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The Encyclopedia of World History.
Radiocarbon
 
Radiocarbon (C-14) dating provides dates for archaeological sites dating from about 1500 C.E. to at least 40,000 years ago, and sometimes earlier. Developed by Willard F. Libby of the University of Chicago, radiocarbon dating is based on the fact that carbon isotope carbon-14 enters the carbon dioxide of the atmosphere with carbon-12, ordinary carbon (c). Living vegetation builds up its own organic matter by photosynthesis and by using atmospheric carbon dioxide. Thus, the ratio of carbon-12 to carbon-14 in living vegetation and the animals that eat it is the same as that in the atmosphere. As soon as an organism dies, no further radiocarbon is incorporated into it and the carbon-14 left therein disintegrates at a known rate, only half remaining after 5,730 years.

The C-14 atoms in radiocarbon samples taken from tiny samples of bone, charcoal, shell, wood, and other organic substances found in archaeological sites are counted by accelerator mass spectrometry, resulting in age determinations that are statistical approximations of the date of the sample. The margin of statistical error can be as high as 2,000 years or more.

Radiocarbon dates from about 9000 B.C.E. can be calibrated with tree-ring chronologies, which are absolutely precise. This calibration is essential, because the concentration of radiocarbon in the atmosphere has varied considerably over time.

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