December 15, 2010

Where Icebergs Go To Die

Icebergs apparently head off to South Georgia when they are going to die.

The blocks of ice break off Antarctica and get swept towards the Atlantic and then ground on the shallow continental shelf that surrounds the 105-mile long island.

They dump billions of tons of freshwater into the local marine environment.

U.K. scientists say that the giants have quite dramatic impacts, even altering the food webs for South Georgia's animals.

"The scale of some these icebergs is something else," oceanographer Dr Mark Brandon from the Open University told BBC News.

"The iceberg known as A-38 had a mass of 300 gigatons. It broke up into two fragments, but it also shattered into lots of smaller bergs. Each smaller berg was still fairly big and each dumped lots of freshwater into the system."

Brandon has been presenting his research at the 2010 American Geophysical Union (AGU) Fall Meeting, which is the largest annual gathering in the world for Earth scientists.

He planted scientific moorings off South Georgia in several hundred feet of water.  The moorings held sensors to monitor the physical properties of the water, including temperature, salinity and water velocity. 

The moorings were in prime position to capture what happened when the mega-berg A-38 turned up in 2004.

It is one of many tabular blocks that have been caught at Sough Georgia, which lies downstream of the Antarctic Peninsula in currents that are known as the Weddell-Scotia Confluence.

The island's continental shelf extends typically over 31 miles from the coast and has an average depth of about 650 feet when mega-bergs reach the island.

"All that freshwater has a measurable effect on the structure of the water column," Brandon told BBC. "It changes the currents on the shelf because it changes the seawater's density. It makes the seawater quite a lot cooler as well." A-38 probably put about 100 billion tons of freshwater into the local area.

Professor Eugene Murphy of the British Antarctic Survey told BBC that mega-bergs have important biological impacts.

Dust and rock fragments picked up in Antarctica act as nutrients when they melt out into the ocean, fueling life like algae and diatoms right at the bottom of food webs.

However, the mega-bergs may have a more negative consequence, especially in the case of A-38.  Some of the data collected by researchers have lead the team to think that the berg's great bulk may have acted as a barrier to the inflow of krill.

These shrimp-like creatures are a vital source of food to many of the island's animals, including its penguins, seals and birds.

"When that berg was sat on the shelf, if was directly in the path of areas that we would normally think of being the main inflow areas for the krill," he told BBC News.

"It does look as though that year was somewhat unusual.

"It was not the worst year but it was one of the more extreme years. And we haven't really got another explanation for what happened in 2004. So this is partly why we're looking at the physics of this problem, to see if we can then examine how it may have affected the biology."


Image Caption: Iceberg A38-B off South Georgia. Credit: Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC


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