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A Lost 'City' of the Deep Reveals Unexpected Forms

 

The remarkable hydrothermal vent structures serendipitously discovered last December during an expedition to a mid-Atlantic Ocean underwater mountain were formed in a very different way than other ocean-floor vents studied since the 1970s, according to findings published July 12 in the journal Nature.

The circulation of fluids that form this new class of hydrothermal vents, which include a massive 18-story vent structure taller than any seen before, apparently is driven by heat generated when seawater reacts with rocks from the underlying mantle, not by volcanic heat.

Though no one has previously seen a field like this one, this kind of vent may be common on the seafloor, said Deborah Kelley, a University of Washington oceanographer and lead author of the Nature report.

If so, scientists may have underestimated the extent of hydrothermal venting, the amount of heat and chemicals pouring into the world's oceans, and the abundance of life that thrives in such conditions.

"Rarely does something like this come along that drives home how much we still have to learn about our own planet," Kelley, a vent expert, said in an interview.

"This means hydrothermal systems may exist in the ocean floor in places where we never expected them," added Jeffrey Karson, a geology professor at Duke's Nicholas School of the Environment and Earth Sciences who accompanied Kelley on a submarine dive to the vent site and was second author of the Nature article.

The Lost City Field, named partly because it sits on an unusually tall dome-shaped seafloor mountain called the Atlantis Massif, was discovered Dec. 4 by an international team of researchers that also included three Duke graduate students.

The expedition, funded by the National Science Foundation, was led by the Scripps Institution of Oceanography geophysicist Donna Blackman to study how the Atlantis Massif formed.

Lost City is like other hydrothermal vent systems where seawater circulating beneath the ocean floor gains chemicals and heat -- enough heat for the fluids to then rise buoyantly and vent into the overlying ocean. As the heated fluids then mix with cold seawater, the chemicals separate from the vent fluids and solidify, sometimes building up into impressive mounds, spires and chimneys of minerals.

Excited expedition scientists immediately realized that the Lost City Field also differs from other hydrothermal vent systems. The first difference was the imposing heights of some of its structures. The structure researchers named Poseidon towers 180 feet, compared to mostly 80 feet or less at other vent fields.

The new vents' chemical compositions are also nearly 100 percent carbonate, the same material as limestone in caves, and range in color from a beautiful white to cream or gray. That contrasts to "black smoker" vents at other places, composed of a darkly mottled mix of sulfide minerals.

Perhaps the Lost City's most distinctive feature is its location on 1.5 million-year-old crust formed from material that migrated far upward from Earth's mid-region, the mantle. Previously known vents formed along the youngest parts of volcanic "spreading centers," where tectonic forces pull apart the sea floor and hot melted magma fills the void.

Heat from underlying magma chambers, located nearer to the surface than the mantle, drives hydrothermal circulation in normal vent fields, generating water temperatures as high as 400 degrees Celsius (752 Fahrenheit).

Those water temperatures are much hotter than recorded at Lost City, located in a place with no obvious heat source. In the area of the Lost City, mantle material was somehow forced up from below. Permeating seawater then appears to have transformed olivine in the mantle rocks into a new material, serpentine, a process called "serpentinization."

Serpentinization causes the altering rock to expand, and the heat that process generates appears to drive hydrothermal circulation at Lost City, Kelley said. Vent fluids there are elevated to lower temperatures of 40 to 75 degrees C (104-167 F).They are also rich in the chemical methane and the element hydrogen.

Based on samples collected by the Woods Hole Oceanographic Institution's research submarine Alvin, that environment supports a diverse and dense microbial community. Microscope examination revealed rocks too covered with life forms for scientists to see the underlying minerals, Kelley noted.

Yet the larger animals, such as clams or tubeworms, that typify other vent environments appear to be rare at Lost City.

Karson, a structural geologist, said in an interview that he was also intrigued by a related aspect of the discovery. The Nature article's authors wrote that "this newly discovered class of seafloor hydrothermal system may provide insights into hydrothermal processes of the early Earth and the life forms they supported."

When the first bacteria appeared on Earth about 3 billion years ago, "olivine rich rocks were more common near the Earth's surface than today because the Earth was much hotter," Karson said.

If the article is right about the forces that drive hot water circulation at the Lost City Field, it's easy to imagine there could be many more such systems, Kelley added.

Within just a 50-mile radius of the Atlantis Massif are three similar mountains subject to the same fracturing and seawater intrusions and perhaps the same reactions with mantle material. And those four represent only a tiny fraction of the potential sites, she said.

A Web site account of the expedition to Atlantis Massif can be found at http://earthguide.ucsd.edu/mar/.