Formation mechanism of rock varnish

Establish a comprehensive formation mechanism for desert/rock varnish that accounts for extreme manganese enrichment, micro-lamination, and compositional variability, reconciling biotic and abiotic pathways and explaining the deposit’s very slow accretion.

Background

Rock varnish is a thin Mn–Fe-rich coating on desert rocks that grows extremely slowly and exhibits micro-laminations and strong Mn enrichment relative to surrounding materials. Abiotic hypotheses include dust leaching and photo-oxidation; biotic mechanisms invoke microbial Mn/Fe oxidation and EPS mediation; polygenetic models combine these influences.

Despite extensive study dating back to Humboldt and Darwin, no single mechanism satisfactorily explains all observed features, prompting calls for integrative models and quantitative assessments.