ESR ISOCHRON DATING: AN INDEPENDENT CONFIRMATION FOR SAMPLE AGE

All ‘trapped charge’ dating methods (TL, OSL, ESR) require knowledge about the sample’s environment, including the radioactive element and sedimentary water concentrations, as well as any changes in these throughout the burial period. During sample excavation, only the modern environmental parameters can be determined accurately. Thus, the changes in these parameters over time must be modelled in order to calculate an age. An alternative approach is to create an isochron, in which the material’s age is determined without reference to the external dose rate, since the time-averaged external dose rate can also be determined from the isochron. In ESR dating, an isochron plots the sample’s internal dose rate against the accumulated dose for several subsamples selected from the same sample. The slope for the best-fit line gives the sample’s age, while the y-intercept yields the total external accumulated dose, from which the time-averaged external dose rate can be determined. Since the precision for isochron ages is generally less than that in the standard ESR method, suggesting that isochrons should be used to confirm ages from the standard method, rather than as definitive results in and of themselves. In ESR isochron dating, tooth enamel from large ungulates can provide five to ten subsamples, which are sufficient to create an isochron. Although frequently smaller teeth may yield three or four subsamples, these rarely give reliable isochrons. Here, we show that three teeth from the same jaw can be combined to create a meaningful plot.

Three large bovid molars were collected during a geoarchaeological survey from Middle Pleistocene sediment along the Aliakmon River, Greece. All three teeth were divided into 5 or 6 subsamples and analyzed by standard ESR. They yielded comparable ages and contained similar uranium concentrations in both the dentine and enamel. The data from the 16 subsamples were used to create an isochron. Within experimental error, the isochron ages agreed with those from the standard ESR method, while the time-average external dose rate from the isochron agreed with modern external dose rate derived from the geochemical analyses of associated sediment surrounding the teeth at the time of recovery.