Cold seep

[[Lamellibrachia|Tube worms (Lamellibrachia luymesi) are among the dominant species in one of four cold seep community types in the Gulf of Mexico.|thumb]]

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 ) 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

thumb|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. 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. 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).

Distribution

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.

In the Gulf of Mexico

thumb|The crewed 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. a month earlier, 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 as part of this project obtained images from the end of a film roll of a deep-sea camera sled (processed on board the vessel November 14, 1984) that resulted in clear images of vesicomyid clam chemosynthetic communities (Rossman et al., 1987) coincidentally in the same manner as the first documentation of chemosynthetic communities at the Galapagos Rift investigating hot water plumes by camera sled in the Pacific in 1976 (Lonsdale 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 () in 1986. The Bush Hill 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 (renamed from Sea Diver and only ), which used one of the Johnson Sea Link submersibles. This site represents the first eyes-on human observations of chemosynthetic communities in the northern Gulf of Mexico and 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. 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. Very-slow-seepage sites do not support complex chemosynthetic communities; rather, they usually only support simple microbial mats (Beggiatoa sp.). and as deep as . 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 . It is hypothesized 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.

In the West Pacific

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

Japan

{| class="wikitable" style="float:right"

|+ Chemosynthetic communities around Japan

|Cold seep

Kuril–Kamchatka Trench-Japan Trench
Sea of Japan
Sagami Bay
Suruga Bay
Nankai Trough
Kagoshima Bay
Ryukyu Trench

|Hydrothermal vent

Izu Islands-Bonin Islands
Mariana Islands
Okinawa Trough

|Whale fall

Sagami Bay
Off Cape Nomamisaki (East China Sea)
Tori-shima seamount (Izu Islands)

Deep sea communities around Japan are mainly researched by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). DSV Shinkai 6500, 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, Suruga Bay, and the Sea of Japan.

thumb|[[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.

In 2011, the area around the Japan Trench suffered from the Tōhoku earthquake. There are cracks, methane seepages, and bacterial mats which were probably created by the earthquake.

New Zealand

Off the mainland coast of New Zealand, shelf-edge instability is enhanced in some locations by cold seeps of methane-rich fluids that likewise support chemosynthetic faunas and carbonate concretions. Dominant animals are tubeworms of the family Siboglinidae and bivalves of families Vesicomyidae and Mytilidae (Bathymodiolus). Many of its species appear to be endemic. Deep bottom trawling has severely damaged cold seep communities, and those ecosystems are threatened. Cold seeps are found at depths down to 2,000 m, and the topographic and chemical complexity of the habitats are not yet mapped. The scale of new-species discovery in these poorly-studied or unexplored ecosystems is likely to be high. Seep fauna include bivalves of families Lucinidae, Thyasiridae, Solemyidae (Acharax sp.), and Vesicomyidae (Calyptogena gallardoi) and polychaetes (Lamellibrachia sp. and two other polychaete species). Gallardo et al. (2007) Unobvious fauna (also unobvious for cold seeps) have been found there with these dominating species: sea snail Fusitriton oregonensis, anemone Metridium giganteum, encrusting sponges, and bivalve Solemya reidi. There have been found, for example, Calyptogena clams Calyptogena kilmeri and Calyptogena pacifica and foraminiferan Spiroplectammina biformis.

map of cold seeps in the Monterey Bay

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

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 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 Jurassic Agoudim Formation of Morocco, the Cretaceous of Colorado and 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.

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, 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.

References

This article incorporates a public domain work of the United States Government from references and CC-BY-2.5 from references

External links

Paul Yancy's vents and seeps page
Monterey Bay Aquarium Research Institute's seeps page
ScienceDaily News: Tubeworms in deep sea discovered to have record long life spans