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Antarctic ice thinning tracked along geological rifts

26 July 2012, by Tom Marshall

Fast thinning of ice in part of West Antarctica is made worse by a hitherto-undiscovered basin in the rock underneath, which stems from an ancient geological fault, a new study shows.

Ice coring

The team hand-augured several ice cores down to 20m to understand the history of how ice has accumulated.

The discovery helps explain why ice is thinning so much faster in some parts of the continent than elsewhere. It turns out that potential warming of oceans due to climate change is only part of the explanation; just as important are geological processes that make some sections of the ice sheet much more vulnerable.

This means that while recent climate change is probably behind current ice losses from the Antarctic coastal regions, how far these losses are transmitted inland depends more on the landscape's longer-term evolution, so a similar amount of warming can have a very different impact in different places.

'We're trying to bridge the gap between the disciplines of glaciology and Antarctic geology,' says Dr Robert Bingham of the University of Aberdeen, the paper's lead author. 'The existence of deep basins under the ice isn't unusual in itself, but we're beginning to appreciate much more than before that a number of them clearly originate in ancient rifting processes, rather than simply being the product of glacial erosion. Their existence, or more specifically the topographic shape beneath the ice that they have created, helps explain why ice thinning is extending so far inland in some areas.'

Map of Eltanin Bay

Map of Eltanin Bay.

Bingham worked with researchers from NERC's British Antarctic Survey to collect and analyse ice-radar data on the Ferrigno Ice Stream, a vast glacier which moves ice from the interior of the continent to the sea. Covering 14,000 square kilometres, the Ferrigno hasn't been studied much before - partly because it's close to the better-known Pine Island Glacier - but satellite measurements of ice height have shown that, like its neighbour, it's thinning exceptionally fast.

Until now scientists thought this could be to do with relatively recent changes in local conditions acting in combination with the impact of global warming, but the researchers behind the new paper found evidence of a huge trough, up to 1.5km deep, in the surface of the Earth far beneath it. This trough was scoured out by ancient ice-flows as they travelled along the course of a rift in the crust during a period when the Antarctic was covered by far more ice than today.

Now submerged, the basin stretches beneath the ice shelf and allows warmer water from the Bellingshausen Sea to flow under it. This makes the ice at its edge melt more quickly, effectively making the glacier's inland surface steeper and increasing the rate at which it carries ice towards the sea. The upshot is that the ice thins more quickly, and that this thinning spreads further inland.


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Inland thinning of West Antarctic Ice Sheet

Bingham says there are several other basins around West Antarctica that are likely to be doing something similar. As part of NERC's forthcoming Ice Sheet Stability and Response (iSTAR) progamme, he's hoping to return to West Antarctica soon with ice-radar equipment to monitor the nearby Pine Island Glacier's thinning - this is known to have a rift basin under it too, but little is known about exactly how this affects ice flowing towards the sea.

'We're talking about geological processes that take place over very long timescales,' he explains, 'but so far we only have data on these areas over an incredibly short period of observation. These are very remote places - Ferrigno is about 800 miles from the Rothera research station - and it's only really in the last decade that it's been logistically possible to get out there with our equipment.'

So scientists are only starting to get the data they need to understand the relationship between these areas' underlying geology and the changes we're seeing in the ice covering them. Achieving this understanding is an important goal, because ice loss from the West Antarctic Ice Sheet is thought to account for ten per cent of the rise in global sea levels observed in recent years, and this loss is dominated by thinning.

The paper appears in Nature.