Hydrothermal Vent Systems Could Have Persisted Millions Of Years, Incubated Life

Discovered just 2½ years ago during a National Science Foundation-funded expedition in the mid-Atlantic Ocean, Lost City has the tallest vents ever seen – the 18-story behemoth at the site dwarfs most vents elsewhere by at least 100 feet. Water is circulated through the vent field by heat from serpentinization, a chemical reaction between seawater and the mantle rock on which Lost City sits, rather than by heat from volcanic activity or magma, responsible for driving hydrothermal venting at sites scientists have been studying since the early 1970s.

If hydrothermal venting can occur without volcanism, it greatly increases the places on the seafloor of early Earth where microbial life could have started. It also means explorers may have more places than previously thought to look for microbial life in the universe.

Although the Lost City vent field is a youthful 30,000 years old, Lost City-type systems might be able to persist hundreds of thousands, possibly millions, of years, says lead author Gretchen Früh-Green of the Swiss Federal Institute of Technology and co-authors from the University of Washington, Duke University and National Atmospheric and Oceanic Administration. One can imagine how such stable, long-lived systems pumping out heat, minerals and organic compounds for millennia might improve the chances for life to spark and to be sustained until it could take hold, say these scientists.

"It's difficult to know if life might have started as a result of one or both kinds of venting," says Deborah Kelley, University of Washington oceanographer, "but chances are good that these systems were involved in sustaining life on and within the seafloor very early in Earth's history."

As far as longevity and stability, it's possible that black-smoker systems might last as long as 100,000 years but it's unlikely, Kelley says. That's because black-smoker systems typically form where new seafloor is being created, a process that – even if a volcanic eruption doesn’t bury a hydrothermal vent field in lava – will eventually shove the seafloor bearing the vents away from the source of volcanic heat needed to power them.

Lost City is already nine miles from the nearest volcanically active spreading center and sits on 1.5 million-year-old crust. Seawater permeating deeply into the fractured surface of the mantle rocks transforms olivine into a new mineral, serpentine. The heat generated during this process is not as great as that found at volcanically active sites – where fluids can reach 700 F – but it is enough to power hydrothermal circulation and produce vent fluids of 105 to 170 F.

Tectonics, the movement of the Earth's great plates, contributes to the fracturing of the mantle rock. But a big reason this kind of system is so self-sustaining, the Science report says, is that fracturing also happens because rocks undergoing serpentinization increase in volume 20 percent to 40 percent. Kelley likens it to water seeping into tiny cracks in roads, then freezing and expanding to cause ruts and frost heaves in the pavement.

Scientists think many Lost City-type systems were possible on early Earth because so much of the mantle had yet to be skinned over with crust, putting it in contact with seawater and making serpentinization possible, Kelley says.

Lost City is the only vent field of its kind known today but scientists say more could exist. Within a 60-mile radius of Lost City are three similar mountains and there are other, potential sites along thousands of miles of ridges in the mid-Atlantic, Indian Ocean and Arctic.

Beyond Earth, peridotite – the mantle material that reacts with seawater during serpentinization – is abundant on all the terrestrial planets in our solar system, says Jeff Karson, Duke University professor. "Peridotite can be exposed by tectonic processes or by major cratering events. This means that Lost City-type venting could occur, or has occurred, in oceans on other planets, and such venting would have the potential to support microbial systems."

Lost City-type systems also may be conducive to life because their fluids are high pH and rich with organic compounds compared to black-smoker systems.

Black smokers get their name because it can appear as if smoke is billowing from the vents. What's actually being seen are dark minerals precipitating when scalding hot vent waters meet the icy-cold ocean depths. Water venting at Lost City, in comparison, is hot enough to shimmer but not "smoke." Because of the different chemistry, black-smoker vents are a darkly mottled mix of sulfide minerals whereas the Lost City vents are nearly 100 percent carbonate, the same material as limestone in caves, and range in colors from white to cream to gray.

The field, named Lost City in part because it sits on a seafloor mountain named the Atlantis Massif, was discovered Dec. 4, 2000, when scientists weren't even looking for hydrothermal vents.

"The discovery of the Lost City vent field is a wonderful example of serendipity in science – studying one problem and discovering something totally new and unexpected," says David Epp, program director in NSF's marine geology and geophysics program. "The detailed work is just beginning and should change the way people think about vent systems."

This spring, the NSF funded the first major scientific expedition to Lost City since its discovery. Led by Kelley and Karson, the expedition is documented at: http://www.lostcity.washington.edu/

Other Science co-authors are the Swiss Federal Institute of Technology 's Stefano Bernasconi, University of Washington's Kristin Ludwig and Giora Proskurowski, and National Atmospheric and Oceanic Administration 's David Butterfield.