T03_03

Radiocarbon dating correlated microlayers in engraved, oxalate-rich accretions: new archives of paleoenvironments and human activity from Australian rock art shelters

Green H¹, Gleadow A¹, Finch D¹, Myers C², McGovern J³, Levchencko V⁴, Heaney P⁵, Pickering R⁶, Balanggarra Aboriginal Corporation⁷

¹The University of Melbourne, Melbourne, Australia, ²Dunkeld Pastoral Company, Kununurra, Australia, ³The University of Queensland, Brisbane , Australia, ⁴Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia, ⁵Lettuce Create, Brisbane, Australia, ⁶The University of Cape Town, Cape Town, South Africa, ⁷Balanggarra Aboriginal Corporation, Kununurra, Australia

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 and other apparently microbial features that support a microbiological origin for the oxalate component. 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.