Anthropogenic interactions with rock varnish

This chapter focuses on the ubiquitous biogeochemical coating known as rock varnish (sometimes called desert varnish in an arid climate) that forms naturally as a result of interaction between budding bacteria and clay minerals. This chapter interfaces with this book through exploring natural processes that generate this coating and the sensitivity of rock varnish to both natural climatic changes and anthropogenic forcings. High-resolution electron microscopy reveals that budding bacteria concentrate both manganese (Mn) and iron (Fe) on cell surfaces. Postdeposition processes mobilize nanoscale Mn and Fe that cements clay minerals to the underlying rock or preexisting varnish. Although other hypotheses exist to explain varnish formation, this model is the only proposed formation process that has a rate-limiting step and does not fall pray to the "varnish rate paradox." Despite rates of varnish accretion of microns per millennia in warm deserts and microns per century in more mesic settings, anthropogenic processes have altered its biogeochemistry in a variety of ways. Globally, lead fallout has contaminated the surface-most micrometer. Regionally, acid fog and anthropogenic dust generation alters varnish textures. Locally, ash from wildfire combines with graffiti such as chalk to coat varnishes. Humans have also applied "artificial varnish" to minimize the aesthetic impact of road construction in wealthier communities.