Dating correlated microlayers in engraved, oxalate-rich accretions: new archives of paleoenvironments and human activity from Australian rock art shelters
Dr Helen Green1, Dr Damien Finch1, Prof Andrew Gleadow1, Ms Jenna Hoy1, Dr Vladimir Levchencko2, Ms Cecilia Myers3, Ms Pauline Heaney4, Dr Robyn Pickering5
1The University Of Melbourne, Melbourne, Australia, 2Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia, 3Dunkeld Pastoral Company, Kununurra, Australia, 4Lettuce Create, Brisbane, Australia, 5The University of Cape Town, Cape Town, South Africa
Distinctive, dark coloured, glaze-like mineral accretions, often found in rock shelters around the world, offer important opportunities for radiocarbon dating of associated rock art.
The mineralogy of these accretions is dominated by well-crystallised calcium oxalate and sulphate minerals, most commonly whewellite and gypsum, with significant occurrences of phosphates in some samples. The accretions are typically several millimetres thick and characterised by distinctive internal laminations that exhibit regular stacked undulations giving a stromatolitic appearance under the microscope. Together with other apparently microbial features observed under the SEM, these features provide strong support for a microbiological origin for these oxalate-rich accretions. Risks surrounding contamination and open system behaviour, previously limiting the application of radiocarbon dating to these accretions, are addressed by the well-crystallised nature of the oxalates and the preservation of fine laminar features within their internal stratigraphies. In a case study from the north Kimberley region of north-western Australia, we demonstrate the use of sample characterisation and chemical pre-treatment techniques to pre-screen for evidence of open system behaviour and address potential contamination. The results provide stratigraphically consistent sequences of radiocarbon dates in mm-scale laminated accretions, with correlations between distinctive patterns in the layer sequences visible in rock shelters up to 90 km apart. This demonstrates that pre-screened samples offer opportunities to reliably date rock art, particularly symbolic markings commonly engraved into these relatively soft deposits and suggests their synchronised formation is not entirely shelter specific but broadly controlled by variations in regional environmental conditions. Consequently, these accretions also offer potential as paleoenvironmental archives, with radiocarbon dating of layers in nine accretions indicating four, approximately synchronous growth intervals covering the last 43 ka.
My research focuses on analysing mineral accretions using a range of geochemical techniques to characterise and understand the formation processes occurring in relation to rock art pigments in north west Australia’s Kimberley region. These techniques include uranium-thorium dating, radiocarbon dating, stable isotope analysis and analytical techniques such as X-ray diffraction analysis and scanning electron microscopy, electron microprobe analysis. I have applied and adapted radiocarbon techniques to oxalate and phosphate bearing layered mineral accretions, with an aim of generating bracketing ages for different rock art styles comprising the established rock art sequence in the Kimberley region.