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 o… Show more
“…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.…”
Section: Discussionmentioning
confidence: 95%
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…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].…”
The Smithsonian Institution Building, commonly referred to as the Castle, is located on the National Mall in Washington, DC, and was constructed in the mid-nineteenth century for the purpose of housing all museum and scientific functions for the newly formed institution. Matching gateposts designed by the Castle's architect were erected more than a century later in the Enid A. Haupt Garden opposite the Castle. Black patches were recently noted on both structures, which are clad with locally quarried Seneca red sandstone. Portable x-ray fluorescence (XRF) spectrometry links the discoloration with elevated Mn concentrations. The discolored patches resemble rock varnish, a Mn-rich coating observed on rock surfaces formed in a variety of environments. Bulk rock and varnish chemistry, in addition to microscopy and microanalysis of the varnish, are presented here. On a bulk chemical basis, the Seneca sandstone is relatively poor in Mn, containing ~500 ppmw. In contrast, the rock varnish is greatly enriched in Mn relative to the stone and to a lesser degree in Pb, Ca, Zn, Cu and Ni. Cross sections of the black encrusted regions show that the stone's red coloration has been modified by black pigmentation from the surface down to ~250 μm. X-ray diffraction of blackened particles produced no discernable pattern, indicating concentrations below the detection limit, poor crystallinity, or both. Scanning electron microscopy and EDS-based x-ray microanalysis of the uppermost portion of the cross section reveal nanometer scale (<20-200 nm) Mn-rich and clay particles concentrated in a thin film (≪1 μm) at the surface. Additionally, Mn oxide particles decorate the surfaces of fine-grained minerals in sandstone pores within the discolored zone. Imaging and microanalysis of the rock surface reveal that the Mn-rich varnish is a discontinuous film ≪1 μm in thickness with an estimated composition of Na 0.2 Ca 0.1 Mg 0.1 Al 0.1 Si 0.5 Mn 1.9 Fe 0.5 O 6.7 . This composition most likely represents a nanoscale mixture of a Mn oxide (e.g., birnessite or todorokite) and an Al-rich silicate mineral. Seneca sandstone on the Smithsonian Castle and gateposts is discolored in patches owing to the Mnrich phase being deposited into two zones: (1) a vanishingly thin patina, and (2) nanoparticles coating grain boundaries and pores in the uppermost ~200-250 μm of the stone. While the mineralogy is similar to well-studied varnish formed in arid settings, rock varnish on the Smithsonian structures is significantly thinner. Because this architectural rock varnish is young, it may represent the earliest stages of formation of the more commonly described varnishes reported in the literature.
“…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.…”
Section: Discussionmentioning
confidence: 95%
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…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].…”
The Smithsonian Institution Building, commonly referred to as the Castle, is located on the National Mall in Washington, DC, and was constructed in the mid-nineteenth century for the purpose of housing all museum and scientific functions for the newly formed institution. Matching gateposts designed by the Castle's architect were erected more than a century later in the Enid A. Haupt Garden opposite the Castle. Black patches were recently noted on both structures, which are clad with locally quarried Seneca red sandstone. Portable x-ray fluorescence (XRF) spectrometry links the discoloration with elevated Mn concentrations. The discolored patches resemble rock varnish, a Mn-rich coating observed on rock surfaces formed in a variety of environments. Bulk rock and varnish chemistry, in addition to microscopy and microanalysis of the varnish, are presented here. On a bulk chemical basis, the Seneca sandstone is relatively poor in Mn, containing ~500 ppmw. In contrast, the rock varnish is greatly enriched in Mn relative to the stone and to a lesser degree in Pb, Ca, Zn, Cu and Ni. Cross sections of the black encrusted regions show that the stone's red coloration has been modified by black pigmentation from the surface down to ~250 μm. X-ray diffraction of blackened particles produced no discernable pattern, indicating concentrations below the detection limit, poor crystallinity, or both. Scanning electron microscopy and EDS-based x-ray microanalysis of the uppermost portion of the cross section reveal nanometer scale (<20-200 nm) Mn-rich and clay particles concentrated in a thin film (≪1 μm) at the surface. Additionally, Mn oxide particles decorate the surfaces of fine-grained minerals in sandstone pores within the discolored zone. Imaging and microanalysis of the rock surface reveal that the Mn-rich varnish is a discontinuous film ≪1 μm in thickness with an estimated composition of Na 0.2 Ca 0.1 Mg 0.1 Al 0.1 Si 0.5 Mn 1.9 Fe 0.5 O 6.7 . This composition most likely represents a nanoscale mixture of a Mn oxide (e.g., birnessite or todorokite) and an Al-rich silicate mineral. Seneca sandstone on the Smithsonian Castle and gateposts is discolored in patches owing to the Mnrich phase being deposited into two zones: (1) a vanishingly thin patina, and (2) nanoparticles coating grain boundaries and pores in the uppermost ~200-250 μm of the stone. While the mineralogy is similar to well-studied varnish formed in arid settings, rock varnish on the Smithsonian structures is significantly thinner. Because this architectural rock varnish is young, it may represent the earliest stages of formation of the more commonly described varnishes reported in the literature.
“…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.…”
Desert ecosystems spanning moisture conditions from dry grasslands to barren hyper-arid landscapes are the largest terrestrial biome with more than 40% of the terrestrial landmass. Remote sensing data provide an efficient costeffective means to assess biophysical indicators of land degradation and desertification, providing that essential ecosystem properties can be monitored. We review the spectral characteristics of plants and soils that are detectable using optical sensors and methods to identify and quantify properties that have potential for monitoring arid ecosystem processes. Vegetation indexes have little sensitivity at low leaf area, particularly when the soil background is highly variable, as is characteristic of many arid systems. Additionally, accumulated dry plant material on the soil surface challenges measurement. Although the absorption characteristics of the major biochemical constituents of plants and soils are generally understood, the methods to retrieve this information from reflectance data and to understand the significance of how the structural organization alters the absorption features remains an area of active research. The overlapping absorption features of plants and soils preclude direct assessment of many biogeochemicals of interest. New biophysical methods that take the full spectral shape into account, including the effect of one compound on the spectral absorption of another, are needed to reduce uncertainty in their estimates. As a result, despite significant progress in developing fundamental understanding of ecosystem processes and optical properties, more research is needed before fully predictable quantitative methods are available.
The chemistry of varnish scrapings from within and adjacent to rock engravings on sandstone outcrops in semiarid Australia was analysed using an electron microprobe (WDS mode). Small-area analyses were used to reduce potential contamination from identifiable within-varnish detrital grains and from substrate material. Ratios of (Ca + K):Ti were compared for rock engravings on older and younger rock surfaces, and between different engraving styles. Although ratios from varnish on the older surface were lower than for the younger surface, differences were not statistically significant. Sorting of analyses for potentially anomalous amounts of minor elements had little effect on the ranking of individual ratio values. Rankings of within-engraving ratio values were generally in accord with relative age expectations but differences between samples were not significant, with the chemical variability of varnish being high even without the contribution of discernible detrital inclusions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.