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Hydrothermal Vent Communities
(Released May 2006)

 
  by Carolyn Scearce  

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The discovery of hydrothermal vents

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Marine geologists studying ocean temperatures at the 2500 meter deep spreading center of the Galapagos Rift acquired their first definitive evidence for the existence of hydrothermal vents in May, 1976. A deep-tow vehicle deployed less than 40 meters above the volcanic ridge detected a buoyant plume of vent discharge. The vehicle, which originated from Scripps Institution of Oceanography's Marine Physical Laboratory, was equipped with a Conductivity Temperature Density device (CTD), sampling bottles, acoustic sensors, and cameras. Water samples and physical measurements helped identify the hydrothermal plume.

While taking these samples, the vehicle also photographed the ocean floor. To everyone's surprise, these photographs revealed a dense, productive benthic community living in close proximity to the hydrothermal vent. Relying on this data, in 1977 Peter Lonsdale published the first scientific paper describing a hydrothermal vent life. The paper describes a community including mussels, anemones, and crabs, as well as evidence of burrowing activity. Lonsdale speculated that increased food resources near vent plumes allowed the animals to flourish, and suggested that searching for these unusual communities might serve as the simplest method of detecting hydrothermal vents.

close-up of black smoker
Black smoker

In 1977 geologists returned to the Galapagos Rift, diving in Alvin, and had the first chance to see hydrothermal vent communities with their own eyes. Two years later a group of biologists, chemists, and geologists came back to the rift with a film crew from National Geographic in tow. A National Geographic documentary and an article by Robert Ballard and Fredrick Grassle (1979) resulted from the 1979 expedition, introducing the general public to the exotic world found at hydrothermal vents. Alvin brought back pictures of giant tube worms, foot-long clams, galatheid crabs, and other unusual creatures.

Though the discovery of vent communities came as a total surprise, the discovery of hydrothermal vents was not unanticipated. Based on measurements of heat flux, marine geologists had hypothesized about the existence of vents years before any were directly located (Lonsdale, 1977). Additionally, the composition of seawater itself indicated that unexplained chemical processes were taking place in the world's oceans. Seawater manganese concentrations were too high, while magnesium concentrations were too low to be accounted for by mineral contributions from river runoff alone. Chemical analysis of vent waters demonstrated that the circulation of water through the ocean crust decreased magnesium levels and increased manganese concentrations in ocean water.

Hydrothermal vents occur at ocean spreading centers, that is, at locations where tectonic plates are pulling apart, creating new ocean floor as volcanic material rises to fill in the space between the plates. At spreading centers ocean water infiltrates the ocean floor and mixes with molten crust, after which hydrothermal fluids rise back to the surface of the sea floor. As hydrothermal fluids return to the ocean floor, they exit through narrow chimneys known as white or black smokers (Metaxas, 2003). Exiting fluids range in temperatures between 300-400 degrees C and are rich in hydrogen sulfide, heavy metals, and other elements. The high temperatures of vent fluids cause them to be more buoyant than ocean water. As hydrothermal fluids escape into ocean waters, they form buoyant plumes that rapidly mix with ambient seawater. The plume rises until the fluids mix sufficiently to reach a state of neutral buoyancy. At this point the plume spreads out horizontally: ocean currents then dictate further mixing and movement (Van Dover, 2000).

The benthic zone surrounding hydrothermal vents is an extremely variable environment. Within this area, vent fluids and oceanic waters mix. These two water types possess very different physical and chemical properties. Consequently, temperature and chemical gradients form within vent environments. Small distances can make a big difference in the characteristics of the water experienced by organisms that live near vents. As currents shift, water properties can change dramatically in a matter of minutes or seconds. Toxic substances precipitate from vent fluids. Living in such a unique physical and chemical environment can require a considerable amount of adaptability.

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