We welcome the opportunity to reply to Dorn (2007) who outlines the classical issues of desert varnish formation. We agree that most previous research has suggested a biological mechanism. Bacterial enhancement of manganese, in manganese-rich coatings is intriguing. However, as we have published (Perry et al., 2006), there is no evidence of a direct link with microbes and varnish formation. In our studies, we rarely see microbes in or on the surface, even after critically point-dried specimens are viewed. Similar observations have been made by others who also noted the lack of bacteria in desert varnish (Taylor-George et al., 1983).
In our recent research, we have also observed silica coatings that are enhanced in manganese in Antarctica. Few microbes are known to exist in these locations, which removes the possibility of biologically mediated processes. We note that manganese-rich varnishes from Antarctica have been investigated previously (Dorn et al., 1992) and it was suggested that budding bacteria might be the cause, but the authors could not rule out contamination added in transit or in the lab.
It has been established experimentally that microbes are not needed to enhance manganese (Krauskopf, 1957; Jones, 1991), and varnish-like coatings have been produced in the laboratory without microbial involvement (Kolb et al., 2004; Perry et al., 2005). However, microbes do co-exist with minerals on rock surfaces, and a microbial involvement in other locations should not be ruled out. Specifically, bacterial DNA has been extracted from varnishes (Perry et al., 2004; Kuhlman et al., 2005, 2006) and other biological compounds (Perry et al., 2003; Schelble et al., 2005) but a biological causal mechanism has not been established. As we suggest in Geology, biological materials are probably entombed within silica in rock coatings and hot-spring silica (Lynne et al., 2006; Perry et al., 2006).
Other authors have suggested that silica might be involved in desert varnish formation and have noted the existence of silica glazes inter-layered with manganese-enhanced silica-rich coatings (Farr, 1981; Smith and Whalley, 1988). Varnishes form by relatively slow accretion, and the source materials are dusts or dissolved constituents in the atmosphere introduced by direct deposition (Thiagarajan and Lee, 2004). It follows that varnish deposition will reflect components available in the environment. If the entities available vary, then silica glaze deficient in manganese, iron, or other trace metals might be deposited under one environmental condition and trace element enhanced deposits in another. Such a process causes a colorless glaze to be interlayered with desert varnish. The environmental control on coating formation is illustrated by the fact that desert varnish and glazes that are juxtaposed are not the same thickness, suggesting that different environmental controls, primarily related to moisture, might control differences in deposition rate.
Conventional interpretations also suggest that manganese and clays are the predominant mineral components in desert varnish. Our work, however, reveals that silicon is usually the predominant element in varnishes, and aluminum is sometimes the second most abundant. Aluminum quantities are highly variable and often not consistent with Al/Si ratios in clays. Clays are present in varnishes (Potter and Rossman, 1977), but we have only found minimal clay content in varnishes using particle size separations. The role of silica in desert varnish formation has been substantially neglected in previous research. We look forward to the contributions of others in furthering our understanding this enigmatic coating and elucidating its silica-based formation mechanism.