Rock Varnish and its Use to Study Climatic Change in Geomorphic Settings

The dusky brown to black coating of rock varnish dominates bare rock surfaces of many desert landforms (Oberlander, 1994). Thicknesses less than even 0.020 mm (or 20 micrometres, μm) are enough to darken light-colored rock types (Fig. 21.1). The gradual pace of change on many desert landforms permits the slow accretion of rock varnish at rates of a few micrometres per thousand years (Dorn, 1998; Liu and Broecker, 2000). Just about any rock type will accumulate varnish, in so long as rock-surface erosion is slow enough to permit varnish accretion. Rock varnish formation starts when bacteria oxidize and concentrate manganese (Mn) and iron (Fe) (Dorn and Oberlander, 1981; Krumbein and Jens, 1981; Palmer et al., 1985; Hungate et al., 1987; Jones, 1991; Drake et al., 1993; Dorn, 1998; Krinsley, 1998). Although random collection of rock varnish reveals an impressive array of dozens of different types of microorganisms and their molecular by-products (Nagy et al., 1991; Dragovich, 1993; Perry et al., 2002; Kuhlman et al., 2005; Kuhlman et al., 2006), the bacteria that actually initiate varnish grow very infrequently (Dorn, 1998). Hence, rock varnish accumulates very slowly. If all of the organisms purported to cause varnish formation (Taylor-George et al., 1983; Eppard et al., 1996; Perry et al., 2003) actually contributed, rates of formation would reach millimetres per thousand years.