Rock varnish

Abstract

Because of its wide distribution, the dark stain known as desert varnish, which covers rock surfaces in arid regions, has intriguing potential as a research tool in diverse fields of investigation. Dark coatings on rock surfaces can be seen in nearly all environments, but are most conspicuous in arid regions where coarse rock debris and solid rock outcrops dominate the landscape. The mode of origin of the coating and the enormous enhancement of manganese in it as compared with ambient levels have been topics of controversy for more than 100 years. More recently, there has been concern regarding the influence of desert varnish on the remote sensing of arid lands (e.g. Warner and El-Baz, 1978;Perry, 1979;Elvidge and Collet, 1981).Desert varnish is the arid phase of a wider phenomenon that we have termed 'rock varnish' (Dom and Oberlander, 1980;Dom, 1980). Rock varnish is usually 10 to 30 microns thick, but we have observed varnish depths ranging from under two to over 500 microns. The constituents of all rock varnishes are derived from sources external to the underlying rock. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, detrital particles such as Si02 and CaC03 , and trace elements, among which Mg, Ca, K, Na, Ti, and Cu are most notable. The classic rock varnish, frequently described in the literature on arid landscapes, is a manganese-rich black coating on rocks, but orange to dusky-brown varnishes are also widespread. Iron and manganese are the distinctive elements of rock varnish. Weathering rinds and encrustations of calcium carbonate, sulphate, and other salts are regarded as distinct from the phenomenon of rock varnish.The degree of development of rock varnish has long been used by archaeologists to estimate the relative ages of stone artifacts and prehistoric rock inscriptions. Similarly, the relative development of varnish in arid regions has been used by geomorphologists to differentiate and correlate surface deposits of alluvium, talus, and eolian sand, as well as erosional features such as cliff faces and relict lake shorelines of varying ages. It has long been hoped that some characteristic of varnish might be used to establish absolute chronologies that could be used in both geomorphological and archaeological research. Wide variations in its chemical composition suggest that rock varnish could also be used as a geochemical prospecting tool. Finally, changes in chemical composition and micromorphology through the

Rock varnish on architectural stone: microscopy and analysis of nanoscale manganese oxide deposits on the Smithsonian Castle, Washington, DC

“…Therefore, the source of Mn must be delivered to the rock surface externally via atmospheric transport and surface leaching/dissolution of airborne dust grains. The final step 3 requires oxidation and precipitation of Mn either by a physiochemical process under acidic oxidized conditions in rainwater [26] or via microbially assisted oxidation as favored by other investigators [7,11,27]. The microstructural imagery collected on the Smithsonian Castle varnish offers no evidence for microbial entombment in Mn oxide, and the organisms observed on the surface (Fig.…”

“…Manganese and iron rock varnishes have been noted to occur in a wide range of environmental settings from well-studied examples in deserts around the globe to less studied examples in rainforests of multiple continents, near active glaciers, and on rocks exposed in rivers [7]. Given the latter, it is perhaps not entirely surprising to learn of rock varnish formation in Washington, DC, which is in the humid subtropical zone according to the Köppen climate classification.…”

“…According to previous investigators, e.g., [7,10], the mechanism of formation of Mn rock varnishes involves multiple steps: (1) accumulation of externally derived clay-bearing dust on a rock surface [10], (2) transport of Mn to the rock surface in solution within rain/ fog droplets, and (3) oxidation and precipitation of Mn oxide cementing previously unconsolidated clay into a heterogeneous nanophase mixture.…”

“…The black discoloration is best described as rock varnish, a thin dark coating on rock surfaces found in all environments but most easily noticed in arid settings where unvegetated geological features prominently stand out [7]. Varnishes are heavily enriched in Mn relative to the rocks they coat, and previous investigations have determined their mineralogy [8][9][10] and outlined the evidence for microbial activity in several rock varnishes [11].…”

Remote sensing based assessment of biophysical indicators for land degradation and desertification

“…Generally, as pavement ages the rock surfaces become smoother with few protruding fragments. Exposed surfaces are coated with thicker varnishes, which are black or brown coatings formed by bacteria interacting with clay minerals (~70%) and iron or manganese oxides [93][94][95]. Spectrally, desert pavement and varnish lower surface albedo relative to younger soil and rock surfaces.…”