Page 2/3 - Posted June 8, 2012
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.”
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.
Photo Credit: Amber Lancaster (PolarTREC 2012), Courtesy of ARCUS
A conductivity, temperature, depth rosette is lowered into the water to take oceanographic measurements and collect water samples.
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.
“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.Back 1 2 3 Next
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