— Researchers camp out on the Rutford Ice Stream in Antarctica (Image: British Antarctic Survey)
Graph showing changes in the outline of the bed of the Rutford Ice Stream between 1991and 1997 (top graph) and between 1997 and 2004 (bottom graph); a considerable amount of erosion was noted in 1997, and a large lump of sediment appeared in 2004 (Image: Geology)
A new study threatens to overturn our understanding of how glaciers deposit whale-shaped hills known as drumlins. The findings could have implications for the computer models used to predict glacier flow and subsequent changes in sea-level.
Andy Smith of the British Antarctic Survey and his colleagues are the first to see a drumlin during formation. They have visited the same spot of the Rutford Ice Stream in Antarctica three times since 1991. Each time, they have mapped the shape of the glacier bed, which lies 2000 metres under the surface of the ice.
The maps are created by sinking explosives in the ice and recording the echo of the detonation after it is reflected off the riverbed. The time it takes for the echo to return to the surface reveals how deep the riverbed is, while the researchers can judge how hard it is from the echo's quality and loudness.
The most recent data, from 2004, revealed a big surprise: a large lump of sediment, 10 metres high and 100 m wide that "plainly wasn't there last time we looked" in 1997, says Smith's colleague David Vaughan, also at BAS.
"This is the first time anyone has observed a drumlin actually forming under the ice," says Smith. Researchers generally assume that ancient drumlins, which are common in the US, the Alps, Ireland, Finland and Patagonia, grew slowly as sediments set in motion by a meltwater river gradually accumulated underneath the ice.
However, the new work shows that this is not necessarily the case, says Vaughan: "What is surprising is that big lumps of sediment are being moved around wholesale rather than being slowly accumulated."
He says the mass of sediment seems like it is encased in the underside of the ice sheet and being dragged along as the glacier moves towards the sea. And it could be bigger yet the researchers do not know how long it is.
Direct observations of activity under ice sheets are rare. Those from the Rutford Ice Stream suggest that theories about the movement of ice sheets that have been extrapolated from the shape, size, age and location of ancient drumlins could be flawed.
"Predicting the future of the worlds ice masses and their impact on sea level requires an understanding of subglacial processes," write the authors in Geology.
The findings are a reminder, says Vaughan, that "all the theories and supposed understanding that we've got about drumlins come from studying relic drumlins that were formed 10,000 years ago and have undergone changes since".
Journal reference: Geology (vol 35, p127)