Radiocarbon (14C) is produced in the upper atmosphere when nitrogen (14N) atoms are bombarded by thermal neutrons from cosmic rays. The 14N atoms absorb the thermal neutrons, resulting in the formation of the unstable 14C atom and a proton:
Radiocarbon then combines with oxygen to form carbon dioxide (14CO2). In turn, this mixes with existing CO2 molecules in the atmosphere and subsequently enters the terrestrial and marine biospheres by incorporation of atmospheric CO2 via photosynthesis (6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O) and incorporation at the air-sea interface.
The uptake of CO2 by primary producers (plants, phytoplankton and algae) is transferred to higher trophic levels (e.g. mammals, fish) through consumption.
The unstable 14C present in animal and plant tissues, and in the oceans, decays to a stable form of nitrogen but is continually being replenished by the atmosphere. Consequently, the amount of 14C stored in this global carbon reservoir remains constant through time.
After death, 14C is no longer exchanged and therefore only decays, 14C → 14N + β, emitting a weak beta particle (β). This allows the age since death to be calculated by comparing the residual 14C with the 14C in a modern standard material. Radioactive decay follows an exponential decay law with a half-life of 5568 ± 30 years, as calculated by Willard F Libby in the 1940s, generally used for 14C age determinations (Willard F Libby was an American professor who first detected radiocarbon in living matter and proposed that 14C measurement could be used as a dating technique. He published the first radiocarbon dates in 1949 and received the Nobel prize for chemistry in 1960).
The quantity of radiocarbon can be measured in 2 ways: by a direct method counting the β particle emissions (gas proportional or liquid scintillation counting), or a relative method where the ratio of 14C to the stable 12C or 13C is measured (accelerator mass spectrometry).
However, there are complications that need to be considered before we can provide an accurate estimate of the age of a material based on its radiocarbon content.
Firstly, the half-life of 14C is not exactly as measured by Libby; it has been recalculated to be 5730 ± 30 years, but the Libby half-life is still used to maintain consistency with the dates that have been produced since 1949. Secondly, the 14C concentration in the atmosphere has varied in the past due to natural and anthropocentric processes.
These complications are resolved by calibrating the radiocarbon dates with known age material using a calibration curve; in effect this transforms a sample’s age from radiocarbon years to calendar years. Calibration programs such as Calib and Oxcal are available for the user to convert radiocarbon years to calendar ages.
Other complications arise with alteration of the 14C in a sample due to contamination or reservoir effects. Contaminants that have leached into the soil or are present from the use of preservation techniques can be removed by pre-treating the samples.
Reservoir effects can be present in samples that have originated from areas such as the oceans, where the carbon is transported to deep layers and mixing is slow. The residence time for carbon in the oceans is high, which can result in deep water carbon having a radiocarbon age of a few millennia. This carbon is then transported back to surface waters by a process known as upwelling. Radiocarbon dates from marine organisms, or individuals who have consumed these organisms, require a marine reservoir correction. The offset to be applied as a correction is calculated using a comparison of terrestrial and marine dates from the same context. Further information and a Marine Reservoir Database, intended to be used with calibration programs, is available here.
A similar complication can arise in freshwater, where there is carbonate geology, resulting in older radiocarbon ages.
Bowman S. 1990. Radiocarbon Dating. London: British Museum
Libby WF, Anderson EC, Arnold JR. 1949. Age Determination by Radiocarbon Content: World-Wide Assay of Natural Radiocarbon. Science 109: 227-228.
Taylor RE. 1987. Radiocarbon Dating: An Archaeological Perspective. New York: Academic Press, Inc.
Walker M. 2005. Quaternary Dating Methods. Chichester: Wiley & Sons