INSTITUTE OF PHYSICS PUBLISHING
MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 14 (2003) 1487–1492
PII: S0957-0233(03)57428-8
Direct radiocarbon dating of prehistoric
cave paintings by accelerator mass
spectrometry
Hélène Valladas
Laboratoire des Sciences du Climat et de l’Environnement, Unité mixte CEA-CNRS,
Bâtiment 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 pigment 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 caves: Cosquer and Chauvet. The presentation of the
dates will 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 cave painting 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 prehistoric art.
Keywords: AMS carbon-14 dating, prehistoric cave paintings, charcoal,
Upper Palaeolithic, Cosquer cave, Chauvet cave
1. Introduction
Until quite recently cave paintings were dated according to
stylistic criteria loosely associated with dates obtained for
archaeological remains found in the vicinity of decorated
surfaces. About two decades ago radiocarbon dating was
revolutionized when accelerator mass spectrometric (AMS)
techniques allowed for the dating of organic samples weighing
as little as 1 mg. Paintings done in charcoal could now be
sampled without visibly damaging the paintings. In addition
to wood charcoal, which has received the most attention (Rowe
2001, Valladas et al 1992, Igler et al 1994), beeswax (Nelson
et al 1995) and plant residues (Watchman and Cole 1993,
Hedges et al 1998) used in the paintings have also been dated.
Below we present the approach used at the Laboratoire des
Sciences du Climat et de l’Environnement (LSCE) to date
Palaeolithic charcoal drawings and paintings and discuss the
results obtained in two French caves.
2. Problems peculiar to the dating of prehistoric
pigments
The first problem, to which there is no simple scientific answer,
has to do with the question of the age of the charcoal at the
0957-0233/03/091487+06$30.00 © 2003 IOP Publishing Ltd
time of execution of the painting. Did the artists use freshly
made charcoal, leftover material from prior cave occupation
(Bednarik 1994) or a mixture of charcoals of several origins?
The possibility that fossil charcoal could have been used
cannot be excluded either (Bednarik 1994). To compound
the problems there is also the possibility that some paintings
were retouched by a later generation of artists. Some of
these questions can be answered by examining the nature
and composition of the pigment under a scanning electron
microscope, others require meticulous in situ examination of
the pigment layer with a good magnifying glass.
The sampling, the first step of the dating process, is done
after preliminary analysis has revealed that the black pigment
contains charcoal. In some instances the wood could be
identified as belonging to the species Pinus.
To protect the visual integrity of the drawings, pigment
is scraped from rock cracks or from the thickest layers.
If the charcoal is well preserved and thick enough, it is best
to collect the sample from a limited area of a figure. When
possible, two or more samples from different portions of a
painting should be taken in order to get several dates and check
the age spread. Otherwise, if the pigment layer is too thin and
Printed in the UK
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H Valladas
The great majority of contaminants introduce carbon younger
than the pigment charcoal and if not eliminated by a proper
treatment will produce a date more recent than the true one.
SAMPLE PREPARATION
Acid (HCl, 0.5M)
High - purity H2O
3. Sample treatment
Deposition on pre-cleaned quartz filter
Basic treatment
Na4P2O7 , NH3 (aq), NaOH
Charcoal
Acid (HCl, 1M)
H2O
Humic fraction
Precipitation and
collection on a
quartz filter
Drying
Drying
Heating, 1hr at 300 - 320°C
in a stream of oxygen
Heating, 1hr at 280°C
in a stream of oxygen
Sample transferred to a combustion tube containing
CuO and Ag wire and sealed under vacuum
Oxidation at 850°C for 7 hours
Purification and determination of CO2 pressure
Catalytic reduction to graphite
Compression to a pellet used as target
AMS measurement
Figure 1. A diagram illustrating the experimental procedure.
has to be scraped from several points, sometimes far apart, one
gets an average date. The collected samples, usually weighing
from 10 to 100 mg, often contain calcite grains or clay from
the rock face, in addition to wood charcoal. To-date, calcium
oxalate, which can be an important contaminant (Watchman
1990, Russ et al 1996, Hedges et al 1998) of outdoor parietal
art in semi-arid regions (particularly in the vicinity of cacti,
lichens, for example), has not been detected in paintings inside
West European caves.
A major problem, inherent in all methods of radiocarbon
dating, is the possible presence of extraneous carbon (Hedges
et al 1989). Exposure of a paintings renders a cave painting’s
pigment particularly vulnerable to contamination. The degree
of contamination depends on when and how the caves were
discovered. For obvious reasons, caves sealed until recently
and not open to the public should give the most reliable dates.
In caves frequently visited in the past the most common
organic contaminants come from contact with visitors’ hands,
cloth fibres, acetylene lamp soot, etc.
Moreover, all
caves harbour a variety of microorganisms whose growth is
stimulated by emanations from the human body (Laiz et al
1999). One must also consider contamination by carbonic,
humic or fulvic acids transported by underground waters.
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The sample pre-processing used to date the Palaeolithic
charcoal drawings and paintings has been described in recent
publications (Valladas et al 1999, 2001a). The treatment of
charcoal varies in intensity according to the sample size. It
involves a succession of ‘acid–base–acid’ treatments which
first dissolve the carbonates that may have come from the
limestone wall or ground water, then humic acids arising from
the transformation of organic matter, and bacteria or other
living microorganisms. A schematic representation of the
treatment steps is shown in figure 1.
The residue from the initial acid bath is retained on a
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
of gradually increased concentration is used next, followed
by sodium hydroxide treatment in cases of alkali-resistant
pigments. As a rule, the treatment stops when the filtrate
becomes highly coloured. The coloration suggests that not
only have the outer grain layers been stripped, but that a good
fraction of the original charcoal has passed into solution. If the
treatment is not interrupted in time, no charcoal might be left
for dating. The remaining charcoal grains are washed again
with aqueous HCl. After the chemical treatment, the purified
charcoal or humic acids collected on another quartz filter are
heated in a stream of oxygen for about an hour between 280
and 320 ◦ C to remove some additional organic contaminants.
Whatever remains is oxidized to carbon dioxide, then
reduced to graphite and compressed into pellets for the
accelerator (Arnold et al 1987). The purification process
eliminates more than 90% of the original mass leaving us with
pellets usually containing from 0.5 to 1 mg of carbon (tables 1
and 2, column 3).
This procedure has been tested on a piece of charcoal
from an Upper Palaeolithic layer (Solutrean). The piece was
broken into several subsamples, of which some were subjected
to very strong chemical treatment, others treated in the same
way as the pigment samples and still others subjected to
chemical but not thermal treatment. We found that a strong
chemical treatment did not give significantly different results
from the weaker treatment usually reserved for the paintings,
and that the thermal treatment did eliminate some additional
contamination by more recent carbon, since the samples thus
treated gave slightly older ages. The results also confirmed the
good reproducibility of our protocol (Valladas et al 2001a).
The extent of contamination by modern carbon during
sample preparation was determined by subjecting several
charcoals over 100 000 years old (‘blank’ sample without 14 C)
to the same treatment as our pigment samples. This yielded
background contamination that was used to make a suitable
correction to the calculated pigment ages.
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.
Reference
Horse 1
Feline
Bison 1
Megaloceros 1
Horse 7
Deer
Star mark
Horse 5
Hand 12
Bison 2
Hand 1
Hand 19
Oval mark
Soil charcoal
Soil charcoal
Soil charcoal
Soil charcoal
GifA 92416
GifA 92417
GifA 92422
GifA 92418
GifA 92419
GifA 92492
GifA 92423
GifA 95135
GifA 95365
GifA 98186
GifA 98196
GifA 98188
GifA 96075
GifA 96072
GifA 95358
GifA 95372
GifA 96069
GifA 95195
GifA 95308
GifA 92409
GifA 92491
GifA 92424
GifA 96073
GifA 96074
Ly-5558
GifA 92348
GifA 92349
GifA 92350
Dateable
carbon (mg)
Date
year (BP)
Error (year)
1 sigma
1.56
0.94
1.23
1.52
0.64
1.22
0.26
1.25
0.12
0.84
0.29
0.25
0.87
0.84
0.63
0.26
1.79
2.04
0.23
0.86
1.59
0.44
1.3
2.12
18 840
18 820
18 760
19 200
18 010
18 530
16 390
19 340
13 460
19 720
19 740
19 290
17 800
24 730
24 840
23 150
26 250
27 350
23 080
27 110
27 110
26 180
27 740
28 370
18 440
20 370
26 360
27 870
250
310
220
240
200
190
260
200
330
210
340
340
160
300
340
620
350
430
640
430
400
370
410
440
440
260
440
470
2.39
2.17
2.06
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
carbon (mg)
Date
year (BP)
Error (year)
1 sigma
GifA 95132
GifA 95133
GifA 95126
GifA 96065
GifA 98157
GifA 98160
GifA 95129
GifA 95130
GifA 95158
1.4
1.22
0.8
0.69
0.27
2.3
1.76
0.308
32 410
30 790
30 940
30 230
20 790
29 670
26 980
26 980
25 700
720
600
610
530
340
950
410
420
850
GifA 96063
Ly-6878
0.85
5.000
31 350
29 000
620
400
GifA 95128
GifA 95155
0.83
0.42
30 340
30 800
570
1.500
GifA 95127
GifA 99081
1.22
1.73
26 120
26 230
400
280
GifA 99809
GifA 99810
GifA 99811
2.27
1.12
2.21
32 360
31 390
32 600
490
420
490
Reference
Hillaire Chamber
Right rhinoceros
Left rhinoceros
Running cow
Horse
Torch scraping 1
Megaceros Gallery*
Megaloceros
Soil charcoal
Salle du Fond
Bison
Cierge Chamber
Torch scraping 2
Hearth
Crâne Chamber
Under bear skull
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H Valladas
Figure 2. Schematic layout of the Gif-sur-Yvette AMS apparatus—‘Tandetron’ (Duplessy and Arnold 1985).
4. Description of tandem mass accelerator
Figure 2 shows a schematic diagram of the Gif-sur-Yvette
tandem accelerator (UMS 2004, CNRS-CEA) which is used
to measure 14 C/12 C and 13 C/12 C isotopic ratios (for a detailed
description see Duplessy and Arnold (1985) and Arnold
et al (1987, 1989)).
The apparatus has three major components:
(a) Low-energy ion source: positive caesium ions that are
focused on the carbon sample. The caesium ion beam
bombards the sample and produces both molecular and
atomic negative ions. This step is followed by a first
mass separation so that ion beams of mass 12, 13 and 14
are well separated. As a result, the mass 12 consists of
only the ions 12 C− , but the mass-13 beam comprises both
13 −
C and 12 CH− and the mass-14 beam comprises 14 C−
together with unwanted molecules such as 13 CH− , 12 CH2− ,
which are roughly 109 –1011 times more abundant than the
14 −
C ions to be measured.
(b) Molecular elimination with the tandem accelerator: the
mass-14 ion beam is then injected in the tube of the tandem
accelerator where the negative ions are first accelerated.
A small stream of argon is injected in the central
section of the instrument so that polyatomic molecules
are dissociated by gas collisions and transformed into a
mixture of positive ions H, 12 C, 13 C and 14 C ions (most
probable charge: +3) which are accelerated in the final
section of the tandem. Their energy is a function of both
mass and charge so that each particle has a unique energy
signature.
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(c) The final separation, detection and counting of 14 C ions
in the high-energy section: the ion beam which leaves
the tandem accelerator is then re-focused and enters an
electrostatic deflector then another magnetic spectrometer
which allows the ions to be separated according to their
energy, their mass and their charge. Finally the beam
which is mainly made of 14 C ions with a small fraction
of 13 C and 12 C ions enters a gas-filled (argon–methane)
ion chamber, where carbon ions lose their energy by gas
collision; the 14 C ions are easily detected and separated
from the unwanted particles.
To get the sample age, as in the case of conventional
radiation-counter radiocarbon dating, one compares the
calculated 14 C/12 C and 13 C/12 C isotope ratios with those
obtained for reference standards of known age.
Two types of operation are needed to determine the carbon
isotopic composition of an unknown sample:
(i) measurement of the number of 14 C ions;
(ii) successive introduction of mass 12 and 13 beams into the
accelerator without changing its tuning and measurements
of the 12 C3+ and 13 C3+ beams collected by Faraday cups.
The two sets of measurements are repeated until sufficient
counts have been obtained for the required statistics and the
apparent 14 C/12 C and 13 C/12 C isotopic ratios to be determined.
Without changing the accelerator tuning, the apparent isotopic
ratios are measured for the reference sample and also for the
blank.
Direct radiocarbon dating of prehistoric cave paintings by accelerator mass spectrometry
5. Discussion
Whenever enough material is available, multiple datings are
done on the same drawing to test the reproducibility and
coherence of the results, and on the humic acid fraction
obtained during the basic treatment to see how much the initial
pigment sample might have been contaminated (Batten et al
1986). In real life situations one encounters three types of
cases, illustrated by the dates obtained for the Cosquer and
Chauvet cave drawings listed in tables 1 and 2 (the ages
obtained with the humic acid fractions are given in italics).
In case 1 the purified charcoal and the humic acid fraction
yield similar results (horse 1, hand R7 in the Cosquer cave,
bison and torch scraping 1 in the Chauvet cave). While good
agreement between the two sets of dates generally increases
one’s confidence in the reliability of the dates, one can never
exclude a remote possibility that both fractions may have
somehow been similarly contaminated.
When the two fractions yield different dates the humic
acid fraction, which one expects to contain more contaminants,
tends to give a lower figure—case 2 (see bisons 1 and 2,
Megaloceros 1 at Cosquer cave). In such cases the age of the
purified charcoal is more trustworthy. The less common case 3
refers to examples where the humic acid fraction yields an age
greater than the purified charcoal (see the horse in the Chauvet
cave and some Altamira bisons (Valladas et al 2001a)). In
general, we have found older dates for a given sample to
be more reliable after noting how much more frequent was
contamination by recent carbon and consequent age-reduction.
Exposed pigments can be polluted by organic materials, some
of which can resist the chemical treatment meant to eliminate
them. Some samples are so small and fragile that if the solid
component is not to dissolve completely the purification has to
be less rigorous. In such cases the humic acid fraction, which
consists of parts of original charcoal that were dissolved in
an alkaline environment and re-precipitated, will give a more
correct greater age.
6. Results
The tabulated ages obtained for drawings in the Cosquer
and Chauvet caves show what type of important information
can nowadays be obtained by the use of AMS radiocarbon
dating. These caves, which are currently being studied
by multidisciplinary teams, provided optimum sampling
conditions, so that more than one sample of certain figures
could be dated. It was also possible to compare the ages of
different fractions of a given scraping to see if the dates are
coherent. Moreover, by dating some of the abundant charcoal
fragments found on the ground near the drawings we were
able to determine the periods of human presence in the cave,
a presence that may have been related to artistic activities.
The Cosquer cave, whose entrance is now 40 m below
sea level, is richly decorated in rock paintings and carvings
(Clottes et al 1992, 1997). About 24 dates (table 1) were
obtained for 13 charcoal drawings including animal figures,
negative hands and geometric signs. Some pigment samples
scraped from several points of a figure were divided in two
and the two halves were treated and dated separately (horse 1,
bisons 1 and 2); they yielded compatible ages, suggesting that
these paintings were done within a relatively short time period
with charcoal coming from the same tree (or contemporaneous
trees). The paintings can be grouped into two time periods
about 10 000 years apart (table 1). This fact is in agreement
with the conclusions based on the observation of the decorated
wall (Clottes and Courtin 1994). The first group consisting
of negative hands, a bison and an oval sign were dated to
between 28 000 and 27 000 years ago, during the Gravettian
period. Except for one horse, the other animals and the starlike sign were dated to between 19 700 and 18 500 years BP,
during the Solutrean period. Taking into account the amplitude
of the errors, it is not possible to conclude if each of the two
painting phases lasted a brief period of time or stretched over
centuries. On the other hand, until more drawings are sampled
we cannot tell if a horse and one stencilled hand, which seem
to date to about 25 000 BP represent an intermediate period of
decoration or are the result of more extensive contamination.
The time span that separates the two bison (1 and 2) which
are similar and depicted on the same wall is rather surprising.
This fact can be interpreted in at least two ways: either the
stylistic conventions were maintained over extremely long time
periods, or the older one was not done with fresh charcoal
(Clottes et al 1997). To help us choose between these
alternatives additional dates will be needed. It is noteworthy
that charcoal fragments collected on the ground nearby also fall
within two distinct time intervals: 18 000–20 000 and 26 000–
28 000 years, respectively.
Chauvet cave was discovered in Ardèche in December,
1994 (Clottes 2001). So far about 40 dates have been obtained
(Clottes et al 1995, Valladas et al 2001b): twelve on pigments
from six drawings from different sections of the cave, two
for charcoal scrapings left by visitors who rubbed their torches
against the wall and the rest for the charcoal found in abundance
on the ground (table 2). The great majority of dates can
be grouped into two tight clusters representing two timeperiods thousands of years apart (29 000–32 500 and 26 000
and 27 000 years BP respectively). The animal representations
were dated to between about 32 000 and 30 500 years BP,
within the Aurignacian period. The torch scrapings were about
27 000 years old, a date not surprising if one notes that in one
case the torch was scraped against a layer of calcite deposited
on top of a drawing! So far, there is no drawing dated to
this second period of human occupation. Most of the ages
obtained for charcoal collected on the ground surface ranged
from 26 000 to 32 000 years BP, suggesting the existence of at
least two major episodes of human intrusion before the cave
was sealed off by a rock-fall. The coherence of the dates
obtained for the drawings of the Cosquer and Chauvet caves
suggests that the samples were not seriously contaminated.
Such satisfactory results can be attributed to the great number
of samples available for dating and to the fact that the cave was
sealed by a natural phenomenon during the Pleistocene period.
7. Conclusion
Even though the direct dating of cave paintings is still in
its infancy, the few dates reported so far have convinced art
historians of the need to revise prior ideas on the evolution of
prehistoric art. The Chauvet cave, in particular, indicates that
theories assuming a linear progression from simple to more
complex composition have to be discarded and that, as early
1491
H Valladas
as the Aurignacian period, some artists had mastered design
and composition (Clottes et al 1995). The AMS radiocarbon
dating also makes it possible to establish distinct periods of
artistic activity within any one cave.
At the present time the AMS technique does not allow for
reliable dating of Palaeolithic drawings done in media other
than charcoal. Unfortunately, the media most commonly used
in the execution of prehistoric paintings are iron oxide and
manganese oxide (Menu 2000). There is hope that some of
these may be dateable in the future, since chemical analyses
have revealed that organic binders of plant or animal origin
were occasionally used with mineral pigments (Pepe et al
1991). The quantities are usually tiny, but improved chemical
techniques will undoubtedly allow us one day to separate,
purify and date such binders.
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