Cold seep

Source: Wikipedia, the free encyclopedia.

Lamellibrachia luymesi1.png
Tube worms
are among the dominant species in one of four cold seep community types in the Gulf of Mexico.

A cold seep (sometimes called a cold vent) is an area of the

ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage 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.[1] The "cold" is relative to the very warm (at least 60 °C or 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

Main articles: Petroleum seep § Offshore seeps, brine pool, pockmark (geology), and mud volcano
These craters mark the formation of brine pools
, from which salt has seeped through the seafloor and encrusted the nearby substrate.

Types of cold seeps can be distinguished according to the depth, as shallow cold seeps and deep cold seeps.[2] Cold seeps can also be distinguished in detail, as follows:

Formation and ecological succession

Cold seeps occur over fissures on the seafloor caused by

tectonic activity. Oil and methane "seep" out of those fissures, get diffused by sediment, and emerge over an area several hundred meters wide.[3]

Methane (CH
4) is the main component of what we commonly refer to as

heterotrophic and symbiont-associated) and background fauna.[4]

Chemosynthetic communities

Blake Ridge
, off South Carolina. The red dots are range-finding laser beams.

Biological research in cold seeps and hydrothermal vents has been mostly focused on the

meiofauna (<1 mm).[2]

Community composition's orderly shift from one set of species to another is called ecological succession:[3]

The first type of organism to take advantage of this deep-sea energy source is

bacterial mats at cold seeps, these bacteria metabolize methane and hydrogen sulfide (another gas that emerges from seeps) for energy.[3] This process of obtaining energy from chemicals is known as chemosynthesis.[3]

A mussel bed at the edge of the brine pool

During this initial stage, when methane is relatively abundant, dense

symbiotic bacteria that also produce energy from methane, similar to their relatives that form mats.[3] Chemosynthetic bivalves are prominent constituents of the fauna of cold seeps and are represented in that setting by five families: Solemyidae, Lucinidae, Vesicomyidae, Thyasiridae and Mytilidae.[5]

This microbial activity produces

seafloor and forms a layer of rock.[3] During a period lasting up to several decades, these rock formations attract siboglinid tubeworms, which settle and grow along with the mussels.[3] Like the mussels, tubeworms rely on chemosynthetic bacteria (in this case, a type that needs hydrogen sulfide instead of methane) for survival.[3] True to any symbiotic relationship, a tubeworm also provides for their bacteria by appropriating hydrogen sulfide from the environment.[3] The sulfide not only comes from the water, but is also mined from the sediment through an extensive "root" system a tubeworm "bush" establishes in the hard, carbonate substrate.[3] A tubeworm bush can contain hundreds of individual worms, which can grow a meter or more above the sediment.[3]

Cold seeps do not last indefinitely. As the rate of gas seepage slowly decreases, the shorter-lived, methane-hungry mussels (or more precisely, their methane-hungry bacterial symbionts) start to die off.[3] At this stage, tubeworms become the dominant organism in a seep community.[3] As long as there is some sulfide in the sediment, the sulfide-mining tubeworms can persist.[3] Individuals of one tubeworm species Lamellibrachia luymesi have been estimated to live for over 250 years in such conditions.[3]

"Roots" of tubeworms also provide a supply of hydrogen sulfide from the sediment to the bacteria inside these tubeworms.
Symbiotic vestimentiferan tubeworm Lamellibrachia luymesi from a cold seep at 550 m depth in the Gulf of Mexico. In the sediments around the base are orange bacterial mats of the sulfide-oxidizing bacteria Beggiatoa spp. and empty shells of various clams and snails, which are also common inhabitants of the seeps.[6]
Tubeworms, soft corals and chemosynthetic mussels at a seep located 3,000 m (9,800 ft) down on the Florida Escarpment. Eelpouts, a Galatheid crab and an alvinocarid
shrimp feed on mussels damaged during a sampling exercise.

The Benthic Filter

The organisms living at cold seeps have a large impact on the carbon cycle and on climate. Chemosynthetic organisms, specifically methanogenic (methane-consuming) organisms, prohibit the methane seeping up from beneath the seafloor from being released into the water above. Since methane is such a potent greenhouse gas, methane release could cause global warming, as hypothesized in earth’s past when gas hydrate reservoirs destabilize.[7] The consumption of methane by aerobic and anaerobic seafloor life is called “the benthic filter”.[8] 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).[8] If the flux of methane flowing through the sediment is too large, and the anaerobic bacteria and archaea are consuming the maximum amount of methane, then the excess methane is consumed by free-floating or symbiotic aerobic bacteria above the sediment at the seafloor. The symbiotic bacteria have been found in organisms such as tube worms and clams living at cold seeps; these organisms provide oxygen to the aerobic bacteria as the bacteria provide energy they obtain from the consumption of methane.

The benthic filter reduces the flux of methane from the seafloor to the above water column

Understanding how efficient the benthic filter is can help predict how much methane escapes the seafloor at cold seeps and enters the water column and eventually the atmosphere. Studies have shown that 50-90% of methane is consumed at cold seeps with bacterial mats. Areas with clam beds have less than 15% of methane escaping.[7] Efficiency is determined by a number of factors. The benthic layer is more efficient with low flow of methane, and efficiency decreases as methane flow or the speed of flow increases.[8] Oxygen demand for cold seep ecosystems is much higher than other benthic ecosystems, so if the bottom water does not have enough oxygen, the efficiency of aerobic microbes in removing methane is reduced.[7] The benthic filter cannot affect methane that is not traveling through the sediment. Methane can bypass the benthic filter if they bubble to the surface or travel through cracks and fissures in the sediment.[7] These organisms are the only biological sink of methane in the ocean.[8]

Comparison with other communities

Main articles:
Deep sea communities and Hydrothermal vent
Lamellibrachia
tube worms and mussel at a cold seep

Cold seeps and

taxa among highly sulphidic habitats.[4]

However, hydrothermal vents and cold seeps differ also in many ways. Compared to the more stable cold seeps, vents are characterized by locally high temperatures, strongly fluctuating temperatures, pH, sulfide and oxygen concentrations, often the absence of sediments, a relatively young age, and often unpredictable conditions, such as waxing and waning of vent fluids or volcanic eruptions.

ephemeral
environments, cold seeps emit at a slow and dependable rate. Likely owing to the cooler temperatures and stability, many cold seep organisms are much longer-lived than those inhabiting hydrothermal vents.

End of cold seep community

Main article:
Deep water coral

Finally, as cold seeps become inactive, tubeworms also start to disappear, clearing the way for

deep water coral reefs is called hydraulic theory.[9][10]

Distribution

Cold seeps were discovered in 1983 by Charles Paull and colleagues on the Florida Escarpment in the

hadal depths.[4] In Chile cold seeps are known from the intertidal zone,[17] in Kattegat the methane seeps are known as "bubbling reefs" and are typically at depths of 0–30 m (0–100 ft),[18] and off northern California they can be found as shallow as 35–55 m (115–180 ft).[14] Most cold seeps are located considerably deeper, well beyond the reach of ordinary scuba diving, and the deepest seep community known is found in the Japan Trench at a depth of 7,326 m (24,035 ft).[19]

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.[14][20]

In the Gulf of Mexico

The manned submersible DSV Alvin
, which made possible the discovery of chemosynthetic communities in the Gulf of Mexico in 1983.

Discoveries

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.[21]

Communities were discovered in the Eastern Gulf of Mexico in 1983 using the manned submersible

benthic ecology
(until this investigation, all effects of oil seepage were assumed to be detrimental), bottom trawls unexpectedly recovered extensive collections of chemosynthetic organisms, including tube worms and clams (Kennicutt et al., 1985). At the same time, LGL Ecological Research Associates was conducting a research cruise as part of the multiyear MMS Northern Gulf of Mexico Continental Slope Study (Gallaway et al., 1988). Bottom photography (processed on board the vessel) resulted in clear images of vesicomyid clam chemosynthetic communities coincidentally in the same manner as the first discovery by camera sled in the Pacific in 1977. Photography during the same LGL/MMS cruise also documented tube-worm communities in situ in the Central Gulf of Mexico for the first time (not processed until after the cruise; Boland, 1986) prior to the initial submersible investigations and firsthand descriptions of Bush Hill (27°47′02″N 91°30′31″W / 27.78389°N 91.50861°W / 27.78389; -91.50861 (Bush Hill)) in 1986 (Rosman et al., 1987a; MacDonald et al., 1989b). The site was targeted by acoustic "wipeout" zones or lack of substrate structure caused by seeping hydrocarbons. This was determined using an acoustic pinger system during the same cruise on the R/V Edwin Link (the old one, only 113 ft (34 m)), which used one of the Johnson Sea Link submersibles. The site is characterized by dense tubeworm and mussel accumulations, as well as exposed carbonate outcrops with numerous gorgonian and Lophelia coral colonies. Bush Hill has become one of the most thoroughly studied chemosynthetic sites in the world.[21]

Distribution

Chemosynthetic communities in the northern part of Gulf of Mexico around cold seeps known in 2000

There is a clear relationship between known hydrocarbon discoveries at great depth in the Gulf slope and chemosynthetic communities, hydrocarbon seepage, and authigenic minerals including carbonates at the seafloor (Sassen et al., 1993a and b). While the hydrocarbon reservoirs are broad areas several kilometers beneath the Gulf, chemosynthetic communities occur in isolated areas with thin veneers of sediment only a few meters thick.[21]

The northern Gulf of Mexico slope includes a

Upper Cretaceous generate oil in most of the Gulf slope fields (Sassen et al., 1993a and b). Migration conduits supply fresh hydrocarbon materials through a vertical scale of 6–8 km (4–5 mi) toward the surface. The surface expressions of hydrocarbon migration are referred to as seeps. Geological evidence demonstrates that hydrocarbon and brine seepage persists in spatially discrete areas for thousands of years.[21]

The time scale for oil and gas migration (combination of buoyancy and pressure) from source systems is on the scale of millions of years (Sassen, 1997). Seepage from hydrocarbon sources through faults towards the surface tends to be diffused through the overlying sediment, carbonate outcroppings, and hydrate deposits so the corresponding hydrocarbon seep communities tend to be larger (a few hundred meters wide) than chemosynthetic communities found around the hydrothermal vents of the

microbial mats (Beggiatoa sp.).[21]

In the upper slope environment, the hard substrates resulting from carbonate precipitation can have associated communities of nonchemosynthetic animals, including a variety of sessile

barite are present.[21]

Chemosynthetic communities in the northern part of Gulf of Mexico around cold seeps known in 2006 include more than 50 communities

The widespread nature of Gulf of Mexico chemosynthetic communities was first documented during contracted investigations by the Geological and Environmental Research Group (GERG) of Texas A&M University for the Offshore Operators Committee (Brooks et al., 1986).

energy reserves in the Gulf of Mexico has also documented numerous new communities through a wide range of depths, including the deepest known occurrence in the Central Gulf of Mexico in Alaminos Canyon Block 818 at a depth of 2,750 m (9,022 ft).[21] The occurrence of chemosynthetic organisms dependent on hydrocarbon seepage has been documented in water depths as shallow as 290 m (951 ft) (Roberts et al., 1990) and as deep as 2,744 m (9,003 ft).[21]
This depth range specifically places chemosynthetic communities in the deepwater region of the Gulf of Mexico, which is defined as water depths greater than 305 m (1,000 ft).[21]

Chemosynthetic communities are not found on the continental shelf although they do appear in the fossil record in water shallower than 200 m (656 ft).[21] One theory explaining this is that predation pressure has varied substantially over the time period involved (Callender and Powell 1999).[21][23] More than 50 communities are now known to exist in 43 Outer Continental Shelf (OCS) blocks.[21] Although a systematic survey has not been done to identify all chemosynthetic communities in the Gulf of Mexico, there is evidence indicating that many more such communities may exist.[21] The depth limits of discoveries probably reflect the limits of exploration (lack of

submersibles capable of depths over 1,000 m (3,281 ft)).[21]

MacDonald et al. (1993 and 1996) have analyzed

oil slicks across the north-central Gulf of Mexico.[21][24][25] Results confirmed extensive natural oil seepage in the Gulf of Mexico, especially in water depths greater than 1,000 m (3,281 ft).[21] A total of 58 additional potential locations were documented where seafloor sources were capable of producing perennial oil slicks (MacDonald et al., 1996).[21] Estimated seepage rates ranged from 4 bbl/d (0.64 m3/d) to 70 bbl/d (11 m3/d) compared to less than 0.1 bbl/d (0.016 m3/d) for ship discharges (both normalized for 1,000 mi2 (640,000 ac)).[21] This evidence considerably increases the area where chemosynthetic communities dependent on hydrocarbon seepage may be expected.[21]

The densest aggregations of chemosynthetic organisms have been found at water depths of around 500 m (1,640 ft) and deeper.

salt diapir in Green Canyon Block 185. The seep site is a small knoll that rises about 40 m (131 ft) above the surrounding seafloor in about 580-m (1,903-ft) water depth.[21]

Stability

According to Sassen (1997) the role of

gas hydrates was first discovered during the MMS study entitled "Stability and Change in Gulf of Mexico Chemosynthetic Communities".[26]
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.[21] At deeper depths (>1,000 m, >3,281 ft), the bottom-water temperature is colder (by approximately 3 °C) and undergoes less fluctuation. The formation of more stable and probably deeper hydrates influences the flux of light hydrocarbon gases to the sediment surface, thus influencing the surface morphology and characteristics of chemosynthetic communities. Within complex communities such as Bush Hill, petroleum seems less important than previously thought (MacDonald, 1998b).[21]

Through

geological time scales. Powell reported evidence of mussel and clam communities persisting in the same sites for 500-4,000 years. Powell also found that both the composition of species and trophic tiering of hydrocarbon seep communities tend to be fairly constant across time, with temporal variations only in numerical abundance. He found few cases in which the community type changed (from mussel to clam communities, for example) or had disappeared completely. Faunal succession was not observed. Surprisingly, when recovery occurred after a past destructive event, the same chemosynthetic species reoccupied a site. There was little evidence of catastrophic burial events, but two instances were found in mussel communities in Green Canyon Block 234. The most notable observation reported by Powell (1995) was the uniqueness of each chemosynthetic community site.[21]

Precipitation of

authigenic
carbonates and other geologic events will undoubtedly alter surface seepage patterns over periods of many years, although through direct observation, no changes in chemosynthetic fauna distribution or composition were observed at seven separate study sites (MacDonald et al., 1995). A slightly longer period (19 years) can be referenced in the case of Bush Hill, the first Central Gulf of Mexico community described in situ in 1986. No mass die-offs or large-scale shifts in faunal composition have been observed (with the exception of collections for scientific purposes) over the 19-year history of research at this site.[21]

All chemosynthetic communities are located in water depths beyond the effect of severe storms, including hurricanes, and there would have been no alteration of these communities caused from surface storms, including

hurricanes.[21]

Biology

The mussel species Bathymodiolus childressi is the dominant species in the mytilid
type of cold seep communities in the Gulf of Mexico

MacDonald et al. (1990) has described four general community types. These are communities dominated by

undescribed.[21]

Individual lamellibranchid

Growth rates determined from recovered marked tube worms have been variable, ranging from no growth of 13 individuals measured one year to a maximum growth of 9.6 cm/yr (3.8 in/yr) in a Lamellibrachia individual (MacDonald, 2002). Average growth rate was 2.19 cm/yr (0.86 in/yr) for the Escarpia-like species and 2.92 cm/yr (1.15 in/yr) for lamellibrachids. These are slower growth rates than those of their hydrothermal vent relatives, but Lamellibrachia individuals can reach lengths 2–3 times that of the largest known hydrothermal vent species.[21] Individuals of Lamellibrachia sp. in excess of 3 m (10 ft) have been collected on several occasions, representing probable ages in excess of 400 years (Fisher, 1995). Vestimentiferan tube worm spawning is not seasonal and recruitment is episodic.[21]

Tubeworms are either male or female. One recent discovery indicates that the spawning of female Lamellibrachia appears to have produced a unique association with the large bivalve Acesta bullisi, which lives permanently attached to the anterior tube opening of the tubeworm, and feeds on the periodic egg release (Järnegren et al., 2005). This close association between the bivalves and tubeworms was discovered in 1984 (Boland, 1986) but not fully explained. Virtually all mature Acesta individuals are found on female rather than male tubeworms. This evidence and other experiments by Järnegren et al. (2005) seem to have solved this mystery.[21]

Growth rates for methanotrophic mussels at cold seep sites have been reported (Fisher, 1995).[21] General growth rates were found to be relatively high. Adult mussel growth rates were similar to mussels from a littoral environment at similar temperatures. Fisher also found that juvenile mussels at hydrocarbon seeps initially grow rapidly, but the growth rate drops markedly in adults; they grow to reproductive size very quickly. Both individuals and communities appear to be very long lived. These methane-dependent mussels have strict chemical requirements that tie them to areas of the most active seepage in the Gulf of Mexico. As a result of their rapid growth rates, mussel recolonization of a disturbed seep site could occur relatively rapidly. There is some evidence that mussels also have some requirement of a hard substrate and could increase in numbers if suitable substrate is increased on the seafloor (Fisher, 1995). Two associated species are always found associated with mussel beds – the gastropod Bathynerita naticoidea and a small Alvinocarid shrimp – suggesting these endemic species have excellent dispersal abilities and can tolerate a wide range of conditions (MacDonald, 2002).[21]

Unlike mussel beds, chemosynthetic clam beds may persist as a visual surface phenomenon for an extended period without input of new living individuals because of low dissolution rates and low sedimentation rates. Most clam beds investigated by Powell (1995) were inactive. Living individuals were rarely encountered. Powell reported that over a 50-year timespan, local extinctions and recolonization should be gradual and exceedingly rare. Contrasting these inactive beds, the first community discovered in the Central Gulf of Mexico consisted of numerous actively plowing clams. The images obtained of this community were used to develop length/frequency and live/dead ratios as well as spatial patterns (Rosman et al., 1987a).[21]

Extensive

autotrophic sulfur bacteria Beggiatoa species, and the orange mats possessed an unidentified nonchemosynthetic metabolism (MacDonald, 1998b).[21]

Heterotrophic species at seep sites are a mixture of species unique to seeps (particularly

galatheid crabs and nerite gastropods had isotopic signatures, indicating that their diets were a mixture of seep and background production. At some sites, endemic seep invertebrates that would have been expected to obtain much if not all their diet from seep production actually consumed as much as 50 percent of their diets from the background.[21]

In the Atlantic Ocean

Blake Ridge
diapir
BT – Barbados trench
OR – Orenoque sectors
EP – El Pilar sector
NIG – Nigerian slope
GUI – Guiness area
REG – Regab pockmark.

Cold-seep communities in the western

Blake Ridge diapir off North Carolina. More recently seep communities have been discovered in the eastern Atlantic, on a giant pockmark cluster in the Gulf of Guinea near the Congo deep channel, also on other pockmarks of the Congo margin, Gabon margin and Nigeria margin and in the Gulf of Cádiz.[12]

The occurrence of chemosymbiotic

bivalves collected from the mud volcanoes of the Gulf of Cadiz were reviewed in 2011.[5]

Cold seeps are also known from the Northern Atlantic Ocean,[2] even ranging into the Arctic Ocean, off Canada and Norway.[14]

Extensive faunal sampling has been conducted from 400 and 3,300 m (1,300–10,800 ft) in the Atlantic Equatorial Belt from the Gulf of Mexico to the Gulf of Guinea including Barbados accretionary prism, the Blake Ridge diapir, and in the Eastern Atlantic from the Congo and Gabon margins and the recently explored Nigeria margin during Census of Marine Life ChEss project. Of the 72 taxa identified at the species level, a total of 9 species or species complexes are identified as amphi-Atlantic.[12]

The Atlantic Equatorial Belt seep megafauna community structure is influenced primarily by depth rather than by geographic distance. The bivalves Bathymodiolinae (within Mytilidae) species or complexes of species are the most widespread in the Atlantic. The Bathymodiolus boomerang complex is found at the Florida escarpment site, the Blake Ridge diapir, the Barbados prism and the Regab site of Congo. The Bathymodiolus childressi complex is also widely distributed along the Atlantic Equatorial Belt from the Gulf of Mexico across to the Nigerian Margin, although not on the Regab or Blake ridge sites. The commensal polynoid, Branchipolynoe seepensis is known from the Gulf of Mexico, Gulf of Guinea and Barbados. Other species with distributions extending from the eastern to western Atlantic are: gastropod Cordesia provannoides, the shrimp Alvinocaris muricola, the galatheids Munidopsis geyeri and Munidopsis livida and probably the holothurid Chiridota heheva.[12]

There have been found cold seeps also in the Amazon deepsea fan. High-resolution seismic profiles near the shelf edge show evidence of near-surface slumps and faulting 20–50 m (66–164 ft) in the subsurface and concentrations (about 500 m2 or 5,400 sq ft) of methane gas. Several studies (e.g., Amazon Shelf Study—AMASEDS, LEPLAC, REMAC, GLORIA, Ocean Drilling Program) indicate that there is evidence for gas seepage on the slope off the Amazon fan based on the incidence of bottom-simulating reflections (BSRs), mud volcanoes, pock marks, gas in sediments, and deeper hydrocarbon occurrences. The existence of methane at relatively shallow depths and extensive areas of gas hydrates have been mapped in this region. Also, gas chimneys have been reported, and exploratory wells have discovered subcommercial gas accumulations and pock marks along fault planes. A sound geological and geophysical understanding of the Foz do Amazonas Basin is already available and used by the energy companies.[28]

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 3,500 m (11,500 ft), and shallower sites in the Gulf of Guinea are need to study in the future.[12]

In the Mediterranean

The first biological evidence for reduced environments in the

Gas hydrates have been sampled at the Amsterdam and Kazan mud volcanoes, and high methane levels have been recorded above the seafloor. Several provinces of the Nile deep-sea fan have been explored recently. These include the very active brine seepage named the Menes Caldera in the eastern province between 2,500 m and 3,000 m, the pockmarks in the central area along mid- and lower slopes, and the mud volcanoes of the eastern province, as well as one in the central upper slope (North Alex area) at 500 m depth.[29]

During these first exploratory dives, symbiont-bearing taxa that are similar to those observed on the Olimpi and Anaximander mud fields were sampled and identified. This similarity is not surprising, as most of these taxa were originally described from dredging in the Nile fan.

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.[29] Cold seeps discovered in the Sea of Marmara in 2008[31] have also revealed chemosynthesis-based communities that showed a considerable similarity to the symbiont-bearing fauna of eastern Mediterranean cold seeps.[29]

In the Indian Ocean

In the Makran Trench, a subduction zone along the northeastern margin of the Gulf of Oman adjacent to the southwestern coast of Pakistan and the southeastern coast of Iran, compression of an accretionary wedge has resulted in the formation of cold seeps and mud volcanoes.[32]

In the West Pacific

Native

continental slope of the South China Sea and Chen et al. (2011)[33] have proposed a theory of its origin as resulting by reduction from tetrahydroxoaluminate Al(OH)4− to metallic aluminium by bacteria.[33]

Japan

Chemosynthetic communities around Japan[34]
Cold seep
Hydrothermal vent
Whale fall

Deep sea communities around Japan are mainly researched by

Kaikō
and other groups have discovered many sites.

Methane seep communities in Japan are distributed along plate convergence areas because of the accompanying tectonic activity. Many seeps have been found in the Japan Trench, Nankai Trough, Ryukyu Trench, Sagami Bay and Suruga Bay, and the Sea of Japan.[35]

Members of cold seep communities are similar to other regions in terms of family or genus, such as Polycheata; Lamellibrachia, Bivalavia; Solemyidae, Bathymodiolus in Mytilidae, Thyasiridae, Calyptogena in Vesicomyidae and so forth.[34] Many of species in cold seeps of Japan are endemic.[35]

In Kagoshima Bay, there are methane gas seepages called "tagiri" (boiling). Lamellibrachia satsuma live around there. The depth of this site is only 80 m, which is the shallowest point Siboglinidae living. L. satsuma may be kept in an aquarium for a long period in 1 atm. Two aquariums in Japan are keeping and displaying L. satsuma. An observation method to introduce it into a transparent vinyl tube is being developed.[36]

DSV Shinkai 6500

DSV Shinkai 6500 discovered vesicomyid clam communities in the Southern Mariana Forearc. They depend on methane which originates in serpentinite. Other chemosynthetic communities would depend on hydrocarbon origins organic substance in crust, but these communities depend on methane originating from inorganic substances from the mantle.[37][38]

In 2011, was performed around the Japan Trench which is epicenter of Tōhoku earthquake. There are cracks, methane seepages and bacterial mats which were probably created by the earthquake.[39][40]

New Zealand

Off the mainland coast of

severely damaged cold seep communities and those ecosystems are threatened. Depths down to 2,000 m including cold seeps belongs among as-yet-unmapped topographic and chemical complexity of habitats. The scale of new-species discovery in these poorly studied or unexplored ecosystems is likely to be high.[44][41]

In the East Pacific

Monterey Bay Aquarium Research Institute has used remotely operated underwater vehicle Ventana in the research of Monterey Bay
cold seeps

In the deep sea the

heterogeneity may influence the biodiversity patterns of the local fauna.[28][45][46][47] Seep fauna include bivalves of families Lucinidae, Thyasiridae, Solemyidae (Acharax sp.), and Vesicomyidae (Calyptogena gallardoi) and polychaetes Lamellibrachia sp. and two other polychaete species.[46] Furthermore, in these soft reduced sediments below the oxygen minimum zone off the Chilean margin, a diverse microbial community composed by a variety of large prokaryotes (mainly large multi-cellular filamentous "mega bacteria" of the genera Thioploca and Beggiatoa, and of "macrobacteria" including a diversity of phenotypes), protists (ciliates, flagellates, and foraminifers), as well as small metazoans (mostly nematodes and polychaetes) has been found.[28][48] Gallardo et al. (2007)[48] argue that the likely chemolithotrophic metabolism of most of these mega- and macrobacteria offer an alternative explanation to fossil findings, in particular to those from obvious non-littoral origins, suggesting that traditional hypotheses on the cyanobacterial origin of some fossils may have to be revised.[28]

Cold seeps (

Metridium giganteum, encrusting sponges and bivalve Solemya reidi.[49]

Cold seeps with chemosynthetic communities along the USA Pacific coast occur in

Calyptogena clams Calyptogena kilmeri and Calyptogena pacifica[51] and foraminiferan Spiroplectammina biformis.[52]

Additionally, seeps have been discovered offshore southern California in the inner California Borderlands along several fault systems including the San Clemente fault,[53] San Pedro fault,[54] and San Diego Trough fault.[55] Fluid flow at the seeps along the San Pedro and San Diego Trough faults appears controlled by localized restraining bends in the faults.[55]

In the Antarctic

The first cold seep was reported from Southern Ocean in 2005.[16] The relatively few investigations to the Antarctic deep sea have shown the presence of deep-water habitats, including hydrothermal vents, cold seeps, and mud volcanoes.[56] Other than the Antarctic Benthic Deep-Sea Biodiversity Project (ANDEEP) cruises, little work has been done in the deep sea.[56] There are more species waiting to be described.[56]

Detection

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

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.[21]

Fossilized records

Late Cretaceous cold seep deposit in the Pierre Shale
, southwest South Dakota

Cold seep deposits are found throughout the

Palaeogene of Honshu,[59] the Neogene of Northern Italy,[60] and the Pleistocene of California.[61] These fossil cold seeps are characterized by mound-like topography (where preserved), coarsely crystalline carbonates, and abundant mollusks and brachiopods
.

Environmental impacts

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, 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.[62][63] 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.[64] Topography (e.g. presence of canyons) and hydrography (e.g., 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 [65] and significant levels of persistent organic pollutants in mesopelagic and bathypelagic cephalopods[66]

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.[67] Primary production will change in the surface layers according to sun exposure, water temperature, major stratification of water masses, for example 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.

See also

References

This article incorporates a

work of the United States Government from references[3][21] and CC-BY-2.5 from references[2][4][6][12][28][29][35][41][56] and CC-BY-3.0 text from the reference[5]

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Further reading

External links

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