The search for early dirt.

Earth was all wet early on, with a single ocean unbroken by any continents. When continental crust finally emerged from the waves, it fundamentally changed all aspects of the planet, from the internal convective roilings to the external soup of gases in the atmosphere. A group of Australian geologists has now identified the oldest evidence of dry land, pushing back the time when scientists believe continents rose up out of the oceans.

Roger Buick of the University of Western Australia in Nedlands and his colleagues discovered a distinct geologic feature called an unconformity, created when layers of sediments build up on an area previously eroded by wind and water. Using the decay of radioactive uranium in the rocks as a clock, the scientists dated the unconformity to 3.46 billion years ago, a half billion years more ancient than the next oldest evidence of dry land, they report in the June 15 Nature.

Geologists working in Canada have found pieces of continental crust from even further back, about 3.96 billion years ago (SN: 10/7/89, p.228). But because such rocks formed deep in the crust, they cannot reveal whether the ancient continental surface sat above sea level. In the Australian case, the presence of an unconformity clearly indicates a land surface reaching above sea level. "The fact that it had an erosion surface means it was exposed to the atmosphere and to weathering," says Buick.

Geologist Paul F. Hoffman of Harvard University calls the Australian find important because it could offer insight into the composition of Earth's early atmosphere. Scientists have speculated that the atmosphere in the planet's first billion years must have been a supergreenhouse, containing hundreds of times more carbon dioxide than today's air. The abundance of this heat-trapping gas would have helped keep oceans from freezing early in the planet's history--a time when the infant sun emitted much weaker light than it does now.

Geochemists can gauge the carbon dioxide content of the ancient atmosphere by studying fossilized soil, or paleosol, preserved in ancient rocks. The Australian rocks appear to contain paleosol, says Buick, who plans to enlist Harvard's Heinrich D. Holland to examine the rocks. Buick also hopes to look below the unconformity for what would be the earliest evidence of life on the planet.