The radiocarbon method of dating was first developed by a group led by Willard F.Libby in 1949, since when it has been used mainly as an archaeological tool. ¹⁴C is an unstable isotope of carbon which is created from ¹⁴N in the upper atmosphere.

Carbon (both ¹⁴C and ¹²C) rapidly oxidises to CO₂ and disperses in the atmosphere. It is then absorbed by all living organisms via photosynthesis (in the case of plants) or through the food chain (in the case of animals). Because ¹⁴C is an unstable isotope it is constantly decaying, but the proportion of ¹⁴C in living organisms stays relatively constant over time through continued metabolic uptake.

In life the ¹⁴C/¹²C ratio therefore remains in equilibrium with the atmosphere. However, upon death there is no further uptake of carbon and the ¹⁴C decays to ¹²N with a half life of 5,730 years.

Measurement of the amount of ¹⁴C remaining in a dead organism will therefore give the date of its death. Using this method, the maximum detectable age is no greater than 50,000 years and over 40,000 years dates are unreliable.

In addition, material from the last 300 years gives unreliable ages, mainly due to the widespread burning of fossil fuels and more recently the explosion of nuclear bombs, both of which have artificially increased the amount of ¹⁴C in the atmosphere. Thus, it is not possible to accurately date organisms which have died since 1950.

Because of the small size of most industrial samples the Accelerator Mass Spectrometry (AMS) method is used. This method can be used to radiocarbon date one milligram of carbon or less and has the added advantage that the small sample size permits more selective sampling. Suitable material includes molluscs, microfossils and wood from terrestrial, non-marine and marine environments.

The results of ¹⁴C dating have traditionally been quoted in “conventional radiocarbon years”, typically in years BP (before present) where the “present” is 1950. The initial assumption that the concentration of atmospheric ¹⁴C has been constant as measured in 1950 has been shown to be incorrect.

Dating annual tree growth rings of known historical age has demonstrated that in the past there have been short term variations in atmospheric ¹⁴C levels. Tree ring dating has been used to construct a probabilistic calibration curve extending back to 11,857 calendar years BP (see figure in diagram).

This method is not without its own inaccuracies and has limited precision, but it does provide dates in terms of calendar years. Extension of the calibration curve further back to about 24,000 calendar years BP has been achieved using U-Th decay series dates from marine organisms (principally corals). The INTCAL98 calibration curve thus constructed has been accepted by international consent (see Stuiver et al., 1998). Two other U-Th dates provide a tentative extension to the INTCAL98 curve which has been used in this report to provide a calibration curve back to about 40k years BP.

Using the INTCAL98 curve it is possible to convert radiocarbon years to calendar years by projecting the radiocarbon age onto the curve and observing the intercept on the calendar year axis. This can be done manually, but there are now computer programs in the public domain which will provide a more consistent and accurate calibration. Because the calibration curve is not a straight line, it is possible to have many intercepts on the calendar year axis, each with its own probability range.

The average difference between a radiocarbon date of a terrestrial organism such as a tree and a marine shell of the same age is about 400 radiocarbon years. This difference which is called the reservoir age is caused both by the delay in exchange rates between atmospheric CO₂ and oceanic bicarbonate, and the dilution effect caused by mixing surface waters with upwelling deep waters which are very old. A reservoir correction factor must therefore be applied to conventional radiocarbon dates based on the remains of marine organisms.

For practical purposes ¹⁴C dating records the time of death of an organism and not the point at which it is finally entombed in sediment. Reworking of older material into younger sediments frequently occurs and can dramatically increase their ¹⁴C age. It is important, therefore, to select specimens for analysis with great care and to record any doubtful material before analysis. For guidance in selecting material for radiocarbon dating and for a summary of the pros and cons of the method please visit this radiocarbon dating report.