"We believe that carbon-14 production is essentially constant over time," said Hodgins. "So the amount of carbon-14 present in living organisms in the past was similar to the levels in living organisms today. When an animal or plant dies, the amount of carbon-14 in its remains drops at a predictable rate, called the radioactive half-life. The half-life of radiocarbon is 5,730 years."
"People from all over the world send our laboratory samples of organic material that they have dug out of the ground and we measure how much carbon-14 is left in them. Based on that measurement, and knowing the radiocarbon half-life, we calculate how much time must have passed since the samples had the same amount of carbon-14 as plants and animals living today."
The researchers use a machine called an accelerator mass spectrometer to measure the amount of radioactive carbon remaining in a sample. The machine works in a manner analogous to what happens when a beam of white light passes through a prism: White light separates into the colors of the rainbow.
The accelerator mass spectrometer generates a beam of carbon from the sample and passes it through a powerful magnet, which functions like a prism. "What emerges from it are three beams, one each of the three carbon isotopes," said Hodgins. "The lightest carbon beam, carbon-12, bends the most, and then carbon-13 bends slightly less and carbon-14 bends slightly less than that."
The relative intensities of the three beams represent the sample's carbon mass spectrum. Researchers compare the mass spectrum of an unknown sample to the mass spectra of known-age controls and from this comparison, calculate the sample's radiocarbon age.
At 33,000 years old, the Siberian skull predates a period known as the Last Glacial Maximum, or LGM, which occurred between about 26,000 and 19,000 years ago when the ice s
|Contact: Daniel Stolte|
University of Arizona