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50 Years of Ocean Discovery: National Science Foundation 1950—2000
The magma chambers feeding the most recent flows were also the obvious heat source for the active vents. The faster the spreading rate, the more likely were vents to be found, shifting the focus of our studies from the slow-spreading Mid-Atlantic Ridge to the faster-spreading East Pacific Rise.
But our studies along the axis of the Galapagos Rift in 1977 and 1979 and our studies at the East Pacific Rise at 21°N in 1978 and 1979 revealed a significant along-strike variation. This was even true for the axis of the Mid-Atlantic Ridge in the FAMOUS where venting had not been found. The ridge is divided into a continuous series of spreading segments bound at each end by transform faults that offset the ridge to either side. As the intersection between the axis and transform fault is approached, the depth of the axis begins to increase. Since the topography of the ridge is the result of thermal expansion, the higher the elevation of the axis, we reasoned, the more likely were we to find hydrothermal activity.
With the SEABEAM system now installed on the N/O Charcot, Jean and I could run along a major length of the East Pacific Rise testing our model in our search for new sites of hydrothermal venting. From May until July 1980, the Charcot slowly zigzagged down the axis of the East Pacific Rise at 22°N to its fastest-spreading segment at 22°S near Easter Island. From these survey lines, we could clearly see individual spreading segments along the strike of axis, each having topographic highs where we felt active venting might be found.
Our first chance to test this model came in April 1981 with a cruise aboard the R/V Melville to the East Pacific Rise at 20°S. Using the Charcot's SEABEAM maps to guide us, we conducted a series of Angus camera runs down the axis of a fast-spreading segment of the ridge near its topographic high and quickly found active hydrothermal vents.
In January 1982, we had another chance to test this model when Jean brought the submersible Cyana aboard the N/O Le Suroit to dive at 13°N on the East Pacific Rise, a site surveyed in 1980 by the Charcot . Once more, the model proved to be an excellent prediction for finding active hydrothermal vents. We even dove in the Cyana where we did not expect to find vents near the axis-transform intersection and didn't.
By now, we were not the only team searching for new vent settings on the Mid-Ocean Ridge. Peter Lonsdale from Scripps, who had played a major role in the discovery of the hydrothermal vents in the Galapagos Rift, was using his considerable skills to search for vent sites in the Gulf of California. The focus of his research was a series of small spreading segments in Guaymas Basin. His efforts proved equally successful in January 1982, when a series of dives by Alvin located and investigated a number of active vents. What made these vents unique was their occurrence in an area of thick sediments.
At the northern end of the Gulf of California is the Colorado River delta. For millions of years this river has deposited a tremendous volume of organic-rich sediments into the gulf, including Guaymas Basin. As a result, the active spreading axis underlying the gulf is buried under a thick accumulation of mud. Hydrothermal fluids that flow out of fissures cutting across the young central volcanic terrain must then rise hundreds of meters through this sediment cover before exiting into the basin's bottom waters. During this final vertical journey, these superheated fluids interact with the overlying organic sediments, greatly altering their chemistry. Oil seeps of thermogenic petroleum hydrocarbons were commonly associated with active vent sites and the soft sediment surface was covered by extensive bright yellow and white bacterial mats.
From 1981 on, the investigation of hydrothermal circulation in the ocean's crust intensified and spread throughout the word. A team headed by Peter Rona of NOAA located hydrothermal vents in the Mid-Atlantic Ridge. Both French and American researchers found additional vents along the East Pacific Rise at 10, 11, and 13°N. From 1982, active hydrothermal vents were discovered on the East Pacific Rise at 13°N followed in 1984 by the discovery of similar vents sites on the Juan de Fuca Ridge and Discover) Ridge off the coast of Washington and British Columbia.
As more active hydrothermal sites were discovered on the East Pacific Rise, the search broadened to include other geologic settings. Dives by Alvin in the Marianas Back-arc basin successfully located active vents. These discoveries were followed by expeditions to the Mid-Atlantic Ridge, which located active vent sites at 26 and 23°N. More recently, hydrothermal vents have been found t,-, the north on the Mid-Atlantic Ridge at 37°17.5'N and 37°50'N. In these latter instances, the vent sites are near the Azores "hotspot" and associated with large lava lakes.
Once high-temperature vents were discovered along the East Pacific Rise at 21°N in 1979, additional important vent animals were added to the list. Perhaps the most impressive was a worm dubbed the "Pompeii Worm" for its ability to live in close proximity to the black smokers, where the exiting vent temperature can exceed 350°C. These worms (Alvinella pompejana) live in tiny tubes that are constantly being covered by fine-grain minerals precipitating out of the vent waters once the hot fluids come into contact with cold ambient seawater.
The dominant organisms associated with the hydrothermal vent communities of the Eastern Pacific (i. e., Galapagos Rift, East Pacific Rise, Guaymas Basin, and Juan de Fuca/Discovery Ridges) include the long, red-tipped vestimentiferan tube worms, large white bivalve clams, and thick accumulations of mussels. Variation in the vent faunal assemblages is thought to be related to differences in vent flow and water chemistry, with higher concentrations c [ biomass associated with lower-temperature vents (i.e., 5-200°C) compared to the higher temperature vents (i.e., 200-360°C).
The two giant-sized mollusks mentioned earlier are the clam-like Calyptogena magnifica and the mussel Bathy