From radiocarbon dating of mud wasp nests to ages for early Australian rock art
Dr Damien Finch1, Professor Andrew Gleadow1, Professor Janet Hergt1, Dr Helen Green1, Dr Vladimir Levchenko2, Ms Pauline Heaney3
1University of Melbourne, Melbourne, Australia, 2Australian Nuclear Science and Technology Organisation, Sydney, Australia, 3Rock Art Australia, Melbourne, Australia
Northern Australia is home to some of the richest rock art provinces in the world. It has long been thought that the oldest of the rock art still visible today may date back to the Pleistocene period, more than 11,000 years ago. Until recently, after more than 30 years of research, there were only one or two quantitative, radiometric, age determinations to support this view. Importantly, there was certainly insufficient data to say when some of the oldest, very distinctive, styles of rock art were created and there were no age estimates at all for the oldest style of figurative rock painting.
Here, we report on the development of radiocarbon dating methods to provide the first estimates of the ages spanned by the two earliest styles of painted rock art in the Kimberley region in northwest Australia. Mud wasp nests overlying, or underlying rock art were dated to provide maximum and minimum age constraints for rock art motifs.
Radiocarbon dating of fossilised mud that sits exposed on rock shelter walls for millennia poses many challenges. The sources of carbon in the nest when it is built need to be identified so that the relationship between the age of the nest, and the age of the carbon in it, is well characterised. As the nest ages, organic and inorganic components will undergo different taphonomic processes that need to be identified to determine if the fossilised nest behaves as a closed system for carbon and to understand which sources of carbon will be preserved. Mineral accretions commonly develop on all exposed surfaces in these rock shelters, including old wasp nests, so the mineralogy of these and any aeolian contamination such as ash and pollen needs to be understood so that appropriate treatments can be designed to remove any such contamination. Wasp nests gradually weather away so the older nests are small and have very low carbon concentrations. Many samples yield less than 10µg of carbon, challenging the limit of what is possible to measure reliably, so pretreatment methods must be optimised to maximise yield while minimising the risk of contamination.
Here we review the methods used to address these challenges. We then report on the results of the dating program, so far, including the first chronometric estimates for the two earliest periods in the stylistic sequence of painted Kimberley rock art. We show that both periods date back to the terminal Pleistocene with evidence of the oldest art painted at the end of the Last Glacial Maximum, around 17,000 years ago.
Dr Damien Finch is a Research Fellow in the Faculty of Science at the University of Melbourne where he is a member of the Archaeological Science group. Damien studies radiocarbon dating and how it can be used to estimate the age of rock art.