Formation Of Rock Varnish Research Articles

Rock varnish revisited

Investigators seeking to solve the mystery of rock varnish formation have yet to embrace hypothesis testing. Thus, this paper presents nine tests that assess the validity of the eight hypotheses proposed over the last four decades to explain varnish formation. An almost singular focus on manganese (Mn)-enrichment by many in varnish research may have led to six of the eight hypotheses failing to explain iron (Fe)-enrichment, as well as why clay minerals dominate the composition of rock varnish. Many varnish researchers displayed a sampling bias by collecting samples only from hot and dry deserts; thus, it should be of no surprise that four hypotheses failed a test of explaining varnishes in different climates; five hypotheses failed to explain varnishes in subsurface locations; and seven hypotheses failed to explain differences in varnish growth rates in hot deserts versus wetter locations. In the end, seven of eight proposed hypotheses to explain varnish formation failed more than five tests, any one of which would falsify the hypothesis. Only one hypothesis “passed” all nine tests.

Biotic-abiotic mingle in rock varnish formation: A new perspective

Rocks in arid to semi-arid regions all over the world have a naturally occurring coating called “rock varnish” or “desert varnish” on their surface. These lustrous, black-to-brown mesoscopic coatings on rocks have long piqued the interest of scholars from a wide range of disciplines. However, there are gaps in our understanding, particularly in relation to the formation of rock varnish. Rock varnish development is supported by competing evidence from the biotic and abiotic schools of thought. In the current investigation, we advance and support a previous polygenetic model by proposing that biotic and abiotic factors collaborate to form these microscopic coatings on the rock surfaces. Physicochemical evidences in this work points toward a new perspective, citing an initial abiotic triggering event followed by biotic processes.

Analysis of Rock Varnish from the Mojave Desert by Handheld Laser-Induced Breakdown Spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) is a form of optical emission spectroscopy that can be used for the rapid analysis of geological materials in the field under ambient environmental conditions. We describe here the innovative use of handheld LIBS for the in situ analysis of rock varnish. This thinly laminated and compositionally complex veneer forms slowly over time on rock surfaces in dryland regions and is particularly abundant across the Mojave Desert climatic region of east-central California (USA). Following the depth profiling examination of a varnished clast from colluvial gravel in Death Valley in the laboratory, our in situ analysis of rock varnish and visually similar coatings on rock surfaces was undertaken in the Owens and Deep Spring valleys in two contexts, element detection/identification and microchemical mapping. Emission peaks were recognized in the LIBS spectra for the nine elements most abundant in rock varnish—Mn, Fe, Si, Al, Na, Mg, K, Ca and Ba, as well as for H, Li, C, O, Ti, V, Sr and Rb. Focused follow-up laboratory and field studies will help understand rock varnish formation and its utility for weathering and chronological studies.

Characteristics of desert varnish from nanometer to micrometer scale: A photo-oxidation model on its formation

Rock varnish is a widespread Mn-rich rock coating commonly developed in arid environments, but the mechanism for its formation is still under debate. In this study, rock varnish and adjacent soil dust collected from Gobi Desert were analyzed for exploring the possible abiotic oxidation mechanism of Mn oxides. The occurrence of rock varnish shows a direct and close relationship with strong irradiation of sunlight, giving the first evidence of photochemical genesis. Abundant caves and tunnels with average diameter of ~1–5 μm are observed on varnish surface regions, which facilitate the penetration of sunlight and water. Metal (oxyhydr)oxides, including birnessite, hematite, goethite, rutile and anatase, account for the major components of rock varnish, which are all solar light-responsive semiconducting minerals. The trace elements enrichment patterns revealed by LA-ICP-MS provide the indication of an aqueous origin. The positive Ce anomalies in varnish in contrast to the rock substrate, as well as the positive correlation between Ce and Mn suggest a strong oxidizing environment in the genesis of rock varnish. Therefore, the photo-generated holes and reactive oxygen species (ROSs) on metal oxides in varnish can promote Mn(II) oxidation, which are further demonstrated from thermodynamic and kinetic considerations. ROSs including 1O2 and OH with strong oxidizing capability are detected by EPR in varnish suspension, providing the direct evidence for Mn(II) oxidation. Laboratory experimental simulations show the photocatalysis of metal oxides in rock varnish greatly promote Mn(II) oxidation by 2.10–7.97 times in comparison with homogenous solution oxidation. All these lines of evidence suggest that light-induced abiotic process may play important roles in the formation of rock varnish together with other abiotic and biotic pathways, which can even provide implications for the evolution of Mn oxides on surface of terrestrial planets.

Natural Extracellular Electron Transfer Between Semiconducting Minerals and Electroactive Bacterial Communities Occurred on the Rock Varnish.

Rock varnish is a thin coating enriched with manganese (Mn) and iron (Fe) oxides. The mineral composition and formation of rock varnish elicit considerable attention from geologists and microbiologists. However, limited research has been devoted to the semiconducting properties of these Fe/Mn oxides in varnish and relatively little attention is paid to the mineral–microbe interaction under sunlight. In this study, the mineral composition and the bacterial communities on varnish from the Gobi Desert in Xinjiang, China were analyzed. Results of principal components analysis and t-test indicated that more electroactive genera such as Acinetobacter, Staphylococcus, Dietzia, and Pseudomonas gathered on varnish bacterial communities than on substrate rock and surrounding soils. We then explored the culture of varnish, substrate and soil samples in media and the extracellular electron transfer (EET) between bacterial communities and mineral electrodes under light/dark conditions for the first time. Orthogonal electrochemical experiments demonstrated that the most remarkable photocurrent density of 6.1 ± 0.4 μA/cm2 was observed between varnish electrode and varnish microflora. Finally, based on Raman and 16S rRNA gene–sequencing results, coculture system of birnessite and Pseudomonas (the major Mn oxide and a common electroactive bacterium in varnish) was established to study underlying mechanism. A steadily growing photocurrent (205 μA at 100 h) under light was observed with a stable birnessite after 110 h. However, only 47 μA was generated in the dark control and birnessite was reduced to Mn2+ in 13 h, suggesting that birnessite helped deliver electrons instead of serving as an electron acceptor under light. Our study demonstrated that electroactive bacterial communities were positively correlated with Fe/Mn semiconducting minerals in varnish, and diversified EET process occurred on varnish under sunlight. Overall, these phenomena may influence bacterial–community structure in natural environments over time.

Rock-art microbiome: influences on long term preservation of historic and culturally important engravings

The Burrup Peninsula in north-west Western Australia is home to one of the most substantial collections of rock engravings, or petroglyphs, in the world. These petroglyphs are carved through the dark coloured patina, commonly referred to as rock varnish, into the weathering rind of the local parent rock. Rock varnish is essentially a thin layer of manganese (Mn) and iron (Fe) oxides and hydroxides with embedded clay minerals, the formation of which is relatively poorly understood. It is generally considered to be a hostile environment for microorganisms due to extreme environmental conditions including low nutrient availability, lack of water, exposure to extreme ultraviolet radiation and intense seasonal and diurnal temperature fluctuations. However, despite these environmental extremes, microorganisms have been found on and in rock varnish and have been reported as playing a significant role in the formation of rock varnish. Given this, it is likely that any change in local environmental conditions will influence the types and activities of microorganisms found in and on rock varnish and associated rock art. This article focuses on the major influences on the microbiome of culturally important rock art in the Burrup Peninsula and the implications of any environmental change on the rock art itself.

Insights into the formation of rock varnish in prevailing dusty regions

AbstractThe characteristics of rock varnish from the Campo de Piedra Pomez (CPP, Andes Argentina) provides new insights into the development of rock varnish under severe dusty conditions. The CPP varnish has been analysed using SEM‐EDAX and Raman techniques. The rock coating is tens of microns in thickness and under the microscope shows a micro to cryptocrystalline appearance and a general granular texture. Silica (quartz, cristobalite and amorphous silica) is the main phase forming these coverings followed by Al, K and Na compounds. Minor Fe oxides and Mn oxides are the source for the orange (Fe) and brown (Fe and Mn) hues of the coating.The results obtained for the CPP coating show that desert varnish developed under dusty circumstances does not have the appearance of typical rock varnish. Textural characteristics include high crystallinity and granular arrangement of the components. Moreover the absence of typical microlaminations is related to the presence of aeolian mineral grains which inhibit their development. The main mechanism of formation of rock varnish under such environmental circumstances is the direct incorporation of aeolian mineral grains into the varnish. However, other physicochemical processes are also required to explain the formation of varnish components such as amorphous silica or iron oxides phases.Although the development of the desert varnish may act as a protector of the underlying pyroclastic rock, the extreme and persistent windy conditions in the CPP field are high enough to weather and erode not only the rock coating but also the original ignimbrite.As other warm desert sites on Earth, the CPP area can also be considered as possible terrestrial analogue to Mars. Some environmental attributes might be similar to those expected on the Martian surface and thus, textural similarities between the CPP varnish and the rock varnish‐like coating of Mars are likely. Copyright © 2014 John Wiley & Sons, Ltd.

From dust to varnish: Geochemical constraints on rock varnish formation in the Negev Desert, Israel

Chemical compositions of rock varnish from the Negev Desert of Israel and local settled dust were used to constrain the mechanisms of varnish formation and patterns of Mn enrichment and accumulation in the varnish. Rock varnish was sampled from coeval, undisturbed prehistoric flint artifacts along a south–north climatic transect (∼30–120mm/yr of rain). Our analyses indicate that Mn, Ba and Pb in the varnish are significantly enriched (∼100×) in respect to the local settling dust and that Mn content systematically fluctuates with depth in the varnish. The varnish and settled dust data combined with basic thermodynamic and kinetic reasoning are used to constrain the following geochemical model of rock varnish formation: dust accumulates in micro-basins on exposed rock surfaces, under pH ∼8 (common Negev value) and during wetting by dew and rain, Mn in the dust is mobilized and leached to a depth of ∼5μm under the varnish surface where Hollandite Mn-oxides precipitate and are adsorbed onto and between the porous clay minerals that comprise most of the varnish. During its mobile phase Mn-oxide is negatively charged and adsorbs rare earth elements. Once the solution dries abrasion removes the upper, weakly cemented dust sediment, which contains mainly Si, Al and Fe (which are not mobile at pH ∼8). Ca is also removed in large quantities. Mn, Ba, Pb and the REE are deposited at a depth and thus, protected from erosion. Reoccurrences of these processes result in a noticeable accumulation of these elements, but not of Si, Al or Fe. The alternating Mn-rich and Mn-poor laminas form as a result of a competition between the leaching rate of Mn and the adhesion rate of the clay minerals. When moisture is high (low), lamina with high (low) Mn/clay mineral ratio forms.

Anthropogenic lead as a tracer of rock varnish growth: Implications for rates of formation

Rock varnish is a thin dark coating best known from deserts, and is believed to grow extremely slowly. Varnish samples from near Socorro, New Mexico (United States), contain as much as 3.7% PbO, derived from nearby smelters operating from A.D. 1870 to 1931. Additional varnish, measuring as much as 4 μm beyond the Pb-rich layer, indicates continued growth from 1931 to 2003. Comparison with other varnish confirms that the Pb is not an artifact. Based on Pb layer thickness, and the period of smelter operation, these very young rock varnishes yield growth rates of 28–639 μm/k.y., substantially higher than previously documented fastest rates of 40 μm/k.y. These rates imply that the average 1–2 μm/k.y. rate for older varnish is not the active growth rate. Rather, it is a long-term value including periods of nondeposition, erosion, and active growth. Therefore, models of rock varnish formation should be reevaluated with consideration of much faster maximum growth rates.

Fungal Communities Associated with Rock Varnish in Black Canyon, New Mexico: Casual Inhabitants or Essential Partners?

Rock varnish is a darkly pigmented coating rich in manganese oxides. Though microbes inhabit varnish deposits, it is unclear whether they are involved in varnish formation. The fungal communities of rock varnish and adjacent rock sites with no visible varnish deposits were examined. Microcolonial fungi were identified at all sampling sites, and were associated with manganese oxides in patches of incipient varnish at non-varnish sites. Fungi were closely related to manganese-oxidizing genera and seventeen isolates oxidized manganese in culture, producing six distinct manganese-oxide morphologies. Our results indicate that microcolonial fungi may play a crucial role in rock varnish formation. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.

Astrobiological Implications of Rock Varnish in Tibet

The study of terrestrial geomicrobiology and its relationship to rock weathering processes is an essential tool in developing analogues for similar processes that may have occurred on Mars. Most studies of manganese-enhanced rock varnish have focused on samples taken from warm arid desert regions. Here, we examine samples obtained from eolian-abraded lava flows of the 4700-4800 m high Ashikule Basin in Tibet. Because it receives approximately 300 mm of precipitation annually, this site is nowhere near as dry as Atacama Desert locales. However, the dusty, sulfate-rich, high-altitude and high-UV flux environment of the Tibetan locale offers new insight into rock varnish formation processes in a terrestrial environment that displays some attributes similar to those expected on early Mars. Microprobe measurements reveal that Mn enhancements in varnish are two orders of magnitude above the dust source, but Fe is only enhanced by a factor of three. Manganese-enhancing bacterial forms are not abundant but are still approximately 3 times more common than in Mojave and Sonoran Desert varnishes. In addition to its occurrence in subaerial positions, Tibetan varnish also occurs in micron-scale "pods" enveloped by silica glaze and as remobilized constituents that have migrated into the underlying weathering rind. A lack of surficial Mn-rich varnish, therefore, might not imply the absence of varnish. In contrast to suggestions that silica glaze might be a good source of microbial fossils and a key to varnish formation, we did not observe any clear microfossil forms entombed in silica glaze; further, there is no gradation between varnish and silica glaze but only distinct contacts.

Models of rock varnish formation constrained by high resolution transmission electron microscopy

Nanometre‐scale transmission electron microscope imagery of manganiferous rock varnishes from Death Valley, California, Peru, Antarctica, and Hawaii confirms prior infrared mineralogy studies. The building blocks of rock varnish are clay minerals that are cemented to the rock by oxyhydroxides of manganese and iron. Rock varnish is layered on the scale of nanometres, with the basic structure defined by the subparallel alignment of detrital clay minerals. Although only a few examples of bacteria were found, possible cell‐wall encrustations are ubiquitous and aligned with the clay minerals. Mn–Fe oxides appear to be mobilized from bacterial casts and then reprecipitate on clay minerals that weather into monolayers. These observations have implications for varnish dating and palaeoenvironmental techniques: K–Ar and uranium‐series dating of rock‐varnish oxides can only yield minimum ages; however, this small spatial scale of mobilization would not alter the signal from micron‐scale microchemical laminations.

Rapid formation of rock varnish and other rock coatings on slag deposits near Fontana, California

AbstractManganiferous rock varnish, silica glaze and iron skins have formed on 20‐ to 40‐year‐old slag piles near Fontana, southern California. Rapid rock‐varnish formation is associated with an unidentified cocci bacterium that grows rapidly in culturing experiments, combined with the likelihood that Mn‐rich solutions flow over slag surfaces. A new model is proposed for the formation of silica glaze, involving soluble Al‐Si complexes and the nature of wetting films. Iron skins occur where runoff is concentrated, and where iron bacteria grow.Observations indicate: (i) substrate can be important in supplying Mn, as well as maintaining a stable surface and channelling runoff to microspots where varnish grows; (ii) ignorance of boundary layer pH/Eh conditions makes chemical‐physical models of varnish formation speculative; (iii) the balance between cementation of recently nucleated varnishes and erosional shear stresses is poorly understood, but is probably an important limiting factor on varnish growth rates; and (iv) there must be at least two steps in biotic varnishing‐accretion of Mn‐Fe in casts, followed by cementation of clays by either in situ cast residue or Mn‐Fe remobilized from casts. Varnish accretion may be limited by the rate at which both steps occur. Rapid development of readily distinguishable manganiferous rock varnishes at this post‐1952 site highlights the danger of speculation on the ages of landforms based solely on the appearance of rock‐varnish coatings.

Distribution and formation of rock varnish in southern Tunisia

AbstractField observations, chemical analysis, electron and light microscopy and radiolabelling experiments provide information on the characteristics, formation and distribution of Fe and Mn rock varnishes in southern Tunisia. Unstable rocks are not surface‐coated, but when broken they commonly reveal internal microbial solution fronts. Stable rocks are coated in Fe, and more rarely Mn, rock varnishes. Radiolabelling indicates the presence of many respiring microbes on both Fe and Mn varnishes. Microscopy shows fungi and bacteria, both on the Fe varnishes and within the rock. Only rare, microcolonial fungi occur on Mn varnishes, suggesting that the microbes are within or beneath the varnish. The abundance of microorganisms supports a biogenic origin for rock varnishes. Different microbial communities appear to be involved in Fe and Mn varnish formation. Microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated, apparently to avoid u.v. light and desiccation. This explains the concentration of elements that form u.v. opaque minerals and clays. The origin of these elements is external to the substrate. These results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern Tunisia.

Characteristics and origin of rock varnish from the hyperarid coastal deserts of northern Peru

The characteristics of a new type of rock varnish from the hyperarid coastal deserts of northern Peru, combined with laboratory experiments on associated soil materials, provide new insights into the formation of rock varnish. The Peruvian varnish consists of an Fe-rich, Mn-poor component covering up to 95% of a varnished surface and a Fe-rich, Mn-rich component found only in pits and along cracks and ridges. The alkaline soils plus the catalytic Fe oxyhydroxides that coat much of the varnish surfaces make the Peruvian situation ideal for physicochemical precipitation of Mn. However, the low Mn content of the dominant Fe-rich, Mn-poor component suggests that such precipitation is minor. This, plus the presence of abundant bacteria in the Mn-rich varnish and the recorded presence of Mn-precipitating bacteria in varnish elsewhere, suggests that bacteria are almost solely responsible for Mn-precipitation in rock varnish. A set of experiments involving Peruvian soil samples in contact with water-CO 2 solutions indicates that natural fogs or dews release Mn but not Fe when they come in contact with eolian materials on rock surfaces. This mechanism may efficiently provide Mn to bacteria on varnishing surfaces. The lack of Fe in solution suggests that a large but unknown proportion of Fe in varnish may be in the form of insoluble Fe oxyhydroxides sorbed onto the clay minerals that form the bulk of rock varnish. The results of this study do not substantively change R. I. Dorn's paleoenvironmental interpretations of varnish Mn:Fe ratios, but they do suggest areas for further inquiry.

Magnetic techniques applied to the study of rock varnish

We report the successful application of magnetic techniques to the study of rock varnish, a surficial coating that forms on rocks in arid environments. Using thin wafers of varnished and unvarnished greywacke sandstone, we have detected a stable remanent magnetization that can be unambiguously attributed to the rock varnish. We have also measured several rock magnetic properties of the rock varnish and have shown that the primary magnetic carrier is either magnetite or maghemite. Our results demonstrate the feasibility of applying magnetic techniques to the study of geomorphic surfaces. Subsequent rock magnetic and paleomagnetic studies of rock varnish should provide new information about the geochemical processes involved in the formation of rock varnish and about the validity of a new chronological technique known as rock varnish dating.