| Marine habitats |
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| Coastal habitats |
| Ocean surface |
| Open ocean |
| Sea floor |
Theseabed (also known as theseafloor,sea floor,ocean floor, andocean bottom) is the bottom of theocean. All floors of the ocean are known as seabeds.
The structure of the seabed of the global ocean is governed byplate tectonics. Most of the ocean is very deep, where the seabed is known as theabyssal plain. Seafloor spreading creates mid-ocean ridges along the center line of major ocean basins, where the seabed is slightly shallower than the surrounding abyssal plain. From the abyssal plain, the seabed slopes upward toward the continents and becomes, in order from deep to shallow, thecontinental rise,slope, andshelf. The depth within the seabed itself, such as the depth down through asediment core, is known as the "depth below seafloor". The ecological environment of the seabed and the deepest waters are collectively known, as ahabitat for creatures, as the "benthos".
Most of the seabed throughout the world's oceans is covered in layers ofmarine sediments. Categorized by where the materials come from or composition, these sediments are classified as either: from land (terrigenous), from biological organisms (biogenous), from chemical reactions (hydrogenous), and from space (cosmogenous). Categorized by size, these sediments range from very small particles calledclays andsilts, known as mud, to larger particles fromsand toboulders.
Features of the seabed are governed by the physics ofsediment transport and by the biology of the creatures living in the seabed and in the ocean waters above. Physically, seabed sediments often come from theerosion of material on land and from other rarer sources, such asvolcanic ash. Sea currents transport sediments, especially in shallow waters wheretidal energy andwave energy cause resuspension of seabed sediments. Biologically, microorganisms living within the seabed sediments change seabed chemistry. Marine organisms create sediments, both within the seabed and in the water above. For example,phytoplankton with silicate or calcium carbonate shells grow in abundance in the upper ocean, and when they die, their shells sink to the seafloor to become seabed sediments.
Human impacts on the seabed are diverse. Examples of human effects on the seabed include exploration, plastic pollution, and exploitation by mining anddredging operations. To map the seabed, ships useacoustic technology to map water depths throughout the world. Submersible vehicles help researchers study unique seabed ecosystems such ashydrothermal vents.Plastic pollution is a global phenomenon, and because the ocean is the ultimate destination for global waterways, much of the world's plastic ends up in the ocean and some sinks to the seabed. Exploitation of the seabed involves extracting valuable minerals fromsulfide deposits via deep sea mining, as well as dredging sand from shallow environments for construction andbeach nourishment.
Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources. The structure of the oceans, starting with the continents, begins usually with acontinental shelf, continues to thecontinental slope – which is a steep descent into the ocean, until reaching theabyssal plain – a topographicplain, the beginning of the seabed, and its main area. The border between the continental slope and the abyssal plain usually has a more gradual descent, and is called thecontinental rise, which is caused bysediment cascading down the continental slope.[citation needed]
Themid-ocean ridge, as its name implies, is a mountainous rise through the middle of all the oceans, between the continents. Typically arift runs along the edge of this ridge. Alongtectonic plate edges there are typicallyoceanic trenches – deep valleys, created by the mantle circulation movement from the mid-ocean mountain ridge to the oceanic trench.[1]
Hotspot volcanic island ridges are created by volcanic activity, erupting periodically, as the tectonic plates pass over a hotspot. In areas with volcanic activity and in the oceanic trenches there arehydrothermal vents – releasing high pressure and extremely hot water and chemicals into the typically freezing water around it.
Deep ocean water is divided into layers or zones, each with typical features of salinity, pressure, temperature andmarine life, according to their depth. Lying along the top of theabyssal plain is theabyssal zone, whose lower boundary lies at about 6,000 m (20,000 ft). Thehadal zone – which includes the oceanic trenches, lies between 6,000 and 11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.[2][3]
Depth below seafloor is avertical coordinate used in geology,paleontology,oceanography, andpetrology (seeocean drilling).Theacronym "mbsf" (meaning "meters below the seafloor") is a common convention used for depths below the seafloor.[4][5]
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Sediments in the seabed vary in origin, from eroded land materials carried into the ocean by rivers or wind flow, waste and decompositions of sea creatures, and precipitation of chemicals within the sea water itself, including some from outer space. There are four basic types of sediment of the sea floor:
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Terrigenous sediment is the most abundant sediment found on the seafloor. Terrigenous sediments come from the continents. These materials are eroded from continents and transported by wind and water to the ocean. Fluvial sediments are transported from land by rivers and glaciers, such as clay, silt, mud, and glacial flour. Aeolian sediments are transported by wind, such as dust and volcanic ash.[7]
Biogenous sediment is the next most abundant material on the seafloor. Biogenous sediments are biologically produced by living creatures. Sediments made up of at least 30% biogenous material are called "oozes." There are two types of oozes: Calcareous oozes and Siliceous oozes.Plankton grow in ocean waters and create the materials that become oozes on the seabed. Calcareous oozes are predominantly composed of calcium shells found in phytoplankton such as coccolithophores and zooplankton like the foraminiferans. These calcareous oozes are never found deeper than about 4,000 to 5,000 meters because at further depths the calcium dissolves.[8] Similarly, Siliceous oozes are dominated by the siliceous shells of phytoplankton like diatoms and zooplankton such as radiolarians. Depending on the productivity of these planktonic organisms, the shell material that collects when these organisms die may build up at a rate anywhere from 1 mm to 1 cm every 1000 years.[8]
Hydrogenous sediments are uncommon. They only occur with changes in oceanic conditions such as temperature and pressure. Rarer still are cosmogenous sediments. Hydrogenous sediments are formed from dissolved chemicals that precipitate from the ocean water, or along the mid-ocean ridges, they can form by metallic elements binding onto rocks that have water of more than 300 °C circulating around them. When these elements mix with the cold sea water they precipitate from the cooling water.[8] Known asmanganese nodules, they are composed of layers of different metals like manganese, iron, nickel, cobalt, and copper, and they are always found on the surface of the ocean floor.[8]
Cosmogenous sediments are the remains of space debris such as comets and asteroids, made up of silicates and various metals that have impacted the Earth.[9]
Another way that sediments are described is through their descriptive classification. These sediments vary in size, anywhere from 1/4096 of a mm to greater than 256 mm. The different types are: boulder, cobble, pebble, granule, sand, silt, and clay, each type becoming finer in grain. The grain size indicates the type of sediment and the environment in which it was created. Larger grains sink faster and can only be pushed by rapid flowing water (high energy environment) whereas small grains sink very slowly and can be suspended by slight water movement, accumulating in conditions where water is not moving so quickly.[11] This means that larger grains of sediment may come together in higher energy conditions and smaller grains in lower energy conditions.
Benthos (from Ancient Greek βένθος (bénthos) 'the depths [of the sea]'), also known as benthon, is thecommunity oforganisms that live on, in, or near the bottom of a sea,river,lake, orstream, also known as thebenthic zone.[12] This community lives in or near marine or freshwatersedimentary environments, fromtidal pools along theforeshore, out to thecontinental shelf, and then down to theabyssal depths.
Light is an important energy source for shallow benthic systems. However, because light isabsorbed before it can reach deep ocean water, the energy source for deep benthic ecosystems is often organic matter from higher up in the water column that drifts down to the depths. Thisdead and decaying matter sustains the benthicfood chain; most organisms in the benthic zone arescavengers ordetritivores.
Many organisms adapted to deep-water pressure cannot survive in the upper parts of thewater column. The pressure difference can be significant (approximately oneatmosphere for every 10 metres of water depth).[13]
The termbenthos, coined byHaeckel in 1891,[14] comes from theGreek nounβένθος 'depth of the sea'.[12][15]Benthos is used infreshwater biology to refer to organisms at the bottom of freshwaterbodies of water, such as lakes, rivers, and streams.[16] There is also a redundant synonym,benthon.[17]
Seabed topography (ocean topography ormarine topography) refers to the shape of the land (topography) when it interfaces with the ocean. These shapes are obvious along coastlines, but they occur also in significant ways underwater. The effectiveness of marine habitats is partially defined by these shapes, including the way they interact with and shapeocean currents, and the way sunlight diminishes when these landforms occupy increasing depths. Tidal networks depend on the balance between sedimentary processes and hydrodynamics however, anthropogenic influences can impact the natural system more than any physical driver.[18]
Marine topographies includecoastal and oceanic landforms ranging from coastalestuaries andshorelines tocontinental shelves andcoral reefs. Further out in the open ocean, they include underwater anddeep sea features such as ocean rises andseamounts. The submerged surface has mountainous features, including a globe-spanningmid-ocean ridge system, as well as underseavolcanoes,[19]oceanic trenches,submarine canyons,oceanic plateaus andabyssal plains.
The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[20]
| Depth Range (meters)[21] | Seafloor Area (km²) | Seafloor Percentage |
|---|---|---|
| 0 – 200 | 26,402,000 | 7.30% |
| 201 – 1000 | 15,848,000 | 4.38% |
| 1001 – 4000 | 127,423,000 | 35.22% |
| 4001 – 6000 | 188,395,000 | 52.08% |
| 6001 – 7000 | 3,207,000 | 0.89% |
| 7001 – 8000 | 320,000 | 0.09% |
| 8001 – 9000 | 111,000 | 0.03% |
| 9000 – 10,000 | 37,000 | 0.01% |
| 10,000 + | 2,000 | < 0.01% |
Each region of the seabed has typical features such as common sediment composition, typical topography, salinity of water layers above it, marine life, magnetic direction of rocks, andsedimentation. Some features of the seabed include flatabyssal plains,mid-ocean ridges, deeptrenches, andhydrothermal vents.
Seabed topography is flat where layers of sediments cover the tectonic features. For example, theabyssal plain regions of the ocean are relatively flat and covered in many layers of sediments.[22] Sediments in these flat areas come from various sources, including but not limited to: land erosion sediments from rivers, chemically precipitated sediments from hydrothermal vents,Microorganism activity,sea currents eroding the seabed and transporting sediments to the deeper ocean, andphytoplankton shell materials.
Where the seafloor is actively spreading and sedimentation is relatively light, such as in the northern and easternAtlantic Ocean, the original tectonic activity can be clearly seen as straight line "cracks" or "vents" thousands of kilometers long. These underwater mountain ranges are known asmid-ocean ridges.[6]
Other seabed environments include hydrothermal vents, cold seeps, and shallow areas. Marine life is abundant in thedeep sea aroundhydrothermal vents.[23] Largedeep sea communities of marine life have been discovered aroundblack and white smokers – vents emitting chemicals toxic to humans and mostvertebrates. This marine life receives its energy both from the extreme temperature difference (typically a drop of 150 degrees) and fromchemosynthesis bybacteria.Brine pools are another seabed feature,[24] usually connected tocold seeps. In shallow areas, the seabed can host sediments created bymarine life such as corals, fish, algae, crabs, marine plants and other organisms.
The seabed has been explored by submersibles such asAlvin and, to some extent,scuba divers with special equipment. Hydrothermal vents were discovered in 1977 by researchers using an underwater camera platform.[23] In recent years satellite measurements ofocean surface topography show very clearmaps of the seabed,[25] and these satellite-derived maps are used extensively in the study and exploration of the ocean floor.
In 2020 scientists created what may be the first scientific estimate of how muchmicroplastic currently resides in Earth'sseafloor, after investigating six areas of ~3 km depth ~300 km off the Australian coast. They found the highly variable microplastic counts to be proportionate to plastic on the surface and the angle of the seafloor slope. By averaging the microplastic mass per cm3, they estimated that Earth's seafloor contains ~14 million tons of microplastic – about double the amount they estimated based on data from earlier studies – despite calling both estimates "conservative" as coastal areas are known to contain much moremicroplastic pollution. These estimates are about one to two times the amount of plastic thought – per Jambeck et al., 2015 – to currently enter the oceans annually.[26][27][28]
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Deep sea mining is the extraction of minerals from the seabed of thedeep sea. The main ores of commercial interest arepolymetallic nodules, which are found at depths of 4–6 km (2.5–3.7 mi) primarily on theabyssal plain. TheClarion–Clipperton zone (CCZ) alone contains over 21 billion metric tons of these nodules, with minerals such ascopper,nickel,cobalt andmanganese making up roughly 30% of their weight.[30] It is estimated that the global ocean floor holds more than 120 million tons of cobalt, five times the amount found in terrestrial reserves.[31]
As of July 2024[update], only exploratory licenses have been issued, with no commercial-scale deep sea mining operations yet. TheInternational Seabed Authority (ISA) regulates all mineral-related activities ininternational waters and has granted 31 exploration licenses so far: 19 for polymetallic nodules, mostly in the CCZ; 7 for polymetallic sulphides inmid-ocean ridges; and 5 for cobalt-rich crusts in theWestern Pacific Ocean.[32] There is a push for deep sea mining to commence by 2025, when regulations by the ISA are expected to be completed.[33][34]
In April 2025, U.S. President Trump signed an Executive Order instructing theNational Oceanic and Atmospheric Administration to expedite permits for companies to mine in both international and U.S. territorial waters, citing the Deep Seabed Hard Minerals Resource Act of 1980.[35]
Deep sea mining is being considered in theexclusive economic zone (EEZ) of countries, such asNorway, where in January 2024 the government announced its intention to allow companies to apply for exploration permits in 2025. In December 2024, Norway's plans to begin awarding exploration licenses were temporarily put on hold after theSocialist Left Party (SV) blocked the planned licensing round as part of negotiations over the government budget.[36][37] In 2022, theCook Islands Seabed Minerals Authority (SBMA) granted three exploration licenses for cobalt-rich polymetallic nodules within their EEZ.[38] In 2025, it was announced that theCook Islands had signed a deal withChina focused on deep-sea mining.[39]Papua New Guinea was the first country to approve a deep sea mining permit in state waters for the Solwara 1 project, despite three independent reviews highlighting significant gaps and flaws in the environmental impact statement.[40]
The most common commercial model of deep sea mining proposed involves a caterpillar-track hydraulic collector and a riser lift system bringing the harvested ore to a production support vessel withdynamic positioning, and then depositing extra discharge down the water column below 2,000 meters. Related technologies include robotic mining machines, as surface ships, and offshore and onshore metal refineries.[41][42] Though largely composed of nickel and manganese which are most widely used as key inputs into the steel industry, wind farms, solar energy,electric vehicles, and battery technologies use many of the deep-sea metals.[41]Electric vehicle batteries are a key driver of the critical metals demand that incentivizes deep sea mining, as well as demands for the production of aerospace and defense technologies, and infrastructure.[43][44]
Theenvironmental impact of deep sea mining is controversial.[45][46] Environmental advocacy groups such asGreenpeace and the Deep Sea Mining Campaign[47] claimed that seabed mining has the potential to damagedeep sea ecosystems and spread pollution from heavy metal-laden plumes.[48] Critics have called for moratoria[49][50] or permanent bans.[51] Opposition campaigns enlisted the support of some industry figures, including firms reliant on the target metals. Individual countries like Norway, Cook Islands, India, Brazil and others with significant deposits within theirexclusive economic zones (EEZ's) are exploring the subject.[52][53]
As of 2021, the majority of marine mining used dredging operations in far shallower depths of less than 200 m, wheresand, silt and mud for construction purposes is abundant, along withmineral rich sands containingilmenite and diamonds.[54][55]On and under the seabed are archaeological sites of historic interest, such as shipwrecks and sunken towns. This underwater cultural heritage is protected by theUNESCO Convention on the Protection of the Underwater Cultural Heritage. The convention aims at preventing looting and the destruction or loss of historic and cultural information by providing an international legal framework.[56]
we follow Ocean Drilling Program (ODP) meters below seafloor (mbsf) convention
metres below the seafloor (mbsf)
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