The Radioactive Clock

Radiocarbon Dating

One of the most common methods for dating archaeological sites is by Carbon-14 (C-14/14C). The method was developed by physicist Willard Libby at the University of Chicago who received the Nobel Prize for the discovery in 1950. The radioactive isotope 14C is created in the atmosphere by cosmic radiation and is taken up by plants and animals as long as they live. Upon death, the isotope begins to decay and after 5730±40 years half of it is gone. The C-14 method cannot be used on material more than about 50,000 years old because of this short half-life. Other isotopes are used by geologists to date older material. 

Each radiocarbon date has a statistical probability shown by the ± number. This number is called a standard deviation and is a measure of the spread of measurements around the mean (average). One standard deviation has a 68% probability and two standard deviations have a 95% probability.

Radiocarbon dating has had an enormous impact on archaeology around the world since it made it possible to date carbon and wood could be directly without dependence on characteristic artifacts or written historical records. But as more dates became available, Egyptologists, who had hieroglyphic records back thousands of years, began to recognize that C-14 dates were generally too young. They proved this by showing that C-14 dates of wooden artifacts with cartouches (dated royal names) did not agree.

The explanation was that the physicists had assumed that the amount of C-14 in the atmosphere had been constant, when in fact it had varied over time. The solution came using dendrochronology (tree ring dating). Since tree rings provide an annual calendar, and some trees live for thousands of years, by C-14 dating the rings themselves one could correct the radiocarbon dates and calibrate the differences. The Bristlecone pine trees in the Sierra Nevada mountains made this possible and today there are international tree ring databases and agreed-upon calibration curves. Another problem derives from the “reservoir effect” in which old material, limestone or graphite, has contaminated the samples. This is particularly true of marine samples and contemporary shells may seem to be hundreds of years old.

When this method was first developed, a fairly large amount of carbon was necessary for dating but use of the AMS (accelerator mass spectrometer) today necessitates only a few milligrams for analysis. 

Every radiocarbon date is written as B.P. (before present). It has been agreed that B.P. should refer to the year the method was recognized, 1950. If the date has been calibrated it should be written as cal. B.P. Radiocarbon dating is still one of the most widely used archaeological methods around the world.

 

Further Reading:

Bowman, Sheridan. 1990. Interpreting the Past: Radiocarbon Dating. University of California Press. Berkeley.

McDougall, Douglas 2008. Nature’s Clocks: How Scientists Measure the Age of Almost Everything. University of California Press. Berkeley and Los Angeles

Ramsey, C. B. 2008. “Radiocarbon dating: revolutions in understanding.” Archaeometry 50 (2): 249-275.

How is physics used in Archaeology? (http://www.physics.org/article-questions.asp?id=117)

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