Deep-sea hydrothermal vents along the Mid-Cayman Rise
A mid-ocean ridge (MOR) is a mountain range on the ocean floor. A typical MOR has a valley known as a rift running along its spine, formed by plate tectonics. These MORs result when convection currents which rise in the mantle as magma uplifted seafloor at a linear weakness in the oceanic crust, and emerge as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates. All the mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system, making the mid-oceanic ridge system the longest mountain range in the world.
A hydrothermal vent: 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 on the ocean floor
A hydrothermal vent on the ocean floor spews super-hot, mineral-rich water that helps support a diverse community of organisms including giant tube worms, clams, limpets and shrimp. 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 these 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. As the vent water bursts out into the ocean, its temperature may be as high as 400°C (750°F). Yet this water does not boil because it is under a lot of pressure due to the tremendous weight of the huge body of ocean above.
While hydrothermal vent sites occupy small areas on the sea floor, the plumes formed when hot acidic vent fluids mix with cold deep-ocean seawater can rise hundreds of meters until they reach neutral buoyancy. Because these plumes contain dissolved chemicals, particulate minerals and microbes, they can then be detected for kilometers or more away from their source as they disperse horizontally in the ocean. The chemical signatures of these plumes vary according to the type of vent site from which they originated.
The three known types of hydrothermal vent sites “Type 1″, “Type 2″and “Type 3″ are distinguished by the kinds of rock that host the sites. The first type of vents occur throughout the world’s mid-ocean ridges and are hosted by rocks that are rich in magnesium and iron –called mafic rocks. The second and third types of vent sites are hosted in rocks called ultramafic that form deep below the seafloor and are composed of material similar to the much hotter lavas that erupted on Earth’s very earliest seafloor, thousands of millions of years ago.
The Mid-Cayman Rise (MCR) is an ultraslow spreading mid-ocean ridge located in the Cayman Trough – the deepest point in the Caribbean Sea and a part of the tectonic boundary between the North American Plate and the Caribbean Plate. At the boundary where the plates are being pulled apart, new material wells up from Earth’s interior to form new crust on the seafloor.
Deep-sea hydrothermal vents along the Mid-Cayman Rise
An interdisciplinary team led by Woods Hole Oceanographic Institution (WHOI) under took the first expedition to search for deep-sea hydrothermal vents along the Mid-Cayman Rise.
The team has reported discovering three distinct types of hydrothermal venting. The team identified the deepest known hydrothermal vent site and two additional distinct types of vents.
The first two sites the team identified are extremely deep and were named Piccard and Walsh in honor of the only two humans to dive to the Challenger Deep – the deepest part of the world’s ocean. The plume detected at the Piccard site – 800 meters deeper than the previously known deepest vent – was comparable to plumes from the “Type 1″ vent sites, first found in the Pacific Ocean in 1977. The Walsh plume also exhibited signals characteristic of a high temperature site, but with a chemical composition (notably the high methane-to-manganese ratio) typically found at a high temperature, ultramafic hosted “Type 2″ vent site. The third site – which the team have named Europa, after the moon of Jupiter – most closely resembles the “Lost City” vent site in the mid-Atlantic ocean— to date the only confirmed low-temperature “Type 3″ site.
Chris German who is the chief scientist as well as the a WHOI geochemist said, “This was probably the highest risk expedition I have ever undertaken. We know hydrothermal vents appear along ridges approximately every 100 km. But this ridge crest is only 100 km long, so we should only have expected to find evidence for one site at most. So finding evidence for three sites was quite unexpected – but then finding out that our data indicated that each site represents a different style of venting – one of every kind known, all in pretty much the same place – was extraordinarily cool. The discovery of ultramafic-hosted vent sites on the Mid-Cayman Rise could provide insight into the very earliest life on our planet and the potential for similar life to become established elsewhere.”
For this mission, German and his colleagues using CTD (conductivity, temperature, and depth) array augmented with sensors to detect suspended particles and anomalous chemical compositions mounted on both a water sampling rosette and a deep-diving robot called Nereus, sniffed out deep-sea plumes originating from the seafloor hydrothermal vents. Using a combination of shipboard and shore-based analyses of water samples for both their chemical and microbial contents, the team was then able to track the plumes toward their sources as well as to determine the likely nature of the venting present at each site. Nereus can operate in both in tethered or “remotely operated” (ROV) mode and free-swimming mode.
The ultimate goal of the mission was to switch Nereus to ROV mode and dive on each vent site to collect samples using Nereus’ robotic manipulator arm. But when they came to within < 250m of the vents at the seafloor they had to stop the mission because of the intervention of the tropical storm Ida. However the research team shared their findings with an international team led by Jon Copley of the National Oceanography Centre in Southampton, UK, who returned to the MCR in Spring 2010 and imaged active vents at both the Piccard and Europa locations using a deep-towed camera called Hybis.
German said, “Given the range and diversity of systems present, and now that we have established exactly where the sites are and what they look like, we really can’t wait to get back and collect first samples with our ROV Jason. This region has the potential to develop into an exciting natural laboratory with plenty of potential for repeat visits and long-term experiments over the decade ahead.”
July 21, 2010