POLENET will monitor bedrock beneath ice sheets to learn more about glacial rebound
Posted June 13, 2008
Terry Wilson’s office at Ohio State University seems pretty typical of a college professor, especially one who splits her time teaching the fundamentals of geology to undergraduates while managing one of the largest and most ambitious projects of the International Polar Year (IPY).
Stacks of papers sit in relatively neat piles on the floor, leaving just enough of an aisle to maneuver in and out of the office. Various filing cabinet drawers are pulled out to their maximum extent, revealing cramped and stuffed folders. The sheer weight of the paperwork seems enough to cause a dimple in the earth that supports her office.
Should that material suddenly blow away, vanish in a freak windstorm, the ground underneath would eventually rebound to its original state, slowly sighing in relief as the burden of weight goes away.
In Antarctica, where the weight of its mighty ice sheets have squashed the earth’s crust below, Wilson and an international team of scientists are studying a real phenomenon called post-glacial rebound. The work is part of an ambitious project, called POLENET, for Polar Earth Observing Network.
POLENET has many goals, from helping to predict sea level rise to learning more about the earth below the crust and down to its core. It’s one of the flagship projects of IPY. Tom Wagner, program director of Earth Sciences at the National Science Foundation’s Office of Polar Programs, doesn’t mince his words when describing the value of POLENET. “Five years from now, this will be the most important science to come out of the International Polar Year,” he said.
The $4.5 million project, led by scientists with the Byrd Polar Research Center at OSU, will install several dozen GPS receivers, seismic sensors and other instruments around the West Antarctic ice sheet, wherever a piece of rock pokes through the ice.
The effort began last austral summer in Antarctica, and will continue through the 2011-12 field season. Collaborators include scientists at NASA’s Jet Propulsion Laboratory, New Mexico Tech, Penn State, University of Memphis, University of Texas Institute for Geophysics and Washington University.
On the rebound
The POLENET GPS receivers aren’t like a handheld unit that you might use to set a waypoint to your favorite backcountry trail or to navigate city streets. These instruments can measure with millimeter-level accuracy, as one important experiment measures the rate at which the bedrock of the continent moves vertically.
How can the earth move in such a way? Well, there’s a lot of ice in Antarctica, but there was even more during what’s known as the last glacial maximum, when ice sheets draped across parts of Europe and North America in the northern hemisphere and Chile and Argentina in the southern hemisphere. The ice was at its maximum extent about 20,000 years ago, then began to shrink.
The ice sheets never disappeared from Antarctica, of course, but they lost a lot of mass during the current interglacial period, well before the Industrial Revolution kicked temperature rise into high gear. How much ice has disappeared? Well, that’s what Wilson and her team hope to find out as they measure the rate of rebound now that all that weight is gone. As you might imagine, it’s a slow, lengthy process. “We’re looking for a long-term signal that shows us this rate of uplift due to ice mass loss since the last glacial maximum,” explained Wilson, associate professor of Earth sciences at Ohio State.
But there are more forces at work than this ancient signature. The Transantarctic Mountains Deformation (TAMDEF) project — a related program that measured bedrock movement with GPS stations along the mountain range that splits the eastern and western halves of the continent — discovered using continuous year-round GPS data that the crust also responds to short-term ice mass loss and gain.
“People with GPS measurements have been able to measure the deflection of the surface from adding snow in the winter and removing snow in the summer,” Wilson said. In fact, the earth at places such as the Amazon Basin reacts strongly to this elastic effect, as a sizable portion of South America sinks when the river floods and then rises more than 5 to 6 centimeters when the waters recede each year.
Many scientists, citing satellite data, believe West Antarctica is losing ice almost as quickly as Greenland because of global warming. The crust below the ice sheet should have an elastic response to that massive, modern evacuation of ice. The idea is if you can capture the rate of elastic response, a relatively short-term event, you can get a true measure of the size of the ice sheet and how much mass it is losing. That would finally put a realistic number on future sea level rise as Antarctica continues to shed weight faster than a contestant on TV’s “The Biggest Loser.”
“It’s kind of serendipity in a way that we’re getting this array of instruments out in time to start recording some of those dynamic changes, particularly in West Antarctica and up toward the Antarctic Peninsula, that are surprising everyone,” Wilson said.
The data from seismometers, which record seismic waves from earthquakes, are also a key part of the equation. The material below the ice sheets is not uniform. It may be thick, dense and stiff. Or perhaps it’s weak and warm. The seismic waves can tell scientists what sits below the ice sheets, which indicates how quickly the crust may move up and down.
“The speed at which [seismic waves] go is a direct recorder of the physical properties of the earth,” Wilson explained. “Integration of seismic and GPS is really important for understanding both short- and long-term ice mass balance. … It’s not entirely straightforward because you have these superimposed signals.”
Seismic waves pass quickly through dense rock, but move slower through warm and “gooey” material. Under one area in West Antarctica, the material is weak and warm where the crust has pulled apart, meaning the rebound, the long-term response, may have already occurred.
“The signal from the last glacial maximum could be gone because it’s relaxed that signal completely in the length of time that’s transpired, versus if we had really cold, dense lithosphere, then you’d have quite a lot of signal going on today,” Wilson said.
The data from POLENET will be particularly useful to a separate project involving a pair of satellites that measure gravity field changes on Earth, according to Wilson.
The Gravity Recovery and Climate Experiment (GRACE), operated by the Center for Space Research at the University of Texas at Austin, measures changes in gravity by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system.
Gravity is related to mass, so as mass grows or shrinks, such as with an ice sheet, the satellites can detect the changes. But one variable missing in its calculation is the rebound effect that POLENET measures. Wilson said a recent series of scientific papers point out that the biggest source of error in models attempting to account for Antarctica’s ice mass is the rebound calculation.
In the next few years that should change. “They will have this very accurate means to track changes in the mass in the ice in a way that they intended to do,” Wilson said of the satellite-based study.
“This is one of the reasons that POLENET got funded. It will make GRACE a very, very powerful tool,” noted Mike Willis, a postdoctoral fellow at Ohio State, who works on POLENET and its sister network in Greenland, GNET.
In the future, using these tools, scientists will be able to say whether Antarctica grew or shrunk on at least a monthly basis, according to Willis. “We don’t even know that at this time. We have hints of it, but that’s based on models — which are probably wrong,” he noted.
Setting up the network
The United States’ component of the five-year project will establish 52 stations, though not all contain both GPS receivers and seismometers. Some 28 countries are involved in POLENET in some form or fashion, though the United States, Italy, the United Kingdom, New Zealand and Germany are the principal partners in the West Antarctica venture.
Each station runs year-round, using solar power in the summer and several hundred pounds of batteries to keep the instruments juiced through the winter. An Iridium modem sends the GPS data in real-time, but the seismic information is too large to send via satellite, so those sites require physical visits.
Stephanie Konfal, a graduate student at OSU, was a member of this past season’s installation crew, which traveled across West Antarctica on ski-equipped aircraft to set up the stations. On a good day, she said, several people could install one station in about three to four hours. “It’s a pretty big setup per site,” she said.
The team spent most of its time working out of Patriot Hills, the only privately run camp on the continent, used mainly by adventurous types for mountaineering expeditions and ski trips to the South Pole.
“It was nice to be out in the field,” Willis said. “I hadn’t lived in a tent since 1998-1999. This was nice to be back to the old school. Old school but cushy — I mean the food was glorious.”
Even more glorious for Wilson is the network that is taking shape. A map of the continent shows variously colored points where POLENET and other GPS and seismometer stations are located. The dots fringe the entire coast of Antarctica, even East Antarctica, while even more freckle the vast interior of West Antarctica.
“It’s pretty spectacular,” Wilson said.
NSF-funded research in this story, Terry Wilson, Ohio State University.
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