Second Thoughts on a Rock-Art Date
Nelson, D. E., Antiquity
D.E. Nelson, radiocarbon scientist in the group which reported early dates for rock-art at two Australian sites in ANTIQUITY 64 (1990), reports a further study which leads him to withdraw the published date for one, at Laurie Creek (Northern Territory). As he notes, his co-authors in the 1990 paper are not all in agreement with his present view. The other site dated in 1990, Judds Cavern (Tasmania), is not addressed in this further report.
A few years ago, as my part of a team effort, I measured a radiocarbon age of about 20 thousand years (RIDDL-1270; 20,300+3100-2300 b.p.) for rock art at the Laurie Creek site in northern Australia (Loy et al. 1990) for a sample (DMP-6) that had been collected during a first reconnaissance of the area. At the time of measurement, we believed that the substance extracted for dating was remnant human blood protein associated with artistic activity, as another team member had detected human blood on the sample and as the chemical extraction method itself selected only high molecular-weight material which was therefore probably proteinaceous (Loy et al. 1990). However, there was a nagging doubt, in that the carbon and nitrogen concentrations were atypical of protein. Given the great age recorded, repeat measurements seemed appropriate, and so more samples of the same material were obtained during a second visit to the site. The results from an examination of these new samples lead me to conclude that we have no demonstrable associative connection between the material dated and human activity, and so the original result can no longer stand as a reliable date for prehistoric art. As this view is not shared by all my colleagues, I report it here under my name only.
The site and the dated material
The Laurie Creek area contains an impressive display of Aboriginal painted rock art in varying states of preservation which suggest that some of the figures are very recent, while others are of considerable antiquity. It is clear that the area has been the focus of artistic activity for a long time. The original sample came from a red mineral 'skin' on a sandstone rock face without visible figurative art; this material was originally believed to be a result of human ritualistic activity associated with the local figurative art. The second suite of samples was collected from the same location on the rock face from which the original was taken. Some went to the Australian National University for further bio-chemical studies, and some to Simon Fraser University to test further the original extraction procedures. This report describes only the results of this latter study.
A microscopic examination of the new samples at low magnifications (6-40 diameters) showed a layered mineral 'skin' of thickness about 1/2 mm on a very friable sandstone base. The red sandstone cement was weakening. It was soluble in HCl, as were the layers which could be removed a little at a time using the acid. The outermost layer was brownish-red; it was underlain in some subsamples by a thin red layer, which was in turn underlain by a thin yellow layer. These inner layers changed to a deep brown on heating to 200 |degrees~ C, and they are probably iron oxides.
While these coloured layers could be interpreted as human paints, such layers also occur naturally. Weathering processes on iron-containing rocks can yield coloured iron oxide minerals (generically called 'limonite') which may be found 'as varnish-like coatings' (Palache et al. 1966). As the cement of red sandstones contains limonite and haematite (Hurlbut 1976), and as the sandstone on which this particular 'skin' was deposited was disintegrating, the coloured layers may be nothing more than dissolved cement reprecipitated at the surface. As there is no visible figurative art on this rock surface, we thus have no evidence for artefactual material, and the only connection to human activity is the positive test for human blood on the surface of the original sample (Loy et al. …