A good proxy
LARISSA project studies ecosystem changes since Larsen A Ice Shelf collapse
Posted June 8, 2012
In 2002, the Larsen B Ice Shelf shattered in spectacular fashion, shocking the polar science community with the rapidity of its disintegration.
A decade later, a multidisciplinary team of scientists aboard the research vessel Nathaniel B. Palmer sailed south from Punta Arenas, Chile, toward the eastern side of the Antarctic Peninsula. Their objective: To return to the region of the Weddell Sea that had once been under the shadow of the ice shelf to understand the changes wrought by its disappearance.
Their previous cruise to the Larsen B embayment in 2010 was blocked by sea ice conditions, and the team was eager to try again.
“It happened again this year. In a certain way, we weren’t able to accomplish our original goals,” said Maria Vernet , chief scientist aboard the Palmer for the second cruise in as many years in support of the LARISSA (LARsen Ice Shelf System, Antarctica) project.
In 2010, conditions had forced the ship to retreat completely to the western side of the peninsula, which generally experiences much lighter sea ice. The ship spent the better part of two months working in the western fjords, collecting sediment cores from the seafloor and supporting glaciological work on the peninsula.
Graphic courtesy: Bruce Huber
Map of the PALMER's cruise around the Antarctic Peninsula and Larsen A region.
While the marine component of LARISSA — one of the major International Polar Year programs funded by the National Science Foundation (NSF) over the two-year period — failed to accomplish its original goals in 2010, the glaciologists recovered a rare ice core from atop the spine of the mountains of the Antarctic Peninsula as planned. The ice-core researchers hope to glean a paleoclimate record that stretches back more than 10,000 years, after the end of the last ice age, referred to by scientists as a glacial period. [See previous article — Tough as ice: OSU team drills record core to bedrock despite operational setbacks.]
Other researchers managed to install several observatories on the eastern glaciers that once fed the Larsen B Ice Shelf, thanks to the help of the British Antarctic Survey and one of its ski-equipped Twin Otters. The instruments, called AMIGOs (Automated Meteorology Ice Geophysics Observation systems), are collecting data on the glaciers and a remaining sliver of the Larsen B now referred to as the Scar Inlet Ice Shelf. Ted Scambos , lead scientist at the National Snow and Ice Data Center (NSIDC) in Boulder, Colo., is overseeing this component of LARISSA. [See previous article — Probing the ice: LARISSA instruments to observe transformation to 'Alaskan style glaciation'.]
Marine biologists aboard the 2010 cruise made some of their own discoveries, such as spying an invading horde of king crabs on the continental shelf that had been absent from the region for millions of years — another sign that the region is gripped by climate change.
Occupying the Larsen A
The 2012 expedition, despite the challenges again presented by the thick sea ice cover, made its own breakthroughs and discoveries.
“We were luckier than two years ago, and were able to work in the Larsen A,” said Vernet, a LARISSA co-principal investigator (PI) and a marine biologist from University of California, San Diego’s Scripps Institution of Oceanography . “Just staying on the Larsen A and eastern side of the peninsula was very helpful.”
The Larsen Ice Shelf is the name early explorers used to describe the nearly continuous ice shelf area in the northwestern Weddell Sea. Glaciologists gave the three distinct embayments different names, as the ice shelves began to change during the last century.
The Larsen A, the northernmost, broke apart in 1995 after decades of retreat. The backup plan in 2012 focused on the changes under way in the Larsen A region as substitute for studying the Larsen B embayment.
“In the absence of our ability to go to Larsen B, Larsen A was a very good proxy,” Vernet noted.
NSF-funded scientists had previously visited the Larsen B region during a series of cruises in the 2000s, led by Eugene Domack , a geosciences professor at Hamilton College in New York. Those expeditions, in part, were the catalyst for the LARISSA project. Domack is the principal investigator on the LARISSA program, though he was not on the 2012 cruise.
Sediment cores recovered during those earlier investigations found evidence that the Larsen B Ice Shelf had been in place for at least ten millennia at the end of the last glacial period, according to Amy Leventer , a LARISSA co-PI and an associate professor of geology at Colgate University .
“The Larsen A Ice Shelf, on the other hand, appears to have been more ephemeral, with [a] long period of open water in the region for much of the mid-Holocene,” said Leventer, referring to the current warm period between glacial periods, called an interglacial. “This kind of information is important in terms of understanding the processes that control the formation and loss of ice shelves.”
The disappearance of the ice shelf particularly affects the marine biology, from the surface to the seafloor.
At the top of the water column are phytoplankton , free-floating marine algae that form the basis of the ocean food web. They are also an important part of the ocean’s carbon cycle, as dead phytoplankton drop to the seafloor as food for other organisms in seafloor sediments. Geologists like Leventer also use their presence or absence in sediment cores to identify periods of ice cover or open water.
Photo Credit: Amber Lancaster (PolarTREC 2012), Courtesy of ARCUS
Filtration system for phytoplankton.
The absence of the Larsen A Ice Shelf in recent years means more open water for blooms of phytoplankton, which require sunlight and nutrients to grow. Satellite observations had found big swings of phytoplankton growth in a given year, referred to as interannual variability, according to Vernet.
“The remote sensing can detect years of high phytoplankton abundance and production as a function of the sea ice being open,” she said.
Sampling from the Palmer from inshore to farther out to the former borders of the ice shelf found marked spatial differences, with higher chlorophyll concentrations away from shore — something also observed from satellites.
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