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.^[10][11] It includes the Atlantic, Pacific, Indian, Antarctic/Southern and Arctic Oceans.^[19] As a general term, "the ocean" and "the sea" are often interchangeable, although speakers of British English refer to "the sea" in all cases,^[20] even when the body of water is one of the oceans.

Strictly speaking, a "sea" is a body of water (generally a division of the world ocean) partly or fully enclosed by land.^[21] 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.^[22]

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 the Earth.^[23][24] The global, interconnected body of salt water is sometimes referred to as the World Ocean, global ocean or the great ocean.^[25][26][27] The concept of a continuous body of water with relatively unrestricted exchange between its components is critical in oceanography.^[28]

Etymology

The word ocean comes from the figure in classical antiquity, Oceanus (/oʊˈsiːənəs/; Greek: Ὠκεανός Ōkeanós,^[29] 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.^[30]

Origin of water

Scientists believe that a sizable quantity of water would have been in the material that formed Earth.^[31] Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. This is called atmospheric escape.

During planetary formation, Earth possibly had magma oceans. Subsequently, outgassing, volcanic activity and meteorite impacts, produced an early atmosphere of carbon dioxide, nitrogen and water vapor, according to current theories. The gases and the atmosphere are thought to have accumulated over millions of years. After Earth's surface had significantly cooled, the water vapor over time would have condensed, forming Earth's first oceans.^[32] The early oceans might have been significantly hotter than today and appeared green due to high iron content.^[33]

Geological evidence helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago.^[34] In the Nuvvuagittuq Greenstone Belt, Quebec, Canada, rocks dated at 3.8 billion years old by one study^[35] and 4.28 billion years old by another^[36] show evidence of the presence of water at these ages.^[34] If oceans existed earlier than this, any geological evidence either has yet to be discovered, or has since been destroyed by geological processes like crustal recycling. However, in August 2020, researchers reported that sufficient water to fill the oceans may have always been on the Earth since the beginning of the planet's formation.^[37][38][39] In this model, atmospheric greenhouse gases kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity.^[40]

Ocean 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.

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.

Since its formation the ocean has taken many conditions and shapes with many past ocean divisions and potentially at times covering the whole globe.^[41]

During colder climatic periods, more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age, glaciers covered almost one-third of Earth's land mass with the result being that the oceans were about 122 m (400 ft) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 5.5 m (18 ft) higher than they are now. About three million years ago the oceans could have been up to 50 m (165 ft) higher.^[42]

Oceanic divisions

Map of Earth centered on its ocean, showing the different ocean divisions

There are different customs to subdivide the ocean and are adjourned by smaller bodies of water such as, seas, gulfs, bays, bights, and straits.

The Ocean is customarily divided into five principal oceans – listed below in descending order of area and volume:

Ocean basins

Bathymetry of the ocean floor showing the continental shelves and oceanic plateaus (red), the mid-ocean ridges (yellow-green) and the abyssal plains (blue to purple)

The ocean fills Earth's oceanic basins. Earth's oceanic basins cover different geologic provinces of Earth's oceanic crust as well as continental crust. As such it covers mainly Earth's structural basins, but also continental shelfs.

In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to form mid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known as subduction. Deep trenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced and magma is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity.^[57][58]

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.^[59]

Oceanographers state that less than 20% of the oceans have been mapped.^[60]

Interaction with the coast

Praia da Marinha in Algarve, Portugal

The zone where land meets sea is known as the coast and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore. A beach is the accumulation of sand or shingle on the shore.^[61] A headland is a point of land jutting out into the sea and a larger promontory is known as a cape. The indentation of a coastline, especially between two headlands, is a bay, a small bay with a narrow inlet is a cove and a large bay may be referred to as a gulf.^[62] Coastlines are influenced by several factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence.^[61]

Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as the swash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normal weathering processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion.^[61]

Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents.^[61] Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of the land by the building of breakwaters, seawalls, dykes and levees and other sea defences. For instance, the Thames Barrier is designed to protect London from a storm surge,^[63] while the failure of the dykes and levees around New Orleans during Hurricane Katrina created a humanitarian crisis in the United States.

Color

Ocean chlorophyll concentration is a proxy for phytoplankton biomass. In this map, blue colors represent lower chlorophyll and reds represent higher chlorophyll. Satellite-measured chlorophyll is estimated based on ocean color by how green the color of the water appears from space.

Water cycle, weather and rainfall

The ocean is a major driver of Earth's water cycle.

Ocean water represents the largest body of water within the global water cycle (oceans contain 97% of Earth's water). Evaporation from the ocean moves 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.^[60] Oceanic evaporation, as a phase of the water cycle, is the source of most rainfall (about 90%),^[68] causing a global cloud cover of 67% and a consistent oceanic cloud cover of 72%.^[69] 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.

Waves and swell

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.^[70] 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.^{[71]: 83–84 [72][73]} 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.^[72]

Constructive interference can lead to the formation of unusually high rogue waves.^[74] Most waves are less than 3 m (10 ft) high^[74] and it is not unusual for strong storms to double or triple that height.^[75] Rogue waves, however, have been documented at heights above 25 meters (82 ft).^[76][77]

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 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.^[74] This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity.^[78]

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.^[79][80]

Sea level and surface

The ocean's surface is an important reference point for oceanography and geography, particularly as mean sea level. The ocean surface has globally little, but measurable topography, depending on the ocean's volumes.

The ocean surface is a crucial interface for oceanic and atmospheric processes. Allowing interchange of particles, enriching the air and water, as well as grounds by some particles becoming sediments. This interchange has fertilized life in the ocean, on land and air. All these processes and components together make up ocean surface ecosystems.

Tides

High tide and low tide in the Bay of Fundy, Canada.

Tides are the regular rise and fall in water level experienced by oceans, primarily driven by the Moon's gravitational tidal forces upon the Earth. Tidal forces affect all matter on Earth, but only fluids like the ocean demonstrate the effects on human timescales. (For example, tidal forces acting on rock may produce tidal locking between two planetary bodies.) Though primarily driven by the Moon's gravity, oceanic tides are also substantially modulated by the Sun's tidal forces, by the rotation of the Earth, and by the shape of the rocky continents blocking oceanic water flow. (Tidal forces vary more with distance than the "base" force of gravity: the Moon's tidal forces on Earth are more than double the Sun's,^[81] despite the latter's much stronger gravitational force on Earth. Earth's tidal forces upon the Moon are 20x stronger than the Moon's tidal forces on the Earth.)

The primary effect of lunar tidal forces is to bulge Earth matter towards the near and far sides of the Earth, relative to the moon. The "perpendicular" sides, from which the Moon appears in line with the local horizon, experience "tidal troughs". Since it takes nearly 25 hours for the Earth to rotate under the Moon (accounting for the Moon's 28 day orbit around Earth), tides thus cycle over a course of 12.5 hours. However, the rocky continents pose obstacles for the tidal bulges, so the timing of tidal maxima may not actually align with the Moon in most localities on Earth, as the oceans are forced to "dodge" the continents. Timing and magnitude of tides vary widely across the Earth as a result of the continents. Thus, knowing the Moon's position does not allow a local to predict tide timings, instead requiring precomputed tide tables which account for the continents and the Sun, among others.

During each tidal cycle, at any given place the tidal waters rise to maximum height, high tide, before ebbing away again to the minimum level, low tide. As the water recedes, it gradually reveals 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.^[82][83] When the sun and moon are aligned (full moon or new moon), the combined effect results in the higher "spring tides", while the sun and moon misaligning (half moons) result in lesser tidal ranges.^[82]

In the open ocean tidal ranges are less than 1 meter, but in coastal areas these tidal ranges increase to more than 10 meters in some areas.^[84] Some of the largest tidal ranges in the world occur in the Bay of Fundy and Ungava Bay in Canada, reaching up to 16 meters.^[85] Other locations with record high tidal ranges include the Bristol Channel between England and Wales, Cook Inlet in Alaska, and the Río Gallegos in Argentina.^[86]

Tides are not to be confused with storm surges, which 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 dramatically above a typical high tide.

Depth

The average depth of the oceans is about 4 km. More precisely the average depth is 3,688 meters (12,100 ft).^[72] Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.^[27] "Deep ocean," which is anything below 200 meters (660 ft), covers about 66% of Earth's surface.^[87] This figure does not include seas not connected to the World Ocean, such as the Caspian Sea.

The deepest region of the ocean is at the Mariana Trench, located in the Pacific Ocean near the Northern Mariana Islands.^[88] The 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.

Oceanic zones

The major oceanic zones, based on depth and biophysical conditions

Oceanographers classify the ocean into vertical and horizontal zones based on 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.

Volumes

The combined volume of water in all the oceans is roughly 1.335 billion cubic kilometers (1.335 sextillion liters, 320.3 million cubic miles).^[72][91][92]

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 ocean water has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.^[14]

The temperature gradient over the water depth is related to the way the surface water mixes with deeper water or does not mix (a lack of mixing is called ocean stratification). This depends on the temperature: in the tropics the warm surface layer of about 100 m is quite stable and does not mix much with deeper water, while near the poles winter cooling and storms makes the surface layer denser and it mixes to great depth and then stratifies again in summer. The photic depth is typically about 100 m (but varies) and is related to this heated surface layer.^[98]

Temperature and salinity by region

The temperature and salinity of ocean waters vary significantly across different regions. This is due to differences in the local water balance (precipitation vs. evaporation) and the "sea to air" temperature gradients. These characteristics can vary widely from one ocean region to another. The table below provides an illustration of the sort of values usually encountered.

Sea ice

Seawater with a typical salinity of 35‰ has a freezing point of about −1.8 °C (28.8 °F).^[89][107] Because sea ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans.^{[108][109][110]} Sea ice usually starts to freeze at the very surface, initially as a very thin ice film. As further freezing takes place, this ice film thickens and can form ice sheets. The ice formed incorporates some sea salt, but much less than the seawater it forms from. As the ice forms with low salinity this results in saltier residual seawater. This in turn increases density and promotes vertical sinking of the water.^[111]

Ocean currents and global climate

Ocean surface currents

A map of the global thermohaline circulation; blue represents deep-water currents, whereas red represents surface currents.

Relationship of currents and climate

Main article: Atlantic meridional overturning circulation

Map of the Gulf Stream, a major ocean current that transports heat from the equator to northern latitudes and moderates the climate of Europe.

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 thermohaline circulation (Atlantic meridional overturning circulation (AMOC), which is part of a global thermoholine circulation, drives water motion.The AMOC is driven by the cooling of surface waters in the polar latitudes in the north and south, 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.^[113] 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. This affects air temperature and precipitation in coastal regions and further inland. Surface heat and freshwater fluxes create global density gradients, which drive the thermohaline circulation that is a part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation.

Oceans moderate the climate of locations where prevailing winds blow in from the ocean. At similar latitudes, a place on Earth with more influence from the ocean will have a more moderate climate than a place with more influence from land. For example, the cities San Francisco (37.8 N) and New York (40.7 N) have different climates because San Francisco has more influence from the ocean. San Francisco, on the west coast of North America, gets winds from the west over the Pacific Ocean, and the influence of the ocean water yields a more moderate climate with a warmer winter and a longer, cooler summer, with the warmest temperatures happening later in the year. New York, on the east coast of North America gets winds from the west over land, so New York has colder winters and hotter, earlier summers than San Francisco.

Warmer ocean currents yield warmer climates in the long term, even at high latitudes. At similar latitudes, a place influenced by warm ocean currents will have a warmer climate overall than a place influenced by cold ocean currents. French Riviera (43.5 N) and Rockland, Maine (44.1 N) have same latitude, but the French Riviera is influenced by warm waters transported by the Gulf Stream into the Mediterranean Sea and has a warmer climate overall. Maine is influenced by cold waters transported south by the Labrador Current giving it a colder climate overall.

Changes in the thermohaline circulation are thought to have significant impacts on Earth's energy budget. Because the thermohaline circulation determines the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations. Modern observations, climate simulations and paleoclimate reconstructions suggest that the Atlantic Meridional Overturning Circulation (AMOC) has weakened since the preindustrial era. The latest climate change projections in 2021 suggest that the AMOC is likely to weaken further over the 21st century.^[114]: 19 Such a weakening could cause large changes to global climate, with the North Atlantic particularly vulnerable.^[114]: 19

Salinity

Annual mean sea surface salinity in practical salinity units (psu) from the World Ocean Atlas.^[115]

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 standard practice to gauge it 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. According to an international agreement, the following formula is used to determine salinity:^[116]

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

The average ocean water chlorinity is about 19.2‰, and, thus, the average salinity is around 34.7‰.^[116]

Salinity has a major influence on the density of seawater. A zone of rapid salinity increase with depth is called a halocline. As seawater's salt content increases, so does the temperature at which its maximum density occurs. Salinity affects both the freezing and boiling points of water, with the boiling point increasing with salinity. At atmospheric pressure,^[117] normal seawater freezes at a temperature of about −2 °C.

Salinity is higher in Earth's oceans where there is more evaporation and lower where there is more precipitation. If precipitation exceeds evaporation, as is the case in polar and some temperate regions, salinity will be lower. Salinity will be higher if evaporation exceeds precipitation, as is sometimes the case in tropical regions. For example, evaporation is greater than precipitation in the Mediterranean Sea, which has an average salinity of 38‰, more saline than the global average of 34.7‰.^[118] Thus, oceanic waters in polar regions have lower salinity content than oceanic waters in tropical regions.^[116] However, when sea ice forms at high latitudes, salt is excluded from the ice as it forms, which can increase the salinity in the residual seawater in polar regions such as the Arctic Ocean.^[89][119]

Due to the effects of climate change on oceans, observations of sea surface salinity between 1950 and 2019 indicate that regions of high salinity and evaporation have become more saline while regions of low salinity and more precipitation have become fresher.^[120] It is very likely that the Pacific and Antarctic/Southern Oceans have freshened while the Atlantic has become more saline.^[120]

Dissolved gases

Sea surface oxygen concentration in moles per cubic meter from the World Ocean Atlas.^[121]

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.^[16] The four most abundant gases in earth's atmosphere and oceans are nitrogen, oxygen, argon, and carbon dioxide. In the ocean by volume, the most abundant gases dissolved in seawater are carbon dioxide (including bicarbonate and carbonate ions, 14 mL/L on average), nitrogen (9 mL/L), and oxygen (5 mL/L) at equilibrium at 24 °C (75 °F)^{[122][123][124]} All gases are more soluble – more easily dissolved – in colder water than in warmer water. For example, when salinity and pressure are held constant, oxygen concentration in water almost doubles when the temperature drops from that of a warm summer day 30 °C (86 °F) to freezing 0 °C (32 °F). Similarly, carbon dioxide and nitrogen gases are more soluble at colder temperatures, and their solubility changes with temperature at different rates.^[122][125]

Oxygen, photosynthesis and carbon cycling

Diagram of the ocean carbon cycle showing the relative size of stocks (storage) and fluxes.^[126]

Photosynthesis in the surface ocean releases oxygen and consumes carbon dioxide. Phytoplankton, a type of microscopic free-floating algae, controls this process. After the plants have grown, oxygen is consumed and carbon dioxide released, as a result of bacterial decomposition of the organic matter created by photosynthesis in the ocean. 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.^[98] This cycling of carbon dioxide in oceans is an important part of the global carbon cycle.

The oceans represent a major carbon sink for carbon dioxide taken up from the atmosphere by photosynthesis and by dissolution (see also carbon sequestration). There is also increased attention on carbon dioxide uptake in coastal marine habitats such as mangroves and saltmarshes. This process is often referred to as "Blue carbon". The focus is on these ecosystems because they are strong carbon sinks as well as ecologically important habitats under threat from human activities and environmental degradation.

As deep ocean water circulates throughout the globe, it contains gradually less oxygen and gradually more carbon dioxide with more time away from the air at the surface. This gradual decrease in oxygen concentration happens as sinking organic matter continuously gets decomposed during the time the water is out of contact with the atmosphere.^[98] Most of the deep waters of the ocean still contain relatively high concentrations of oxygen sufficient for most animals to survive. However, some ocean areas have very low oxygen due to long periods of isolation of the water from the atmosphere. These oxygen deficient areas, called oxygen minimum zones or hypoxic waters, will generally be made worse by the effects of climate change on oceans.^[127][128]

pH

The pH value at the surface of oceans (global mean surface pH) is currently approximately in the range of 8.05^[129] to 8.08.^[130] This makes it slightly alkaline. The pH value at the surface used to be about 8.2 during the past 300 million years.^[131] However, between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.^[132] Carbon dioxide emissions from human activities are the primary cause of this process called ocean acidification, with atmospheric carbon dioxide (CO₂) levels exceeding 410 ppm (in 2020).^[133] CO₂ from the atmosphere is absorbed by the oceans. This produces carbonic acid (H₂CO₃) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H⁺). The presence of free hydrogen ions (H⁺) lowers the pH of the ocean.

There is a natural gradient of pH in the ocean which is related to the breakdown of organic matter in deep water which slowly lowers the pH with depth: The pH value of seawater is naturally as low as 7.8 in deep ocean waters as a result of degradation of organic matter there.^[134] It can be as high as 8.4 in surface waters in areas of high biological productivity.^[98]

The definition of global mean surface pH refers to the top layer of the water in the ocean, up to around 20 or 100 m depth. In comparison, the average depth of the ocean is about 4 km. The pH value further down below (lower than 100 m) has not yet been affected by ocean acidification in the same way. There is a large body of deeper water where the natural gradients of pH from 8.2 to about 7.8 still exists and it will take a very long to acidify these waters, and equally a long time to recover from that acidification. But as the top layer of the ocean (the photic zone) is crucial for its marine productivity, any changes to the pH value and temperature of the top layer can have many knock-on effects, for example on marine life and ocean currents (see also effects of climate change on oceans).^[98]

The key issue in terms of the penetration of ocean acidification is the way the surface water mixes with deeper water or does not mix (a lack of mixing is called ocean stratification). This in turn depends on the water temperature and hence is different between the tropics and the polar regions (see ocean#Temperature).^[98]

The chemical properties of seawater complicate pH measurement, and several distinct pH scales exist in chemical oceanography.^[135] There is no universally accepted reference pH-scale for seawater and the difference between measurements based on multiple reference scales may be up to 0.14 units.^[136]

Alkalinity

Alkalinity is the balance of base (proton acceptors) and acids (proton donors) in seawater, or indeed any natural waters. The alkalinity acts as a chemical buffer, regulating the pH of seawater. While there are many ions in seawater that can contribute to the alkalinity, many of these are at very low concentrations. This means that the carbonate, bicarbonate and borate ions are the only significant contributors to seawater alkalinity in the open ocean with well oxygenated waters. The first two of these ions contribute more than 95% of this alkalinity.^[98]

The chemical equation for alkalinity in seawater is:

A_T = [HCO₃^-] + 2[CO₃^2-] + [B(OH)₄^-]

The growth of phytoplankton in surface ocean waters leads to the conversion of some bicarbonate and carbonate ions into organic matter. Some of this organic matter sinks into the deep ocean where it is broken down back into carbonate and bicarbonate. This process is related to ocean productivity or marine primary production. Thus alkalinity tends to increase with depth and also along the global thermohaline circulation from the Atlantic to the Pacific and Indian Ocean, although these increases are small. The concentrations vary overall by only a few percent.^[98][134]

The absorption of CO₂ from the atmosphere does not affect the ocean's alkalinity.^{[137]: 2252} It does lead to a reduction in pH value though (termed ocean acidification).^[133]

Residence times of chemical elements and ions

Residence time of elements in the ocean depends on supply by processes like rock weathering and rivers vs. removal by processes like evaporation and sedimentation.

The ocean waters contain many chemical elements as dissolved ions. Elements dissolved in ocean waters have a wide range of concentrations. Some elements have very high concentrations of several grams per liter, such as sodium and chloride, together making up the majority of ocean salts. Other elements, such as iron, are present at tiny concentrations of just a few nanograms (10⁻⁹ grams) per liter.^[116]

The concentration of any element depends on its rate of supply to the ocean and its rate of removal. Elements enter the ocean from rivers, the atmosphere and hydrothermal vents. Elements are removed from ocean water by sinking and becoming buried in sediments or evaporating to the atmosphere in the case of water and some gases. By estimating the residence time of an element, oceanographers examine the balance of input and removal. Residence time is the average time the element would spend dissolved in the ocean before it is removed. Heavily abundant elements in ocean water such as sodium, have high input rates. This reflects high abundance in rocks and rapid rock weathering, paired with very slow removal from the ocean due to sodium ions being comparatively unreactive and highly soluble. In contrast, 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. 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 in the ocean are estimated to be millions of years, while for highly reactive and insoluble elements, residence times are only hundreds of years.^[116]

Nutrients

North Atlantic
gyre

North Atlantic
gyre

North Atlantic
gyre

Indian
Ocean
gyre

North
Pacific
gyre

South
Pacific
gyre

South Atlantic
        gyre

Ocean gyres rotate clockwise in the north and counterclockwise in the south

A few elements such as nitrogen, phosphorus, iron, and potassium essential for life, are major components of biological material, and are commonly known as "nutrients". Nitrate and phosphate have ocean residence times of 10,000^[140] and 69,000^[141] years, respectively, while potassium is a much more abundant ion in the ocean with a residence time of 12 million^[142] 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.^[143] Production of rock phosphate, an essential ingredient in inorganic fertilizer,^[144] is a slow geological process that occurs in some of the world's ocean sediments, rendering mineable sedimentary apatite (phosphate) a non-renewable resource (see peak phosphorus). This continual net deposition loss of non-renewable phosphate from human activities, may become a resource issue for fertilizer production and food security in future.^[145][146]

Climate change

Marine pollution

Plastic pollution

This section is an excerpt from Marine plastic pollution.[edit]

Marine plastic pollution 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.^[183][184] Microplastics and nanoplastics result from the breakdown or photodegradation of plastic waste in surface waters, rivers or oceans. Recently, scientists have uncovered nanoplastics in heavy snow, more specifically about 3,000 tons that cover Switzerland yearly.^[185]

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.^[186] Global consumption of plastics is estimated to be 300 million tonnes per year as of 2022, with around 8 million tonnes ending up in the oceans as macroplastics.^[187][188] Approximately 1.5 million tonnes of primary microplastics end up in the seas. Around 98% of this volume is created by land-based activities, with the remaining 2% being generated by sea-based activities.^{[188][189][190]} It is estimated that 19–23 million tonnes of plastic leaks into aquatic ecosystems annually.^[191] The 2017 United Nations Ocean Conference estimated that the oceans might contain more weight in plastics than fish by the year 2050.^[192]

A woman and a boy collecting plastic waste at a beach during a cleanup exercise

• Schmidt, Christian; Krauth, Tobias; Wagner, Stephan (October 11, 2017). "Export of Plastic Debris by Rivers into the Sea" (PDF). Environmental Science & Technology. 51 (21): 12246–12253. Bibcode:2017EnST...5112246S. doi:10.1021/acs.est.7b02368. ISSN 0013-936X. PMID 29019247. The 10 top-ranked rivers transport 88–95% of the global load into the sea
• "Supporting Information: Export of plastic debris by rivers into the sea" (PDF).</ref>^[193] Asia was the leading source of mismanaged plastic waste, with China alone accounting for 2.4 million metric tons.^[194]

Overfishing