August 7, 2003
Seafloor vent systems may have spawned earliest life
Black smoker hydrothermal vent systems may have the fire power, but the staying power of seafloor hydrothermal vent systems like the bizarre Lost City vent field — discovered just two and a half years ago — is one reason they may have been incubators of some of Earth’s earliest life, say UW scientists and their co-authors in a recent issue of Science.
Lost City, in the mid-Atlantic Ocean about 1,500 miles from the United States, has the tallest vents ever seen. The 18-story tower at the site dwarfs most vents elsewhere by at least 100 feet. Water circulates through the vent field because of heat from a chemical reaction between seawater and mantle rock, rather than by heat from volcanic activity or magma, as in the black-smoker vents found on the seafloor 200 miles off the coast of Washington. Both systems teem with microorganisms capable of living off the vent fluids, which would be toxic to all other life.
If hydrothermal venting can occur without volcanism, it greatly increases the places on the seafloor of early Earth where microbial life could have started, says Deborah Kelley, UW oceanographer and one of the co-authors. “It’s difficult to know if life might have started as a result of one or both kinds of venting,” she says, “but chances are good that these systems were involved in sustaining life on and within the seafloor very early in Earth’s history.”
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. 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 — eventually shoves the seafloor bearing the vents away from the source of volcanic heat needed to power them.
Lost City is 9 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, and generates heat to power hydrothermal circulation.
A big reason this kind of system is so self-sustaining is that fracturing happens during tectonic movements and because rocks undergoing serpentinization increase in volume from 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.
Other Science co-authors are UW graduate students Kristin Ludwig and Giora Proskurowski, the Swiss Federal Institute of Technology’s Gretchen Früh-Green and Stefano Bernasconi, and National Oceanic and Atmospheric Administration’s David Butterfield.
This spring, the National Science Foundation 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/