Precision dating of the Palaeolithic: A new radiocarbon chronology for the Abri Pataud (France), a key Aurignacian sequence
Introduction
A proper understanding of the cultural and behavioural changes that accompanied the Middle to Upper Palaeolithic transition in Europe is predicated upon a reliable chronological framework. The pattern of dispersal by anatomically modern humans in Western Europe, the spread of the Aurignacian and the subsequent disappearance of Neanderthals, can only be disentangled with a solid and reliable chronology. Radiocarbon dating is the most important chronometric method for the period, but since the inception of the method, dates have been plagued by problems of precision, a lack of calibration and associated difficulties encountered during the measurement of samples with low levels of remaining 14C. A lack of precise dates has made it impossible to compare the appearance of similar items in different regions and track cultural changes (Pettitt, 1999). In addition, it has been extremely difficult to work out the temporal relationship between transitional industries such as the Châtelperronian and the Aurignacian, and to test models of ‘acculturation’ or the independent development of Upper Palaeolithic ‘modern’ behaviour by Neanderthals. As such, debates over these issues have run for decades with little or no resolution (Mellars, 1973, Mellars, 1989, Mellars, 1999, Mellars, 2004, Mellars, 2005, Mellars, 2006, White, 1982, d’Errico et al., 1998, Zilhão and d’Errico, 1999, Zilhão and d’Errico, 2003, d’Errico, 2003, Teyssandier et al., 2006, Zilhão, 2006).
Sadly, the databases of radiocarbon dates that are available for sites from the period are filled with many dates that are now known to be problematic or at least potentially quite problematic. Recent work has shown that there are severe doubts regarding the reliability of the majority of the radiocarbon dates obtained thus far on material dating to the Middle and Upper Palaeolithic of Europe. The problems are mainly related to consistent underestimating of the true age for specific samples, which are almost certainly due to the difficulties in effectively decontaminating samples of bone from the deleterious effects of modern carbon contamination (Higham et al., 2006, Higham et al., 2009, Higham, 2011). Similarly, there have been significant challenges in overcoming the effect of the radiocarbon background limit. It is only comparatively recently that programmes to attempt to quantify the background limits for dating bone beyond 40,000 BP (before present) have been developed (see Hedges and Pettitt, 1998). Given that the transition is thought to lie between about this age and ∼35,000 BP, one can see that the potential for age underestimation is only too real. Chappell et al. (1996) have drawn attention to this problem in dating archaeological contexts approaching the radiocarbon limit. Further problems are inherent in the lack of careful selection of material and the use of bulked samples for dating prior to the accelerator era.
Recent work suggests that the problems specific to pre-treatment chemistry, however, are being overcome. We have worked to develop and apply newer methods, including using an ultrafiltration protocol in our collagen extraction (Brown et al., 1988, Bronk Ramsey et al., 2004, Higham et al., 2006), as well as utilising more rigorous ABOx-SC preparation methods for dating charcoal (Bird et al., 1999, Higham et al., 2006, Higham et al., 2009). We inherit a legacy of problematic data, and so to address this we have been working on redating some of the key western European sequences within the orbit of a larger project (funded by the Natural Environment Research Council of the UK). Perhaps as many as 70% of previous radiocarbon dates may be aberrant at some Middle to Upper Palaeolithic sites (Higham, 2011) and those that are not are saved usually by having very large quoted uncertainties. At some sites, all of the dates are almost certainly erroneous, and at others perhaps 40% are questionable. As we shall see, the Abri Pataud is no exception in all of this. The principal reason is apparent when one compares radiocarbon dates of samples prepared before and after the application of ultrafiltration, in the case of bone, and ABOx, in the case of charcoal. Very often, the newly treated samples are significantly older, which we take to mean that they have more successfully been decontaminated of the modern carbon taken up during the burial history of the sample (Higham, 2011).
Section snippets
The Abri Pataud
The Abri Pataud is a key archaeological sequence in Les Eyzies-de-Tayac, in the French Dordogne. The site itself sits at the base of imposing limestone cliffs flanking the Vézère River (Fig. 1). Pataud was discovered at the end of the 19th century but it was not until the 1950s that extensive and deep excavations took place under the direction of Hallam L. Movius (Movius, 1977). These excavations were rigorous and innovative, and the recording techniques used at the time have allowed subsequent
Radiocarbon dating
This paper focuses on the chronology of the key Aurignacian and Early Gravettian layers of the site. Producing a reliable radiocarbon chronology is crucial because lithic sequences from many other sites dating to the period are compared against the Pataud sequence, since it is so extensive and deep, and it is one of the key Périgord sites. A large group of previous dates have been obtained (Table 1 and Fig. 3). Movius applied the radiocarbon method widely at the site. The dates initially
Methods
Initially, we planned to date directly samples of osseous bone implements, such as points, from the Abri Pataud sequence, but these were smaller than expected and had evidence for extensive glues and preservatives on the surfaces from conservation work. This could influence the accuracy of radiocarbon measurements. We decided instead to focus on humanly-modified and cut bone. Our first selection of samples in 2007 covered levels 6–14. Later, material from these levels and level 5 was resampled
Results
We have obtained 31 new AMS determinations. These are shown in Table 3. All analytical data associated with each of the dated bones is also provided. Three bones yielded collagen below our minimum required threshold of 1.0% weight. These were given OxA-X, rather than OxA- numbers, but overall the collagen yields were acceptable. Ultrafiltration reduces collagen yields compared with other techniques, but we think that this allows us to exclude potentially problematic, poorly preserved samples.
Calibration
Calibration of radiocarbon has proven particularly problematic within Marine Oxygen Isotope Stage 3 (MIOS3). In 2004, INTCAL04 was published, with a curve allowing calibration back to 26,000 cal BP, based on preliminary data mainly from coral and foraminifera (Reimer et al., 2004). In this paper, we use the INTCAL09 dataset, which spans back to 50 ka cal BP (Reimer et al., 2009). A note of caution is warranted regarding the use of calibration in this period. Calibration is an ongoing process
Bayesian modelling
A Bayesian modelling framework and OxCal 4.1 (Ramsey, 2001, Bronk Ramsey, 2009a) were used to analyse the radiocarbon data. This approach enables the incorporation of archaeological stratigraphic information along with the radiocarbon likelihoods obtained, within a statistical model. This method has been outlined in previous publications (e.g., Buck et al., 1996, Bronk Ramsey, 2009a). The model we developed is shown in Fig. 4. It consists of a series of phases, represented by the successive
Discussion
As mentioned above, boundaries mark the start and end dates of Eboulis through the sequence as well as archaeological levels (Fig. 5). These boundaries are not dated themselves, but are produced as part of the modelling as probability distribution functions (PDFs). They allow us to provide temporal constraints for when events occurred, began or ended, through the archaeological sequence.
We have tentatively compared the new chronological sequence with the NGRIP δ18O palaeotemperature profiles
Conclusions
New radiocarbon dating, involving the pre-treatment of bone using an ultrafiltration protocol, as well as the application of Bayesian modelling to the results, has provided for the first time a precise chronological sequence for Aurignacian levels 6–14, and Gravettian level 5, at the site of Abri Pataud. Previous dates are shown to be almost completely unreliable. The principal reason is that the type of material that was dated was not reliable. In some contexts (e.g. Level 6) the previous
Acknowledgements
We acknowledge the entire team at the Oxford Radiocarbon Accelerator Unit, University of Oxford, particularly F. Brock, M. Humm, D. Baker, A. Bowles, B. Emery, P. Leach and J. Davies. The Natural Environment Research Council (NERC) (grant NE/D014077/1) funded this work. We acknowledge, with gratitude, the help of Dr. C. Vercoutère (Paris), Dr. R. Dinnis (Sheffield) and Mrs. Margaret Chapman (Cheddar) for discussions and in helping us sort and select good material for dating at the Musée de
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