Antarctic ice seals life's fate

by Paul Francuch francuch@uic.edu of the University of Illinois at Chicago

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Microbes discovered packed in an ice-sealed, briny lake in Antarctica may help advance techniques to search for signs of life locked in the subterranean ice on Mars, and provide a model for what lakes on Earth may have looked like during severe glacial periods. The findings were reported in the online edition of the Proceedings of the National Academy of Sciences the week of Dec. 16.

A team of researchers led by Peter Doran, assistant professor of earth and environmental sciences at the University of Illinois at Chicago, discovered an unusual and extreme aquatic ecosystem in Lake Vida -- among the largest of many lakes in Antarctica's McMurdo Dry Valleys located about 2,000 miles due south of New Zealand.

The team found a liquid lake of super-concentrated salt water, seven times saltier than normal seawater, locked beneath 19 meters (62 feet) of lake ice -- a record lake ice cover on Earth. The salty water has been isolated from the atmosphere for at least 2,800 years.

Ice core samples taken from above the pool of brine revealed frozen bacteria and algae that came back to life after gradual melting. The scientists believe the chilly brine may harbor life as well.

"Any primary producers potentially living in the brine down deep are not getting sunlight, so they would have to be chemosynthetic, that is, getting energy from the surrounding chemicals instead of sunlight," said Doran. "The microbes frozen in the ice above seem to be life that was living shallow in the lake at some time, then water flowed on top of the thick ice cover and froze."

Carbon-14 dating showed microbes gathered from ice near the brine to be more than 2,800 years old. Doran said the brine is likely to be at least that old, forming as salts settled down into it while thick ice grew outward from the sealed, freeze-resistant salty lake.

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Doran and his research team bored holes to take ice samples from Lake Vida back in 1996 after scanning the lake with ground penetrating radar. "It's a beautiful tool for this because radar doesn't like salt," said Doran. "As soon as radar sees salt, it disappears, so there's a really clear line of where the salt begins."

With those findings, the research team mapped Lake Vida with indications of where salt water appeared trapped. They bored down through ice, taking core samples but stopping in slushy water about three meters above the brine lake. One reason for not drilling lower was fear that special ice-melting chemicals used to help the drilling might have contaminated samples of brine.

The research overturns earlier assumptions that Lake Vida was frozen solid. "That's an important conclusion because Lake Vida is in an area that has a fairly cold mean annual temperature," said Doran. "It actually pushes back our estimates of what it takes to freeze a lake ecosystem."

NASA is interested in the research because the Lake Vida ecosystem serves as a classroom of sorts, providing lessons for launching Martian ice probes that may yield frozen microbes.

"It's pretty much common knowledge that Mars had a water-rich past and likely went from a warmer to a colder climate, although this last point is coming under much debate lately," said Doran. "Mars is very cold. It has a thin atmosphere now and cannot support liquid water on the surface, although there's evidence that there may be something in the near-surface -- maybe a brine, or something like that."

"If the planet went through a cooling, Mars must have had perennially ice-covered lakes, then this ice-sealed type of environment," said Doran. "Unless we find evidence of life still there today, I see Lake Vida as being like life's last romp on Mars, potentially, in an ice-sealed lake. Lake Vida gives us an excellent model of how long a lake ecosystem can survive before you snuff it out by turning down the temperature. How does the ecosystem respond, what does it look like and, more importantly, if we go back and collect samples, where do we look and what are we looking for to find evidence of these types of lakes?"

NASA and the National Science Foundation are jointly funding a follow-up study to test miniature coring equipment and to dip a straw-like device into the brine of Lake Vida to suck-out samples.

"We'll do it like we're on a planetary mission," said Doran. "While we don't expect to find 'Lake Vidas' on Mars today, it's a good analogue for something in Mars' past. Also, if there are brines leaking out from Mars' subsurface today, they could well come from a buried 'Lake Vida.'"

Besides Doran, the research team authors include Christian Fritsen of the Desert Research Institute of Reno, Nev.; Christopher McKay of the NASA Ames Research Center; and John Priscu and Edward Adams of Montana State University. Support for the research came from grants from the NSF and the NASA Exobiology Program.