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Ocean

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World map of the five-ocean model with approximate boundaries


The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth and contains 97% of Earth's water. Another definition is "any of the large bodies of water into which the great ocean is divided".[1] Separate names are used to identify five different areas of the ocean: Pacific (the largest) Atlantic, Indian, Southern (Antarctic), and Arctic (the smallest).[2][3] Seawater covers approximately 361,000,000 km2 (139,000,000 sq mi) of the planet. The ocean is the principal component of Earth's hydrosphere, and therefore integral to life on Earth. Acting as a huge heat reservoir, the ocean influences climate and weather patterns, the carbon cycle, and the water cycle.

Oceanographers divide the ocean into different vertical and horizontal zones based on physical and biological conditions. The pelagic zone consists of the water column from surface to ocean floor throughout the open ocean. The water column is further categorized in other zones depending on depth and on how much light is present. The photic zone includes water from the surface to a depth of 200 m, where photosynthesis can occur. This makes the photic zone the most biodiverse. Photosynthesis by plants and microscopic algae (free floating phytoplankton) creates organic matter from chemical precursors like water and carbon dioxide. This upper sunlit zone is the origin of the food supply which sustains most of the ocean ecosystem. Light only penetrates to a depth of a few hundred meters; the remaining ocean below is cold and dark. The continental shelf where the ocean approaches dry land is more shallow, with a depth of a few hundred meters or less. Human activity has a greater impact on the continental shelf.  

Ocean temperatures depend on the amount of solar radiation reaching the ocean surface. In the tropics, surface temperatures can rise to over 30 °C (86 °F). Near the poles where sea ice forms, the temperature in equilibrium is about −2 °C (28 °F). Deep seawater temperature is between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the ocean.[4] Water continuously circulates in the oceans creating ocean currents. These directed movements of seawater are generated by forces acting upon the water, including temperature differences, atmospheric circulation (wind), the Coriolis effect and differences in salinity.[5] Tidal currents originate from tides, while surface currents are caused by wind and waves. Major ocean currents include the Gulf Stream, Kuroshio current, Agulhas current and Antarctic Circumpolar Current. Collectively, currents move enormous amounts of water and heat around the globe. This circulation significantly impacts global climate and the uptake and redistribution of pollutants such as carbon dioxide by moving these contaminants from the surface into the deep ocean.

Ocean water contains large quantities of dissolved gases, including oxygen, carbon dioxide and nitrogen. This gas exchange takes place at the ocean surface and solubility depends on the temperature and salinity of the water.[6] The increasing concentration of carbon dioxide in the atmosphere due to fossil fuel combustion leads to higher concentrations in ocean water, resulting in ocean acidification.[7] The ocean provides society with important environmental services, including climate regulation. It also offers a means of trade and transport and access to food and other resources. Known to be the habitat of 230,000 species, it may contain far more--perhaps over two million species.[8] However, the ocean is subject to numerous environmental threats, including marine pollution, overfishing, ocean acidification and other effects of climate change. The continental shelf and coastal waters that are most influenced by human activity are especially vulnerable.

Terminology

The Atlantic, one component of the system, makes up 23% of the "global ocean".
Surface view of the Atlantic Ocean

Ocean and sea

The terms "the ocean" or "the sea" used without specification refer to the interconnected body of salt water covering the majority of the Earth's surface.[2][3] It includes the Atlantic, Pacific, Indian, Southern and Arctic Oceans.[9] As a general term, "the ocean" is mostly interchangeable with "the sea" in American English, but not in British English.[10] Strictly speaking, a "sea" is a body of water (generally a division of the world ocean) partly or fully enclosed by land.[11] The word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas) or wholly (as inland seas) bordered by land.[12]

World ocean

The contemporary concept of the World Ocean was coined in the early 20th century by the Russian oceanographer Yuly Shokalsky to refer to the continuous ocean that covers and encircles most of Earth.[13] The global, interconnected body of salt water is sometimes referred to as the world ocean or global ocean.[14][15] The concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.[16]

Etymology

The word ocean comes from the figure in classical antiquity, Oceanus (/ˈsənəs/; Greek: Ὠκεανός Ōkeanós,[17] pronounced [ɔːkeanós]), the elder of the Titans in classical Greek mythology. Oceanus was believed by the ancient Greeks and Romans to be the divine personification of an enormous river encircling the world.

The concept of Ōkeanós has an Indo-European connection. Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases.[18]

Geography

Oceanic divisions

The major oceanic divisions – listed below in descending order of area and volume – are so named based on nearest continents, various archipelagos, and other criteria.[19][20][21] Oceans are fringed with coastlines that run for 360,000 kilometres in total distance.[22][23] They are also connected to smaller, adjoining bodies of water such as, seas, gulfs, bays, bights, and straits. Seawater covers approximately 361,000,000 km2 (139,000,000 sq mi) and is customarily divided into five principal oceans, as below:

Oceans by size
# Ocean Location Area
(km2)
(%)
Volume
(km3)
(%)
Avg. depth
(m)
Coastline
(km)
1 Pacific Ocean Extends between Asia and Australasia and the Americas[24] 168,723,000
(46.6)
669,880,000
(50.1)
3,970 135,663
2 Atlantic Ocean Extends between the Americas and Europe and Africa[25] 85,133,000
23.5
310,410,900
23.3
3,646 111,866
3 Indian Ocean Extends between southern Asia, Africa and Australia[26] 70,560,000
19.5
264,000,000
19.8
3,741 66,526
4 Southern Ocean Encircles Antarctica. Sometimes considered an extension of the Pacific, Atlantic and Indian Oceans,[27][28] 21,960,000
6.1
71,800,000
5.4
3,270 17,968
5 Arctic Ocean Borders northern North America and Eurasia and covers much of the Arctic. Sometimes considered a sea or estuary of the Atlantic.[29][30][31] 15,558,000
4.3
18,750,000
1.4
1,205 45,389
Total 361,900,000
100
1.335×10^9
100
3,688
 
377,412
[32]
NB: Volume, area, and average depth figures include NOAA ETOPO1 figures for marginal South China Sea.
Sources: Encyclopedia of Earth,[24][25][26][27][31] International Hydrographic Organization,[28] Regional Oceanography: an Introduction (Tomczak, 2005),[29] Encyclopædia Britannica,[30] and the International Telecommunication Union.[32]

Ocean ridges and ocean basins

World distribution of mid-oceanic ridges; USGS

Every ocean basin has a mid-ocean ridge, which creates a long mountain range beneath the ocean. Together they form the global mid-oceanic ridge system that features the longest mountain range in the world. The longest continuous mountain range is 65,000 km (40,000 mi). This underwater mountain range is several times longer than the longest continental mountain range--the Andes.[33]

Oceanographers state that less than 20% of the oceans have been mapped.[34]

Formation

The origin of Earth's oceans is unknown. Oceans are thought to have formed in the Hadean eon and may have been the cause for the emergence of life. Scientists believe that a sizable quantity of water would have been in the material that formed the Earth.[35] Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. This is called atmospheric escape.

Plate tectonics, post-glacial rebound, and sea level rise continually change the coastline and structure of the world ocean. A global ocean has existed in one form or another on Earth for eons.

Physical properties

Volumes

The volume of water in all the oceans together is approximately 1.335 billion cubic kilometers (320.3 million cubic miles).[19][36][37]

It has been estimated that there are 1.36 billion cubic kilometers (332 million cubic miles) of water on Earth.[38] This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams. Saltwater accounts for 97.5% of this amount, whereas fresh water accounts for only 2.5%. Of this fresh water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic and mountain glaciers; 30.8% is in the form of fresh groundwater; and only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems.[38]

The total mass of Earth's hydrosphere is about 1.4 × 1018 tonnes, which is about 0.023% of Earth's total mass. At any given time, about 20 × 1012 tonnes of this is in the form of water vapor in the Earth's atmosphere (for practical purposes, 1 cubic meter of water weighs one tonne). Approximately 71% of Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%).[39]

Depth

False color photo
Map of large underwater features (1995, NOAA)

The average depth of the oceans is about 4 km. More precisely the average depth is 3,688 meters (12,100 ft).[19] Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.[15] "Deep ocean," which is anything below 200 meters (660 ft.), covers about 66% of Earth's surface.[40] This figure does not include seas not connected to the World Ocean, such as the Caspian Sea.

The deepest point in the ocean is the Mariana Trench, located in the Pacific Ocean near the Northern Mariana Islands.[41] Its maximum depth has been estimated to be 10,971 meters (35,994 ft). 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.

Color

Season-long composites of ocean chlorophyll concentrations. The purple and blue colors represent lower chlorophyll concentrations. The oranges and reds represent higher chlorophyll concentrations. These differences indicate areas with lesser or greater phytoplankton biomass.

Most of the ocean is blue in color, but in some places the ocean is blue-green, green, or even yellow to brown.[42] Blue ocean color is a result of several factors. First, water preferentially absorbs red light, which means that blue light remains and is reflected back out of the water. Red light is most easily absorbed and thus does not reach great depths, usually to less than 50 meters (164 ft.). Blue light, in comparison, can penetrate up to 200 meters (656 ft.).[43] Second, water molecules and very tiny particles in ocean water preferentially scatter blue light more than light of other colors. Blue light scattering by water and tiny particles happens even in the very clearest ocean water,[44] and is similar to blue light scattering in the sky.

The main substances that affect the color of the ocean include dissolved organic matter, living phytoplankton with chlorophyll pigments, and non-living particles like marine snow and mineral sediments.[45] Chlorophyll can be measured by satellite observations and serves as a proxy for ocean productivity (marine primary productivity) in surface waters. In long term composite satellite images, regions with high ocean productivity show up in yellow and green colors because they contain more (green) phytoplankton, whereas areas of low productivity show up in blue.

Oceanic zones

Drawing showing divisions according to depth and distance from shore
The major oceanic zones, based on depth and biophysical conditions

Oceanographers divide the ocean into different vertical and horizontal zones defined by physical and biological conditions. The pelagic zone consists of the water column of the open ocean, and can be divided into further regions categorized by light abundance and by depth.

Grouped by light penetration

  • The photic zone includes the oceans from the surface to a depth of 200 m; it is the region where photosynthesis can occur and is, therefore, the most biodiverse. Photosynthesis by plants and microscopic algae (free floating phytoplankton) allows the creation of organic matter from chemical precursors including water and carbon dioxide. This organic matter can then be consumed by other creatures. Much of the organic matter created in the photic zone is consumed there but some sinks into deeper waters.
  • Below the photic zone is the mesopelagic or twilight zone where there is a very small amount of light. Below that is the aphotic deep ocean to which no surface sunlight at all penetrates. Life that exists deeper than the photic zone must either rely on material sinking from above (see marine snow) or find another energy source. Hydrothermal vents are a source of energy in what is known as the aphotic zone (depths exceeding 200 m). The pelagic part of the photic zone is known as the epipelagic.[46]

Grouped according to depth and temperature

The pelagic part of the aphotic zone can be further divided into vertical regions according to depth and temperature:[46]

  • The mesopelagic is the uppermost region. Its lowermost boundary is at a thermocline of 12 °C (54 °F) which generally lies at 700–1,000 meters (2,300–3,300 ft) in the tropics. Next is the bathypelagic lying between 10 and 4 °C (50 and 39 °F), typically between 700–1,000 meters (2,300–3,300 ft) and 2,000–4,000 meters (6,600–13,100 ft). Lying along the top of the abyssal plain is the abyssopelagic, whose lower boundary lies at about 6,000 meters (20,000 ft). The last and deepest zone is the hadalpelagic which includes the oceanic trench and lies between 6,000–11,000 meters (20,000–36,000 ft).
  • The benthic zones are aphotic and correspond to the three deepest zones of the deep-sea. The bathyal zone covers the continental slope down to about 4,000 meters (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 oceanic trenches.

If a zone undergoes dramatic changes in temperature with depth, it contains a thermocline. The tropical thermocline is typically deeper than the thermocline at higher latitudes. Polar waters, which receive relatively little solar energy, are not stratified by temperature and generally lack a thermocline because surface water at polar latitudes are nearly as cold as water at greater depths. Below the thermocline, water everywhere in the ocean is very cold, ranging from −1°C to 3°C. Because this deep and cold layer contains the bulk of ocean water, the average temperature of the world ocean is 3.9°C.[47] If a zone undergoes dramatic changes in salinity with depth, it contains a halocline. If a zone undergoes a strong, vertical chemistry gradient with depth, it contains a chemocline. Temperature and salinity control the density of ocean water, with colder and saltier water being more dense, and this density in turn regulates the global water circulation within the ocean.[46]

The halocline often coincides with the thermocline, and the combination produces a pronounced pycnocline.

Grouped according to distance from land

The pelagic zone can be further subdivided into two sub regions based on distance from land: the neritic zone and the oceanic zone. The neritic zone encompasses the water mass directly above the continental shelves and hence includes coastal waters, whereas the oceanic zone includes all the completely open water.

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.[46]

Temperature

Ocean temperatures depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with the sea ice is about −2 °C (28 °F). There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep seawater has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.[4]

Seawater with a typical salinity of 35‰ has a freezing point of about −1.8°C (28.8°F).[46] When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions this freezes into a thin flat sheet known as nilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15‰, but by the time the sea ice is one year old, this falls to 4–6‰.[48]

Ocean warming accounts for 90% of the energy accumulation from climate change between 1971 and 2010.[49] About one third of that extra heat has been estimated to propagate to depth below 700 meters.[50]

Ocean currents and global climate

Ocean surface currents
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, whereas warm water sinks and cools near Greenland in the North Atlantic and near Antarctica in the South Atlantic.
A map of the global thermohaline circulation; blue represents deep-water currents, whereas red represents surface currents.

Types of ocean currents

An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, temperature and salinity differences.[5] Ocean currents are primarily horizontal water movements. They have different origins, such as tides for tidal currents, or wind and waves for surface currents.

Tidal currents are in phase with the tide, hence are quasiperiodic; associated with the influence of the moon and sun pull on the ocean water. Tidal currents may form various complex patterns in certain places, most notably around headlands.[51] Non-periodic or non-tidal currents are created by the action of winds and changes in density of water. In littoral zones, breaking waves are so intense and the depth measurement so low, that maritime currents reach often 1 to 2 knots.[52]

The wind and waves create surface currents (designated as "drift currents"). These currents can decompose in one quasi-permanent current (which varies within the hourly scale) and one movement of Stokes drift under the effect of rapid waves movement (which vary on timescales of a couple of seconds). The quasi-permanent current is accelerated by the breaking of waves, and in a lesser governing effect, by the friction of the wind on the surface.[52]

This acceleration of the current takes place in the direction of waves and dominant wind. Accordingly, when the ocean depth increases, the rotation of the earth changes the direction of currents in proportion with the increase of depth, while friction lowers their speed. At a certain ocean depth, the current changes direction and is seen inverted in the opposite direction with current speed becoming null: known as the Ekman spiral. The influence of these currents is mainly experienced at the mixed layer of the ocean surface, often from 400 to 800 meters of maximum depth. These currents can considerably change and are dependent on the yearly seasons. If the mixed layer is less thick (10 to 20 meters), the quasi-permanent current at the surface can adopt quite a different direction in relation to the direction of the wind. In this case, the water column becomes virtually homogeneous above the thermocline.[52]

The wind blowing on the ocean surface will set the water in motion. The global pattern of winds (also called atmospheric circulation) creates a global pattern of ocean currents. These are not only driven by the wind but also by the effect of the circulation of the earth (coriolis force). Theses major ocean currents include the Gulf Stream, Kuroshio current, Agulhas current and Antarctic Circumpolar Current. The Antarctic Circumpolar Current encircles Antarctica and influences the area's climate as well as connecting currents in several oceans.[52]

Relationship of currents and climate

Collectively, currents move enormous amounts of water and heat around the globe influencing climate. These wind driven currents are largely confined to the top hundreds of meters of the ocean. At greater depth the drivers of water motion are the thermoahline circulation. This is driven by the cooling of surface waters at northern and southern polar latitudes creating dense water which sinks to the bottom of the ocean. This cold and dense water moves slowly away from the poles which is why the waters in the deepest layers of the world ocean are so cold. This deep ocean water circulation is relatively slow and water at the bottom of the ocean can be isolated from the ocean surface and atmosphere for hundreds or even a few thousand years.[52] This circulation has important impacts on global climate and the uptake and redistribution of pollutants such as carbon dioxide by moving these contaminants from the surface into the deep ocean.      

Ocean currents greatly affect Earth's climate by transferring heat from the tropics to the polar regions and thereby also affecting air temperature and precipitation in coastal regions and further 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 Earth's energy budget. Since the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations. However, climate change might result in the shutdown of thermohaline circulation in the future. This would in turn trigger cooling in the North Atlantic, Europe, and North America.[53]

Waves and swell

Movement of molecules as waves pass

The motions of the ocean surface, known as undulations or wind waves, are the partial and alternate rising and falling of the ocean surface. The series of mechanical waves that propagate along the interface between water and air is called swell - a term used in sailing, surfing and navigation.[54] These motions profoundly affect ships on the surface of the ocean and the well-being of people on those ships who might suffer from sea sickness.

Wind blowing over the surface of a body of water forms waves that are perpendicular to the direction of the wind. The friction between air and water caused by a gentle breeze on a pond causes ripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in the Roaring Forties, long, organized masses of water called swell roll across the ocean.[55]: 83–84 [19][56] If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. The size of the waves depends on the fetch, the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between the two can produce broken, irregular seas.[19]

Constructive interference can cause individual (unexpected) rogue waves much higher than normal.[57] Most waves are less than 3 m (10 ft) high[57] and it is not unusual for strong storms to double or triple that height.[58] Rogue waves, however, have been documented at heights above 25 meters (82 ft).[59][60]

The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave is pushed across the surface of the ocean by the wind, but this represents a transfer of energy and not a horizontal movement of water. As waves approach land and move into shallow water, they change their behavior. If approaching at an angle, waves may bend (refraction) or wrap around rocks and headlands (diffraction). When the wave reaches a point where its deepest oscillations of the water contact the ocean floor, they begin to slow down. This pulls the crests closer together and increases the waves' height, which is called wave shoaling. When the ratio of the wave's height to the water depth increases above a certain limit, it "breaks", toppling over in a mass of foaming water.[57] This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity.[61]

Earthquakes, volcanic eruptions or other major geological disturbances can set off waves that can lead to tsunamis in coastal areas which can be very dangerous.[62][63]

Tides

Tides are the regular rise and fall in water level experienced by oceans in response to the gravitational influences of the moon and the sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level. As the water recedes, it uncovers more and more of the foreshore, also known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude.[64][65]

In the open ocean tidal ranges are less than 1 m but in coastal areas these tidal ranges increase to more than 10 m in some areas.[66]

Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minute period that it takes for the Earth to make a complete revolution and return the moon to its previous position relative to an observer. Tidal force or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an effect on tides as the Sun.[67] When the sun, moon and Earth are all aligned (full moon and new moon), the combined effect results in the high "spring tides".[64] A storm surge can occur when high winds pile water up against the coast in a shallow area and this, coupled with a low pressure system, can raise the surface of the ocean at high tide dramatically.

Water cycle, weather and rainfall

Ocean water represents the largest body of water within the global water cycle (oceans contain 97% of Earth's water), with evaporation from the ocean moving water into the atmosphere to later rain back down onto land and the ocean.[68] Oceans have a significant effect on the biosphere. The ocean as a whole is thought to cover approximately 90% of the Earth's biosphere.[34] Oceanic evaporation, as a phase of the water cycle, is the source of most rainfall (about 90%).[68] Ocean temperatures affect climate and wind patterns that affect life on land. 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).

As the world's ocean is the principal component of Earth's hydrosphere, it is integral to life on Earth, forms part of the carbon cycle and water cycle, and - as a huge heat reservoir - influences climate and weather patterns.

Chemical composition of seawater

Salinity

Salinity is a measure of the total amounts of dissolved salts in seawater. It was originally measured via measurement of the amount of chloride in seawater and hence termed chlorinity. It is now routinely measured by measuring electrical conductivity of the water sample. Salinity can be calculated using the chlorinity, which is a measure of the total mass of halogen ions (includes fluorine, chlorine, bromine, and iodine) in seawater. By international agreement, the following formula is used to determine salinity:[69]

Salinity (in ‰) = 1.80655 × Chlorinity (in ‰)

The average ocean water chlorinity is about 19.2‰, and, thus, the average salinity is around 34.7‰.[69]

Salinity has a major influence on the density of seawater. A zone of rapid salinity increase with depth is called a halocline. The temperature of maximum density of seawater decreases as its salt content increases. Freezing temperature of water decreases with salinity, and boiling temperature of water increases with salinity. Typical seawater freezes at around −2 °C at atmospheric pressure.[70] If precipitation exceeds evaporation, as is the case in polar and temperate regions, salinity will be lower. If evaporation exceeds precipitation, as is sometimes the case in tropical regions, salinity will be higher. Thus, oceanic waters in polar regions have lower salinity content than oceanic waters in temperate and tropical regions.[69] However, the formation of sea ice at high latitudes excludes salt from the ice and thereby increases salinity in the residual seawater in some polar regions.[46]

General characteristics of ocean surface waters

The waters in different regions of the ocean have quite different temperature and salinity characteristics. This is due to differences in the local water balance (precipitation vs evaporation) and the "sea to air" temperature gradients. These characteristics can vary a lot within ocean regions but the table below provides an illustration of the sort of values usually encountered.

Generalized characteristics of ocean surface waters in different climatic regions[71][72][73][74][75]
Characteristic Polar regions Temperate regions Tropical regions
Precipitation vs. evaporation P > E P > E E > P
Sea surface temperature in winter −2 °C 5 to 20 °C 20 to 25 °C
Average salinity 28‰ to 32‰ 35‰ 35‰ to 37‰
Annual variation of air temperature ≤ 40 °C 10 °C < 5 °C
Annual variation of water temperature < 5 °C 10 °C < 5 °C

Oxygen, carbon dioxide, other gases and the carbon cycle

Ocean water contains large quantities of dissolved gases, including oxygen, carbon dioxide and nitrogen. These dissolve into ocean water via gas exchange at the ocean surface, with the solubility of these gases depending on the temperature and salinity of the water.[6] The increasing carbon dioxide concentrations in the atmosphere due to fossil fuel combustion lead to higher concentrations in the ocean waters and ocean acidification.[7] The dissolving atmospheric carbon dioxide then reacts with bicarbonate and carbonate ions in seawater.

The process of photosynthesis in the surface ocean also consumes some carbon dioxide and releases oxygen which may then return to the atmosphere. This photosynthesis in the ocean is dominated by microscopic phytoplankton, a type of free floating algae. The subsequent bacterial decomposition of organic matter formed by photosynthesis in the ocean consumes oxygen and releases carbon dioxide. The sinking and bacterial decomposition of some organic matter in deep ocean water, at depths where the waters are out of contact with the atmosphere, leads to a reduction in oxygen concentrations and increase in carbon dioxide, carbonate and bicarbonate.[76] This cycling of carbon dioxide in oceans is an important part of the global carbon cycle. The oceans represent a major sink for carbon dioxide taken up from the atmosphere by photosynthesis and by dissolution. There is also increasing attention focused on carbon dioxide uptake in coastal marine habitats such as mangroves and saltmarshes, a process sometimes referred to as “Blue carbon”. The attention on these habitats is because these are strong carbon sinks and also habitats under considerable threat from human activities and environmental degradation.

This decrease in oxygen concentration increases with the amount of sinking organic matter and the time the water is out of contact with the atmosphere.[76] Most of the deep waters of the ocean still contain relatively high concentrations of oxygen sufficient for most animals to survive. However, there are some ocean areas with water with very low oxygen due to long periods of isolation from the atmosphere, and this oxygen deficiency could be made worse by climate change.[77]

Concentrations of dissolved gases in the ocean[78][79]
Gas Concentration of seawater, by mass (in parts per million), for the whole ocean % dissolved gas, by volume, in seawater at the ocean surface
Carbon dioxide (CO2) 64 to 107 15%
Nitrogen (N2) 10 to 18 48%
Oxygen (O2) 0 to 13 36%
Solubility of oceanic gases (in mL/L) with temperature at salinity of 33‰ and atmospheric pressure[80]
Temperature O2 CO2 N2
0 °C 8.14 8,700 14.47
10 °C 6.42 8,030 11.59
20 °C 5.26 7,350 9.65
30 °C 4.41 6,600 8.26

Residence times of chemical elements and ions

The ocean waters contain all of the chemical elements as dissolved ions, but the concentration in which they occur range from some with very high concentrations of several grammes per liter, such as sodium and chloride, to others, such as iron, with tiny concentration of a few ng (10−9) g/l. The concentration of any element depends on its rate of supply to the ocean from rivers, the atmosphere and hydrothermal vents, and the rate of its removal. Hence very abundant elements in ocean water like sodium, have quite high rates of input, reflecting high abundance in rocks and relatively rapid weathering, coupled to very slow removal from the ocean because sodium ions are rather unreactive and very soluble. By contrast some other elements such as iron and aluminium are abundant in rocks but very insoluble, meaning that inputs to the ocean are low and removal is rapid. Oceanographers consider the balance of input and removal by estimating the residence time of an element as the average time the element would spend dissolved in the ocean before it is removed. This removal is usually to the sediments, but in the case of water and some gases to the atmosphere. These cycles represent part of the major global cycle of elements that has gone on since the Earth first formed. The residence times of the very abundant elements like sodium in the ocean are estimated to be millions of years, while for highly reactive and insoluble elements, residence times are only hundreds of years.[69]

Mean oceanic residence time for various chemical elements and ions[81][82]: 225–230 
Chemical element or ion Residence time (in years)
Iron (Fe) 200
Aluminum (Al) 600
Manganese (Mn) 1,300
Water (H2O) 4,100
Silicon (Si) 20,000
Carbonate (CO32−) 110,000
Calcium (Ca2+) 1,000,000
Sulfate (SO42−) 11,000,000
Potassium (K+) 12,000,000
Magnesium (Mg2+) 13,000,000
Sodium (Na+) 68,000,000
Chloride (Cl) 100,000,000

Nutrients

A few elements such as nitrogen, phosphorus and potassium are essential for life, are major components of biological material, and are commonly called “nutrients”. Nitrate and phosphate have ocean residence times of 10,000[83] and 69,000 [84]years, respectively, while potassium is a much more abundant ion in the ocean with a residence time of 12 million[85] years. The biological cycling of these elements means that this represents a continuous removal process from the ocean's water column as degrading organic material sinks to the ocean floor as sediment. Phosphate from intensive agriculture and untreated sewage is transported via runoff to rivers and coastal zones to the ocean where it is metabolized. Eventually, it sinks to the ocean floor and is no longer available to humans as a commercial resource.[86] Production of rock phosphate, an essential ingredient in inorganic fertilizer[87] is a slow geological process occurring in some of the world's ocean sediments thus making minable sedimentary apatite (phosphate) in effect a non-renewable resource (see peak phosphorus). This continuous net deposition loss of non-renewable phosphate from human activities may become a resource problem in the future for fertilizer production and food security.[88][89]

Marine life

Life within the ocean evolved 3 billion years prior to life on land. Both the depth and the distance from shore strongly influence the biodiversity of the plants and animals present in each region.[90] The diversity of life in the ocean is immense, including:

Killer whales (orca) are highly visible marine apex predators that hunt many large species. But most biological activity in the ocean takes place among microscopic marine organisms that cannot be seen individually with the naked eye, such as marine bacteria and phytoplankton.[91]

Marine life, sea life, or ocean life is the plants, animals, and other organisms that live in the salt water of the sea or ocean, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. Marine organisms, mostly microorganisms, produce oxygen and sequester carbon. Marine life in part shape and protect shorelines, and some marine organisms even help create new land (e.g. coral building reefs). Most life forms evolved initially in marine habitats. By volume, oceans provide about 90% of the living space on the planet.[92] The earliest vertebrates appeared in the form of fish,[93] which live exclusively in water. Some of these evolved into amphibians, which spend portions of their lives in water and portions on land. Other fish evolved into land mammals and subsequently returned to the ocean as seals, dolphins, or whales. Plant forms such as kelp and other algae grow in the water and are the basis for some underwater ecosystems. Plankton forms the general foundation of the ocean food chain, particularly phytoplankton which are key primary producers.

More than 200,000 marine species have been documented, and perhaps two million marine species are yet to be documented.[94] Marine species range in size from the microscopic like phytoplankton, which can be as small as 0.02 micrometres, to huge cetaceans like the blue whale – the largest known animal, reaching 33 m (108 ft) in length.[95][96] Marine microorganisms, including protists and bacteria and their associated viruses, have been variously estimated as constituting about 70% [97] or about 90% [98][91] of the total marine biomass. Marine life is studied scientifically in both marine biology and in biological oceanography. The term marine comes from the Latin mare, meaning "sea" or "ocean".
Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term marine comes from the Latin mare, meaning sea or ocean). A habitat is an ecological or environmental area inhabited by one or more living species.[99] The marine environment supports many kinds of these habitats. Marine habitats can be divided into coastal and open ocean habitats. Coastal habitats are found in the area that extends from as far as the tide comes in on the shoreline out to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only seven percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf.
Coral reefs form complex marine ecosystems with tremendous biodiversity
Marine ecosystems are the largest of Earth's aquatic ecosystems and exist in waters that have a high salt content. These systems contrast with freshwater ecosystems, which have a lower salt content. Marine waters cover more than 70% of the surface of the Earth and account for more than 97% of Earth's water supply[100][101] and 90% of habitable space on Earth.[102] Seawater has an average salinity of 35 parts per thousand of water. Actual salinity varies among different marine ecosystems.[103] Marine ecosystems can be divided into many zones depending upon water depth and shoreline features. The oceanic zone is the vast open part of the ocean where animals such as whales, sharks, and tuna live. The benthic zone consists of substrates below water where many invertebrates live. The intertidal zone is the area between high and low tides. Other near-shore (neritic) zones can include mudflats, seagrass meadows, mangroves, rocky intertidal systems, salt marshes, coral reefs, lagoons. In the deep water, hydrothermal vents may occur where chemosynthetic sulfur bacteria form the base of the food web.

Human uses of the oceans

The ocean has been linked to human activity throughout history. These activities serve a wide variety of purposes, including navigation and exploration, naval warfare, travel, shipping and trade, food production (e.g. fishing, whaling, seaweed farming, aquaculture), leisure (cruising, sailing, recreational boat fishing, scuba diving), power generation (see marine energy and offshore wind power), extractive industries (offshore drilling and deep sea mining), freshwater production via desalination.

Many of the world's goods are moved by ship between the world's seaports.[104] Large quantities of goods are transported across the ocean, especially across the Atlantic and around the Pacific Rim.[105] A lot of cargo, such as manufactured goods, is usually transported within standard sized, lockable containers, loaded on purpose-built container ships at dedicated terminals.[106] Containerization greatly increased the efficiency and decreased the cost of moving goods by sea, and was a major factor leading to the rise of globalization and exponential increases in international trade in the mid-to-late 20th century.[107]

Oceans are also the major supply source for the fishing industry. Some of the major harvests are shrimp, fish, crabs, and lobster.[34] The biggest commercial fishery globally is for anchovies, Alaska pollock and tuna.[108]: 6  A report by FAO in 2020 stated that "in 2017, 34 percent of the fish stocks of the world’s marine fisheries were classified as overfished".[108]: 54  Fish and other fishery products from both wild fisheries and aquaculture are among the most widely consumed sources of protein and other essential nutrients. Data in 2017 showed that "fish consumption accounted for 17 percent of the global population’s intake of animal proteins".[108] In order to fulfill this need, coastal countries have exploited marine resources in their exclusive economic zone, although fishing vessels are increasingly venturing further afield to exploit stocks in international waters.[109]

The ocean offers a very large supply of energy carried by ocean waves, tides, salinity differences, and ocean temperature differences which can be harnessed to generate electricity.[110] Forms of sustainable marine energy include tidal power, ocean thermal energy and wave power.[110][111] Offshore wind power is captured by wind turbines placed out on the ocean; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore.[112] There are large deposits of petroleum, as oil and natural gas, in rocks beneath the ocean floor. Offshore platforms and drilling rigs extract the oil or gas and store it for transport to land.[113]

"Freedom of the seas" is a principle in international law dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought in international waters.[114] Today, this concept is enshrined in the United Nations Convention on the Law of the Sea (UNCLOS).[114]

There are two major international legal organizations that are involved in ocean governance on a global scale, namely the International Maritime Organization and the United Nations. The International Maritime Organization (IMO), which was ratified in 1958 is responsible mainly for maritime safety, liability and compensation and they have held some conventions on marine pollution related to shipping incidents. Ocean governance is the conduct of the policy, actions and affairs regarding the world's oceans.[115]

Threats

Global cumulative human impact on the ocean[116]

Human activities affect marine life and marine habitats through many negative influences, such as marine pollution (including marine debris and microplastics) overfishing, ocean acidification and other effects of climate change on oceans.

Marine pollution

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, although marine transportation significantly contributes as well.[117] Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean.[118] The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blown debris and dust can also play a role, as these pollutants can settle into waterways and oceans.[119] Pathways of pollution include direct discharge, land runoff, ship pollution, atmospheric pollution and, potentially, deep sea mining.

The types of marine pollution can be grouped as pollution from marine debris, plastic pollution, including microplastics, ocean acidification, nutrient pollution, toxins and underwater noise. Plastic pollution in the ocean is a type of marine pollution by plastics, ranging in size from large original material such as bottles and bags, down to microplastics formed from the fragmentation of plastic material. Marine debris is mainly discarded human rubbish which floats on, or is suspended in the ocean. Plastic pollution is harmful to marine life.

Plastic pollution

Marine plastic pollution (or plastic pollution in the ocean) is a type of marine pollution by plastics, ranging in size from large original material such as bottles and bags, down to microplastics formed from the fragmentation of plastic material. Marine debris is mainly discarded human rubbish which floats on, or is suspended in the ocean. Eighty percent of marine debris is plastic.[120] Microplastics and nanoplastics result from the breakdown or photodegradation of plastic waste in surface waters, rivers or oceans. It is estimated that there is a stock of 86 million tons of plastic marine debris in the worldwide ocean as of the end of 2013, assuming that 1.4% of global plastics produced from 1950 to 2013 has entered the ocean and has accumulated there.[121] The 2017 United Nations Ocean Conference estimated that the oceans might contain more weight in plastics than fish by the year 2050.[122]

Oceans are polluted by plastic particles ranging in size from large original material such as bottles and bags, down to microplastics formed from the fragmentation of plastic material. This material is only very slowly degraded or removed from the ocean so plastic particles are now widespread throughout the surface ocean and are known to be having deleterious effects on marine life.[123] Discarded plastic bags, six pack rings, cigarette butts and other forms of plastic waste which finish up in the ocean present dangers to wildlife and fisheries.[124] Aquatic life can be threatened through entanglement, suffocation, and ingestion.[125][126][127] Fishing nets, usually made of plastic, can be left or lost in the ocean by fishermen. Known as ghost nets, these entangle fish, dolphins, sea turtles, sharks, dugongs, crocodiles, seabirds, crabs, and other creatures, restricting movement, causing starvation, laceration, infection, and, in those that need to return to the surface to breathe, suffocation.[128] There are various types of ocean plastics causing problems to marine life. Bottle caps have been found in the stomachs of turtles and seabirds, which have died because of the obstruction of their respiratory and digestive tracts.[129] Ghostnets are also a problematic type of ocean plastic as they can continuously trap marine life in a process known as ‘ghost fishing.’[130]

Overfishing

Overfishing is the removal of a species of fish from a body of water at a rate that the species cannot replenish, resulting in those species becoming underpopulated in that area. Overfishing can occur in water bodies of any sizes, such as ponds, rivers, lakes or oceans, and can result in resource depletion, reduced biological growth rates and low biomass levels. Sustained overfishing can lead to critical depensation, where the fish population is no longer able to sustain itself. Some forms of overfishing, such as the overfishing of sharks, has led to the upset of entire marine ecosystems.[131] Types of overfishing include: Growth overfishing, recruitment overfishing, ecosystem overfishing.

Climate change

The effects of climate change on oceans include the rise in sea level from ocean warming and ice sheet melting, and changes in ocean acidification, circulation, and stratification due to changing temperatures leading to changes in oxygen concentrations. There is clear evidence that the Earth is warming due to anthropogenic emissions of greenhouse gases and leading inevitably to ocean warming.[132] The greenhouse gases taken up by the ocean (via carbon sequestration) help to mitigate climate change but lead to ocean acidification.

Physical effects of climate change on oceans include sea level rise which will in particular affect coastal areas, ocean currents, weather and the seafloor. Chemical effects include ocean acidification and reductions in oxygen levels. Furthermore, there will be effects on marine life. The consensus of many studies of coastal tide gauge records is that during the past century sea level has risen worldwide at an average rate of 1–2 mm/yr reflecting a net flux of heat into the surface of the land and oceans. The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because the chemical equilibria that govern seawater pH are temperature-dependent.[133] Increase of water temperature will also have a devastating effect on different oceanic ecosystems like coral reefs. The direct effect is the coral bleaching of these reefs, which live within a narrow temperature margin, so a small increase in temperature would have a drastic effects in these environments.

Ocean acidification

Between 1751 and 1996, the pH value of the ocean surface is estimated to have decreased from approximately 8.25 to 8.14,[134] representing an increase of almost 30% in H+ ion concentration in the world's oceans (note the pH scale is logarithmic so a change of one in pH unit is equivalent to a ten fold change in H+ ion concentration).[135][136] Increasing acidity is thought to have a range of potentially harmful consequences for marine organisms such as depressing metabolic rates and immune responses in some organisms and causing coral bleaching.[137] By increasing the presence of free hydrogen ions, the additional carbonic acid that forms in the oceans ultimately results in the conversion of carbonate ions into bicarbonate ions. Ocean alkalinity (roughly equal to [HCO3] + 2[CO32−]) is not changed by the process, or may increase over long time periods due to carbonate dissolution.[138] This net decrease in the amount of carbonate ions available may make it more difficult for marine calcifying organisms, such as coral and some plankton, to form biogenic calcium carbonate, and such structures become vulnerable to dissolution.[139] Ongoing acidification of the oceans may threaten future food chains linked with the oceans.[140][141]

Extraterrestrial oceans

Extraterrestrial oceans may be composed of water or other elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for oceans' existence elsewhere in the Solar System.

Although Earth is the only known planet with large stable bodies of liquid water on its surface and the only one in the Solar System, other celestial bodies are thought to have large oceans.[142] In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.[143][144]

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