Laboratory images Three isotopes of carbon are found in nature; carbon, carbon and carbon Hereafter these isotopes will be referred to as 12C, 13C, and 14C. The half-life is the time taken for an amount of a radioactive isotope to decay to half its original value. A unique characteristic of 14C is that it is constantly formed in the atmosphere. Production and decay 14C atoms are produced in the upper atmosphere where neutrons from cosmic rays knock a proton from nitrogen atoms.
Photosynthesis incorporates 14C into plants and therefore animals that eat the plants. From there it is incorporated into shell, corals and other marine organisms. When a plant or animal dies it no longer exchanges CO2 with the atmosphere ceases to take 14C into its being. Schematic of 14C production and decay in the atmosphere.
The newly formed 14C is oxidized to 14CO2 where it then enters the biosphere. Following an organisms death, radioactive decay occurs converting the 14C back to 14N. His first publication showed the comparisons between known age samples and radiocarbon age Libby et al, ; Libby, This invention was revolutionary. In Libby was awarded the Nobel Prize for chemistry for this contribution. Measuring 14C To obtain the radiocarbon age of a sample it is necessary to determine the proportion of 14C it contains.
The gas counter detects the decaying beta particles from a carbon sample that has been converted to a gas CO2, methane, acetylene. A liquid scintillation measurement needs the carbon to be converted into benzene, and the instrument then measures the flashes of light scintillations as the beta particles interact with a phosphor in the benzene.
The main limitation of these techniques is sample size, as hundreds of grams of carbon are needed to count enough decaying beta particles. This is especially true for old samples with low beta activity. This means that it can be difficult to effectively clean the samples and remove enough contaminating carbon to obtain an accurate date.
The absolute radiocarbon standard is wood, the OX-I standard has an activity of 0. A variant of this equation is also used when the samples are analysed by AMS. Calibration In the s it was observed that the radiocarbon timescale was not perfect.
The age of known artefacts from Egypt were too young when measured by radiocarbon dating. A scientist from the Netherlands Hessel de Vries tested this by radiocarbon dating tree rings of know ages de Vries, This brings us to two reasons why a radiocarbon date is not a true calendar age.
The true half-life of 14C is years and not the originally measured years used in the radiocarbon age calculation, and the proportion of 14C in the atmosphere is not consistent through time. The latter is due in part to fluctuations in the cosmic ray flux into our atmosphere e. Since then there have been many studies examining the variations in the 14C production and its effects on the radiocarbon age to calendar age calibration e.
Stuiver, ; Edwards et al. Since fossil fuel is derived from millions of year old organic carbon it contains no 14C. It is essential to have radiocarbon ages calibrated to calendar ages so as to have an accurate measure of time. It is also important to be able to compare ages with samples dated by other means, e. It therefore became necessary to create a calibration between radiocarbon dates and calendar age.
The ideal calibration material must have a precise calendar age and sample the atmosphere carbon reservoir of interest. Tree-ring Calibration Fortunately annual tree rings provide a perfect calibration material available in nature. Since those first measurements in the s a detailed, precise calibration between radiocarbon and calendar age has been developed using many long-lived tree species.
Dendrochronology provides the accurate calendar age for each ring in the tree, and then a radiocarbon age can be assigned to each calendar age. Several tree-ring chronologies have been constructed including the Belfast Irish Oak chronology Baillie et al. Friedrich et al, ; Schaub et al. However this is as far back in time as the continuous tree-ring radiocarbon calibration can be extended at present. More old trees are being discovered every year and this may eventually increase this calibration dataset at a later date.
They are called floating because they do not have a direct calendar age and must use the radiocarbon to match their ages. For example, many sections of old sub-fossil New Zealand Kauri trees have been found that span time from , years old Hogg et al.
Other calibration curves have been proposed by individual research groups for example Fairbanks et al. These calibration curves form the basis of several online calibration programs that take the radiocarbon age and output a calibrated age, the major online calibration programs are;.