A comprehensive model for CO2− radicals in fossil tooth enamel: Implications for ESR dating

In the past two years, we have carried out a series of irradiation experiments on fossil tooth enamel fragments, using gamma irradiation for the whole enamel thickness, UV irradiation through the buccal–enamel boundary (BEB) and beta irradiation through the BEB as well as the dentine–enamel junction (DEJ). Using a simulated annealing procedure, the ESR spectra were analysed for three types of CO2− radicals, consisting of two types of anisotropic CO2− radicals (AICORs, orthorhombic and axial) plus non-oriented CO2− radicals (NOCORs).In this paper we complemented the beta irradiation experiments with gamma irradiations on fragments representing the domains close to the BEB as well as the DEJ. The two beta irradiation experiments generated qualitatively similar radical distributions to the natural sample. The effects of gamma irradiation were significantly different. The ESR spectra of both fragments contained only orthorhombic AICORs (non-axial CO2− radicals) plus a very large amount of NOCORs. Relative to the natural fragments, the domain close to the BEB had eight times more NOCORs and the domain close to the DEJ twenty-nine times more.Heating experiments (at 125 °C) showed that NOCORs do not transfer into AICORs but lead to a general decrease of the overall intensity. Orthorhombic CO2− radicals convert into axial. The strong difference between the beta and gamma irradiations leads to the postulation of two different types of NOCORs, one unstable and decaying without transfer into stable AICORs and a second stable variety converting into AICORs. It seems that beta irradiation resembles gamma irradiation with concurrent heating. A preliminary experiment confirmed elevated temperatures during beta irradiation.The appearance of unstable NOCORs, which are not removed by conventional post-irradiation heating steps or long storage times between irradiation and measurement, has serious consequences for ESR dating. It points to significant age underestimations. For the fragments we have investigated thus far, the underestimation is in the range of 30%. However, at this stage it is not possible to provide a correction that can be easily applied to published results, as there are indications that the offset could depend on age, tooth type (molars versus incisors) as well as species (bovid, horse, hippopotamus, humans, etc.). The implications for ESR dating in general, for published ESR results as well as possible remedies are discussed in detail.