Radiocarbon dating sample contamination

A mass of evidence seems to confirm that the mixing rate of radiocarbon in the atmosphere is rapid, and that with respect to its radiocarbon content the atmosphere can be considered as a homogeneous entirety. The contamination of samples with matter from an extraneous source can nevertheless invalidate this assumption. Two types of contamination can be differentiated physicochemical contamination and mechanical intrusion. There are two forms of physicochemical contamination. One is due to the dilution of the concentration of radiocarbon in the atmosphere by very old carbon, practically depleted of radiocarbon, released by the combustion of fossil fuel, such as coal and oil. The other is by the contamination with radiocarbon produced by nuclear bomb tests during the 1950s and later in the twentieth century. The uncertainties introduced by these forms of contamination complicate the interpretation of data obtained by the radiocarbon dating method and restrict its accuracy and the effective time range of dating. 

Olsson, E., The Problem of Sample Contamination in Radiocarbon Dating, unpublished doctoral dissertation, Columbia University, 1963. 

Additional Methods of Analysis. The use of radiocarbon dating where sufficient sample is available to permit adequate confidence in the date obtained is not precluded by the caution above. It is essential, however, that radiocarbon dates be obtained only for the proteinaceous or cellulosic substrate with care taken to exclude contaminants and coloring materials. Note should be made of the possibility of the introduction of lubrication materials in the conservation of brittle textiles. 

By contrast, the subject of contamination of radiocarbon samples by non-indigenous organics has enjoyed great vogue among archaeologists especially as an explanation of why their radiocarbon dates are wrong,... 

The initial activity is a known constant through the past 70,000 years independent of passing time and with a value independent of geographical location and the species of dead plant or animal from which the relevant tissues are taken. It is assumed that the sample to be dated was not contaminated by modern radiocarbon and that its observed activity is unaffected by radioactive impurities. Research has shown that the initial carbon-14 activity of modern plant and animal tissues is constant to a first approximation and also that the radiocarbon activity in archeological materials of known age is measurable with sufficient precision to provide radiocarbon dates that agree satisfactorily with the known historical ages. 

Despite the simplicity of the carbonation process previously described, in practice the radiocarbon dating of lime mixtures is not so simple. Several studies have recognized the drawbacks and limitations of radiocarbon dating of lime based materials. These are mainly attributed to contamination of samples from carbonaceous substances such as incompletely burnt limestone or grains of carbonate sand. 

For this reason the lumps of pure lime embedded in old lime mixtures are the most suitable samples for the radiocarbon dating of lime based materials. With their use the contamination problem previously described is greatly reduced. The only contaminants remain on the external surface where the lump was originally jointed with the mortar matrix... 

The reasons will be clearer after Section 16.4, where AMS radiocarbon sample preparation procedures will be described. Now we would simply like to recall that in preparing graphite pellets for the sputtering source, after a physical chemical cleaning, samples to be dated are usually combusted to obtain C02, which is then converted to graphite by a further step. In this process, the main problem with small samples (a few tens of micrograms) is the possible introduction of contamination. 

Radiocarbon analysis is possible for dating ancient textiles only where sufficient sample is available. Unfortunately, the uncertainty in dating becomes unacceptably large with small sample size, contamination, and specimens of lesser antiquity (5). Chemical methods of dating are mainly limited to the observation that reducing the sulfur content of wool textiles appears to coincide, in some cases, with embrittlement and/or age (6). The presence of certain kinds of dyes and the manner in which they fade are sometimes used as the evidence of provenance (7). 

At the three laboratories, samples were cleaned and treated following standard procednres to remove contaminants. The cloth samples were then combusted to gas and their radiocarbon content was measured in an Accelerator Mass Spectrometer. The almost identical AMS measurements at the three laboratories provided a calendar age range of ad 1260-1390 with at least 95% confidence (Fig. 5.17). The results from the three control samples agree with previous radiocarbon measurements and/or historical dates. The AMS dating provides conclusive evidence that the linen of the Shroud of Turin is medieval. 

For dating with bone samples, one is required to carry out chemical pretreatment of bone, removal of preservatives and contaminants from objects, radiocarbon calibration, stable isotope analysis, and interpretation of contaminants. 

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