Rock Varnish Research Articles

Exploring the astrobiological potential of rock varnish from a mars analogue field site of Ladakh, India

Rock varnish, a dark-coloured natural feature rich in manganese (Mn), iron (Fe), and clay minerals that forms on rock surfaces and subsurface rock fractures in extremely dry and cold environments, is believed to provide nutritional support to microbiota. Because varnish supports an extensive microbial community, this rock coating is considered a substrate for potential microbial life to thrive in extreme environments on Earth. Although research in the past decades have advanced understanding of the varnish microbiome, little is known about this microbial community in settings that are high altitude (lower oxygen), dry, and cold. We present here new morphological, chemical, and rock magnetic results of rock varnish from this environmental setting, the Ladakh, a potential analogue site for life in extreme environments. Our results include the presence of putative magnetofossils-in the form of nanochains present in the rock varnish layer. Further, the higher concentrations of oxidised Mn4+ and carboxylic acid functionality on the varnish surface revealed organic signatures. These collective results point towards the enriched concentration of magnetic minerals on the varnish layer that are possibly sourced through biotic forms. Consequently, the rock varnish can serve as an archive of ancient environmental records, as well as a potential geomaterial for astrobiological studies from the Martian analogue field location of Ladakh, which needs to be explored further for extensive biogeochemical studies.

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.

Insights on the Formation Conditions of Manganese Oxides from Crimora, VA (USA)

Many regions of the United States contain manganese deposits economically valuable in New England, Appalachian, and Piedmont regions in the Eastern United States, in Northern Arkansas, and, to a small extent, in Central–Western California. Mn oxide/hydroxide (commonly referred to as Mn oxide minerals) are found in a wide variety of geological settings and occur as fine-grained aggregates, veins, marine and freshwater nodules and concretions, crusts, dendrites, and coatings on rock surfaces (e.g., desert varnish). How manganese oxides form and what mechanisms determine which oxides are likely to form are limited and still debated. This paper focuses on Mn oxides collected at the southern bound of the abandoned open-pit site called Crimora Mine (Augusta County, Virginia). This study uses mineralogical and chemical features to shed light on the origin of manganese deposits in Crimora along the western foot of the Blue Ridge in South–West Virginia. We report the first detailed study on the genesis of the Crimora manganese deposit conducted since the mine was closed in the 1950s. Crimora Mine sample is dark black fine- to medium-grained round and oblong nodules coated with a fine-grained intermix of yellowish earthy limonite, clays, and quartz. Scanning electron microscopy (SEM) revealed that the Crimora Mn-oxides exhibit concentric layering, breccia-like matrices, and veins. X-ray powder diffraction (XRPD) identified the set of Mn minerals as hollandite and birnessite. The concentration and range of dissolved chemical species in freshwater, seawater, and hydrothermal depositional fluids impart a geochemical signature to the Mn-oxides, providing a diagnostic tool to shed light on their genetic origin. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of the Crimora manganese oxides shows Mn, Fe, and Ti, as well as trace elements such as Co, Ba, Y, Zn, Cr, Ni, Tl, La, V, and Li. A bivariate analysis based on the geochemical correlation of Mn and other common substituting cations (e.g., Fe, Co, Ti) shows a mixed genesis in different environments with varying biological and sedimentary supergene (freshwater and marine) conditions. These data suggest that the Mn-rich deposit in Crimora, VA, was formed in a continental margin environment of surficial deposits and reprecipitated in mixed biogenic and supergene conditions.

Decay of ignimbrite fairy chimneys of Arizona's Basin and Range Province, USA

AbstractFairy chimneys of ignimbrite are well studied in Cappadocia in central Anatolia, Turkey. The ignimbrite landforms of Arizona's Basin and Range, USA, in contrast, remain relatively unexplored in geomorphic scholarship, despite decades of geological research on the characteristics of the ignimbrites themselves. This research focuses on the rock‐decay processes that modify Arizona's Basin and Range fair chimneys. Dissolution of glass and groundmass, measured along joints using digital image processing of back‐scattered electron microscope imagery, reveal a 3× higher porosity than inside the rock interior. This method also measured a 3‐4× increase in dissolution near the base of the chimney compared with the rock interior, leading to notch development that promotes chimney mass wasting and subsequent ballistic impacts. Epilithic organisms studied with electron microscopy both enhance rock decay and promote case hardening. These observations are consistent with prior understanding of how fairy chimneys evolve over time: columnar joints exposed on cliff faces experience enough decay to allow erosion of joint material, this erosion separates the chimney from the rock face and notch development near the chimney base promotes mass wasting. The rock‐decay processes studied here likely operate on very different timescales. Mass wasting events leading to ballistic impacts occur at observable timescales. Varnish microlamination dating, using ultrathin sections of rock varnish formed on fairy chimneys, indicates that the case hardening of the top of fairy chimneys are much more stable than the sides; the tops last experienced surface detachment during the early Holocene, as opposed to the sides that experienced flaking throughout the late Holocene. The timescale of basal notch development is not known, with the exception that the very surface of notches erode at rates too fast to accumulate rock varnish. The timescales of epilithic organisms enhancing rock decay and dissolution of columnar joints are not known.

Age estimates for the rock art at the Rocky Ridge site (Utah) based on archaeological and archaeometric evidence

Rock art provides unique information on human prehistory, but the difficulty of dating these images limits our ability to place them into a larger archaeological context. In this study, we apply a combination of archaeological information and archaeometric measurements to address this challenge in a field study at the rock art site at Rocky Ridge, Utah. We developed a preliminary classification scheme of descriptive figure types and performed measurements of Mn and Fe areal densities by portable X-ray fluorescence (pXRF) to examine the suitability of the rock varnish for deriving age estimates based on the degree of varnish re-accumulation on the petroglyphs. The pXRF-based age estimates indicate rock art creation from the Early Archaic through the Late Prehistoric periods and are generally consistent with independent archaeological information, but also highlight some of the limitations of the technique.

Lithodiversity and cultural use of desert varnish in the Northern Desert of Mexico

Rock varnish is a thin layer (1-40 microns every 1000 years), dark-reddish in color (30% Mn and Fe oxides, 70% clay minerals), that coats a rock surfaces in the northern Mexican deserts. This accretion has been used as a canvas by ancestral human groups to record different types of motifs (zoomorphic, geometric and anthropomorphic) in the northern Mexican desert (Sonora and Chihuahua). These petroglyphs reflect the cultural and artistic nature of past inhabitants and the varnish preserves them through the millennia. Rock varnish coats a variety of rock types, and this paper explores differences in the chemical composition and interaction of varnish and the underlying rock. We studied varnish from three different sites: Samalayuca (Chihuahua); El Álamo (Sonora); and La Proveedora (Sonora). The analytical techniques of microscopy, X-Ray Diffraction (XRD) and Laser Breakdown Spectra (LIBS), reveals a similar chemical composition (Mn, Fe, Ca and Al mainly) that it does not depend on the lithodiversity or location (Samalayuca -sandstone-, El Álamo -sandstone- and La Proveedora -granite-). We observed differences in the contact between the varnish and the underlying rock. Varnish penetrate into decayed minerals such as plagioclase; thick accumulations can occur on hard minerals such as quartz. Like prior research, we find little important contribution of material from the underlying to its varnish coating and conclude its main component derives mostly from aeolian deposition.

Artificial weathering of rock types bearing petroglyphs from Murujuga, Western Australia

AbstractMurujuga in Western Australia has the largest concentration of ancient rock engravings (petroglyphs) in the world. However, the Murujuga rock art is potentially threatened by local industrial air pollution, in particular by acid rain, but unambiguous scientific evidence is still missing. Here, we report on results of an accelerated weathering experiment, simulating Murujuga weather and climate conditions that was designed and performed to test whether the expected small changes in chemical, mineralogical, and physical characteristics of the rock surface can be detected and reliably quantified by various analytical means. Locally acquired Murujuga granophyre and gabbro samples with natural varnish were artificially weathered for up to four months in a climate chamber under conditions that simulated 2 years of natural weathering. Mineralogical, chemical, and physical changes were qualitatively monitored by X-ray diffraction and confocal Raman spectroscopy, and quantified by colorimetry, portable X-ray fluorescence spectrometry, and micro-computed tomography. In addition, artificial rainwater that was sprinkled over the rock samples was collected and analysed by inductively-coupled plasma mass spectrometry. The results show significant chemical and physical changes of the surfaces of the rock varnish after 1 month of artificial weathering. The analytical results demonstrate that it is possible to quantitatively monitor small changes caused by the weathering of gabbro and granophyre. Therefore, such a semi-actualistic experimental approach, when carefully designed, potentially allows testing the hypothesis that the weathering rate of the Murujuga petroglyphs is increased by local industrial air pollution. Further experimental work is currently under way.

Archaeometric studies on rock art at four sites in the northeastern Great Basin of North America.

Rock art originated some 46,000 years ago and can provide unique insights into the minds of our human ancestors. However, dating of these ancient images, especially of petroglyphs, remains a challenge. In this study, we explore the potential of deriving age estimates from measurements of the areal densities of manganese (DMn) and iron (DFe) in the rock varnish on petroglyphs, based on the concept that the amount of varnish that has regrown on a petroglyph since its creation, relative to the surrounding intact varnish, is a measure of its age. We measured DMn and DFe by portable X-ray fluorescence (pXRF) on dated Late Pleistocene and Holocene rock surfaces, from which we derived accumulation rates of Mn and Fe in the rock varnish. The observed rates were comparable to our previous findings on basalt surfaces in North America. We derived age estimates for the rock art at four sites in the northern Great Basin region of North America based on DMn measurements on the petroglyphs and intact varnish. They suggest that rock art creation in this region began around the Pleistocene/Holocene transition and continued into the Historic Period, encompassing a wide range of styles and motifs. Evidence of reworking of the rock art at various times by Indigenous people speaks of the continued agency of these images through the millennia. Our results are in good agreement with chronologies based on archeological and other archaeometric techniques. While our method remains subject to significant uncertainty with regard to the absolute ages of individual images, it provides the unique opportunity to obtain age estimates for large ensembles of images without the need for destructive sampling.

Birnessite-clay mineral couple in the rock varnish: a nature's electrocatalyst

Efficient water splitting by birnessite-clay minerals in natural rock varnish, paving the way for the synthesis of a new generation of cost-effective MnO2 electrocatalysts inspired by nature.

Characterization of Rock Varnish Ferromanganese Crusts on Ancient Egyptian Wall Paintings from Bahariya Oasis, Egypt

Characterization of Rock Varnish Ferromanganese Crusts on Ancient Egyptian Wall Paintings from Bahariya Oasis, Egypt

When Red Turns Black: Influence of the 79 AD Volcanic Eruption and Burial Environment on the Blackening/Darkening of Pompeian Cinnabar.

It is widely known that the vivid hue of red cinnabar can darken or turn black. Many authors have studied this transformation, but only a few in the context of the archeological site of Pompeii. In this work, the co-occurrence of different degradation patterns associated with Pompeian cinnabar-containing fresco paintings (alone or in combination with red/yellow ocher pigments) exposed to different types of environments (pre- and post-79 AD atmosphere) is reported. Results obtained from the in situ and laboratory multianalytical methodology revealed the existence of diverse transformation products in the Pompeian cinnabar, consistent with the impact of the environment. The effect of hydrogen sulfide and sulfur dioxide emitted during the 79 AD eruption on the cinnabar transformation was also evaluated by comparing the experimental evidence found on paintings exposed and not exposed to the post-79 AD atmosphere. Our results highlight that not all the darkened areas on the Pompeian cinnabar paintings are related to the transformation of the pigment itself, as clear evidence of darkening associated with the presence of manganese and iron oxide formation (rock varnish) on fragments buried before the 79 AD eruption has also been found.

Threshold of Weathering Needed for Fire to Erode Rock Art: Case Study of a Hohokam Petroglyph, Central Arizona, USA

ABSTRACTA Sonoran Desert petroglyph panel experienced an intense wildfire event in July 2021 that eroded the entire surface, removing the Hohokam-style rock art. Field observations during sampling in 1995 indicated that the panel: (1) was coated with a heavy rock varnish, (2) had a ‘fresh’ visual appearance, and (3) had some granite-derived sand (angular grus) at the panel’s base. Micron-scale back-scatter and nanoscale transmission electron microscopy of pre-fire samples revealed a minimal amount of decay (granite grussification): mainly minor grain-to-grain separation; minor internal dissolution; and a little feldspar grain cracking. Our basic finding is that even this minimal amount of grussification was enough to set the stage for the wildfire to erode the entire panel. Pre-fire micron-scale cracking may have enabled the fire’s steep thermal gradient to spall the surface. Panel erosion was likely enhanced by pre-existing grain-to-grain porosity to facilitate further fire-induced granular disintegration. Pre-fire nanoscale dissolution within mineral grains, formed along crystal defects, provided a weakness that then led to grain cracking of quartz and other granitic minerals. The implication for the conservation of rock art on granitic panels is worrisome, but clear and simple: condition assessments need to indicate whether any granitic sand occurs at a panel’s base. Given that many places experiencing climate change are also experiencing drought and enhanced risk from wildfire, the appropriate management recommendation would then be to remove all vegetation near the panel on a regular basis.

X-ray Microspectroscopy and Ptychography on Nanoscale Structures in Rock Varnish

X-ray microspectroscopy is a powerful analytical method in geoscientific and environmental research as it provides a unique combination of nanoscale imaging with high spectroscopic sensitivity at relatively low beam-related sample damage. In this study, “classical” scanning transmission soft X-ray microscopy (STXM) with X-ray absorption spectroscopy and the recently established soft X-ray ptychography are applied to the analysis of selected rock varnish samples from urban and arid desert environments. X-ray ptychography enhances the spatial resolution relative to STXM by up to 1 order of magnitude. With its high chemical sensitivity, it can resolve nanoscale differences in valence states of the key varnish elements manganese (Mn) and iron. Our results emphasize the complex nanoarchitecture of rock varnish as well as the diverse mineralogy of the Mn oxy–hydroxide matrix and its embedded dust grains. In contrast to the fast-growing urban varnish, the slow-growing arid desert varnish revealed a remarkable nanoscale stratification of alternating Mn valence states, providing hints on the layer-wise and still enigmatic growth process.

Floodwater farming and quarrying at Jabal Hamra Arlbieg in the Jordanian desert: Economic support for the classical period Faynan Orefield

Recent research has shown that the Faynan Orefield was a scene of intensive metal production during the Classical Period, but the infrastructure supporting this activity is less well known. We present evidence for previously undetected floodwater-farming and quarrying, which contributed to the economic life of the Faynan complex during Classical times. The site is located on an ancient route, south of the main orefield. We describe the hydrological control features of the floodwater farm and a possible place of habitation. Pollen analysis suggests that olives and cereals were cultivated. Exposures of sediment sequences containing buried walls, and ceramics both within and upon these sediments, all indicate that the activity took place during Nabatean to Late Roman/Byzantine times. Two ancient quarried areas were distinguished from natural landforms by a combination of geomorphic properties and variations in the geochemistry of the Mn-Fe rich desert varnish on long-exposed and quarried rock surfaces. Essentially uncontaminated by metal pollution, the site provided uncontaminated produce and building stone to the Faynan Orefield. Sediments and soils on the site are essentially unpolluted.

Life-sized Neolithic camel sculptures in Arabia: A scientific assessment of the craftsmanship and age of the Camel Site reliefs

The life-sized, naturalistic reliefs at the Camel Site in northern Arabia have been severely damaged by erosion. This, coupled with substantial destruction of the surrounding archaeological landscape, has made a chronological assessment of the site difficult. To overcome these problems, we combined results from a wide range of methods, including analysis of surviving tool marks, assessment of weathering and erosion patterns, portable X-ray fluorescence spectrometry, and luminescence dating of fallen fragments. In addition, test excavations identified a homogenous lithic assemblage and faunal remains that were sampled for radiocarbon dating. Our results show that the reliefs were carved with stone tools and that the creation of the reliefs, as well as the main period of activity at the site, date to the Neolithic. Neolithic arrowheads and radiocarbon dates attest occupation between 5200 and 5600 BCE. This is consistent with measurements of the areal density of manganese and iron in the rock varnish. The site was likely in use over a longer period and reliefs were re-worked when erosion began to obscure detailed features. By 1000 BCE, erosion was advanced enough to cause first panels to fall, in a process that continues until today. The Camel Site is likely home to the oldest surviving large-scale (naturalistic) animal reliefs in the world.

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.

Oxidizing atmosphere and life on land during the late Paleoproterozoic outset of the “boring billion”

The so called “boring billion” (1800–800 Ma) of Late Paleoproterozoic and early Mesoproterozoic time can now be reassessed from the evidence of diverse paleosols in the Ruyang Group (1749–1561 Ma) in Henan Province, China. Widespread marine sulfidic facies is the reason for the term “boring billion”, but terrestrial facies and paleosols were far from uniform or anoxic at that time. The Ruyang Group in Henan has 12 distinct kinds of paleosols, with 5 distinct pedotypes in alluvial fan facies of the Bingmagou Formation, 6 in fluvial facies of the Yunmengshan Formation, 3 in coastal plain facies of the Baicaoping Formation, and 4 in the coastal to intertidal Beidajian Formation. These paleosols are evidence of hyperarid climate that was cool temperate despite proximity to the paleoequator. These ancient soils were alive with iron- and manganese-fixing bacteria that created desert varnish and vesicular structure, as well as sulfur-oxidizing bacteria creating desert roses of gypsum. The paleosols may also have supported cyanobacteria, algae and fungi, known from palynomorphs in shales of the Baicaoping and Beidajian formations. Consumption of alkali and alkaline earth elements in the paleosols are evidence of 1700–20,600 ppm (6–73 times preindustrial level or PAL) of atmospheric CO2, consistent with the amount needed for a greenhouse effect to offset a faint young sun. A comparable model based on iron and manganese oxidation in the paleosols indicates 318–16,623 ppm (0.002–0.08 PAL) O2. This was enough to support animals (0.005 PAL O2), considering that these are minimal estimates of O2 diluted by some soil respiration. Animals had not yet evolved, but that evolutionary process may not have been limited by atmospheric composition.

Trace elements migrating from tailings to rock varnish laminated sediments in an old mining region from Nelson, Nevada, USA

The dispersion and influence of soluble and particulate trace elements (TEs) present in the materials from abandoned mines is an imminent problem on terrestrial and aquatic environments. The goal of this research is to provide a broader view of such transport, assessing particle size distribution and TE interactions (e.g., scavenging) with mine tailings and locally derived sediments transported by water in arid region. Sand grains from wash sediment were collected from a dry ephemeral wash in Nelson, Nevada where the Techatticup Mine and Mill worked between 1850 and 1960. Samples were evaluated to determine the association and behavior of certain metals and metalloids including aluminum, arsenic, barium, cadmium, copper, chromium, iron, manganese, lead, selenium, vanadium, and zinc to rock varnish laminated sediments. Results show that some metals and metalloids were more concentrated on larger particles whereas others indicated the opposite tendency. For example, As was greatest on silt fractions and least on coarse fractions, while Se was detected only on silts. Cr, Cu, Fe, Mn, Pb, and Zn concentrations all increased with decreasing particle size (silt > sand > coarse), whereas Al, Ba, and V showed the opposite trend (silt < sand < coarse). The enrichment factor (EF) of Cu, Zn, Pb, As, and Se were lower in mine tailing sediment treated with wash than those obtained for source material. However in treated wash sediment samples, the high EFs of Ba, Cr, V, and Mn suggested a possible greater affinity of these metals to the Al, Fe, and Mn oxides from the RV sediment. The data found in this study show that transport of contaminated sediments in arid regions could lead to the enrichment of some TEs in down-gradient areas from source materials.

Profiling Bacterial Diversity in Relation to Different Habitat Types in a Limestone Cave: İnsuyu Cave, Turkey

Caves are semi-closed ecosystems that experience the absence of sunlight and often, poor organic matter conditions due to physical isolation. Therefore, microbial activity is a crucial factor for sustaining life in caves. However, the heterogeneity of microbial diversity and community structure in relation to different types of habitats in karstic caves (e.g., drip water residue, mineral deposits, rock varnish) remains largely unknown. In this study, cave deposit samples in various morphology were analyzed to reveal its bacterial and archaeal diversity of İnsuyu Cave in Burdur Province that is located in central Turkey. Illumina MiSeq sequencing of seven cave deposit samples (S1–S7) from the zones with a diverse array of environmental conditions revealed a total of four bacterial phyla including (1) Proteobacteria ranging from 80.20% to 98.72%, (2) Actinobacteria ranging from 0.23–13.03%, (3) Firmicutes ranging from 0.18 to 12.61% and (4) Bacteroidetes ranging from 0.00% to 3.80%, while none of the samples exhibits sign for archaeal life form. The heatmap diagram on abundance and distribution of the genera, referring to the results of 16S rRNA gene-based metagenomic analysis of the cave-dwelling microbial communities in İnsuyu Cave, revealed that the structures of the bacterial communities were significantly different across the samples (S1–S7). The high abundance of taxa that is relevant to anthropogenic activity revealed a strong impact of agricultural activity on the bacterial composition of İnsuyu Cave.