Artificial weathering of rock types bearing petroglyphs from Murujuga, Western Australia

Abstract

AbstractMurujuga in Western Australia has the largest concentration of ancient rock engravings (petroglyphs) in the world. However, the Murujuga rock art is potentially threatened by local industrial air pollution, in particular by acid rain, but unambiguous scientific evidence is still missing. Here, we report on results of an accelerated weathering experiment, simulating Murujuga weather and climate conditions that was designed and performed to test whether the expected small changes in chemical, mineralogical, and physical characteristics of the rock surface can be detected and reliably quantified by various analytical means. Locally acquired Murujuga granophyre and gabbro samples with natural varnish were artificially weathered for up to four months in a climate chamber under conditions that simulated 2 years of natural weathering. Mineralogical, chemical, and physical changes were qualitatively monitored by X-ray diffraction and confocal Raman spectroscopy, and quantified by colorimetry, portable X-ray fluorescence spectrometry, and micro-computed tomography. In addition, artificial rainwater that was sprinkled over the rock samples was collected and analysed by inductively-coupled plasma mass spectrometry. The results show significant chemical and physical changes of the surfaces of the rock varnish after 1 month of artificial weathering. The analytical results demonstrate that it is possible to quantitatively monitor small changes caused by the weathering of gabbro and granophyre. Therefore, such a semi-actualistic experimental approach, when carefully designed, potentially allows testing the hypothesis that the weathering rate of the Murujuga petroglyphs is increased by local industrial air pollution. Further experimental work is currently under way.