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

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.

The Burrup Peninsula (see Figure 1) is estimated to contain over 1 million petroglyphs. These form one of the longest sequences of art in the world, extending back probably as much as 40 000 years. This makes the area one of the most significant rock art regions in the world¹. The engravings are diverse in form and include those of many animal and bird species including extinct animals (e.g. Thylacines, the Tasmanian tiger)². The petroglyphs (see Figure 2 as an example) on the Burrup Peninsula are carved into the rock varnish of the parent granophyre and gabbro igneous rocks. Rock varnish usually forms very slowly, at a rate of 1–10 μm per thousand years, particularly in arid desert environments where rainfall is low^3,4. At its most basic rock varnish can be described as a dark coloured coating with thickness that seldom exceeds 200 μm that is composed mostly of Mn and Fe oxides that are cemented to clay minerals in a laminated structure³. The Mn and Fe present in rock varnish comes from a range of sources likely including the atmosphere, precipitation, dust and from surrounding soils^5,6. Mn and Fe concentrations can vary greatly but in general Mn is enriched over Fe relative to their natural distribution with Mn oxides usually accounting for around 20% of the total oxides⁷.

Figure 1. Map showing the Dampier Archipelago and adjacent mainland, Pilbara region Western Australia.

Figure 2. Burrup rock with petroglyph. Image by Mike Donaldson, reproduced with the permission of Murujuga Aboriginal Corporation.

There is some evidence suggesting that the microlaminations in rock varnish correlate with alterations in environmental conditions³. Liu et al.⁸ hypothesise that Fe‐rich layers are formed during dry conditions whereas darker Mn‐rich layers are formed during periods of wet³. Precipitation and solubility of Mn is generally controlled by pH and Eh conditions with low Eh and pH usually leading to dissolution of Mn whereas high Eh and pH promotes precipitation. The ratio of Mn to Fe in the varnish thus varies with climatic conditions⁹. It is likely that when conditions are moist this promotes the activity of bacteria and fungi which in turn produce organic acids that work to decrease local pH. Field observations by Northup and colleagues at the Black Canyon (New Mexico, US) reported a strong correlation of visibly more substantial Mn varnishing on preferential water pathways flowing on rock surfaces and cliff faces, and in ephemeral rock pothole pools¹⁰.

The mechanism of rock varnish formation and growth is still under discussion with both biotic and abiotic processes being postulated. It has been proposed that if abiotic processes were responsible for producing rock varnish that rates would be much more rapid than empirical measurements suggest¹¹. Evidence of a biological role has been mounting⁵ with significant interest being generated around likely biological mechanisms. For example the formation of β-alanine and δ-butyric acid by enzymatic carboxylation is indicative of biological activity¹². There is also significant evidence that suggests that microorganisms can directly or indirectly control Mn precipitation^13,14 with biomineralisation of Mn being proposed in a wide range of environments including hot springs¹⁵ and soils¹⁶. The elevated concentration of Mn that is well above that of the geological background also suggests that the formation of rock varnish is likely a biological process. This is because Mn and Fe compounds concentrated in bacteria and fungi become chemically bound in the crystalline structure and external coating of the clay minerals that cements the clays to rock surfaces¹⁷. The hypotheses that microbial communities play a role in the formation of rock varnish are also supported by the fact that a number of microbial metabolic pathways use Fe and Mn as electron donors^18,19.

Many different bacteria and fungi have been isolated and characterised from rock varnish^10,20–22 but those with the ability to oxidise and precipitate Mn and Fe^13,19 are of particular interest. Within this group budding bacteria, for example the genera Hyphomicrobium and Pedomicrobium, have been extensively studied as they have the ability to encrust Mn and Fe oxides within their cells^23,24. These types of budding bacteria have been identified growing on rock varnishes found in warm deserts^5,25 and are known to produce Mn-rich deposits on rocks present in mountain soils²⁶, acid mine drainage¹⁹, and caves^27,28. Krinsley et al.¹¹ reported the first evidence of budding bacteria present in situ within rock varnish that were directly enhancing Mn and Fe. The authors investigated a site at Erie Barge Canal (New York) where rock varnish had completely coated the quarried sandstone over the course of approximately 100 years¹¹. This site has rock varnish that is approximately 15 μm thick and the authors conclude that only one or two budding bacteria encrusting Mn and Fe oxides each year would be needed to generate the rock varnish¹¹. By extrapolating from this data the authors speculate that it would take only one budding-bacterium every 400 years to explain a 20 μm thick, 10 000-year-old warm desert varnish similar to that found in the Burrup Peninsula¹¹. We can therefore hypothesise that unique and rarely occurring environmental conditions are required to promote rock varnish formation by specific budding bacteria, for example, optimum moisture, UV and solar radiation exposure.

Recent expansion of industry in the Burrup Peninsula may potentially upset the delicate balance of environmental conditions that led to rock varnish formation. Acid rain and nitrogen deposition as a result of industrial expansion has the potential to stimulate microorganisms that may not be compatible with rock varnish formation and or which may produce organic acids that could be detrimental to the survival of the rock varnish^29,30. The combined influence of acid rain and microbial organic acid production will decrease the pH of the rock art environment potentially resulting in dissolution of the Mn and Fe within the rock varnish ultimately leading to deterioration and consequent destruction of the rock art. Research is required to identify the specific organisms responsible for rock varnish formation and to assess the impact of pollution on the rock art microbiome and likely impacts on rock varnish so that we can better understand how to protect the culturally significant rock art present in the Burrup Peninsula.