Radiocarbon dating of Kimberly rock art

Abstract

Throughout the world, ancient rock art records some of the earliest attempts at complex human communication. However, constraining the age of older rock art has remained a largely intractable scientific problem thereby limiting our ability to integrate rock art into the rest of the archaeological record.

Researchers have studied the globally significant Aboriginal rock art in the Kimberley region of Western Australia for more than 40 years and have comprehensively documented a sequence of rock art stylistic periods. It has long been thought that the oldest styles in this sequence date back to the Pleistocene but only two such dates, relating to identifiable motifs, have been published and both are problematic.

The surviving pigment in paintings of all but the most recent Kimberley rock art style contains no material that can be radiometrically dated. There are, however, mineral accretions and mud wasp nests in the same Kimberley rock shelters that house rock art and, occasionally, these under or overlie paintings. This study explores the development of radiocarbon dating techniques to reliably date remnant mud wasp nests found to be in contact with rock art.

Recently constructed mud wasp nests were collected and analysed to understand the source and age of carbon-bearing material they contain. Unburned plant material and charcoal were found in similar volumes, but charcoal is the carbon-bearing constituent most likely to provide a reliable radiocarbon age for old nests. Old wasp nests were analysed using a wide range of techniques to determine how taphonomic processes alter their physical and chemical composition. These results guided experimentation with pretreatment methods designed to remove sources of carbon contamination while preserving as much of the carbon in the original charcoal as possible. A total of 120 old mud wasp nests were prepared for radiocarbon dating of which 75 contained sufficient carbon for measurement. The distribution of the 75 ages indicated nests were built quasi-continuously over, at least, the last 20,000 years.

The motifs in contact with the 75 nests were classified into one of the six main Kimberley rock art stylistic periods by two subject matter experts. Just 3 nests overlay motifs from each of the Cupules and Wanjina periods suggesting only that some motifs in these styles are older than 7,200 years and 500 years, respectively. The 4 dates available for each of the Static Polychrome and Painted Hand periods permit a very tentative hypothesis for their chronology while the 16 dates relating to Irregular Infill Animal Period (IIAP) motifs and the 20 dates for Gwion motifs provide a more secure estimate.

The concise hypothesis proposed for the chronology of the Kimberley rock art styles is that the IIAP style was in use from at least 17,000 to 13,000 years ago. It was followed by the Gwion period from 13,000 – 12, 000 years ago and then the Static Polychrome period 11,000 to 9,000 years ago. The Painted Hand period followed at around 8,500 to 9,000 years ago.