The Eocene Greenhouse

Fifty-five million years ago, during the Eocene epoch, Earth underwent a warming more pronounced than any other on record. The average global temperature rose by 1 to 4 degrees C. Palm trees and alligators flourished within the Arctic circle. Researchers point to a greenhouse effect as the cause of this warming—an excess of heat-trapping carbon dioxide in the atmosphere, not unlike the situation we may face in the coming century. But what could have caused such an imbalance so long before humans and their combustion engines?

How about chemical changes wrought by the genesis of the world's largest mountains?

Shortly (in geologic terms) before the Eocene warming began, Earth suffered a colossal shock when the Indian tectonic plate, drifting north through the Tethyan Ocean, ran smack into Asia. In the crumpling and grinding and folding that followed, the Indian plate was subducted. Great quantities of its crust were wedged under the Asian plate, throwing up in turn a 1600-mile- long orogenic belt including two massive mountain ranges, the Karakoram and the Himalaya, and the immense Tibetan plateau.

Geochemically, the result was metamorphism. Crustal rock driven to depths of 100 kilometers or more began to heat, and so to change. Limestones and dolomites, plentiful in the Himalayan region, would have released massive amounts of carbon dioxide. And where the gas could reach a fault or fissure to the surface, it would have escaped into the atmosphere.

"Typically this happens by a process called seismic pumping," says Derrill Kerrick, Penn State professor of geosciences. "A pop or shift opens up some space at depth, and gases are released. I call it metamorphic burps."

The indigestion under the newly formed Himalayan belt, Kerrick calculates, would have lasted at least 10 million years. Working from that duration and a rough idea of the amount of carbonate source rock involved— and knowing of the presence of huge fault zones running the length of the Himalayan mountains—Kerrick estimated how much carbon dioxide might have escaped. Then he turned to Penn State colleague Ken Caldeira, who specializes in global modeling, to determine how much carbon dioxide would have been needed for the Eocene warming and see whether his calculations matched up.

The degassing following the Himalayan upwelling, Caldeira's model showed, could easily have filled the bill. In fact, it could have produced four times the carbon dioxide needed to make the Eocene swelter. "And that's only one part of the global total," Kerrick says. "There are other orogenic belts that would have contributed, too."

Kerrick has presented his preliminary findings at numerous meetings in the United States and Europe. This fall he will travel to the Himalayas on a grant from the National Science Foundation to begin more detailed characterizations of rock composition. "There's lots of work to do to test the model," he acknowledges. But metamorphism, he argues, seems at least as plausible an explanation for Eocene warming as volcanic activity, which up to now has been the most frequently hypothesized source.

Derrill M. Kerrick, Ph.D., is professor of geosciences in the College of Earth and Mineral Sciences, 243 Deike Building, University Park, PA 16802; 814-865-7574. Kenneth Caldeira, Ph.D., was a postdoctoral research fellow at the College's Earth System Science Center. The project reported above is being funded by the National Science Foundation.

Last Updated September 01, 1993