Biological contributions to manganese oxides in rock varnish at Murujuga (Western Australia)

Rock varnish is a ubiquitous Mn-rich coating on exposed rock surfaces in arid environments, yet the mechanisms underlying its formation remain debated. Here, we investigate rock varnish from Murujuga, Western Australia, to assess the role of microbial processes in manganese (Mn) accumulation. Bulk compositional and mineralogical analyses confirm high concentrations of Mn, Fe, Al, and Si; however, the Mn matrix is predominantly composed of amorphous to poorly crystalline phases that fall below the indexing or detection thresholds of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Nanoscale characterization reveals a Mn-rich matrix encasing discrete Fe and Al-Si grains, featuring nanometre scale laminations and particle size distribution characteristic of biogenic Mn oxides. High-quality metagenome-assembled genomes (MAGs) reveal a pronounced dominance of Chroococcidiopsidaceae and Rubrobacter_F , pioneer taxa known to accumulate intracellular Mn for defence mechanisms. Furthermore, targeted functional annotation using Hidden Markov Models (HMMs) confirms a widespread, community-level genomic potential for biologically influenced Mn accumulation and utilization. Because this biomineralisation is an ongoing process governed by local environmental stressors, these rock coatings have high potential as long-term paleoenvironmental and climate proxies. This is the first microbiomic characterisation of the rock varnish from the Murujuga Cultural Landscape, and an important step in unlocking the potential of this deposit as a chronological marker for this region’s petroglyphs.