Desert varnish to date Murujuga rock engravings

Murujuga, the Dampier Archipelago in northern Western Australia, is home to one of the world’s largest, densest and most diverse collections of engraved rock art. Rock engravings, made in both deep time and the recent past, reflect the continuing cultural and spiritual connection to Pilbara Traditional Owners and Custodians. In partnership with the Murujuga Aboriginal Corporation, the Centre for Rock Art and Management at the University of Western Australia (CRAR+M UWA) is working to study, monitor, and care for these engravings.   

The engravings are made on rock surfaces by chipping away the weathered crust, revealing the lighter rock below. Some of these engraved surfaces host a thin black coating called desert varnish. This is significant because desert varnish has been observed to form over the top of, and within, older rock engravings. Therefore, studying this varnish might give clues to the minimum and maximum dates of the currently undatable engravings.

To investigate the use of this varnish as a dating method, an interdisciplinary team of archaeologists, geologists, and microbiologists on the Dating Murujuga’s Dreaming ARC Linkage Project (LP190100724) are using various microscopy techniques at the Microscopy Australia facility at UWA to investigate the varnish’s microstructure, geochemistry, and biofilms. 

Elemental analysis of the desert varnish (blue) on the underlying rock (rich in silicon (Si)).

Scanning electron microscopy revealed complex layered structures, and elemental mapping showed layers rich in clay, manganese, and iron. Additionally, confocal microscopy showed that the surface of the varnish harbours bacterial biofilms, indicating that the varnish has a biological component. By understanding the detailed structure of the varnish, it may be possible to identify and date constituents that formed when the varnish was developing and therefore help to date the engravings themselves.

Confocal image of biofilm in desert varnish

The Dating Murujuga’s Rock Art Project (LP190100724) is an Australian Research Council Linkage Project between The University of Western Australia, The University of Melbourne, The University of Wollongong, Murujuga Aboriginal Corporation, Rio Tinto and Woodside.