Measured

Observatory: Unearthing History
by Henry Fountain, The New York Times, 12/8/98

Seeking new clues to changes in vegetation and climate that occurred thousands of years ago, scientists have gone underground. Eighty feet underground, in fact, in a cave in Missouri. The scientists, from the University of Minnesota and the University of Iowa, analyzed stalagmites from Crevice Cave, in the southeastern part of the state, for evidence of what things were like topside 75,000 to 25,000 years ago. They reported their results in the current issue of Science.

A stalagmite tells a tale through time, growing on the floor of a cave through the constant drip of mineral-laden liquid from above. That liquid, of course, began as rain, and filtered through decaying plant matter in the soil like water through coffee grounds. Analysis of carbon isotopes in sections of the stalagmite can reveal what kinds of plant matter the water dripped through. And the sections can be dated by measuring the amount of a radioactive thorium isotope they contain. All that analysis showed that the region around Crevice Cave was mostly forest from 75,000 to 71,000 years ago, shifting to savannah and then prairie for about 15,000 years before returning to forest.

A similar analysis of oxygen isotopes in the stalagmites showed average annual atmospheric temperatures varied by about 7 degrees Fahrenheit, peaking around 57,000 years ago and reaching a low point about 46,000 years ago.