Radiocarbon Dating

Dr Tom Higham

Oxford Radiocarbon Accelerator Unit
 
Dating the Amesbury Archer was an important part of the overall archaeological research programme. Samples of bone from both the Archer and his companion were AMS radiocarbon dated at the Oxford Radiocarbon Accelerator Unit, University of Oxford. In addition, another burial found nearby was also dated to determine whether it was the same age as the Archer, or whether it represented a later phase of burial activity at the site.
The Oxford Radiocarbon Accelerator Unit has recently installed a new state-of-the-art AMS machine.The Oxford Radiocarbon Accelerator Unit has recently installed a new state-of-the-art AMS machine.
 
The first stage in the process of radiocarbon dating using AMS (Accelerator Mass Spectrometry) is to drill a small sample of bone powder from the bone itself. This is achieved in the laboratory using an electric drill. Before drilling the exterior of the bone is carefully cleaned and then about 500 milligrams is drilled. The bone powder is pretreated and cleaned over about 4 days. First, a dilute acid is used to remove the carbonate fraction of the bone, which accounts for about 75% of its entire mass. Studies have shown this material is unreliable for radiocarbon dating. Then, the collagen fraction is isolated and purified using filters. The purified collagen extracted from the Archer weighed about 30 milligrams. This collagen is then converted into carbon dioxide in a combustion furnace, and finally converted into graphite, before being radiocarbon dated.
 
The Oxford Radiocarbon Accelerator Unit has recently installed a new state-of-the-art AMS machine. This machine was used to date these bones, which were amongst the first samples to be analysed using it. AMS enables each carbon-14 atom in the sample to be counted directly, so the measurement process only takes an hour or so. A radiocarbon age is calculated by comparing the sample activity with that measured on standards of known-age.
 
Both the Archer and his companion were dated twice, to add greater confidence to the final date. The results were consistent and in good agreement, and suggested that both individuals date to the middle part of the second millennium BC (2400-2200 BC). The third burial produced a much later result of 1700 years before present. Further dating work is being conducted in an attempt to determine whether or not the Archer and his companion were in fact direct contemporaries or separated by a period of time.
 
Dr Tom HighamDr Tom Higham
 

Tom Higham

Dr Tom Higham works at the Oxford Radiocarbon Accelerator Unit. He administers the Unit's archaeological dating programmes, and is the secretary to the NERC Oxford Radiocarbon Accelerator Dating Service advisory panel. His research interests revolve around archaeological dating using AMS, including colonisation and settlement in the Pacific Islands, the application of Bayesian calibration methods to archaeological dating, sample pre-treatment chemistry and dating novel sample types.