‘Fossil’ mountains entombed by ice

Cold temperatures have preserved rugged Antarctic range

Buried deep beneath East Antarctica’s ice sheet, the Gamburtsev Mountains are the world’s most invisible range. New research suggests that overlying ice like that hiding them from view today could have preserved their rugged topography for the past 300 million years.

UNDER ICE Antarctica’s Gamburtsev Mountains may have been protected from erosion by the ice sheet that covers them, preserving jagged peaks for the past 300 million years. Above, the mountains are shown in a slightly exaggerated view based on radar data. Michael Studinger/Lamont Doherty Earth Observatory

SOUTHERN PEAKS The Gamburtsev Mountains in eastern Antarctica. E. Feliciano

The work bolsters the counterintuitive notion that glaciers, rather than just carving down young peaks into eroded hills like a buzzsaw, could sometimes protect high jagged terrain.

“It’s feasible for topography to be preserved,” says Stephen Cox, a graduate student at Caltech and coauthor of a paper scheduled to appear in Geophysical Research Letters. A supercold cap of ice could have allowed the ancient Gamburtsevs to look like the Alps instead of the highly eroded Appalachians.

Russian scientists first identified the Gamburtsevs in 1958 as part of a survey during the International Geophysical Year, and geologists have been puzzled ever since about how the range came to be. The mountains are in a stable part of the continent that hasn’t seen much tectonic activity — usually the way mountains are born — in more than 500 million years. “The Gamburtsevs are either really old, or some big part of the tectonic puzzle is missing,” says Cox.

His team tackled the question by looking at how quickly the mountains eroded over time. Because the range is buried, researchers have to study it indirectly — in this case by probing mineral grains at the bottom of Prydz Bay in East Antarctica, where pieces of rock washing off the Gamburtsevs ended up.

Grains of the mineral apatite preserve a record, known as a cooling age, of how fast the mountains were eroded. Cox’s team analyzed the apatite in two ways — the amounts of uranium, thorium and helium it contained, and the number of “fission tracks” left by decaying uranium — to build a cooling history of the Gamburtsevs.

The team concluded that over the past 250 million years, mountains inland of Prydz Bay eroded just 2.5 to five kilometers — an order of magnitude slower than modern erosion in places like the Alps. Earlier studies had suggested slow Antarctic erosion over the past 118 million years, but the new study goes further back in time and supports the idea that the Gamburtsevs really are ancient.

Cold glaciers or ice sheets atop the mountains could have protected them from wearing away, Cox suggests.

A paper published in Nature last month describes how glaciers could similarly be preserving topography in the southernmost Andes today.

“When you get to colder climates, glaciers are actually frozen to the rock,” says geologist Stuart Thomson of the University of Arizona in Tucson, a coauthor of that paper and a member of Cox’s team. “They flow a little, but they don’t erode much at all.”

Radar surveys of the Gamburtsevs conducted in 2008 and 2009 confirm that the range is unusually rugged, with V-shaped valleys rather than the U-shaped ones that are characteristic of glacial erosion.

Still, another Antarctic expert warns against drawing too many conclusions about ice atop the Gamburtsevs, especially over the past few tens of millions of years. The new work can’t reveal anything explicit about when big ice sheets or smaller mountain glaciers were actually present, says John Goodge, a geologist at the University of Minnesota in Duluth.

Yet studying erosion rates could help researchers better figure out the history of Antarctic ice, says Thomson. He is now working on more detailed studies of erosion over the past 34 million years, when the great East Antarctic ice sheet is thought to have started growing.

“We’re trying to look at where sediments come from and what they tell us,” he says. Then researchers who use computer models can include those data and see whether current ideas about how Antarctica got icy are correct.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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