Greg Balco collects a
rock sample Jan. 10th while
conducting fieldwork in
the McMurdo Dry Valleys
at an area near
Researchers Track Soil Movement and Land Erosion in the McMurdo Dry Valleys
By Peter Rejcek, Sun staff
“It’s such an outrageous thought, that nothing is moving here,” said Putkonen,the project’s principal investigator. “It’s so foreign to the geological understanding of the surface processes.”
Putkonen does not know whether there is significant soil movement occurring in the Dry Valleys or not. He wants an answer to that question for several reasons,but mainly because it will broaden the overall picture of landscape erosion, which is a well-documented process at the lower latitudes. Most soil movement in those areas is by water. At one extreme,he explained, is something like a monsoon that causes mudslides and radical geomorphic changes. The Himalayan Mountains are an example of this kind of process.
Scientist Jaakko Putkonen
uses trekking poles to make
his way across the rocky
terrain in the McMurdo
The current perception is that the Dry Valleys is at the other extreme, where erosion and soil movement seems miniscule.
“We’re interested in how fast the land-scape is changing, what processes are responsible for these landforms and how long does it take to make them,” said Greg Balco, a geologist on the team.
This is the second and final field season for the project. Putkonen and his four team members spent nearly six weeks in the Dry Valleys. They established three primitive camps during that time in Arena Valley, the Labyrinth and near Mount Electra, where they believed liquid water would play the most minimal role in erosion and movement. They’ve been collecting rock samples for later lab analysis but also following up from last season on what Putkonen calls two low-tech methods to determine how quickly soil is moving.
The first method involved burying small, rectangle-shaped wooden boxes flush with the ground surface. They placed the dirt traps last season in areas where they believed there was a high probability to capture movement, such as at the bottom of a steep slope. The second low-tech approach involved taking photos of the ground at 17 sites.
The team returned this season to the same spots to re-photograph them. They’ll compare the two pictures and see what changes may have occurred from one year to the next. They’ve also been following up on their 20 traps scattered across the Arena Valley and the Labyrinth, where deeply eroded dolerite gives the area a maze-like appearance. The sites were flagged and their coordinates recorded on a hand-held Global Positioning System unit.
“If something is in the box, [the soil] must have moved since I was there a year ago,” Putkonen said. So far, they’ve found “significant amounts” of soil in the traps, according to Balco.
“We didn’t know if we would observe any sediment transport,” Balco said during a field visit near the Mount Electra campsite, where the team was busy collecting samples and doing reconnaissance for possible future work. The area is reminiscent of the high desert in the United States, with mesa-like features, and is about 1,400 meters above sea level. The ground is covered in sandstone and dolerite rocks of varying hues and sizes.
It’s slow going over such rough ground. A typical field day can involve about a 15-kilometer roundtrip on uneven terrain, hauling large backpacks filled with supplies, equipment (including hammers and chisels) and canvas bags of rock.
The collection process itself is straightforward. Balco, for example, is on the lookout for places where he can collect samples on varying angles of slope or with different wind exposures. He makes notes, observes the surrounding landscape, picks his sample, chisels it off if necessary, photographs the site and then bags the rock.
All those heavy samples are eventually headed to the lab. That’s when a third, far more high-tech process will be used, called cosmogenic nuclide dating. Cosmogenic isotopes are rare radioactive isotopes created when cosmic radiation interacts with an atomic nucleus.
Terrestrial rocks are pelted by radiation from outer space that changes the oxygen in the rock’s quartz, producing a substance called beryllium-10 (Be-10). Because these cosmogenic isotopes have long half-lives (anywhere from thousands to millions of years), they are useful for dating geological features and activities. Be-10 production occurs at a steady rate. By counting the number of Be-10 atoms, scientists can determine the length the rock has been exposed at the surface. For example, if Be-10 occurs at a rate of 10 per year and there are 100,000 atoms present in a particular specimen, then the rock has been exposed for 10,000 years.
Rosemary Garofalo, who works with the Berg Field Center
at McMurdo Station, takes a break from helping collect rock
samples in an area of the McMurdo Dry Valleys near Mount
Electra. The terrain is reminiscent of the southwest
desert of the United States.
While the three techniques for collecting data seem disparate, they are related. For example, if the group collected a lot of rock and dirt in one trap, the radiation exposure in that area should be less because of the higher rate of erosion — the rock hasn’t been at the surface as long as in areas where little soil movement is occurring. “These are two different ways of looking at the same problem,” Putkonen said.
Putkonen’s group collected about 500 kilograms of rock last season. Dating the material is a slow and meticulous process, he said, but “we seem to be getting really interesting results.”
The current understanding is that the Dry Valleys have been ice-free for about 10 million years. But if the cosmogenic nuclide dating shows the rocks have only been exposed at the surface for, say, two million years, then there might have been a more recent ice influx. Balco said the group is simply not finding rocks that have been exposed for 10 million years.
“One way to interpret the data is to say [the Dry Valleys were] ice-covered some time in the past,” said Putkonen. Another possible interpretation is that the erosion rates are much higher than believed.
“What will probably turn out to be the case is that the Dry Valleys are far patchier than we think,” Balco explained, meaning erosion is relatively fast in some areas and nearly non-existent in others.
Putkonen is more interested in understanding the landscape evolution and geomorphology of the Dry Valleys than just dating various rock surfaces there. If he can determine that the soil is indeed eroding and moving at appreciable rates, other questions arise, like how is it moved and where is the dirt ending up? There are places he calls “deflation hollows” where the soil is completely gone and the bedrock is exposed. How are these bald spots formed? Wind? Frost heaves?
“If I understand what the soil is doing, I can understand the more mobile part of the landscape,” Putkonen said.
NSF-funded research in this story: Jaakko Putkonen, University of Washington.
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