Rock-art microbiome: influences on long term preservation of historic and culturally important engravings

Abstract

The Burrup Peninsula in north-west Western Australia is home to one of the most substantial collections of rock engravings, or petroglyphs, in the world. These petroglyphs are carved through the dark coloured patina, commonly referred to as rock varnish, into the weathering rind of the local parent rock. Rock varnish is essentially a thin layer of manganese (Mn) and iron (Fe) oxides and hydroxides with embedded clay minerals, the formation of which is relatively poorly understood. It is generally considered to be a hostile environment for microorganisms due to extreme environmental conditions including low nutrient availability, lack of water, exposure to extreme ultraviolet radiation and intense seasonal and diurnal temperature fluctuations. However, despite these environmental extremes, microorganisms have been found on and in rock varnish and have been reported as playing a significant role in the formation of rock varnish. Given this, it is likely that any change in local environmental conditions will influence the types and activities of microorganisms found in and on rock varnish and associated rock art. This article focuses on the major influences on the microbiome of culturally important rock art in the Burrup Peninsula and the implications of any environmental change on the rock art itself.

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

“…e Effect of physical weathering of an unworked surface, causing varnish flaking off, which would have destroyed any petroglyphs that might have been present concentrations of petroglyphs in all rocky ephemeral stream valleys, especially around the point at which these valleys join more open plains [4]. The granophyre and gabbro are characterised by a thin, whitish, weathered layer of about one centimetre maximum thickness that is covered by a 1 to 200 µm thick outer darker red-brown layer, known as the patina or rock varnish, that consists of about 25 wt.% Fe and Mn oxides and 75 wt.% clay minerals (mostly montmorillonite and kaolinite) [1,7]. Other studies report about 70% clay minerals and 25% Fe and Mn oxides and hydroxides [8], while analyses from rock varnish from different sources all over the world revealed that 10 to 18 wt.% of the varnish are Fe and 10 to 19 wt.% are Mn oxides [9].…”

“…Some models for varnish formation are based on abiotic processes [11], whereas others consider biotic processes to be central to their origin [10]. There is evidence for the presence of an unusually aerobic microbial ecosystem on the surface of the rock varnish from Murujuga [7,12,13], including photosynthetic cyanobacteria that can enrich manganese and iron [12]. Recent evidence shows that various cyanobacteria accumulate extraordinary amounts of intracellular Mn that is used by the bacteria as a catalytic antioxidant necessary to cope with the substantial oxidative stress present in a desert environment [14].…”