Radiocarbon Dating for Archaeology

• Advantages of Accelerator Mass Spectrometry
• Sample Material and Type
• Oxford Radiocarbon Accelerator Unit
• Archaeological Applications

The range of radiocarbon dating, which is the last 50,000 years, spans most of the activities of modern humans (and the extinction of Neanderthals in Europe). Radiocarbon dating has been widely used to date prehistoric archaeological sites throughout this period.

Increases in precision, though improved measurement techniques and better statistical methods, have also resulted in an increasing use of radiocarbon dating in historical periods where other dating techniques are in doubt.

Advantages of Accelerator Mass Spectrometry

Accelerator measurement differs from the older decay counting method in that it enables samples a thousand times smaller to be dated. This improvement often allows archaeologists to select more samples from each context in a site. The ability to obtain a radiocarbon date using only a small quantity of material has made possible the direct dating of valuable archaeological artefacts. Consequently, the last twenty years have seen a greatly renewed interest and activity in dating archaeological sites and museum material.

Since only 1-2 milligrams of carbon are required, it is possible to date a very large range of samples. This means:

• sites which previously lacked adequate organic remains are more likely to be dateable
• archaeologists have a greater choice of samples which are securely associated with the cultural event or horizon they wish to date
• more informative samples can be directly dated; for example, identifiable seeds of a particular cultivar, or bones of an animal species with butchery marks
• a greater number of radiocarbon measurements can be made, allowing a more detailed study of chronological sequences and identification of mobile material
• more stringent chemical treatment can be applied in order to remove any likely contaminants
• valuable archaeological finds can be sampled whilst still allowing for museum display

Because of these advantages, the dates are usually more reliable and archaeologically useful than dates on larger samples. In terms of precision and time range covered, they are similar to conventional measurements.

Smaller samples (e.g. charcoal) can be mobile within deposits. This problem can usually be overcome by formulating a specific dating strategy in consultation with the radiocarbon lab.

Sample Material and Type

Ideally, the sample should embody as much archaeological information as possible. For example, animal bones with human cut-marks date:

• the time of human presence
• the chronological occurrence of the particular animal
• the stratigraphic level
• Quantities Required

The quantity of material required depends strongly on the composition; this table gives approximate values for some common materials. In some cases, substantially smaller quantities can be used.

Many other materials can be dated, for example food residues from ceramics and mastic on lithics. The amount needed depends on the carbon content. Samples treated with preservatives, and ones from certain special environments require larger quantities.

Oxford Radiocarbon Accelerator Unit (ORAU)

Copyright (c) James King-Holmes, 2005

There are several accelerator facilities worldwide performing radiocarbon analyses, with about six in Europe. The Oxford Laboratory is unique in putting the majority of our effort into the dating of archaeological samples. We have an international reputation for this and have specialised both in developing chemical pre-treatment processes, and in providing archaeological advice, scrutiny and interpretation.

We measure about 1000 dates per year in house with a further 1000 AMS measurements on samples prepared elsewhere. About 200 of these are measured for British archaeologists in conjunction with NERC Scientific Services at greatly reduced cost. Many of the rest are performed for archaeologists from all around the world. We also undertake research into methods that improve the accuracy and usefulness of radiocarbon dating. We have particular interest and expertise in the following areas amongst others:

• advice on the selection of material for radiocarbon dating
• chemical pre-treatment of archaeological bone
• removal of preservatives and contaminants from objects
• stable-isotope analysis and interpretation - particularly in relation to past human diet and identification of contaminants.
• dating of extremely small samples - using our unique gas ion source
• radiocarbon calibration and methods of analysing site chronologies - using our computer program OxCal
• general archaeological interpretation of radiocarbon dating evidence

We perform regular quality control tests both through the main international laboratory inter-comparisons (TIRI, FIRI etc), informal laboratory inter-comparisons and with regular in-house measurements on known-age material. We are also ISO-9001 accredited.

Archaeological Applications

Past archaeological research programmes of the Laboratory have included:

• Mesolithic and Upper Palaeolithic human skeletal remains
• Upper Palaeolithic cave sequences and open sites
• worked bone, antler and ivory artefacts
• early agricultural sites in Europe, Asia and the Near East
• early ceramic sites in South America
• Late Bronze Age chronology of the Aegean
• human burials in peat bogs
• late prehistoric wooden artefacts