Deep-sea hydrothermal vents are fissures in the sea floor from which geothermally heated water is emitted. The majority of discovered vents exist at mid-ocean ridges (sites of seafloor spreading) as their formation is associated with the divergence of tectonic plates and establishment of new ocean crust (Van Dover et al. 2002). Since the discovery of the first deep-sea hydrothermal vents at the Galápagos Rift in 1977 (Corliss et al. 1979), two major forms of vents have been identified; “black smokers” and “white smokers”. Both types of vents exist as chimney-like structures extending from the seabed; however, they are differentiated by their unique physical and chemical properties and subsequent plume colour (Corliss et al. 1979; Spiess et al. 1980; Haymon & Kastner 1981; Jannasch & Wirsen 1981). Deep-sea hydrothermal vents are environments of extreme variability due to the mixture of superheated, acidic vent fluid and surrounding sea water (Kingston Tivey 2013). Despite these extreme environmental conditions, vents often host complex, highly endemic ecosystems (Johnson et al. 1986; Bates et al. 2005; Sarradin et al. 2008); a discovery which has greatly expanded the understanding of what limits life on earth (Van Dover 2000).
The known geographical distribution of deep-sea hydrothermal vents has continuously expanded due to new discoveries. Most vents have been discovered at low to mid-latitudes. However, in 1999, vents were discovered at the East Scotia Ridge; a back-arc spreading centre (a spreading centre that creates a basin between an island arc and the continental mainland) in the Atlantic sector of the Southern Ocean (German et al. 2000). It is now known that deep-sea hydrothermal vents occur along all back-arc spreading centres and mid-ocean ridges (Van Dover et al. 2002) and since the global mid-ocean ridge system (a 65,000 km long mountain chain connecting all mid-ocean ridges) is present in each ocean, deep-sea hydrothermal vents are widely distributed (Figure 1). In addition to mid-ocean ridges and back-arc spreading centres, vents also occur at hotspots for volcanic activity, such as the Yellowstone Hotspot (44.43°N, 110.67°W). Although deep-sea hydrothermal vents are widely distributed, studies have revealed significantly higher numbers of vents at particular mid-ocean ridges. The Mid-Atlantic Ridge, beginning northeast of Greenland and terminating at the Bouvet Triple Junction in the South Atlantic, hosts one of the largest single deposits of black smokers known (Kelley 2001). Large vent deposits also exist at the East Pacific Rise, extending from the northern end of the Gulf of California to Antarctica. More recent discoveries of vents include those at the Cayman Trough (18.5°N, 83.0°W), also the world’s deepest seafloor spreading centre (Murton et al. 2012), the Gulf of California (28.0°N, 112.0°W) and the Alarcon Rise (situated between the Pescadero and Tamayo transform vaults in the Gulf of California) (Thompson et al. 2012; Shukman 2013). Currently, deep-sea vents have been discovered at around 200 sites; however, much of the mid-ocean ridge crest is still yet to be studied (Rogers et al. 2012).
Figure 1: Map of the global distribution of mid-ocean ridges.
Map source: the U.S Geological Survey (add specific reference for figure).
The establishment of deep-sea hydrothermal vents begins with the movement of plates in the Earth’s crust. At locations where the Earth’s geological plates move away from one another, oceanic spreading centres are created. Here, magma from the mantle rises to the seafloor, then cools and solidifies to create a new seabed. During this cooling and solidifying process, cracks form in the new seabed, allowing seawater to seep through. As this seawater circulates through the upper ocean crust, it is heated by molten rock and becomes enriched in dissolved minerals (Van Dover et al. 2002). The chemical composition of this circulating seawater will be determined by the geology and temperature of the rocks, and the amount of water that has previously passed through the vent (Kelley 2001). The geothermally heated water, at temperatures as high as 464°C, has increased buoyancy, which forces it to be expelled through the vent (Haase et al. 2007). However, once mixed with seawater, the fluid cools and much of the dissolved minerals precipitate out, creating plumes of smoke. Once cooled, these minerals deposit, creating solid, chimney-like structures. Anhydrite is the first mineral layer to be deposited around the opening of deep-sea hydrothermal vent chimneys (Jannasch & Taylor 1984), but the final combination of deposited minerals is unique to each type of smoker. At locations such as the Galápagos Rift, deep-sea hydrothermal vents can also exist as very small crevices in the seafloor, which do not form chimneys, instead emitting fluid between 10-20°C over a large surface area (Corliss et al. 1979; Jannasch & Taylor 1984).
Black smokers and white smokers
i. Black smokers
Black smokers were first discovered at the East Pacific Rise at 21°N and are characterised by large chimneys and dark-coloured plumes rich in sulphur and copper (Spiess et al. 1980; Haymon & Kastner 1981) (Figure 2). Vent fluid typically exits black smokers through a tight axial channel at temperatures between 350 and 400°C, precipitating out sulfates and sulphide minerals that form chimneys rich in pyrite, chalcopyrite, sphalerite, anhydrite and barite (Kelley 2001). At the Trans-Atlantic Geotraverse (TAG) hydrothermal field, located at the slow-spreading Mid-Atlantic Ridge, black smokers are present at the top of vent mounds as distinctive spire-shaped chimneys, often reaching heights of 15 m. The largest known black smoker at the TAG hydrothermal field, nicknamed “Godzilla”, reached 45 m before collapsing in 1996. At the East Pacific Rise (a fast-spreading ridge system), vents are typically smaller than those at the Mid-Atlantic Ridge.
Figure 2: Black smokers at the Mid-Cayman Rise in the Caribbean sea, January 2012. Image taken by Chris German, ©Woods Hole Oceanographic Institute.
ii. White smokers
White smokers were also first discovered at the East Pacific Rise at 21°N. They emit plumes of milky-coloured smoke rich in barium, silicon and calcium (Spiess et al. 1980; Haymon & Kastner 1981; Jannasch & Wirsen 1981) (Figure 3). These plumes lack copper-, iron- and sulphur-bearing minerals, most likely as the result of chalcopyrite and pyrite precipitation within the mound prior to venting (Tivey 1998). At locations including the TAG hydrothermal field, white smoker chimneys have been found to contain desirable metals for mining, including gold, silver and barium (Hannington et al. 1995; Tivey 1998). In addition to a different chemical composition, white smokers and black smokers also differ in that white smokers are typically smaller, emit cooler (265-300°C), slower flowing liquid through a highly porous axial zone (Koski et al. 1994), and exist close to the margins of vent mounds (Kelley 2001).
Figure 3: White smokers at the Champagne vent site, Northwest Eifuku volcano, located within The Marianas Trench Marine National Monument.
Image source: The National Oceanic and Atmospheric Administration (add specific source of figure).
Prior to the discovery of deep-sea hydrothermal vents, deep-sea environments were considered “too extreme” to support highly productive ecosystems (Lutz 2012). The initial 1977 expedition to the Galápagos Rift vent field revealed complex and highly adapted vent-supported ecosystems (Van Dover et al. 2002), challenging existing theories of what conditions limited life on earth. During this expedition, unique megafauna surrounding deep-sea vents were also discovered, including white clams (Calyptogena magnifica) exceeding 30 cm in length (Figure 4), mussels (Bathymodiolus thermophilus) exceeding 10 cm in length (Figure 5) and vestimentiferan tube worms (Riftia pachyptila) reaching 2 m in length (Figure 6). Deep-sea hydrothermal vents were also the first intricate ecosystems discovered to depend on microbial chemoautotrophic production (Van Dover 2000). Productivity within vents begins with chemoautotrophs extracting energy from reduced inorganic compounds concentrated in vent fluids (Hessler & Kaharl 2013). These chemoautotroph microorganisms are then filtered from the vent water or grazed upon by invertebrates such as barnacles and limpets, and many invertebrates (e.g. vestimentiferan tubeworms) host chemoautotrophic microorganisms as part of a symbiotic or epi-symbiotic relationship (Van Dover et al. 2002).
Organisms at deep-sea hydrothermal vents are typically concentrated at the middle and outer layers of chimneys due to the high temperatures (>250°C) of the inner layers (Kormas et al. 2006). At the middle chimney layers, biological communities are rich in methanogens, anaerobic hydrogen oxidisers, reducers and fermentative heterotrophs (McCollom & Shock 1997). As outer layers are cooler and exposed to oxygen from surrounding seawater, biological communities here are characterised by oxidisers and aerobic methanotrophs (Reysenbach et al. 2007). The outer layers of white smokers can also host encrusting worms and crabs (Corliss et al. 1979; Spiess et al. 1980; Haymon & Kastner 1981). Sources of nutrients utilised by deep-sea vent biota include marine snow, whale falls and cold seeps.
More than 400 morphological species from deep-sea vents have been identified since 1977 (McArthur & Tunnicliffe 1998), but there is evidence that numerous deep-sea hydrothermal vent taxa originated from environments other than the deep sea. Some taxa appear to have undergone evolutionary radiation at the familial and generic level and have a long history of deep-sea vent endemicity (Newman 1985), while other taxa appear to have more recently radiated at the species level, possibly originating from shallow-water species (McArthur & Tunnicliffe 1998).
Figure 4: Caylptogena magnifica at the vent site "Calyfield", located at the Galápagos Rift. Image taken by Tim Shank, ©Woods Hole Oceanographic Institute.
Figure 5: A colony of deep-sea Bathymodiolus thermophilus mussels. ©Woods Hole Oceanographic Institute.
Figure 6: Giant tube worms, Riftia pachyptila, at the Galápagos Rift during 1977. Image taken by John M. Edmund ©Woods Hole Oceanographic Institute.
Access to deep-sea hydrothermal vents is highly sought after for purposes including scientific research, bioprospecting and mining. Regulating access to, and activity at, deep-sea hydrothermal vents is complicated by the fact that some deep-sea vents fall within national Exclusive Economic Zones (EEZs), while others are located in international waters. Under the United Nations Convention on the Law of the Sea (UNCLOS) (United Nations 1982), States have rights to exploit the seabed within their EEZs. Despite this right to exploitation, signatories to UNCLOS and additional legally binding conventions, such as the Convention on Biological Diversity (CBD)(United Nations 1992a), are obligated to protect the environment (particularly fragile ecosystems) and to use environmental resources sustainably (Korn et al. 2003). To fulfill this obligation, parties planning on undertaking activity at deep-sea hydrothermal vents are required to evaluate the potential environmental impact of any proposed project via an Environmental Impact Assessment, and submit this to the appropriate body for review. Outside of national jurisdiction, a framework of both legally binding conventions and (hortatory) - replace this with some other word - policies regulate activity at deep-sea hydrothermal vents (Korn et al. 2003). The principle legal instruments of this framework are UNCLOS, Part XI Implementation Agreement (established under UNCLOS) (United Nations 1994), Rio Declaration of Principals (United Nations 1992b), Agenda 21 (United Nations 1993), the CBD, customary law and the rules enforced by the International Seabed Authority. Under the Part XI Implementation Agreement, the seabed outside of national jurisdiction is defined as “The Area”, and all mineral-related activities in “The Area” are to be controlled by the International Seabed Authority. At present, there is not a legally binding agreement in force that specifically applies to marine scientific research undertaken at deep-sea hydrothermal vents in "The Area".
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