How Antarctica grew its ice – and lost its hanging gardens

By Catherine Brahic Up to 3000 metres beneath the ice, at the coldest point on Earth, towering peaks, hanging valleys and deep gorges have been frozen in ice for 14 million years. Now the first detailed view of this frozen landscape is revealing how the world’s biggest chunk of ice – the Antarctic ice sheet – was born. The radar images suggest that Antarctica “grew” its ice cap in three stages, carving out the rock below in distinct ways as glaciers expanded, retracted, and flowed downstream. The images were collected between 2004 and 2008 by researchers who drove huge trains of caterpillar tractors in tight lines over Dome A, a plateau of ice at the heart of Antarctica. The tractors carried radars that pinged down through the ice and sent back profiles of the frozen rock landscape below. Dome A, the highest point on the continent, is also one of the coldest places on Earth, with temperatures as low as -90 °C. Far beneath its frozen surface lie the Gamburtsev mountains, where glaciologists believe the Antarctic ice sheet was born. Its distance to the ocean and high altitude would have made it the coolest spot on the continent 34 million years ago, when the ice began to grow. Martin Siegert et al. Because Dome A is so remote and so cold, and because kilometres of ice separate the surface from the mountain tops, we know more about the surface of Mars than we do about the Gamburtsev. Until recently, only a single radar flight some 30 years ago had probed the chain. In the past few years glaciologists have used planes and tractors to map out the iced mountains more fully. Earlier this year, the British Antarctic Survey revealed images of the mountains’ profile. Now, a Chinese, Japanese and UK team have published results detailing the 900-square-kilometre area beneath Dome A. By studying the images, the researchers have determined how the ice cap formed. First, some 34 million years ago, small mountain-top glaciers developed. They froze and thawed with variations in Earth’s orbit, sometimes filling the range’s main valley and its tributaries, sometimes disappearing entirely. These variations would have created distinct, high-altitude cirques, hanging valleys and deepened the main valley. Isotope records from the deep ocean show that global temperatures dropped by up to 8 °C about 14 million years ago. This froze the ice to the rock, and it will have moved very little since then, preserving the landscape below. Rocks are not all that will have been frozen in time and space. Martin Siegert of the University of Edinburgh, UK, says it is very likely that there are bits of frozen vegetation down there too – far out of reach. “It would have looked much like Patagonia today, with quite lush forests and small valley glaciers cutting into the alpine topography,” he says. Journal reference: Nature (DOI: 10.1038/nature08024) More on these topics:
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