Ice Sheet’s erratic behaviour

British Antarctic Survey field assistant Alan Hill sampling an erratic granite boulder on James Ross Island, Antarctica.

British Antarctic Survey field assistant Alan Hill sampling an erratic granite boulder on James Ross Island, Antarctica.

29 March 2011

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

The Antarctic Ice Sheet is of exceptional interest to geoscientists due to its size and northern location, which means that it reacts quickly and dynamically to climate change.

The team of four has just spent seven weeks in a field camp on the Ulu Peninsula, part of the James Ross Island, and mapped in detail an area of 600 km2.

One of their major efforts was dedicated to examining the distribution of “erratic” rocks that can be used to demonstrate that the Antarctic Ice Sheet expanded and then retreated across James Ross Island, probably around 12,000 years ago.

According to Principal Investigator, Professor Neil Glasser from the Institute of Geography and Earth Sciences at Aberystwyth University, the term “erratic” describes rocks and boulders that have been moved from their original location to a new area by glaciers and ice sheets.

“James Ross Island is made of volcanic rocks but we were pleased to find hundreds of large granite boulders scattered all across the island. They were on the coast, in the interior of the island and even on mountains at high elevations”, he said.

“These granite erratics were ripped up by the Antarctic Ice Sheet and moved onto James Ross Island at some time in the past when the Antarctic 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 m in diameter; the size of a small family car” explained Professor Glasser.  

Dr Jonathan Carrivick from the University of Leeds said: “The Antarctic Peninsula is geologically completely different to James Ross Island so we know that wherever we find these erratic rocks we can be pretty sure that an ice sheet from the Antarctic Peninsula brought them onto James Ross Island.”

“It is then a relatively easy task to use the exact composition of the erratic rocks to match them to the same rock types on the Antarctic Peninsula; i.e. to trace the movement of the ice sheet across the Prince Gustav Channel.”

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 question the team collected rock samples from the surface of more than 50 of the granite boulders (see attached photograph). These samples will be used to date the former expansion of the Antarctic Ice Sheet using a laboratory technique called cosmogenic exposure age dating.

Dr Bethan Davies from Aberystwyth University explained: “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.”

 “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 sea level rise”.

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.