Anal. Chem. 2009, 81, 1728–1735
Radiocarbon Dating of Ancient Rock Paintings Marvin W. Rowe Texas A&M University College Station and Qatar
Rock art images are among the most enigmatic and personal artifacts studied in paleoarchaeology. Rock paintings (pictographs) left by ancient prehistoric cultures are found all over the worldsvirtually everywhere there are rocks. One example of a polychrome painting in the lower Pecos River region of southwest Texas (where thousands of other impressive painted images are found on the limestone shelter walls) is shown in Figure 1. Two frequently asked questions are, without regard to location, how old is it and what does it mean? This review deals with attempts to answer the question of age.1 Rock art is an important, irreplaceable part of our heritage. When there is inadequate ethnology for a region, as is most often the case, it can be argued that rock art is the most important evidence available for discerning the thought processes and the aesthetic, symbolic, and religious ideas of the prehistoric cultures of the peoples who painted them.2 But to incorporate rock art into mainstream archaeological thought, one must be able to assign it to a specific ancient culture and time span. Archaeology gained an important technique for dating archaeological artifacts in the 1940s when Willard Libby and his colleagues developed radiocarbon dating,3,4 a revolutionary method for which he was awarded the 1960 Nobel Prize in Chemistry. Thus, when we were asked about two decades ago by anthropologist Harry Shafer to date a piece of a pictograph that had been picked up from the floor of an ancient painted shelter in southwest Texas, we primarily considered radiocarbon dating while trying to devise a means of dating that pictograph. The principle of radiocarbon dating is deceptively simple. Radioactive 14C, or radiocarbon, is continuously produced in the earth’s upper atmosphere by interactions of secondary cosmic ray neutrons with the most common atmospheric isotope, 14N. The 14C produced is rapidly oxidized to 14CO2 and quickly distributed throughout the atmosphere, thus becoming 1728
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MARVIN W. ROWE
A technique based on cold argon and oxygen plasmas permits radiocarbon dates to be obtained on paintings that contain inorganic pigments. (To listen to a podcast about this feature, please go to the Analytical Chemistry website at http://pubs.acs.org/journal/ancham.)
Figure 1. Polychrome pictograph from shelter 41VV83 in the lower Pecos River region of southwest Texas. Colors are dark red, yellow, and black. The central figure is ∼1/2 m tall and is called the Ecstatic Shaman or the Electric Jesus. The ages determined for paintings of this style are 3000-4000 years.1
incorporated into the earth’s biological carbon cycle. For this paper, we assume that most living matter has a very similar 14 C/12C level. When an organism dies, the contemporary 14C 10.1021/ac802555g CCC: $40.75 2009 American Chemical Society Published on Web 02/09/2009
Figure 2. A polished cross section of a pictograph with red pigment. Three layers are clearly seen: an overlying calcite/calcium oxalate mineral accretion, the iron ocher pigment, and the underlying limestone.
level is no longer fixed into biological tissues, and that which remains decreases through radioactive decay; the half-life is 5730 years. Thus the measured level of 14C in a dead organism compared with the constant level in living material leads to an estimate of the date of death of the organism. In practice, there are many refinements, most of which have been worked out in considerable detail over the past six decades. It remains the most accurate and reliable dating technique in archaeology. Only with the advent of accelerator MS (AMS) for measuring radiocarbon did it become feasible to date pictographs.5-12 AMS has greatly reduced the amount of carbon necessary for determining a radiocarbon date, from several grams in conventional dating to ∼0.05 mg. The first radiocarbon dates on charcoal pigments from pictographs were published in 1987.11,12 Inorganic (mineral) pigments are far more frequently used for pictographs than is charcoal, even for black pigment. Reds, oranges, yellows, browns, and purples are iron oxide/hydroxides, and black paintings are often manganese oxide/hydroxides.13 Figure 2 is a photograph of a polished section of an iron oxide pictograph from southwest Texas. Three layers are clearly seen. The outside is an accretion layer usually composed of calcite, gypsum, and calcium oxalate. Then comes an underlying red iron oxide/hydroxide layer (the paint pigment), and finally the rock to which paint was applied, in this case limestone. Sometimes a preexisting accretion layer underlies the paint.14,15 Rock paintings are among the most difficult archaeological artifacts to date. None of the inorganic pigments typically found in rock art can be radiocarbon dated; those pictographs can be dated only if organic material was added in the preparation of the paint. If this was not done, creating a chronology will depend on a wider-ranging archaeological investigation of the context in which the image was created. If organic material was added,
Figure 3. Radiocarbon dates obtained by plasma chemical extraction of widely differing kinds of organic archaeological artifacts compared with those from previous work, or destructive AMS dating.
enough of it must have survived for an AMS measurement from a reasonably small area (∼2 × 2 cm); the amounts of carbon extracted are routinely near the limit of accurate measurement by AMS (60-150 µg carbon). In addition, the rock that was painted upon must not contain significant indigenous organic matter itself. These criteria are often, but not always, met.16,17 Other properties of the rock and paint that could be problematic must be considered. There must be no exchange in the carbon isotopes after death of the organism, that is, there must be no change in the 14C/12C except that due to radioactive decay. Our dates are compared with materials of known age (Figure 3) to ensure that this criterion is met.16-20 Extraction of the organic carbon must not introduce significant mass fractionation, and, as seen in Figure 2, one needs to be able to separate the organic matter from the paint sample without contamination from limestone or calcium oxalate, either of which would nullify a date. Our measurements of δ13C indicate that plasma chemical extraction does not introduce significant fractionation of the carbon isotopes.18-20 In addition, the technique was primarily aimed at overcoming the potential contamination from carbonates or oxalates, so there is no need for the harsh strong acid/strong base treatments normally used to date charcoal and other organic archaeological artifacts.16-20 RADIOCARBON DATING TECHNIQUES Two basic means have been used to radiocarbon date rock paintings. The most common one is based on the analysis of organic pigments. Charcoal is often the pigment used for producing black rock paintings (MnO2 is also common). Charcoal paintings were the first to be radiocarbon dated, and they represent a substantial fraction of the dated pictographs, especially for Paleolithic rock paintings.11,12,21-31 In the European Paleolithic caves, researchers have concentrated entirely on Analytical Chemistry, Vol. 81, No. 5, March 1, 2009
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dating charcoal pigments. The experimental procedure for charcoal pigments is the same as for other archaeological charcoal, that is, alternating acid, base, and acid soaks at 50-70 °C; thus it can be reliably done. The difference is that samples from rock paintings are virtually always much smaller than those taken from other archaeological charcoal.1 Charcoal is a commonly dated archaeological material, but it dates the death of the plant from which it was made, not the time of an archaeological event of interest, in our case a rock painting. Thus, the charcoal used as pigment could have been much older than the painting itself. It is also possible that the wood used to make the charcoal might have died a long time before it was burned into charcoal.32 Either of these cases, old charcoal or old wood, can complicate the analysis, and rarely can these possibilities be totally eliminated. A second possibility in the case of old charcoal is that the wood could have been burned at an earlier unknown time and the resultant charcoal used much later to execute a painting.33-35 Caution is necessary when interpreting charcoal-derived radiocarbon dates, and the possibility of old wood or old charcoal must be considered for each situation. The old-charcoal problem was illustrated by historic graffiti in Australia.36 “Mr. C. B. Ross”, written in charcoal in a shelter, was radiocarbon dated to 1310 years BP. (BP refers to radiocarbon years before present, defined as A.D. 1950.) Because the Ross family has lived in the region only since the late 1800s, a more recent date was expected. However, two samples of near-surface charcoal from the shelter were tested, yielding dates of 690 and 1470 years BP.37 Old charcoal was readily available from the shelter floor and was obviously used to write the graffiti. In spite of these problems dating charcoal paint pigments, most results obtained so far appear to be valid and accurate. In addition to charcoal, many dates have been obtained on beeswax images. There is little reason to doubt the validity of the dates, but beeswax figures are found only in a very isolated area in Australia.38,39 PLASMA EXTRACTION OF ORGANIC CARBON FROM PAINTS BASED ON INORGANIC PIGMENTS The second major way of dating pictographs is to analyze the radiocarbon content of organic material that may have been added to the paints as binders or vehicles for the pigments by the original artists. Almost none of the colored rock art images in the world contain visible organic matter. When we first attempted to develop a dating method, we examined the Pecos River pictographs of southwest Texas, an area rich in enigmatic, polychromatic, often larger-than-life images. However, charcoal was apparently not used as a pigment in those paintings, even for black; MnO2 was used instead.13 The sample preparation for radiocarbon dating of charcoal is conventionally done by a strong acid wash, followed by a strong base wash and a final strong acid wash, usually at 50-70 °C. We were concerned that this harsh treatment, which destroys perhaps 30% of even very stable charcoal, would adversely affect the organic carbon of unknown composition present in a paint sample. Not knowing whether all or a substantial fraction of the unknown organic material might be removed by the harsh treatments, we selected a technique in which those treatments were unnecessary. We focused on extracting small amounts of 1730
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organic carbon in the paints from the far greater amounts of inorganic carbon in the rock. We scraped a small (∼2 × 2 cm) area of the painted surface with clean dental picks or new surgical blades. The samples were wrapped in aluminum foil and stored in plastic bags in a desiccator to await analysis. It is virtually inevitable that much greater amounts of calcium carbonate and calcium oxalate are included in the scraped rock art sample than organic material in the paint. We opted to extract organic carbon using low-temperature (e150 °C), low-pressure (∼ 1 torr) oxygen plasmas coupled with static high vacuum to isolate CO2 formed by the plasma oxidation of organic matter. Figure 4 is a schematic of a plasma apparatus.40 Radio frequency power for the plasmas was supplied by commercial 27.1 and 13.5 MHz generators. We ran typically at powers of 100 W or less but used higher-power plasmas to clean the sample chamber of adsorbed atmospheric CO2. Low-temperature oxygen plasmas contain 1-2% ionized oxygen, as well as atomic oxygen, and are highly reactive with organic carbon. However, we showed that the plasma did not affect the much larger amounts of limestone and calcium oxalate;18,41 thus, we could cleanly extract the organic carbon under very mild conditions. Under standard operating conditions, we first ran higher power (150 W) oxygen plasmas to rid the interior of the sample chamber of organic contamination from previous runs. After multiple oxidations, the carbon background was reduced to
