km k m (1 mi) deep. Small segments of the ridge extend above sea level to form islands (for example Iceland but most of the ridge crest is at a depth of o f approximately 2.5 km (1.5 mi). R idges, idges, Plains,
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The seafloor continues to deepen away from the crests of the mid-oceanic ridge system out to the extensive flat abyssal plains which constitute the largest segment of the sea floor. floor. The T he depths of the individual plains are roughly rough ly uniform the deepest of them occur in the Pacific Ocean (6 000 m/20 000 ft) and the shallowest occur in the Atlantic Ocean (5 000 m/ 16 000 ft). ,
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In selected areas usually at the margins of the ocean o cean basins the abyssal plains descend into steep oceanic trenches where the greatest depths in the oceans are found. Examples of o f these trenches include the the Tonga Trench (Pacific) 10 880 m (35 700 ft); and the deepest hole in the ocean the Marianas Trench (Pacific) 11 022 m (36 163 ft). Associated with most of these major trench systems are volcanic island arcs found on o n the landward side of the trenches. ,
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Zones and Seamounts. Superimposed on the features of the seafloor previously mentioned are many other smaller-s s maller-scale cale but very important physiographic features. Segments of the mid-oceanic ridges are commonly offset o ffset laterally by parallel linear fracture zones thousands of kilometers long. These oceanic fracture zones are characterized by ridges and valleys separated by steep rock ro ck cliffs that are hundreds to several thousands of meters high. These fracture zones can be traced out into the abyssal plains where the traces of the cliffs c liffs are lost beneath the sediment. Fracture
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The abyssal plains are also dotted with numerous isolated mountains called seamounts that extend in some cases above sea level to beco me islands. Characteristically these seamounts belong to large groups of such features which may be randomly dispersed over a large area or arranged in a line. ,
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Seafloor Spreading. The origins of many of o f these strikingly different different physiographic features are linked together by the concept of seafloor spreading. The seafloor is produced by the cooling of upwelling molten material at spreading centers charact erized by the mid-oceanic ridges. The ridges are broken and discontinuous d iscontinuous due to fracture zones along which w hich transform faulting occurs. Newly formed material moves away from the r idge crest and tens of millions of years later it is subducted or downwarped in an oceanic trench. ,
co ntinents extend seaward as a Continental Shelves, Slopes, and R ises. ises. The margins of all the continents broad flat shallow shallow shelf. These co ntinental shelves dip gently seaward and are usually less than 200 m (660 ft) below sea level. The continental shelves comprise only a small portion (7.6%) of the seafloor but their importance is far greater because the shallow depths make it possible po ssible to exploit their natural resources. ,
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The shelves abruptly end at t he shelf breaks where the seafloors rapidly descend along the t he continental slopes to the abyssal depths. At the bases of the continental slopes which cover about 15.3% of the oceanic area are the continental rises which represent a series of sediment slumps from the slopes above that have spilled down onto the deep seafloor or ocean basins. The ,
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basins cover about 75.9% of the oceanic area. Only 1.2% of the ocean is greater than 6 000 m (19 686 ft) in depth. ,
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Submarine Canyons (from Pur S)
A stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast isolated by erosion.They are formed when part o f a headland is eroded by hydraulic action The force of the water weakens cracks in the headland causing them to later collaps co llapsee forming free-standing stacks and even a small islandstacks also form when a natural arch collapses under gravity under gravity due to sub-aerial processes like wind erosio ,
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Cuspate forelands are geographical features found on coastlines and created by long shore drift. drift. Made out of sand of sand and shingle and later stabilised by vegetation cuspate forelands are triangularshaped accretions and extend seawards. ,
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Importance
of the Oceans and Seas Oceans and seas are now understood to be integral parts of the entire geologic process of continental weathering runoff and deposition followed by either uplift and subaerial exposure or subduction into the depths dept hs of the Earth during the process of plate t ectonics and seafloor spreading. ,
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The oceans and seas also known as the hydrosphere are responsible for the regulation of many major processes that occur on the t he surface of the Earth. Much of the precipitation precipitation that falls upon land areas is derived from oceanic evaporation. e vaporation. The hydrosphere acts as a t remendous heat reservoir exerting a dominant effect on t emperature extremes over large land areas. The movement of ocean currents also creates moderating effects in some areas at latitudes where weather extremes might otherwise make life unpleasant. In addition the oceans act as reservoirs reservo irs for numerous other substances that provide a buffer ing effect on the levels of various gases in the atmosphere and in some cases a dilution of otherwise toxic materials that humans have introduced into salt and fresh waters. ,
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The oceans and seas represent a place of recreation a means of transportation and a storehouse of food mineral resources and energy sources. Their potential as a source so urce of immeasurable resources is just beginning to be realized. ,
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The ocean provides pro vides tremendous potential as a means of transportation. The major portion of goods distributed worldwide are shipped by water the least expensive method o f transport and one that provides a livelihood livelihood for a significant percentage of the world's population. Transportation.
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As a biological entity the oceans represent a highly productive environment. The basic biological habitats of the ocean can be divided into pelagic (water region) and benthic (seafloor region) environments. The pelagic zone can be divided into the neritic zone (down to 200 m/660 ft and over the continental shelf) and the oceanic zone (below 200 m). The neritic and the upper oceanic ( epipelagic zone) regions correspond to the photic zone in which photosynthesis is possible. The benthic habitat is also divided into the intertidal zone (between high a nd low tide) the sublittoral zone (down to 200 m; see littoral zone); the bathyal zone (200 to 4 000 m) the abyssal zone (4 000 to 5 000 m/13 000 to 16 000 ft) and the hadal zone (deeper than 5 000 m). The photic zone is the most highly productive area containing numerous benthic planktonic (free-floating) and nektonic (free-swimming) marine creatures. Below the phot ic zone the biomass decreases considerably with the only food source for marine life being the constant rain of organic matter from above. The lack of light and food has produced numerous adaptations in deep-sea life resulting in some very strange forms at these dept hs. Food.
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Throughout recorded history people have used the ocean as a source of food. But even at present rates of removal the food-resource potential of the oceans has barely been touched. At present most countries remove only certain choice cho ice species of fish from the ocean. This practice has led to the depletion of fish populations popu lations of these few species whereas at the same time other ot her species have remained almost untouched. The depletion of fish populations in once-cho ice fishing grounds has caused considerable international dispute between the governments and fishing fishing fleets of many countries ultimately leading to expa nsion of offshore fishing limits in many parts ,
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of the world and has a lso forced the development of techniques for fish farming and the cultivation and seeding of many bottom areas in order to enhance shellfish she llfish production. A change in attitudes att itudes about the consumption of various species of fish may a lso help alleviate fishing pressures on certain species and encourage a more balanced exploitation of fisheries resources. The infancy of research in fisheries biolo gy is such that the behavior food requirements and breeding habits of most fish are not well known. Investigation may lead to a situation where human beings can depend more heavily on the ocean as a food source without creating a drastic impact on the overall ecology of the ocean. ,
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Water. The ocean is a source of fresh water in many highly arid nearshore areas where the cost of transporting water from regions where naturally occurring fresh water is abundant is greater than the cost of desalination. Most current desalination methods resemble a distilling process. Other methods that are available but not in widespread use are freezing reverse osmosis and ionic processes. Current costs of desalinization are not so high as to be prohibitive to domestic and some industrial users; but whether new develop ments will bring costs down to a level where agricultural and other industrial users will find the use of desalinized water practical remains uncertain. ,
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Energy. Theoretically the ocean represents a tremendous source of energy. Current use of the oceans by the energy energ y industry is restricted for the most part however to the use of seawater as a coolant in nearshore nuclear power po wer plants. This particular use has produced considerable response from groups concerned with the environmental impact of the discharge of heated water from these reactors. ,
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Some energy can be extracted from the ocean by making use of the change in sea level caused by tidal cycles. The use of this method however is currently limited. Numerous other methods are mostly in experimental stages at present. Severa l of these methods make use of the temperature t emperature differential that exists across the thermocline. Other methods would make use o f wave and current energy. Most of these designs suffer from o ne or more disadvantages such as high generation costs great distances between suitable generating sites and t he power market or a number of engineering problems. ,
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The Sun replenishes the oceanic energy reserves at a much greater rate than human beings could ever remove energy from the oceans. The problems of economically removing this energy most of which is diffused over the entire ent ire ocean are however at present so insurmountable as to make widespread dependence on this energy resource impractical. ,
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Oil. Since the 1960s the production of oil from wells on the continental co ntinental shelves has increased drastically to the point where it represents a substantial percentage of the world's production. More recent evidence indicates t hat oil deposits also reside on the continental slope. Current drilling and maintenance technology however has not yet advanced to a state where tapping of these additional reserves is practical in a ro utine manner. Current estimates of offshore reserves far surpass presently known onshore reserves. A need for additional energy supplies will no doubt stimulate the technological advancements necessary t o take advantage of o f these reserves and to minimize damage to the environment. ,
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Economically important minerals are constantly introduced into the o cean from a variety of sources and most of the material accumulates on the ocean bottom. Rivers dump vast quantities of particulate mineral materials into the oceans each year. Volcanic eruptions and hydrothermal solutions introduce many metals into solution and in so lid lid form. Minerals.
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Heavy minerals have a tendency to accumulate in superficial placer deposits which can be located using a variety of geophysical techniques and then can be mined by dredging. Phosphate rocks are also mined from nearshore areas. Salt domes which are subbottom bedrock deposits are mined by pumping hot water down to melt melt the sulfur and to force the molten material back up the drill dr ill string. ,
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The deep sea as well as nearshore environments stores vast amounts of economically important oceanic mineral resources. Large areas of the ocean bottom mostly in temperate and tropical regions are covered with calcareous deep-sea oozes which are the product of deposition of calcium carbonate skeletons accreted by planktonic and benthonic microorganisms. microorganisms. This material could be useful in the manufacture of various building supplies most notably cement and concrete. In higher latitudes the ocean-bottom o cean-bottom sediments are dominated by siliceous oozes. T hese oozes the product o f silica minerals secreting microorganisms serve as an efficient filtering and insulating material. ,
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Ocean Pollution Oceans clearly play an essential ro le in life on Earth yet because of their vastness humans have tended to use their waters as dumping d umping grounds for many waste materials. This practice has increased as land areas for such wastes diminish. diminish. Oceans Ocea ns also receive all of the pollutants p ollutants that are fed to them by the r ivers of the world. Even when ships are no t actively engaged in dumping wastes they are themselves sources of pollution most notab ly the giant tankers that have caused numerous massive oil spills. ,
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As a result by the late 20th century ocean studies indicate that what had once been thought impossible is becoming a reality. The oceans as a whole are showing signs of environmental pollution. Even the surface waters of o f the oceans are increasingly plagued by obvious o bvious litter. Some of this litter washes ashore to render beaches unsight ly while other such debris entangles and kills many sea birds and mammals every year. ,
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More insidious than these litter problems are the effects o f toxic contaminants from wastes that are dumped in the ocean. These chemicals can upset delicate marine ecosystems as they are absorbed by organisms all along the food chain. Even the paints that are being used on many ships can be hazardous. The need to address the matter of ocean pollution has been recognized at national and international levels. The U.S. Congress Co ngress for example passed an act in 1988 that would prohibit ocean dumping by 1991 and in that same year 65 nations agreed to cease burning toxic wastes at sea by 1994 should acceptable alternative practices be found. In the late 1990s the latter action remains under debate however as several nations continue the practice o f ocean burning of toxic wastes. The U.S. act may prove as unenforceable as a prior p rior one in 1977 that attempted the t he same prohibition. Worldwide the problem of ocean po llution remains. remains. ,
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Over 60% of the world¶s w orld¶s population is estimated to live near coastal areas. The oceans govern the health of our planet.
By 2025 the world population is projected to be between 7.5-8.5 billion
World population growth Coastal zones in the U.S. are associated with trillion-do trillion-dollar llar industries. industries. Worldwide over 2 billion people rely on seafood and products from the sea for their sustenance and this number continues to increase. ,
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World population is estimated to increase from 7.5 billion to 8.5 billion billion by 2025. The past rise in human population has already a lready triggered dramatic changes in coastal ecosystems resulting in significant depletion in stocks of fish and shellfish increased pollution in waterways and coastal environments and increased pressures on marine mammals. ,
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In the past few decades there has ha s been a gro wing awareness of the strong linkages between Earth¶s oceans and human health . Examples of such connections include the following: y y
Harmful algal blooms that produce red tides that impact our ability to harvest shellfish Cholera outbreaks related to ocean temperature-chemical variations that occur during El Niño events in the Southern Ocean and perhaps when the Indian Ocean is in its Dipole/Zonal mode Incorporation of toxins into fish and marine mammals that are then ingested by humans. ,
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The complexity vastness and dynamic nature of the ocean provide challenges in understanding and monitoring environmental changes and the impact these changes have on human health . New multidisciplinary centers such as the University of Washington¶s Pacific Northwest Center for Human Health and Ocean Studies are conducting research aimed at better understanding these issues. ,
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The advanced technologies opportunities for deployment of o f state-of-the-art sensor arrays adaptive sampling and real-time communications provide by Ocean Observatories infrastructure and resultant new predictive models will form a cr itical component to investigating how changing environmental conditions in the oceans impact human health. Such information is essential to making informed policy and management decisions in the future. ,
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Oceans and seas are the bodies bo dies of salt water that cover about 71 percent of the Earth's surface and are referred to in tot al as the world ocean. Several centuries ago t he "seven seas" were considered the navigable oceans namely the Atlantic Pacific Indian and Arctic oceans the Mediterranean and Caribbean seas and the Gulf of Mexico. At present however oceanographers consider all other oceans and seas as belonging to the Atlantic Pacific or Indian oceans. The Arctic Ocean the Mediterranean and Caribbean seas and the Gulf of Mexico are considered marginal seas of the Atlantic At lantic Ocean. These in turn have their own marginal bays and seas. Narrow shallow shallow straits stra its separate the marginal seas from the Atlantic: the Straits of Florida (Gulf of Mexico) the Strait of o f Gibraltar (Mediterranean) and many gaps between the islands of the Greater and Lesser Antilles for the Caribbean Sea. ,
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Many other large bodies of water have been designated as seas but all are marginal to the great oceans. The largest of these are the Bering Sea the Coral Sea the East China Sea and South China Sea the Sea of Okhotsk the Sea of Japan the Yellow Sea and the Philippine Sea bordering the Pacific; the Arabian Sea the Red Sea and the Bay of Bengal bordering the Indian Ocean; the Scotia Sea the North Sea the Labrador Sea the Weddell Sea the Norwegian Sea and the Greenland Sea bordering the Atlantic Ocean. Marginal seas differ d iffer from the major oceans primarily in size but also in depth and bottom topography. ,
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The boundaries between the oceans are based on geographic criteria and have little to do with physical water-mass boundaries. The Atlantic is separated from the Indian Ocean by the 20° E meridian and from the Pacific Ocean ( in the south) by a line extending from Cape Horn at the tip of South America to t he South Shetland Islands off Antarctica's tip and (in the nort h) by the narrowest part of the Bering Strait. The dividing line between the Pacific and Indian oceans extends along an arc through the Malay Peninsula Sumatra Java and Timor to Cape Londonderry in Australia to Tasmania and then along the 147° 14 7° E meridian to Antarctica. ,
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Reference is often made to the t he Antarctic or Southern Ocean which encircles the Antarctic continent and consists of the southernmost sectors of the three principal oceans. In spite of the lack of definitive geographic boundaries the meteorological and oceanographic conditions in the high southern latitudes combine to produce a well-defined circumpolar current called the West ,
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Wind Drift. This current distinguishes the Antarctic Ocean as a physical ent ity but the ocean's geographic borders are less easily defined. ,
Oceanic regions constitute a much larger percentage of the Earth's surface in the So uthern Hemisphere (81%) than in the Northern No rthern Hemisphere Hemisphere (61%). This factor is reflected by major differences in oceanic circulation and weather patterns between the two hemispheres.
