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Ocean floor area where hydrogen sulfide, methane and other hydrocarbon rich fluid seepage occurs

A cold seep (sometimes called a cold vent) is an area of the ocean floor where seepage of fluids rich in hydrogen sulfide, methane, and other hydrocarbons occurs, often in the form of a brine pool. Cold does not mean that the temperature of the seepage is lower than that of the surrounding sea water; on the contrary, its temperature is often slightly higher. The "cold" is relative to the very warm (at least 60 °C (140 °F)) conditions of a hydrothermal vent. Cold seeps constitute a biome supporting several endemic species. Cold seeps develop unique topography over time, where reactions between methane and seawater create carbonate rock formations and reefs. These reactions may also be dependent on bacterial activity. Ikaite, a hydrous calcium carbonate, can be associated with oxidizing methane at cold seeps.

Types of cold seeps can be distinguished according to the depth, as shallow cold seeps and deep cold seeps. The consumption of methane by aerobic and anaerobic seafloor life is called “the benthic filter”. The first part of this filter is the anaerobic bacteria and archaea underneath the seafloor that consume methane through the anaerobic oxidation of methane (AOM).

Cold seeps were discovered in 1983 by Charles Paull and colleagues on the Florida Escarpment in the Gulf of Mexico at a depth of Since then, seeps have been discovered in many other parts of the world's oceans. Most have been grouped into five biogeographic provinces: Gulf of Mexico, Atlantic, Mediterranean, East Pacific, and West Pacific, the Arctic Ocean, the North Sea, Skagerrak, Kattegat, the Gulf of California, the Red Sea, the Indian Ocean, off southern Australia, and in the inland Caspian Sea. In the Pacific Northwest, a cold seep called Pythia's Oasis was discovered in 2015. With the recent discovery of a methane seep in the Southern Ocean, cold seeps are now known in all major oceans. in Kattegat, the methane seeps are known as "bubbling reefs" and are typically at depths of , and off northern California, they can be found as shallow as
In addition to cold seeps existing today, the fossil remains of ancient seep systems have been found in several parts of the world. Some of these are located far inland in places formerly covered by prehistoric oceans.

The chemosynthetic communities of the Gulf of Mexico have been studied extensively since the 1990s, and communities first discovered on the upper slope are likely the best understood seep communities in the world. The history of the discovery of these remarkable animals has all occurred since the 1980s. Each major discovery was unexpected―from the first hydrothermal vent communities anywhere in the world to the first cold seep communities in the Gulf of Mexico. Very slow seepage sites do not support complex chemosynthetic communities; rather, they usually only support simple microbial mats (Beggiatoa sp.). More than 50 communities are now known to exist in 43 Outer Continental Shelf (OCS) blocks. Results confirmed extensive natural oil seepage in the Gulf of Mexico, especially in water depths greater than 1,000 m (3,281 ft). It is hypothesized (MacDonald, 1998b) that the dynamics of hydrate alteration could play a major role as a mechanism for regulation of the release of hydrocarbon gases to fuel biogeochemical processes and could also play a substantial role in community stability. Recorded bottom water temperature excursions of several degrees in some areas such as the Bush Hill site (4–5 °C at 500 m (1,640 ft) depth) are believed to result in dissociation of hydrates, resulting in an increase in gas fluxes (MacDonald et al., 1994). Although not as destructive as the volcanism at vent sites of the mid ocean ridges, the dynamics of shallow hydrate formation and movement will clearly affect sessile animals that form part of the seepage barrier. There is potential of a catastrophic event where an entire layer of shallow hydrate could break free of the bottom and considerably affect local communities of chemosynthetic fauna. The chemosymbiotic bivalves collected from the mud volcanoes of the Gulf of Cadiz were reviewed in 2011. Exploration of new areas, such as potential seep sites off of the east coast of the U. S. and the Laurentian fan where chemosynthetic communities are known deeper than , and shallower sites in the Gulf of Guinea are need to study in the future. Moreover, the study of symbioses revealed associations with chemoautotrophic bacteria, sulfur oxidizers in Vesicomyidae and Lucinidae bivalves and Siboglinidae tubeworms, and highlighted the exceptional diversity of bacteria living in symbiosis with small Mytilidae. The Mediterranean seeps appear to represent a rich habitat characterized by megafauna species richness (e. g., gastropods) or the exceptional size of some species such as sponges (Rhizaxinella pyrifera) and crabs (Chaceon mediterraneus), compared with their background counterparts. This contrasts with the low macro and mega faunal abundance and diversity of the deep eastern Mediterranean. Seep communities in the Mediterranean that include endemic chemosynthetic species and associated fauna differ from the other known seep communities in the world at the species level but also by the absence of the large size bivalve genera Calyptogena or Bathymodiolus. The isolation of the Mediterranean seeps from the Atlantic Ocean after the Messinian crisis led to the development of unique communities, which are likely to differ in composition and structure from those in the Atlantic Ocean. Further expeditions involved quantitative sampling of habitats in different areas, from the Mediterranean Ridge to the eastern Nile deep sea fan. have also revealed chemosynthesis based communities that showed a considerable similarity to the symbiont bearing fauna of eastern Mediterranean cold seeps.

Native aluminium has been reported also in cold seeps in the northeastern continental slope of the South China Sea and Chen et al. (2011)

With continuing experience, particularly on the upper continental slope in the Gulf of Mexico, the successful prediction of the presence of tubeworm communities continues to improve; however, chemosynthetic communities cannot be reliably detected directly using geophysical techniques. Hydrocarbon seeps that allow chemosynthetic communities (Guaymas Basin) to exist do modify the geological characteristics in ways that can be remotely detected, but the time scales of co occurring active seepage and the presence of living communities is always uncertain. These known sediment modifications include (1) precipitation of authigenic carbonate in the form of micronodules, nodules, or rock masses; (2) formation of gas hydrates; (3) modification of sediment composition through concentration of hard chemosynthetic organism remains (such as shell fragments and layers); (4) formation of interstitial gas bubbles or hydrocarbons; and (5) formation of depressions or pockmarks by gas expulsion. These features give rise to acoustic effects such as wipeout zones (no echoes), hard bottoms (strongly reflective echoes), bright spots (reflection enhanced layers), or reverberant layers (Behrens, 1988; Roberts and Neurauter, 1990). Potential locations for most types of communities can be determined by careful interpretation of these various geophysical modifications, but to date, the process remains imperfect and confirmation of living communities requires direct visual techniques. the Cretaceous of Hokkaido, the Palaeogene of Honshu, the Neogene of Northern Italy, and the Pleistocene of California. These fossil cold seeps are characterized by mound like topography (where preserved), coarsely crystalline carbonates, and abundant mollusks and brachiopods.

Major threats that cold seep ecosystems and their communities face today are seafloor litter, chemical contaminants, and climate change. Seafloor litter alters the habitat by providing hard substrate where none was available before or by overlying the sediment, thereby inhibiting gas exchange and interfering with organisms on the bottom of the sea. Studies of marine litter in the Mediterranean include surveys of seabed debris on the continental shelf, slope, and bathyal plain. In most studies, plastic items accounted for much of the debris, sometimes as much as 90% or more of the total, owing to their ubiquitous use and poor degradability. Weapons and bombs have also been discarded at sea, and their dumping in open waters contributes to seafloor contamination. Another major threat to the benthic fauna is the presence of lost fishing gear, such as nets and longlines, which contribute to ghost fishing and can damage fragile ecosystems such as cold water corals. Chemical contaminants such as persistent organic pollutants, toxic metals (e. g., Hg, Cd, Pb, Ni), radioactive compounds, pesticides, herbicides, and pharmaceuticals are also accumulating in deep sea sediments. Topography (such as canyons) and hydrography (such as cascading events) play a major role in the transportation and accumulation of these chemicals from the coast and shelf to the deep basins, affecting the local fauna. Recent studies have detected the presence of significant levels of dioxins in the commercial shrimp Aristeus antennatus and significant levels of persistent organic pollutants in mesopelagic and bathypelagic cephalopods. Climate driven processes and climate change will affect the frequency and intensity of cascading, with unknown effects on the benthic fauna. Another potential effect of climate change is related to energy transport from surface waters to the seafloor. Primary production will change in the surface layers according to sun exposure, water temperature, major stratification of water masses, and other effects, and this will affect the food chain down to the deep seafloor, which will be subject to differences in quantity, quality, and timing of organic matter input. As commercial fisheries move into deeper waters, all of these effects will affect the communities and populations of organisms in cold seeps and the deep sea in general.