Carbon-14 Dating

Radiocarbon is the best-known radiometric dating technique due to its successful application to problems in human history and prehistory for over 50 years. [Page 437]Willard Libby's development of the technique in the late 1940s permitted relative time to be sorted radiometrically in archaeological contexts in a manner that eclipsed the more traditional relative dating methods that had developed over the previous century.

The decay of carbon-14 through 6.64 half-lives or 37,000years, after which only 1% of the original carbon-14remains. The production, mixing, and fate of the carbon-14 as part of the carbon cycle is shown in the insert.

Carbon-14 is a radioactive isotope produced in the upper atmosphere when cosmic rays generate neutrons as they bombard that outer atmosphere. These neutrons can enter abundant nitrogen-14 nuclei (78% of the atmosphere), converting some of them to carbon-14. The carbon-14 combines with oxygen to form carbon-14 dioxide, which is assimilated by plants and the animals that consume those plants. The carbon-14 production is relatively constant over short periods of time and is in equilibrium with the carbon sinks in the environment (see figure).

The Libby radiometric clock is started when the organic item (animal or plant) dies. The clock is set to “0” at death and begins to tick, with a half-life of 5,568 years. After 5,568 years, one half of the original radiocarbon-14 remains; after 2 × 5568 years, one quarter is left, and so on. There are two methods employed today to determine the amount of radiocarbon remaining in a sample: (1) decay counting, the traditional method, which relies on the decay of carbon-14 to nitrogen-14 with the emission of a beta ray that can be measured in decays per minute per gram and compared with a standard that has a decay rate of 15 decays per minute per gram; and (2) accelerator mass spectrometry (AMS), the atom-counting method, which counts the atoms of radiocarbon-14 in a sample and compares that result with those of a standard.

Libby made a fundamental assumption when he developed his technique. He assumed that the rate of carbon-14 production was constant through time. Subsequently, this assumption was shown to be untrue, but the difficulty could be overcome by instituting a calibration that compared the radiocarbon age of an item with the radiocarbon age of a wood sample of known age. Today this calibration curve is known as the dendrochronological correction curve, and it allows the dated sample's age to be expressed either in calendrical years B.C.(or B.C.E.) or A.D.(or C.E.), or as a conventional radiocarbon-14 age expressed as B.P. (before 1950 in radiocarbon years). Today, there is a convention, not universally accepted, that radiocarbon-14 dates that have not been dendro-corrected (for example, very old dates [> 40,000], groundwater dates, or atmospheric pollution dates) should be designated with a lowercase bp (before present). AMS [Page 438]does not escape this dendro-correction limitation, but does offer advantages in that smaller samples can be used (milligrams vs. grams); shorter counting times are possible due to improved counting statistics (hours vs. days); the backgrounds associated with cosmic rays, which must be shielded against in the traditional method, are not a factor; and ages up to 70,000 years are measurable because of lower backgrounds and greater counting statistics.

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