Archaeology: Radiocarbon Dates

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ARCHAEOLOGY ARTICLE

Matching Radiocarbon Dates to Calendar Dates
By MIKE BALDWIN

9,600 radiocarbon years	= 11,000 calendar years
10,200 radiocarbon years	= 12,000 calendar years
11,000 radiocarbon years	= 13,000 calendar years
12,000 radiocarbon years	= 14,000 calendar years
12,700 radiocarbon years	= 15,000 calendar years
13,300 radiocarbon years	= 16,000 calendar years
14,200 radiocarbon years	= 17,000 calendar years
15,000 radiocarbon years	= 18,000 calendar years
15,900 radiocarbon years	= 19,000 calendar years
16,800 radiocarbon years	= 20,000 calendar years
17,600 radiocarbon years	= 21,000 calendar years
18,500 radiocarbon years	= 22,000 calendar years
19,300 radiocarbon years	= 23,000 calendar years
20,000 radiocarbon years	= 24,000 calendar years

02.21.2001 -- The question of when people first reached the Americas is complicated by a problem with dates.

Archaeologists generally rely on radiocarbon dating to determine the age of such artifacts as bones, charcoal or wood. But one radiocarbon year is not the same as one calendar year.

Radiocarbon dating works because all living things absorb carbon from the atmosphere around them. Living things (plant and animal) absorb two different isotopes of carbon: carbon 14 and carbon 12. While an animal or plant is alive, the ratio of carbon 14 to carbon 12 in its tissues reflects the ratio present in the atmosphere. Once it dies, that ratio changes.

Carbon 14 is radioactive (but not dangerous) and undergoes radioactive decay; carbon 12 is stable. During a creature's lifetime, processes such as breathing replenish carbon 14. After death, however, the amount drops, and the ratio between carbon 14 and carbon 12 falls as well. Scientists know the rate at which carbon 14 decays, and by determining how much has been lost compared with carbon 12, they can decide upon the age of an object.

The ratio of carbon 14 to carbon 12 in the atmosphere is not constant, which alters the baseline for calibrating dates. To match radiocarbon years to calendar years, researchers have turned to independent timescales based on tree rings, ice cores and uranium-thorium dating.

Unfortunately for scientists studying the peopling of the Americas, the period between 10,000 and 20,000 years ago has been difficult to calibrate. For many years archaeologists simply presented their results in uncorrected radiocarbon years.

Recent findings, however, make it easier to adjust dates from this era. The distinction between radiocarbon years and calendar years is very important. A report in early 2000 described a 13,000-year-old skeleton found in California and compared it to 12,500-year-old Monte Verde, without mentioning that the former date was in calendar years and the latter, radiocarbon years. Some readers understandably thought that the California skeleton was older than the campsite at Monte Verde. But in calendar years, Monte Verde is 14,700 years old.

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© 1998-2006 Memphis Archaeological and Geological Society. This page last updated 06.24.2006.