Dating correlated microlayers in engraved, oxalate-rich accretions: new archives of paleoenvironments and human activity from Australian rock art shelters
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Date
2021-11-17
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Nuclear Science and Technology Organisation
Abstract
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.
Description
Keywords
Age estimation, Archaeology, Environment, Humans, Rocks, Shelters, Calcium, Phosphates, Cultural objects
Citation
Green, H., Finch, D., Gleadow, A., Hoy. J., Levchencko, V., Myers, C., Heaney, P., & Pickering, R. (2021). Dating correlated microlayers in engraved, oxalate-rich accretions: new archives of paleoenvironments and human activity from Australian rock art shelters. Paper presented to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 131). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf