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A good proxy

LARISSA project studies ecosystem changes since Larsen A Ice Shelf collapse

 

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.

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.

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.

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.” 

Filtering phytoplankton

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.

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.

“We were able to find results beyond the remote sensing [data], and we were able to confirm some of the results we see from remote sensing,” Vernet said.

Cruising through time

Meanwhile, the benthic biologists aboard the ship turned their attention to the organisms that had taken up residence in the Larsen A embayment.

“We’re curious about what effect the ice shelf had on limiting carbon flow and energy to the organisms below when the ice shelf was intact,” explained Michael McCormick, an associate professor of biology at Hamilton College who was on both LARISSA cruises.

McCormick said the LARISSA team has a geological record of how the Larsen A Ice Shelf changed over more than a century before its final break-up in 1995. The ship made a spatial transect from near shore to the former ice shelf’s outer edges, essentially a trip back through time. The sites farthest from shore had the most benthic biology.

“They’ve had longer time to adapt and change to the conditions of having all of this [phytoplankton] productivity at the surface that wouldn’t be there when the ice shelf was there,” said McCormick, whose expertise is on the benthic microorganisms. “We’ll be looking at hundreds of thousands of microbial [gene] sequences to help construct this [ice-shelf retreat] gradient in microbial community composition.”

Doing oceanography

One of the main goals for this year’s cruise was to understand the connection between the melt and retreat of the Larsen B glaciers and the regional oceanography. Again, the LARISSA team had to improvise, since the glacier data was coming from the AMIGO stations in an area that the ship couldn’t reach.

Instead, the researchers focused their ocean-glacier interaction studies in regions farther north, in the Drygalski Trough, carved by the Drygalski Glacier, and an area informally known as Bombardier Bay based on its proximity to Bombardier Glacier. 

“There are not many available physical oceanographic measurements in the Larsen A, at least not modern data, that I’m aware of,” noted Bruce Huber, an oceanographer from Lamont-Doherty Earth Observatory at Columbia University who recently returned from the 2012 cruise. “Our [measurements], while not comprehensive, will provide a good look at the water masses on the inner shelf in one season.”

Unlike on the western side of the Antarctic Peninsula, where circumpolar deep water is being driven onto the continental slope and melting ice shelves from below, surface melt plays a more significant role in weakening ice shelves on the eastern side.

“But ocean melting still occurs, as it does wherever you have floating ice shelves, and is a vital part of the total mass balance of the ice sheet,” Huber noted, adding that the same circumpolar deep water mass that flows through the Weddell Sea cools down quite a bit before it circulates below the eastern ice shelves.

The oceanographers also had the opportunity to collect some data in the Prince Gustav Channel, which connects the Larsen A to the Erebus and Terror Gulf on the north side of James Ross Island, according to Huber, revealing some interesting bathymetry. 

“The channel is separated by a submarine ridge at its northern end, which reaches to within 250 meters of the surface,” he said. “Data we collected show very different water characteristics on either side of the ridge, demonstrating that the deeper waters of Larsen A are really separated from the northern part of the channel.”

Searching for seeps

One of the major results from those pre-LARISSA trips to the Larsen B embayment in the mid-2000s was the discovery of a cold seep community, an unusual ecosystem that survives by chemical energy venting from the seafloor.

The researchers recorded a video of the seafloor at the end of the 2005 expedition and only later discovered a thriving clam community, mud volcanoes and a thin layer of bacterial mats at 850 meters below where the ice shelf once floated.

It was the first cold seep ever found in Antarctica, but repeated attempts to return to the site have been thwarted by pack ice, including the most recent cruise.

“The geologic conditions that permitted this to form must mean there are other places where we can find other cold seeps in Antarctica,” McCormick noted.

In fact, that turned out to be the case.

On the southbound voyage to the western side of the peninsula, through a passage called Antarctic Sound, the Palmer’s sonar picked up the signature of what is believed to be a methane bubble plume rising from the seafloor, which is associated with cold seeps.

Harder evidence of yet another cold seep in the middle of the Larsen A embayment came from a camera deployed by Craig Smith, a LARISSA co-PI from the University of Hawaii.

“We think it’s a cold seep, but it’s a completely different setting than we expected,” Vernet said.

The camera captured images of the clams that congregate around these anoxic environments but no methane bubbles. Nor were there any bacterial mats that seem to grow around the seafloor fissures where organisms find better — or, at least, different — living through chemistry.

“The exciting thing is that there is evidence of other cold seeps that we could pursue,” McCormick said. “We’re really eager to sample another one to see if it has similar microbial composition to the Larsen B cold seep.”

But the discoveries, like the one in 2005, came too late for further investigation. Sea ice conditions prevented the Palmer from returning to the Larsen A site. A visit at the end of the research cruise to the area where the methane plume had been picked up by sonar also proved impossible, as sea ice choked even the extreme northeast peninsula by mid-April.

Drilling for ikaite

Another rare phenomenon in the Southern Ocean proved a little less elusive.

Several shallow sediment cores taken by a device called a kasten corer from the seafloor contained the rare metastable mineral ikaite, a calcium carbonate that forms under near-freezing conditions that scientists are beginning to use to tease out information about climate and even ice cover in the past as another way to construct a paleoclimate record.

“Our interest in ikaite is related to its very limited geographic occurrence and the interpretation, in the geologic literature, of its pseudomorph, glendonite, as a paleo-indicator of ‘Antarctic-like’ conditions in ancient rock sequences,” Leventer explained.

The exact conditions under which ikaite forms and morphs into other minerals are still a mystery to science, according to McCormick.

“There’s quite a bit of interest in learning about how these crystals are formed, so if we have a contemporary model of ikaite formation, then when we find these older ikaites, we have some idea of the actual conditions under which they were made,” he said.

One common denominator, aside from extremely cold waters, is organically rich sediments, Leventer added.

“But these two qualities characterize many more regions of the Antarctic continental margin than sites where ikaite is found,” she said, “so a project goal is to capitalize on the interdisciplinary nature of our research team to study the new cores that contain ikaite, using as many tools as possible.”

McCormick said there are a number of possibilities that may be involved in ikaite formation. For instance, there are high concentrations of ammonium in the cores containing ikaite, along with methane and sulfur.

“[The sediment cores] were actively off-gassing and hissing. You could see the mud squeezing out between the panels. We had to work very quickly to get our samples,” he said. “You could tell when we got one of these cores up. The whole back half of the ship reeked of sulfide.”

The assumption is that the ikaite mineralization process is abiotic, without any biological participation, but microorganisms have been known to play a role in the formation of other minerals, McCormick noted.

“We’re looking at the microbial community right at those levels where the ikaite is being formed,” he said.

Landing biological specimens

Microbes to larger marine fauna are the subject of an altogether different experiment involving a whale carcass that had been purposefully left on the seafloor in the Antarctic Sound back in 2010.

Based on a hypothesis first pioneered by Smith about 20 years ago, sunken whale corpses, also known as whale falls, are thought to serve as a catalyst for exotic marine critters capable of existing in extreme environments. In fact, the carcasses can be seen as another sort of proxy — in this case, for the elusive cold-seep communities.

The first LARISSA cruise had left behind a pile of whalebones in a lander, an aluminum frame surrounded by an acoustic release that grips ballast. A coded ping from the ship triggers the acoustic release, bringing the now-buoyant lander to the surface.

The researchers discovered that indeed some of the same suspects known to inhabit cold-seep communities made a home on the whale carcass, including different species of marine worm.

McCormick’s microbes also play an important part in this micro-ecosystem, from the surface to within the core of the bone. The latter appear to thrive in anoxic environments, with the ability to use the fatty lipids in the bones.

“It’s a nice story,” McCormick said. “We’re getting a nice idea of how the microbial communities structure themselves within the whalebone to optimize the retrieval of nutrients and energy out of the components of the bone.”

Even the oceanographers benefitted from recovering the whale lander, which had refused to budge during an earlier attempt to retrieve the experiment during the southbound trip.

“We also recovered a temperature-pressure record from a whalebone lander … yielding a very nice two-year record of bottom temperature in the Sound,” Huber said. “It showed a very dramatic seasonal signal, with the water at the bottom [at 1,000 meters] reaching near-freezing temperatures during the winter months.”

Continuing the adventure

The LARISSA story is not over yet.

The research team has one more expedition planned in 2013 under the current proposal — a trip aboard the new South Korean research vessel Araon, a 109.5-meter-long icebreaker commissioned in 2009.

The scientists will hope the third time is indeed the charm next year.

NSF-funded research for the LARISSA program: Eugene Domack, Hamilton College, Award No. 0732467; Maria Vernet, University of California-San Diego Scripps Institution of Oceanography, Award No. 0732983; Ted Scambos, University of Colorado at Boulder, Award No. 073292; Amy Leventer, Colgate University, Award No. 0732625; Michael McCormick, Hamilton College, Award No. 0732917; Cindy Van Dover, Duke University, Award No. 0732450; Arnold Gordon and Bruce Huber, Columbia University, Award No. 0732651; Ellen Mosley-Thompson and Lonnie Thompson, Ohio State University, Award No. 0732655; Craig Smith, University of Hawaii, Award No. 0732711; Martin Truffer, University of Alaska Fairbanks, Award No. 0732602; Erin Pettit, University of Alaska Fairbanks, Award No. 0855265; Stefanie Brachfeld, Montclair State University, Award No. 0732605; Julia Wellner, University of Houston, Award No. 0732614; and Scott Ishman, Southern Illinois University at Carbondale, Award No. 0732554.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs