Sediment core samples drawn from Lake Titicaca, South America's largest freshwater lake, have revealed a 25,000-year precipitation record that may rewrite the books on ancient climate. According to results reported Friday in the journal Science, the South American tropics, long thought to have been arid during the last Ice Age, may have in fact been wet.

Working in the Bolivian section of Titicaca (it straddles Bolivia and Peru), Duke University geologist Paul Baker and his colleagues extracted cores up to 46 feet long from the lake bottom. Subsequent analyses of the cores' oxygen isotope ratios, magnetic values, calcium carbonate concentrations and fossilized diatoms (microscopic, silica-encased algae) indicated that the lake's environs were wet both during the last Ice Age and during later cold periods in the North Atlantic Ocean region. During warm periods in the North Atlantic, on the other hand, the Titicaca region experienced drought. Data from other paleoclimatic research suggest that cyclical changes in the tilt of the earth's axis and seawater temperatures drove these wet conditions in the South American tropics.

The researchers acknowledge that equating the moisture levels of the Lake Titicaca region with climate conditions in the jungles to the east is "stepping out on a limb," but Baker notes that most evidence supports that link. He warns, however, that the climate conditions of the past should not be used to forecast the future. "Human influence is so dominant now," Baker asserts, "that whatever is going to go on in the tropics has much less to do with sea surface temperatures and the earth's orbital parameters and much more to do with deforestation, increasing atmospheric carbon dioxide and global warming."