A
hydrothermal vent is a
fissure in a planet's surface from which
geothermally heated
water issues. Hydrothermal vents are commonly found near
volcanically active places, areas where
tectonic plates are moving apart, ocean basins, and
hotspots.
Hydrothermal vents are locally very common because the earth is both geologically active and has large amounts of water on its surface and within its crust. Common land types include
hot springs,
fumaroles and
geysers. The most famous hydrothermal vent system on land is probably within
Yellowstone National Park in the
United States. Under the sea, hydrothermal vents may form features called
black smokers.
Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive, often hosting complex communities fueled by the chemicals dissolved in the vent fluids.
Chemosynthetic archaea form the base of the food chain, supporting diverse organisms, including
giant tube worms,
clams,
limpets and
shrimp.
Active hydrothermal vents are believed to exist on
Jupiter's moon
Europa, and ancient hydrothermal vents have been speculated to exist on
Mars.
Exploration
In 1949, a deep water survey reported anomalously hot brines in the central portion of the
Red Sea. Later work in the 1960s confirmed the presence of hot, 60°C (140°F), saline brines and associated metalliferous muds. The hot solutions were emanating from an active subseafloor
rift. The highly saline character of the waters was not hospitable to living organisms. The brines and associated muds are currently under investigation as a source of mineable precious and base metals.
The chemosynthetic ecosystem surrounding submarine hydrothermal vents were discovered along the Galapagos Rift, a spur of the
East Pacific Rise, in 1977 by a group of marine geologists led by
Jack Corliss of Oregon State University. In 1979, biologists returned to the rift and used
ALVIN, an
ONR research submersible from
Woods Hole Oceanographic Institute, to see the hydrothermal vent communities with their own eyes. In that same year, scientist
Peter Lonsdale published the first scientific paper on hydrothermal vent life.
In 2005, Neptune Resources NL, a mineral exploration company, applied for and was granted 35,000 km² of exploration rights over the Kermadec Arc in
New Zealand's
Exclusive Economic Zone to explore for
seafloor massive sulfide deposits, a potential new source of
lead-
zinc-
copper sulfides formed from modern hydrothermal vent fields.
The discovery of a vent in the Pacific Ocean offshore of
Costa Rica, named the Medusa hydrothermal vent field (after the serpent-haired
Medusa of
Greek mythology), was announced in April 2007.
White smokers at Champagne Vent on
DominicaPhysical properties
Hydrothermal vents in the deep ocean typically form along the
Mid-ocean ridges, such as the
East Pacific Rise and the
Mid-Atlantic Ridge. These are locations where two
tectonic plates are diverging and new crust is being formed.
The water that issues from seafloor hydrothermal vents consists mostly of
sea water drawn into the hydrothermal system close to the volcanic edifice through faults and porous sediments or volcanic strata, plus some magmatic water released by the upwelling
magma.
In terrestrial hydrothermal systems the majority of water circulated within the fumarole and geyser systems is
meteoric water plus
ground water that has percolated down into the thermal system from the surface, but it also commonly contains some portion of
metamorphic waters, sedimentary formational
brines and
magmatic water that is released by the magma. The proportion varies from location to location.
The water emerges from a hydrothermal vent at temperatures ranging up to 400°C, compared to a typical 2°C for the surrounding deep ocean water. The high pressure at these depths significantly expands the thermal range at which water remains liquid, and so the water doesn't boil. Water at a depth of 3,000 m and a temperature of 407°C becomes
supercritical. However the increase in salinity pushes the water closer to its
critical point.
Some hydrothermal vents form roughly cylindrical chimney structures. These form from minerals that are dissolved in the vent fluid. When the super-heated water contacts the near-freezing sea water, the minerals precipitate out to form particles which add to the height of the stacks. Some of these chimney structures can reach heights of 60 m. An example of such a towering vent was "Godzilla", a structure in the Pacific Ocean near
Oregon that rose to 40 m before it fell over.
The initial stages of a vent chimney begin with the deposition of the mineral
anhydrite.
Sulfides of
copper,
iron and
zinc then precipitate in the chimney gaps, making it less porous over the course of time. Vent growths on the order of 30 cm per day have been recorded.
Chimney structures that emit a cloud of black material are called "
black smokers", named for the dark hue of the particles they emit. The black smokers typically emit particles with high levels of sulfur-bearing minerals, or sulfides. "White smokers" refer to vents that emit lighter-hued minerals, such as those containing barium, calcium, and silicon. These vents also tend to have lower temperature plumes.
An April 2007 exploration of the deep-sea vents off the coast of
Fiji found those vents to be a significant source of dissolved
iron.
Biological communities
Life has traditionally been seen as driven by energy from the sun, but deep sea organisms have no access to sunlight, so they must depend on nutrients found in the dusty chemical deposits and hydrothermal fluids in which they live. Previously marine biologists assumed that vent organisms were dependent on a "rain" of detritus from the upper levels of the ocean, like deep sea organisms are. This would leave them dependent on plant life and thus the sun. Some hydrothermal vent organisms do consume this "rain," but with only such a system, life forms would be very sparse. Compared to the surrounding sea floor, however, hydrothermal vent zones have a density of organisms 10,000 to 100,000 times greater.
Hydrothermal vent communities are able to sustain such vast amounts of life because vent organisms depend on chemosynthetic bacteria for food. The water that comes out of the hydrothermal vent is rich in dissolved minerals and supports a large population of chemo-autotrophic bacteria. These bacteria use sulfur compounds, particularly hydrogen sulfide, a chemical highly toxic to most known organisms, to produce organic material through the process of
chemosynthesis.
The ecosystem so formed is reliant upon the continued existence of the hydrothermal vent field as the primary source of energy, which differs from most surface life on Earth which is based on solar energy. However, although it is often said that these communities exist independently of the sun, some of the organisms are actually dependent upon oxygen produced by photosynthetic organisms. Others are anaerobic as was the earliest life.
The chemosynthetic bacteria grow into a thick mat which attracts other organisms such as
amphipods and
copepods which graze upon the bacteria directly. Larger organisms such as
snails,
shrimp,
crabs,
tube worms,
fish, and
octopuses form a
food chain of predator and prey relationships above the primary consumers. The main families of organisms found around seafloor vents are
annelids,
pogonophorans,
gastropods, and
crustaceans, with large
bivalves, vestimentiferan worms, and "eyeless" shrimp making up the bulk of non-microbial organisms.
Sulfide chimney of the
Magic Mountain hydrothermal field, British Columbia, Canada
Some theories indicate that life originated at hydrothermal vents from inorganic precursors.
Tube worms form an important part of the community around a hydrothermal vent. The tube worms, like parasitic worms, absorb nutrients directly into their tissues. This is because tube worms have no mouth or even a digestive tract, so the bacteria live inside them. There are approximately 285 billion bacteria per ounce of tubeworm tissue. Tubeworms have red plumes which contain hemoglobin. Hemoglobin combines hydrogen sulfide and transfers it to the bacteria living inside the worm. In return the bacteria nourish the worm with carbon compounds.
The two species that inhabit a hydrothermal vent are Tevnia jerichonana
, and Riftia pachyptila
. One community has been discovered dubbed 'Eel City', which consists predominantly of eels. Though eels are not uncommon, as mentioned earlier invertebrates typically dominate hydrothermal vents. Eel City is located near Nafanua volcanic cone, American Samoa.
Other examples of the unique fauna who inhabit this ecosystem are a snail armored with scales made up of iron and organic materials, and the Pompeii worm (Alvinella Pompejana), which is capable of withstanding temperatures up to 80°C (176°F).
Over 300 new species have been discovered at hydrothermal vents, many of them "sister species" to others found in geographically separated vent areas. It has been proposed that before the North American plate overrode the mid-ocean ridge, there was a single biogeographic vent region found in the eastern Pacific. The subsequent barrier to travel began the evolutionary divergence of species in different locations. The examples of convergent evolution seen between distinct hydrothermal vents is seen as major support for the theory of natural selection and evolution as a whole.Biological theories
Although the discovery of hydrothermal vents is a relatively recent event in the history of science, the importance of this discovery has given rise to, and supported, new biological and bio-atmospheric theories. The deep hot biosphere
At the beginning of his 1992 paper The Deep Hot Biosphere
, Thomas Gold referred to ocean vents
in support of his theory that the lower levels of the earth are rich in living biological material that finds its way to the surface. Gold's theory however went beyond hydrothermal vents and proposed abiogenic petroleum origin (i.e. that petroleum is not just fossil based, but is manufactured deep in the earth), as further expanded in the book The Deep Hot Biosphere. According to Gold: "Hydrocarbons are not biology reworked by geology (as the traditional view would hold) but rather geology reworked by biology.
" This hypothesis has been rejected by petroleum geologists, who hold that, even if does occur, the amount of petrochemical produced in this manner is negligible; no naturally occurring abiotic petroleum has ever been found.
An article on abiogenic hydrocarbon production'' in the February 2008 issue of
Science Magazine used data from experiments at
Lost City (hydrothermal field) to report how the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.
Hydrothermal origin of life
Günter Wächtershäuser proposed the
Iron-sulfur world theory and suggested that life might have
originated at hydrothermal vents. Wächtershäuser proposed that an early form of metabolism predated genetics. By metabolism he meant a cycle of chemical reactions that produce energy in a form that can be harnessed by other processes.
It has been proposed that amino-acid synthesis could have occurred deep in the Earth's crust and that these amino-acids were subsequently shot up along with hydrothermal fluids into cooler waters, where lower temperatures and the presence of clay minerals would have fostered the formation of peptides and protocells. This is an attractive hypothesis because of the abundance of CH
4 and NH
3 present in hydrothermal vent regions, a condition that was not provided by the Earth's primitive atmosphere. A major limitation to this hypothesis is the lack of stability of organic molecules at high temperatures, but some have suggested that life would have originated outside of the zones of highest temperature. There are numerous species of extremophiles and other organisms currently living immediately around deep-sea vents, suggesting that this is indeed a possible scenario.
Exploitation
Hydrothermal vents, in some instances, have led to the formation of exploitable mineral resources via deposition of
seafloor massive sulfide deposits. The
Mount Isa orebody located in
Queensland, Australia, is an excellent example.
Recently, mineral exploration companies, driven by the elevated price activity in the base metals sector during the mid 2000s, have turned their attention to extraction of mineral resources from hydrothermal fields on the seafloor. Significant cost reductions are, in theory, possible. Consider that in the case of the Mount Isa orebody, large amounts of capital are required to sink shafts and associated underground infrastructure, then laboriously drill and blast the ore, crush and process it, to win out the base metals, an activity which requires a large workforce.
The
Marshall hydrothermal recovery system is a patented proposal to exploit hydrothermal vents for their energy and minerals. A hydrothermal field, consisting of chimneys and compacted chimney remains, can be reached from the surface via a dynamically positioned ship or platform, using conventional pipe, mined using modified soft rock mining technology (continuous miners), brought to the surface via the pipe, concentrated and dewatered then shipped directly to a smelter. While the concept sounds far-fetched, it uses already proven technology derived from the offshore oil and gas industries, and the soft-rock mining industries.
Two companies are currently engaged in the late stages of commencing to mine seafloor massive sulfides. Nautilus Minerals is in the advanced stages of commencing extraction from its Solwarra deposit, in the
Bismarck Archipelago, and Neptune Minerals is at an earlier stage with its Rumble II West deposit, located on the
Kermadec Arc, near the
Kermadec Islands. Both companies are proposing using modified existing technology. Nautilus Minerals, in partnership with
Placer Dome (now part of
Barrick Gold), succeeded in 2006 in returning over 10 tonnes of mined SMS to the surface using modified drum cutters mounted on an ROV - a world first. Neptune Minerals in 2007 succeeded in recovering SMS sediment samples using a modified oil industry suction pump mounted on an ROV - also a world first.
Seafloor mining has environmental impact connotations. A large amount of work is currently being engaged in by both the above mentioned companies to ensure that potential environmental impacts of seafloor mining are well understood and control measures are implemented, before exploitation commences.
Attempts have been made in the past to exploit minerals from the seafloor. The 1960s and 70s saw a great deal of activity (and expenditure) in the recovery of
manganese nodules from the abyssal plains, with varying degrees of success. This does demonstrate however that recovery of minerals from the seafloor is possible, and has been possible for some time. Interestingly, mining of manganese nodules served as a cover story for the elaborate attempt by the CIA to raise the sunken
Soviet submarine K-129, using the
Glomar Explorer, a ship purpose built for the task by
Howard Hughes. The operation was known as
Project Jennifer, and the cover story of seafloor mining of manganese nodules may have served as the impetus to propel other companies to make the attempt.
See also
