LARISSA takes unique approach for research on ice shelf ecosystem
Posted September 18, 2009
Eugene Domack and his colleagues aboard the ARSV Laurence M. Gould were about six hours north of where the Larsen B Ice Shelf once floated on the northwest Weddell Sea when they made perhaps the biggest discovery of the 2005 science cruise.
A few minutes of videotape taken underwater revealed extraordinary signs of life on the floor of the continental shelf — bacteria so thick it was visible to the naked eye in what biologists call bacterial mats, along with bivalves such as clams. The scientists had unexpectedly discovered a cold seep biological community, subsisting not on light as most ecosystems but on methane vented through the seafloor.
Thick sea ice stymied a return voyage in 2006 to sample the unique ecosystem, which would have existed under the lightless shadow of the Larsen B Ice Shelf until it abruptly collapsed in 2002.
The discovery turned even more shocking — and, for curious scientists, even more interesting — when Domack’s team determined, based on benthic sediment cores, the ice shelf had been in place for at least 10,000 years.
Adding to the mystery: In 2007, a German team that did manage to reach the spot where the seep community was found reported that it was dead — and for what appeared far longer than it seemed possible based on its discovery only two years ago.
Questions emerged, as they always do: What happened to the newly discovered ecosystem? Is it truly dead or still alive? Did it evolve because of or in spite of the ice shelf? What did the collapse of the ice shelf do to the local ecosystem? How will it evolve now that that portion of the ice shelf is gone? Did global warming cause the ice shelf to break up abruptly? Or was the collapse part of a millennia-old cycle?
Some of the answers may come from LARISSA. The LARsen Ice Shelf System, Antarctica, project is an interdisciplinary program to study as many facets of the system as possible, from the remaining ice shelf itself to marine sediments piling up on the continental shelf below and from the critters that call the Larsen Embayment home to ocean circulation patterns.
LARISSA is funded by NSF’s Antarctic Integrated System Science (AISS) program.
“To be an AISS project, the ‘I’ must be met — a project must be integrated,” explained AISS program manager Lisa Clough. “It’s a bit difficult to describe, but I think a useful question to ask is what happens to the project if one of the pieces is removed — if you can’t answer the overarching questions unless all components are present — that’s an integrated project.
“LARISSA clearly meets the integration challenge,” she added. “All the scientific pieces are interesting in their own right, but putting all the pieces together will make the whole much greater than the sum of its parts, and provide critical insight into climate change questions.”
Chief scientist Domack said getting answers to many of those questions will have implications beyond the Antarctic Peninsula and the Weddell Sea.
“It’s a way for scientists to look at a small system and all the bits and pieces that contribute to its fundamental change and refine our models and estimates of how the larger parts of the Antarctic cryosphere will respond to the future,” explained Domack, a geosciences professor at Hamilton College in New York.
The LARISSA team will deploy on a two-month research cruise aboard the RVIB Nathaniel B. Palmer beginning in January 2010.
The ice shelf goeth
The Larsen B Ice Shelf was one of three ice shelves that made up the total Larsen Ice Shelf, which mainly consists now of Larsen C. Still, the remaining ice shelf is the third largest ice shelf in West Antarctic after the Ross and Ronne-Filchner ice shelves at about 48,000 square kilometers.
The Larsen A, the smallest of the three, disintegrated in January 1995 after decades of retreat. The collapse of 3,250 square kilometers of the Larsen B during a few short weeks in early 2002 represented the largest such event in more than 30 years of ice-shelf observations, according to the Boulder-based National Snow and Ice Data Center (NSIDC).
LARISSA will offer NSIDC lead scientist Ted Scambos an opportunity to get up close and personal with a region he has observed and studied for 14 years, mainly using satellites.
“I’m really looking forward to it,” said Scambos, a principal investigator on the project and the lead on the cryosphere and ocean investigations. “It’s an area that’s on the cutting edge of climate change. Things that are happening there on the peninsula are a precursor to what’s going to happen over larger parts of Antarctica over the coming decades.”
Scientists estimate the peninsula has warmed by more than 2.5 degrees Celsius since the 1950s and more than double that average during the winter season. The verdict is mixed on whether the rest of the continent is heating up, with many on the scientific jury of the opinion that parts of West Antarctica are changing rapidly, particularly an area around Pine Island Bay on the Amundsen Sea where glaciers are retreating.
A paper in the journal Nature in January 2009 reported that overall Antarctica is warming in step with the rest of the planet, with the smaller western half heating up more than the high-altitude eastern side is cooling. “If that’s the case, other ice shelves will feel the brunt of rising summer temperatures,” Domack said.
Fingerprints in the mud
The Larsen B collapse was unusual because the ice melted from the top down after an unseasonably warm 2002. That’s different from what’s happening on the other side of West Antarctica, where warm seawater is melting glaciers from the bottom up.
“There are a lot of things that may have contributed to the ice shelf breakup, which seems to be an unprecedented event in the last 10,000 years,” explained Domack. “We’re not sure whether this is a climate tipping point or not — a permanent passing of the region from one climate state to another. The sedimentology record is vital in determining that.”
The scientists can learn much about the region by drilling various cores into the seafloor sediment that accumulated before and after the Larsen B disappeared — a job made much easier now that the ice shelf is gone. One goal will be to determine if there is a signature — a sort of sedimentological fingerprint — of ice shelf collapse in the mud and sand.
“[If] so, we can recognize similar events in other parts of the Antarctic or the world,” Domack said.
“Some of [the sediment record] might represent subglacial lake deposits, and some of it might represent a rather catastrophic flush of sediments as the ice flow from the land accelerated following the ice shelf demise in the first few months,” he added.
The subglacial lake, in particular, is intriguing. Did it play a role in the demise of Larsen B, perhaps destabilizing the grounding line where ice met bedrock?
“We’ll have access to this subglacial lake with the ship now that the ice is gone, so we can sample it in detail that you probably couldn’t do by drilling through ice,” Domack said. “No matter how many holes you punch through [an ice shelf], you can’t get the kind of detail that we’re going to have access to.”
Thanks to the LARISSA project, Bruce Huber and his team will have the opportunity to study the ocean in a scarcely sampled region of the Weddell Sea. A principal investigator from Lamont-Doherty Earth Observatory of Columbia University and co-chief scientist on the cruise, Huber is particularly interested in how the physical ocean properties may have changed since the ice shelf disappeared and sea ice took its place.
In addition to standard CTD measurements — profiles of ocean conductivity (to determine salinity), temperature and depth (measured as pressure) taken by a profiler deployed from the vessel — the oceanographers hope to set out as many as eight moorings with a suite of instruments. Anchored to the seafloor by weights and well below the reach of passing icebergs, the instruments will measure things like salinity, temperature, and current speed and direction over two years before the Palmer retrieves them in 2012.
Huber said the moorings will also trap organic material as they fall through the water column for later study.
“The sediment traps are of interest to all the components of this program because the sediment rain will consist of particulates brought into Larsen B area by melting shelf ice,” he explained. “It will also include remains of biological organisms — phytoplankton, zooplankton — that grow and die in the water column and sink when they’re finished.
“By looking at a time series of this accumulation we can get a better idea of the processes ongoing in this area,” he added.
Maria Vernet will observe some live organisms during the LARISSA cruise. A research biologist from Scripps Institution of Oceanography at UC San Diego and lead on the ecosystem studies comprising LARISSA, Vernet is interested in the seep community and the biological changes under way now that the ice shelf is gone.
“It is a very special community, organized to take advantage of gases that come off the crust,” she explained. “We want to characterize that community, so we want to find a live one that is still functional.”
Vernet’s particular specialty revolves around phytoplankton — free-floating, unicellular algae that bloom in the upper reaches of the ocean in an area scientists call the photic zone where light is available and photosynthesis occurs. The recent introduction of this new source of organic carbon is expected to change the dynamics of the benthic, or seafloor, community of animals, according to Vernet.
“The communities would now look more similar to the western side of the peninsula,” she explained. Other members of the ecosystem team will look more closely at the seep community, if one can be found, studying the relationship between the chemical-loving bacteria and the larger animals that likely have bacteria living in them so they can turn the chemical carbon into energy.
“It’s a very particular, symbiotic relationship between bacteria and animals,” Vernet said.
Work from the Palmer for this part of the cruise will include samples using the CTD profiler and capturing animals using nets. Sediment traps will also be a key experiment to determine how much and what kind of organic carbon sinks to the floor — and where — to feed the benthic animals.
“So, for example, if there are more phytoplankton close to the coast, then they’re going to be able to maintain a higher density of benthic animals,” Vernet said.
Getting to the core of climate
Not all of the action will happen on the ship.
Ellen Mosley-Thompson and Lonnie Thompson, leading ice-core paleoclimatologists from The Ohio State University, will spend about six weeks on a 2,000-meter-high ice ridge to drill an ice core to bedrock. The roughly 400- to 500-meter-long core will provide the LARISSA team with a detailed climate history of the region.
How far back in time will they travel? It’s difficult to determine, Mosley-Thompson said, but the hope is the core will reach into the Last Glacial Maximum (LGM), about 20,000 years ago when Earth’s major ice sheets advanced to their most recent extent.
“If [the glacial ice is] not there and we’ve drilled to bedrock, that’s going to be a real conundrum,” Mosley-Thompson said. The absence of LGM ice would mean one of two things, she explained. One explanation would be that ice didn’t cover the northern tip of the Antarctic Peninsula during the LGM, which seems unlikely.
The other option is that a previously warm period during the Holocene — the last 12,000 years of stable climate in which human civilization has emerged — melted it away. That would present an interesting paradox if the ice had indeed melted yet the Larsen B Ice Shelf had survived.
“If there’s glacial stage ice that means our core to bedrock will contain more than 10,000 years,” Mosley-Thompson said.
However, she said it’s still possible — in fact, quite likely — that the subsequent studies of the ice core will reveal previous periods in the Holocene that were as warm as the 20th century. Again, the head-scratcher would be why the Larsen B persevered during those times but collapsed in 2002.
It’s a problem the Thompsons have encountered before, such as with Mount Kilimanjaro, where an ice core drilled there revealed that the mountain’s largest ice field has remained intact throughout the Holocene, including a 300-year-long drought event about 4,200 years ago.
“Why are they not surviving today? What’s different?” Mosley-Thompson mused. Since 1912, about 85 percent of Mount Kilimanjaro’s ice cover has disappeared.
“Our projections are within two decades [Mount Kilimanjaro’s ice fields] will all be gone,” she said.
Between a rock and a hard place
Meanwhile, while the Thompsons and their six-member team labor on an ice ridge, Greg Balco will skip across the peninsula by helicopter to every spot he can safely find a mountain top poking through the ice, called a nunatak.
A postdoc at the Berkeley Geochronology Center in California, Balco specializes in cosmogenic-nuclide dating, which determines how long surfaces have been exposed based on the concentration of cosmogenic nuclides produced from cosmic rays hitting the rock.
In particular, Balco is interested in collecting glacial debris — rocks left behind when glaciers retreated since the LGM — and dating how long they’ve been exposed at the surface of the nunataks.
“We find exposure ages of glacial debris near the top of these things are older than it is at the bottom,” he explained. “That’s because the ice sheet surface has been gradually lowering and exposing consecutively more and more rock.”
In essence, Balco will be able to say how quickly the ice in this region retreated and when, which will complement other climate history records taken in other parts of Antarctica using similar dating methods.
“We’ll look for these glacial deposits over as great an elevation range as we can, which is equivalent to a time range,” Balco said. “We’re basically going to do what we’ve done in a lot of other places in Antarctica before.”
There are some differences, however. “One challenge of the Antarctic Peninsula is it’s really rugged,” Balco said, adding that the heavily crevassed glaciers require air support from the ship. He hopes to find suitable samples at several sites, but that may require visiting 10 or more areas to ensure success.
“That’s the big challenge with this kind of a project. We haven’t been to these sites; we don’t know what’s there.”
They’ll soon find out.
NSF-funded research in this story: Eugene Domack, Hamilton College, Award No. 0732467; Arnold Gordon and Bruce Huber, Columbia University, Award No. 0732651; Ted Scambos, University of Colorado at Boulder, Award No. 0732921; Ellen Mosley-Thompson and Lonnie Thompson, Ohio State University, Award No. 0732655; Maria Vernet, University of California-San Diego Scripps Institution of Oceanography, Award No. 0732983.
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