Giant icebergs head to watery end at island graveyard

By Jonathan Amos
Science correspondent, BBC News, San Francisco


South Georgia is the place where colossal icebergs go to die.

The huge tabular blocks of ice that frequently break off Antarctica get swept towards the Atlantic and then ground on the shallow continental shelf that surrounds the 170km-long island.

As they crumble and melt, they dump billions of tonnes of freshwater into the local marine environment.

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

Those familiar with the epic journey of Ernest Shackleton in 1916 will recall that it was at South Georgia that the explorer sought help to rescue his men stranded on Elephant Island.

The same currents that assisted Shackleton's navigation across the Scotia Sea in the James Caird lifeboat are the same ones that drive icebergs to South Georgia today.

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

"The iceberg known as A-38 had a mass of 300 gigatonnes. 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."

Dr Brandon has been presenting his research here in San Francisco at the American Geophysical Union (AGU) Fall Meeting, the world's largest annual gathering of Earth and planetary scientists.

Slow death

With a group of colleagues he planted scientific moorings off South Georgia in several hundred metres of water. The moorings held sensors to monitor the physical properties of the water, including temperature, salinity and water velocity. The presence of plankton was also measured.

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, such as B-10A and A-22B, which have been caught at South Georgia, which lies downstream of the Antarctic Peninsula in currents known as the Weddell-Scotia Confluence.

The island's continental shelf extends typically more than 50km from the coast and has an average depth of about 200m, and when the mega-bergs reach the island, they ground and slowly decay.

"All that freshwater has a measurable effect on the structure of the water column," said Dr Brandon. "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 tonnes of freshwater into the local area.

Image caption,
The scale of the bergs that arrive at South Georgia is hard to grasp

Professor Eugene Murphy, from the British Antarctic Survey, says mega-bergs have important biological impacts.

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

But at South Georgia, the giants may on occasions have a more negative consequence, especially in the case of A-38. Some of the data collected by researchers across the territory leads the team to think the berg's great bulk may have acted as a barrier to the inflow of krill.

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

Image caption,
South Georgia is a highly productive area

In years when there are few krill at South Georgia, the predators that eat them will suffer poor breeding success. In really bad years, the beaches of South Georgia can be littered with dead pups and chicks, Professor Murphy says.

"When that berg was sat on the shelf, it 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."

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