Team probes buried Antarctic lake
Scientists study body of water under ice sheet to understand how subterranean lakes fit into the continental puzzle
Posted February 4, 2007
On the frozen Antarctic continent, subglacial lakes are a hot spot of scientific interest, but the information they contain remains untapped.
“The ice sheet in Antarctica can be as much as 5 kilometers thick, and at the bottom point, it can be quite warm, as warm as the melting point of ice,” said Sridhar Anandakrishnan, who led a science team to the South Pole this season to learn more about one lake that rests about 16 kilometers away from the U.S. Antarctic Program station.
Geothermal activity and the 3 kilometers of insulating ice at Pole can trap heat and melt sections of the ice sheet’s base. If some of that water begins to collect in basins, it can form a lake as it would anywhere else on Earth.
“Subglacial lakes are potentially very valuable treasure troves for interesting biota and paleoclimate information,” Anandakrishnan said. He added that sediments left in the lakes may give clues as to the history of the ice sheet itself.
Aerial radar surveys have flagged hundreds of lakes with large surface areas dwelling under the ice. The subglacial lake at the South Pole was first identified using this method in the late 1970s.
The largest is Lake Vostok, which sits 4 kilometers beneath Russia’s Vostok Station. At 10,000 square kilometers, it is a little bigger than Lake Ontario.
“Lake Vostok is sort of the crown jewel of subglacial lakes, so we have to be very careful before we try to sample there,” Anandakrishnan said of the lake that reaches more than 500 meters deep.
“However, there are hundreds of these other lakes,” he added, “and while we need to be very careful with them as well, we can experiment a little bit with sampling technologies.”
But before any sampling efforts can even be proposed near the South Pole, scientists need to get a better idea of the subglacial lake’s properties.
“The problem is that Vostok is the only one that has been definitively defined as a lake with significant volumes of water,” Anandakrishnan said.
Radar was used to scout out the 15-by-15-kilometer lake near the South Pole as well as the other lakes spread across the continent, but it can only provide a two-dimensional picture because water reflects radar signals. Therefore, radar can provide information on a lake’s area, but it can’t penetrate the surface to explore what lies beneath.
Depth is crucial in determining the scientific value of a subglacial lake. Deep lakes mean longevity, giving the organisms there time to take hold. Shallow lakes are much harder on any life that does exist there because the water is more susceptible to drying up or refreezing.
Leo Peters wears a backpack-mounted GPS antenna for surveying subglacial lakes.
Anandakrishnan’s team turned to a method called seismic reflection profiling to obtain the vital third dimension.
“Radar kind of gives you indication that there is a lake and seismic really nails it down,” Anandakrishnan said. “We don’t really know what is there at the South Pole yet. As far as the radar is concerned, water that is a meter or a few meters thick is the same as water that is tens of meters thick.”
The team also went back over sections of the lake with ground-based radar, which gives a much finer picture of the lake’s shape than its airborne cousin.
“In a way, I think that will be the most valuable thing we have. There’s lots and lots of radar data on the rest of the continent, but there’s very little seismic data,” Anandakrishnan said. “We’ll have a seismic record, and we’ll have a radar record in exactly the same spot. … If we could do a really good job of identifying what the characteristics of this normal lake are, then we hope all the other normal lakes around the continent would pop into focus.”
The principle behind seismic reflection profiling is much more straightforward than its name suggests – make a sound at the surface and listen for the echo.
Anandakrishnan and his team needed an extremely loud sound to make it down several kilometers to the lake and then echo back to the surface again, and so they turned to Pentaerythritol Tetranitrate (PETN) – one of the strongest known high explosives.
The team made 5-centimeter-wide holes 18 to 30 meters into the ice with a hot water drill that spits out 93-degree-Celsius water and lowered an explosive into each one.
Each of the approximately 100 charges was detonated one at a time and ranged in weight from a fifth of a kilogram to almost 5 kilograms.
“At the surface we don’t feel very much, but the sound travels down into the ice,” Anandakrishnan said.
When the sounds hit the lake’s surface, part of the signals were reflected, but part of them continued to the bottom of the lake, where they then bounced back to the scientists on the surface.
The reflected signals were recorded on the surface of the ice by 150 geophones, which operate much like microphones but record vibrations in the ice instead of in the air.
Anandakrishnan and his team are now back in the United States with the seismic records of their month in the field. It takes a lot of data processing to turn the EKG-like seismic graphs into a profile of the lake.
To determine the lake’s depth, the team looks at the difference between arrival times of the signal reflected off the surface of the lake and the signal reflected off the bottom.
“In the field, we have limited capability for processing the data,” Anandakrishnan said. “We can look at them, see that they are of high quality, and say, ‘Yep, that looks good. We got data.’ But we can’t really interpret them for the properties of the subglacial material until we get home and can do a more thorough job.
“I’m hopeful that we will have an answer soon, but I’m not sure what it is yet.”
NSF-funded research in this story: Sridhar Anandakrishnan, Pennsylvania State University.