Lakes under ice streams appear to speed ice flow as they drain and fill
Posted March 13, 2009
Scientists have known that lakes exist beneath Antarctica's ice sheets since the late 1960s, but only more recently have glaciologists like Slawek Tulaczyk discovered that the subglacial waterworks appears to play a key role in ice sheet dynamics.
Tulaczyk and his small science team spent about a month over November and December of 2008 on the Whillans Ice Stream, one of about a half-dozen large, fast-moving rivers of ice pouring from the West Antarctic Ice Sheet into the Ross Ice Shelf.
These ice streams, scientists have found, exhibit a strange behavior: Large swaths of ice experience sudden changes in elevation, up to 10 meters, in just a year or two. That's much faster than what could be explained by simple ablation or thickening.
“That's why these things immediately sparked our interest,” Tulaczyk explained after returning from his second field season for the project. “They had to represent some process that was very quick … and most likely happening at the base of the ice sheet. It meant there was a large volume of something moving around beneath the ice sheet, and the most likely candidate was subglacial water.”
The idea is that some of these subglacial lakes are filling and draining, like the water that fills and drains the locks used to move ships through the Panama Canal. But in the case of these dynamic Antarctic lakes, scientists aren't certain what's throwing the switch. Certain lakes reach a threshold, and then the water pours out into a conduit and to the next lake downstream.
One head scratcher: Why do the lakes drain and fill at all? A related puzzle: The lake can drain below the level that it had previously been able to trap the water. How? “It drains more water than was needed to overcome that [hydrostatic] seal,” Tulaczyk said.
“If you have a lake that goes from being full to being empty over a large area, that means you are changing the basal [or bottom] conditions for the ice sheet very drastically,” he said. “The ice will have a much easier time moving over a large body of water than it will over a sediment layer or bedrock at the bottom of the lake.”
Satellite data used by other researchers found an apparent connection between the changing velocity of a glacier and the draining of a lake, Tulaczyk said. Learning more about the relationship between the lakes and the ice sheet is important for scientists who model ice dynamics. The more accurate the model, the better scientists can estimate possible sea-level rise as the ice flows into the ocean.
There are two large lakes in the area of the ice stream, Whillans and Mercer lakes, along with a handful of smaller ones. Tulaczyk and his team are interested in mapping out the depth of the lakes, as well as measuring the elevation changes should one of these filling and draining events occur.
The four-member team established a camp in the area, traveling by snowmobile to each of their study sites — as much as 150 kilometers roundtrip in a single day. Jake Walter, a graduate student at the University of California, Santa Cruz, described the conditions of the ride in a dispatch on the Exploratorium's Ice Stories Web site .
“It is hard not to hesitate getting on the Skidoos [snowmobiles] in the morning, when the temperature is at 20 degrees below zero Celsius and there is already a 20-knot wind blowing, knowing you will be going 30 mph sometimes directly into it!” he wrote in his Ice Stories dispatch .
For mapping the lakes, the scientists use a technique called an active seismic survey, which Antarctic researchers have used for more than 50 years since the International Geophysical Year .
The technique involves using carefully set explosives to create images of the ice and water below the surface. A string of 48 geophones capture the sound waves created by the detonation as they reflect off the layers of ice, water and bedrock.
Tulaczyk's team had set up a network of 10 GPS stations over the lakes during the first field season in 2007-08 to monitor vertical and horizontal movement of the ice stream. The continuously recording GPS instruments are powered by wind and solar.
Unfortunately, the wind generation did not work well in the winter during the 24-hour darkness. The oscillation in power caused the hard drives to fill up too quickly, and some of the stations stopped recording as early as June, six months before the scientists returned to check on the instruments.
The team will return for a third and final field season next year so they can have a two-year time series of data. They will also retrieve their instruments at that time — a hard job considering each GPS station weights about 600 pounds. The scientists will have to disassemble each instrument and haul it back to their camp by snowmobile, where a ski-equipped airplane can land.
But Tulaczyk is already looking past that field season and to a proposal currently pending with the National Science Foundation to drill through the ice and into the lake to sample the ice, water and sediment. He noted that the ice over the lakes is only about 700 meters thick, which would only require a hot-water drill for access — an operation that could take only a day or two to accomplish.
Russian scientists are only about 100 meters away from reaching Antarctica's most famous body of water, Lake Vostok. Located some 4,000 meters below the surface, Lake Vostok is roughly the size of Lake Ontario and many researchers believe may be home to a unique ecosystem. There are concerns about possible contamination of that lake because of the drilling fluids used to cool the drill.
Tulaczyk said hot water drilling proposed at Whillans is a clean technology in that it is free from chemical contaminants. However, there is concern about the possible introduction of microbial or biologic contaminants. Officials at NSF have said the environmental impact assessment for the project will be conducted with the intent of reducing all potential impacts, including contamination of the lake from both chemical and biological agents.
Unlike Lake Vostok, which appears to experience little hydrologic turnover, the dynamic lakes under the Whillans Ice stream are self-cleaning systems, Tulaczyk said.
But the Russians aren't the only ones poised to sample a subglacial lake. The British recently announced plans to use a hot-water drill to reach Lake Ellsworth, located about 3 kilometers below the surface in West Antarctica in 2012-13. The magazine New Scientist declared the race is on to reach Antarctica's hidden lakes .
“If everything goes well, the U.S. Antarctic Program may be the first one to penetrate and sample a subglacial lake,” Tulaczyk said.
NSF-funded research in this story: Slawek Tulaczyk, University of California, Santa Cruz; Award No. 0636970 .
About the Sun