The relevance of parametric U-uptake models in ESR age calculations
Introduction
Ever since the inception of ESR dating of tooth enamel (Ikeya, 1982), it was clear that the unknown uranium uptake history has to be addressed in dose rate calculations. Over time, various parametric models have been proposed for the reconstruction of U-uptake of dental tissues, notably early (EU), linear (LU, Ikeya, 1982) and very recent (RU, Blackwell and Schwarcz, 1993) uptake. The EU model has some physical meaning, it presents the closed system and provides a minimum age. The RU model is physically less meaningful, but provides the maximum age. The main virtue of these three parametric models lies mainly in their reasonable simple computability (for explicit formulae, see Grün, 1989). Nevertheless, many publications have either favoured a particular U-uptake model, for the convenience to explain the result, or claimed that the correct age of the sample lay somewhere between the calculated EU and LU ages (e.g. Grün and Stringer, 1991). However, without any knowledge of the U-uptake history, it is only safe to assume that the correct age of a sample lies somewhere between the EU and RU. Depending on the contribution of the U in the dental tissues to the total dose rate, this difference ranges between negligible and utterly enormous.
Section snippets
Open system modelling
Uranium uptake can be modelled by combining ESR and U-series data. Although the explicit U-uptake in nature may occur in multiple phases, two models have been developed, which can bracket virtually all possible scenarios, as long as no U-leaching occurs. For the modeling of U-uptake, Grün et al. (1988) used a smooth diffusion function: U(t) = Um(t/T)p+1, where U(t) is the uranium concentration at the time t, Um the measured, present day U-concentration, T the age of the sample and p the uptake
U and U-series mapping
The introduction of laser ablation analysis to teeth (Eggins et al., 2003, Eggins et al., 2005) provides some insights about the general mode of U-uptake into tooth enamel. Fig. 1A shows a bovid tooth from the archaeological site of Holon (Porat et al., 1999). The samples in Fig. 1 are discussed in more detail in Eggins et al. (2003) and Grün et al. (2008a). While the uranium in the dentine is approximately uniform, the U-concentration in the enamel drops by about two orders of magnitude at the
Evaluation of p-values
To get more general insights into the general behaviour of U-uptake, published p-values were compiled and separated into two groups, from cave sites (and rock shelters) and open air sites. The p-values from the sites of Atapuerca Sima, Azokh, Bois Roche, Cave of Hearth, Cuddie Springs, Holon and Klasies River are hitherto unpublished data, the other data are from the following publications: Ambrona, Isernia (Falgueres et al., 2007), Atapuerca Grand Dolina (Falgueres et al., 1999, Falgueres
Conclusions
The statement that the correct ESR age of a sample probably lies somewhere between the EU and LU uptake age calculations is incorrect. It is not even true that the correct age lies always somewhere between the EU and RU model calculations, because there have been occasions of U-leaching. ESR results with significant differences between EU and RU age estimates can only be refined by U-series analyses of the dental tissues. Anything else is an oversimplified approach to ESR dating.
Acknowledgment
I thank J.J. Bahain, Département de Préhistoire du Muséum National d'Histoire Naturelle, Paris, for critical comments. I thank the anonymous reviewer for thoughtful comments. This study was partly funded by the ARC discovery projects DP0666084 Out of Africa and into Australia: robust chronologies for turning points in modern human evolution and dispersal and ARC DP06604144 Microanalysis of human fossils: new insights into age, diet and migration.
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