Theoretical effects of industrial emissions on colour change at rock art sites on Burrup Peninsula, Western Australia
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Journal of Archaeological Science: Reports, ISSN: 2352-409X, Vol: 12, Page: 457-462
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- Acid load; Colour change; Desert varnish; Industrial pollution; Petroglyphs; Pourbaix diagrams; Rock art; Rock varnish; Arts and Humanities; Social Sciences
Burrup Peninsula in northwest Western Australia, with an estimated one million petroglyphs, has the world's largest concentration of ancient rock art. It is one of a few places in the world where a continuous history of people living with a changing environment for over 40,000 years is recorded through rock art. The art is under threat due to high concentrations of acidic and nitrate-rich pollution from nearby industrial complexes. Maintenance of the outer, rock or desert varnish, layer of the rocks is essential for preservation of the art. An increase in acidity of rock surfaces through acid rain and organic acids from nitrate-stimulated microbial growth may alter the mineral composition, integrity and colour of the rock varnish. This paper describes analyses of distilled water washings from rocks on Burrup Peninsula compared with rocks collected prior to industrialisation. Acidity of rock surfaces has increased from near neutral to a pH just above 4. The increasing acidity has been associated with a logarithmic increase in solubilisation of manganese (Mn) and iron (Fe) compounds from the rock surfaces. A theoretical evaluation using electrochemical equilibrium principles confirms that increasing acidity will increase the solubilisation of Mn and Fe compounds. Removal of darker Mn and Fe mixed M(II)/M(III) compounds from the outer, rock varnish layer and the relative increase in ferrous oxide and illite/kaolin compounds will result in the rock surface layers becoming thinner, lighter, redder and more white/yellow over time. The impact on engraved surfaces would be expected to be greater because the rock varnish is thinner than on the non-engraved surface rock. Pollution from industry on Burrup Peninsula is likely to destroy the rock art over time.