Antarctic ice sheet's erratic behaviour

A team from the University of Leeds and Aberystwyth University has returned from the Antarctic Peninsula with exciting new information on the behaviour of the giant Antarctic Ice Sheet.

The Antarctic Ice Sheet is of exceptional interest to geoscientists due to its size and location, which mean that it reacts quickly and dynamically to climate change. The team of four found that the ice sheet had expanded and then retreated across neighbouring James Ross Island several times over the last 25,000 years. The findings are crucial for understanding the thickness and extent of the ice sheet through time, and so its past and future contribution to sea level rise.

The group spent seven weeks in a field camp on the Ulu Peninsula - which forms part of James Ross Island - mapping in detail an area of 600 km2. They examined the distribution of so-called 'erratic' rocks on the Peninsula, a term used to refer to stones and boulders that have been moved from their original site by glaciers and ice sheets. The team found hundreds of large granite boulders scattered all across the Peninsula despite the islands' volcanic make-up, indicating that they were ripped up by the Antarctic Ice Sheet and moved to their new location.

"Geologically, the Antarctic Peninsula is completely different to James Ross Island - so we know that wherever we find these erratic rocks we can be fairly sure that an ice sheet from the Antarctic Peninsula brought them onto the island" said Dr Jonathan Carrivick, from the University of Leeds. "It is then a relatively easy task to match the exact composition of the rocks to those on the Antarctic Peninsula. In doing so, we can trace the historic movement of the ice sheet across the Prince Gustav Channel".

Principal Investigator Professor Neil Glasser, from Aberystwyth University, added: "The granite 'erratics' were ripped up by the Antarctic Ice Sheet and moved onto James Ross Island at some time in the past when the Ice Sheet was much more extensive and thicker. We were surprised both by the number and the size of these erratic boulders. Some are up to 3 metres in diameter - the size of a small family car".

The burning question is when the Antarctic Ice Sheet was big enough to have brought all these granite erratic boulders onto James Ross Island.

To try and answer this, the team collected rock samples from the surface of more than 50 granite boulders. These samples will be used to date precisely the former expansion of the Antarctic Ice Sheet, using a laboratory technique called cosmogenic exposure age dating.

"This dating technique works because we can use the build-up of cosmogenic isotopes in the granite rocks to gain an understanding of the length of time for which the boulders have been exposed on the surface of the Earth," said Dr Bethan Davies, from Aberystwyth University. "It is a relatively new technique but it will help us answer the important question of when the Antarctic Ice Sheet was bigger in the past. We will then be in a position to understand better the possible future behaviour of the ice sheet and its likely effect on rises in sea level".

The research was funded by the UK Natural Environment Research Council and supported by the British Antarctic Survey (BAS), who organised the fieldwork on James Ross Island. BAS supplied all the fieldwork logistics including transport, the field camp and safety equipment. The team of three scientists were also accompanied by a Field General Assistant from BAS.

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