ESR chronology of the Somme River Terrace system and first human settlements in Northern France

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Quaternary Geochronology 2 (2007) 356–362 www.elsevier.com/locate/quageo

Research paper

ESR chronology of the Somme River Terrace system and first human settlements in Northern France Jean-Jacques Bahaina,, Christophe Falgue`resa, Michel Laurenta, Pierre Voincheta, Jean-Michel Dolob, Pierre Antoinec, Alain Tuffreaud a

De´partement de Pre´histoire du Muse´um National d’Histoire Naturelle, UMR 5198 du CNRS, 1 rue Rene´ Panhard, 75013 Paris, France b Laboratoire National Henri Becquerel, CEA/Saclay, 91191Gif-sur-Yvette, France c Laboratoire de Ge´ographie Physique, UMR 8591 du CNRS, 1 Place Aristide Briand, 92195 Meudon cedex, France d Laboratoire ‘Pre´histoire et Quaternaire’, UMR 8018 du CNRS, Universite´ de Lille 1, 59655 Villeneuve d’Ascq, France Received 12 October 2005; accepted 26 April 2006 Available online 30 June 2006

Abstract The Somme Valley, Northern France, is famous for its archaeological sequence, where numerous rich Palaeolithic sites, such as SaintAcheul, the type site of the Acheulian, have been discovered. The archaeological levels are often directly associated with fossil alluvial sediments of the River Somme or with slope deposits, including loess and palaeosols. In the middle reaches of the valley, near Amiens, the system of fossil-stepped fluvial terraces is particularly well developed and preserved, and occurs on 10 alluvial formations. These terraces, from +5 to +55 m above the present-day valley bedrock, allow the study of the environmental changes and the human settlement of this area through the Pleistocene. Since 1988, ESR dating was systematically applied on bleached quartz extracted from the fossil fluvial deposits, in order to better describe the geological evolution of the stepped system. More recently, U-series/ESR dating has also been performed on teeth collected from the different terrace deposits. Here we present a synthetic review of the main ESR results, and propose an ESR chronology for the geological evolution of the Somme fluvial system and for the Middle Pleistocene human settlement of northern France. r 2006 Elsevier Ltd. All rights reserved. Keywords: Somme Valley; Stepped terrace system; ESR dating; Fluvial sediments; Teeth; Palaeolithic

1. Introduction The River Somme is one of the major rivers that drain the northern part of France towards the English Channel. Its present-day catchment is relatively small (5800 km2), but during the cold stages of the Pleistocene, the Somme River was connected to the large Channel River System, which included the Seine, Thames and Rhine systems (Fig. 1) (Antoine et al., 2000, 2003a; Bourillet et al., 2003). In the middle reaches of the Somme Valley, near the town of Amiens, a stepped terrace system is well developed, Corresponding author. Tel.: +33 155432736; fax: +33 143312279.

E-mail addresses: [email protected] (J.-J. Bahain), [email protected] (C. Falgue`res), [email protected] (P. Voinchet), [email protected] (J.-M. Dolo), [email protected] (P. Antoine), [email protected] (A. Tuffreau). 1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.04.012

including on each bedrock step one alluvial formation overlain by a loess–palaeosols sequence. During the XIXth century, numerous Palaeolithic remains were discovered from several alluvial formations of this system. These discoveries have played a prominent role in the recognition of prehistoric archaeology by the French scientific community. Between 1830 and 1860, the works of pioneers including Casimir Picard, Jacques Boucher de Perthes and Marcel Rigollot, in the area between Abbeville and Amiens (Fig. 1), have demonstrated the coexistence of extinct faunal assemblages and human lithic industries, including handaxes, in the fossil fluvial deposits (see details in Tuffreau, 2001). In 1872, these discoveries lead Gabriel de Mortillet to propose the name of one of the main Somme Paleolithic sites, Saint-Acheul, to a Lower Palaeolithic culture, the Acheulian, in his chronological classification system of prehistoric cultures. Several famous French

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5° W ENGLAND

Thames

London

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The English Channel

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Pleistocene terraces Saint Valery

50° N

Abbeville

Somme

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Paris

0

Saint Sauveur Amiens Montières Cagny Renancourt Saint 100 km Acheul

0° Fig. 1. General location of the Somme Valley (Northern France) (after Antoine et al., 2000, 2003b).

Fig. 2. Synthetic cross-section throughout the Middle Somme Terrace System near Amiens (after Antoine et al., 2003b).

archaeologists such as Breuil, Bourdier or Bordes subsequently worked on the Somme Valley sites. Since the 1970s, extensive excavations have been undertaken by Alain Tuffreau on several important archaeological localities within the Somme terrace system, including Cagny La Garenne, Cagny l’Epinette or Saint-Acheul (for details see Tuffreau, 2001; Tuffreau and Antoine, 1995). 2. The Somme terrace system At the same time as the archaeological excavations, the Somme Valley terrace system was investigated by numerous geologists. Among them, the work of Victor Commont before the First World War was crucial for the understanding of the chronology of the archaeological remains in the various alluvial formations (Tuffreau, 2001). More recently, extensive geological studies have been conducted in the valley (Bourdier et al., 1974; Somme´ et al., 1984; Haesaerts and Dupuis, 1986; Antoine, 1990, 1994), and a multi-disciplinary approach combining environmental, geological and geochronological analyses

has been adopted. This approach has greatly improved the description of both the fluvial system and the associated slope-deposit sequences (Antoine et al., 2000, 2003a, b). Near Amiens, the Somme system is represented by a very regular succession of 10 stepped fossil alluvial formations, classified from I to X and ranging from+5 to+55 m relative height above the modern valley bedrock floor (Fig. 2). On the basis of a multidisciplinary approach of the whole system, it has been demonstrated that the formation of this stepped terrace system has been driven by glacial–interglacial climatic cycles, superimposed upon a slow and regular uplift since the end of the Lower Pleistocene. Within this system, each alluvial formation represents the budget of a glacial/interglacial cycle, especially between the middle and lower parts of the system (Antoine, 1994; Antoine et al., 2000, 2003b). The Somme system appears today as one of the best reference in NW Europe for the study of the response of fluvial systems to climatic variations and uplift since about 1 Ma (Bridgland, 2000; Bridgland et al., in press).

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The typical alluvial sequence overlying each bedrock step can be summarised by the following succession of sedimentary units (from base to top) (Antoine et al., 2000, 2003a, b): (1) coarse chalky slope deposits with interstratified fluvial silts, deposited under an early glacial environment (rarely preserved), (2) coarse fluvial gravels and sands (braided channel pattern) corresponding to Pleniglacial sedimentation (main part of the sedimentary budget), (3) calcareous silts and sandy silts of fluvial origin locally capped by organic soils and (or) calcareous tufas representing interglacial deposition. Each alluvial sequence is separated from the subsequent unit by incision of 5/6 m, which has resulted in the formation of a new bedrock step. These alluvial sequences are overlain by slope deposits including loess, gravel beds and palaeosols. The number of palaeosols recorded within this overlying sequence increases with the antiquity of the underlying alluvial formation. The chronostratigraphical interpretation of the terrace system of the Somme was thus initially been based on the recognition of a cyclic glacial–interglacial pattern within the fluvial sequences and the overlying loess–palaeosol deposits (Haesaerts and Dupuis, 1986; Antoine, 1990, 1994). It has been supported by amino-acid geochronology based on mollusc shells from the fluvial sediments (Bates, 1993), biostratigraphy (Auguste, 1995), palaeomagnetism, and most recently by electron spin resonance (ESR) and U-series dating (Laurent et al., 1994; Antoine et al., 2000, 2003a, b, in press). As mentioned above, numerous Palaeolithic sites have been discovered in the Somme valley. The main archaeological sites are concentrated in the district between Abbeville and Amiens. Near Abbeville, the sites of Carrie`re Carpentier (Formation VII), and of Stade and Champ de Mars (Formation VI) yield the most ancient evidences of human occupation in the area. Around Amiens, many sites have been found at Saint-Acheul (Formations VI and V), Cagny-la-Garenne and Cagny-Cimetie`re (Formation IV), and Cagny-L’Epinette (Formation III) (Tuffreau and Antoine, 1995). Although numerous handaxes have been recovered within the terrace deposits at Abbeville and Amiens since the beginning of the 20th century, it is not simple to determine the age of the oldest human occupation in the Somme basin. Indeed, the earliest evidence for human occupation is generally based on previous discoveries from the Carrie`re Carpentier at Abbeville (Commont, 1910) that have not been reinvestigated. A unique short excavation in 1989 also allowed the rediscovery of some large mammal remains from these fine-grained fluvial deposits. However, no Palaeolithic artefacts were recovered (Antoine, 1990). It has then been demonstrated that the oldest reliable Acheulian industries from the Somme are associated with the alluvial Formation VI.

3. ESR dating of the Somme Valley terrace system— previous results and experimental strategy The rigorous stratigraphical age control of each of the alluvial formations makes the Somme Valley system an ideal test case to check the potential of new geochronological methods. For this reason, the system was chosen for the systematic application of ESR method on bleached quartz. A preliminary study, performed in a larger programme of investigations, including stratigraphical fieldwork, excavations, and geological, palaeontological and archaeological studies, was undertaken between 1988 and 1998, during which samples were collected from the various alluvial formations (Laurent et al., 1994, 1998; Antoine et al., 2000). The results obtained, supported by independent U-series and magnetostratigraphical evidences, have confirmed the previous geological and biostratigraphical results. From 1998 to 2001, a CNRS research programme, entitled Paleoenvironment and Hominid Evolution offered the opportunity to summarise the previous results and to develop a multidisciplinary approach of the Somme Valley terrace system and its associated Palaeolithic sites (Antoine et al., 2003b). New ESR analyses were performed on fluvial sediments from beneath the different terrace levels (Formations V, VII and X), and were complemented by U-series/ESR dating on teeth collected from several archaeological sites (Saint-Sauveur, Formation I: Antoine et al., 1995; Cagny l’Epinette, Formation IV: Tuffreau and Antoine, 1995; Cagny La Garenne, Formation V: Tuffreau and Antoine, 1995) or from palaeontological horizons (Graˆce-Autoroute, Formation X: Antoine et al., 2000). The site of Cagny-La Garenne II (Tuffreau, 2001) was then chosen as reference site. Sediment samples were selected from various geological units within the same section located close to the excavation zone from which one tooth was recovered. In common with the equivalent-age Saint-Acheul site, located some kilometres to the west of Cagny (Antoine and Limondin-Lozouet, 2004), this Cagny-La Garenne site was also the subject of stratigraphical, sedimentological, malacological and biostratigraphical studies. 3.1. ESR dating of bleached quartz The ESR dating procedure used for fluvial bleached quartz (Yokoyama et al., 1985; Laurent et al., 1998) was recently improved (Voinchet et al., 2004). For each sample, it includes a study of the artificial UV bleaching of the quartz aluminium centre and the determination of an individual non-bleached ‘residual’ dose, prior to the determination of the DE value. The bleaching percentage was determined using the experimental procedure described in Voinchet et al. (2004) after an artificial-light exposure for 800 h. Equivalent doses (DE) were determined using an exponential fitting with the additive-dose method. External dose rates correspond to both sediment (b+g) dose and cosmic dose. Three types of

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measurements have been performed. About 100 g of sediments, including rock fragments if present, were measured at least one month after insertion in a box. The sediments were sampled from the same square and at the same height as the teeth. At the Cagny archaeological excavations, CaSO4 dosimeters have been placed in the studied levels at the exact location of the analysed sediments. Internal doses were calculated from g-spectrometry measurements of the U, Th and K contents of the samples using data of Adamiec and Aitken (1998). Experimental parameters and methods are presented by Voinchet et al. (2004). 3.2. ESR/U-series dating of teeth The teeth were analysed by both U-Series and ESR in order to calculate the US-model ages (Gru¨n et al., 1988). In this model, a particular parameter p, related to the Uranium-uptake kinetic, is calculated for each dental tissue, based on data derived from U-series and ESR analyses (radioelement content, isotopic ratios, palaeodoses, external g-dose rate, etc.). It implies a complex analytical procedure including a- and g-ray spectrometry measurements (for details, see Falgue`res et al., 1997). The different dental tissues (enamel, dentine and cementum) were separated mechanically. The enamel, after cleaning its outer surface to eliminate the effect of external a-radiation, was ground and sieved (100–200 mm). Then, the enamel powder was split into 10 aliquots for irradiation. The samples were irradiated using a calibrated 60Co g-ray source. The equivalent doses (DE) were determined from the asymmetric enamel signal at g ¼ 2:0018 with the additive method using an exponential fitting (Yokoyama et al., 1985). U-series analyses were performed on the different dental tissues using a-ray

5

Age (ka) 1000

δ18Ο 3

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Upper Pleist.

B R U H N E S

P l Me i si d t do l c e e n e

M A T U Y A M A

P l Ee a i r s l t yo .

spectrometry and according to the standard methods described by Bischoff et al. (1988). Then, ESR/US ages were calculated using the following parameters: k-value of 0.1370.02 (Gru¨n and Schwarcz, 2000), and water content of 3 wt% in the enamel, 10 wt% in the dentine and in the cementum, and 20 wt% in the sediment. b-attenuations in the enamel layers were calculated taking into account the part destroyed by sample preparation. Dose rates were obtained using different ways: g-ray spectrometry with a low background, and a high-purity germanium detector was used to evaluate the external doses of the sediments studied or containing the teeth and the dose rate was then calculated from the U, Th and K contents of the environmental soil; At the Cagny archaeological sites, CaSO4 dosimeters were used. The effect of Ra and Rn losses in each tissue was determined by combining a- and g-ray data (Bahain et al., 1992). 4. Results and discussion The results obtained from sediments and teeth are reported in Tables S1 and S2, respectively, and in Fig. 3.





The attribution of Formation I to the MIS 6/5 is reinforced by the U-Series (TIMS) data of a new tufa sequence at Caours, near Abbeville, which confirm the Eemian age (120–125 ka BP) of the top of the alluvial sequence (Antoine et al., in press) and by the US-ESR age obtained from the Saint-Sauveur tooth (94714 ka). This age confirms the correlation of these archaeological deposits on the basis of the palaeontology to a cold stadial of the Weichselian Early Glacial (MIS 5d), as proposed by Antoine et al. (1995). The US-age of the teeth collected from the Cagnyl’Epinette site (318748, 289743 and 291744 ka

ESR results obtained on the Somme fluvial system

3.5 Fm I

0

500

4.5

359

Fm II

Fm III

Fm IV

Fm V

Fm VI

Fm VII

Fm VIII

Fm IX

Fm X

Chronostratigraphical interpretation Formation OIS Fm 0 4

5 7 9 11 13 15 17 19 21 25 29 31 37 41 Marine Isotopic Stages (MIS)

? Previous ESR ages (Laurent et al., 1994, 1998; Antoine et al., 2000) Teeth US-ESR ages (this work)

Fm I Fm II Fm III Fm IV

6 7b 8 10

Fm V Fm VI Fm VII

12 14 16

Fm VIII 18 Fm IX 20 or22 Fm X

?

22 or more

Quartz ESR ages (this work)

Fig. 3. ESR results obtained on the Somme Valley System and chronostratigraphical evolution of the system versus oxygen isotope stages taken from Shackleton (1995).

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for the IB silty fluvial unit; 253738 and 266740 for the H slope-deposit unit), is in agreement with the previous date obtained by Laurent et al. (1994, 1998) (296753 ka). These data allow to date the deposition of the fine fluvial deposits of Formation IV to MIS 9, and the beginning of the slope deposit sedimentation to the beginning of MIS 8, and, in this context, confirm the previous proposal of Antoine (1994) and Antoine et al. (2000, 2003a) that Formations III & II correspond to MIS 8/7.3 and MIS 7.2/7.1, respectively, based on the ages obtained by Laurent et al. (1994, 1998) (266740 ka and 203738, respectively) and according to the multidisciplinary study. Hence, the response of the fluvial system appears more complex between MIS 8 and 5e. For this period, the fluvial record in the Somme Valley is represented by the succession of three alluvial sequences separated by about 5/6 m bedrock incisions (Formation III, MIS8/7.3; Formation II, MIS 7.2/7.1; and Formation I, MIS 6/5e). It appears that MIS 7 must not be apprehended as a classical interglacial period and, that during MIS 7, according to these data, the previous 100 ka glacial–interglacial pattern of the fluvial record is replaced by shorter stadial–interstadial cyclicity. Indeed, from the palaeoclimatological point of view, MIS 7 includes a short but very strong cold period (MIS 7.2) between about 220 and 230 ka BP (EPICA, 2004), which is associated with a low sea-level stage (70 to 80 m: Waelbroeck et al., 2002). This cold event, apparently as strong as MIS4, has also been recorded in continental sequences by the deposition of typical loess beds in Europe (Antoine, 1994), and in China (Soil S2, Liu, 1991). Our interpretation is that it was sufficiently intense to lead to the incision of the bedrock, and then to the deposition of the fluvial sequence of Formation II. The results obtained from the samples collected from the Cagny-La Garenne II site (Formation V), shown in Figure S1 (Antoine, 2001), demonstrate that any contamination of the sample (even traces) by chalky fragments derived from the substrate leads to an overestimation of the ESR age (due to a high bleaching percentage and to high DE value). Fluvial loams and silts seem to be better suited to ESR analyses. The ages obtained from these sediments, from La Garenne U1a unit (443743 ka) and from the equivalent LV unit in Saint Acheul (403773 ka), are in agreement with the stratigraphical and paleoenvironmental evidences. These results and the Garenne tooth US-ESR age (448768 ka) confirm the previous results obtained by Laurent et al. (1994, 1998) (4007100 ka) and indicate that the fluvial sedimentation of the Formation V occurred during MIS 12 and 11. The geological and palaeoenvironmental evidence thus allows a reconstruction of the history of the Garenne sequence evolution and of the timing of sedimentation (Antoine and Limondin-Lozouet, 2004). Two sediment samples from Formation VII yield ESR ages of 5097110 and 5897134 ka, which correspond



to the previous ESR age of 600790 ka obtained by Laurent et al. (1994, 1998). Based on the correlation of Formation IV to MIS 12/11, it is reasonable to suggest correlation of terraces VI and VII to MIS 14/13 and 16/15, respectively. Two ESR ages (9977141 and 11057186 ka) have been previously obtained on the highest terrace sediments of the Somme system (Formation X: Antoine et al., 2000). The US-ESR age obtained during this study (9437141 ka) confirms the antiquity of this terrace. Since the Brunhes–Matuyama boundary has been identified in the fluvial deposits of the Formation IX (Bourdier et al., 1974), it is possible to attribute Formation IX to MIS 20/19 or 22/21, and Formation VIII, although not directly dated currently, to MIS 18/17. Hence, Formation X can be attributed probably to MIS 22/21, but may also be older.

Hence, the chronostratigraphical interpretation of the Somme system by Antoine (1990, 1994), essentially based on the recognition of a cyclic morphosedimentary pattern driven by glacial–interglacial climatic cycles and on the pedostratigraphy of the slope-deposit sequences (Antoine et al., 2000), is supported by the ESR dates obtained. It also allows dating of the beginning of Acheulian settlement in the Somme area to MIS 15, and the appearance of the Middle Palaeolithic assemblages to the MIS 9/8 transition. This area therefore seems to have been populated relatively late in comparison to the southern European zone, where human occupation has been demonstrated to date from the Lower Pleistocene, before 780 ka BP, on the basis of lithic materials from several sites (Orce and Atapuerca Elefante, Spain; Monte Poggiolo, Italy; Pont-de-Lavaud and Vallonnet, France). It is also supported by palaeoanthropological remains (Atapuerca Gran Dolina TD6, Spain; Ceprano, Italy: Falgue`res, 2003). 5. Conclusion For almost 20 years, the systematic application of the ESR method to date the fluvial deposits and the associated palaeontological remains from the Somme Valley terrace system, in association with palaeomagnetism, stratigraphical and paleoenvironmental studies, has allowed the development of a chronostratigraphical framework, into which the archaeological remains can be easily placed. It is confirmed that each alluvial formation represents the depositional sequence of a glacial/interglacial cycle, as it was previously proposed according to pedostratigraphical and palaeoenvironmental evidences. The exceptions are Formations II and III, which probably correspond to MIS 7.2/7.1 and 8/7.3, respectively, and seem to indicate for this period a stadial–interstadial cyclicity. The first attested human settlement demonstrated in this area, recognised from the Acheulian assemblages of Formation VII, is dated to MIS 16/15 (ca. 600 ka BP). In the future, it is hoped to develop this geochronological approach by the dating of

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palaeontological remains and fluvial sediments from the alluvial formations II, III, VI–VIII, in order to validate the ESR method applied to bleached quartz by direct age comparison with US-ESR data, and to constrain the ESR results using pedostratigraphical evidence. Acknowledgements The ESR study of the Somme Valley system was initiated in 1988 following an initiative by Henry de Lumley (French National Museum of Natural History) and Yuji Yokoyama (CNRS). This work was partly supported between 1997 and 2000 by the Palaeoenvironment and Hominid Evolution programme funded by the CNRS, France. During fieldwork, the authors benefited from the assistance of the Lille University Team, including Patrick Auguste and Agne`s Lamotte, and of the excavators at the different archaeological sites. Experimental analyses were performed with the help of Bernard Morin (ESR measurements, SIARE, University Paris VI), Manfred Frechen, Annette Engelmann (g-spectrometry, Institut fu¨r Geowissenschaftliche Gemeinschaftsaufgaben, Hannover), He´le`ne Valladas, Norbert Mercier (in situ TL-dosimetry, LSCE, CEACNRS), Omar Ajaja, Hassan Masaoudi and Anta Saw (U-series, French National Museum of Natural History), and they are gratefully thanked. The ESR spectrometer of the French National Museum of Natural History was bought with the financial support of the Sesame Iˆlede-France programme. Finally, the authors thank the anonymous referee for the valuable comments, and Phillip Gibbard (Department of Geography, University of Cambridge) for the revision of the manuscript. Editorial handling by: R. Gru¨n Appendix A. Supplementary Materials Supplementary data associated with this article can be found in the online version at doi:10.1016/j.quageo. 2006.04.012. References Adamiec, G., Aitken, M., 1998. Dose-rate conversion factors: update. Ancient TL 16, 37–50. Antoine, P., 1990. Chronostratigraphie et environnement du Pale´olithique du bassin de la Somme, vol. 2. Publications du Centre d’Etudes et de Recherches Pre´historiques, Lille, 231pp. Antoine, P., 1994. The Somme Valley terrace system, Northern France; a model of river response to Quaternary climatic variations since 800,000 BP. Terra-Nova 6, 453–464. Antoine, P., 2001. La stratigraphie de la se´quence fluviatile de Cagny-La Garenne II (Somme). In: Tuffreau, A. (Ed.), L’Acheule´en dans la valle´e de la Somme et Pale´olithique moyen dans le Nord de la France: donne´es re´centes, vol. 6. Publications du Centre d’Etudes et de Recherches Pre´historiques, Lille, pp. 45–49. Antoine, P., Limondin-Lozouet, N., 2004. Identification of MIS 11 Interglacial tufa deposit in the Somme Valley (France): new results from the Saint-Acheul fluvial sequence. Quaternaire 15, 41–52.

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