Science Interviews

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Sun, 6th Nov 2011

The Stability of the West Antarctic Ice Sheet - Planet Earth Online

Chris Fogwell, University of Exeter

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Ben -   How stable is the West Antarctic ice sheet?  Its one of the biggest questions in Climate Science.  After all, if the ice itself melted then global sea levels could rise by between 3 and 5 metres, and that would be a catastrophe.  To work out how stable the ice sheet has been in the past, scientists at the University of Exeter has been using a process known as cosmogenic isotope dating.  The technique involves studying isotopes, those are different forms of the same element and Richard Hollingham met up with glacial geologist Chris Fogwell to see what he has found.

Chris F -   This is a cosmogenic isotope extraction laboratory and this is one of the first in England.  It's basically a clean air laboratory, so dust-free conditions; we use this airlock or antechamber.

Richard -   So let's go inside through the sliding door, and you've got a sticky door mat.  This is to take dust of our shoes.

Chris F -   Yeah, basically dust is our big enemy in these places because it's absolutely loaded with beryllium which is one of the isotopes which we're trying to measure so we try and avoid getting dirt and dust in here at all.

West AntarcticaRichard -   And as you might expect the lab itself is sparse, white surfaces, fume cupboards and a blast of air from the vents above us designed to keep the room free of contamination.  Here, Chris' team is studying rock samples found on or near the ice sheet.

Chris F -   You can kind of visualise the West Antarctic ice sheet as being a giant glacier.  It's an ice sheet, so it drowns the topography but it still quarries rocks from the base of the ice sheet and brings them to the surface at certain points.  Now, we can pick up those rocks from both up on mountainsides, so where the former ice sheet used to be when it was far bigger and on the modern ice sheet as well which are coming up through the ice.  Now, we can test these and look at the long term configuration of the surface of the ice sheet and how it's changed.

Richard -   Can we have a look at these rocks then.  So, let's wander over to the bench here.  This one is quite beautiful.  Some sort of granite, is it?

Chris F -   It's basically full of quartz and different feldspar minerals.  We're principally interested in the quartz because this contains the isotopes that we're analysing here which is beryllium-10 and aluminium-26 and they're produced within the latter structure of the quartz.  So, essentially we collect these rocks from different important geomorphological locations across the mountain range and extract the quartz from them.  Now once we've got the quartz and we've cleaned the quartz we can extract the minute amounts of isotope from within the quartz.

Richard -   What's this isotope, this beryllium isotope?

Chris F -   Yes, we have beryllium-10 and aluminium-26.  Now these isotopes are only produced by the interaction with cosmic rays which come through the atmosphere.  Now they produce the isotope year on year at a reasonably well known rate which allows us to essentially use it as a clock, use the build up of it as an exposure clock since the ice sheet changed its shape or structure.

Richard -   So when they're exposed they're absorbing cosmic rays, when they're buried by the ice they're not.

Chris F -   Yeah, this is the theory that we base this technique on.  Now it's giving us a very good idea for where the upper surface of the ice sheet has been and when it was there, so it can allow us to reconstruct the three dimensional shape of the ice sheet.

Richard -   So you got this bit of rock, it's about the size of a house brick.  What can that rock tell you?

Chris F -   Well it can tell us a few things.  Basically, we can get an idea of ice flow direction because we know where the rocks outcrop from, so we know where the rock has been transported from which is one very useful thing.  It can give us the structure of the ice sheet back through time, but more importantly, by analysing the concentration of cosmogenic isotopes,  it can give us the idea of the time since it's been exposed.

Richard -   And what have you found?

Chris F -   We've found an interesting pattern which shows that the current configuration of the West Antarctic ice sheet in this sector of Antarctica has remained the same for hundreds of thousands of years.  Now, this evidence is based upon the presence of features called moraines on the sides of the mountains.  Now above these moraines you get long, long exposure ages, long cosmogenic exposure ages, which suggest that there's been no real glaciation of the mountains, of the upper part of the mountains, for a long period of time. Now, if the West Antarctic ice sheet was to melt then the likelihood is, you would have small mountain glaciers growing as the ice sheet re-grew essentially around its base.  We don't see any evidence of that and we use this and the long term preservation of these moraines on the slopes of these mountains as evidence for stability of the West Antarctic ice sheet in this sector.

Ben -   Chris Fogwell at the University of Exeter.  He was talking to Planet Earth podcast presenter Richard Hollingham and we should say that Chriss work does not definitively prove anything about the stability of the West Antarctic ice sheet.  As far as that's concerned, the jury is definitely still out.

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Very interesting research. 

This needs to be correlated with the previous research on diatom genomic drift on both sides of the West Antarctic Ice Sheet. 

And there needs to be a better hand-waving explanation of higher sea levels during previous interglacial periods. 

However, it is good to realize the slow nature of events in the Antarctic, with hundreds of thousands of year ice build-up not being lost in a few decades.

How would seasonal ice affect the beryllium research?  Has any of this been correlated with the Antarctic Peninsula which has more of a seasonal ice covering, at least during the Holocene.  Are there sections of the Antarctic Peninsula that would have had an ice cap during the glacial periods, and seasonal ice during the interglacial periods? CliffordK, Tue, 8th Nov 2011



The Earth has a very asymmetric geography. The Northern Hemisphere is roughly 50/50 land/water; the Southern Hemisphere is roughly 10/90. There is almost no land in the zone between 45S and 70S where one would expect any ice age extension of the ice cap to occur. Between 45N and 70N it is quite a different story!

During the ice ages, an ice cap covered much of Europe, Asia and North America. During interglacials it retreats to a small area in Greenland.

In the Southern hemisphere the Antarctic ice cap presumably extended to cover the Antarctic peninsula and Patagonia. It did not reach Australia as far as I know. The greater proportion of the Southern ice cap remains over most of the Antarctic continent during interglacials.

It follows then that Southern events make only a minor contribution to the sea level rise in the interglacials, and that nearly all of the rise can be attributed to ice cap melting in the North. damocles, Tue, 8th Nov 2011

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