Dating Pictographs with Radiocarbon

Abstract

We report here progress on our technique for 14C dating pictographs. We use low-temperature oxygen plasmas coupled with high-vacuum techniques to selectively remove carbon-containing material in the paints without contamination from rock substrates or accretions. We dated >16 pictograph samples that generally agree with ages expected on the basis of archaeological inference. We have shown that carbonate and calcium oxalate decomposition does not occur during our procedure; little mass fractionation is produced. We also used the technique on samples of known 14C activity. In each case our results agree with previously determined ages of archaeological charcoal samples. Two samples of the standard Third International Radiocarbon Intercomparison wood yielded ages in near accord with the accepted value. We used 14C-free samples to establish that the method and apparatus do not have a significant live carbon background. Each of these determinations supports our conclusion that the technique has the potential of producing accurate and reliable ages. However, background organic material in the basal rocks and accretions can be troublesome, often completely negating the dates obtained.

Radiocarbon Dating of Ancient Rock Paintings

“…The resulting CO 2 sample was then sent to an AMS laboratory for radiocarbon measurement. [42][43][44][45] …”

“…When an organism dies, the contemporary 14 C 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 14 C 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.…”

“…Radioactive 14 C, or radiocarbon, is continuously produced in the earth's upper atmosphere by interactions of secondary cosmic ray neutrons with the most common atmospheric isotope, 14 N. The 14 C produced is rapidly oxidized to 14 CO 2 and quickly distributed throughout the atmosphere, thus becoming incorporated into the earth's biological carbon cycle. For this paper, we assume that most living matter has a very similar 14 C/ 12 C level. When an organism dies, the contemporary 14 C level is no longer fixed into biological tissues, and that which remains decreases through radioactive decay; the half-life is 5730 years.…”

“…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.…”

“…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 14 C/ 12 C except that due to radioactive decay. Our dates are compared with materials of known age ( Figure 3) to ensure that this criterion is met.…”