办事指南

Clear as mud

点击量:   时间:2017-08-07 01:02:01

By Jonathan Knight ANCIENT muck from a Japanese lake bed has made it possible to accurately carbon date objects up to 45 000 years old. Scientists may now be able to learn in detail how Europe was settled and when the first humans crossed the Bering Strait. Archaeologists date organic objects by measuring the proportion of radioactive carbon they contain. Like normal carbon, the radioactive isotope is picked up from the atmosphere during the lifetime of a plant or animal and it remains in organic material like hair, bones or wooden tools. As the radioactive carbon decays, its falling levels reveal the age of the objects. But atmospheric levels of radioactive carbon are continuously changing, so carbon dating must be calibrated with an independent measure of time, such as tree rings. Using very old trees like the bristlecone pine, as well as preserved trunks whose ring patterns overlap those of living trees, archaeologists had managed to calibrate the radiocarbon scale for the past 10 000 years. Now Hiroyuki Kitagawa of the International Research Center for Japanese Studies in Kyoto says he has found well-preserved seasonal layers of sediment in the bed of nearby Lake Suigetsu. “There was beautiful black and white lamination, like a bar code,” he says. Kitagawa reports in last week’s Science(vol 279, p 1187) that by studying the carbon in the sediment layers, he was able to push the radiocarbon dating calibration back by another 35 000 years to 45 000 years. The new data will be invaluable to archaeologists, says Rosemary Joyce, director of the Hearst Museum of Anthropology at the University of California at Berkeley. She says that more accurate dating of American and Asian finds will help resolve debates about whether people first crossed the Bering Strait on a land bridge 12 000 years ago, or as much as 30 000 years before that. Joyce says it should also be easier to pin down the migration patterns of early European cave painters. “They all lived within this new extended term,