New type of hydrothermal vent looms large.

A newly found hydrothermal vent system, which its discoverers have dubbed the Lost City, rises on an undersea mountain in the Atlantic Ocean. The mineral deposits produced by these vents include, at 18 stories tall, the largest ever observed. The deposits rise like chimneys above the seafloor and are host to rich communities of microbial life.

This scene represents a new type of hydrothermal vent system, reports Deborah S. Kelley, a geologist at the University of Washington in Seattle. The mineral-laden waters that spew from the vents are driven by heat-generating chemical reactions in the rocks of Earth's crust. The discovery may help explain some mysteries about the chemistry of deep ocean waters. It also suggests that ocean-bottom hydrothermal activity may be much more widespread than geologists had expected.

The concentrations of chemicals in the warm, alkaline water gushing from the Lost City vents are similar to those that some scientists contend may have been present at similar hydrothermal systems during the early years of Earth's history, says Kelley.

The new hydrothermal field was discovered last December near the top of a seamount called the Atlantis massif. This submerged mountain lies about 2,500 kilometers east of Bermuda. Although the mountain sharply rises from the seafloor, its flat top still lies about 700 meters below the ocean's surface. Shallow-water shells and signs of erosion atop the seamount indicate that at least some portions have been at or near sea level. Kelley and her team report their find in the July 12 NATURE.

Most known systems of hydrothermal vents have been found along midoceanic ridges, where large blocks of Earth's crust--tectonic plates--spread apart to form new seafloor. At these sites, molten rock at or just beneath the ocean bottom heats water that has seeped downward through fissures. Such vent systems emit a mineral-rich, highly acidic brew as hot as 400 [degrees] C. In the ocean depths, iron, sulfur, and other materials precipitate from these waters to form dark clouds, so scientists refer to such vents as black smokers.

Kelley says the Lost City is different from all other known hydrothermal fields. It's located about 15 km away from a midoceanic ridge and on ocean crust that's about 1.5 million years old. What's more, rather than being driven by the heat of volcanism, hydrothermal activity at the Lost City vents appears to be fueled by a chemical reaction that's taking place in the seamount's rocks.

Those rocks, which were once deep within the Earth's mantle, were exposed by erosion after the seamount rose to near or above the sea's surface. Now in contact with seawater that circulates through fissures in the rock, a mineral called olivine changes into another mineral, serpentine. This reaction generates heat and produces a number of substances including methane and hydrogen. Kelley's team identified these gases dissolved in the highly alkaline water that spouts from the Lost City vents at temperatures between 40 [degrees] C and 75 [degrees] C.

When the heated water emerges from the vents into the cool ocean water, chemical reactions deposit vast amounts of minerals and form an otherworldly landscape of chimneys, mounds, and spires. Analysis of the deposits shows a mixture that includes calcium carbonate, the same white mineral that forms stalactites and stalagmites in terrestrial limestone caves.

Because calcium carbonate is much stronger than the sulfide minerals typically formed by other hydrothermal vents, the Lost City deposits can grow exceptionally high. The tallest of the 30 or so carbonate structures there, a 60-m chimney, is still growing. It easily tops the largest chimney ever found at a black smoker--a 41-m-tall sulfide specimen off Washington State that broke in half and toppled over a few years ago.

Hydrothermal activity plays an important part in moving dissolved minerals and other substances through the Earth's crust and into the ocean, says Andrew T Fisher, a hydrogeologist at the University of California, Santa Cruz. There's enough water flowing through such seafloor vent systems to recycle the ocean s volume every 500,000 to 1 million years, he notes.

Furthermore, the newly discovered, nontectonic type of hydrothermal vent found at the Lost City could help researchers better understand the wide range of chemical processes that take place in the ocean, says Karen L. Von Damm, a geochemist at the University of New Hampshire in Durham. Scientists have realized for years that the known rates of production and loss among some of the ocean's dissolved minerals don't seem to match, she adds.

The methane, hydrogen, and minerals emitted by the Lost City vents support a thriving community of microbes, says Kelley. Although a few crabs and sea urchins walk the streets of the Lost City, Kelley says that these complex deep-sea creatures don't seem to be closely associated with the vent system. The Lost City appears devoid of other multi-cellular animals, such as the tube worms that populate some black-smoker sites (SN: 9/28/96, p. 201).

"This find is incredibly exciting because it shows us just how rich and diverse the deep-sea ecosystem might be," says Robert M. Hazen, a geophysicist at the Carnegie Institution of Washington (D.C.). "There's every reason to suspect that life could have evolved at sites like this."