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New type of hydrothermal vents found

A five-foot-wide flange, or ledge, on the side of a chimney in the Lost City Field is topped with dendritic carbonate growths  

By Alex Walker
CNN Science and Technology

SEATTLE, Washington (CNN) -- Hydrothermal vents discovered last December in the mid-Atlantic Ocean puzzled scientists because their formation was unlike any other ocean-floor vents previously studied. Oceanographers may now know why.

According to scientists writing in the British journal Nature this week, the new class of vents is not formed by volcanic activity, but rather by the heat generated when seawater reacts with mantle rocks.

"It's a brand new kind of hydrothermal system," said University of Washington oceanographer Deborah Kelley, who authored the Nature article. "Seawater causes a chemical reaction with the unstable mantle rocks, and gases such as methane and hydrogen form. The process, which turns mantle rock into the mineral serpentine, gives off heat and contributes to the formation of vents."

Since the 1970s, scientists believed that hydrothermal vents were unique to "spreading centers" -- places on the ocean floor where volcanic activity occurs. Those areas, such as the East Pacific Rise, experience eruptions every 10 to 20 years and vent hot fluids that are scalding hot -- about 700 degrees Fahrenheit.

The newly discovered vent field, called the Lost City, is located in a slow-spreading, much cooler environment. Eruptions there occur every 20,000 to 50,000 years, and temperatures range from only 110 to 170 degrees Fahrenheit.

"It was surprising to discover hydrothermal vents sitting on a million to a million-point-five year-old crust," Kelley said. "Based on this finding, there must be other similar vent fields out there. We had been looking for them only in volcanic regions. Now we know they could be elsewhere."

The physical structure of the newly found vents is different than the dark "black smokers" in volcanic areas that spew sulfide materials from deep within the earth. The Lost City vents are a light-colored carbonite, which is the same limestone material found in caves.

Important discovery for microbiologists too

Also, the new vents are much taller than any other vents ever discovered.

One called Poseidon is 180 feet tall, the equivalent of an 18-story building.

Previously studied vents range from 60 to 80 feet.

The carbonate structures at the Lost City Field include this chimney more than 30 feet in height -- the top shows evidence of collapse and re-growth  

"Height requires a stable plumbing system for the vent fluid for a prolonged period of time," Kelley said. "Volcanic vents, or black smokers, are either covered by flows from frequent eruptions, or the plumbing system is cut off due to earthquakes. That's why they are much shorter than the stable vents in Lost City."

The discovery of these new vents is important not only to oceanographers and to geologists, but to microbiologists as well. Chemical reactions in vent fluids provide nutrients to single-cell organisms, called thermophiles, which do not need sunlight to survive. The Lost City Field is at a depth of more than 3,200 feet.

"On the vents, you notice filaments that look like small kelp beds waving in the water," Kelley said. "We took a small rock sample to the lab, and could not even see the surface of the rock. It was completely covered with single-cell organisms."

Thermophiles, the only inhabitants of the Lost City, include archaea, a recently discovered class of organisms which scientists believe are among the most primitive on earth.

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