Radiocarbon dating

Since this scientific method of dating organic materials was first suggested by W.F.Libby in 1946 a vast amount of invaluable evidence has been obtained on man’s past. The method gave the first universal means of absolute dating quite independent of subjective archaeological methods. many of the results surprised archaeologists, but the fact that the dates are so consistent with each other has overruled the early reluctance to accept inconvenient ones.

All living organisms contain carbon, and radiocarbon dating is based around the fact that Carbon 14, a radioactive carbon isotope, is around in the environment naturally, and is absorbed by all living things. C14 is produced in the upper atmosphere by the action of cosmic rays on Nitrogen 14. The C14 then combines with oxygen to produce carbon dioxide and is taken in by plants during photosynthesis. From plants this C14 is absorbed into every living thing via the food chain. It is useful for organisms up to around 40,000 years old.

Because the isotope is radioactive it is unstable and decays away. While an organism is alive the C14 lost by radioactive decay is constantly replaced through the food chain. However, when the organism dies this replenishment ceases and the amount present goes down, altering the carbon ratio. Radiocarbon isotopes decay at a known rate but the amount of radiocarbon in the atmosphere has varied over time, thus, with calibration, the ratio can be used to date when the organism died. Some faulty dates have also appeared due to contamination of the sample and archaeological context, and the like, but the more samples tested, the fuller and securer the chronological framework becomes.  

The date given by a radiocarbon test is never exact. A +/- figure appears after every date and means that there is a 2 to 1 chance that the correct date lies within the quoted range. Secondly the rate of decay of C14 is based in all published examples on a half-life of 5568+/-30 years. The half-life means that after 5568 years, one half of the C14 will have disintegrated, after another 5568 years one half of the remainder. It is becoming clear that the figure is too small. 5730 has been suggested but it has been agreed not to change the figure until a new one can be internationally accepted.