Colossal icebergs careening along the Antarctic coastline can shut down the deep, cold currents that help drive ocean circulation, a new study reports.
These aren't everyday icebergs, even by Antarctica's mighty standards — these huge icebergs are the size of small European countries. Unleashed by fracturing glaciers, they interfere with the Antarctic Ocean's sea-ice factories, called polynyas, according to the study.
The open-water polynyas persist year-round, allowing bitter winter winds to freeze seawater into freshwater ice, leaving behind dense, salty seawater that sinks to the ocean depths. These sinking currents, called the Antarctic Bottom Water, are a major driver of oceanic circulation, said study co-author Guillaume Massé, a paleoclimatologist at the University of Laval in Quebec, Canada.
The scientists studied the recent history of one of the Antarctic Bottom Current's most important polynyas, near East Antarctica's Mertz Glacier. About one-quarter of the cold current comes from this polynya, according to earlier studies. By analyzing microscopic plankton fossils and geochemical tracers in seafloor sediments, the researchers could determine whether the polynya was covered or ice-free. Different plankton grow when the ocean surface is choked with ice compared with when the surface is open water and exposed to sunlight. [The World's 10 Biggest Oceans and Seas]
Mertz Glacier stretches far out into the sea, and helps protect the polynya against surface currents with a long, floating tongue of ice. At least, it did until 2010, when an iceberg the size of Luxembourg broke off the glacier and drifted west into the polynya next door.
In the past 250 years, similar-size icebergs have launched from Mertz Glacier about every 70 years, the researchers discovered. "It's predictable," said lead study author Philippine Campagne, a graduate student in paleoclimatology at the University of Bordeaux in France.
It appears that an underwater plateau called Mertz Bank acts like a doorstop, preventing the glacier from grinding any farther than the plateau. Stuck in place, the glacier eventually cracks under its own pressure. (Though, in 2010, Mertz was helped along by a crash with another iceberg.)
And when Mertz Glacier unleashes one of these country-size icebergs, the giant ice floe interferes with sea-ice production in the polynya and shuts down the cold-water current, the researchers said.
Currently, the open-water area west of Mertz Glacier is now smaller than it was in 2010 and clogged with sea ice. This is where two icebreaker ships, the Akademik Shokalskiy and the Xue Long, were trapped in January 2014.
Shutting down the bottom water may also shift the local food chain, because the current influences plankton growth. Scientists in Australia are currently studying the impact of calving events on the local wildlife, including local penguin colonies.
Campagne said the seafloor records analyzed in the study suggest it takes about 40 years for the polynya system to naturally recover. "It has a major impact," she said. Without protection from the long glacier tongue, the polynya ice factory shuts down until the glacier regrows, Massé said. Mertz Glacier currently advances at less than a mile (about 1.1 kilometers) every year.
The researchers said they think similar cycles play out in other polynya-glacier systems around Antarctica. "This is a natural phenomenon that [occurs] all around Antarctica," Campagne said.
The findings were published March 24 in the journal Nature Communications.
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