Hydrothermal discoveries from the deep.
Since undersea "chimneys" were firstdiscovered to be spewing out hot, mineral-laden water from the ocean floor (SN: 1/12/80, p.28), scientists have come to suspect that they are as important a source for ocean chemicals and heat as are land-based processes. This idea was fortified last week as researchers reported on hydrothermal venting at a variety of sites in the Pacific and Atlantic oceans. They presented their findings in San Francisco at the joint meeting of the American Geophysical Union and the American Society of Limnology and Oceanography.
One of the most exciting recent findswas made by the National Oceanic and Atmospheric Administration (NOAA) research ship Discover, which lived up to its name last August during a cruise over the Juan de Fuca spreading ridge, 400 miles off the coast of Oregon. Discover's towed instruments detected a new kind of hydrothermal plume that was much hotter, larger, shorter-lived and more symmetrically shaped than the plumes that normally emerge from seafloor vents. According to Edward Baker of NOAA's Pacific Marine Environmental Laboratory in Seattle, this plume -- which was apparently released over a few days -- contained 100 cubic kilometers of water. The amount of heat and chemicals in the plume was equivalent to that emitted by as many as 2,000 "black smoker" vents in one year.
Computer models suggest that theplume could have been formed by a long crack -- about a kilometer long by one-third meter wide -- that for a few days issued a "black curtain" of hot water and minerals, says Baker. Because of the plume's large size and its placement over the Juan de Fuca ridge -- part of the 60,000-kilometer-long, seam-like network of spreading centers that churn out new seafloor worldwide -- researchers suspect it was released during a small episode of seafloor spreading.
"Certainly there have been underwatervolcanic eruptions, but we've not observed anything like this on a ridge crest before," says Stephen hammond, director of the NOAA VENTS Program in Newport, Ore. Most geologic evidence for seafloor spreading is in the form of records of processes that act over millions of years, he says. The new plume offers a glimpse at seafloor spreading on the scale of days. "Since we really don't know the time rates of seafloor spreading," he adds, "this may be a very key observation."
Researchers at the meeting also highlightedstudies of vents on the Mid-Atlantic Ridge, which spreads much more slowly than its Pacific cousins. Last summer, with the help of the submersible Alvin, they found that the water chemistry and heat from Atlantic vents are similar to that emerging from vents along the East Pacific Rise.
"That's very unexpected," says PeterRona of NOAA in Miami. "We thought these vents would be completely different, since they're on a slow-spreading ridge ... which is a kilometer deeper and supposedly has a different kind of magma chamber under it."
Rona's group has also discovered twolarge mounds of polymetallic sulfides -- compounds of potential economic importance that are deposited on the ocean bottom when the hydrothermal plumes hit the cold seawater. One mound, still being deposited, contains 4.5 million tons of sulfides and is the size and shape of the Houston Astrodome, says Rona. The group also found a much larger, inactive mound that they think was built up over tens of thousands of years.
The implications of this find, saysRona, "are that slow-spreading ridges may be more effective at concentrating larger sulfide deposits than are fast-spreading ridges, where the crust moves away much sooner from the heat source driving hydrothermal circulation."
The recent Alvin dives in the Atlanticgave biologists a closer look at Atlantic vent communities, including a newly discovered genus of shrimp found swarming around "black smoker" chimneys. Perhaps the most intriguing biological mystery in the vent area, however, was the finding of thousands of highly symmetric, Chinese-checkerboard-like patterns on the seafloor, which were first photographed several years ago. Rona thinks the pattern may be either an animal itself or the burrows made by an animal. He says the patterns are "dead ringers for a 70-million-year-old [Paleodictyon] trace fossil that is exposed in the Alps."
Scientists took two core samples ofthese Atlantic patterns during the recent Alvin dives, but Rona says the biologists have not yet opened them. This part of the deep Atlantic, he says, may be a stable sanctuary for what may turn out to be the first living examples of an animal long extinct on the rest of the planet.
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|Date:||Dec 20, 1986|
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