Desert Varnish - a “Microsoil” of the Arid Environments: Origin, Geographical Variability, and Paleoecological Signi cance (Review)

Desert varnish or rock varnish is a thin reddish-black microlayer that cover rocks and is composed mostly of clay minerals, iron and manganese oxides. In this paper, both terms - desert and rock varnish - are used interchangeably. For more than two centuries, rock varnish has been an object of study and interest. Because it develops mainly in arid and desert areas, the interactions between radiation, wind, humidity and microbial activity are determining factors in its formation. On the other hand, the growth rates (few microns per millennium) suggest that it can function as a tool for recording past environments. The current work presents the state of knowledge of rock varnish focusing mainly on inorganic studies. The formation theories, geographical location, magnetic properties, lithodiversity, climatic variability, dating insights, pedofeatures, and some historical aspects are discussed. In addition, the different analytical techniques that have been used to study varnish chemical and mineralogical composition are grouped.