Characterisation of rock varnish at Murujuga (Western Australia): Implications for climate records and petroglyph dating

The aim of this study is to describe the textural, chemical, mineralogical and microbiome characteristics of rock (desert) varnish and assess its potential as a paleoclimate archive, with particular attention to the challenges of reconstructing climate variability in Northwest Australia. Rock samples from Murujuga (Dampier Archipelago, Western Australia) with visible varnish were collected from Rosemary Island, the Burrup Peninsula and Dolphin Island across three lithologies: microgranite, gabbro and microgabbro. Scanning electron microscopy revealed three recurring varnish fabrics: stromatolitic, laminar and detrital-rich. Stratigraphic analysis shows that stromatolitic fabrics are more prevalent at the lower varnish–rock interface, reflecting in situ varnish formation. No consistent relationship was found in varnish fabrics between rock types and site locations or microclimates. Geochemical analysis reveals that stromatolitic and detrital-rich layers exhibit different Mn-Fe-Al-Si proportions, reflecting shifts in formation and or remobilisation that may track moisture availability and dust influx during warmer Quaternary climate intervals. XRD analysis did not identify any Mn-bearing crystalline phases, revealing that the Mn component is amorphous. Confocal microscopy identified active phototrophic microbes on varnish surfaces, and cyanobacteria may promote stromatolitic growth on sun-exposed surfaces. Together, these results demonstrate that stromatolitic and laminar fabrics are the primary microbial varnish formations and highlight the potential of varnish to record environmental conditions linked to moisture availabilities in the Pilbara region. Next is to Identify suitable regional climate proxies in the Pilbara to calibrate varnish features and test their viability as a Quaternary climate archive. • Varnish at Murujuga has three dominant fabrics: stromatolitic, laminar, and detrital. • Mn and Fe vary in stromatolitic layers, while Al and Si vary in detrital layers. • Mn phases are analytically amorphous, while detrital areas contain quartz and clay. • Phototrophic microbiome capable of accumulating Mn-oxide resides on varnish surfaces. • Rock varnish is biogenic and has the potential to record past climatic changes.