The Rainbow hydrothermal vent field is a system of

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are compos ...

-hosted hydrothermal vents located at 36°14'N on the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North A ...

(MAR). It was discovered in 1994 from temperature readings of ten high-temperature

black smokers A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspo ...

at a depth of approximately , where fluids can exceed . The site is shallower and larger in area than many other vent fields along the Azores section of the MAR with an area of . Located southeast of

Faial Island Faial Island (), also known in English as Fayal, is a Portuguese island of the Central Group (Portuguese: ''Grupo Central'') of the Azores. The Capelinhos Volcano, the westernmost point of the island, may be considered the westernmost point of E ...

, it is a popular geochemical sampling and modeling site due to close proximity to the

Azores ) , motto= ( en, "Rather die free than subjected in peace") , anthem=( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...

and definitive representation of

serpentinization Serpentinization is a hydration and metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the serpentine group miner ...

from

hydrothermal circulation Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

and

synthesis Synthesis or synthesize may refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors **Organic synthesis, the chemical synthesis of organi ...

. Vent geology, biology, and fluid content make Rainbow comparable to other hot hydrothermal vents of the Azores such as Lucky Strike and Menez Gwen. However;

chlorinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...

concentration, and pH distinguish it from neighboring vent fields. As a hot, ultramafic-hosted vent field, pH levels of fluids are extremely low with much H₂ and CH₄ generated from water interactions with mafic

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...

s. Though not actively considered for development, Rainbow lies within the MoMAR (Monitoring of the Mid Atlantic Ridge) survey area for a marine observatory.

Discovery and expeditions

Rainbow has had a number of visits since its initial discovery in 1994. Remotely Operated Vehicles (ROVs),

submersible A submersible is a small watercraft designed to operate underwater. The term "submersible" is often used to differentiate from other underwater vessels known as submarines, in that a submarine is a fully self-sufficient craft, capable of in ...

s, and Conductivity Temperature Depth (CTD) probes have been deployed to sample, characterize, and explore the vent field. * Rainbow was first identified in 1994 from TOBI

side-scan sonar Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side imaging sonar, side-imaging sonar and bottom classification sonar) is a category of sonar system that is used to efficiently create an image of large areas of the sea ...

and CTD data on the HEAT Cruise, which returned bathymetric

geomorphology Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or ...

of the Rainbow

Massif In geology, a massif ( or ) is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole. The term also refers to a ...

. * Premier fluid sampling occurred during the FLORES cruise in 1997, also sampling Azores MAR sites Menez Gwen and Lucky Strike. * The area was surveyed in greater detail in 2001 on the IRIS cruise, marking a debut in magnetism,

gravimetry Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. Units of measurement Gr ...

, and water sample data from the ROV ''Victor 6000''. Some vent fluid and gas sampling was also performed on the IRIS cruise of 2001. * In 2002, the SEAHMA 1 cruise was conducted to sample geology and biology at the

Azores triple junction The Azores Triple Junction (ATJ) is a geologic triple junction where the boundaries of three tectonic plates intersect: the North American Plate, the Eurasian Plate and the African Plate. This triple junction is located along the Mid-Atlantic R ...

where the African, Eurasian, and North American plates meet. * On the MoMARDREAM cruises of 2007 and 2008, the ROV Victor and submersible

Nautile ''Nautile'' is a crewed submersible owned by Ifremer, the French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and acce ...

were used in collecting a variety of geological samples. During the MoMARDREAM cruises, a total of 29

dredges Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing d ...

(14 from 2007, 15 in 2008) were collected. * Fluid sampling was also performed on the KNOX18RR cruise in 2008, with samples collected by the ROV ''Jason''. * 2009 saw the first Portuguese journey to the Rainbow massif from the EMEPC/LUSO/2009 Expedition to study Azores vent biology. * In 2012, trace metals were assessed from numerous field studies of the MAR using the '' R/V Knorr'' and ROV ''Jason II''.

Geologic setting

Rainbow is located on a massif at depth, shared with two fossil (mostly inactive) vent sites Ghost City and Clamstone. As a slow-spreading ridge at approximately 2.2 cm/yr, extensive

faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...

has uplifted

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

and

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...

and exposed ultramafic rock to cold seawater. Faulting may also be responsible for magnitude 3 - 3.5

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

s observed in hydroacoustic data, suggesting that the region is tectonically active. Unlike high-temperature

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

systems, this ultramafic setting is associated with a positive

magnetic anomaly In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying ...

; postulated to come from

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...

precipitation. The active Rainbow site exhibits numerous active and inactive chimneys at serpentinized peridotite outcrops, distinguishable from

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

cover either by protruding from sediment or at a

scarp Scarp may refer to: Landforms and geology * Cliff, a significant vertical, or near vertical, rock exposure * Escarpment, a steep slope or long rock that occurs from erosion or faulting and separates two relatively level areas of differing elevatio ...

Chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...

concentrations from vent fluids suggest a common heat source for the site, though the location and geometry of heat sources is unknown.

Hydrothermal circulation

Due to the extensive faulting at the Rainbow massif, cold ocean seawater is able to permeate deep into the seafloor. Water circulates down a fault line, actively reacting with various sediment and rock layers until it is warmed by an underlying heat source. When warmed, it can undergo a phase transition - often resulting in dramatic changes to fluid chemistry. Super-heated vent fluids then rise and are ejected from the seafloor, where a dramatic drop in temperature from cold-water mixing can cause some fluid chemicals to precipitate out and form chimneys. With relatively little basalt within a kilometer of the vent field, most reactions influencing the vent fluids during hydrothermal circulation come from differing degrees of serpentinization and veining of peridotites.

Olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...

-rich rocks such as

troctolite Troctolite (from Greek τρώκτης 'trout' and λίθος 'stone') is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with minor pyroxene. It is an olivine-rich anorth ...

s undergo significant alteration, being partially replaced by serpentine and magnetite. There is evidence of high-temperature serpentinite alteration on some samples with pre-existing serpentinite, demonstrating

overprinting Overprinting refers to the process of printing one colour on top of another in reprographics. This is closely linked to the reprographic technique of 'trapping'. Another use of overprinting is to create a rich black (often regarded as a colour ...

of serpentinites with higher iron content. Mylonic peridotites at the vent field show plastic deformation then overprinted by serpentine and

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chl ...

. Rainbow exhibits very acidic vent fluids (pH ~2.8) from

hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is di ...

ions released from numerous ultramafic rock interactions during vent circulation. Fluids also contain a number of organic carbon molecules, from

alkane In organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms tha ...

s and

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...

to complicated polycyclic aromatic hydrocarbons (PAHS) and biogenic

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

s. These organic carbon molecules suggest organisms living within the vents, harnessing

chemosynthetic In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic compounds (e.g., hydrogen gas, hydrog ...

reactions to

metabolize Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

. Serpentinization reactions occur with hydrothermal circulation causing water to react with hot iron-containing minerals, releasing H₂ gas and transforming the base rock. Serpentinization may also be responsible for a significant amount of methane produced at Rainbow. Vent fluids are known to travel many kilometers northeast from their associated vents, depositing any unreacted elements to the seafloor far away from their source.

Phase separation

Phase separation can occur when seawater is heated to a high enough temperature it will form a second phase. At pressure below the critical point (298 bar, 407 °C for seawater), the seawater boils and produces a vapor phase. At pressure above the critical pressure a saline brine forms as the second phase. In the subsurface, gravitational forces can lead the two phases to separate. Waters circulating deep within Rainbow rise to a high enough pressure or temperature so that they react with the rock and chemical constituents are dissolved into or

precipitate In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...

out of the vent fluid. Chemical concentrations in the fluid are further modified when it undergoes phase separation because volatile constituents are concentrated in the vapor rich phase and metal

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s in the brine. Phase separation carries great significance for

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

, an abundant element in seawater with few reactions outside of phase separation, and is often normalized to assess

thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...

within a ridge system. Depending on the

chemical stability In chemistry, chemical stability is the thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state A quantum mechanical system or particle that is bound—that is, confined spati ...

of the elements, water entering the seafloor will therefore exhibit different chemical characteristics when it comes back out. At Rainbow, phase separation is a suggested cause for particularly high concentrations of chloride, trace elements, and hydronium, as they differ greatly from similar MAR vents like Logatchev. Furthermore, Rainbow vent fluids have the highest concentrations of many elements found at the Azores vents, such as

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that c ...

s, and

rare earth elements The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

(REE). Due to the extreme endmember pH, chloride is hypothesized to act as a dominant

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

and therefore forms many weak complexes with other elements at high temperatures. These complexes become unstable when pH rises or temperature decreases, therefore releasing many transition metals and REEs. Observed

endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...

fluid samples taken at different vent sites are of very similar

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy u ...

and

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

concentrations, which suggests a common heat source for the vents. There may, however, be many heat sources at Rainbow considering the complex faulting tectonics, and extensive amounts of gabbro and peridotite.

Vent Field biology

Rainbow has been a long-lived environment for microbial life, with a great diversity of archaea and

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

at the vent field. Microbes are known to utilize energy from H₂ gas and H₂S oxidation, with some

chemotroph A Chemotroph is an organism that obtains energy by the oxidation of electron donors in their environments. These molecules can be organic molecule, organic (chemoorganotrophs) or inorganic compound, inorganic (chemolithotrophs). The chemotroph de ...

s belonging to the Asgard archaea tree. The Asgard archaea are a very recent discovery with a theorized ancestry in evolution between

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...

s and

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...

s. Some of the biogenic

alkanes In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whic ...

found within vent fluids are suggestive of sulfide-oxidizing bacteria or archaea. However, there is also abundant evidence of abiotic production of organic and inorganic molecules at Rainbow, such as

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...

and

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonat ...

, which may have been supportive of early life. Due to the hot temperatures, low pH, and longevity of the vent activity, there is a strong case for life to originate at sites similar to the Rainbow Massif. Regarding

macrofauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''biota''. Zoolo ...

, the Rainbow Massif has been supportive of many kinds of

decapods The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is es ...

and

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

, such as Alvinocarididae and

Bathymodiolus ''Bathymodiolus'' is a genus of deep-sea mussels, marine bivalve molluscs in the family Mytilidae. Many of them contain intracelluar chemoautotrophic bacterial symbionts. Species Modern (non-fossil) species within the genus ''Bathymodiolus'' ...

respectively, feeding where nutrient-rich vent fluids interact with the cold

bathypelagic The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelag ...

waters. Rainbow exhibits fossils of many kinds of vesicomyid and thyasirid shells. Fossils have been dated at other sites on the Rainbow massif, with Ghost City containing gastropod and clam remains that are aged at nearly 111,000 years old. Shell-rich carbonates have been found at Clamstone, which may be as old as 25,000 years.

Human uses and preservation

Rainbow, as with all other deepwater vent systems, is a location of highly-specialized biology and sensitive geological structures. Due to its significance as an accessible and examplar ultramafic system, Rainbow is a very popular site for scientific expeditions involving intrusive long-term monitoring, environmental manipulation, and geological sampling. It is also the only vent field on the MAR which has been visited by tourists. Due to some veining processes,

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...

extraction and mining are another activity than may upset ecosystems of the massif. Rainbow has a tricky history regarding preservation, as the site is within the

OSPAR The Convention for the Protection of the Marine Environment of the North-East Atlantic or OSPAR Convention is the current legislative instrument regulating international cooperation on environmental protection in the North-East Atlantic. Work ...

Maritime Area and just outside of Portugal's exclusive economic zone similarly to another vent field, Saldanha. Portugal was unable to distinguish Rainbow as residing off of the extended Azores shelf region - therefore rendering it unqualified for OSPAR protection as a High Seas location. The

World Wide Fund for Nature The World Wide Fund for Nature Inc. (WWF) is an international non-governmental organization founded in 1961 that works in the field of wilderness preservation and the reduction of human impact on the environment. It was formerly named the Wor ...

(WWF) lobbied for Rainbow's protection in 2005 and as of 2006, Rainbow is listed by OSPAR as protected marine nature preserve with an

MPA MPA or mPa may refer to: Academia Academic degrees * Master of Performing Arts * Master of Professional Accountancy * Master of Public Administration * Master of Public Affairs Schools * Mesa Preparatory Academy * Morgan Park Academy * Mo ...

size of 22.15 km².

References

{{reflist

External links

Official Marine Protected Areas Portugal Page on the Azores
Hydrothermal vents Azores Geology of the Atlantic Ocean Geology of the Azores Mid-Atlantic Ridge