Radiocarbon (14C) is a naturally occurring isotope of carbon that is formed in the upper atmosphere when cosmic radiation interacts with nitrogen atoms (Figure 1). Once produced, it mixes rapidly across each of the hemispheres, quickly entering the terrestrial food chain through photosynthesis, with the result that the 14C content of land plants and animals is in equilibrium with the contemporary atmosphere. When an organism dies, it ceases to take up radiocarbon. Since 14C is an unstable (radioactive) isotope, with a half-life of 5730±40 years, the proportion of radiocarbon in the deceased organism decreases over time. It is by measuring the amount of radiocarbon that remains that scientists are able to estimate the amount of time that has passed since the organism’s death.
The naturally occurring concentration of 14C in living material is extremely low, with there being approximately 1,000,000,000,000 stable carbon atoms for every 14C atom. As we move further in time from the death of the organism, the amount of radiocarbon in the sample decreases further through decay. In general, the time limit to which 14C dating is effective is 50,000 years.
Sample processing and analysis
To accurately date an archaeological sample, it is important that only the 14C that was part of the organism when it died is measured. Therefore, the first task is to effectively pretreat the sample to remove any exogenous carbon that has entered the sample since death, and which might otherwise bias the results. This contamination usually comes from the burial environment, but can also come from such things as post-excavation conservation practices. Pretreatment follows a mixture of physical and chemical processes. The end result is to isolate a contaminant-free chemical fraction of a sample for dating.
For the first 40 years or so, conventional radiocarbon dating involved converting the purified sample into either a gas (CO2, C2H2, or CH4) or a carbon-rich aromatic liquid (C6H6) and actively counting the individual decays of 14C atoms in the samples, by either gas proportional or liquid scintillation counting, respectively. While conventional dating methods are highly accurate, they require large samples and can require long processing and counting times, depending on the age of sample. Today, nearly all 14C dating utilises accelerator mass spectrometry (AMS) for measuring samples. AMS dating is fundamentally different from conventional methods in that the 14C atoms in the sample are directly measured, rather than their decay. The result is that much smaller sample sizes are possible (e.g. 1 g of bone rather than 200 g).
Preparation of AMS samples involves combusting a sample to CO2, which is then graphitized using an iron catalyst. The resulting graphite is pressed into an aluminium target and loaded into a wheel for measuring on the AMS.
While the radiocarbon age is an accurate measurement of the proportion of 14C remaining in a sample, for archaeologists, what is most important is having a calendrical time-scale for that age, a date. The amount of radiocarbon in the atmosphere has not been constant over time. By measuring the 14C in samples from tree-rings that have been dendro-dated, scientists have produced an internationally agreed calibration curve for both the northern and southern hemisphere (IntCal09 and ShCal04, respectively). Various computer programs (e.g. BCal, Calib, CalPal, OxCal) can be used to convert a radiocarbon age into a radiocarbon date (Figure 2).
Bayliss, A., McCormac, G., van der Plicht, J., 2004. An illustrated guide to measuring radiocarbon from archaeological samples, Physics Education, 39, 137–144.
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