Ancient river valleys discovered beneath Antarctic ice sheet

A landscape shaped by rivers and glaciers over millions of years appears to have been preserved beneath the East Antarctic ice sheet. The finding suggests that this area of Antarctica may have been covered in ice for more than 30 million years, which could inform models of how the ice will respond to climate change in the future.

Aside from a few mountain peaks, nearly all of the geological features of Antarctica are covered in a sheet of ice more than 2 kilometres thick. Radar measurements have revealed the broad contours of this hidden landscape, but the details remain largely obscure.

Stewart Jamieson at Durham University in the UK and his colleagues looked at satellite radar measurements of the surface of the East Antarctic ice sheet to get a new view. “Any ice sitting on the top is going to very generally undulate over those mountains or those hills,” says Jamieson.

They then checked the topography inferred from the ice surface against direct measurements from ice-penetrating radar recorded by aerial surveys.

Together, the radar measurements revealed a striking region dubbed “Highland A”, which was distinct from the surrounding landscape. The 32,000-square-kilometre area features three blocks of land, separated by wide, fjord-like troughs. If the ice were removed, the topography wouldn’t be dissimilar to the peaks and valleys of the Lake District in the UK, says Jamieson.

The researchers posit that these features describe a geological history stretching back to the separation of Antarctica from the Gondwana supercontinent around 180 million years ago. They suggest the “fjords” between blocks of land were shaped by rivers flowing through rifts created by the break-up, then carved out further by glaciers that formed when the climate cooled around 34 million years ago.

As cooling accelerated and the ice sheet grew above the region, the topography in the surrounding areas was scrubbed away. But Jamieson says the features of Highland A were preserved because the glaciers formed a cold base beneath the ice sheet that locked onto the rock and prevented erosion. “The glaciers switch from being an erosive mechanism to being a protective mechanism.”

The preserved landscape also suggests that the region has been covered in ice for at least the past 14 million years, if not 34 million years, says Jamieson. “If you had a huge retreat [of the ice sheet], then our landscape would have got scrubbed away.”

Most research on melting ice in Antarctica has focused on the West Antarctic ice sheet. “East Antarctica is the elephant in the room, and we need to understand how stable it’s been over the past several million years,” says Mathieu Morlighem at Dartmouth University in New Hampshire. The findings could improve models of how the ice sheet will respond as climate change raises average temperatures to those last seen millions of years ago, he says.

The preservation of Highland A could mean the ice sheet is exceedingly stable, at least over that tiny patch of the continent. However, Sean Gulik at the University of Texas at Austin thinks the ice sheet may have seen fluctuations as recently as 1.5 million years ago, which would suggest it is more sensitive to today’s warming. “We have so little real knowledge about what’s below the ice sheet,” he says.