The deep sea is usually the largely to completely lightless areas of the sea that are below a depth of at least 200 m. According to this definition, the deep sea covers around 88% of the area of the oceans.
Apart from hydrothermal sources, also known as black smokers , there is no primary biological production in the deep sea , as the absence of light excludes plant life . Despite the apparently very hostile conditions, there is still a diverse animal world , including predators and scavengers, as well as animals that specialize in their diet on symbiotic bacteria or archaea .
The temperature is consistently low (−1 ° C to 4 ° C). At a depth of 10,000 m there is a pressure of around 1,000 bar . Strong currents are rare, and seasonal fluctuations are limited to the amount of detritus that sinks here from the illuminated areas and is the main food source of the deep sea.
The areas of the deep sea lying at a depth of 1,000 m and more cover an area of around 318 million km², which represents around 62% of the total surface of the earth.
In the deep sea or at its edges there are tectonically remarkable zones:
- Mid-ocean ridge , first mountain system of the ocean in - Atlantic discovered
- Deep-sea basins - large and deep basins that exist in all parts of the ocean
- Deep sea trenches - especially so-called trenches located in the Pacific . The (according to a controversial measurement from 1957) 11,034 m deep Witjastief 1 and the (according to measurements from 2010) 10,984 ± 25 m deep Challenger deep (both in the Mariana Trench , Pacific) are considered the deepest known places in the world's oceans.
The ridges and gullies have only been investigated using geophysical methods in the last few decades and have proven to be the "seams of plate tectonics ". Among other things, they make themselves felt in the earth's gravitational field and in the distribution of earthquakes .
The counterpart of the deep sea is the “flat sea”, especially the shelf areas off the continents .
The ocean is divided into two large areas:
In the sea, the pelagic is divided into five zones, according to the morphological division of the water floor:
- The epipelagial are the top 200 meters of the water column of free water (pelagial) in the sea. This illuminated ((eu) photic) deep zone is characterized by positive bioproductivity ( trophogenic zone , i.e. positive material and energy balance, strongest build-up of biomass ) and the highest biodiversity within the ecosystem . In the epipelagial there is sufficient light for higher plants and photoautotrophic microorganisms to carry out photosynthesis . In addition to plankton and the active floating lives here Nekton , so fish , crustaceans and cephalopods . At the bottom, the epipelagial is bounded by the mesopelagial .
- The mesopelagial is the area between approx. 200 meters to approx. 1,000 meters sea depth; from here the actual deep sea begins. A small amount of blue light penetrates this depth. The pressure is around 100 bar at a depth of 1,000 meters. This corresponds to around 1,000 tons per square meter or 100 kilograms per square centimeter. There is no vegetation due to the lack of light, and only a small amount of plankton . The temperature in the depth zone from 500 m to 1500 m suddenly drops from 5 ° C to just over 0 ° C. For example, the deep-sea hatchet fish live in the mesopelagic .
- The bathypelagial ranges from 1,000 to 4,000 meters deep. The pressure is around 400 bar at a depth of 4,000 meters. There is no more sunlight, only fish and bacteria generate light in the form of bioluminescence . Among the living at this depth deep sea fishing also find the deep-sea angler fish .
- The Abyssopelagial (4,000–6,000 m) is the depth zone from 4,000 to 6,000 meters. Here the temperature is close to freezing. In the Abyssopelagial z. B. the deep sea crab .
- The Hadopelagial (6,000–11,000 m) is the deepest zone in the sea and ranges from 6,000 to 11,000 meters, the deepest point in the ocean. As in the abyssopelagial, the temperature is close to freezing point. Nevertheless there are living things here, e.g. B. the bristle worm .
According to some marine biologists , abyssopelagial, bathypelagial and pelagic hadal (also hadopelagial, hadal pelagial, hadopelagic zone) cannot be delimited due to their similar hydrological and biological properties, which is why they are combined into one zone of the deep sea.
- In a trophogenic zone ( nutrient layer , i.e. more oxygen and biomass is produced than consumed)
- In a tropholytic zone ( depletion layer , i.e. less oxygen and biomass is produced than consumed).
The boundary between the two zones is called the compensation level (the biomass built up through photosynthesis is broken down again through respiration processes, the net biogenic production is approximately zero). The remaining amount of light available for photosynthesis is so small that only a small amount of biomass can be built up by the plants ( producers ). This, like the oxygen generated , is completely consumed again by them through respiration. In the greater (i.e. tropholytic) water depths, all the oxygen and all nutrients necessary for the organisms to live must therefore come from the trophogenic layer by means of substance transport, or the inhabitants ascend (vertical migration) and actively fetch the required substances. The actual position of the trophogenic layer and thus also the compensation level depends on the current photosynthetic output, which in turn is influenced by various factors. The light permeability of the water is determined by the local conditions (water turbidity, plankton density ), the photosynthesis output fluctuates in higher latitudes directly with the course of the seasons.
- the bathyal (Gr. bathys 'depth') is the area of the continental slope where the seabed sinks from the shallow area of the continental shelf to the deep sea level and ranges from 200 m to 2,000 / 3,000 m depth.
- the abyssal region ( ancient Greek ἄβυσσος , Latin abyssus 'abyss') is the area of the deep sea basins with a depth of 2,000 / 3,000 m to 6,000 m
- the Hadal Zone (Greek hades , underworld), which includes the deep-sea trenches that extend from 6,000 m to about 10,000 m depth.
In the deep sea, 90 percent of the near-ground biomass consists of sea cucumbers .
There are some ecosystems on the seabed that use inorganic substances for their energy production ( chemosynthesis , chemolithotrophy ). The starting point are chemoautotrophic bacteria that live in symbiosis with beard worms and mussels , on which other animals can feed. These ecosystems arise where water emerges from the soil, which is enriched with methane and hydrogen sulfide , for example . These locations can be found in the area of the mid-ocean ridges as well as in the subduction zones and continental slopes .
In the area of the ridge there are hot springs , where water up to 400 ° C exits, and warm springs , where the hot water begins to cool deeper in the ground and, after emerging from the ground, is only slightly warmer than the surrounding cold sea water . The warm thermal springs can also be created by an exothermic chemical process, serpentinization , in which peridotite is converted to serpentinite by absorbing large amounts of seawater , which also generates methane (CH 4 ), hydrogen sulfide (H 2 S) and hydrogen gas (H 2 ) can be. This mechanism is not tied to the mid-ocean ridges, so that such thermal springs can be observed, for example, in the spreading zone of a backarc basin . Such a source was first discovered in 2000 ( Lost City ). The heated water creates so-called black and white smokers , on average 20 m to 25 m high chimneys, which consist of precipitated minerals.
Cold springs can be found in subduction zones and continental slopes (see methane spring or Cold Seep ), whereby the water emerging from the ground is not heated. The ecosystem found at a cold spring is also based on the symbiosis with chemoautotrophic bacteria, but since these occur both at a depth of several thousand meters and on the edge of the deep sea on the continental slopes at a depth of several hundred meters, the range of species to be found can be larger. The cold spring area is characterized by the fact that calcium carbonate is deposited in crusts and that methane hydrates can be found.
The history of deep sea research is relatively young, since the extreme conditions prevailing in the deep sea place enormous technical demands on people.
- 1521: Ferdinand Magellan lets a 700 m long rope from his ship down into the sea to sound out the oceans. Since it does not reach the bottom, he concludes that the sea is infinitely deep.
- 1818: Life is detected in the deep sea for the first time. The English researcher Sir John Ross uses a kind of gripping device to bring worm and jellyfish species on board from a depth of 2000 m.
- 1844: Although John Ross has detected living things, Edward Forbes contradicts him, stating that the number of living things decreases with depth. Therefore there could be no life from a depth of 600 m (abyss theory).
- 1850: Michael Sars finds a rich animal world at a depth of 800 m in the Lofoten Islands and thus refutes the abyssal theory.
- In 1860 a telegraphic cable that had been laid only three years earlier was recovered in the Mediterranean. Several animal species have already settled in places that were 2000 m deep. That counts as final evidence.
- 1872–1876: A first expedition to systematically explore the deep sea with the corvette HMS Challenger under the direction of marine biologist Charles Wyville Thomson brings many new results.
- 1890–1898: During the Austro-Hungarian Pola expeditions under the scientific direction of Franz Steindachner , the deep sea in the eastern Mediterranean, the Adriatic and the Red Sea is explored.
- 1898–1899: The German Valdivia expedition led by the zoologist Carl Chun delivers, among other things, rich animal material from depths of more than 4000 m off the coast of Antarctica .
- 1930: People reach the “deep sea” for the first time. William Beebe and Otis Barton dive with a steel ball with a porthole, the bathysphere , 435 m into the depth and are there surrounded by jellyfish and shrimp .
- 1934: The Bathysphere reaches a depth of 923 m.
- 1948: Otis Barton reaches a depth of 1370 m for the first time and breaks the record of 1934.
- 1960: Jacques Piccard and Don Walsh manage to dive with the Trieste to the Challenger Deep in the Mariana Trench, one of the deepest points in the sea. Even at a depth of 10,740 m, they still discover fish and other living beings in shapes that are quite strange for our eyes (e.g. with very large mouths and luminous organs, adapted to the special living environment of these sea depths).
- 2012: The Deepsea Challenger , manned exclusively by James Cameron , dives to the bottom of the Challenger Deep. After the first exploration in 1960, it is the second manned and first-time 1-person exploration of the sea at this depth.
- 2019: Between April 28, 2019 and May 7, 2019, the Limiting Factor diving boat made four dives in Challenger Deep and one dive in Sirena Deep .
Importance of the deep sea
Oceans can be roughly divided into the near-surface layers and the deep sea. While the former react with fluctuations in currents, temperatures and salinity within weeks and months due to the direct coupling to the rapidly changing atmospheric conditions, changes in the deep sea are caused and played out by fluctuations in surface conditions in limited areas of polar and subpolar latitudes because of the enormous masses of water involved in periods of many decades to centuries. The deep sea plays an important role, especially with regard to anthropogenic climate change , especially when it comes to questions relating to global climate change .
The importance of the polar or sub-polar areas is based on the density-related anomaly of the water (greatest density at approx. 4 ° C) or its modification by the salinity of the oceans. The salt content of the oceans averages around 34.7 ‰, which significantly changes the properties of the water. The temperature of the density maximum shifts at an average salinity of 34.7 ‰ to −3.8 ° C and thus falls below the freezing point of −1.9 ° C. As a result, when the sea cools down until the ice begins to form, a convection movement occurs : cooled (and therefore denser) water sinks, warmer (and less dense) rises from deeper layers. The warmer water releases its heat content into the atmosphere and sinks again when it cools down again. The water absorbs atmospheric gases (e.g. carbon dioxide ) and thus ensures that they are transported into the deep sea. For this reason, the convection areas are also those marine areas in which the highest proportions of anthropogenic carbon dioxide can be found.
In addition to these vertical convection movements, horizontal ocean currents also play an important role in the deep sea (see also: global conveyor belt ). Depending on the current surface conditions, cold water masses of different characteristics arise, which can be traced along their distribution routes in the deep sea. These cold water masses can be distinguished by their temperature, salinity and density values, the oxygen content or the content of anthropogenic trace gases from their area of origin. The deepest waters are predominantly Antarctic origin, they are also called " Antarctic bottom water , called", and in English as "Antarctic Bottom Water" (AABW). The water masses of arctic origin have a somewhat lower density. These are called " North Atlantic Deep Water , referred to" or in English as "North Atlantic Deep Water" (NADW) which is as powerful intermediate layer above the ground water.
Although the deep sea takes up most of our planet, less is known about it than about the surface of the moon. This is due to their relative inaccessibility: Few countries have landers , deep-sea submarines or ships large enough to retrieve samples from the deep sea. Sampling at a depth of 8000 m already requires 11 km of cables. It also takes 24 hours to lower a device to this depth and to bring it up again. The equipment and ships are very expensive, a large research ship costs several tens of thousands of euros per day. Animals whose behavior is to be investigated must also be observed in their habitat or brought up in special pressure vessels, as they would not survive the enormous changes in pressure alive. Also, due to the scarcity of food, deep-sea animals are usually not very numerous, so many samples are required to even detect a species.
The deep sea in art and literature
The fascination of the unknown is also evident in literature, film and music:
- In 1870 the novel 20,000 Leagues Under the Sea by Jules Verne was published for the first time . Verne's book was made into a film several times, most recently in the 1990s. The 1954 film adaptation received an Oscar for best visual effects the following year .
- The 1989 film Abyss is set in the deep sea. He also received an Oscar in 1990 for his visual effects and was nominated for three more.
- Frank Schätzing treats the unexplored depths of the seas in his novel The Swarm . In it he describes so-called Yrr - fictional deep-sea creatures of unknown intelligence, far superior to human ones.
- The German-Swiss progressive metal band The Ocean released the concept album Pelagial in 2013 , in which they set a journey from the surface of the sea to the bottom of the deep sea. The individual songs are divided into the five zones of the pelagial .
- Heat content of the oceans
- Deep sea fishing
- Epibenthos sledge
- CTD rosette
- International Seabed Authority
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