Erratic ice sheet raising sea levels

U. LEEDS (UK) — At some point in time, the Antarctic ice sheet was extensive enough and thick enough to move boulders the size of a small family car.

Distribution of these boulders, called erratic rocks, on the Ulu Peninsula portion of James Ross Island indicate the ice sheet has repeatedly expanded and retreated during the past 25,000 years.

Because of its size and location, the ice sheet is of exceptional interest to geoscientists in that it reacts quickly and dynamically to climate change. Erratic rocks are stones and boulders that have been moved from their original site by ice sheets and glaciers.

Researchers recently spent several weeks mapping in detail an area of 600 km2 and 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 a 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,” says Jonathan Carrivick, professor of geography at 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.”

“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,” says Neil Glasser, professor of geography at Aberystwyth University.

“We were surprised both by the number and the size of these erratic boulders. Some are up to 3 meters in diameter.”

To determine when the Antarctic ice sheet was big enough to have brought the boulders onto James Ross Island, rock samples from the surface of more than 50 of them will be dated using 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,” says Bethan Davies of 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 behavior 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).

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