INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND Meas.Sci.Technol. 14 (2003) 1487–1492 PII: S0957-0233(03)57428-8 Direct radiocarbon dating of prehistoric paintings by accelerator mass spectrometry

Hel´ ene` Valladas

Laboratoire des Sciences du Climat et de l’Environnement, Unitemix´ te CEA-CNRS, Batimentˆ 12, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France

Received 13 December 2002, accepted for publication 26 February 2003 Published 29 July 2003 Online at stacks.iop.org/MST/14/1487 Abstract Advances in radiocarbon dating by accelerator mass spectrometry now make it possible to date prehistoric cave paintings by sampling the itself instead of relying on dates derived from miscellaneous prehistoric remains recovered in the vicinity of the paintings. Presented below are some radiocarbon dates obtained at the ‘Laboratoire des Sciences du Climat et de l’Environnement’ for charcoal used in the execution of prehistoric paintings decorating two French : Cosquer and Chauvet. The presentation of the dateswill be preceded by a short discussion of the experimental procedure used in our laboratory (pigment sampling, chemical treatment, etc). The ages obtained so far have shown that the art of appeared early in the Upper Palaeolithic period, much earlier than previously believed. The high artistic quality of the earliest paintings underlines the importance of absolute chronology in any attempt to study the evolution of .

Keywords: AMS carbon-14 dating, prehistoric cave paintings, charcoal, Upper Palaeolithic, ,

1. Introduction time of execution of the painting. Did the artists use freshly Until quite recently cave paintings were dated according to made charcoal, leftover material from prior cave occupation stylistic criteria loosely associated with dates obtained for (Bednarik 1994) or a mixture of charcoals of several origins? archaeological remains found in the vicinity of decorated The possibility that fossil charcoal could have been used surfaces. About two decades ago radiocarbon dating was cannot be excluded either (Bednarik 1994). To compound revolutionized when accelerator mass spectrometric (AMS) the problems there is also the possibility that some paintings techniques allowed for the dating of organic samples weighing were retouched by a later generation of artists. Some of as little as 1 mg. Paintings done in charcoal could now be these questions can be answered by examiningthenature sampled without visibly damagingthe paintings. In addition and composition of the pigment under a scanning electron to wood charcoal, which has received the most attention (Rowe microscope, others require meticulous in situ examination of 2001, Valladas et al 1992, Igler et al 1994), beeswax (Nelson thepigment layer with a good magnifying glass. et al 1995) and plant residues (Watchman and Cole 1993, The sampling, the first step of the dating process, is done Hedges et al 1998) used in the paintings have also been dated. after preliminary analysis has revealed that the black pigment Below we present theapproach used at the Laboratoire des contains charcoal. In some instances the wood could be Sciences du Climat et de l’Environnement (LSCE) to date identified as belonging to the species Pinus. Palaeolithic charcoal drawings and paintings and discuss the To protect the visual integrity of the drawings, pigment results obtained in two French caves. is scraped from rock cracks or from the thickest layers. If the charcoal is preserved and thick enough, it is best 2. Problems peculiar to the dating of prehistoric to collect the sample from a limited area of a figure. When possible, two or more samples from different portions of a The first problem, to which there is no simple scientific answer, painting should be taken in order to get several dates and check has to do with the question of the age of the charcoal at the the age spread. Otherwise, if the pigment layer is too thin and

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SAMPLE PREPARATION The great majority of contaminants introduce carbon younger than the pigment charcoal and if not eliminated by a proper

Acid (HCl, 0.5M) treatment will produce a date more recent than the true one. High-purity H2O 3. Sample treatment Deposition on pre-cleaned quartz filter The sample pre-processing used to date the Palaeolithic Basictreatment charcoal drawings and paintings has been described in recent Na4P2O7 ,NH3 (aq), NaOH publications (Valladas et al 1999, 2001a). The treatment of charcoal varies in intensity according to the sample size. It involves asuccession of ‘acid–base–acid’ treatments which Charcoal Humic fraction Acid (HCl, 1M) Precipitation and first dissolve the carbonates that may have come from the H2O collection on a limestone wall or ground water, then humic acids arising from quartz filter the transformation of organic matter, and bacteria or other living microorganisms. A schematic representation of the Drying Drying treatment steps is shown in figure 1. The residue from the initial acid bath is retained on a Heating, 1hr at 300 - 320°CHeating, 1hr at 280°C in a stream of oxygen in a stream of oxygen pre-cleaned quartz-frit filter and subjected to the subsequent basic treatment. This treatment, gentle at first, is increased in intensity according to the fragility of the sample. One begins with a dilute solution of sodium pyrophosphate whose concentration is increased progressively. Aqueous ammonia Sample transferred to a combustion tube containing of gradually increased concentration is used next, followed CuO and Ag wire and sealed under vacuum by sodium hydroxide treatment in cases of alkali-resistant pigments. As a rule, the treatment stops when the filtrate Oxidation at 850°Cfor 7 hours becomes highly coloured. The coloration suggests that not only have the outer grain layers been stripped, but that a good Purification and determination of CO2 pressure fraction of the original charcoal has passed into solution. If the Catalytic reduction to graphite treatment is not interrupted in time, no charcoal might be left for dating. The remaining charcoal grains are washed again Compression to a pellet used as target with aqueous HCl. After the chemical treatment, the purified charcoal or humic acids collected on another quartz filter are AMSmeasurement heated in a stream of oxygen for about an hour between 280 and 320 ◦Ctoremove some additional organic contaminants. Figure 1. Adiagram illustrating the experimental procedure. Whatever remains is oxidized to carbon dioxide, then reduced to graphite and compressed into pellets for the has to be scraped from several points, sometimes far apart, one accelerator (Arnold et al 1987). The purification process gets an average date. The collected samples, usually weighing eliminates more than 90% of the original mass leaving us with from 10 to 100 mg, often contain calcite grains or clay from pellets usually containing from 0.5 to 1 mg of carbon (tables 1 the rock face, in addition to wood charcoal. To-date, calcium and 2, column 3). oxalate, which can be an important contaminant (Watchman 1990, Russ et al 1996, Hedges et al 1998) of outdoor parietal This procedure has been tested on a piece of charcoal art in semi-arid regions (particularly in the vicinity of cacti, from an Upper Palaeolithic layer (). The piece was lichens, for example), has not been detected in paintings inside broken into several subsamples, of which some were subjected West European caves. to very strong chemical treatment, others treated in the same Amajor problem, inherent in all methods of radiocarbon wayasthe pigment samples and still others subjected to dating, is the possible presence of extraneous carbon (Hedges chemical but not thermal treatment. We found that a strong et al 1989). Exposure of a paintings renders a cave painting’s chemical treatment did not give significantly different results pigment particularly vulnerable to contamination. The degree from the weaker treatment usually reserved for the paintings, of contamination depends on when and how the caves were and that the thermal treatment did eliminate some additional discovered. For obvious reasons, caves sealed until recently contamination by more recent carbon, since the samples thus and not open to the public should give the most reliable dates. treated gave slightly older ages. The results also confirmed the In caves frequently visited in the past the most common good reproducibility of our protocol (Valladas et al 2001a). organic contaminants come from contact with visitors’ hands, The extent of contamination by modern carbon during cloth fibres, acetylene lamp soot, etc. Moreover, all sample preparation was determined by subjecting several cavesharbour a variety of microorganisms whose growth is charcoals over 100 000 years old (‘blank’ sample without 14C) stimulated by emanations from the body (Laiz et al to the same treatment as our pigment samples. This yielded 1999). One must also consider contamination by carbonic, background contamination that was used to make a suitable humic or fulvic acids transported by underground waters. correction to the calculated pigment ages. 1488 Direct radiocarbon dating of prehistoric cave paintings by accelerator mass spectrometry Table 1. Radiocarbon dates for prehistoric paintings at the Cosquer cave. Humic acid dates are written in italics. For pictures of the dated paintings see Clottes and Courtin (1994). Ly = Lyon, France; GifA = Gif-sur-Yvette, France. Dateable Date Error (year) Reference carbon (mg) year (BP) 1 sigma Horse 1 GifA 92416 1.56 18 840 250 GifA 92417 0.94 18 820 310 GifA 92422 1.23 18 760 220 Feline GifA 92418 1.52 19 200 240 Bison 1 GifA 92419 0.64 18 010 200 GifA 92492 1.22 18 530 190 GifA 92423 0.26 16 390 260 Megaloceros 1GifA 95135 1.25 19 340 200 GifA 95365 0.12 13 460 330 Horse 7 GifA 98186 0.84 19 720 210 GifA 98196 0.29 19 740 340 Deer GifA 98188 0.25 19 290 340 Star mark GifA 96075 0.87 17 800 160 Horse 5 GifA 96072 0.84 24 730 300 Hand 12 GifA 95358 0.63 24 840 340 GifA 95372 0.26 23 150 620 Bison 2 GifA 96069 1.79 26 250 350 GifA 95195 2.04 27 350 430 GifA 95308 0.23 23 080 640 Hand 1 GifA 92409 0.86 27 110 430 GifA 92491 1.59 27 110 400 GifA 92424 0.44 26 180 370 Hand 19 GifA 96073 1.3 27 740 410 Oval mark GifA 96074 2.12 28 370 440 Soil charcoal Ly-5558 18 440 440 Soil charcoal GifA 92348 2.39 20 370 260 Soil charcoal GifA 92349 2.17 26 360 440 Soil charcoal GifA 92350 2.06 27 870 470

Table 2. Radiocarbon dates for prehistoric paintings at the Chauvet cave (Clottes et al 1995). Humic acid dates are written in italics. (∗Thirteen other dates have been obtained by the LSCE on charcoal samples collected on the ground of the Megaceros Gallery; 11 of them range between 29 700 and 32 900 and the two other between 25 400 and 26 600 years BP.) Dateable Date Error (year) Reference carbon (mg) year (BP) 1 sigma Hillaire Chamber Right rhinoceros GifA 95132 1.4 32 410 720 GifA 95133 1.22 30 790 600 Left rhinoceros GifA 95126 0.8 30 940 610 Running cow GifA 96065 0.69 30 230 530 Horse GifA 98157 20 790 340 GifA 98160 0.27 29 670 950 Torch scraping 1 GifA 95129 2.3 26 980 410 GifA 95130 1.76 26 980 420 GifA 95158 0.308 25 700 850 Megaceros Gallery* Megaloceros GifA 96063 0.85 31 350 620 Soil charcoal Ly-6878 5.000 29 000 400 Salle du Fond Bison GifA 95128 0.83 30 340 570 GifA 95155 0.42 30 800 1.500 Cierge Chamber Torch scraping 2 GifA 95127 1.22 26 120 400 GifA 99081 1.73 26 230 280 Crˆane Chamber Under bear skull GifA 99809 2.27 32 360 490 GifA 99810 1.12 31 390 420 GifA 99811 2.21 32 600 490

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Figure 2. Schematic layout of the Gif-sur-Yvette AMS apparatus—‘Tandetron’ (Duplessy and Arnold 1985).

4. Description of tandem mass accelerator (c) The final separation, detection and counting of 14Cions in the high-energy section: the ion beam which leaves Figure 2 shows a schematic diagram of the Gif-sur-Yvette the tandem accelerator is then re-focused and enters an tandem accelerator (UMS 2004, CNRS-CEA) which is used electrostatic deflector then another magnetic spectrometer to measure 14C/12Cand13C/12Cisotopic ratios (foradetailed which allows the ions to be separated according to their description see Duplessy and Arnold (1985) and Arnold energy, their mass and their charge. Finally the beam et al (1987, 1989)). which is mainly made of 14Cions with a small fraction The apparatus has three major components: of 13Cand12Cions enters a gas-filled (argon–methane) (a) Low-energy ion source: positive caesium ions that are ion chamber, where carbon ions lose their energy by gas focused on the carbon sample. The caesium ion beam collision; the 14Cions are easily detected and separated bombards the sample and produces both molecular and from the unwanted particles. atomic negative ions. This step is followed by a first mass separation so that ion beams of mass 12, 13 and 14 To get the sample age, as in the case of conventional are well separated. As a result, the mass 12 consists of radiation-counter radiocarbon dating, one compares the − only the ions 12C ,butthemass-13 beam comprises both calculated 14C/12Cand13C/12Cisotope ratios with those 13 − 12 − 14 − C and CH and the mass-14 beam comprises C obtained for reference standards of known age. 13 − 12 − together with unwanted molecules such as CH , CH2 , Two types of operation are needed to determine the carbon 9 11 whichare roughly 10 –10 times more abundant than the isotopic composition of an unknown sample: 14C− ions to be measured. (b) Molecular elimination with the tandem accelerator: the (i) measurement of the number of 14Cions; mass-14 ion beam is then injected in the tube of the tandem (ii) successive introduction of mass 12 and 13 beams into the accelerator where the negative ions are first accelerated. accelerator without changing its tuning and measurements Asmallstream of argon is injected in the central of the 12C3+ and 13C3+ beams collected by Faraday cups. section of the instrument so that polyatomic molecules are dissociated by gas collisions and transformed into a The two sets of measurements are repeated until sufficient mixture of positive ions H, 12C, 13Cand14Cions (most counts have been obtained for the required statistics and the probablecharge: +3) which are accelerated in the final apparent 14C/12Cand13C/12Cisotopic ratios to be determined. section of the tandem. Their energy is a function of both Without changing the accelerator tuning, the apparent isotopic mass and charge so that each particle has a unique energy ratios are measured for the reference sample and also for the signature. blank.

1490 Direct radiocarbon dating of prehistoric cave paintings by accelerator mass spectrometry 5. Discussion with charcoal coming from the same tree (or contemporaneous trees). The paintings can be grouped into two time periods Whenever enough material is available, multiple datings are about 10 000 years apart (table 1). This fact is in agreement done on the same drawing to test the reproducibility and with the conclusions based on the observation of the decorated coherence of the results, and on the humic acid fraction wall (Clottes and Courtin 1994). The first group consisting obtained during the basic treatment to see how much the initial of negative hands, a bison and an oval sign were dated to pigment sample might have been contaminated (Batten et al between 28 000 and 27 000 years ago, during the 1986). In real life situations one encounters three types of period. Except for one horse, the other animals and the star- cases, illustrated by the dates obtained for the Cosquer and like sign were dated to between 19 700 and 18 500 years BP, Chauvet cave drawings listedintables1and2(theages during the Solutrean period. Taking into account the amplitude obtained with the humic acid fractions are given in italics). of the errors, it is not possible to conclude if each of the two In case 1 the purified charcoal and the humic acid fraction painting phases lasted a brief period of time or stretched over yield similar results (horse 1, hand R7 in the Cosquer cave, centuries. On the other hand, until more drawings are sampled bison and torch scraping 1 in the Chauvet cave). While good we cannot tell if a horse and one stencilled hand, which seem agreement between the two sets of dates generally increases to date to about 25 000 BP represent an intermediate period of one’s confidence in the reliability of the dates, one can never decoration or are the result of more extensive contamination. exclude a remote possibility that both fractions may have The time span that separates the two bison (1 and 2) which somehow been similarly contaminated. are similar and depicted on the same wall is rather surprising. When the two fractions yield different dates the humic This fact can be interpreted in at least two ways: either the acid fraction, which one expects to contain more contaminants, stylistic conventions were maintained over extremely long time tends to give a lower figure—case 2 (see bisons 1 and 2, periods, or the older one was not done with fresh charcoal Megaloceros 1atCosquer cave). In such cases the age of the (Clottes et al 1997). To help us choose between these purified charcoal is more trustworthy. The less common case 3 alternatives additional dates will be needed. It is noteworthy refers to examples where the humic acid fraction yields an age that charcoal fragments collected on the ground nearby also fall greater than the purified charcoal (see the horse in the Chauvet within twodistinct timeintervals: 18 000–20 000 and 26 000– cave and some Altamira bisons (Valladas et al 2001a)). In 28 000 years, respectively. general, we have found older dates for a given sample to Chauvet cavewasdiscovered inArdeche` in December, be more reliable after noting how much more frequent was 1994 (Clottes 2001). So far about 40 dates have been obtained contamination by recent carbon and consequent age-reduction. (Clottes et al 1995, Valladas et al 2001b): twelve on pigments Exposed pigments can be polluted by organic materials, some from six drawings from different sections of the cave, two of which can resist the chemical treatment meant to eliminate for charcoal scrapings left by visitors who rubbed their torches them. Some samples are so small and fragile that if the solid against the wall and the rest for the charcoal found in abundance component is not to dissolve completely the purification has to on the ground (table 2). The great majority of dates can be less rigorous. In such cases the humic acid fraction, which be grouped into two tight clusters representing two time- consists of parts of original charcoal that were dissolved in periods thousands of years apart (29 000–32 500 and 26 000 an alkaline environmentandre-precipitated, will give a more and 27 000 years BP respectively). The animal representations correct greater age. were dated to between about 32 000 and 30 500 years BP, within theAurignacian period. The torch scrapings were about 6. Results 27 000 years old, a date not surprising if one notes that in one The tabulated ages obtained for drawings in the Cosquer case the torch was scraped against a layer of calcite deposited and Chauvet caves show what type of important information on top of a drawing! So far, there is no drawing dated to can nowadays be obtained by the use of AMS radiocarbon this second period of human occupation. Most of the ages dating. These caves, which are currently being studied obtained for charcoal collected on the ground surface ranged by multidisciplinary teams, provided optimum sampling from 26 000 to 32 000 years BP, suggesting the existence of at conditions, so that more than one sample of certain figures least two major episodes of human intrusion before the cave could be dated. It was also possible to compare the ages of was sealed off by a rock-fall. Thecoherence of the dates different fractions of a given scraping to see if the dates are obtained for the drawings of the Cosquer and Chauvet caves coherent. Moreover, by dating some of the abundant charcoal suggests that the samples were not seriously contaminated. fragments found on the ground near the drawings we were Such satisfactory results can be attributed to the great number able to determine the periods of human presence in the cave, of samples available for dating and to the fact that the cave was apresence that may have been related to artistic activities. sealed by a natural phenomenon during the period. The Cosquer cave, whose entrance is now 40 m below sea level, is richly decorated inrockpaintings and carvings 7. Conclusion (Clottes et al 1992, 1997). About 24 dates (table 1) were obtained for 13 charcoal drawings including animal figures, Even though the direct dating of cave paintings is still in negative hands and geometric signs. Some pigment samples its infancy, the few dates reported so far have convinced art scraped from several points of a figure were divided in two historians of the need to revise prior ideas on the evolution of and the two halves were treated and dated separately (horse 1, prehistoric art. The Chauvet cave, in particular, indicates that bisons 1 and 2); they yielded compatible ages, suggesting that theories assuming a linear progression from simple to more thesepaintings were done within a relatively short time period complex composition have to be discarded and that, as early

1491 HValladas as the period, some artists had mastered design Hedges R E M, Bronk C R, Van Klinken G J, Pettitt P B, and composition (Clottes et al 1995). The AMS radiocarbon Nielsen-Marsh C, Etchegoyen A, Fernandez Niello J O, dating also makes it possible to establish distinct periods of Boschin M T and Llamazares A M 1998 Methodological issues in the radiocarbon dating of rock paintings Radiocarbon artistic activity within any one cave. 40 35–44 At the present time the AMS technique does not allow for Hedges R E M, Law I A, Bronk C R and Housley R A 1989 reliable dating of Palaeolithic drawings done in media other TheOxford accelerator mass spectrometry facility: than charcoal. Unfortunately,themedia most commonly used technical developments in routine dating Archaeometry 31 in the execution of prehistoric paintings are iron oxide and 99–113 Igler W, Dauvois M, Hyman M, Menu M, Rowe M, Vezian J and manganese oxide (Menu 2000). There is hope that some of Walter P 1994 Datation radiocarbone de deux figures parietales´ these may be dateable in the future, since chemical analyses de la grotte du Portel (Commune de Loubens, Ariege)` Bull. have revealed that organic binders of plant or animal origin Soc. Pr´ehist. Ari`ege-Pyr´en´ees XLIX 231–6 were occasionally used with mineral pigments (Pepe et al Laiz L, Groth I, Gonzales I and Saiz-Jimenez C 1999 1991). The quantities are usually tiny, but improved chemical Microbiological study of the dripping waters in Altamira cave (Santillana del Mar, ) J. Microbiol. Methods 36 techniques will undoubtedly allow us one day to separate, 129–38 purify and date such binders. Menu M 2000 Le savoir faire des premiers peintres La Recherche, Hors-S´erie 4 56–8 Nelson D E, Chaloupka G, Chippindale C, Alderson M S and References Southon J R 1995 Radiocarbon dates for beeswax figures in the prehistoric of Northern Australia Archaeometry 37 Arnold A, Bard E, Maurice P, Valladas H and Duplessy J C 1989 151–6 14Cdating with the Gif-sur-Yvette Tandetron accelerator: Pepe C, Clottes J, Menu M and Walter P 1991 Le liant des peintures status report and study of isotopic fractionations in the sputter prehistoriques´ ariegeoises´ C. R. Acad. Sci. Paris II 312 929–34 ion source Radiocarbon 31 284–9 Rowe MW2001 Dating by AMS analysis Handbook of Rock Art Arnold M, Bard E, Maurice P and Duplessy J C 1987 14Cdating Research ed D S Whitley (Walnut Creek, CA: AltaMira) pp with the Gif-sur-Yvette Tandetron accelerator: status report 139–66 Nucl. Instrum. Methods B 29 120–3 Russ J, Palma R L, Loyd D H, Boutton T W and Coy M A 1996 Batten R J, Gillespie R, Gowlett J A J and Hedges R E M 1986 The Origin of the whewellite-rich rock crust in the lower Pecos AMS dating of separate fractions in archaeology Radiocarbon region of Southwest Texas and its significance to paleoclimate 28 698–701 reconstructions Quat. Res. 46 27–36 Bednarik R 1994 About rock art dating Int. Newsletter Rock Art 7 Valladas H, Cachier H, Maurice P, Bernaldo De Quiros F, Clottes J, 16–18 Cabrera-Valdes V, Uzquiano P and Arnold M 1992 Direct Clottes J (ed) 2001 La Grotte Chauvet. L’art des Origines (Paris: radiocarbon dates for prehistoric paintings at the Altamira, El Seuil) Castillo and Niaux caves Nature 357 68–70 Clottes J, Chauvet J M, Brunel-Deschamps E, Hillaire C, Valladas H, Tisnerat N, Cachier H and Arnold M 1999 Datation Daugas J P, Arnold M, Cachier H, Evin J, Fortin P, directe des peintures prehistoriques´ par la methode´ du carbone Oberlin C, Tisnerat N and Valladas H 1995 Les peintures 14 en spectrometrie´ de masse par accel´ erateur´ Rev. paleolithiques´ de la grotte Chauvet-Pont d’Arc, a` Arch´eom´etrie Suppl. 1999 39–44 Vallon-Pont-d’Arc (Ardeche,` France): datations directes et Valladas H, Tisnerat-Laborde N, Cachier H, Arnold M, indirectes par la methode´ du radiocarbone C. R. Acad. Sci., Bernaldo De Quiros F, Cabrera-Valdes V, Clottes J, Courtin J, Paris II 320 1133–40 Fortea-Perez J, Gonzales-Sainz C and Moure-Romanillo A Clottes J and Courtin J 1994 La Grotte Cosquer (Paris: Seuil) 2001a Radiocarbon AMS dates for cave paintings Clottes J, Courtin J, Collina-Girard J, Arnold M and Radiocarbon 43 977–86 Valladas H 1997 News from Cosquer cave; climatic studies, Valladas H, Clottes J, Geneste J M, Garcia M, Arnold M, recording, sampling, dates Antiquity 71 321–6 Cachier H and Tisnerat-Laborde N 2001b Evolution of Clottes J, Courtin J, Valladas H, Cachier H, Mercier N and prehistoric cave art Nature 413 479 Arnold M 1992 La grotte Cosquer datee´ Bull. Soc. Pr´ehist. Watchman A 1990 A summary of occurrences of oxalates-rich Fran¸caise 89 230–4 crusts in Australia Rock Art Res. 7 44–50 Duplessy J C and Arnold M 1985 Radiocarbon dating by accelerator Watchman A and Cole N 1993 Accelerator radiocarbon dating of mass spectrometry Nuclear Methods of Dating ed E Roth and plant-fibre binders in rock paintings from northeastern BPoty (Dordrecht: Kluwer) pp 437–53 Australia Antiquity 67 355–8

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