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Using light to date Murujuga’s Dreaming

Murujuga (Dampier Archipelago) is one of the most abundant petroglyph (rock engraving) provinces in the world. The CRAR+M Murujuga Dating team, in partnership with Murujuga Aboriginal Corporation (MAC), is working to improve our understanding of this significant cultural land- and seascape.

Murujuga Landscape on Dolphin Island with the Dating team being introduced to geological variability (photo: Luke Gliganic)

It is understood that people have made rock art at Murujuga throughout the 50,000 years that this land and sea scape has been occupied. It is a research priority of the MAC Cultural Management Plan to understand the age of the engraved rock art that they are managing. However, it is nearly impossible to date rock engravings using conventional dating methods, and the exact age of Murujuga’s rock art has remained a mystery. The Dating project aims to rectify this by using an exciting new technique called optical-surface exposure dating (optical-surf, for short) to help understand when people engraved the rock surfaces across the archipelago.

Macropod and echidna motifs at Murujuga (photo: Luke Gliganic)

So how does it work? You might have heard of optical dating – an approach that can be used to tell for how long sand grains have been buried. This is based on the principle that a latent-luminescence signal accumulates in mineral grains when they are shielded from sunlight (e.g. when they are buried). The size of the luminescence signal is proportional to duration of burial; the longer a grain is buried, the larger its luminescence signal will be. When a mineral grain is exposed to sunlight, its latent-luminescence signal is removed. We can collect buried sand grains, measure their luminescence signal, and determine when the grains were last exposed to sunlight (i.e., and know how long they have been buried). This process can also be used to date buried rock surfaces and even surface artefact scatters.

Optical-surf uses the same luminescence signals as classical optical dating but flips the script of the dating approach. When a rock surface is exposed to sunlight, its latent-luminescence signal is bleached. Optical-surf is based on the observation that sunlight does not stop at the surface, but actually passes into the rock surface. The longer a rock surface is exposed to sunlight, the deeper into that rock surface the luminescence signal is bleached. If we can measure the luminescence signal as a function of depth into a rock surface and calibrate this with known-age samples, we can determine how long the target rock surface has been exposed to sunlight.

How can we use this to date petroglyphs?

When an artist engraves a rock they remove surface material, thereby exposing a previously unexposed rock surface. This, in turn, sets the optical-surf clock ticking at the site of the engraving.

Riso TL/OSL Reader (Photo: Michael Meyer)

The Dating project is now in the experimental phase of collecting small core samples from rocks of the different geologies across Murujuga as well as known-age rock surface exposures, such as road cuttings. These cores will be processed at the OSL laboratory at the University of Wollongong by Dr Luke Gliganic, who is the Murujuga Research Fellow on the Dating Project running this experimental study. These small cores will be sliced in 1 mm increments using a low-speed precision saw. Each slice will be stimulated with LEDs and their luminescence signals will be measured. By setting up these exposure calibrations, we will develop a precise model to start asking more directly when the rock was engraved.

At this stage of the Project (which will take at least two years of laboratory-controlled and field experiments) we are only using non-cultural stone material. All work is being undertaken in close collaboration with the Murujuga Circle of Elders, the cultural custodians of the Murujuga landscape. We patch the sampling scars to leave almost no visible impact when we are finished. Once our calibration experiments are confirmed, we will then enter a new phase of the project and approach MAC for permission to sample rock art.

Preparation and sampling a field-based experiment on non-cultural stones (photo: Luke Gliganic)

Our samples collected in Year 1 of the project are showing promising results. This year’s field sampling will widen the sample sites to different geologies across the archipelago and continue our field experiments. We are excited to be undertaking this cutting-edge science to yield new data, which should help address one of MAC’s intriguing research priorities and provide new results to support their World Heritage nomination.

Want to know more about the process that our team will be using? Check out this article in The Conversation for more information.

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