The ocean ( plural "the oceans", from ancient Greek Ὠκεανός Ōkeanós , German 'the world current flowing around the disc' , personified as the ancient god Oceanus ) is the name given to the largest seas on earth . The term Weltmeer is used synonymously and as a translation in German .
In general, large amounts of water on other celestial bodies are also called “oceans”.
Location of the oceans
A total of 71 percent of the earth's surface is covered by seas (the oceans and their tributaries ). They focus on the water hemisphere , the center of which is in the vast Pacific near New Zealand . On the opposite land hemisphere , only the Atlantic , the Arctic Ocean and parts of the Southern Ocean and the Indian Ocean are located .
The five oceans of the earth are:
In contrast to technical terminology, a distinction is often only made between three oceans in everyday language : the Atlantic , Pacific and Indian oceans . In this view, without the North and the Southern Ocean , the Arctic Ocean is considered part of the Atlantic and counted the Southern or Antarctic Ocean to the Atlantic, Pacific and Indian Ocean.
Historically, one speaks of the " Seven Seas ", which, in addition to the Pacific, Atlantic and Indian Oceans, also include the Caribbean Sea , the Mediterranean Sea , the Yellow Sea and the North Sea (or other seas that are considered secondary seas such as the Black Sea or the Baltic Sea ).
An alternative view divides the two largest oceans on earth according to their affiliation to the northern and southern hemispheres into the north and south Atlantic and the north and south Pacific, also considers the northern and southern polar oceans as oceans and counts seven oceans together with the indicator. This corresponds to counting seven continents (North America, South America, Europe, Africa, Asia, Oceania ( Australia and Oceania ), Antarctica).
The individual oceans that lie between the continents differ from other parts of the world ocean in terms of volume, salinity , their own tidal system, waves ( swell ) and ocean currents, as well as geological history .
Within the oceans and their tributaries or on the ocean floor there are sometimes very high and elongated mid-ocean ridges , sometimes very many and lower thresholds , large and small deep-sea basins , deep-sea channels and various sea lows as well as the Pacific ring of fire in the Pacific . In addition, numerous islands , archipelagos and archipelagos protrude from these seas and peninsulas into them. The Arctic Ocean and the Southern Ocean are partly or completely covered by pack and drift ice .
The bottom of an ocean is the top of a piece of oceanic crust . Its shape is explained by the theory of plate tectonics . After that, new ocean floor is created on the mid-ocean ridges and drifts away until it dips into the Earth's interior in a deep-sea channel ( subduction zones ). This means that an ocean can get bigger or smaller, emerge again and also disappear (see also → Wilson cycle ). It is believed that the Atlantic Ocean is around 150 million years old. Earlier oceans are for example the Mirovia , the Panthalassa , the Rheic Ocean , the Iapetus or the Tethys with the “European” marginal sea Paratethys .
The course of the coast depends not only on the shape and location of the continents, but also on the volume of seawater. For example, there is less seawater at low temperatures, as large amounts of water are stored as ice sheets and glaciers on the continents, while at rising temperatures, on the other hand, there is a rise in sea level ( transgression ) due to thermal expansion and the melting of the ice masses . Other factors are uplifts and subsidence of the ocean floor due to geological events.
The volume of the oceans was estimated at 1.33 · 10 9 km 3 in 2009 , corresponding to an average depth of 3680 m - less than 10% were measured exactly.
The body of water in an ocean is not uniform, but changes with depth. There are large, stable movements of water, the ocean currents . Most important is the " Global Conveyor Belt ", a combination of ocean currents that connect four of the five oceans and in which surface currents and deep currents form a global water cycle. This can lead to the formation of large water eddies or eddies at a depth of several 1000 m. Mid-ocean ridges can also lead to turbulence. Large water eddies from 50 km to 200 km in diameter, which last for several weeks and carry cold, nutrient-rich deep water to the sea surface, are also observed. There can also be "internal waves" in the body of water. The largest submarine waves of more than 200 meters were z. B. measured in the 320 km wide Luzon Strait in the South China Sea. In this strait, a deep current accumulates large amounts of heavy, cold deep water in front of sub-sea cliffs, which at some point spills over and then sinks back down to the old depth, causing an internal wave to be triggered. Such internal waves can travel thousands of kilometers in the ocean.
Water waves appear on the sea surface . It can be irregular water movements generated by the wind , which can be quantified by a sea state scale. Individual waves or groups of waves, the so-called “ monster waves ”, are particularly dangerous waves that are created when several waves are superimposed and can reach heights of more than 25 m. The tsunamis are waves caused by seaquakes and volcanic eruptions, which only pile up to dangerous heights near the coast.
By Serpentinization 60 cubic kilometers of sea water are chemically per year seafloor bound. In addition, the sediments on the sea floor are saturated with water. This water is released again in the subduction zones .
The oxygen content of the sea water near the sea surface is determined by the transfer of oxygen from the air into the water and the biological production of oxygen from carbon dioxide (CO 2 ) by the marine phytoplankton . Therefore, especially in the tropics, surface water can temporarily become oversaturated ( oxygen saturation > 100 percent), so that more oxygen is released into the air. However, the phytoplankton themselves use up part of the oxygen produced in the dark.
With increasing water depth and the associated decrease in sunlight, the oxygen saturation of the sea water decreases. In addition to breathing in oxygen through the zooplankton and part of the bacterioplankton , the increasing biodegradation of biomass also contributes to the reduction of the oxygen content. In the ocean, the deep-sea water does not overturn , as oxygen-rich surface water is created in the Labrador Sea , the Greenland Sea and the Weddell Sea , which sinks into the deep sea and is distributed worldwide via the deep currents of the Global Conveyor Belt. The oxygen distribution in the deep sea is not uniform; There are so-called oxygen minimum zones , where, for example, anaerobic ammonia oxidation and denitrification occur ( molecular nitrogen is produced through anaerobic respiration of bacteria, which escapes from the water into the air). These areas are often found in the tropics, with the Arabian Sea having a significant oxygen minimum zone at a depth of 200 m to 1150 m.
According to researchers at the Geomar Helmholtz Center for Ocean Research Kiel, the oxygen content of the oceans worldwide decreased by around 2% between 1960 and 2010, with major consequences e.g. B. for fish or other organisms in already oxygen-poor marine regions. Rising water temperatures are responsible for this, as warmer surface water absorbs less oxygen than colder water and, moreover, warmer water manifests the temperature stratification of the sea water, so that its circulation is reduced and thus less oxygen is transported from the sea surface to great sea depths . It is expected that the oxygen content of the oceans will decrease by about 3-4% by the end of this century.
Sunlight, which decreases with increasing depth, is of great importance for the ocean ecosystem. In the uppermost, sunlight-filled part of the ocean, the euphotic zone , plants use photosynthesis to absorb energy. This is followed by the dysphotic zone , where sunlight is only sufficient for seeing. In the layer below, the aphotic zone , there is no longer any sunlight.
Another important characteristic of the oceans is that the sea water behaves chemically differently at different depths. Marine life such as mussels , corals , calcareous algae and diatoms use calcium carbonate and silicon dioxide through biomineralization to build shells and skeletons. However, these biominerals can be broken down chemically by the seawater. For the calcium carbonates aragonite and calcite in the oceans there is a lower depth from which they dissolve, the calcite and aragonite compensation depth .
The depth of an ocean is divided into several levels. It begins with the shelf area reaching down to a depth of 200 meters . This is followed by the continental slope , which merges into the shallower continental foot at a depth of 2000 to 4000 m. This is followed by the Abyssal with a maximum depth of 6000 m and below that the Hadal .
The very rare, mostly seasonal upwelling areas are very rich in nutrients. Cold deep currents rise in them and replace the nutrient-poor warm surface water.
Ocean ecosystems are severely affected by global warming . Climate change is primarily due to the energy imbalance in the earth's climate system caused by increasing concentrations of greenhouse gases ; around 93% of the energy imbalance was absorbed as heat content from the oceans between 1971 and 2010 . As a result, a sharp rise in temperatures in the oceans has been observed over the past few decades, the continuation of which will lead to a significant rise in sea levels due to the thermal expansion of water. The warming of the oceans is also associated with the death of marine life. This means that by the year 2100 the oxygen content of the oceans is expected to decrease by 7%.
New and Pleuston
On the sea surface, " Neuston " (ancient Greek "the swimming one") denotes the entirety of the organisms that live in a thin layer of between about five centimeters to only a few millimeters directly below the water surface; " Pleuston " (agr. "The sailing"), on the other hand, means the entirety of the larger living beings floating on or on the water surface.
The open ocean covers about 80 percent of the area of the world's ocean, but only 1 percent of the biomass is produced there. In this oligotrophic area, it is mainly the lack of nitrogen and phosphorus in the seawater that limits the growth of marine plants ( phytoplankton ). But the lack of important metals, such as iron, also has a growth-inhibiting effect, which is why iron fertilization of HNLC areas is being experimented with. In the relatively nutrient-poor open ocean, the importance of viruses in the upper water layers is important, as an infection of the bacteria, e.g. B. the blue-green algae (cyanobacteria) causes them to burst open and thus make their contents available as nutrients.
Large water eddies, in which cold, nutrient-rich seawater is pumped from the depths to the sea surface, act like a short-term upwelling area and lead to an explosive increase in phytoplankton. Tropical cyclones have the same effect .
Large elevations of the sea floor, which sometimes reach up to the surface of the water, such as individual underwater mountains ( seamounts and guyots ) and large underwater mountains are significant. These elevations influence the ocean currents, so that nutrient-rich deep water transported over great distances rises to shallower depths and thus an oasis of life can arise in an otherwise nutrient-poor part of an ocean.
The transition between the mainland and the deep sea is formed by the shelf reaching up to 200 meters in depth, the subsequent continental slope and the continental foot.
The shelf areas of the oceans are very rich in nutrients and are of great economic importance for the neighboring countries. In this respect, the legal construct of an exclusive economic zone was created in order to subordinate the now mostly overfished fishing grounds and possible deposits of oil and natural gas to national sovereignty. The common fisheries policy applies in the European Union .
On soft ground in the shallow sea or in the mudflats, plants from the seagrass family sometimes form extensive seagrass meadows . In addition to their great ecological importance, they are also important for coastal protection .
The deep sea is a so far little explored area of the oceans. With manned deep-sea submarines for medium and great depths as well as with unmanned autonomous and remote-controlled submersible vehicles , images have been taken and samples have been collected on site since the 20th century. Until then, only nets, for example on the Challenger Expedition (1872–1876) from up to 8000 m depth or the Valdivia Expedition (1898–1899) from around 4600 m depth, could catch more or less squashed creatures from the deep sea .
In contrast to the illuminated upper area of the ocean, the deep sea receives too little or no sunlight at all , so that photosynthesis is not possible there. Most deep-sea animals migrate from the weak-light zone upwards at sunset to the area that is illuminated during the day in order to feed there, and then submerge again at sunrise. During this hike, you will encounter robbers lurking in the air. The most common hikers are copepods , jellyfish and krill . It is vital for the animals living here that they do not stand out in color from the weak blue light coming from above. Important camouflage techniques are transparency and backlighting, in that the light organs present on the underside of the body shine blue to different degrees depending on the lighting conditions. This bioluminescence becomes even more important in the sunlight-free zone of the deep sea. There are deep-sea fish there that use light signals to attract prey or partners.
The almost colorless, transparent ear jellyfish in the blue spotlight
Vampire squid hunt with light and emit glowing particles as they escape.
Deep sea frogfish with a light organ above the mouth
Ghost fish have large, upward-pointing eyes
The ocean floor is the largest habitat on earth and includes the soils of the coasts, the shelves, the continental slopes, the great deep sea plains and the deep sea trenches.
The ocean floor on a continental slope usually consists of sand and gravel , in the intertidal zones also of silt and mud . Further away from the continents, it mainly consists of clays and residues of microorganisms that slowly sink from the surface to the bottom of an ocean in the form of so-called sea snow. In this way, a layer of deep sea sediments , on average 800 m thick , is created, which is an important part of the deep biosphere .
The organisms in the ocean floor feed on the falling remains of plants and animals, and occasionally also on dissolved volcanic gases . It is also conceivable that hydrogen generated by radiolysis is used by bacteria as an energy source. Bacteria and a few archaea live in the uppermost sediment layer , which is still enriched with oxygen , while only archaea can be found below. In the open ocean of the South Pacific, in an area where there is little sea snow annually, a lot of oxygen could be measured in the sediment at depths of up to eight meters, while carbon was again hardly available. There were few, but very active, oxygen-dependent bacteria. Smaller animals in the upper sediment layer are, for example, worms, snails and mussels.
On the ocean floor up to 50 m depth grow tropical coral reefs and on the continental shelves to a depth of 1,000 meters by the trawling endangered cold water reefs . Other typical living on the seabed marine animals are sea anemones , tube worms , sponges , sea urchins , sea cucumbers , starfish , brittle stars and bottom-dwelling fish such as gurnard , flatfish or ipnopidae .
There are hot springs in some submarine mountains, the Mediterranean ridges . These deposit ore sludge and form the basis for the Black Smoker's ecosystem, which is completely independent of sunlight (see also Lost City ). In the vicinity of deep sea channels and in places where methane hydrate becomes unstable as a result of landslides, there are cold springs , the so-called cold seeps , also known as methane springs . They arise when water, enriched, for example, with methane and hydrogen sulfide , flows out of the sea floor . At the hot and cold springs there are beard worms , which live in symbiosis with bacteria. At the hot springs there is a diverse and biomass-rich fauna, for example, yeti crabs and certain types of mussels, snails and shrimp. The ecosystem of the cold springs is similar to that of the hot springs, only there is no increased temperature of the sea water, it is more permanent and the transition to the non-specialized fauna is easier. Another important ecosystem is the cadavers of large organisms, such as whales , which sink to the ocean floor and serve various organisms as a source of food for months to decades. These are, for example, sharks , hagfish and bone-eating worms .
Oceans on other planets and satellites
An ocean probably exists, hidden under a mighty ice crust, on Jupiter's moon Europa , perhaps also on the other moons Ganymede and Callisto . Such an ocean is very likely on Saturn's moon Enceladus . Much evidence suggests that Mars contained open bodies of water in the early stages of its development. Smaller oceans or only lakes made of hydrocarbons (methane, ethane) could exist on Saturn's moon Titan all year round or only temporarily ( methane lakes on Titan ). It can only be speculated whether the gas planets Jupiter , Saturn , Uranus and Neptune might harbor layers of liquid phases, possibly made of helium or hydrogen . For the origin of the oceans see origin of terrestrial water .
- John Farndon: Atlas of Oceans - An Ecological Survey of Underwater Life . Yale University Press 2011
- Gotthilf Hempel , Kai Bischof , Wilhelm Hagen (ed.): Fascination marine research. An ecological reader . Springer Verlag Berlin 2017, ISBN 978-3-662-49714-2
- Stephen Hutchinson: Atlas of the Oceans - Geography, Living Things, Climate and Natural Phenomena. National Geographic, Sydney 2009, ISBN 978-3-86690-167-4
- Manfred Leier: World Atlas of the Oceans - with the depth maps of the world's oceans. Frederking and Thaler, Munich 2007, ISBN 978-3-89405-541-7
- Ian S. Robinson: Understanding the Oceans from Space. Springer, Berlin 2008, ISBN 978-3-540-24430-1
- Dorrik Stow: Encyclopedia of the oceans. Oxford University Press, Oxford 2004, ISBN 0-19-860687-7
- blueactionfund.org: Safeguarding Marine Biodiversity ("Safeguarding Maritime Biodiversity ")
- vsr-gewässerschutz.de: Stop the over-fertilization of the seas , association for the protection of ground and surface water
Photos and videos
- Adventure ocean - underwater photography by David Hettich
- Institute for Oceanography, University of Hamburg
- Helmholtz Center for Oceanography
- German Oceanographic Data Center (DOD)
- International Association for the Physical Sciences of the Oceans (IAPSO)
- Institut Océanographique Paris / Monaco (French)
- Monterey Bay Aquarium Research Institute (English)
- Universe of the Oceans , Terra X documentary (ZDF)
- Friedrich Kluge : Etymological dictionary of the German language . 24th edition. Berlin 2002.
- Matthew A. Charette, Walter HF Smith: The Volume of Earth's Ocean . In: Oceanography. 23, 2010, doi: 10.5670 / oceanog.2010.51
- Leibniz Institute for Marine Sciences : Eddy in the deep sea ( Memento from September 30, 2007 in the Internet Archive )
- scinexx.de: Deep sea gorges as a gigantic mixing plant
- CSIRO Australia: Craig Macaulay: Ocean robots explain NSW cold water temperatures ( Memento from May 14, 2008 in the Internet Archive )
- Axel Bojanowski : Researchers discover sea waves 200 meters high. In: Spiegel online. May 2015.
- Roland Oberhänsli: Why haven't the oceans been dry long ago? (Page 30) ( Memento from February 15, 2013 in the Internet Archive )
- National Oceanographic Data Center, nodc.noaa.gov: World Ocean Atlas 2005 (various interactive graphics on oxygen saturation according to sea depth and season )
- ifm-geomar.de , Sulamith Antal: The oxygen supply of the ocean ( Memento from January 24, 2009 in the Internet Archive )
- Institute for Chemistry and Biology of the Sea at the University of Oldenburg: Arabian Sea, oxygen minimum zone ( Memento from January 25, 2009 in the Internet Archive )
- Matt McGrath: Oceans running out of oxygen say scientists . December 7, 2019 ( bbc.com [accessed December 7, 2019]).
- Sunke Schmidtko, Lothar Stramma, Martin Visbeck: Decline in global oceanic oxygen content during the past five Decades. In: Nature. 542, 2017, p. 335, doi : 10.1038 / nature21399 . From: spiegel.de , Wissenschaft , February 16, 2017: Less oxygen in the oceans: the fish stay out of breath (February 17, 2017)
- Fiona Harvey: Oceans losing oxygen at unprecedented rate, experts warn . In: The Guardian . December 7, 2019, ISSN 0261-3077 ( theguardian.com [accessed December 7, 2019]).
- scinexx.de: Marine viruses exposed as climate actors , from September 2, 2011.
- NASA Data Shows Hurricanes Help Plants Bloom In 'Ocean Deserts'
- Carl Wirsen: Is Life Thriving Deep Beneath the Seafloor?
- UHNAI-Exploring the Deep Subseafloor
- scinexx.de: teeming life in the oceanic desert
- Antje Lenhart: Ecology of deep sea hydrothermal springs (PDF file; 315 kB)
- ESA : Cassini's new view of land of lakes and seas