D.E. Nelson (1993), in the last ANTIQUITY, declared doubts about an old date for rock art in northern Australia we published in 1990. T.H. Loy, another co-author of the original paper, confirms his continuing confidence in the determination.
Accelerator mass spectrometry radiocarbon dating of pigments containing protein taken from a large rock art complex at Laurie Creek in northern Australia yielded an age of c. 20,000 years ago (RIDDL 1270) (Loy et al. 1990). Further fieldwork was undertaken in 1990 to re-sample from the original location, and to sample other figures at the dated site and at both other art and natural localities in the valley.
Nelson (1993) has recently reported amino acid, carbon/nitrogen ratio and isotopic analyses of one sample taken contiguous to the dated sample location, and questioned the human connection between the source material and the measured age. He states the date no longer has any archaeological meaning. I disagree, and argue that the combination of microstratigraphic, chemical, isotopic and immunological data give consistent and compelling evidence for the presence of human activity directly associated with the dated sample, especially in light of the clear absence of such evidence on all off-art and off-site samples (Loy 1993).
Microscopy reveals a complex microstratigraphy where multiple layers of red hematite, yellow limonite and an unidentified white mineral lie over the uncoloured parent sandstone; they are divided by calcium oxalate layers and have an outer layer of oxalate which obscures the brilliant colours of the underlying minerals. Comparison with nearby 'Late Period' art figures reveals a similar variation in pigment overlayering. There is no similarity with the natural samples. The pigment colours themselves are almost certainly not the product of in situ oxidation and reduction of iron ions (see Watchman et al. 1993).
To date, 15 samples from the 1990 collection have been tested using the SpA conjugate and monoclonal antihuman serum albumin tests (Loy et al. 1990). All the contiguous samples have yielded positive results. Off-art and off-site natural samples are consistently negative. In situ immunological tests to detect mammalian IgG (Loy in preparation) reveal a discrete zone containing this protein lying within a visually undifferentiated red hematite layer in the contiguous sample. No IgG was detected in any of the other pigments in that sample, nor in off-art and off-site samples. Re-testing the residual original sample solution has yielded positive reactions. …