The Big Thaw; November 1995; Scientific American Magazine; by Horgan; 3 Page(s)
The vast shield of ice capping the Antarctic is the largest body of freshwater on the planet. If it melted, sea levels would surge by 60 meters, submerging coastal areas around the world. Some scientists have therefore become increasingly alarmed in recent years as David M. Harwood of the University of Nebraska and others have presented evidence that a mere three million years ago, during the Pliocene epoch, the Antarctic ice sheet melted, transforming the frozen continent into a collection of tree-covered islands. The disturbing implication is that global warming, which may push temperatures to Pliocene levels by the middle of the next century, might trigger a catastrophic meltdown of the ice sheet.
Now a group led by David E. Sugden of the University of Edinburgh has challenged this scenario. Sugden and his six co-workers report in Nature that they have discovered ice at least eight million years old in a region that, in Harwood's view, should have been clear of ice as recently as three million years ago. The Copyright 1995 Scientific American, Inc. new finding has intensified what was already a fierce debate between "stabilists " and "dynamists" over the ice cap's past and, more important, its future. The Harwood group based its claim of a big thaw on fossilized beech trees and marine diatoms found high in the Transantarctic Mountains, a rocky spine that cuts the Antarctic roughly in half. The beech fossils were undatable, but the diatoms were of a type known to have existed in the southern oceans three million years ago. According to Harwood, the beech trees grew on the ice-free shores of Antarctic islands during the warm Pliocene, and the diatoms thrived in the marine basins surrounding the landmasses. When the balmy weather of the Pliocene gave way to a more frigid climate, the beech trees all died off; the expanding sea ice pushed sediments laden with diatoms up over the islands, where the diatoms mingled with the beech fossils. Those Pliocene islands became the peaks of the Transantarctic Mountains. But George H. Denton of the University of Maine, a member of Sugden's group, questions Harwood's analysis. Denton says that even today diatoms can be blown from the open sea surrounding the Antarctic far inland. The three-million-year-old diatoms found by Harwood might also have been transported from open sea into the Transantarctic Mountains, mingling with the much older fossilized beech trees, Denton explains.