Two types of CO2− radicals threaten the fundamentals of ESR dating of tooth enamel

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Grun, R, Joannes-Boyau, R & Stringer, C 2008, 'Two types of CO2 radicals threaten the fundamentals of ESR dating of tooth enamel', Quaternary Geochronology, vol. 3, no. 1-2, pp. 150-172.

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In ESR dating of tooth enamel, dose values are usually obtained from powdered samples. It has been shown that the qualitative response of enamel powder to environmental and laboratory dosing is closely similar, thus apparently validating ESR protocols for dose estimation. When working on human fossils, their cultural and scientific value prevents the powdering of any samples. Measurements are carried out on enamel fragments instead. In fragments, natural and laboratory irradiations cause significantly different ESR responses. These can be attributed to two distinct CO2 radicals, one with apparently axial symmetry (orientated) and one that has no preferential orientation (non-orientated). The spectra of the naturally irradiated samples investigated here show a mix of about 90% of the orientated and about 10% of the non-orientated CO2 radicals. In contrast, laboratory irradiation induces a mix of about 60% orientated and 40% non-orientated CO2 radicals. Heating experiments show that the non-orientated CO2 radicals are significantly less stable than the orientated. This fact on its own would have serious implications for dose estimations, implying massive underestimations. It turns out, however, that with the heating induced decline of the non-orientated CO2 radicals, a larger number of orientated CO2 radicals is created. It is presently unclear whether these two processes are directly connected. In spite of very large internal reorganisations during these processes, powder spectra are only little affected by heating, because the powder spectra of both CO2 radicals are closely similar.

The maximum differences that are observed on powder spectra in post-irradiation heating experiments, are in the range of 3%, dose values may be affected by a similar amount. However, when using preheating steps, which would occur when applying post-irradiation heating protocols to single aliquots, the thermal behaviour of the irradiated samples is quite different. While the annihilation of the non-orientated CO2 radicals has about the same decay rate in preheated and non-preheated samples, the increase of the orientated CO2 radicals is less pronounced in the preheated samples. As a result, overall intensity changes become critically dependent on the preheating times. The observed increase in signal intensity is in contrast to long-term fading experiments that showed that the ESR intensity of irradiated samples actually decreased over 6–12 years after laboratory irradiation. In view of this complexity and the small amount of quantitative information available, it is presently not possible to recommend any pre- or post-irradiation protocols.

The good news for ESR dating is that any systematic errors introduced by a different mix of orientated and non-orientated CO2 radicals in the natural and irradiation spectra seem relatively small (<5%). At present, the most reliable way for the dose estimation of fragments seems to measure the samples rotating around the three principal axes and to merge the spectra to produce a powder-equivalent spectrum and use this for the construction of the dose response curve.