Rock Varnish Research Articles

Influence of desert varnish on the reflectance of gossans in the context of Landsat TM data, southern Red Sea Hills, Sudan

Laboratory spectra have been used for studying the characteristics of desert varnish in the context of Landsat TM data in order to establish its influence on the reflectance of gossans and their discrimination. Supported by limited chemical analysis, reflectance values from selected rock samples have been processed using statistical methods, and spectral plots produced and visually interpreted. Spectral analysis shows that paucity and Mn/Fe proportion in desert varnish, lightness or darkness of the rock, its mineral composition, and the wavelength under consideration control the effect of desert varnish on Landsat TM data. In general, overall reflectance of rocks is reduced by up to 70% in the VIS parts and about 20% in the SWIR parts of the electromagnetic spectrum due to desert varnish. For acidic rocks this effect seems to be intense, whereas mafic rocks are less influenced. The latter generally display spectral crossovers partly due to Mn/Fe content. In the VIS region absorption bands turn to featureless spectra due to desert varnish, whereas in the SWIR spectra are generally subdued but preserved. This has apparently little consequence on the discrimination of gossans from other rock types on composites constituted by bands from the VIS, IR and SWIR parts.

Rock Varnish: Recorder of Desert Wetness?

Rock varnish is a thin coating (<200 µm) of a cocktail rich in Mn, Fe, and clay minerals that is ubiquitous in desert regions. It has become the center of a contentious controversy revolving around its use to date geomorphic surfaces and/or to evaluate past climate conditions. We observe pronounced temporal variations in Mn and Ba concentration that are similar over large regions and that likely relate to variations in paleo-wetness. The mode of formation of varnish remains uncertain, but anthropogenic Pb concentrated in outermost varnish layers indicates its continued formation, and experiments using cosmogenic Be suggest that, while precipitation is a primary control, dust, dew, and aerosols may also be important in delivering the ingredients of varnish. We suggest several steps that may lead to rejuvenation and future breakthrough in varnish studies.

Desert pavements and associated rock varnish in the Mojave Desert: How old can they be?

Desert pavements are common features of arid landscapes and have been widely used as a relative age indicator of the geomorphic surfaces upon which they are developed. In this study I examined the patterns of pavement development as a function of elevation in the Mojave Desert as well as the causes for the gradual disappearance of pavement at high elevations. Pavement density, as measured by percentage of pebble coverage, decreases systematically with elevation gain by ;3% per 100 m, from 95% coverage below 500 m to less than 60% at 1700 m. Plants appear to be the main agent of pavement disruption; plant density decreases as pavement density increases. Burrowing by rodents and crusting by cryptobiota also disrupt pavement development at higher elevation. During the last glacial maximum, plant communities were displaced 1000‐1400 m downward in the Mojave Desert. Pavements today generally do not survive above the blackbush (Coleogyne ramossisma)-sagebrush (Artemisia tridentata) zone. Evidence from packrat middens shows that these and other plants typical of high elevations today grew as low as 300‐400 m during the last glacial maximum. I suggest that during the last glacial maximum, desert pavements were confined to the lowest alluvial fans of Death Valley and adjoining low valleys. No alluvial desert pavements above ;400 m in the region are older than the latest Pleistocene. By the same reasoning, desert varnish on desert pavements above 400 m may all be Holocene in age, except where developed on stable boulders.

Limestone weathering in contemporary arid environments: a case study from southern Tunisia

Weathering features are described from a meteorologically arid area in southern Tunisia. Active weathering is concentrated in topographic lows (pits and pans) that concentrate available moisture and are associated with endolithic and epilithic algae responsible for algal boring, plucking and etching of the limestone substrate. Deposits of gypsum have concentrated through evaporation within hollows and occur as isolated patches of tabular crystals indicating an additional weathering agent characteristic of arid environments. The active development of a complex microkarst and associated salt weathering are combining to destroy linear karstic features such as rillenkarren, widespread case-hardening and in places patches of iron- and manganese-rich rock varnishes. The complexity of active and inherited weathering features belies the perceived simplicity and lack of weathering opportunities associated with aridity. Instead it focuses on the need to consider the nature of microclimatic conditions at the rock/air interface especially when seeking to define desert weathering environments. It also highlights the difficulties of interpreting polygenetic landforms and the importance of microscale studies for identifying the impact of contemporary climatic conditions. Copyright © 2000 John Wiley & Sons, Ltd.

Evolution of desert colluvial boulder slopes

We employed rhyolite domes formed during the past million years in eastern California to study the evolution of boulder-dominated slopes. The slopes studied are steep (from ∼25° to ∼35°) and are made of coarse, boulder-sized blocks. These slopes include well-varnished vertically-oriented colluvial deposits that appear similar to relict periglacial stone stripes. In this case, they are the result of ongoing desert slope processes. Such deposits are common throughout the arid southwestern US, but their morphometric character, fabric and rates of formation have not been assessed systematically. Basic field and isotopic dating methods are combined with two relatively new surface dating techniques, cosmogenic chlorine-36 and rock varnish microlaminae, to produce a detailed million-year-long slope development history with broad implications for geomorphic theory. Grain size, grain shape, grain angularity, and slope deposit area do not change significantly from genesis to ∼0.5 Ma; trends in the data change markedly after that time. A geomorphic threshold occurs between ∼0.5 and ∼1 Ma. as indicated by significant changes in all parameters but grain orientation data. On-going erosion over the million-year continuum is evidenced by active debris flow features, by terminal Pleistocene ages of microlaminae on all deposits, and by 36Cl ages that are progressively younger than potassium–argon ages for slope genesis.

Rock Engraving Chronologies and Accelerator Mass Spectrometry Radiocarbon Age of Desert Varnish

It has been proposed that the minimum age of rock engravings can be established from dating organic carbon associated with desert varnish, a naturally occurring coating on rocks in arid regions. In this study, five radiocarbon ages were obtained from organic carbon in previously undated varnishes from the Pilbara, Western Australia. Four varnish samples were scraped from dull black coatings and one from patchy, shiny black varnish. All radiocarbon ages were younger than 2800 yearsbp . Ages were younger at the rock/varnish interface than in overlying varnish, a reversal of expected stratigraphic relations. These results highlighted a number of difficulties associated with the interpretation of varnish radiocarbon ages, including the potential absence of a link between organic carbon age and varnish deposition. Age linkages are complicated by the time-transgressive nature of varnish formation, its weathering over time, and the probable mixed ages of dated organic carbon. Engraving chronologies based solely on radiocarbon ages of varnish should be treated with extreme caution.

Desert pavement characteristics on wadi terrace and alluvial fan surfaces: Wadi Al-Bih, U.A.E. and Oman

In arid mountain areas, the dating and correlation of alluvial depositional surfaces is often uncertain. Especially in regions where the geomorphology is not well known, surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages. Pleistocene wadi terraces and associated alluvial fans occur in Wadi Al-Bih, U.A.E. and Oman, for which correlations and age relationships are not known. Three age-related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships. Deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after Miocene uplift of the region. The younger two groups of terraces and fans are inset within the older group. To identify the gross effects of pavement development, comparisons have been made between terrace surface and subsurface particle-size distributions. The older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces. This reflects the degree of pavement development. Particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces. Criteria for the classification of pavements were developed based on clast fracturing and angularity, size, sorting, packing, and surface texture, from which a simple index of pavement development has been derived. Other properties, rock varnish and weathering characteristics, were also recorded; but these proved to be less discriminatory than pavement characteristics. The pavement data have been augmented by observations on soils. Detailed studies of pavements on terraces (8 sites, 12 samples covering the three age groups) and fans (5 sites, 10 samples covering the three age groups) allow differentiation between age-groups. The three terraces show three different age-related pavement types, expressed by differences in the pavement development index. Weakly-developed pavements (little fracturing, sub-rounded clasts, some modification of the depositional fabric, incipient soil development, stage I CaCO 3 accumulation) occur on the youngest terrace and fan surfaces. Moderately-developed pavements (clast fracturing, sub-angular clasts, moderate sorting and packing, deeper soil development, stage II CaCO 3 accumulation) occur on the middle terrace and fan surfaces. Well-developed pavements (complete clast fracturing into small angular fragments, mature sorting and packing of the pavement surface, deep soil development with strong horizonation, stage III CaCO 3 accumulation) occur on the highest terrace and oldest fan surfaces. There are minor differences between the youngest pavements on terraces and fans, which reflect initial sedimentological differences. These differences become less as the pavements develop. On the basis of comparative studies, the oldest terrace is estimated to date from sometime prior to ca. 100 ka BP, the second terrace and the most extensive fan surface from the Late Pleistocene, and the youngest terrace and fan phase from the Latest Pleistocene or Early Holocene.

Terminal Pleistocene wet event recorded in rock varnish from Las Vegas Valley, southern Nevada

Analyses of rock varnish samples from latest Pleistocene alluvial-fan surfaces in Las Vegas Valley, southern Nevada, reveal replicable lamination patterns that are characterized by low-Mn orange surface layers and high-Mn dark basal layers. Radiocarbon dating from beneath the sampled alluvial-fan surfaces suggests that the Mn-rich basal layers accumulated during a short wet phase 10–11 14C ka when extensive black mats were deposited throughout the region, and paleolake records in the Great Basin also indicate wet conditions during this time period. In contrast, the Mn-poor orange surface layers formed under relatively dry conditions in the Holocene. Thus, these varnish microlaminations are connected with environmental fluctuations that appear to be related to climate change. Evidence from Las Vegas Valley, together with that from Death Valley and the Mojave Desert, suggests that the deposition of these Mn-rich dark basal layers in rock varnish likely corresponded in time to the terminal Pleistocene Younger Dryas-aged wet event in the Great Basin.

Characterization of alluvial bajada facies distribution using TM imagery

Automatic mapping techniques using multiband satellite image information have been used to study sediment grain‐size variations on an alluvial bajada. A previous study of sparsely vegetated alluvial surfaces in central‐western Argentina showed that the reflectivity recorded in seven‐band Landsat TM images is controlled by sediment composition, age and grain size. At diameters &gt;4 mm, clast composition influences image information, while at grain sizes smaller than sand, clay mineralogy begins to influence spectral characteristics. The progressive increase in desert varnish and the loss of fine grain‐size fractions as a result of deflation cause the age influence. The bajada sedimentary environment is well suited for testing the influence of grain‐size variability on image recovery, as most drainage basins are small and sourced by a single geological unit, which produces compositionally homogeneous alluvial surfaces. Additionally, most drainage lines are active within intervals of 10–50 years, reducing the effect of surface ageing. Larger than average drainage basins produce oversized drainage lines that disrupt the bajada, generate individual alluvial fans and have slightly different compositions. Two typical bajadas were selected to map grain‐size characteristics using automatic classification techniques. The obtained classes were checked in the field, and grain‐size and compositional counts were completed. Although both bajadas are very different in composition, most grain‐size curves showed similar shapes, suggesting that deposition took place by the same process (hyperconcentrated flash floods). The values of the median and mean were consistent across the same class between both bajadas. Thus, unsupervised classification techniques are useful for mapping sediment grain size, although minor field control is needed. Image classes represent areas of similar grain size, which are elongated parallel to the mountain front in an active alluvial bajada, indicating a homogeneous distribution of sedimentary processes along strike. Changes in the width of image classes indicate different downstream fining rates closely related to the topographic gap and the slope change rate. In contrast to bajadas, alluvial fans have semi‐circular belts and a pie‐piece‐shaped area in which most active streams are located. Thus, unlike fans, bajadas lack autocyclic mechanisms for producing heterogeneous sedimentary sequences. The sedimentary log of an ancient bajada was measured in order to show the influence of allocyclic factors in the absence of autocyclicity.

Rock coatings and aeolian abrasion on Mars: Application to the Pathfinder landing site

Rock coatings can be used to constrain the rate of abrasion by wind on Mars. The susceptibility to abrasion for potential rock coatings on Mars (salt/salt‐cemented coatings, rock varnish, and amorphous silica) were determined experimentally. Rock coatings generally abrade more easily than the host rock, although amorphous silica is an exception. If coatings exist on rocks at the Mars Pathfinder landing site, then the rate of abrasion is extremely low, consistent with previous studies of erosion rates. Alternatively, rock varnish or silica coatings could protect the rocks from sandblasting, although this may require coating formation under current to recent conditions. The Pathfinder area may lack efficient abrasive particles, and the observed duneforms may be composed of sand‐size dust aggregates rather than crystalline particles.

Revisiting William Morris Davis and Walther Penck to Propose a General Model of Slope ‘Evolution’ in Deserts

Based on research from slopes on rhyolite domes of known age formed over a million-year continuum in eastern California, a classic geomorphic debate is reconsidered and a general model of desert slope development proposed. This study examines steep (∼25° to ∼35°) boulder-dominated slopes that include well, varnished, vertically oriented colluvial deposits. Such deposits are common throughout the arid southwestern United States. Basic field and isotopic dating methods are combined with two surface-dating techniques, cosmogenic chlorine-36 and rock varnish microlaminae, to produce a detailed slope development history with broad implications for geomorphic theory that includes the unresolved geomorphic debate between Walther Penck and William Morris Davis. Slopes in this study are dominated by the on-going desert slope processes of debris flows and in-situ grain disintegration as evidenced by active debris flow features, terminal Pleistocene ages of microlaminae, and chlorine-36 ages progressively younger than potassium-argon ages for slope genesis. Results also indicate that slopes retreat in a parallel fashion as postulated by Penck. Furthermore, the deposits do not exhibit significant changes in grain size, shape, or angularity from genesis to ∼0.6 Ma but change markedly after that time possibly indicating a geomorphic threshold between ∼0.6 and ∼1 Ma, or episodic erosional events throughout the mid to late Pleistocene.

The weathering of granodiorite porphyry in the thiel mountains, inland antarctica

The weathering of granodiorite porphyry is examined in the extremely cold and dry environment of the inland part of Antarctica. Weathering features include granular disintegration, rock varnish, and sheeting. Sheeting has gradually proceeded since the exposing of the nunatak, but the other types of weathering have not actively advanced in recent times.Granular disintegration primarily comprises the release of individual crystals. The mechanism of this release is that cracks are created along crystal boundaries and cleavages in phenocrysts mainly due to differential thermal expansion. Plagioclase and hypersthene are released more readily than quartz. Disintegration of plagioclase has produced many prism‐shaped holes, 0.5 to 2.0 mm in length, while fine‐grained quartz crystals, 0.05 to 0.1 mm in diameter, are incidentally released without cracking.The reddish brown (10R4/4) rock varnishes result from oxidation and consist of limonite, which fills cracks and penetrates into crystals. Ferrous iron in hypersthene, biotite, ilmenite and magnetite is transformed into limonite by oxidation. Manganese is not found in the varnishes. Sulfur, which is important for oxidation and which may have originated from adhered snow, is concentrated on the surface of the rock.Sheeting has precipitated rock falls, and has gradually formed a gray mosaic on the varnished wall. The sheeting was caused by gravitational body force, which is internal stress of the rock body due to the mass of the overburden.

A review of the history of dating rock varnishes

The history of rock varnish dating spans almost 20 years from the initial suggestion to use uranium series isotopes through the use of a geochemical ratio (cation-ratio dating) to the current application of accelerator mass spectrometry radiocarbon dating (AMS 14 C ). Uranium isotopes were ignored while other dating methods were tested exhaustively on several continents. Evidence of the unreliable dating of the formation of varnishes using cation-ratios was demonstrated by examples of environmental and textural observations reflecting localised leaching, and by chemical analyses that contradicted the fundamental assumptions of the method. The radiocarbon dating of substances embedded in varnishes is also problematic where carbon compounds and sources are not identified. The incompatible coexistence of different carbonaceous components, a recent controversial issue, is explained with reference to different environments of formation. The future of AMS 14 C dating of rock varnishes centres around overcoming the practical limitations of sampling and analysing extremely small quantities of carbon-bearing substances. Future directions for dating varnished rock surfaces may include innovative refinements to the U-series, cosmogenic isotope and luminescence methods.

Rock varnish microlaminations from northern Tianshan, Xinjiang and their paleoclimatic implications

Laterally continuous dark and orange microlaminations are observed in rock varnish from the northern TianGhan, Xinjiang. Microprobe chemical analyses indicate that dark layers in the varnish are rimh in Mn and orange layers are poor in Mn. Microscopic examinations of varnish ultra-thin sections reveal a lamination pattern that is regionally consistent and comparable. Based on geomorphic age control, the Holocene varnish lamination sequence is tentatively established and its paleoclimatic implications are explored within the framework of existing paleoclimati data in the region.

How fast does rock varnish grow?

How fast does rock varnish grow?

How fast does rock varnish grow?

Research Article| February 01, 2000 How fast does rock varnish grow? Tanzhuo Liu; Tanzhuo Liu 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Wallace S. Broecker Wallace S. Broecker 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (2): 183–186. https://doi.org/10.1130/0091-7613(2000)28<183:HFDRVG>2.0.CO;2 Article history received: 28 Jun 1999 rev-recd: 08 Oct 1999 accepted: 11 Nov 1999 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Tanzhuo Liu, Wallace S. Broecker; How fast does rock varnish grow?. Geology 2000;; 28 (2): 183–186. doi: https://doi.org/10.1130/0091-7613(2000)28<183:HFDRVG>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Rates of rock-varnish accumulation have never been systematically documented owing to the difficulty in accurately determining both varnish thickness and age. In this study, we quantitatively assess varnish-accumulation rates through thin sectioning and microscopic examination of rock varnish from radiometrically dated geomorphic features of late Quaternary age in the western United States drylands. Our data indicate that rock varnish grows at rates ranging from <1 to 40 µm/k.y. on subaerially exposed rock surfaces and rarely reaches a thickness exceeding 200 µm regardless of age. Our data also indicate that varnish-accumulation rates vary greatly from sample to sample at a given site, suggesting that varnish thickness does not correlate with the age of the associated geomorphic feature, invalidating the potential use of varnish thickness as a relative age indicator in geomorphology and archaeology. Nevertheless, being the slowest known accumulating terrestrial sedimentary deposit, rock varnish constitutes a unique long-term microscale sedimentary archive of past environmental changes in deserts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Hanaupah-Fan shoreline deposit at Tule Spring, a gravelly shoreline deposit of Pleisticene Lake Manly, Death Valley, California, USA

The Hanaupah-Fan Shoreline Deposit (HSD) is an as yet undescribed occurrence of shoreline sediments of late Pleistocene Lake Manly in Death Valley, California. It is located in the southern part of Death Valley, at the northeastern periphery of Hanaupah Fan. The HSD is a gently sloping, WSW-ENE elongated ridge, about 600 m long, 165 m wide and 8 m high. Its surface extends from -12 to +28 m in elevation, i.e. it has a vertical range of 40 m. We interpret the deposit as a sediment body that extended from the Hanaupah Fan east into the lake. Rising lake level, and waves approaching both from the north and south eroded fan materials, and produced a sediment body with a complex architecture. Fetch for waves approaching from either direction was about 40 km. The sedimentary inventory consists of cross-stratified gravel beds of various size ranges, dipping towards the north, south, and east, and of horizontal berm gravel beds, and horizontal silt layers. A discordant gravel layer covers the entire surface of the deposit, probably produced by wave action during the last phase of lake regression. This uniform gravel layer forms a surface that is distinctly different from the surrounding fan surfaces. It is relatively fine grained, much better sorted, and densely packed. Rock varnish is very well developed, and imparts a dark color to the surface, which makes it easily recognizable on aerial photographs. No absolute age date is available as yet, but circumstantial evidence places the formation of the deposit at the peak of marine isotope stage 2 (Wisconsinan/Weichsellian glacial maximum)

Limestone weathering in contemporary arid environments: a case study from southern Tunisia

Weathering features are described from a meteorologically arid area in southern Tunisia. Active weathering is concentrated in topographic lows (pits and pans) that concentrate available moisture and are associated with endolithic and epilithic algae responsible for algal boring, plucking and etching of the limestone substrate. Deposits of gypsum have concentrated through evaporation within hollows and occur as isolated patches of tabular crystals indicating an additional weathering agent characteristic of arid environments. The active development of a complex microkarst and associated salt weathering are combining to destroy linear karstic features such as rillenkarren, widespread case-hardening and in places patches of iron- and manganese-rich rock varnishes. The complexity of active and inherited weathering features belies the perceived simplicity and lack of weathering opportunities associated with aridity. Instead it focuses on the need to consider the nature of microclimatic conditions at the rock/air interface especially when seeking to define desert weathering environments. It also highlights the difficulties of interpreting polygenetic landforms and the importance of microscale studies for identifying the impact of contemporary climatic conditions.

A clue regarding the origin of rock varnish

137Cs and excess 210Pb have been found in rock varnish samples from several localities in the desert regions of the western United States. As 137Cs was absent in the environment prior to the atmospheric testing of nuclear weapons, and as 210Pb has a half life of only 22 years, these isotopes must have been added to this slow growing patina during the last half of the present century, demonstrating that, at least in part, the elements constituting varnish are supplied from the atmosphere. The discovery opens a window into the mechanism by which this mysterious substance forms. Not only radioisotopes but also anthropogenic metals can be harnessed to assess the ratio of rate of accumulation of a given element on varnish to its delivery rate from the atmosphere. Based on measurements reported here, for 210Pb this ratio is about 0.1 and for 137Cs about 0.01.

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.