Ocean

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For other uses, see Ocean (disambiguation).
Surface of the Atlantic ocean meeting the Earth's planetary boundary layer and troposphere

An ocean (from Greek Ὠκεανὸς, "okeanos" Oceanus)[1] is a body of saline water that composes a large part of a planet's hydrosphere.[2] In the context of Earth, it also refers to major divisions of the planet's World Ocean, such as the Atlantic Ocean.[3][4] The word "sea" is often used interchangeably with "ocean", but a sea is a body of saline water in a more inland location rather than a location in which it encompasses the land around it.[5]

Earth's global ocean is the largest confirmed surface ocean on all observable planets. Approximately 71% of the planet's surface (~3.6x108 km2) is covered by saline water that is customarily divided into several principal oceans and smaller seas. Because it is the principal component of Earth's hydrosphere, the world ocean is integral to all known life, forms part of the carbon cycle, and influences climate and weather patterns. The total volume is approximately 1.3 billion cubic kilometres (310 million cu mi)[6] with an average depth of 3,790 metres (12,430 ft). It is the habitat of 230,000 species known to science, however much of the ocean's depths remain unexplored and it is estimated that over two million marine species may exist.[7] The origin of Earth's oceans is still unknown though they are believed to have first appeared in the Hadean period and may have been the point of origin for the emergence of life.

Extraterrestrial oceans may be composed of a wide range of elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan though there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories, Mars and Venus are theorised to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was covered by water though water on Mars is no longer oceanic. A runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia mixed water are known to lower the freezing point potentially allowing water to exist in large quantities in extraterrestrial environments as brine or convecting ice. The Solar System's gas giant planets are also believed to possess liquid atmospheric layers of yet to be confirmed compositions. Unconfirmed oceans are speculated beneath the surface of many dwarf planets and natural satellites, notably the ocean of Europa believed to have over twice the water volume of Earth. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.

Contents

Earth's surface ocean

Rotating series of maps showing alternate divisions of the oceans
Maps exhibiting the world's oceanic waters. A continuous body of water encircling the Earth, the world (global) ocean is divided into a number of principal areas, which depend on temperature, size, and contents. Five oceanic divisions are usually recognized: Pacific, Atlantic, Indian, Arctic, and Southern; the last two listed are sometimes consolidated into the first three.
Oceanus.png
Earth's oceans
(World Ocean)
Main article: World Ocean
See also: Borders of the oceans

Though generally described as several 'separate' oceans, these waters comprise one global, interconnected body of salt water sometimes referred to as the World Ocean or global ocean.[8][9] This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[10]

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. These divisions are (in descending order of size):

The Pacific and Atlantic may be further subdivided by the equator into northern and southern portions. Smaller regions of the oceans are called seas, gulfs, bays, straits and other names.

Geology

Geologically, Earth's ocean is the area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the mantle. The ocean floor spreads from mid-ocean ridges where two plates adjoin. Where two plates move towards each other, one plate subducts under another plate (oceanic or continental) leading to an oceanic trench.

Continental crust is thicker but less dense. From this perspective, the earth has three oceans: the World Ocean, the Caspian Sea[citation needed], and Black Sea. The latter two were formed by the collision of Cimmeria with Laurasia.

The Mediterranean Sea is at times a discrete ocean, because tectonic plate movement has repeatedly broken its connection to the World Ocean through the Strait of Gibraltar. The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is a natural canal cut through continental rock some 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.

Despite their names, smaller landlocked bodies of saltwater that are not connected with the World Ocean, such as the Aral Sea, are actually salt lakes.

Origin

Main article: Origin of water on Earth

There are several acknowledged theories as to how the world's oceans were formed over the past 4.4 billion years.[12]

Some of the most likely contributory factors to the origin of the oceans are as follows:

The ocean and life

The ocean has a significant effect on the biosphere. Oceanic evaporation, as a phase of the water cycle, is the source of most rainfall, and ocean temperatures determine climate and wind patterns that affect life on land. Life within the ocean evolved 3 billion years prior to life on land. Both the depth and distance from shore strongly influence the amount and kinds of plants and animals that live there.[13]

Physical properties

Further information: Sea water

The area of the World Ocean is 361 million square kilometres (139 million square miles)[14] Its volume is approximately 1.3 billion cubic kilometres (310 million cu mi).[6] This can be thought of as a cube of water with an edge length of 1,111 kilometres (690 mi). Its average depth is 3,790 metres (12,430 ft), and its maximum depth is 10,923 metres (6.787 mi)[14] Nearly half of the world's marine waters are over 3,000 metres (9,800 ft) deep.[9] The vast expanses of deep ocean (anything below 200 metres (660 ft)) cover about 66% of the Earth's surface.[15] This does not include seas not connected to the World Ocean, such as the Caspian Sea.

The total mass of the hydrosphere is about 1,400,000,000,000,000,000 metric tons (1.5×1018 short tons) or 1.4×1021 kg, which is about 0.023 percent of the Earth's total mass. Less than 3 percent is freshwater; the rest is saltwater, mostly in the ocean.

Color

Main article: Color of water

The bluish color of the ocean is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.[16]

Glow

Sailors and other mariners have reported that the ocean often emits a visible glow, or luminescence, which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow.[17] It may be caused by bioluminescence.[18][19][20]

Exploration

Main article: Ocean exploration
False color photo
Map of large underwater features. (1995, NOAA)

Ocean travel by boat dates back to prehistoric times, but only in modern times has extensive underwater travel become possible.

The deepest point in the ocean is the Mariana Trench, located in the Pacific Ocean near the Northern Mariana Islands. Its maximum depth has been estimated to be 10,971 metres (35,994 ft) (plus or minus 11 meters; see the Mariana Trench article for discussion of the various estimates of the maximum depth.) The British naval vessel, Challenger II surveyed the trench in 1951 and named the deepest part of the trench, the "Challenger Deep". In 1960, the Trieste successfully reached the bottom of the trench, manned by a crew of two men.

Much of the ocean bottom remains unexplored and unmapped. A global image of many underwater features larger than 10 kilometres (6.2 mi) was created in 1995 based on gravitational distortions of the nearby sea surface.[citation needed]

Regions and depths

Drawing showing divisions according to depth and distance from shore
The major oceanic divisions

Oceanographers divide the ocean into regions depending on physical and biological conditions 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 zone covers 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) 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. 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 and 4 °C (50 and 39 °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, 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. This lies between 6,000–11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.

Major sea divisions, including sea life that lives in that division

Along with pelagic aphotic zones there are also benthic aphotic zones. These correspond to the three deepest zones of the deep-sea. The bathyal zone covers the continental slope down to about 4,000 metres (13,000 ft). The abyssal zone covers the abyssal plains 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 the oceanic zone. The neritic encompasses the water mass directly above the continental 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.

Climate effects

World map with colored, directed lines showing how water moves through the oceans. Cold deep water rises and warms in the central Pacific and in the Indian, while warm water sinks and cools near Greenland in the North Atlantic and near Antarctica in the South Atlantic.
A summary of the path of the thermohaline circulation/ Great Ocean Conveyor. Blue paths represent deep-water currents, while red paths represent surface currents

Ocean currents greatly affect the Earth's climate by transferring heat from the tropics to the polar regions, and transferring warm or cold air and precipitation to coastal regions, where winds may carry them inland. Surface heat and freshwater fluxes create global density gradients that drive the thermohaline circulation part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation. Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations.

For a discussion of the possibilities of changes to the thermohaline circulation under global warming, see shutdown of thermohaline circulation.

It is often stated that the thermohaline circulation is the primary reason that the climate of Western Europe is so temperate. An alternate hypothesis claims that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin, and because atmospheric waves bring warm air north from the subtropics.[21][22]

The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.

One of the most dramatic forms of weather occurs over the oceans: tropical cyclones (also called "typhoons" and "hurricanes" depending upon where the system forms).

Biology

Main article: Marine biology

Lifeforms native to oceans include:

Economy

The oceans are essential to transportation: most of the world's goods move by ship between the world's seaports.

Oceans are also the major supply source for the fishing industry. Some of the more major ones are shrimp, fish, crabs and lobster.

Ancient oceans

Diagram showing three stages of oceanic evolution, including rift valley, new ocean basin, and mature ocean with sediment and evolving ridge
Genesis of an ocean

Continental drift continually reconfigures the oceans, joining and splitting bodies of water.[citation needed] Ancient oceans include:

Historical context

The original concept of "ocean" goes back to notions of Mesopotamian and Indo-European mythology, imagining the world to be encircled by a great river. Okeanos in Greek, reflects the ancient Greek observation that a strong current flowed off Gibraltar and their subsequent assumption that it was a great river. (Compare also Samudra from Hindu mythology and Jörmungandr from Norse mythology.) The world was imagined to be enclosed by a celestial ocean above the heavens, and an ocean of the underworld below.

Artworks which depict maritime themes are known as marine art, a term which particularly applies to common styles of European painting of the 17th to 19th centuries.

Extraterrestrial oceans

See also Extraterrestrial liquid water
Two models for the composition of Europa predict a large sub-surface ocean of liquid water. Similar models have been proposed for other celestial bodies in the Solar System

While Earth is the only known planet with large stable bodies liquid water on its surface and the only one in our the Solar System, other celestial bodies are believed to possess large oceans.

Planets

The gas giants, Jupiter and Saturn, are thought to lack surfaces and instead have a stratum of liquid hydrogen, however their planetary geology is not well understood. Likewise the ice giants of Uranus and Neptune may also possess vast oceans of liquid water under their thick atmospheres, though their internal structure has not been confirmed.

There is currently much debate over whether Mars once had an ocean in its northern hemisphere, and over what happened to it; recent findings by the Mars Exploration Rover mission indicate Mars had long-term standing water in at least one location, but its extent is not known.

Astronomers believe that Venus had liquid water and perhaps oceans in its very early history. If they existed, all later vanished via resurfacing.

Natural satellites

A global layer of liquid water thick enough to decouple the crust from the mantle is believed to be present on Titan, Europa and, with less certainty, Callisto, Ganymede[23] and Triton.[24][25] A magma ocean is thought to be present on Io. Geysers have been found on Saturn's moon Enceladus, though their origins are not well understood. Other icy moons may also have internal oceans, or have once had internal oceans that have now frozen.[23]

Large bodies of Liquid hydrocarbons are thought to be present on the surface of Titan, though they are not large enough to be described as oceans and are sometimes referred to as lakes or seas. The Cassini–Huygens space mission initially discovered only what appeared to be dry lakebeds and empty river channels, suggesting that Titan had lost what surface liquids it might have had. Cassini's more recent fly-by of Titan offers radar images that strongly suggest hydrocarbon lakes near the colder polar regions. Titan is thought to have a subterranean water ocean under the ice and hydrocarbon mix that forms its outer crust.

Dwarf planets and trans-Neptunian objects

Diagram showing a possible internal structure of Ceres

Ceres appears to be differentiated into a rocky core and icy mantle and may harbour a liquid water ocean under its surface.[26][27]

Not enough is known of the larger Trans-Neptunian objects to determine whether they are differentiated bodies capable of possessing oceans although models of radioactive decay suggest that Pluto,[28] Eris, Sedna, and Orcus have oceans beneath solid icy crusts at the core-boundary approximately 100 to 180 km thick.[23]

Extrasolar

Rendering of a hypothetical large extrasolar moon with surface liquid water oceans

Some planets and natural satellites beyond the Solar System are likely to possess oceans, including possible water ocean planets similar to Earth in the habitable zone or "Liquid Water belt". The detection of oceans, even through the spectroscopy method, however is likely to prove extremely difficult and inconclusive.

Theoretical models have been used to predict with high probability that GJ 1214 b, detected by transit, is composed of exotic form of ice VII, making up 75% of its mass.[29]

Other possible candidates are merely speculated based on their mass and position in the habitable zone include planet though little is actually known of their composition. Some scientists have speculated Kepler-22b to be an "ocean-like" planet.[30] Models have been proposed for Gliese 581 d that could include surface oceans. Gliese 436 b is speculated to have an ocean of "hot ice".[31] Extrasolar moons orbiting planets, particularly gas giants within their parent star's habitable zone may theoretically possess surface oceans.

See also

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References

 This article uses bare URLs for citations. Please consider adding full citations so that the article remains verifiable. Several templates and the Reflinks tool are available to assist in formatting. (Reflinks documentation) (February 2012)
  1. ^ Ὠκεανός
  2. ^ "WordNet Search — ocean". Princeton University. http://wordnetweb.princeton.edu/perl/webwn?s=ocean. Retrieved February 21, 2012. 
  3. ^ "ocean, n.". Oxford English Dictionary. http://www.oed.com/view/Entry/130201?redirectedFrom=ocean#eid. Retrieved February 5, 2012. 
  4. ^ "ocean". Merriam-Webster. http://www.merriam-webster.com/dictionary/ocean. Retrieved February 6, 2012. 
  5. ^ "WordNet Search — sea". Princeton University. http://wordnetweb.princeton.edu/perl/webwn?s=sea. Retrieved February 21, 2012. 
  6. ^ a b Qadri, Syed (2003). "Volume of Earth's Oceans". The Physics Factbook. http://hypertextbook.com/facts/2001/SyedQadri.shtml. Retrieved 2007-06-07. 
  7. ^ Drogin, Bob (October 24, 2003). "Census of Marine Life maps an ocean of species". Los Angeles Times. http://www.latimes.com/new/la-na-fish2-2009aug02,0,5785256.story?page=1&track=ntothtml. Retrieved August 18, 2009. [dead link]
  8. ^ "Ocean". The Columbia Encyclopedia. 2002. New York: Columbia University Press
  9. ^ a b "Distribution of land and water on the planet". UN Atlas of the Oceans
  10. ^ Spilhaus, Athelstan F. (July 1942). Maps of the whole world ocean. 32 (3). American Geographical Society. pp. 431–5. 
  11. ^ "Limits of Oceans and Seas, 3rd edition". International Hydrographic Organization. 1953. http://www.iho-ohi.net/iho_pubs/standard/S-23/S23_1953.pdf. Retrieved 7 February 2010. 
  12. ^ "Wilde, S. A.; Valley, J.W.; Peck, W.H. and Graham, C.M. (2001) "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago" ''Nature'' 409: pp. 175-178" (PDF). http://www.geology.wisc.edu/zircon/Wilde_et_al.PDF. Retrieved 2012-02-19. 
  13. ^ Biology: Concepts & Connections. Chapter 34: The Biosphere: An Introduction to Earth's Diverse Environment. (sec 34.7)
  14. ^ a b "The World's Oceans and Seas". The World's Oceans and Seas. Encarta. http://encarta.msn.com/media_461547746/The_World's_Oceans_and_Seas.html. 
  15. ^ Drazen, Jeffrey C.. "Deep-Sea Fishes". School of Ocean Earth Science and Technology, University of University of Hawaiʻi at M?noa. http://www.soest.hawaii.edu/oceanography/faculty/drazen/fishes.htm. Retrieved 2007-06-07. 
  16. ^ Paula G. Coble "Marine Optical Biogeochemistry:  The Chemistry of Ocean Color" Chemical Reviews, 2007, volume 107, pp 402–418. doi: 10.1021/cr050350
  17. ^ Britt, Robert Roy (October 4, 2005). "Mystery Ocean Glow Confirmed in Satellite Photos". http://www.livescience.com/strangenews/051004_sea_glow.html. 
  18. ^ 21 November 2005, Usa today: A glowing sea, courtesy of algae Quote: "...The water glowed green in the direction of the movement...A: Little microscopic creatures (called Lingulodinium polyedrum) that glow in the dark caused the alluring strange display that night..."
  19. ^ 05 October 2005, New Scientist: Sea's eerie glow seen from space Quote: "...The ancient mariners were right. Tales of "milky seas" that glow bluish-white at night and extend as far as the horizon have been spun by sailors for centuries. Now this eerie glow has been spotted from space....The glowing area spanned 15,400 square kilometres (5,900 sq mi), an area the size of Connecticut (Image: Steven D Miller, US Naval Research Laboratory)..."
  20. ^ NASA, DAAC Study: The Incredible Glowing Algae "...Each year, the North Atlantic Ocean announces springtime by producing “blooms” large enough to be seen from space. These explosive increases in microscopic marine algae, called phytoplankton, appear as sudden bright blossoms in satellite imagery..."
  21. ^ Seager, R. (2006). "The Source of Europe's Mild Climate". American Scientist. http://www.americanscientist.org/issues/feature/2006/4/the-source-of-europes-mild-climate. 
  22. ^ Rhines and Hakkinen (2003). "Is the Oceanic Heat Transport in the North Atlantic Irrelevant to the Climate in Europe?". ASOF Newsletter. http://www.realclimate.org/Rhines_hakkinen_2003.pdf. 
  23. ^ a b c Hussmann, H.; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus 185 (1): 258–273. Bibcode 2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.  edit
  24. ^ McKinnon, William B.; Kirk, Randolph L. (2007). "Encyclopedia of the Solar System". In Lucy Ann Adams McFadden, Lucy-Ann Adams, Paul Robert Weissman, Torrence V. Johnson. Encyclopedia of the Solar System (2nd ed.). Amsterdam; Boston: Academic Press. pp. 483–502. ISBN 978-0-12-088589-3. 
  25. ^ Javier Ruiz (December 2003). "Heat flow and depth to a possible internal ocean on Triton". Icarus 166 (2): 436–439. Bibcode 2003Icar..166..436R. doi:10.1016/j.icarus.2003.09.009. 
  26. ^ McCord, Thomas B. (2005). "Ceres: Evolution and current state". Journal of Geophysical Research 110 (E5): E05009. Bibcode 2005JGRE..11005009M. doi:10.1029/2004JE002244. 
  27. ^ Castillo-Rogez, J. C.; McCord, T. B.; and Davis, A. G. (2007). "Ceres: evolution and present state" (PDF). Lunar and Planetary Science XXXVIII: 2006–2007. http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2006.pdf. Retrieved 2009-06-25. 
  28. ^ "The Inside Story". pluto.jhuapl.edu — NASA New Horizons mission site. Johns Hopkins University Applied Physics Laboratory. 2007. http://www.pluto.jhuapl.edu/science/everything_pluto/9_insideStory.html. Retrieved 2007-03-29. 
  29. ^ David A. Aguilar (2009-12-16). "Astronomers Find Super-Earth Using Amateur, Off-the-Shelf Technology". Harvard-Smithsonian Center for Astrophysics. http://www.cfa.harvard.edu/news/2009/pr200924.html. Retrieved January 23, 2010. 
  30. ^ Abel Mendez Torres (2011-12-08). "Updates on Exoplanets during the First Kepler Science Conference". Planetary Habitability Laboratory at UPR Arecibo. http://phl.upr.edu/library/notes/updatesonexoplanetsduringthefirstkeplerscienceconference. 
  31. ^ Reuters. http://in.today.reuters.com/news/newsArticle.aspx?type=worldNews&storyID=2007-05-17T114149Z_01_NOOTR_RTRJONC_0_India-298599-1.xml&archived=False. 

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