NPS Archeology Program: Archeology for Interpreters

Absolute dating

Absolute dating techniques attempt to pinpoint a discrete, known interval in time such as a day, year, century, or millennia. Very few artifacts recovered from an archeological site can be absolutely dated. Archeologists use several methods to establish absolute chronology including radiocarbon dating, obsidian hydration, thermoluminescence, dendrochronology, historical records, mean ceramic dating, and pipe stem dating. Each of these methods is explained in this section.

Radiocarbon dating

Radiocarbon dating is a widely applied absolute dating method in archeology. It is based on the knowledge that living organisms build up their own organic matter by photosynthesis or by using atmospheric carbon dioxide. Radiocarbon dates can be obtained from many types of organic material including charcoal, shell, wood, bone and hair. The amount of carbon dioxide in the living organism is equal to that in the atmosphere. When the organism dies, the carbon 14 (C14) atoms disintegrate at a known rate, with a half-life of 5,700 years. It is possible then to calculate the date of an organic object by measuring the amount of C14 left in the sample. Because the concentration of radiocarbon in the atmosphere has varied considerably over time, radiocarbon dates as far back as 7,000 years may be corrected by calibrating them against accurate dates from radiocarbon-dated tree rings and developing a master correction curve.

Archeologists use a statistical standard deviation to increase the range of dates for a sample that has been given a C14 date. Radiocarbon dates are usually calculated to one standard deviation. For example, if a sample is tested and given a radiocarbon date of 1000 BC. with a standard deviation of plus or minus 120 years, the chances are two in three that that sample dates from between 1120 and 880 BC. Here's how:

Calculations based on one standard deviation of 120 years:

1000 + 120 = 1120 BC (Oldest date) 1000 - 120 = 880 BC (Most recent date)

To increase the range of possible dates of a sample, archeologists may calculate the radiocarbon date to two standard deviations. Calculations based on two standard deviations increases the possible date range, increasing the probability of the sample lying within this range to 95 percent. Here's how:

Calculations based on two standard deviations of 120 years (120 x 2 = 240)

1000 + 240 = 1240 BC (Oldest date) 1000 - 240 = 760 BC (Most recent date)

As a rule, the more standard deviations used, the larger the probable date range for the sample and consequently, the higher the probability is for that sample to fall within the expanded date range.

FUN FACT	Researchers have developed a method to eliminate or reduce organic contamination on prehistoric rock paintings, enabling them to use radiocarbon dating technology to date rock art sites. (4/30/01)
	Rock paintings at Nevada's Christmas Tree Pass (National Center for Preservation Technology and Training)

TRY IT YOURSELF

Radiocarbon Dating

This web site describes radiocarbon dating and includes six short movies illustrating different stages in the radioactive and radiocarbon dating processes. (4/30/01)

Quick Time Player© required.

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