December 12, 2002
Different kind of venting provides alternatives for finding life elsewhere
It was two years ago this month that scientists surveying the seafloor in the mid-Atlantic were startled to come upon a field of hydrothermal vents with pale “chimneys” the height of skyscrapers, far taller than any seen before, leading scientists to believe they were looking at a field unlike any previously discovered.
Hydrothermal vent fields are made up of chimneys, spires and mounds that form as heated seawater loaded with minerals vents from the seafloor into the ocean. Previously studied vent fields have been on or close to volcanically active ridges on the seafloor. The field found Dec. 4, 2000, which was named “Lost City,” was miles from such a ridge.
Lost City is giving scientists reason:
- To believe that similar systems may be or have been present on water-bearing, tectonically active planets. Researchers also believe that systems like Lost City may be common on Earth, according to Deborah Kelley, University of Washington associate professor of oceanography and one of the three people on the first manned dive to the field the day after it was discovered.
- To speculate that life on this planet may have started in the relatively warm, alkaline waters rich with methane and hydrogen that result when seawater reacts with mantle rock. Today the mantle in most places is capped by oceanic crust. But early in Earth’s history mantle rocks may have been much more exposed to seawater, providing ample opportunity for conditions to support microbial life at the seafloor.
Kelley explained the differences from previously studied vent systems during last week’s news conference “Life in extreme environments on Earth – and where else in the solar system?” at the annual fall meeting of the American Geophysical Union in San Francisco. Scientific presentations during the meeting on Lost City included one by Kelley and one by UW graduate student Matt Shrenk.
Kelley said Lost City is distinctive because it sits on mantle rock that’s about 1.5 million years old and its hydrothermal venting — in which water circulates into the seafloor, gaining heat and chemicals until there is enough heat for the fluids to rise buoyantly and vent back into the ocean — doesn’t appear to be driven by volcanic activity.
Fluids from there give no chemical evidence of having been in contact with magma chambers, Kelley said. This is unlike any system found at the Earth’s spreading centers where magma chambers are present. Very young seafloor is created – often dramatically during volcanic eruptions – at spreading centers and vented water can be as hot as 700 degrees F.
But venting at Lost City occurs because of heat generated by chemical changes in the rocks: seawater permeates deeply into the fractured surface of the mantle rocks where it transforms the mineral olivine into a new mineral, serpentine, in a process called serpentinization.
The heat generated during serpentinization is not as great as that at volcanically active sites but it is enough to power hydrothermal circulation and produce vent fluids of 105 to 170 degrees F, Kelley said. The result is a field of dramatic vents not made of sulfide but of carbonate minerals, or limestone. The most massive, at 18 stories, is the tallest vent structure ever seen anywhere. The vents support a community of microorganisms able to live off the fluids rich with methane and hydrogen, both byproducts of serpentinization.
The Lost City hydrothermal vent field was discovered in the mid-Atlantic, at about 30 degrees north latitude, during a National Science Foundation-funded expedition that had not set out to seek a hydrothermal vent field. It is nine miles from the nearest volcanically active spreading center.
It is on the summit of the mountain known as Atlantis Massif, one reason for the name Lost City. Mantle rock is usually many kilometers beneath the seafloor but at the Atlantis Massif, the Earth’s forces thrust mantle rock up exposing it directly at the seafloor. Spreading and faulting stripped much of the mountain down to the underlying mantle rocks.
The extent of the hydrothermal field is unknown. In the limited time researchers were there they saw about 30 active and inactive carbonate chimneys. Tallest is the 180-foot vent scientists have named Poseidon. Previously studied vents mostly reach 80 feet or less with the tallest being a 135-foot vent on the seafloor off the coast of Washington (which toppled in recent years).
The new vents are nearly 100 percent carbonate, the same material as limestone in caves, and range in color from a beautiful clean white to cream or gray, in contrast to black smoker vents that are a darkly mottled mix of sulfide minerals.
It’s easy to imagine there could be many more such systems, Kelley said. Within a mere 60-mile radius of the Atlantis Massif are three similar mountains subject to the same fracturing, the same intrusion of seawater and perhaps the same reactions with mantle material. And those four represent a tiny fraction of the potential sites along the 6,200 mile Mid-Atlantic Ridge, Indian ridges and the Arctic Ridge, also considered slow- and ultra-slow-spreading centers.
Kelley leads the second scientific expedition to Lost City from April 9 to May 9, 2003.