The core truth
LARISSA component to drill into ice sheet may reveal past abrupt climate changes
Posted September 18, 2009
It turns out you can’t just drill an ice core in any old place in Antarctica.
You have to plan the logistics — how do you get there and how do you support an operation for weeks at a time? You have to flesh out the science goals — do you want detailed annual layers of climate history or a record hundreds of thousands of years old? And then you rely on decades of ice-coring experience around the globe — and a little bit of luck.
The Thompsons from Byrd Polar Research Center (BPRC) at The Ohio State University certainly possess plenty of the former, having led expeditions to both poles and to the high-altitude ice fields of South America, Asia and Africa’s Kilimanjaro. Their latest work to drill a 400- to 500-meter-long ice core as part of the multi-disciplinary LARISSA program is routine in most respects.
Yet Ellen Mosley-Thompson is still cautiously optimistic about their possible success, because in reality there’s nothing standard about working in remote, cold places and drilling into ice sheets with equipment that can wear and break down far from the closest repair shop.
“If we get the core to bedrock, I’ll be thrilled,” said Mosley-Thompson during a phone interview from her office at BPRC, while colleague and husband Lonnie Thompson was away drilling an ice core on Nevado Hualcán in northern Peru at 5,800 meters, not far below the 6,122-meter summit.
“Weather dominates,” Mosley-Thompson said of ice-coring expeditions. “It doesn’t matter how well you plan, probably the plan will change. It always does, but you have to have a starting point.”
The starting point for the LARISSA ice core — which will provide insight to the climate history of the Antarctic Peninsula region and provide context for the Larsen B Ice Shelf breakup in 2002 — is 66.04 degrees south and 64.003 degrees west. Dubbed Site Beta, that’s the point on the 2,000-meter-high Bruce Plateau where the scientists believe they’ll have the best success of finding an ice core with distinct and thick annual layers.
It will actually be up to glaciologist Ted Scambos and his ground-penetrating radar team to mark the “X” on the ice cap for the coring site before the Thompsons arrive. Scambos’ team will pull the radar on a sled behind a snowmobile, imaging the layers of ice and bedrock below, and measuring the exact altitude with GPS.
“They’re going to pick the exact spot,” Mosley-Thompson said.
The ideal site should have thick, annual layers of ice, which provide a detailed year-to-year record of climate, recorded by dust particles, chemical compounds such as sulfate and nitrate, and the isotopic ratios of oxygen and hydrogen. The properties of the ice can discern dry, cold conditions or warm, wet conditions in the past.
The abundance of dust, sulfate and isotopes change over the course of the year, and can be used to help date the core. Gases trapped in bubbles in the ice can also be measured and provide a history of the atmosphere’s past gaseous composition.
Smooth bedrock is also important, Mosley-Thompson explained, otherwise the ice core record becomes convoluted. “That’s going to keep the layers pretty horizontal. Flat bedrock is critical.
“It was a challenge,” she added. “We spent a lot of time looking at different locations, looking at all of the different radar profiles, looking back at shallow cores that had been drilled in the region.”
While the high snow accumulation rate on the ice ridge — possibly more than a meter per year — is good for that “high-resolution” detail, it could mean the record left in the ice is somewhat brief, according to Scambos.
“That’s part of the problem. It’s warm; it’s very high snow accumulation, so the ice near the bottom flows away more rapidly,” said Scambos, lead scientist at the National Snow and Ice Data Center in Boulder, Colo. “It’s unclear of how much of that older record will still be left at the base of the ice.”
Mosley-Thompson thinks it will be possible to find glacial stage ice — ice at least 20,000 years old, dating back to when the northern and southern ice sheets reached their maximum extent before retreating and ushering in a relatively stable era of climate.
Still, even during the current Holocene, which began about 12,000 years ago, the climate made some wild swings. For example, based on a large dust event found in tropical and subtropical cores the Thompsons have recovered and analyzed, there was a severe drought about 4,000 years ago that lasted for some three centuries. Yet that event doesn’t show up in any of the continental cores drilled in Antarctica.
Might it show up in the more northerly LARISSA ice core?
“It was a very large drought. It was associated with some societal collapses in the Middle East, the beginning of the first Dark Age, and it is very well recorded in the Andes,” Mosley-Thompson said. “We’re very curious to see if this large event shows up in Antarctica.”
By studying these previous, abrupt natural events such as the drought, the paleoclimatologists hope to learn more about human influences, from increasing atmospheric carbon dioxide to clear-cutting Amazonian rainforests.
“They’re very important,” Mosley Thompson said of abrupt climate change events. “First, it is essential just to know that they happened. And then, hopefully, by piecing together multiple records from different locations we can get a better sense of what the driver or drivers might have been.”
Those answers — and undoubtedly new questions — will come later after the Thompsons and their colleagues have analyzed the core. First, of course, they have to drill into the ice and get the cores back to the United States.
The ice-coring operation is scheduled to take about 45 days with a six-person crew, living in tents and working under a geodesic dome about seven meters in diameter. The team will build a trench where the ice cores will be stored until a plane arrives each week with fresh supplies and fuel.
The cores will be packed into insulated boxes and flown back to the nearby British Antarctic Survey’s Rothera Base, where they will be stored in freezers until the RVIB Nathaniel B. Palmer makes a quick port call to retrieve them.
Generators power the elecromechanical drill, which consists of outer and inner barrels. The outer barrel has anti-torques at the top that hold it stationary in the hole, with an electrical motor at the top, all of it suspended on a cable, and lowered and raised by a winch topside.
The inner barrel does the work of cutting into the ice. After the drill has swallowed about a meters-length of ice, the scientists pull the drill to the surface, disconnect the barrel, push the ice out and process the core — making various measurements about its weight, appearance, density and other notable characteristics. The ice goes into a plastic sleeve and then into a heavy cardboard tube that is placed in the storage trench. All the equipment is cleaned and the drill is sent back down to take another bite out of the ice.
“Then it’s just repeat, repeat, repeat,” Mosley-Thompson said.
Eventually, the deeper ice that is very cold and under great stress begins to fracture as the drill bores down. When this happens, the team will switch to a thermal drill, which applies heat to an electric element that melts its way down into the ice. The amount of power provided to the heater element dictates the drilling rate. Too much power could burn up the heater element.
“There is a science to ice-core drilling, but there’s also an art to it,” Mosley-Thompson said. “Experience is the best teacher.”
NSF-funded research in this story: Ted Scambos, University of Colorado at Boulder, Award No. 0732921; Ellen Mosley-Thompson and Lonnie Thompson, Ohio State University, Award No. 0732655.
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