Desert varnish is a microbially-mediated rind that accumulates on exposed rock surfaces in arid environments. Varnishes are complex combinations of clays with Fe oxides (magnetite, maghemite, hematite) and the Mn oxide birnessite. Mn-oxidizing microorganisms and fungi have been implicated in the formation of desert varnish through the oxidation of Mn+2 to Mn+4 from detrital particulates and aerosols. We are investigating whether desert varnishes can preserve a fossil record of the organisms that created them.

Several Mn- and Fe-rich varnishes on granitic substrates from the Sonoran desert (Arizona) were examined by TEM, SEM, and powder XRD. The varnishes appear as "biofabrics" composed of clay minerals, in which are embedded single and colonial microorganisms in various stages of decay and fossilization. The predominant clay is a K-bearing illite (~ 1.0 nm basal spacing), which is interspersed with a sheet-like Mn oxide containing trace concentrations of Ba, likely birnessite (~0.6- 0.7 nm basal spacing). Features that appear to be both single and colonial organisms are embedded throughout and upon the varnish surfaces. Most common are toroidal-shaped cells (~0.5-2 mm long) in various states of degradation as indicated by C and S abundances. Colonial organisms include those in chains, each cell up to ~10 mm in length, and large honeycomb-textured features, ~100 mm in diameter, containing many ~3 mm holes. These desert varnish samples thus form suitable media for the preservation of microfossils.

The large hematite deposit in Sinus Meridiani, on the planet Mars, is being evaluated as a site for a NASA rover mission in 2003. One proposed origin for this deposit involves surface weathering and the formation of iron-rich coatings on the rocks. If the hematite proves to be a surface coating similar to desert varnish, the site will become a prime location in the search for evidence of life on Mars.