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A Note About C14 Dating    

The dates in this feature are provided in calendar years, but are based on radiocarbon dating technology. Radiocarbon dating has revolutionized archeology, but discrepancies can occur when calendar dates and radiocarbon dates are compared. The farther we go back in time, the harder it is to say that a calendar date is the same as a radiocarbon date.

Calendar time refers to measured years before the present - one revolution of the earth around the sun equals one year. Radiocarbon years are based on the rate at which the carbon 14 isotope decays. Living things, such as seeds, plants, and bone, absorb carbon 14 from the atmosphere. When organisms die, absorption stops and the C14 begins to decay at a constant rate. Archeologists estimate when the living material died by measuring the amount of C14 remaining.

Radiocarbon dates are calibrated because there is progressively less agreement between them and calendar dates as we go back in time. A calendar date of 10,000 years before present is just that. A radiocarbon date of 10,000 years before present is actually about 11,450 calendar years ago.

 

 
(photo) Two fieldworkers screen soil for artifacts.

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Left: Sifting soil at Virginia's Thunderbird site.

Right: Excavating Ohio's Manning site.

 
(photo) Archeologist working in Ohio. Use prohibited without permission. Photograph by Bradley Lepper, Ohio Historical Society.

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