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Quaternary Geochronology 30 (2015) 493e497
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Dating results on sedimentary quartz, bones and teeth from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France): A comparative study between TT-OSL and ESR/U-series methods Marion Hernandez a, b, *, Jean-Jacques Bahain c, Norbert Mercier a, Olivier Tombret c, res c, Jacques Jaubert d Christophe Falgue IRAMAT-CRP2A, UMR 5060 CNRS - Universit e Bordeaux Montaigne, Maison de l'arch eologie, Esplanade des Antilles, 33607 Pessac cedex, France Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany c Departement de Pr ehistoire du Mus eum National d’Histoire Naturelle, UMR 7194 CNRS, 1 rue Ren e Panhard, 75013 Paris, France d PACEA, UMR 5199 CNRS- Universit e de Bordeaux, All ee Geoffroy Saint Hilaire, 33615 Pessac cedex, France a
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Article history: Received 24 October 2014 Received in revised form 28 May 2015 Accepted 1 June 2015 Available online 3 June 2015
This study presents palaeodosimetric results from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France). Nine sedimentary quartz samples (41e60 mm) have been analyzed using a multiple aliquot protocol based on the measurement of the TT-OSL signal. In addition, 7 teeth and 7 bones have been dated by combining the ESR method with U-series analyses. Both methods gave consistent age results allowing correlation of the Early Middle Paleolithic Human occupation of the site to MIS 6 and part of the Lower Paleolithic tools to MIS 7. Beyond the establishment of a radiometric chronology for the Coudoulous I sequence, this study focuses on the information extracted from the intercomparison of the methods. Our data suggest that 1) the TT-OSL signal is stable over at least the last 230 ka (considering the age range of the studied samples), 2) there are not significant problems of incomplete bleaching leading support to the applicability of the TT-OSL technique for sedimentary deposits associated with karstic contexts. This approach highlights the interest of combining luminescence and ESR/U-series methods to discuss the reliability of the dating results. © 2015 Elsevier B.V. All rights reserved.
Keywords: TT-OSL dating ESR/U-series dating SW France Early Middle Palaeolithic
1. Introduction The establishment of precise and detailed chronologies for Middle Pleistocene archaeological sites is particularly challenging since it requires non-routine approaches. During the last few decades, various methodological improvements have broadened the tools available for dating such sites. Among others, the combination of the ESR method with U-series analyses enables the direct dating of faunal remains (Grün et al., 1988). In addition, the potential to implement luminescence dating on sedimentary quartz beyond the limit of the conventional OSL approach has been recently demonstrated through the use of the thermally
Bordeaux * Corresponding author. IRAMAT-CRP2A, UMR 5060 CNRS - Universite ologie, Esplanade des Antilles, 33607 Pessac cedex, Montaigne, Maison de l’arche France. E-mail address: [email protected] (M. Hernandez). http://dx.doi.org/10.1016/j.quageo.2015.06.003 1871-1014/© 2015 Elsevier B.V. All rights reserved.
transferred OSL (TT-OSL) signal (Wang et al., 2006). Despite the progress acquired for each method independently, studies showing application of both methods are still scarce (see Arnold et al., 2014). This study focuses on the comparison of ESR/Useries performed on 7 teeth and 7 bones and TT-OSL on 9 sedimentary quartz samples from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France). This Paleolithic site exemplifies some of the major issues that must be addressed when applying palaeodosimetric methods in a karstic context, i.e. dose rate heterogeneity and bleaching issues. The ESR/U-series and TT-OSL dating methods have been implemented independently in two different laboratories (IRAMAT-CRP2A in Bordeaux and IPH in Paris) and the resulting data were then compared. In this paper, we took advantage of the complementary nature of these methods in order to discuss the results and to finally establish a detailed chronology for the human occupation of the site.
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2. Site and samples 2.1. Summary of historical research and geological context Coudoulous is located in South Western France on the eastern border of the Aquitaine basin (Fig. 1). The site is part of a karstic system developed in the Quercy region and is composed of three recesses among which Coudoulous I has revealed the most complete sedimentary sequence and the richest archaeological collection associated with Middle Pleistocene deposits. The site was discovered in 1966 by G. Maury and, after a short period of restoration (Bonifay and Clottes, 1981), it was excavated yearly from 1993 to 2004 under the direction of J. Jaubert (Jaubert et al., 2005). The cavity of Coudoulous I forms an aven (collapsed vertical shaft) of around 20 m in diameter (Figure S1). The sedimentary infill originated from external and internal sources of the karst. Angular limestone blocks and broken speleothems from the walls of the cave are mixed with rubefied clays and limestone of the karren of nearby uplands (Jaubert et al., 2005). The detritic sequence is divided in 10 stratigraphic layers grouped in 4 main units (Fig. 2, S2). The archaeological sequence is bracketed by two stalagmitic floors dated by the UeTh method (Fig. 2). The ages obtained for the upper speleothem, which correspond to MIS 5 (Jaubert et al., 2005; Couchoud, 2006), provide a terminus ante quem for the units below. The results obtained for the lower stalagmitic floor overlying the basal unit in the northern part of the site show that the initial formation of this floor is beyond the limit of the method (>350 ka; see Jaubert et al., 2005). 2.2. The Paleolithic site of Coudoulous I Lower and Early Middle Paleolithic (EMP) artefacts associated with numerous faunal remains have been discovered at the site. The EMP industry, restricted to archaeological layer 4, is dominated by Levallois core reduction on flints. While a different method is identified for the quartzite lithic industry, both revealed a high technological commitment and show adaptation to the local raw material (Jaubert et al., 2005). Lithic materials made from quartz, quartzite and volcanic rocks have been found in smaller proportion in the middle and lower Units (layers 5a to 8d). The typotechnological signature of this material is less evident (due to the absence of biface tools) but it has been assigned to the Acheulean
techno-culture, with the low technological complexity of the ratoire” being attributed to the function of the site “chaîne ope (Jaubert, 1995; Jaubert and Servelle, 1996). Coudoulous I represents a key site for the study of interactions between Humans and large mammals during the Middle Pleistocene period. From an interdisciplinary study, it has been demonstrated that the cavity has been used for marginal scavenging during the Lower Paleolithic and was used as a kill-butchery site during the early Middle Paleolithic (Brugal and Jaubert, 1991; Jaubert et al., 2005). 2.3. Samples Seven fragments of bones and three teeth associated with the EMP remains (layer 4) have been taken for ESR/U-series analysis. In addition, four teeth originating from Unit 7 (3 associated to layer 7 a/b and 1 to 7c) were also collected for dating. Nine sediment samples distributed throughout the middle and lower units of the sequence (Fig. 2, S6) have been collected under controlled light conditions for luminescence dating. Two samples from layers 4 and 7c allow direct comparisons with the ESR-U series results. 3. ESR/U-series dating 3.1. ESR and ESR/U-series age calculation ESR and ESR/U-series ages were calculated using the following parameters: k-value of 0.13 ± 0.02 for enamel (Grün and Katzenberger-Apel, 1994) and equal to 0 for bones (assuming inverse correlation hypothesis; see Bahain et al., 1992); water content of 16 ± 10 wt% for sediments (measured data, see in section 4) and 3 ± 1 wt% for enamel (assumed data from Driessens, 1980), 7 ± 5 wt % for bone, dentine and cementum (assumed data in respect with the measured sediment water content). Dose rates were obtained in different ways: the U, Th and K contents of the environmental sediments were determined using a high purity and low background germanium detector and the dose rates were then calculated considering the conversion factors of Adamiec and Aitken (1998); in situ g-dosimetry was also carried out directly for each level. Beta attenuation factors for the enamel layers were calculated using a Monte Carlo approach (Brennan et al., 1997) and taking into account the part removed during preparation; the cosmic dose rate was estimated using the formula of Prescott and Hutton (1994) as
Fig. 1. Geographical location of Coudoulous I and other Middle Pleistocene archaeological sites in SW France.
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with the OSL samples; the effect of Rn losses in each tissue was determined by combining alpha and gamma spectrometry data (Bahain et al., 1992). The ESR/U-series ages were calculated in two different ways. When the samples had experienced U uptake only, the Uranium Series (US) model was used (Grün et al., 1988) (“DATA” software; Grün, 2009). For the samples displaying U-leaching, the Accelerating Uptake (AU) model (Shao et al., 2012) was applied on whole teeth or on some tissues when it was required (CAM software, by courtesy of Qingfeng Shao). For each tissue, a specific and single U-Uptake parameter (p in US model and n in AU model) is calculated from both U-series and ESR data, allowing the calculation of dose rate contributions and ages (see details on calculation in Shao et al., 2011). 3.2. Age results Table S1 and S2 summarize the parameters used for the ESR/Useries age calculation (De, U-uptake parameter, annual dose rates, ages). Seven bones and three teeth from layer 4 were analyzed. For the bones, two groups of ages can be distinguished: the first group (5 samples) provides similar results around 160 ka while the second group (n ¼ 2), ages are younger (~90 ka). While all the bones experienced the same type of U-uptake, with an early incorporation or a light U-leaching, the second group is characterized by a very high U-content in spite of their individual De values being similar to the average De of the first group (Figure S4). The relatively young ages obtained for these two samples therefore seem to be related to a time-dependent change in the dosimetric parameters, likely induced by a complex U-uptake history. Indeed, the US-ESR approach models a single (monotonous) U-uptake into the sample. Here some part of U could have been incorporated then leached from the samples. Ages obtained for the teeth recovered in Layer 4 range from 145 ± 11 to 192 ± 22 ka and can be clearly linked to the first group of bone ages; moreover, their modeled U-uptake behavior is similar to those of the bones (Figure S5). All these results lead to a mean age of 159 ± 21 ka (n ¼ 8, the second bone group was not used) for this layer. The mean age calculated for the three teeth recovered from layer 7a/b is 193 ± 32 ka. By contrast, the date obtained for the tooth from layer 7c (168 ± 18 ka) seems underestimated, probably because of the high radionuclide contents of the associated sediment and of the late U-uptake kinetics modeled for its tissues that increases the impact of the external dose on dose rate. It can be observed in Figure S5 and Table S2 that the U-uptake in layers 7 teeth varies from an early uptake to a late uptake, according to tissues, the later uptake being modeled for layer 7c sample. The reason for this age discrepancy of the 7c tooth cannot be ascertained from the available data and further teeth analyses would be required to understand the offset/cause of the reconstructed U-uptake dynamics. 4. TT-OSL dating 4.1. Equivalent doses Unsurprisingly, preliminary tests showed that the natural fast component of the OSL signal of all the sedimentary quartz samples (the measured fraction was 41e60 mm) had reached its saturation level. For this reason, a methodology based on the TT-OSL signal, originating from deeper traps with higher saturation level (Wang et al., 2006), has been applied. A multiple aliquot protocol based on the construction of a single regenerative growth curve (SGC) for each sample has been implemented in this study (see details in sup. material, Table S3, Figure S7). This approach had been designed in order to avoid potential sensitivity changes during single aliquot processes that can lead to overestimated De values (Hernandez
Fig. 2. Stratigraphic log of Coudoulous I showing the location of the samples and of the archaeological remains. Limestones and stalagmitic floors correspond to the layers highlighted with a grey band.
et al., 2012a). Dose-recovery tests have not been performed on these samples and we therefore acknowledge that the suitability of the chosen TT-OSL protocol cannot necessarily be guaranteed for these particular samples. In the absence of dose recovery tests, it follows that some of the TT-OSL age discrepancies discussed in the 'age results' section may be explained by unfavourable responses to the chosen De measurement protocol. We note, however, that this TT-OSL dating approach has previously provided reliable ages in known-age studies performed by Hernandez et al. (2012b; 2014). The dispersion of the Lx/Tx ratios (for all regenerative doses but excluding the 0 dose ratios) has been calculated and, depending on each sample, this value is between 5% and 12% (Figure S7, Table S5). Such high values clearly reflect some variability in the TT-OSL response to the dose (Table S3). In the same way, the scatter of the Ln/Tn ratios have been calculated and range from 8% to 15%. The fact that both sets of values display similar dispersion is
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encouraging but it cannot rule out the possibility of additional external or internal sources of De scatter - at least at the resolution of the analyses performed using multi-grain discs. Indeed, the radial plots for these samples appear to be overdispersed and large proportions (53e87%) of De values lie outside the 2 sigma standardized estimate bands in all samples except COSL8 and COSL2 (Figure S8). Given the difficulties of ascertaining the source of De scatter in this multi-grain study, we have simply used the central age model (Galbraith et al., 1999) to calculate the mean De value of each sample (Table S5). 4.2. Dose rate A fraction of each sediment sample was dried, homogeneised and compacted in 12 cc plastic boxes sealed with a paraffin wax and stored for 4 weeks to allow post-radon emitters to build up. The radioelemental activities (K, U, Th) were determined with highresolution gamma ray spectrometers (Table S4). For most of the samples, the U-series is affected by a disequilibrium (between 226Ra and 210 Pb for 8 out of 9 samples, and between 238U and 226Ra for 6 samples) indicating that the contribution of this series to the dose rates likely changed over the time. However, without a precise knowledge of the history of these changes, we considered the present Ra226 activities as representative of the mean values experienced by these samples during the past, even though this assumption is not satisfying. These values were then converted to equivalent-U concentrations. The equivalent-U, Th and K contents were used to calculate the external alpha and beta dose rates using the conversion factors of Adamiec and Aitken (1998). The attenuation factors of Mejdhal (1979) and Brennan et al. (1991) were taken into account for calculating these beta and alpha dose rates, respectively, considering the grain size (41e60 mm) selected for the luminescence measurements. For the alpha dose rate, an a-value of 0.038 ± 0.01 (Tribolo et al., 2001) was assumed. Due to the sedimentological heterogeneity of the karstic infilling, dosimeters containing an aluminum oxide (Al2O3:C) powder were inserted in the different archaeological layers and left for approximately one year; their analysis gave information on the present gamma and cosmic dose rates. Since the sediments of Coudoulous I had been artificially dried because of the excavations, a blank sediment sample was collected in Coudoulous II (a cave nearby the studied site) in order to measure its water content. This value (16 ± 10 wt%) was assumed to be representative of the past water content of all the sediment samples studied here. The cosmic dose rates received by the dosimeters and by the sediment samples were calculated from the data published by Prescott and Hutton (1994) after evaluating their depth. Partial shielding has not been accounted for in the cosmic ray dose rate calculation because the roof collapsed in a relatively closed cavity and most of the blocks remain in place in the site and continue to shield the underlying layers. The total dose rates range from 950 ± 23 mG y/a to 2692 ± 50 mG y/a, and show a high spatial variability throughout the site (Table S5). 4.3. Age results The TT-OSL age results range from 137 ± 11 to 209 ± 14 ka (Fig. 3). The ages of samples COSL9 to COSL3 show a chronological gap between the deposit of layer 4 (sample COSL9 dated to 144 ± 11 ka) and the sediments present below in the stratigraphy. All ages associated from layer xalpha to layer 7e (samples COSL 8 to 3) are indistinguishable at 1 sigma level and range from 174 ± 14 to 209 ± 14 ka. The sediment samples COSL2 and COSL1 associated with the bottom of the sequence (layer 8a and 8b’2) are dated to 153 ± 14 and 137 ± 11 ka, respectively. Although they are compatible at the 2 sigma level with the set of ages from layer 7,
Fig. 3. ESR/U-series and TT-OSL age results in stratigraphic order.
those last two age estimates can be seen as underestimated results. The reason of this underestimation is not clear but one can at least hypothesize the fact that the protocol used for the De values determination was not appropriate for these samples, and/or the changes in the dose rates induced by the disequilibria detected in the U-series had a larger impact for these particular samples, possibly because these disequilibria appeared quite soon in the depositional history of these samples. As it is difficult to validate these hypotheses, one prefer to consider these last two results (samples COSL 2 and 1) as preliminary age estimates. 5. Discussion and conclusions Considering the interpretations proposed above independently for each dating method, the ESR/U-series and TT-OSL ages show good agreement (Fig. 3). For layer 4, the mean ESR/U-series age (157 ± 14 ka) is consistent with the associated TT-OSL age (144 ± 11 ka). According to the temperate climatic proxies that were observed from both the microfaunal and geological data (Jaubert et al., 2005; Bernard et al., 2009), we are able to correlate the deposition of layer 4, and therefore the EMP Human occupation, with the first part of the glacial Marine Isotopic Stage (MIS) 6, before the Glacial Maximum, geologically recorded at Coudoulous I in layer 3. The ages obtained with the TT-OSL technique for the subdivisions of layer 7 indicate that they are all contemporary with MIS 7 and/or beginning of MIS 6. While the mean ESR/U-series age (193 ± 32 ka) obtained on teeth associated with layer 7 ab fits well with this chronological attribution, the corresponding result for layer 7c is also compatible at 2 sigma with the sedimentary quartz age but falls more likely into MIS 6. However, as discussed in section 3, this last result is considered to be underestimated. The correlation of layer 7 preferentially to the temperate climate during MIS 7 is supported by the faunal analyses based on the study of Canis lupus lunellensis remains (Boudadi-Maligne, 2010). Overall, the record of MIS5e speleothem in layer 2 (at the top of the stratigraphical sequence) and the palaeontological assemblage typical of the late Middle Pleistocene agree with our results to place layers 4 and 7 in MIS6 and MIS7 respectively. Beyond the geochronological data acquired for the archaeological sequence of Coudoulous I, this study provides some insights about the TT-OSL method. The bleaching state of the grains at deposition is crucial for luminescence dating in general and especially for the TT-OSL technique since it has been shown that this signal is characterized by a slower bleaching rate compared to the fast OSL component (e.g. Tsukamoto et al., 2008). The general agreement between TT-OSL ages and the results obtained by the ESR/U-series which are not based on the same resetting phenomenon, suggest complete bleaching of the grains
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(at least for the large majority of the samples). Elsewhere, several studies have suggested that the thermal lifetime of TT-OSL traps could generate age underestimation (Adamiec et al., 2010; Li and Li, 2006; Shen et al., 2011; Thiel et al., 2012). However, this limitation is at present still unclear because various values of the lifetime have been published (3.74 Ma at 10 CeLi and Li, 2006; 4.49 MaeAdamiec et al., 2010 and 1.88 MaeShen et al., 2011). Given that the lifetime should be at least 10 times greater than the age of the dated sediment, those experimental results suggest that the method could be inappropriate for samples older than ~200 ka. However, some comparison studies suggest a much greater potential for using this signal to reliably date greater age ranges (e.g. Wang et al., 2006; Arnold et al., 2014). Due to the complexity of measuring the thermal lifetime experimentally, it is of great interest to multiply the comparisons with independent age controls. Even if the age range of the studied samples presented in this paper is limited to the last 250 ka, the comparison between ESR/U-series and the TT-OSL dating suggest that the method is applicable at least over the MIS 7 period. Acknowledgment This work was supported by a Ph.D. scholarship awarded by the Region Aquitaine, France (grant number 20081101013) and has benefited from the help of the French government's National Research Agency, LabEx ANR-10-LABX-0003-BCDiv, within the project “Investissements d'avenir” n ANR-11-IDEX-0004-02. We gional d’Arche ologie de Midi-Pyre ne es wish to thank the Service Re de ric Maksud (Toulouse) for authorising the sampling program. Fre is gratefully acknowledged for his assistance during field work. We also like to thank Angela Perri for language editing of the text and the anonymous reviewer for his helpful comments which lead to improve this paper. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quageo.2015.06.003. References Adamiec, G., Aitken, M., 1998. Dose-rate conversion factors: update. Anc. TL 16, 37e50. Adamiec, G., Duller, G.A.T., Robert, H.M., Wintle, A.G., 2010. Improving the TT-OSL SAR protocol through source trap characterisation. Radiat. Meas. 45, 768e777. s, J.M., Pe rez-Gonz Arnold, L.J., Demuro, M., Pare alez, A., Arsuaga, J.L., Bermúdez de Castro, J.M., Carbonell, E., 2014. Evaluating the suitability of extended-range luminescence dating techniques over early and Middle Pleistocene timescales: published datasets and case studies from Atapuerca, Spain. Quat. Int. http://dx.doi.org/10.1016/j.quaint.2014.08.010. res, C., Sarcia, M.N., 1992. ESR dating of tooth Bahain, J.-J., Yokoyama, Y., Falgue enamel: a comparison with K-Ar dating. Quat. Sci. Rev. 11, 245e250. cuyer, C., Brugal, J.-P., Genty, D., Wainer, K., Gardien, V., Bernard, A., Daux, V., Le Fourel, F., Jaubert, J., 2009. -Pleistocene seasonal temperature variations 18 recorded in the d O of Bison priscus teeth. Earth Planet. Sci. Lett. 283, 133e143. Tour de Faure (Lot). La Bonifay, E., Clottes, J., 1981. Le gisement de Coudoulous a histoire du Quercy dans le contexte de Midi-Pyre ne es. In: XXIe me session pre s pre historique de France. Montauban-Cahors, September 1979, vol. 1, du Congre pp. 27e28. istoce nes du sud de la France : approche Boudadi-Maligne, M., 2010. Les canis ple matique, e volutive et biochronologique. PhD Thesis. University of biosyste Bordeaux I, p. 446. Brennan, B.J., Lyons, R.G., Philipps, S.W., 1991. Attenuation of alpha particle track
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Dating results on sedimentary quartz, bones and teeth from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France): A comparative study between TT-OSL and ESR/U-series methods Marion Hernandez a, b, *, Jean-Jacques Bahain c, Norbert Mercier a, Olivier Tombret c, res c, Jacques Jaubert d Christophe Falgue IRAMAT-CRP2A, UMR 5060 CNRS - Universit e Bordeaux Montaigne, Maison de l'arch eologie, Esplanade des Antilles, 33607 Pessac cedex, France Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany c Departement de Pr ehistoire du Mus eum National d’Histoire Naturelle, UMR 7194 CNRS, 1 rue Ren e Panhard, 75013 Paris, France d PACEA, UMR 5199 CNRS- Universit e de Bordeaux, All ee Geoffroy Saint Hilaire, 33615 Pessac cedex, France a
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a b s t r a c t
Article history: Received 24 October 2014 Received in revised form 28 May 2015 Accepted 1 June 2015 Available online 3 June 2015
This study presents palaeodosimetric results from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France). Nine sedimentary quartz samples (41e60 mm) have been analyzed using a multiple aliquot protocol based on the measurement of the TT-OSL signal. In addition, 7 teeth and 7 bones have been dated by combining the ESR method with U-series analyses. Both methods gave consistent age results allowing correlation of the Early Middle Paleolithic Human occupation of the site to MIS 6 and part of the Lower Paleolithic tools to MIS 7. Beyond the establishment of a radiometric chronology for the Coudoulous I sequence, this study focuses on the information extracted from the intercomparison of the methods. Our data suggest that 1) the TT-OSL signal is stable over at least the last 230 ka (considering the age range of the studied samples), 2) there are not significant problems of incomplete bleaching leading support to the applicability of the TT-OSL technique for sedimentary deposits associated with karstic contexts. This approach highlights the interest of combining luminescence and ESR/U-series methods to discuss the reliability of the dating results. © 2015 Elsevier B.V. All rights reserved.
Keywords: TT-OSL dating ESR/U-series dating SW France Early Middle Palaeolithic
1. Introduction The establishment of precise and detailed chronologies for Middle Pleistocene archaeological sites is particularly challenging since it requires non-routine approaches. During the last few decades, various methodological improvements have broadened the tools available for dating such sites. Among others, the combination of the ESR method with U-series analyses enables the direct dating of faunal remains (Grün et al., 1988). In addition, the potential to implement luminescence dating on sedimentary quartz beyond the limit of the conventional OSL approach has been recently demonstrated through the use of the thermally
Bordeaux * Corresponding author. IRAMAT-CRP2A, UMR 5060 CNRS - Universite ologie, Esplanade des Antilles, 33607 Pessac cedex, Montaigne, Maison de l’arche France. E-mail address: [email protected] (M. Hernandez). http://dx.doi.org/10.1016/j.quageo.2015.06.003 1871-1014/© 2015 Elsevier B.V. All rights reserved.
transferred OSL (TT-OSL) signal (Wang et al., 2006). Despite the progress acquired for each method independently, studies showing application of both methods are still scarce (see Arnold et al., 2014). This study focuses on the comparison of ESR/Useries performed on 7 teeth and 7 bones and TT-OSL on 9 sedimentary quartz samples from the Middle Pleistocene archaeological site of Coudoulous I (Lot, SW France). This Paleolithic site exemplifies some of the major issues that must be addressed when applying palaeodosimetric methods in a karstic context, i.e. dose rate heterogeneity and bleaching issues. The ESR/U-series and TT-OSL dating methods have been implemented independently in two different laboratories (IRAMAT-CRP2A in Bordeaux and IPH in Paris) and the resulting data were then compared. In this paper, we took advantage of the complementary nature of these methods in order to discuss the results and to finally establish a detailed chronology for the human occupation of the site.
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2. Site and samples 2.1. Summary of historical research and geological context Coudoulous is located in South Western France on the eastern border of the Aquitaine basin (Fig. 1). The site is part of a karstic system developed in the Quercy region and is composed of three recesses among which Coudoulous I has revealed the most complete sedimentary sequence and the richest archaeological collection associated with Middle Pleistocene deposits. The site was discovered in 1966 by G. Maury and, after a short period of restoration (Bonifay and Clottes, 1981), it was excavated yearly from 1993 to 2004 under the direction of J. Jaubert (Jaubert et al., 2005). The cavity of Coudoulous I forms an aven (collapsed vertical shaft) of around 20 m in diameter (Figure S1). The sedimentary infill originated from external and internal sources of the karst. Angular limestone blocks and broken speleothems from the walls of the cave are mixed with rubefied clays and limestone of the karren of nearby uplands (Jaubert et al., 2005). The detritic sequence is divided in 10 stratigraphic layers grouped in 4 main units (Fig. 2, S2). The archaeological sequence is bracketed by two stalagmitic floors dated by the UeTh method (Fig. 2). The ages obtained for the upper speleothem, which correspond to MIS 5 (Jaubert et al., 2005; Couchoud, 2006), provide a terminus ante quem for the units below. The results obtained for the lower stalagmitic floor overlying the basal unit in the northern part of the site show that the initial formation of this floor is beyond the limit of the method (>350 ka; see Jaubert et al., 2005). 2.2. The Paleolithic site of Coudoulous I Lower and Early Middle Paleolithic (EMP) artefacts associated with numerous faunal remains have been discovered at the site. The EMP industry, restricted to archaeological layer 4, is dominated by Levallois core reduction on flints. While a different method is identified for the quartzite lithic industry, both revealed a high technological commitment and show adaptation to the local raw material (Jaubert et al., 2005). Lithic materials made from quartz, quartzite and volcanic rocks have been found in smaller proportion in the middle and lower Units (layers 5a to 8d). The typotechnological signature of this material is less evident (due to the absence of biface tools) but it has been assigned to the Acheulean
techno-culture, with the low technological complexity of the ratoire” being attributed to the function of the site “chaîne ope (Jaubert, 1995; Jaubert and Servelle, 1996). Coudoulous I represents a key site for the study of interactions between Humans and large mammals during the Middle Pleistocene period. From an interdisciplinary study, it has been demonstrated that the cavity has been used for marginal scavenging during the Lower Paleolithic and was used as a kill-butchery site during the early Middle Paleolithic (Brugal and Jaubert, 1991; Jaubert et al., 2005). 2.3. Samples Seven fragments of bones and three teeth associated with the EMP remains (layer 4) have been taken for ESR/U-series analysis. In addition, four teeth originating from Unit 7 (3 associated to layer 7 a/b and 1 to 7c) were also collected for dating. Nine sediment samples distributed throughout the middle and lower units of the sequence (Fig. 2, S6) have been collected under controlled light conditions for luminescence dating. Two samples from layers 4 and 7c allow direct comparisons with the ESR-U series results. 3. ESR/U-series dating 3.1. ESR and ESR/U-series age calculation ESR and ESR/U-series ages were calculated using the following parameters: k-value of 0.13 ± 0.02 for enamel (Grün and Katzenberger-Apel, 1994) and equal to 0 for bones (assuming inverse correlation hypothesis; see Bahain et al., 1992); water content of 16 ± 10 wt% for sediments (measured data, see in section 4) and 3 ± 1 wt% for enamel (assumed data from Driessens, 1980), 7 ± 5 wt % for bone, dentine and cementum (assumed data in respect with the measured sediment water content). Dose rates were obtained in different ways: the U, Th and K contents of the environmental sediments were determined using a high purity and low background germanium detector and the dose rates were then calculated considering the conversion factors of Adamiec and Aitken (1998); in situ g-dosimetry was also carried out directly for each level. Beta attenuation factors for the enamel layers were calculated using a Monte Carlo approach (Brennan et al., 1997) and taking into account the part removed during preparation; the cosmic dose rate was estimated using the formula of Prescott and Hutton (1994) as
Fig. 1. Geographical location of Coudoulous I and other Middle Pleistocene archaeological sites in SW France.
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with the OSL samples; the effect of Rn losses in each tissue was determined by combining alpha and gamma spectrometry data (Bahain et al., 1992). The ESR/U-series ages were calculated in two different ways. When the samples had experienced U uptake only, the Uranium Series (US) model was used (Grün et al., 1988) (“DATA” software; Grün, 2009). For the samples displaying U-leaching, the Accelerating Uptake (AU) model (Shao et al., 2012) was applied on whole teeth or on some tissues when it was required (CAM software, by courtesy of Qingfeng Shao). For each tissue, a specific and single U-Uptake parameter (p in US model and n in AU model) is calculated from both U-series and ESR data, allowing the calculation of dose rate contributions and ages (see details on calculation in Shao et al., 2011). 3.2. Age results Table S1 and S2 summarize the parameters used for the ESR/Useries age calculation (De, U-uptake parameter, annual dose rates, ages). Seven bones and three teeth from layer 4 were analyzed. For the bones, two groups of ages can be distinguished: the first group (5 samples) provides similar results around 160 ka while the second group (n ¼ 2), ages are younger (~90 ka). While all the bones experienced the same type of U-uptake, with an early incorporation or a light U-leaching, the second group is characterized by a very high U-content in spite of their individual De values being similar to the average De of the first group (Figure S4). The relatively young ages obtained for these two samples therefore seem to be related to a time-dependent change in the dosimetric parameters, likely induced by a complex U-uptake history. Indeed, the US-ESR approach models a single (monotonous) U-uptake into the sample. Here some part of U could have been incorporated then leached from the samples. Ages obtained for the teeth recovered in Layer 4 range from 145 ± 11 to 192 ± 22 ka and can be clearly linked to the first group of bone ages; moreover, their modeled U-uptake behavior is similar to those of the bones (Figure S5). All these results lead to a mean age of 159 ± 21 ka (n ¼ 8, the second bone group was not used) for this layer. The mean age calculated for the three teeth recovered from layer 7a/b is 193 ± 32 ka. By contrast, the date obtained for the tooth from layer 7c (168 ± 18 ka) seems underestimated, probably because of the high radionuclide contents of the associated sediment and of the late U-uptake kinetics modeled for its tissues that increases the impact of the external dose on dose rate. It can be observed in Figure S5 and Table S2 that the U-uptake in layers 7 teeth varies from an early uptake to a late uptake, according to tissues, the later uptake being modeled for layer 7c sample. The reason for this age discrepancy of the 7c tooth cannot be ascertained from the available data and further teeth analyses would be required to understand the offset/cause of the reconstructed U-uptake dynamics. 4. TT-OSL dating 4.1. Equivalent doses Unsurprisingly, preliminary tests showed that the natural fast component of the OSL signal of all the sedimentary quartz samples (the measured fraction was 41e60 mm) had reached its saturation level. For this reason, a methodology based on the TT-OSL signal, originating from deeper traps with higher saturation level (Wang et al., 2006), has been applied. A multiple aliquot protocol based on the construction of a single regenerative growth curve (SGC) for each sample has been implemented in this study (see details in sup. material, Table S3, Figure S7). This approach had been designed in order to avoid potential sensitivity changes during single aliquot processes that can lead to overestimated De values (Hernandez
Fig. 2. Stratigraphic log of Coudoulous I showing the location of the samples and of the archaeological remains. Limestones and stalagmitic floors correspond to the layers highlighted with a grey band.
et al., 2012a). Dose-recovery tests have not been performed on these samples and we therefore acknowledge that the suitability of the chosen TT-OSL protocol cannot necessarily be guaranteed for these particular samples. In the absence of dose recovery tests, it follows that some of the TT-OSL age discrepancies discussed in the 'age results' section may be explained by unfavourable responses to the chosen De measurement protocol. We note, however, that this TT-OSL dating approach has previously provided reliable ages in known-age studies performed by Hernandez et al. (2012b; 2014). The dispersion of the Lx/Tx ratios (for all regenerative doses but excluding the 0 dose ratios) has been calculated and, depending on each sample, this value is between 5% and 12% (Figure S7, Table S5). Such high values clearly reflect some variability in the TT-OSL response to the dose (Table S3). In the same way, the scatter of the Ln/Tn ratios have been calculated and range from 8% to 15%. The fact that both sets of values display similar dispersion is
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encouraging but it cannot rule out the possibility of additional external or internal sources of De scatter - at least at the resolution of the analyses performed using multi-grain discs. Indeed, the radial plots for these samples appear to be overdispersed and large proportions (53e87%) of De values lie outside the 2 sigma standardized estimate bands in all samples except COSL8 and COSL2 (Figure S8). Given the difficulties of ascertaining the source of De scatter in this multi-grain study, we have simply used the central age model (Galbraith et al., 1999) to calculate the mean De value of each sample (Table S5). 4.2. Dose rate A fraction of each sediment sample was dried, homogeneised and compacted in 12 cc plastic boxes sealed with a paraffin wax and stored for 4 weeks to allow post-radon emitters to build up. The radioelemental activities (K, U, Th) were determined with highresolution gamma ray spectrometers (Table S4). For most of the samples, the U-series is affected by a disequilibrium (between 226Ra and 210 Pb for 8 out of 9 samples, and between 238U and 226Ra for 6 samples) indicating that the contribution of this series to the dose rates likely changed over the time. However, without a precise knowledge of the history of these changes, we considered the present Ra226 activities as representative of the mean values experienced by these samples during the past, even though this assumption is not satisfying. These values were then converted to equivalent-U concentrations. The equivalent-U, Th and K contents were used to calculate the external alpha and beta dose rates using the conversion factors of Adamiec and Aitken (1998). The attenuation factors of Mejdhal (1979) and Brennan et al. (1991) were taken into account for calculating these beta and alpha dose rates, respectively, considering the grain size (41e60 mm) selected for the luminescence measurements. For the alpha dose rate, an a-value of 0.038 ± 0.01 (Tribolo et al., 2001) was assumed. Due to the sedimentological heterogeneity of the karstic infilling, dosimeters containing an aluminum oxide (Al2O3:C) powder were inserted in the different archaeological layers and left for approximately one year; their analysis gave information on the present gamma and cosmic dose rates. Since the sediments of Coudoulous I had been artificially dried because of the excavations, a blank sediment sample was collected in Coudoulous II (a cave nearby the studied site) in order to measure its water content. This value (16 ± 10 wt%) was assumed to be representative of the past water content of all the sediment samples studied here. The cosmic dose rates received by the dosimeters and by the sediment samples were calculated from the data published by Prescott and Hutton (1994) after evaluating their depth. Partial shielding has not been accounted for in the cosmic ray dose rate calculation because the roof collapsed in a relatively closed cavity and most of the blocks remain in place in the site and continue to shield the underlying layers. The total dose rates range from 950 ± 23 mG y/a to 2692 ± 50 mG y/a, and show a high spatial variability throughout the site (Table S5). 4.3. Age results The TT-OSL age results range from 137 ± 11 to 209 ± 14 ka (Fig. 3). The ages of samples COSL9 to COSL3 show a chronological gap between the deposit of layer 4 (sample COSL9 dated to 144 ± 11 ka) and the sediments present below in the stratigraphy. All ages associated from layer xalpha to layer 7e (samples COSL 8 to 3) are indistinguishable at 1 sigma level and range from 174 ± 14 to 209 ± 14 ka. The sediment samples COSL2 and COSL1 associated with the bottom of the sequence (layer 8a and 8b’2) are dated to 153 ± 14 and 137 ± 11 ka, respectively. Although they are compatible at the 2 sigma level with the set of ages from layer 7,
Fig. 3. ESR/U-series and TT-OSL age results in stratigraphic order.
those last two age estimates can be seen as underestimated results. The reason of this underestimation is not clear but one can at least hypothesize the fact that the protocol used for the De values determination was not appropriate for these samples, and/or the changes in the dose rates induced by the disequilibria detected in the U-series had a larger impact for these particular samples, possibly because these disequilibria appeared quite soon in the depositional history of these samples. As it is difficult to validate these hypotheses, one prefer to consider these last two results (samples COSL 2 and 1) as preliminary age estimates. 5. Discussion and conclusions Considering the interpretations proposed above independently for each dating method, the ESR/U-series and TT-OSL ages show good agreement (Fig. 3). For layer 4, the mean ESR/U-series age (157 ± 14 ka) is consistent with the associated TT-OSL age (144 ± 11 ka). According to the temperate climatic proxies that were observed from both the microfaunal and geological data (Jaubert et al., 2005; Bernard et al., 2009), we are able to correlate the deposition of layer 4, and therefore the EMP Human occupation, with the first part of the glacial Marine Isotopic Stage (MIS) 6, before the Glacial Maximum, geologically recorded at Coudoulous I in layer 3. The ages obtained with the TT-OSL technique for the subdivisions of layer 7 indicate that they are all contemporary with MIS 7 and/or beginning of MIS 6. While the mean ESR/U-series age (193 ± 32 ka) obtained on teeth associated with layer 7 ab fits well with this chronological attribution, the corresponding result for layer 7c is also compatible at 2 sigma with the sedimentary quartz age but falls more likely into MIS 6. However, as discussed in section 3, this last result is considered to be underestimated. The correlation of layer 7 preferentially to the temperate climate during MIS 7 is supported by the faunal analyses based on the study of Canis lupus lunellensis remains (Boudadi-Maligne, 2010). Overall, the record of MIS5e speleothem in layer 2 (at the top of the stratigraphical sequence) and the palaeontological assemblage typical of the late Middle Pleistocene agree with our results to place layers 4 and 7 in MIS6 and MIS7 respectively. Beyond the geochronological data acquired for the archaeological sequence of Coudoulous I, this study provides some insights about the TT-OSL method. The bleaching state of the grains at deposition is crucial for luminescence dating in general and especially for the TT-OSL technique since it has been shown that this signal is characterized by a slower bleaching rate compared to the fast OSL component (e.g. Tsukamoto et al., 2008). The general agreement between TT-OSL ages and the results obtained by the ESR/U-series which are not based on the same resetting phenomenon, suggest complete bleaching of the grains
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(at least for the large majority of the samples). Elsewhere, several studies have suggested that the thermal lifetime of TT-OSL traps could generate age underestimation (Adamiec et al., 2010; Li and Li, 2006; Shen et al., 2011; Thiel et al., 2012). However, this limitation is at present still unclear because various values of the lifetime have been published (3.74 Ma at 10 CeLi and Li, 2006; 4.49 MaeAdamiec et al., 2010 and 1.88 MaeShen et al., 2011). Given that the lifetime should be at least 10 times greater than the age of the dated sediment, those experimental results suggest that the method could be inappropriate for samples older than ~200 ka. However, some comparison studies suggest a much greater potential for using this signal to reliably date greater age ranges (e.g. Wang et al., 2006; Arnold et al., 2014). Due to the complexity of measuring the thermal lifetime experimentally, it is of great interest to multiply the comparisons with independent age controls. Even if the age range of the studied samples presented in this paper is limited to the last 250 ka, the comparison between ESR/U-series and the TT-OSL dating suggest that the method is applicable at least over the MIS 7 period. Acknowledgment This work was supported by a Ph.D. scholarship awarded by the Region Aquitaine, France (grant number 20081101013) and has benefited from the help of the French government's National Research Agency, LabEx ANR-10-LABX-0003-BCDiv, within the project “Investissements d'avenir” n ANR-11-IDEX-0004-02. We gional d’Arche ologie de Midi-Pyre ne es wish to thank the Service Re de ric Maksud (Toulouse) for authorising the sampling program. Fre is gratefully acknowledged for his assistance during field work. We also like to thank Angela Perri for language editing of the text and the anonymous reviewer for his helpful comments which lead to improve this paper. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quageo.2015.06.003. References Adamiec, G., Aitken, M., 1998. Dose-rate conversion factors: update. Anc. TL 16, 37e50. Adamiec, G., Duller, G.A.T., Robert, H.M., Wintle, A.G., 2010. Improving the TT-OSL SAR protocol through source trap characterisation. Radiat. Meas. 45, 768e777. s, J.M., Pe rez-Gonz Arnold, L.J., Demuro, M., Pare alez, A., Arsuaga, J.L., Bermúdez de Castro, J.M., Carbonell, E., 2014. Evaluating the suitability of extended-range luminescence dating techniques over early and Middle Pleistocene timescales: published datasets and case studies from Atapuerca, Spain. Quat. Int. http://dx.doi.org/10.1016/j.quaint.2014.08.010. res, C., Sarcia, M.N., 1992. ESR dating of tooth Bahain, J.-J., Yokoyama, Y., Falgue enamel: a comparison with K-Ar dating. Quat. Sci. Rev. 11, 245e250. cuyer, C., Brugal, J.-P., Genty, D., Wainer, K., Gardien, V., Bernard, A., Daux, V., Le Fourel, F., Jaubert, J., 2009. -Pleistocene seasonal temperature variations 18 recorded in the d O of Bison priscus teeth. Earth Planet. Sci. Lett. 283, 133e143. Tour de Faure (Lot). La Bonifay, E., Clottes, J., 1981. Le gisement de Coudoulous a histoire du Quercy dans le contexte de Midi-Pyre ne es. In: XXIe me session pre s pre historique de France. Montauban-Cahors, September 1979, vol. 1, du Congre pp. 27e28. istoce nes du sud de la France : approche Boudadi-Maligne, M., 2010. Les canis ple matique, e volutive et biochronologique. PhD Thesis. University of biosyste Bordeaux I, p. 446. Brennan, B.J., Lyons, R.G., Philipps, S.W., 1991. Attenuation of alpha particle track
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