Quick
Search: 		  
advanced search
 GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help 
Geology Don't get GSW? Talk to your librarian.
JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Geology; March 2008; v. 36; no. 3; p. 215-218; DOI: 10.1130/G24409A.1
© 2008 Geological Society of America
This Article
Right arrow Abstract
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Garvie, L. A.J.
Right arrow Articles by Buseck, P. R.
Right arrow Search for Related Content
GeoRef
Right arrow GeoRef Citation

Nanometer-scale complexity, growth, and diagenesis in desert varnish

Laurence A.J. Garvie1, Donald M. Burt1 and Peter R. Buseck2

1 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA
2 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA and Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
Nanometer-scale element mapping and spectroscopy of desert varnish from the northern Sonoran Desert in southwestern Arizona reveal a dynamic disequilibrium system characterized by postdepositional mineralogical, chemical, and structural changes activated by liquid water. Lack of equilibrium is suggested by the large variety of coexisting Mn phases. Sparse secondary Ba and Sr sulfates also occur, as do carbonaceous particles. Individual Mn-oxide particles contain variable concentrations of Ba and Ce, reflecting their role as repositories of trace elements, presumably derived from atmospheric aerosols. Desert varnish is analogous to more familiar sediments in displaying authigenic and diagenetic structures, but with total sediment thicknesses <1 mm and structures at the nanometer scale. As such, it is neither a weathering rind nor patina, but a unique subaerial sediment that is in dynamic disequilibrium. Our results suggest continuing adjustment of varnish to changing environmental conditions.

Key Words: desert varnish • rock varnish • Mn • subaerial sediment • diagenesis • Sonoran desert


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
A slowly accumulating, dark Mn- and Fe-rich layered coating can occur on rock surfaces worldwide (Glasby et al., 1981; Jones, 1991; Reneau et al., 1992; Dorn, 2007). It is most typical in arid environments, which explains its common name, desert varnish. It has been used to indicate past environmental conditions (Liu et al., 2000; Broecker and Liu, 2001; Liu, 2003) as well as to date landscape surfaces and ancient human activity (Watchman, 2000). A recent resurgence in the study of desert varnish has taken place, partly because similar rock coatings are thought to occur on Mars (McSween et al., 2004; Haskin et al., 2005).

Desert varnish, also called rock varnish, is characterized by internal structures that resemble stratification and stromatolitic forms (Krinsley, 1998; Fleisher et al., 1999; Liu et al., 2000). It consists of authigenic Mn and Fe oxides and a variety of eolian-derived (detrital) minerals (Potter and Rossman, 1977, 1979a, 1979b). It is characteristically laminated, with layers rich in either Mn or Fe oxides alternating with silicate-rich layers (Reneau et al., 1992; Fleisher et al., 1999; Liu and Broecker, 2000; Liu et al., 2000). This layering reveals an accretionary process, with new layers added on to the surface of existing varnish. Infrared spectroscopy indicates that the dominant Mn-bearing minerals are structurally similar to birnessite (Potter and Rossman, 1979b), and EXAFS (extended X-ray absorption fine structure) spectroscopy suggests that the Mn minerals have tunnel or layer structures (McKeown and Post, 2001). The Mn oxides are inferred to be rich in Ba (Raymond et al., 1991; Reneau et al., 1992), and hematite has been shown to be the primary Fe oxide in varnish from hot arid environments (Potter and Rossman, 1979b). Opal might be an important cement in some rock coatings (Perry et al., 2006). Formation of the varnish is thought to range from abiotic to biotic or a combination of the two (Hungate et al., 1987; Jones, 1991; Nagy et al., 1991; Krinsley, 1998). The high numbers of microorganisms on varnish surfaces together with the discovery of fungi and bacteria that can oxidize Mn have led some to invoke microorganisms as the primary agents in the concentration and precipitation of the Mn and Fe minerals (Grote and Krumbein, 1992).

Despite the number of studies of desert varnish, it is surprising that many fundamental features of its mineralogy and chemistry are poorly known. Here we provide new evidence from high-spatial-resolution imaging and spectroscopy of a dynamic disequilibrium system characterized by postdepositional mineralogical and textural changes.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
We collected samples with different lithologies and geomorphologic settings from areas in the Sonoran Desert in southwestern Arizona. Three representative samples were selected for electron microscopy. Sample 1 (32°48'36''N, 112°15'10''W) is a 5 cm amphibolite gneiss pebble with an ~50 µm varnish coating. This is a typical pebble from a desert pavement and has a shiny band ~1 cm wide that circles the stone. Sample 2 (33°03'38''N, 112°10'58''W) is a quartz biotite gneiss with as much as 200 µm of well-developed varnish. Sample 3 (32°59'10''N, 112°30'38''W) is characterized by ~500-µm-thick varnish and is from a region where varnish formed on the tops of granite boulders.

Mineralogical and chemical characterization of the samples was undertaken by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron energy-loss spectroscopy (EELS). Millimeter-sized pieces of varnish were gently disaggregated in methanol, and a drop of the varnish in suspension was dried on Cu grids coated with lacy carbon. High-resolution (HR) TEM images and EELS spectra were acquired from electron-transparent areas of sample protruding into the holes of the lacy carbon film. HRTEM images were acquired with a Topcon 002B TEM operating at an accelerating voltage of 200 kV. EELS data were acquired with a GATAN 766 DigiPEELS spectrometer attached to a Philips 400-ST field-emissiongun (FEG) TEM operated at an accelerating voltage of 100 kV. The width at half maximum, and hence the energy resolution, of the zero-loss peak was 0.8 eV. Spectra were acquired in diffraction mode, with beam-convergence semiangle, {alpha}, and collection semiangle, β, of 16 and 11 mrad, respectively.

An electron-beam-transparent section, ~10 x 20 µm, was prepared by focused ion beam thinning (FIB) of a typical area from sample 2. This sample was chosen for the FIB sectioning because the petrographic thin sections showed clearly developed laminations. The FIB section was placed on a carbon film on a Cu TEM grid. TEM images and energy-filtered (EF) TEM data from the FIB section were acquired with an FEI Technai F20 TEM operated at 200 kV. EFTEM data were acquired with a post-column Gatan Imaging Filter (GIF), model 860 GIF 2001 containing a 1000 x 1000 pixel MultiScanTM CCD (charge coupled device). EFTEM images for Figure 2 were acquired at 512 x 512 resolution (2 x 2:1 hardware binning), microscope magnification of 15,000X, spot size and gun lens 1, and large objective aperture. Element maps were computed and plotted using the "acquire elemental map" command based on the IF Mapping module in DigitalMicrograph (DM) using two pre-edge images and one post-edge image. A dark-current image was acquired and subtracted from each image. Prior to computing the energy-filtered images, the three images were manually corrected for specimen drift using the "measure relative drift" command in DM. The "acquire elemental map" produces a grayscale image for each element, and these maps were manually assigned a color and combined into a composite image using the color mix procedure in DM. Contiguous element maps were combined using the Photomerge function in Photoshop CS2.


Figure 02
View larger version (92K):
[in this window]
[in a new window]

  		Figure 2. TEM and EFTEM images of the varnish FIB section. A: Low-magnification, bright-field TEM image of the FIB section. The section is resting on an amorphous-C film that is supported on a Cu TEM grid. The dark bars at the sides of the FIB section are thicker areas of the section used for support. The blue arrows show the same point on the low-resolution (A) and higher-resolution (B) TEM images. B: Bright-field TEM image of a portion of the FIB section showing the location (white outline) of the composite EFTEM image in (C). The white areas at the bottom are holes. C: Color Mn-Fe map of a portion of the desert varnish FIB section outlined in (B). The image illustrates the distribution of iron (red) and manganese (green). Two pores, indicated by arrows, are rimmed by Mn-rich material. The majority of the black areas in the EFTEM image are rich in Si. This image is a composite of 20 separate Fe and Mn EFTEM images.

 
XRD profiles were obtained using a Rigaku powder diffractometer employing Cu K{alpha} radiation and a graphite monochromator. The varnish was separated from samples 2 and 3 using small surgical knives, gently disaggre-gated using a pestle and mortar, placed on a single-crystal, low-background quartz plate, dispersed with water, and dried to form a film ~1 x 1 cm. Care was taken to separate and analyze the varnish without contributions from the underlying weathered rock. It was not possible to scrape sufficient material for powder XRD from sample 1. Diffraction patterns were recorded from 4° to 45° 2{theta}, step scan of 0.02°, with 25 s per step.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES CITED
 
Ultrathin (<10 µm) petrographic sections (Figs. 1A, 1B) show alternating dark and light layers roughly parallel to the varnish surface, locally with flaring-upward structures. The contact with underlying rock is generally sharp. The XRD profiles from samples 2 and 3 are similar. The pattern from sample 2 is therefore shown as an example (Fig. 1C). The pattern reveals a predominance of phyllosilicates, including illite, illite-smectite, and kaolinite, with minor quartz. Few other peaks are present, but a weak and broad peak near 33° 2{theta} is at the same wavelength as the main hematite reflection.


Figure 01
View larger version (95K):
[in this window]
[in a new window]

  		Figure 1. Optical micrographs (A and B) from ultrathin section and powder X-ray diffraction profile (C) of typical varnish from sample 2. A: Low-magnification image showing the varnish layers parallel to the air-varnish surface (LV—layered varnish) and rubbly bottom between varnish and underlying rock. RV—rubbly varnish; Q—quartz grains from the rock. B: Higher-magnification optical micrograph of flaring-upward structure common in some varnish. Bands are 1–2 µm thick. C: Diffraction pattern shows broad peaks from phyllosilicates and iron oxides and sharp reflections from quartz. I—illite, S—smectite, K—kaolinite, Q—quartz, H—hematite, G—goethite.

 
TEM of a sample thinned using FIB (Figs. 2A, 2B1) provides the first detailed view of the relationship between the varnish minerals and structure such as microvugs and crosscutting veins. The layering generally coarsens from the top to the bottom of this section. Clay flakes and mottled material dominate, with minor amounts of rounded, presumably detrital, particles. The clays are similar in size and identity to those in aerosol particles of desert dust (Reynolds et al., 2006). Clay is the dominant mineral, as indicated by the XRD data. Individual clay particles are separated by mottled, electron-dense material.

Compositional maps based on EFTEM imaging clearly show the nanometer-scale segregation of Mn and Fe (Fig. 2C) and provide new insights regarding varnish structures. Near the top of the section, the varnish shows laminations rich in Fe or Mn, where Mn-rich layers <20 nm thick are evident and Fe-rich patches <10 nm are resolvable. Stringers of Mn-rich material cut across the varnish roughly perpendicular to the rock surface. Near the bottom of the section is a micrometer-thick area of Fe-rich material. The Fe- and Mn-rich material between the clay flakes is typically low in Si, although Si occurs in the clay flakes and thin stringers parallel to the Mn- and Fe-rich layers. Pores rimmed by Mn-rich material occur (Fig. 2C). Nanometer-scale mixtures of Si- and Fe-rich material surrounded by Mn-rich material are common (Fig. 3). Nanometer-sized Ti-rich grains, presumably detrital in origin, are randomly distributed throughout.


Figure 03
View larger version (165K):
[in this window]
[in a new window]

  		Figure 3. Comparison of transmission electron microscopy (TEM) and energy-filtered (EF) TEM images from sample 2. A, C: Bright-field TEM image. B, D: Corresponding EFTEM images. EFTEM images show distribution of Fe (red), Mn (green), Si (blue), and Ti (yellow). Panel C shows mottled Mn-rich material (white arrow), clay flake (black arrow), and Ti-oxide particle (blue arrow).

 
The TEM data show that the Mn minerals in the varnish are extremely fine grained, typically <100 nm (Fig. 4). Based on EELS analyses, electron diffraction, and morphology, the minerals are members of the hollandite-todorokite groups, birnessite and buserite, poorly crystallized phyllomanganates of unspecified type, and Mn, Fe spinel. Manganese is most abundant as fluffy coatings on clays, and this feature suggests a role for clays in Mn oxidation and fixation, similar to that in soils (Reddy and Perkins, 1976). Sparse but ubiquitous euhedral barite and strontiobarite occur in the mottled material. Analyses of individual Mn-oxide particles indicate that they contain variable concentrations of Ba and Ce (Fig. 4), reflecting the role of these particles as repositories of trace elements, presumably derived from atmospheric aerosols.


Figure 04
View larger version (116K):
[in this window]
[in a new window]

  		Figure 4. Transmission electron microscopy (TEM) and spectroscopic data of particles of disaggregated varnish from sample 3 adhering to a lacy-C film. A: Bright-field TEM image of desert varnish fragments. Black arrow shows single Mn-oxide particle. B: Mn map (green). C: High-resolution TEM image of particle indicated by arrow in A. Blue bar shows area analyzed in D. D: Histogram of intensities from barred region in C showing the 1 nm periodicity. E: Portion of electron energy-loss spectroscopy (EELS) spectrum from the Mn-oxide particle showing the presence of Ba and Ce.

 
The mineralogical heterogeneity of varnish is well displayed on the pebble (sample 1). Samples for TEM analysis were prepared from several areas of the pebble, and these showed differences in mineralogy, composition, and concentration of Mn oxides. The majority of discrete Mn oxides occur in the shiny mantle, which is an ~1-cm-wide zone that circles the stone ~1 cm above the desert floor. Ba-Mn oxide particles are common in some areas on sample 1 and occur as tissue-like aggregates, elongated particles, and small fluffy particles adhering to the clays. Many of these particles have lattice spacings between 5.5 and 7 Å, visible at the edges of folded flakes. These spacings are consistent with a phyllomanganate-like structure, similar to birnessite. Birnessite typically has a 7 Å spacing between adjacent layers; the lower values measured using the TEM are consistent with collapse of the layers in the high vacuum of the TEM (Post and Veblen, 1990). Occasional elongated Mn-rich particles with ~10 Å spacings, consistent with todorokite, are also present (Turner and Buseck, 1981).

Carbon grains with varnish samples were identified using EELS. Some contain minor K, N, and O. The small particle sizes and occurrence of K are consistent with a biomass-burning origin (Pósfai et al., 2003). Rare stringers of C-rich materials with small N and O peaks also occur. They have C:N:O ratios similar to those of chitin and may represent remnants of fungal hyphae. The C in the clay may represent intercalated material from bacteria occurring with the varnish.

Our data are consistent with varnish formation through repeated wetting and drying of the rock surfaces, and leaching and oxidation of Fe and Mn. There are several potential sources for the Mn and Fe that include leaching from clays in dust, transport from the substrate by capillary action, or derivation from soluble salts in aerosols. Previous studies show no evidence for the substrate as the source of these elements (Thiagarajan and Lee, 2004; Dorn, 2007). Given their typically low Mn concentrations, clays also appear to be an inadequate source of these elements. The lack of free silica in the varnish also argues against significant postdepositional weathering of the entrained aerosol clays. The remaining possibility is aerosols, which contain soluble Mn and Fe (Guieu et al., 1994; Baker et al., 2006). An aerosol source is consistent with the analysis of trace elements and detection of radionuclides in desert varnish (Thiagarajan and Lee, 2004; Hodge et al., 2005). The nanometer-scale segregation of Fe and Mn (Fig. 2C) suggests growth involving evaporation and oxidation of soluble Fe2+ and Mn2+ to insoluble high-valence oxides, with Fe precipitating first followed by Mn. An unknown factor in the Mn and Fe cycles in the varnish is the role of microorganisms (Adams et al., 1992; Grote and Krumbein, 1992).

Evidence for diagenesis includes the crosscutting Mn stringers (Fig. 2C), Mn-rich rimmed pores, textural coarsening deeper in the varnish, and the presence of Ba and Sr sulfates. Alteration apparently involved dissolution of some Mn minerals, possibly through reduction by organic matter left by organisms inhabiting the varnish surface. Little Fe would redissolve because of the presence of oxidized Mn. The dissolution of Ba-bearing Mn oxides liberates Ba to form sulfates, with reprecipitation of Mn oxides filling cracks and parts of the deeper cavities. Desert varnish accumulates at rates of <1–40 µm/k.y. (Liu and Broecker, 2000). It requires many thousands of years to form and is repeatedly subjected to wetting and large diurnal changes in temperature from below freezing to <80 °C. It is likely that the large clay fraction, some of which is smectite, swells during wetting. Repeated swelling and contraction probably contributed to the textural changes evident in the varnish. The disequilibrium nature of varnish also implies that ancient varnish might be difficult to find and recognize because once it was buried and exposed to groundwater or other moisture for long periods, it would probably have dissolved or recrystallized to phases such as coarse pyrolusite dendrites or specular hematite.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES CITED
 
Desert varnish undergoes diagenesis, as indicated by fracture and cavity fillings, recrystallization, coarsening, and growth of secondary phases such as Ba-Sr sulfate. It resembles typical sediments in displaying authigenic minerals and diagenetic structures, but with total sediment thicknesses <1 mm and structures at the nanometer scale. As such, varnish is neither a weathering rind nor patina, but a unique subaerial sediment from an arid environment that is subject to continuing change. The abundance of different Mn phases shows that varnish does not attain mineralogical equilibrium, presumably owing to large extremes of wetting and drying plus freezing and baking. These characteristics of varnish need to be considered in any attempt to use it as an indicator of age or climate.


   ACKNOWLEDGMENTS
 
We thank D. Smith and K. Weiss for providing access to the facilities within the Center for Solid State Science at Arizona State University. Garvie and Buseck acknowledge financial support from the National Aeronautics and Space Administration (NNG06GE37G) and National Science Foundation (grant EAR-0440388). We thank G.P. Glasby, D. Krinsley, and an anonymous reviewer for their constructive and encouraging reviews. We are grateful to T. Malis and Jian Li, CANMET Materials Technology Laboratory, Natural Resources Canada, for preparing the focused ion beam section using the instrumentation of Fibics Incorporated, Ottawa, Canada.


   FOOTNOTES
 
Figure 2 is a separate loose insert. Back


   REFERENCES CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 

Adams, J.B., Palmer, F., and Staley, J.T., 1992, Rock weathering in deserts: Mobilization and concentration of ferric iron by microorganisms: Geomicrobiology Journal, v. 10 pp. 99-114.[ISI][GeoRef]

Baker, A.R., Jickells, T.D., Witt, M., and Linge, K.L., 2006, Trends in the solubility of iron, aluminium, manganese and phosphorus in aerosol collected over the Atlantic Ocean: Marine Chemistry, v. 98 pp. 43-58 doi: 10.1016/j.marchem.2005.06.004.[CrossRef][ISI][GeoRef]

Broecker, W.S., and Liu, T., 2001, Rock varnish: Recorder of desert wetness?: GSA Today, v. 11, no. 8 pp. 4-10.[GeoRef]

Dorn, R.I., 2007, Rock varnish: in Nash, D.J., and McLaren, J.S., eds., Geochemical sediments and landscapes: London, Blackwell, pp. 246-297.

Fleisher, M., Liu, T.Z., and Broecker, W.S., 1999, A clue regarding the origin of rock varnish: Geophysical Research Letters, v. 26 pp. 103-106 doi: 10.1029/1998GL900229.[CrossRef][ISI][GeoRef]

Glasby, G.P., McPherson, J.G., Kohn, B.P., Johnston, J.H., Keys, J.R., Freeman, A.G., and Tricker, M.J., 1981, Desert varnish in southern Victoria Land, Antarctica: New Zealand Journal of Geology and Geophysics, v. 24 pp. 389-397.[GeoRef]

Grote, G., and Krumbein, W.E., 1992, Microbial precipitation of manganese by bacteria and fungi from desert rock and rock varnish: Geomicrobiology Journal, v. 10 pp. 49-57.[ISI][GeoRef]

Guieu, C., Duce, R., and Arimoto, R., 1994, Dissolved input of manganese to the ocean: Aerosol source: Journal of Geophysical Research, v. 99, no. D9 pp. 18789-18800 doi: 10.1029/94JD01120.[CrossRef]

Haskin, L.A., and 29 others2005, Water alteration of rocks and soils on Mars at the Spirit rover site in Gusev crater: Nature, v. 436 pp. 66-69 doi: 10.1038/nature03640.[CrossRef][Medline]

Hodge, V.F., Farmer, D.E., Diaz, T., and Orndorff, R.L., 2005, Prompt detection of alpha particles from 210Po: Another clue to the origin of rock varnish?: Journal of Environmental Radioactivity, v. 78 pp. 331-342 doi: 10.1016/j.jenvrad.2004.05.009.[CrossRef][ISI][Medline]

Hungate, B., Danin, A., Pellerin, N.B., Stemmler, J., Kjellander, P., Adams, J.B., and Staley, J.P., 1987, Characterization of manganese-oxidizing (MnII -> MnIV) bacteria from Negev Desert rock varnish: Implications in desert varnish formation: Canadian Journal of Microbiology, v. 33 pp. 939-943.[ISI]

Jones, C.E., 1991, Characteristics and origin of rock varnish from the hyperarid coastal deserts of northern Peru: Quaternary Research, v. 35 pp. 116-129 doi: 10.1016/0033-5894(91)90099-Q.

Krinsley, D., 1998, Models of rock varnish formation constrained by high resolution transmission electron microscopy: Sedimentology, v. 45 pp. 711-725 doi: 10.1046/j.1365-3091.1998.00172.x.[CrossRef][ISI][GeoRef]

Liu, T., 2003, Blind testing of rock varnish microstratigraphy as a chronometric indicator: Results on late Quaternary lava flows in the Mojave Desert, California: Geomorphology, v. 53 pp. 209-234 doi: 10.1016/S0169-555X(02)00331-8.[CrossRef][ISI][GeoRef]

Liu, T., and Broecker, W.S., 2000, How fast does rock varnish grow?: Geology, v. 28 pp. 183-186 doi: 10.1130/0091-7613(2000)28<183:HFDRVG>2.0.CO;2.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]

Liu, T., Broecker, W.S., Bell, J.W., and Mandeville, C.W., 2000, Terminal Pleistocene wet event recorded in rock varnish from Las Vegas Valley, southern Nevada: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 161 pp. 423-433 doi: 10.1016/S0031-0182(00)00097-3.[CrossRef][GeoRef]

McKeown, D.A., and Post, J.E., 2001, Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy: American Mineralogist, v. 86 pp. 701-713.[Abstract/Free Full Text][ISI][GeoRef]

McSween, H.Y., and 35 others2004, Basaltic rocks analyzed by the Spirit rover in Gusev crater: Science, v. 305 pp. 842-845 doi: 10.1126/science.3050842.[Abstract/Free Full Text][CrossRef][ISI][Medline][GeoRef]

Nagy, B., Nagy, L.A., Rigali, M.J., Jones, W.D., Krinsley, D.H., and Sinclair, N.A., 1991, Rock varnish in the Sonoran Desert: Microbiologically mediated accumulation of manganiferous sediments: Sedimentology, v. 38 pp. 1153-1171 doi: 10.1111/j.1365-3091.1991.tb00376.x.[CrossRef][ISI][GeoRef]

Perry, R.S., Lynne, B.Y., Sephton, M.A., Kolb, V.M., Perry, C.C., and Staley, J.T., 2006, Baking black opal in the desert sun: The importance of silica in desert varnish: Geology, v. 34 pp. 537-540 doi: 10.1130/G22352.1.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]

Pósfai, M., Simonics, R., Li, J., Hobbs, P.V., and Buseck, P.R., 2003, Individual aerosol particles from biomass burning in southern Africa: 1. Compositions and size distributions of carbonaceous particles: Journal of Geophysical Research, v. 108, no. D13p. 8483 doi: 10.1029/2002JD002291.[CrossRef]

Post, J.E., and Veblen, D.R., 1990, Crystal structure determination of synthetic sodium, magnesium, and potassium birnessite using TEM and Rietveld method: American Mineralogist, v. 75 pp. 477-489.[Abstract][ISI][GeoRef]

Potter, R.M., and Rossman, G.R., 1977, Desert varnish: The importance of clay minerals: Science, v. 196 pp. 1446-1448 doi: 10.1126/science.196.4297.1446.[Abstract/Free Full Text][CrossRef][ISI][Medline][GeoRef]

Potter, R.M., and Rossman, G.R., 1979a, Mineralogy of manganese dendrites and coatings: American Mineralogist, v. 64 pp. 1219-1226.[Abstract][ISI][GeoRef]

Potter, R.M., and Rossman, G.R., 1979b, The manganese- and iron-oxide mineralogy of desert varnish: Chemical Geology, v. 25 pp. 79-94 doi: 10.1016/0009-2541(79)90085-8.[CrossRef][ISI][GeoRef]

Raymond, R.J.J., Reneau, S.L., and Harrington, C.D., 1991, Elemental relationships in rock varnish as seen with scanning electron microscopy and energy dispersive X-ray elemental line profiling: Scanning Microscopy, v. 5 pp. 37-46.[ISI][GeoRef]

Reddy, M.R., and Perkins, H.F., 1976, Fixation of manganese by clay minerals: Soil Science, v. 121 pp. 21-24 doi: 10.1097/00010694-197601000-00004.[ISI][GeoRef]

Reneau, S.L., Raymond, R.J., and Harrington, C., 1992, Elemental relationships in rock varnish stratigraphic layers, Cima volcanic field, California: Implications for varnish development and the interpretation of varnish chemistry: American Journal of Science, v. 292 pp. 684-723.[Abstract/Free Full Text][ISI][GeoRef]

Reynolds, R.L., Reheis, M., Yount, J., and Lamothe, P., 2006, Composition of aeolian dust in natural traps on isolated surfaces of the central Mojave Desert—Insights to mixing, sources, and nutrient inputs: Journal of Arid Environments, v. 66 pp. 42-61 doi: 10.1016/j.jaridenv.2005.06.031.[CrossRef][ISI]

Thiagarajan, N., and Lee, C.T.A., 2004, Trace-element evidence for the origin of desert varnish by direct aqueous atmospheric deposition: Earth and Planetary Science Letters, v. 224 pp. 131-141 doi: 10.1016/j.epsl.2004.04.038.[CrossRef][ISI][GeoRef]

Turner, S., and Buseck, P.R., 1981, Todorokites: A new family of naturally occurring manganese oxides: Science, v. 212 pp. 1024-1027 doi: 10.1126/science.212.4498.1024.[Abstract/Free Full Text][CrossRef][ISI][Medline][GeoRef]

Watchman, A., 2000, A review of the history of dating rock varnishes: Earth-Science Reviews, v. 49 pp. 261-277 doi: 10.1016/S0012-8252(99)00059-8.

Received for publication 5 September 2007

Revised manuscript received 2 November 2007

Manuscript accepted 7 November 2007





This Article
Right arrow Abstract
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Garvie, L. A.J.
Right arrow Articles by Buseck, P. R.
Right arrow Search for Related Content
GeoRef
Right arrow GeoRef Citation

JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2008 by Geological Society of America