The making of Antarctica's Fjords.

• Position: Geoscience

Antarctica's topography began changing from flat to fjord-filled starting about 34,000,000 years ago, according to a report in Nature Geoscience. Knowing when Antarctica's topography started shifting from a flat landscape to one with glaciers, fjords, and mountains is important for modeling how the Antarctic ice sheet affects global climate and sea-level rise.

Although radar surveys have revealed a rugged alpine landscape under Antarctica's two-mile-thick ice sheet, the surveys tell nothing about when the continent's deep valleys formed. 'We have worked out how the landscape under the ice has changed through time," says lead author Stuart N. Thomson. "People have speculated when the big fjords formed under the ice, but no one knows for sure until you sample the rocks or the sediments."

He and his colleagues sampled East Antarctica's rocks by examining the sediments that built up offshore for millions of years as rocks and dirt eroded off the continent into Prydz Bay. "We use the sediments to trace what was happening under the ice in the past," notes Thomson.

The team found that, between 250,000,000 and 34,000,000 years ago, erosion from the region now covered by the huge Lambert Glacier was anything but fast, suggesting the area was relatively flat and drained by slow-moving rivers. About 34,000,000 years ago, at the same time the climate shifted and Antarctica was becoming covered with ice, the rate of erosion...