Rock Varnish Research Articles

An ecophysiological explanation for manganese enrichment in rock varnish

SignificanceRock varnish is a prominent feature of desert landscapes and the canvas for many prehistoric petroglyphs. How it forms—and, in particular, the basis for its extremely high manganese content—has been an enduring mystery. The work presented here establishes a biological mechanism for this manganese enrichment, underpinned by an apparent antioxidant strategy that enables microbes to survive in the harsh environments where varnish forms. The understanding that varnish is the residue of life using manganese to thrive in the desert illustrates that, even in extremely stark environments, the imprint of life is omnipresent on the landscape.

Iconographic and archaeometric studies on the rock art at Musayqira, Al‐Quwaiyah Governorate, central Saudi Arabia

AbstractWe investigated the rock art (petroglyphs) in the Al‐Quwaiyah Governorate (Saudi Arabia) with focus on the Musayqira site. Iconographic analysis showed a broad variety of human depictions, game animals (ibex, ass, ostrich, lion, etc.), domestic animals (cattle, camel, horse, dog, etc.), inscriptions and abstract symbols. Archaeometric measurements by pXRF provided the areal densities of Mn on petroglyphs and intact rock varnish. Varnish accumulation rates were derived from Mn density measurements on inscriptions of known approximate age. Applying these rates to varnish densities on the petroglyphs yielded age estimates. Some of the rock art was incised in fracture fill rather than true varnish, complicating the interpretation of the data. The combination of iconographic and archaeometric data indicated multiple phases of rock art creation, corresponding to the Late Neolithic, Bronze Age, Iron Age, and Islamic Period, with different artistic traditions, reflecting changing socioeconomic and ecological conditions.

Rock varnish record of the African Humid Period in the Lake Turkana basin of East Africa

Rock varnish is a manganiferous dark coating accreted on subaerially exposed rocks in drylands. It often contains a layered microstratigraphy that records past wetness variations. Varnish samples from latest Pleistocene and Holocene geomorphic features in the Lake Turkana basin, East Africa display a regionally replicable microstratigraphy record of Holocene millennial-scale wetness variability and a broad interval of wetter conditions during the African Humid Period (AHP). Three major wet pulses in the varnish record occurred during the generally wet interval of the early Holocene (11.5–8.5 ka) when the lake attained its maximum high stand (MHS) at 455–460 m. A >23 m drop from the MHS occurred between 8.5 and 8 ka. Subsequently two additional wet pulses occurred during the early to middle Holocene (8–5 ka) when the lake occupied its secondary high stand at 445 m. Collectively, these five wet phases represent an extended wet interval coincident with the AHP in the region. One moderate wet phase occurred during the subsequent climatic transition from the humid to arid regime (5–4.3 ka) after the lake level dropped rapidly from 445 m to <405 m. Five minor wet phases took place during the overall arid period of the late Holocene (4.3–0 ka) when the lake level oscillated below 405 m. These findings indicate that the AHP terminated rapidly around 5 ka in the Turkana basin in terms of lake level drop, but the regional shift in relative humidity from the AHP mode to its present-day condition lagged for about 700 years until 4.3 ka, hinting at a gradual phasing out in terms of moisture condition. These findings further suggest that Lake Turkana overflowed intermittently into the Nile drainage system through its topographic sill at 455–460 m during the early Holocene and has become a closed-basin lake thereafter for the past 8 ky.

Age estimation of tectonically exposed surfaces using cation-ratio dating of rock varnish

Cation-ratio dating (RCD) is a non-destructive and rapid means of dating rock varnish. This study explored that use of rock varnish RCD as a means of aging tectonically exposed surfaces in a semi-arid region of northwestern Greece with varying degrees of tectonic activity. Tectonic structures in the study region were identified and aged using conventional dating techniques, and faults were classified as active, potentially active, or inactive. A stringent field sampling methodology was adopted which successfully determined the concentrations of calcium, potassium, and titanium cations in rock varnish on the exposed surfaces of these faults. Results showed that RCD was a valuable tool for dating tectonic structures, and could be used to determine their activation age. The rapidly obtained results of this study could be used to expedite the identification of active and potentially active faults. Furthermore, these findings would contribute to neotectonic mapping efforts and other studies that relate to the tectonics of a specific area. The RCD method was less destructive and more accurate compared to alternative numerical dating methods and was a rapid means of identifying tectonically active faults. It has the potential to be employed in managing future disasters and mitigating serious economic repercussions from tectonic activity.

Clay minerals identification in rock varnish by XRD: A one-step reduction approach

Rock varnishes are known to be fine, dark, glossy submicron films found in deserts bare rock surfaces. The oxides and hydroxides of manganese and iron bind together the clay minerals present in the varnish layer. The processes of oxide-hydroxide accumulation at varnish sites are due to iron and manganese oxidizing bacteria which may require clay minerals for additional nutrition. Quantification and identification of clay minerals in this biofilm is needed to understand its formation. Past attempts to analyze the mineralogical composition of rock varnish have led to inconclusive results as varnish is a submicron thin layer composed of a complex mineral matrix. The elimination of non-crystalline cementing groups comprising of free iron oxides is a key step in the identification of many types of clay minerals, particularly in soil/sediment mineral studies.•The Fe-Mn oxide-hydroxide coatings, acting as cementing materials, can be easily removed using a one-step reduction method employing Na2S2O4 at 70 °C, leading to separation of clay minerals.•We have taken the lead from earlier reported Jackson (1958) method, wherein a combination of reagents was used such as sodium acetate, sodium citrate, hydrogen peroxide, sodium bicarbonate, and sodium dithionite for removing carbonate, organic carbon and Fe-Mn oxy-hydroxide coatings respectively from sediment grains to segregate individual grains from each other.•Our modification helps in the unveiling of clay minerals from a solid substrate and reports the X-ray diffraction peaks, which are elsewise hard to detect and therefore earlier studies are inconclusive.

Imaging and analysis of near-infrared fluorescence from tiger chert and Native American tiger chert artifacts

Tiger chert was used extensively by Native American people of the Clovis lithic industry as the source material for stone tools. Although tiger chert occurs naturally only within the Bridger Formation of Southwest Wyoming, tiger chert artifacts have been found throughout the American west. This paper explores the previously unreported near-infrared fluorescence emitted from tiger chert upon visible excitation. The two objectives of this study were to demonstrate the utility of near-infrared fluorescence imaging (NIRFI) to tiger chert artifacts and to identify the chemical origin of the observed fluorescence. Tiger chert was analyzed by visible and NIRFI, SEM-EDX, ICP-OES, and fluorescence spectroscopy. NIRFI can be easily used to discriminate between tiger chert artifacts and the environment, since most materials such as soil and most rocks appear black in the resultant image. This suggests NIRFI should be utilized by archeologists at any site where tiger chert has been previously identified. Additionally, NIRFI was used to differentiate between tiger chert artifacts and an artificially weathered, recently-knapped tiger chert point, suggesting that NIRFI could be used to distinguish genuine tiger chert artifacts from counterfeits. Tiger chert fluorescence was confined to the manganese-and-iron-rich surface, suggesting the presence of desert varnish; however, red agate from the same site did not exhibit fluorescence, instead implicating tiger-chert-specific weathering. Thus, the chemical origin of fluorescence was not conclusively identified.

Rock varnish in La Proveedora/Sonora in the context of desert geobiological processes and landscape evolution

Desert varnish is a dark microlayer that forms on rocky surfaces that is usually associated with arid and desert environments. It consists mainly of clay minerals (60%), while the rest are Fe and Mn oxides. Growth rates are very slow and vary from <1 to 40 μm/ky. Although different proposals exist to explain their formation mechanisms, these processes are still unknown. Around the world, various groups and human communities have created petroglyphs with different meanings in desert varnish. In the Sonoran Desert, the archaeological site La Proveedora is known for having many petroglyphs made in granite rock varnishes. It is believed that people groups occupied the area during the mid-Holocene (ca 5000-3000 BC). The research was carried out by analyzing and looking for possible signals and contributions from past environments and current processes that are preserved in the varnish layers. The X-ray diffraction, laser breakdown spectra and scanning electron microscopy with energy dispersive spectroscopy analyses in varnish samples, showed that the distribution and concentrations of Fe and Mn exhibit an alternating behavior on a depth scale from the surface until contact with granite. The DNA analysis showed a major presence of the Proteobacteria and Actinobacteria groups. Notably, some of these microorganisms can incorporate Fe and Mn into their metabolic processes and mobilize them through the varnish for its formation. The 14C radiocarbon dating (370 ± 54 cal BP) indicates a very young age associated primarily with recent microorganisms. However, a first approximation was obtained for the minimum age (5000-1000 yr BP). The comparison between the areas of varnish and the surfaces within the petroglyphs suggests that the varnish formation occurred under conditions in the past when the humidity was higher, and that it is probably a very slow, intermittent or uncoated formation mechanism today.

Archaeometric studies on the petroglyphs and rock varnish at Kilwa and Sakaka, northern Saudi Arabia

AbstractWe conducted rock varnish measurements at four rock art sites in north‐western Saudi Arabia, including Kilwa and the Camel Site near Sakaka. We determined the areal densities of Mn and Fe in rock varnish that had accumulated on petroglyph surfaces since their creation, complemented by a detailed analysis of varnish samples. We inferred varnish accumulation rates by relating the Mn areal density on inscriptions to their ages estimated based on the type of script used. Applying these rates to the varnish densities on the rock art indicated that the art was produced during two distinct periods, corresponding to the Pre‐Pottery/Late Neolithic and the Bronze/Iron Age, respectively, with different artistic traditions, reflecting distinct socioeconomic and ecological conditions. Our dating approach, while admittedly burdened with substantial uncertainties, yields ages consistent with archaeological and historical evidence and it provides a unique quantitative tool to obtain at least rough ages for otherwise undatable rock art.

Rock varnish as a natural canvas for rock art in La Proveedora, northwestern Sonoran Desert (Mexico): Integrating archaeological and geological evidences

Rock surfaces covered with desert varnish frequently feature petroglyphs and thus comprise an important resource for the artistic expression of ancient societies. Knowledge about the processes of varnish formation would be helpful to assess the production, regeneration and durability of this “natural canvas” and related rock art. We performed a multidisciplinary study of rock varnish associated with petroglyphs at the archaeological site of La Proveedora (Sonora, Mexico) that has been occupied by humans for at least 4500 years. An inventory and mapping of these ancient engravings revealed 5873 individual images on 1331 rock surfaces. Classification of these motifs showed that geometric forms were the most commonly featured, followed by zoomorphic and anthropomorphic designs. Colorimetric measurements according to the L*a*b* system confirmed that the carvings have a strong color contrast with the background patina. Petrographic observations revealed the variable thickness of the varnish coating, which penetrates deeper into the granitic rock at contacts with plagioclases and ferromagnesian minerals, forming a “rooted” configuration of the lower boundary. Under SEM thin globular laminae were observed; plagioclase and biotite adjacent to the varnish showed signs of alteration. EDS microanalysis, LIBS and X-ray diffraction demonstrated that the patina consists of Fe and Mn oxihydroxides and clay minerals with differing amounts of Fe and Mn within the coating. These results support the polygenetic model of rock varnish development. Weathering of granite and eolian particles give rise to clay mineral formation, whereas Fe and Mn mobilization and reprecipitation is controlled by microorganisms. Varnish formation was more intensive under moister conditions which prevailed in Sonora during the terminal Pleistocene. Thus, rock varnish comprised a non-renewable resource for ancient artists, being a relict feature. The Holocene rates of patina development were too slow to re-pigment engravings, so the petroglyphs represent a persistent element of a cultural landscape. We attribute the surprising durability of the dark coatings to the “rooted” configuration of their lower boundaries that could be important for petroglyph preservation.

Geochemical studies on rock varnish and petroglyphs in the Owens and Rose Valleys, California.

We investigated rock varnish, a thin, manganese- and iron-rich, dark surface crust, on basaltic lava flows and petroglyphs in the Owens and Rose Valleys (California) by portable X-ray fluorescence (pXRF) and femtosecond laser-ablation inductively-coupled-plasma mass spectrometry (fs-LA-ICPMS). The major element composition of the varnish was consistent with a mixture of Mn-Fe oxyhydroxides and clay minerals. As expected, it contained elevated concentrations of elements that are typically enriched in rock varnish, e.g., Mn, Pb, Ba, Ce, and Co, but also showed unusually high enrichments in U, Cu, and Th. The rare earth and yttrium (REY) enrichment pattern revealed a very strong positive cerium (Ce) anomaly and distinct negative europium (Eu) and Y anomalies. The light rare earth elements (REE) were much more strongly enriched than the heavy REY. These enrichment patterns are consistent with a formation mechanism by leaching of Mn and trace elements from aeolian dust, reprecipitation of Mn and Fe as oxyhydroxides, and scavenging of trace elements by these oxyhydroxides. We inferred accumulation rates of Mn and Fe in the varnish from their areal densities measured by pXRF and the known ages of some of the lava flow surfaces. The areal densities of Mn and Fe, as well as their accumulation rates, were comparable to our previous results from the desert of Saudi Arabia. There was a moderate dependence of the Mn areal density on the inclination of the rock surfaces, but no relationship to its cardinal orientation. We attempted to use the degree of varnish regrowth on the rock art surfaces as an estimate of their age. While an absolute dating of the petroglyphs was not possible because of the lack of suitable calibration surfaces and a considerable amount of variability, the measured degree of varnish regrowth on the various petroglyphs was consistent with chronologies based on archeological and other archaeometric techniques. In particular, our results suggest that rock art creation in the study area continued over an extended period of time, possibly starting around the Pleistocene/Holocene transition and extending into the last few centuries.

Properties and Occurrences of Rock Coatings in Jeli, Kelantan As A Record of Environmental Processes

Rock coatings can be defined as a product material deposition from surroundings. It is resulted from the wide variety of reactions which occur at the interface between the lithosphere and the biosphere. The coatings are biochemically and mineralogically complex and have the potential to record changes in their immediate environment. The changing of the coating gives information about the factors involved during the process which include dissolution and chemical reaction. The range of their activities and their proper significance in the environment as well as their relationship with the microorganisms that inhibit rocks and minerals provide a lot of information of the process involved. Types of coatings that have been identified on the outcrops around Kampung Kalai, Jeli are case hardening, iron film, dust film, rock varnish and lithobiontic coatings. The environmental exposures and inputs to the coatings, determines the coating mineralogy that formed on the parent rock. Scanning Electron Microscope (SEM) was used to analyse the microscopic properties of the coating samples. The presence of iron oxides have been found on the brick orange coating which potentially occurred from chemical weathering such as oxidation and the leaching of iron-bearing minerals. Besides that, geochemical analyses using X-Ray Fluorescence (XRF) on selected samples have shown limited information about the changing elemental composition. Iron, aluminium and silica are the major elements composition in the rock coatings reflecting the similar elemental composition of the existing parent rock.

AEOLIAN SANDS IN NORTH AFRICA AND THE SOUTHWEST OF THE ARABIAN PENINSULA

Background. The majority of the world’s deserts were formed within the valleys and coasts of ancient alluvial rivers and lakes. The material carried across many hundreds and thousands of kilometres by the wind was mixing with sediments of various genetic types. In addition, due to the similar physical nature of the wind and water transport of sediments, the recognition of the aeolian environment can be problematic. Aim. Using innovative research approaches, to provide a qualitative structural description of sands; to identify and characterize factors contributing to the formation and preservation of a ferruginous film on grains; to establish the genesis of the transferred sediments, as well as the relationship with the parent rocks. Materials and methods. Samples for analysis were collected from aeolian deposits in three deserts: Rub al Khali (United Arab Emirates), Nubian desert (Egypt) and Grand Erg Oriental (Algeria). Particle size analysis was used to characterize samples in terms of three parameters (length, width, thickness). X-ray diffraction analysis was carried out using an ARL X’tra diffractometer (Switzerland). The collected samples were also studied using a binocular microscope in reflected light and a scanning electron microscope. Results. The mineral composition of the sands under study was examined. The graphs (histograms) of the size distribution of quartz grains (being the most common sand mineral in terms of size and frequency of occurrence) were plotted. The shape and nature of the grain surface were analysed. Conclusion. The frosting on aeolian sand grains can result not only from micro-cracking as the most common sign of wind transfer, but also from the presence of a calcite film on grains. Factors contributing to the formation and preservation of a ferruginous film (desert varnish) were identified. The conditions for the formation of aeolian sands, as well as the genesis of such sand sediments, were established.

Geochemical insights into the relationship of rock varnish and adjacent mineral dust fractions

Rock varnishes are μm-thin, dark, manganese(Mn)-rich crusts that accrete in the order of few μm/ka on weathering-resistant lithologies. Although these crusts can form in all climates, they are best known in arid to semi-arid settings. Aeolian dust is understood as a major contributor to the distinct trace metal and REE enrichments in rock varnish. However, the exact proportions of abiotic and biotic formation mechanisms that may explain the oxidation-reactions of Mn2+ to Mn4+, present as Mn oxyhydroxides in the varnish, are still a matter of ongoing debate.We present here the first systematic study of trace element enrichment processes between the uppermost layer of the varnish sequence and their adjacent <50 μm and >50 μm dust fractions across a selection of 41 major and trace elements. This approach is used to investigate samples from three fully arid deserts: the An Nafud in Saudi Arabia, the Negev in Israel, and the Mojave in the USA, which are compared to the significantly different environment of the semi-arid Knersvlakte in South Africa. A new in situ trace element analysis protocol was developed to perform femto- and nanosecond sector-field LA-ICP-MS microanalyses at high-spatial resolution, which allows us to measure the most recent varnish layers for their comparison with recent dust. In agreement with previous studies, all varnishes are enriched in Mn, Pb, Ce, Co, Ba, Zn, Ni, and the Rare-Earth Elements (REE). Here we demonstrate that fine (<50 μm) dust is characterized by similar trace element trends as the varnishes, at overall lower mass fractions. Dust >50 μm has low trace element mass fractions, and enrichment patterns plotting distinctly away from varnish and fine dust. Based on these geochemical patterns, our results indicate a general enrichment mechanism from fine dust to varnish. Previous studies suggested dust to play an integral role in providing the trace elements that are incorporated into the varnish by pH-Eh fluctuations in short-term rain and fog events. We amalgamate and refine previous growth models by providing direct evidence that leaching of about 10% of the Mn and other trace elements from clay minerals in rain or fog droplets at pH ~5 and subsequent scavenging on varnish surfaces at pH ~8 leads integrated over time to the distinct enrichment patterns of the varnish, while initial Mn oxyhydroxide formation is suggested to follow pathways of metal oxide mediated photo-catalysis.For the semi-arid occurrence in the Knersvlakte we present a distinct growth model as both environment, varnish, and dust composition differ significantly from the arid settings. Here, thick, metallic-looking varnish occurs mainly on the rim of quartz pebbles, lacks microlamination, and likely has upscaled growth processes. Among other aspects, we suggest a more complex interplay between photo-catalysis, nocturnal condensation events on quartz pebbles with subsequent water trapping at the rock-soil-atmosphere interface, due to a salic top-soil layer, to mainly account for these differences.

Nanoscale structure and compositional analysis of manganese oxide coatings on the Smithsonian Castle, Washington, DC

The Smithsonian Castle is a historic building in Washington, D.C. that was erected 165 years ago and served as the first home for the Smithsonian Institution. Currently, the building exhibits dark surface discoloration due to manganese-rich coatings that are growing in patches across its Seneca sandstone exterior. Bulk and microanalysis of this coating indicate that it is consistent with rock varnish, but the coating is difficult to analyze without contribution from the underlying minerals that comprise the sandstone. To address this limitation, SEM/FIB-prepared cross-sections of the coating were analyzed using HRTEM, STEM-EDX, and SAED analyses in order to study the nanoscale structure and composition of the Mn-rich coating in isolation, without convolution from the sandstone. Structurally, the coating is <1 μm thick, polycrystalline, and composed primarily of birnessite, a layered manganese oxide. High spatial resolution data reveal that the coating is Ca-bearing, with only minor quantities of Fe, Si, and Al, distinguishing it compositionally from typical manganese rock varnish and classifying it instead as a manganese skin. TEM revealed layers of aeolian detritus below and a thin silica glaze above the Mn-rich coating, providing a relative timeline of formation. Nanoscale particulates of barite found within the Mn-rich coating, along with the micro- and nanoscale detritus below the coating, indicate atmospheric deposition as the main source for both Mn and Ba. The compositional and structural information gathered from these analyses helps constrain mechanisms of formation towards a biological origin; however, no direct evidence for Mn oxide producing microorganisms was observed.

Macro-classification of meteorites by portable energy dispersive X-ray fluorescence spectroscopy (pED-XRF), principal component analysis (PCA) and machine learning algorithms

The research on meteorites from hot and cold deserts is gaining advantages from the recent improvements of portable technologies such as X-ray fluorescence spectroscopy (XRF). The main advantages of portable instruments include the fast recognition of meteorites through their classification in macro-groups and discrimination from materials such as industrial slags, desert varnish covered rocks and iron oxides, named “meteor-wrongs”. In this study, 18 meteorite samples of different nature and origin were discriminated and preliminarily classified into characteristic macro-groups: iron meteorites, stony meteorites and meteor-wrongs, combining a portable energy dispersive XRF instrument (pED-XRF), principal component analysis (PCA) and some machine learning algorithms applied to the XRF spectra. The results showed that 100% accuracy in sample classification was obtained by applying the cubic support vector machine (CSVM), fine kernel nearest neighbor (FKNN), subspace discriminant-ensemble classifiers (SD-EC) and subspace discriminant KNN-EC (SKNN-EC) algorithms on standardized spectra.

Racing environmental changes and paleoclimate using the micromorphology of soils and desert varnish in central Iran

Soil and desert varnish are powerful records capable of saving invaluable data regarding environmental factors and processes during their formation stages. The present research was carried out to identify the environmental variations and paleoclimate reconstruction in the central deserts of Iran using soil and varnish micromorphological characteristics. Mantled pediment, alluvial fan, and alluvial plain landforms were selected. A minimum of one representative pedon was described and sampled on each geomorphic surface, amounting to a total of eight pedons. Varnished rocks were further collected from all geomorphic surfaces and studied by petrography microscope. Clay (coatings and micro layers), calcite (nodules, coatings, quasicoatings, and infillings), anhydrite (nodules), halite (coatings) pedofeatures, clay coating-calcite infilling, and anhydrite nodule-clay coating compound pedofeatures were investigated in the thin sections of the soil. Lenticular, vermiform, and platy gypsum crystals were identified as nodules and interlocked plates. Desert varnishes (100-600 µm) were different from host rocks as far as color, texture, and formative components are concerned. According to micromorphological evidence, the area probably experienced two different climates. Coatings and infillings of clay in soils and rock crevices were developed in an environment with more available humidity. Evaporite minerals were formed in soils and clay coatings on rock surfaces in the following period with less available moisture. The study results showed that micromorphology could be a necessary and useful tool in pedology and paleopedology investigations.

Taxonomic and functional insights into rock varnish microbiome using shotgun metagenomics.

Rock varnish is a microbial habitat, characterised by thin (5-500 μm) and shiny coatings of iron (Fe) and manganese (Mn) oxides associated with clay minerals. This structure is well studied by geologists, and recently there have been reports about the taxonomical composition of its microbiome. In this study, we investigated the rock varnish microbiome using shotgun metagenomics together with analyses of elemental composition, lipid and small molecule biomarkers, and rock surface analyses to explore the biogeography of microbial communities and their functional features. We report taxa and encoded functions represented in metagenomes retrieved from varnish or non-varnish samples, additionally, eight nearly complete genomes have been reconstructed spanning four phyla (Acidobacteria, Actinobacteria, Chloroflexi and TM7). The functional and taxonomic analyses presented in this study provide new insights into the ecosystem dynamics and survival strategies of microbial communities inhabiting varnish and non-varnish rock surfaces.

The “Kalgutinsky” Style in the Rock Art of Central Asia

On the basis of petroglyphic sites Kalgutinsky Rudnik (Kalgutinsky mine) on the Ukok Plateau, Baga-Oigur and Tsagaan-Salaa in northwestern Mongolia, a distinct “Kalgutinsky” style of rock art of the Russian and Mongolian Altai is described. The distance between these sites is about 20 km. This group is marked by very specifi c stylistic features, common technological properties, a narrowly defi ned motif, featuring only animals, and a very intense desert varnish. All these features and the proximity of the sites suggest that they should be regarded as a special group, which we term the “Kalgutinsky” style and date to the Upper Paleolithic on the basis of several criteria. Images of mammoths at Baga-Oigur and Tsagaan-Salaa are similar to those known in the classic Upper Paleolithic cave art of Western Europe. An entire set of stylistic features typical of the “Kalgutinsky” canon is seen also in the representations of mammoths, and this manner is consonant with that of European Upper Paleolithic rock art. Our fi ndings suggest that a peculiar “Kalgutinsky” style existed and, moreover, that it represented a separate Central Asian locus of Upper Paleolithic rock art.

Nanoscale Analysis of Manganeous Oxide Rock Varnish on the Smithsonian Castle, Washington, DC

Nanoscale Analysis of Manganeous Oxide Rock Varnish on the Smithsonian Castle, Washington, DC

Age, palaeoenvironment, and preservation of prehistoric petroglyphs on a boulder in the oasis of Salut (northern Sultanate of Oman)

The preservation of rock art in open-air contexts is a global issue controlled by several environmental processes, which are less investigated than the cultural significance of engravings and paintings. For that reason, we discuss the age, preservation, and palaeoenvironmental context of petroglyphs discovered on the flat, almost vertical face of a large boulder fallen along the western slope of Jabal Hammah, a rocky hill that borders the oasis of Salut (northern Sultanate of Oman). Geoarchaeological investigation highlighted that, in the region, the preservation of petroglyphs is due to the interplay of two contrasting weathering processes. On one hand, karst dissolution – even if it is a very slow process in arid and semi-arid lands – gradually levels the surface of boulders. On the other hand, a biomineralized Mn- and Fe-rich rock varnish has developed inside the grooves of the engravings, thus sheltering them from extreme dissolution and promoting the preservation of the pristine shape of the representations. Moreover, organics trapped within the rock varnish have been radiocarbon dated to 2600 ± 60 uncal. years BP. This result allows establishing a limit ante quem for the production of these specific engravings and to root it to the Bronze or Iron Age exploitation of the area. This result is of particular relevance in a region where well-dated rock art is virtually absent. Today, the biogeochemical processes leading to the formation of the protective crust are almost inactive, and not consistent with the present dry environmental settings. Their occurrence is in accordance with other local palaeoclimatic record, and suggests Bronze and Iron Age climatic conditions wetter than today. A broader implication of our work is that it shows how a multidisciplinary approach to the study of rock art provides the opportunity of understanding the age of rock art and its paleoenvironmental significance. We demonstrate that physical, chemical, and biological weathering processes are in charge of the preservation and/or destruction of rock art; such processes have to be seriously taken into account in projects of rock art field assessment.