Pine Island cruise
Scientists deploy robotic sub, moorings to study fast-moving glacier
Posted April 17, 2009
Stan Jacobs and his colleagues first reached the calving front of the Amundsen Sea’s Pine Island Glacier in 1994, and after several tries finally got there again earlier this year on the U.S. Antarctic Program’s RVIB Nathaniel B. Palmer.
The glacier, fronted by a small floating ice shelf, is one of the fastest moving rivers of ice on the continent, and one of the main sources contributing to sea-level rise out of West Antarctica. But the heat from below, not above, is thinning the ice shelf and allowing the glacier to flow more quickly into the ocean, according to researchers like Jacobs.
During that first trip in 1994, when scientists knew “next to nothing” about the ocean circulation and seafloor topography of the bay where the glacier empties, they noticed a mass of “warm” Circumpolar Deep Water (CDW) on the continental shelf. A few degrees Celsius higher than the frigid surface conditions, this water intrudes on the shelf at depths of about 300 meters or more, running through glacier-carved troughs that allow it access to the ice above.
“We’re fairly sure that the process we happened upon was ongoing at the time we arrived, but we don’t have any easy way to determine what the ocean conditions were on the shelf prior to 1994 because there were no measurements then,” said Jacobs, a research scientist at Lamont-Doherty Earth Observatory at Columbia University.
All of their visits have occurred during the austral summer, when sea ice is at its minimum, to allow the research vessel easier access to Pine Island and other ice shelves in the region. The researchers have compiled a good time series of data around that particular time of the year. “What we don’t really know is how the deep water varies year-round and from year to year,” Jacobs said.
The scientists hope to fill the gap after this latest expedition, he added. “The main thing we were trying to do this time was set out a series of moorings that could record the winter properties and the currents over a couple of years. We can then gain a better understanding of how the deep water forcing changes over time.”
Part of the International Polar Year (IPY) science program involving some 60 countries, the project included collaboration with British scientists, led by Adrian Jenkins with the British Antarctic Survey (BAS). A colleague of Jacobs’ since the 1994 expedition that discovered the CDW on the continental shelf, Jenkins brought a unique tool to the 2009 study — a robotic submarine capable of operating underneath an ice shelf.
“On the six missions beneath the ice, the sub performed exactly as it was programmed; the only problems were caused by the unknown nature of the environment into which we were sending it,” Jenkins reported via e-mail in March following the expedition.
The sub — part of a £5.86 million British program to explore the marine environment beneath floating ice shelves — is 7 meters long, powered by 5,000 D-cell batteries, and can range up to 400 kilometers and to depths of 1,600 meters. The sub’s main job was to explore the underside of the ice shelf and the deep channels in the seafloor that can funnel the warm water to the ice.
The yellow-colored submarine penetrated nearly 60 kilometers to the vicinity of the grounding line, where ice meets seafloor, after making three forays of about half that length. It was near the start of the return leg of the fourth trip when the sub collided with the ice base, according to Jenkins.
“We believe it ascended into a basal crevasse,” he explained. “We cannot be sure of the exact dimensions, but it had to dive over 100 [meters] down to find an escape route. It then travelled 55 [kilometers] back to the rendezvous with the ship, despite having sustained damage to the nose.”
The sub covered about 510 kilometers, spending 94 hours beneath the ice shelf. This was only the second time such a machine has ventured into this little-explored realm. The previous adventure was a 50-kilometer run beneath Antarctica’s Fimbul Ice Shelf several years ago.
The robot apparently proved its worth, yielding “a wealth of data on the [ice shelf] cavity shape and its seawater properties. … [It has] substantially altered our thinking about ocean circulation under the [Pine Island Glacier],” Jenkins wrote in one of the weekly ship’s logs. “Unexpected features in the cavity morphology have major implications for basal melting and glacier advance,” he added.
While still crunching the data, Jenkins said the sub tracked the depth of the ice shelf base and seabed, and the temperature and salinity of the seawater. “This will enable us to determine the route by which ocean currents transport heat to the ice shelf base to fuel the high melt rates.”
The results come at a time of increased interest in the polar regions thanks to a series of recent studies that warn Antarctica is more vulnerable to climate change than previously thought. NASA scientists, in a study led by Eric Rignot, reported last year that ice loss in Antarctica increased by 75 percent in the last 10 years due to a speed-up in the flow of its glaciers. The loss is now about equal to that from the Greenland Ice Sheet, and most of it appears to be occurring in the Amundsen Sea.
More recently, in February and March of this year during separate meetings of scientists in Europe, researchers reported that current estimates of sea-level rise appear too low. The International Scientific Congress on Climate Change in Copenhagen released a statement in March that said the average sea level around the world could rise by more than a meter by century’s end, double the 2007 prediction by the United Nation’s Intergovernmental Panel on Climate Change (IPCC).
“The numbers from the last IPCC are lower … because it was recognized at the time that there was a lot of uncertainty about ice sheets,” said Rignot in a release from the Copenhagen meeting. He and Konrad Steffen, director of the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado in Boulder, led a study of the Larsen C Ice Shelf along the Antarctic Peninsula this past season.
“As a result of the acceleration of outlet glaciers over large regions, the ice sheets in Greenland and Antarctica are already contributing more and faster to sea level rise than anticipated,” added Rignot, a professor of Earth System Science at the University of California Irvine and senior research scientist at NASA’s Jet Propulsion Laboratory.
Another scientist with NASA but with support from the National Science Foundation, Robert Bindschadler, will lead a team to Pine Island Glacier to drill through the ice sheet with a hotwater drill to study the ice-ocean interaction from a different perspective. That multi-year project is scheduled to begin work in 2009-10 after some preliminary work back in 2007-08.
Jacobs said it is likely the CDW has been coming on the Antarctic continental shelf in the southeast Pacific for quite some time, as records dating back more than a century in the Bellingshausen Sea report similar water temperatures as today. What may be different, he said, is the volume of water being drawn onto the shelf, possibly due to changes in atmospheric circulation.
The researchers expect to return in 2011 to retrieve the 14 moorings deployed during this cruise, and hope the data they record can help answer some of the questions about the CDW. More immediate information may come from an ice-tethered profiler (ITP), an instrument developed at the Woods Hole Oceanographic Institution.
The ITP consists of a cylindrical capsule, outfitted with oceanographic sensors that slide up and down on a line attached to a small capsule anchored on the surface of an ice floe. A weight at the bottom helps keep the line vertical. The instrument, the first of its kind used in the Antarctic, transmits measurements to the scientists in near-real time.
The scientists had a difficult time finding a suitable place for the ITP system because of the low ice cover this season, and the ‘fast’ ice on which the ITP was tethered has since gone adrift. But the good weather was a blessing in every other way, particularly for using the British autosub, Jacobs said.
NSF-funded research in this story: Stan Jacobs, Adrian Jenkins and Hartmut Hellmer, Columbia University, Award No. 0632282.
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