The Antarctic ice sheet sits like a shallow dome on the continent’s surface. Over eons, the weight of accumulating ice in the continental interior forces outlying ice down the dome’s slope and into the sea. No one really knows how long ice has covered Antarctica. Some geologists say that as recently as 3 million years ago, Antarctica had a relatively mild climate and was even forested. David Sugden thinks that’s unlikely, particularly in light of a recent expedition he made to the frozen continent. He discovered a fragment of a long-vanished glacier that appears to be at least 8 million years old.
Sugden, a geologist at the University of Edinburgh, was trying to date an ancient volcanic eruption in East Antarctica when he and his colleagues uncovered the old ice. They were in Beacon Valley, a boulder- filled area between glaciers, collecting volcanic ash from a seven-foot- deep, V-shaped fissure in rocky debris deposited by an ancient glacier. Using radiometric dating methods, Sugden determined that the ash settled into the fissure after a volcanic eruption 8.1 million years ago.
Beneath the ash--and presumably at least as old--the researchers found a layer of ice at least 300 feet thick. In East Antarctica, where the average temperature hovers around zero degrees Fahrenheit, ice today never melts. But it does sublime--it converts directly into water vapor as the wind sweeps over it. Sugden believes that as most of the ice that once covered this area sublimed, the rocky debris suspended in the glacier was left behind. That debris protected the remaining glacier ice for millions of years. Had the climate ever gotten as warm as some researchers have claimed, though, Sugden argues, the ice would have melted anyway.
The evidence for a warm Antarctic has been fossilized pollen and plankton found high in the Transantarctic Mountains, near the coast, and dated to around 3 million years ago. At that time, the theory goes, Antarctica was ice-free and forested, with internal seaways covering low- lying areas; when the climate got colder, glaciers formed in the interior of the continent and then advanced across the seaways, sweeping plankton and pollen into the mountains. But Sugden says the plankton could instead have come from sea ice off the Antarctic coast and been blown into the mountains by wind. As for the pollen, Sugden says it has never been accurately dated and may come from a much earlier clement period.
Sugden’s ancient ice suggests that Antarctica’s climate has remained fairly stable for at least 8 million years. He is now studying the amounts of carbon dioxide and other gases in air bubbles in the ice, seeking to understand the ancient atmosphere. Says Sugden: It’s rather exciting to find something that old, and it does give us the very exciting chance to have a look at the climate and atmosphere of the time.