Warm cretaceous earth: don't hold the ice.

Warm Cretaceous earth: Don't hold the ice

The long-held assumption that the earth has been ice-free for significant stretches of geologic time may be on the rocks. Large, out-of-place boulders in Australia have led scientists to suggest that ice existed there even during the warmest known section of earth history, the Cretaceous period -- a time so temperate that forests thrived in Antarctica.

During this period, which spanned from 144 million to 66 million years ago, ocean levels peaked and large seas filled the interior of Australia as well as other continents. Sediments that settled to the floor of the ancient Australian sea formed layers of small-grained rocks such as sandstone. But geologists there also have found many boulders of exotic rock that sit in the middle of these sedimentary layers. These rocks, measuring up to 3 meters in diameter, are sometimes located more than 100 kilometers from what was then the coastline.

The only viable explanation for the presence of these boulder is that small rafts of ice carried the rocks out to sea and then dropped them as the ice melted, say L.A. Frakes and J.E. Francis of the University of Adelaide in Australia, who report their analysis of the rocks in the June 9 NATURE. The scientists say they ruled out the possibility of other modes of trnasportation such as mudflows, strong water currents or volcanoes.

It is not possible to tell whether the ice on Australia could have lasted throughout the year or only during the winter. "Either one is a big change from how people viewed the Cretaceous climate," says Eric J. Barron from Pennsylvania State University in University Park.

Using computer models, Barron has estimated that the mean global temperatures during the Cretaceous averaged some 6[deg.] C to 12[deg.] C warmer than today -- a whopping figure by climate standards. But he and other researchers now believe that in spite of the average global warmth, the interior of certain continents may have been cool.

Scientists have found similar dropstones in Cretaceous deposits from the Northern Hemisphere, and the rocks also are found in sediments from other warm periods. According to Frakes and Francis, these finds suggest that "the possibility of an ice-free earth having ever existed appears small."

Alfred M. Zeigler at the University of Chicago, a supporter of the ice-fre-earth theory, says "people are kind of skeptical" about the report by Frakes and Francis. "On the other hand," he adds, "the claim they make [about ice-rafting] seems to be the most likely explanation."