Manganese accumulation in rock varnish on a desert piedmont, Mojave Desert, California, and application to evaluating varnish development
This paper presents an initial attempt to quantify the progressive development of varnish coats over time by measuring the amount of Mn accumulated in varnish collected from geomorphic surfaces of different ages. The data demonstrate that rock
varnish can show significant spatial variation in degree of development on geomorphic surfaces of similar age, and imply that collecting varnish as old as a geomorphic surface may be difficult on surfaces as young as late Pleistocene.
Factors that interfere with the age determination of rock varnish
Discussion of the potential of rock varnish as a Quaternary dating technique for geomorphical and archaeological studies. New backscattered electron microscopy imagery of varnishes from southwestern North America illustrates the need for great
caution in the selection of samples for all varnish dating methods.
Microchemistry of small desert varnish samples, Western New South Wales, Australia
The aim of this study is to investigate varnish chemistry, especially the ratio of (Ca+K):Ti, in known or inferred relative age situations. Instead of examining the entire varnish deposit, small-area chemical analysis points were selected
on the scrapings to include the varnish matrix or cement and to exclude as much other material as possible. The results were then examined to establish whether element ratios of varnish were in accord with archaeological (rock engraving) and geomorphic evidence
A note on the characteristics and possible origins of desert varnishes from southeast Morocco
Identification of two substrate - controlled varnish types : a patchy varnish interspersed with silica glaze on silicified limestone and a complete varnish cover on partially - silicified limestone. Electron probe microanalysis and scanning electron
microscopy were used in conjunction to investigate their chemical composition. In both instances, varnishes are predominantly silica and iron-rich with localized concentrations of manganese. Inorganic fixation of iron and manganese is the preferred
Distribution and formation of rock varnish in southern Tunisia
Field 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. The abundance
of microorganisms supports a biogenic origin for rock varnishes.
Characteristics and origin of rock varnish from the hyperarid coastal deserts of northern Peru
This paper describes a new variety of rock varnish from the hyperarid coastal deserts of Peru, presents experimental evidence for a physicochemical mechanism that enhances Mn relative to Fe, and takes a closer look at the controls on the Mn : Fe
ratios in rock varnish. This work is particularly relevant to paleoenvironmental interpretations of the variations in Mn : Fe ratios in rock varnishes.
Evidence suggesting that methods of rock-varnish cation-ratio dating are neither comparable nor consistently reliable
Using samples from a prehistoric quarry site in the Mojave Desert, the AA. tested and compared the two principal methods of rock-varnish cation-ratio dating, analysis of rock varnish scrappings and analysis of rock varnish in situ. Discussion
A preliminary electron probe study of microchemical variations in desert varnish in western New South Wales, Australia
This study seeks to establish whether systematic variations in varnish chemistry occur at a microstratigraphic scale within varnish deposits, and specifically investigates variations in (a) iron and manganese contents and (b) elemental ratios
, between the upper and lower portions of individual varnishes from an area near Broken Hill.
The purpose of this paper is first to show that whether or not biomineralization processes are instrumental in the formation of all rock varnish, they apparently affect the morphology of varnish and, hence, the distribution of Mn within the varnish
. Second, the AA. show through XRD, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) that the Mn phase in rock varnish is by most definitions a non-crystalline phase with an affinity for a number of cations not normally present
Cation-ratio dating of rock varnish : a geographic assessment
This paper presents the results of a three-year study of the potential and limitations of CR dating. The unique role of rock varnish as compared to other surface exposure dating methods is its ability to determine the timing of last abrasion
Ferromanganese rock varnish in north Norway : a subglacial origin
A thin, dark brown rock varnish is described from ice-smoothed bedrock on the forefield of a glacier in North Norway. Glacier retreat rates indicate that the varnish has remained exposed subaerially at the surface for about twenty years, and its
A comment on a note on the characteristics and possible origins of desert varnishes from southeast Morocco by DRS. SMITH and WHALLEY and reply
This note queries the discussion of the origins of desert varnishes (Smith and Whalley, 1988), with particular reference to manganese - rich conditions. The case for a biotic model of varnish development is proposed and illustrated as opposed
Uncertainties in the radiocarbon dating of organics associated with rock varnish : a plea for caution
The conclusion of the A. is that the interpretation of radiocarbon ages associated with rock varnish is unclear, and there may be similar problems associated with other rock coatings. To obtain reliable ages in the future, the effort must be based
on a careful characterization of different types of organic matter associated with rock varnish. In addition, the ages of different types of organics must be measured where there are independent age controls, and in blind tests.
Rapid formation of rock varnish and other rock coatings on slag deposits near Fontana, California
Manganiferous 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
Understanding the spatial variability of environmental change in drylands with rock varnish microlaminations
This paper presents the findings on the spatial comparibility of visual microlaminations in rock varnishes. It also applies this experimental tool to understand spatial variabilities in environmental change. Following a discussion of methods
and results, the AA. present the pattern of varnish layering in the Death Valley region, an analysis of the method's uncertainties, and an exploration of its utility for mapping alluvial fans and other cases.
Holocene rock varnish microstratigraphy and its chronometric application in the drylands of western USA
Analyses of hundreds of rock varnish samples from latest Pleistocene and Holocene geomorphic features in the drylands of western USA reveal a regionally replicable Holocene microlamination sequence. This radiometrically calibrated and climatically
, the varnish microlamination dating technique has the great potential to yield numerical age assignments for surface stone tools, petroglyphs, and geoglyphs of prehistoric age in the drylands of western USA.
Varnish microlaminations : new insights from focused ion beam preparation
Arid area ; California ; Desert ; Electron microscope ; Palaeoclimate ; Research technique ; United States of America ; Varnish ; Weathering
The cross-sectional texture of rock varnish varies considerably with the scale of analysis and technique used to image a sample. One key to nanoscale analysis involves focused ion beam (FIB) techniques, but FIB preparation remains challenging
for samples like rock coatings with heterogeneous density and abundant porosity. A new technique involving multiangle ion thinning and in situ plan-view lift-out facilitated a scanning transmission electron microscopy study of rock varnish from Death Valley
. The results reveal variability in lateral continuity of nanometer microlaminae that can be interrupted by post-depositional diagenesis involving leaching of Mn and Fe, and this variability could explain why some of the visual varnish microlaminations (VML