Desert varnish

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@article{doi101126science1203109183,
    author = "Hunt, Charles B.",
    title = "Desert Varnish",
    year = "1954",
    journal = "Science",
    abstract = "71MetricsTotal Downloads7Last 6 Months2Last 12 Months2Total Citations1Last 6 Months0Last 12 Months0View all metrics",
    url = "https://doi.org/10.1126/science.120.3109.183",
    doi = "10.1126/science.120.3109.183",
    openalex = "W4240229988",
    references = "openalexw633827141"
}

Desert varnish forms a dark coating up to 0.10 mm thick on the exposed surfaces of many stones and outcrops in southern California deserts. Wet chemical analyses were made of varnish, the underlying weathered rind, and fresh rock for a rhyolite and two andesites. The principal elements in varnish are O, H, Si, Al, Fe, and Mn, and the last two give the deposit its distinctive physical characteristics. H_2O, Fe_2O_3, and especially MnO show the greatest enrichment. Field observations and a number of partial analyses indicate that the best varnishes are on fine-grained rocks relatively rich in Fe and Mn. \n \nSpectrographic analyses were made of 22 varnishes, 14 rocks, 8 soils, and 5 samples of air-borne material. In the varnishes Ti, Ba, and Sr are by far the most abundant trace elements, followed by Cu, Ni, Zr, Pb, V, Co, La, Y, B, Cr, Sc, and Yb. Cd, W, Ag, Nb, Sn, Ga, Mo, Be, and Zn were recorded in some but not all varnishes. The trace-element content of all varnishes is similar, and the variations recorded are related to differences in the local geology. Most trace elements are considerably enriched in varnish—Cu and Co especially, and Ni, Pb, Ba, Cr, Yb, B, Y, Sr, and V. \n \nThe chemical data suggest that (1) varnish on stones seated in soil or colluvium is derived largely from that material, (2) varnish on large bedrock exposures come from weathered parts of the rock, (3) air-borne material is probably a minor contributor. \n \nThe formation of desert varnish is primarily a weathering process involving the solution, transportation, and deposition of Mn and Fe in particular and a host of trace elements. Most of these elements are derived from local sources, and the small amount of movement required can occur by transport in solution or possibly by ionic diffusion through moisture films. Dew may be as important a source of moisture as rain. Organic agents, such as bacteria, may cause deposition of varnish, but this has not yet been demonstrated. In the desert, evaporation and the catalytic action of MnO_2 should be capable of performing the task. \n \nThe rate of varnish formation varies widely with local conditions. Hundreds and thousands of years may be required to form a dark coating in some situations, but at one locality in the Mojave Desert a good varnish formed on the surface stones of an alluvial deposit in 25 years. Although the widespread evidence of varnish deterioration may be due to climatological change, conditions in some parts of this desert area are currently favorable to varnish formation.

@article{doi10113000167606195869487cdodv20co2,
    author = "Engel, Celeste G. and Sharp, Robert P.",
    title = "CHEMICAL DATA ON DESERT VARNISH",
    year = "1958",
    journal = "Geological Society of America Bulletin",
    abstract = "Desert varnish forms a dark coating up to 0.10 mm thick on the exposed surfaces of many stones and outcrops in southern California deserts. Wet chemical analyses were made of varnish, the underlying weathered rind, and fresh rock for a rhyolite and two andesites. The principal elements in varnish are O, H, Si, Al, Fe, and Mn, and the last two give the deposit its distinctive physical characteristics. H\_2O, Fe\_2O\_3, and especially MnO show the greatest enrichment. Field observations and a number of partial analyses indicate that the best varnishes are on fine-grained rocks relatively rich in Fe and Mn. \n \nSpectrographic analyses were made of 22 varnishes, 14 rocks, 8 soils, and 5 samples of air-borne material. In the varnishes Ti, Ba, and Sr are by far the most abundant trace elements, followed by Cu, Ni, Zr, Pb, V, Co, La, Y, B, Cr, Sc, and Yb. Cd, W, Ag, Nb, Sn, Ga, Mo, Be, and Zn were recorded in some but not all varnishes. The trace-element content of all varnishes is similar, and the variations recorded are related to differences in the local geology. Most trace elements are considerably enriched in varnish—Cu and Co especially, and Ni, Pb, Ba, Cr, Yb, B, Y, Sr, and V. \n \nThe chemical data suggest that (1) varnish on stones seated in soil or colluvium is derived largely from that material, (2) varnish on large bedrock exposures come from weathered parts of the rock, (3) air-borne material is probably a minor contributor. \n \nThe formation of desert varnish is primarily a weathering process involving the solution, transportation, and deposition of Mn and Fe in particular and a host of trace elements. Most of these elements are derived from local sources, and the small amount of movement required can occur by transport in solution or possibly by ionic diffusion through moisture films. Dew may be as important a source of moisture as rain. Organic agents, such as bacteria, may cause deposition of varnish, but this has not yet been demonstrated. In the desert, evaporation and the catalytic action of MnO\_2 should be capable of performing the task. \n \nThe rate of varnish formation varies widely with local conditions. Hundreds and thousands of years may be required to form a dark coating in some situations, but at one locality in the Mojave Desert a good varnish formed on the surface stones of an alluvial deposit in 25 years. Although the widespread evidence of varnish deterioration may be due to climatological change, conditions in some parts of this desert area are currently favorable to varnish formation.",
    url = "https://doi.org/10.1130/0016-7606(1958)69[487:cdodv]2.0.co;2",
    doi = "10.1130/0016-7606(1958)69[487:cdodv]2.0.co;2",
    openalex = "W2016751313"
}

Abstract Desert pavement and vesicular layer are described as distinct horizons of some Gray Desert soils. Distribution of > 2‐mm. and < 2‐mm. particles in the profile along with laboratory tests suggest that accumulation of gravel and stones at the surface to form a desert pavement is not due solely to removal of finer material by wind or water. There has also been some upward movement of coarse fragments from the nearly stone‐free layers below. Some properties of vesicular layers are pointed out by field and laboratory studies. Natural vesicular structure was destroyed by sieving and a new, but similar, structure formed by merely wetting and drying the soil. This led to a hypothesis for origin of the vesicular layer as a pedogenic horizon.

@article{doi102136sssaj195803615995002200010017x,
    author = "Springer, M. E.",
    title = "Desert Pavement and Vesicular Layer of Some Soils of the Desert of the Lahontan Basin, Nevada",
    year = "1958",
    journal = "Soil Science Society of America Journal",
    abstract = "Abstract Desert pavement and vesicular layer are described as distinct horizons of some Gray Desert soils. Distribution of > 2‐mm. and < 2‐mm. particles in the profile along with laboratory tests suggest that accumulation of gravel and stones at the surface to form a desert pavement is not due solely to removal of finer material by wind or water. There has also been some upward movement of coarse fragments from the nearly stone‐free layers below. Some properties of vesicular layers are pointed out by field and laboratory studies. Natural vesicular structure was destroyed by sieving and a new, but similar, structure formed by merely wetting and drying the soil. This led to a hypothesis for origin of the vesicular layer as a pedogenic horizon.",
    url = "https://doi.org/10.2136/sssaj1958.03615995002200010017x",
    doi = "10.2136/sssaj1958.03615995002200010017x",
    openalex = "W2162057244"
}

Desert varnish samples from Deep Springs Valley and Death Valley in California were studied with the use of the electron microprobe. Varnish can be interpreted as having two layers, an inner subordinate layer rich in $$SiO_{2}$$ and usually $$Al_{2}O_{3}$$ and an outer main layer rich in FeO and MnO. The subordinate layer may be altered rock which has lost all evidence of crystal structure and to which Fe and Mn have been added. These layers are not distinguishable optically. In varnish on quartzites, FeO, MnO, $$Al_{2}O_{3}$$, and $$K_{2}O$$ all increase in concentration outward through the varnish from the rock-varnish contact. However, in varnish on argillites, $$Al_{2}O_{3}, K_{2}O$$, and sometimes FeO decrease in concentration outward. Comparison of these variations with data on the bulk composition of the fresh rock suggests that elements which increase outward may be supplied largely from external sources and that elements which decrease outward may be supplied predominantly from the underlying rock. The FeO: MnO ratio generally decreases outward through the varnish. This suggests that some of the Mn in varnish is dissolved whenever Fe-Mn-bearing solutions arrive on the rock and that fractionation occurs during subsequent precipitation such that Mn is precipitated last. This fractionation may be responsible for the existence of the orange bottom coating, the groundline band, and the darker varnish in hollows on the rock surface.

@article{doi101086627435,
    author = "Hooke, Roger LeB. and Yang, Houng‐Yi and Weiblen, P. W.",
    title = "Desert Varnish: An Electron Probe Study",
    year = "1969",
    journal = "The Journal of Geology",
    abstract = "Desert varnish samples from Deep Springs Valley and Death Valley in California were studied with the use of the electron microprobe. Varnish can be interpreted as having two layers, an inner subordinate layer rich in $$SiO\_{2}$$ and usually $$Al\_{2}O\_{3}$$ and an outer main layer rich in FeO and MnO. The subordinate layer may be altered rock which has lost all evidence of crystal structure and to which Fe and Mn have been added. These layers are not distinguishable optically. In varnish on quartzites, FeO, MnO, $$Al\_{2}O\_{3}$$, and $$K\_{2}O$$ all increase in concentration outward through the varnish from the rock-varnish contact. However, in varnish on argillites, $$Al\_{2}O\_{3}, K\_{2}O$$, and sometimes FeO decrease in concentration outward. Comparison of these variations with data on the bulk composition of the fresh rock suggests that elements which increase outward may be supplied largely from external sources and that elements which decrease outward may be supplied predominantly from the underlying rock. The FeO: MnO ratio generally decreases outward through the varnish. This suggests that some of the Mn in varnish is dissolved whenever Fe-Mn-bearing solutions arrive on the rock and that fractionation occurs during subsequent precipitation such that Mn is precipitated last. This fractionation may be responsible for the existence of the orange bottom coating, the groundline band, and the darker varnish in hollows on the rock surface.",
    url = "https://doi.org/10.1086/627435",
    doi = "10.1086/627435",
    openalex = "W1964349919"
}

ABSTRACT Stone pavements are armored surfaces comprising intricate mosaics of coarse particles, usually only one or two stones thick, set on or in fine material. They occur widely in many unvegetated areas, and preeminently in hot deserts. Pavement studies in several deserts, and especially in Chile and California, suggest that: 1) deflation may be a relatively unimportant process of pavement formation; 2) running water, and processes causing upward migration of coarse particles through the soil may effect coarse particle concentration at the surface; 3) various superficial disintegration processes may break up coarse particles and increase particle density; 4) the ways in which pavements are established, their rates of formation, and the relative importance of the different processes, may vary greatly from place to place.

@article{doi101111j146783061970tb00741x,
    author = "Cooke, Ronald U.",
    title = "STONE PAVEMENTS IN DESERTS 1",
    year = "1970",
    journal = "Annals of the Association of American Geographers",
    abstract = "ABSTRACT Stone pavements are armored surfaces comprising intricate mosaics of coarse particles, usually only one or two stones thick, set on or in fine material. They occur widely in many unvegetated areas, and preeminently in hot deserts. Pavement studies in several deserts, and especially in Chile and California, suggest that: 1) deflation may be a relatively unimportant process of pavement formation; 2) running water, and processes causing upward migration of coarse particles through the soil may effect coarse particle concentration at the surface; 3) various superficial disintegration processes may break up coarse particles and increase particle density; 4) the ways in which pavements are established, their rates of formation, and the relative importance of the different processes, may vary greatly from place to place.",
    url = "https://doi.org/10.1111/j.1467-8306.1970.tb00741.x",
    doi = "10.1111/j.1467-8306.1970.tb00741.x",
    openalex = "W2028260376"
}

Endolithic algae from fossiliferous limestones, crystalline limestones and sandstones collected in desert areas of Israel and California were studied with reflected light and scanning electron microscopes. The rocks which are colonized by endolithic algae are light-coloured and their texture is porous. On the surface, there is a 0.1–3.0 mm thick crust where the airspaces between rock particles are filled by mineral substance. The crust is permeable to liquids and gases but impervious to cellular organisms. Algal species of the genus Gloeocapsa which form monospecific populations appear as a 0.1–2.5 mm thick blue-green zone under the surface crust. They grow in the internal airspace system of the rock, cell colonies adhering to the crystals. Whereas the desert macroclimate is “hostile” to free-living microscopic algae, the endolithic environment is well suited to support algal life. Irradiation is filtered to a proper level and the porous rock covered by the surface crust constitutes a system which retains humidity and ensures suitable temperature. It is suggested that endolithic algae contribute to rock weathering.

@article{doi102216i003188841044111,
    author = "Friedmann, E. Imre",
    title = "Light and scanning electron microscopy of the endolithic desert algal habitat",
    year = "1971",
    journal = "Phycologia",
    abstract = "Endolithic algae from fossiliferous limestones, crystalline limestones and sandstones collected in desert areas of Israel and California were studied with reflected light and scanning electron microscopes. The rocks which are colonized by endolithic algae are light-coloured and their texture is porous. On the surface, there is a 0.1–3.0 mm thick crust where the airspaces between rock particles are filled by mineral substance. The crust is permeable to liquids and gases but impervious to cellular organisms. Algal species of the genus Gloeocapsa which form monospecific populations appear as a 0.1–2.5 mm thick blue-green zone under the surface crust. They grow in the internal airspace system of the rock, cell colonies adhering to the crystals. Whereas the desert macroclimate is “hostile” to free-living microscopic algae, the endolithic environment is well suited to support algal life. Irradiation is filtered to a proper level and the porous rock covered by the surface crust constitutes a system which retains humidity and ensures suitable temperature. It is suggested that endolithic algae contribute to rock weathering.",
    url = "https://doi.org/10.2216/i0031-8884-10-4-411.1",
    doi = "10.2216/i0031-8884-10-4-411.1",
    openalex = "W2022178682"
}

@article{doi101038259387a0,
    author = "Bauman, A. J.",
    title = "Desert varnish and marine ferromanganese oxide nodules: congeneric phenomena",
    year = "1976",
    journal = "Nature",
    url = "https://doi.org/10.1038/259387a0",
    doi = "10.1038/259387a0",
    openalex = "W1971904241"
}

Desert varnish has been characterized by infrared spectroscopy, x-ray diffraction, and electron microscopy. It is a distinct morphological entity having an abrupt boundary with the underlying rock. Clay minerals comprise more than 70 percent of the varnish. Iron and manganese oxides constitute the bulk of the remainder and are dispersed throughout the clay layer.

@article{potter1977desert,
    author = "Potter, Russell M. and Rossman, George R.",
    title = "Desert Varnish: The Importance of Clay Minerals",
    year = "1977",
    journal = "Science",
    abstract = "Desert varnish has been characterized by infrared spectroscopy, x-ray diffraction, and electron microscopy. It is a distinct morphological entity having an abrupt boundary with the underlying rock. Clay minerals comprise more than 70 percent of the varnish. Iron and manganese oxides constitute the bulk of the remainder and are dispersed throughout the clay layer.",
    url = "https://doi.org/10.1126/science.196.4297.1446",
    doi = "10.1126/science.196.4297.1446",
    number = "4297",
    openalex = "W2056380135",
    pages = "1446-1448",
    volume = "196",
    references = "doi101016037119516480165x, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi10113000167606197687725mobmoa20co2, doi101346ccmn19680160104, doi101346ccmn19680160305, doi101346ccmn19700180104, openalexw161080799, openalexw2240482887, openalexw2932319362"
}

@misc{potter1977desert4,
    author = "Potter, R. M. and Rossman, G. R",
    title = "Desert varnish",
    year = "1977",
    howpublished = "the importance of clay minerals: Science, v. 196, p. 1446-1448",
    note = "talkorigins\_source = {true}; raw\_reference = {Potter, R. M., and Rossman, G. R., 1977, Desert varnish: the importance of clay minerals: Science, v. 196, p. 1446-1448.}"
}

@article{doi101038276489a0,
    author = "Perry, Randall S. and Adams, J. B.",
    title = "Desert varnish: evidence for cyclic deposition of manganese",
    year = "1978",
    journal = "Nature",
    url = "https://doi.org/10.1038/276489a0",
    doi = "10.1038/276489a0",
    openalex = "W1997772382",
    references = "doi101007bf01820710, doi101016s0422989408x70485, doi101038259387a0, doi101086627435, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi101130001676061972831493gofoca20co2, doi102307279516, openalexw653970988, potter1977desert"
}

Optical and electron probe microanalysis of the dark patination coating rocks from the Sonoran Desert reveal a distinct, though sometimes discontinuous layer approximately 20 microns thick. This layer is characterized by concentrations of iron, manganese and magnesium considerably higher than their concentrations within the host rocks. The metal-rich layer is contained within a clay-like matrix and is, in some samples, separated from the host rock by a weathering cortex up to 1 mm thick. A large fraction of the metal in the outer layer appears to have been derived from sources external to the varnished rocks, probably wind-transported dust and clay.

@article{doi101086649741,
    author = "Allen, Carlton C.",
    title = "Desert Varnish of the Sonoran Desert: Optical and Electron Probe Microanalysis",
    year = "1978",
    journal = "The Journal of Geology",
    abstract = "Optical and electron probe microanalysis of the dark patination coating rocks from the Sonoran Desert reveal a distinct, though sometimes discontinuous layer approximately 20 microns thick. This layer is characterized by concentrations of iron, manganese and magnesium considerably higher than their concentrations within the host rocks. The metal-rich layer is contained within a clay-like matrix and is, in some samples, separated from the host rock by a weathering cortex up to 1 mm thick. A large fraction of the metal in the outer layer appears to have been derived from sources external to the varnished rocks, probably wind-transported dust and clay.",
    url = "https://doi.org/10.1086/649741",
    doi = "10.1086/649741",
    openalex = "W1997686100"
}

@article{doi1010160009254179900858,
    author = "Potter, Russell M. and Rossman, George R.",
    title = "The manganese- and iron-oxide mineralogy of desert varnish",
    year = "1979",
    journal = "Chemical Geology",
    url = "https://doi.org/10.1016/0009-2541(79)90085-8",
    doi = "10.1016/0009-2541(79)90085-8",
    openalex = "W2058306764",
    references = "doi101016b9780080092355500267, doi101016s0070457109x70019, doi101021ac60294a030, doi101086627339, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi101346ccmn19580070122, doi102136sssaj197203615995003600050024x, openalexw2613288873, potter1977desert"
}

@article{doi101007bf00378791,
    author = "Krumbein, Wolfgang E. and Jens, K.",
    title = "Biogenic rock varnishes of the negev desert (Israel) an ecological study of iron and manganese transformation by cyanobacteria and fungi",
    year = "1981",
    journal = "Oecologia",
    url = "https://doi.org/10.1007/bf00378791",
    doi = "10.1007/bf00378791",
    openalex = "W2082823380",
    references = "doi101007bf01611203, doi1010160009254179900858, doi101016b978012135902750011x, doi101038276489a0, doi1010990022128711111, doi101126science1203109183, doi10113000167606195869487cdodv20co2, doi101146annurevmi32100178003225, doi1023071797455, openalexw1526419501, openalexw368099268, potter1977desert"
}

The ζ potentials and the streaming potential coefficients of silicate mineral‐water and rock‐water systems were measured for purposes of estimating the magnitude of electrokinetic effects induced by water diffusion in the earth's interior. In rock‐water systems the adsorption reaction of H + at a solid surface is proved to be a main factor determining the ζ potential. For typical crustal rocks the ζ potential in water of p H higher than 2 is commonly negative and increases in absolute magnitude as supporting electrolyte concentration decreases and as temperature increases. A physicochemical theory was developed on the above features of the ζ potential on the basis of the adsorption reaction at the solid‐liquid interface. The experimental and theoretical results obtained in the present study of the ζ potential and the streaming potential coefficient in rock‐water systems are applied to some geophysical problems: electromagnetic variations induced by water diffusion prior to earthquakes and the self‐potential anomaly associated with hydrothermal convection.

@article{doi101029jb086ib03p01763,
    author = "Ishido, Tsuneo and Mizutani, Hitoshi",
    title = "Experimental and theoretical basis of electrokinetic phenomena in rock‐water systems and its applications to geophysics",
    year = "1981",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "The ζ potentials and the streaming potential coefficients of silicate mineral‐water and rock‐water systems were measured for purposes of estimating the magnitude of electrokinetic effects induced by water diffusion in the earth's interior. In rock‐water systems the adsorption reaction of H + at a solid surface is proved to be a main factor determining the ζ potential. For typical crustal rocks the ζ potential in water of p H higher than 2 is commonly negative and increases in absolute magnitude as supporting electrolyte concentration decreases and as temperature increases. A physicochemical theory was developed on the above features of the ζ potential on the basis of the adsorption reaction at the solid‐liquid interface. The experimental and theoretical results obtained in the present study of the ζ potential and the streaming potential coefficient in rock‐water systems are applied to some geophysical problems: electromagnetic variations induced by water diffusion prior to earthquakes and the self‐potential anomaly associated with hydrothermal convection.",
    url = "https://doi.org/10.1029/jb086ib03p01763",
    doi = "10.1029/jb086ib03p01763",
    openalex = "W1973789490",
    references = "doi1010160021979772901749"
}

Scanning electron microscopy and energy dispersive x-ray analyses of desert varnish reveal that microorganisms concentrate ambient manganese that becomes greatly enhanced in brown to black varnish. Specific characteristics of desert varnish and of varnish bacteria support a microbial origin for manganese-rich films. Varnish microbes can be cultured and produce laboratory manganese films. Accordingly, natural desert varnish and also manganese-rich rock varnishes in nondesert environments appear to be a product of microbial activity.

@article{doi101126science21345131245,
    author = "Dorn, Ronald I. and Oberlander, Theodore M.",
    title = "Microbial Origin of Desert Varnish",
    year = "1981",
    journal = "Science",
    abstract = "Scanning electron microscopy and energy dispersive x-ray analyses of desert varnish reveal that microorganisms concentrate ambient manganese that becomes greatly enhanced in brown to black varnish. Specific characteristics of desert varnish and of varnish bacteria support a microbial origin for manganese-rich films. Varnish microbes can be cultured and produce laboratory manganese films. Accordingly, natural desert varnish and also manganese-rich rock varnishes in nondesert environments appear to be a product of microbial activity.",
    url = "https://doi.org/10.1126/science.213.4513.1245",
    doi = "10.1126/science.213.4513.1245",
    openalex = "W1982129609"
}

@misc{dorn1981microbial2,
    author = "Dorn, R. I. and Oberlander, T. M",
    title = "Microbial origin of desert varnish",
    year = "1981",
    howpublished = "Science, v. 213, p. 1245-1247",
    note = "talkorigins\_source = {true}; raw\_reference = {Dorn, R. I., and Oberlander, T. M., 1981, Microbial origin of desert varnish: Science, v. 213, p. 1245-1247.}"
}

@article{doi101177030913338200600301,
    author = "Dorn, Ronald I. and Oberlander, Theodore M.",
    title = "Rock varnish",
    year = "1982",
    journal = "Progress in Physical Geography Earth and Environment",
    url = "https://doi.org/10.1177/030913338200600301",
    doi = "10.1177/030913338200600301",
    openalex = "W4211115059",
    references = "doi1010029781444313574, doi101007bf00378791, doi1010160026265x67900793, doi101021ba19680073ch021, doi101086627271, doi101126science1203109183, doi101126science7268409, doi1023071797455, openalexw2912219260, potter1977desert"
}

@misc{dorn1982rock3,
    author = "Dorn, R. I. and Oberlander, T. M",
    title = "Rock varnish",
    year = "1982",
    howpublished = "Progress in Physical Geography, v. 6, no. 3, p. 317- 366",
    note = "talkorigins\_source = {true}; raw\_reference = {Dorn, R. I., and Oberlander, T. M., 1982, Rock varnish: Progress in Physical Geography, v. 6, no. 3, p. 317- 366.}"
}

@article{doi101007bf02097739,
    author = "Taylor‐George, Susan and Palmer, F. and Staley, James T. and Borns, David James and Curtiss, Brian and Adams, J. B.",
    title = "Fungi and bacteria involved in desert varnish formation",
    year = "1983",
    journal = "Microbial Ecology",
    url = "https://doi.org/10.1007/bf02097739",
    doi = "10.1007/bf02097739",
    openalex = "W2076852250"
}

Abstract Rock varnish coats many surfaces of geomorphic and archaeologic interest in arid lands. All varnish dating techniques are limited by the time lag between the exposure of a surface to subaerial processes and the onset of varnishing. They are valid only where manganese is not remobilized after deposition, for example, in most arid environments. The premise of a new age-determination method, cation-ratio dating, is that the ratio of the more mobile cations (e.g., K and Ca) to titanium in varnish decreases with time. Although there are many inherent assumptions and potential limitations, cation-ratio dating has been verified on relative age-sequences from a Death Valley debris cone, Negev Desert talus flatirons, and prehistoric lake levels at Searles Lake in California. Varnish cation ratios have been calibrated to independently dated surfaces in the Coso volcanic field and vicinity in California. Tentative absolute dates have been assigned to geomorphic surfaces in the Coso area. Cation ratios have been used to distinguish relative ages of archaeologic artifacts in southwestern North America and to demonstrate that varnish at the South Stoddard locality, Mojave Desert, did not form in 25 yr.

@article{doi1010160033589483900650,
    author = "Dorn, Ronald I.",
    title = "Cation-Ratio Dating: A New Rock Varnish Age-Determination Technique",
    year = "1983",
    journal = "Quaternary Research",
    abstract = "Abstract Rock varnish coats many surfaces of geomorphic and archaeologic interest in arid lands. All varnish dating techniques are limited by the time lag between the exposure of a surface to subaerial processes and the onset of varnishing. They are valid only where manganese is not remobilized after deposition, for example, in most arid environments. The premise of a new age-determination method, cation-ratio dating, is that the ratio of the more mobile cations (e.g., K and Ca) to titanium in varnish decreases with time. Although there are many inherent assumptions and potential limitations, cation-ratio dating has been verified on relative age-sequences from a Death Valley debris cone, Negev Desert talus flatirons, and prehistoric lake levels at Searles Lake in California. Varnish cation ratios have been calibrated to independently dated surfaces in the Coso volcanic field and vicinity in California. Tentative absolute dates have been assigned to geomorphic surfaces in the Coso area. Cation ratios have been used to distinguish relative ages of archaeologic artifacts in southwestern North America and to demonstrate that varnish at the South Stoddard locality, Mojave Desert, did not form in 25 yr.",
    url = "https://doi.org/10.1016/0033-5894(83)90065-0",
    doi = "10.1016/0033-5894(83)90065-0",
    openalex = "W2067887526",
    references = "doi101007bf00378791, doi1010160009254179900858, doi101016s0140196318317452, doi101021ba19680073ch021, doi101038276489a0, doi10108000401706196210490038, doi101086627271, doi101126science2044394701, doi10113000167606195869487cdodv20co2, doi101177030913338200600301, doi102136sssaj195803615995002200010017x, doi105962bhltitle132168, potter1977desert"
}

Abstract Manganese‐oxidizing microorganisms have been implicated in the deposition of manganese in dark manganiferric rock varnish coatings on desert rocks. For this study, a collection of bacteria able to oxidize manganese has been obtained from rock varnish samples from the Sonoran and Mojave Deserts. Two groups of organisms predominated among the isolates. One group was identified as Arthrobacter spp. based on their rod‐coccus transformations, cell shapes, postfission division stages, and physiological characteristics. Another major group consisted of gram‐positive cocci tentatively ascribed to the genus Micrococcus. In addition, single isolates of Bacillus sp., Planococcus sp., Streptococcus sp., and Hyphomonas sp. were identified as manganese‐oxidizing bacteria. Some isolates grew too poorly to be easily characterized and identified.

@article{doi10108001490458609385943,
    author = "Palmer, F. and Staley, James T. and Murray, R. G. E. and Counsell, T. J. and Adams, J. B.",
    title = "Identification of manganese‐oxidizing bacteria from desert varnish",
    year = "1986",
    journal = "Geomicrobiology Journal",
    abstract = "Abstract Manganese‐oxidizing microorganisms have been implicated in the deposition of manganese in dark manganiferric rock varnish coatings on desert rocks. For this study, a collection of bacteria able to oxidize manganese has been obtained from rock varnish samples from the Sonoran and Mojave Deserts. Two groups of organisms predominated among the isolates. One group was identified as Arthrobacter spp. based on their rod‐coccus transformations, cell shapes, postfission division stages, and physiological characteristics. Another major group consisted of gram‐positive cocci tentatively ascribed to the genus Micrococcus. In addition, single isolates of Bacillus sp., Planococcus sp., Streptococcus sp., and Hyphomonas sp. were identified as manganese‐oxidizing bacteria. Some isolates grew too poorly to be easily characterized and identified.",
    url = "https://doi.org/10.1080/01490458609385943",
    doi = "10.1080/01490458609385943",
    openalex = "W1979122919"
}

@misc{dorn1986cationratio1,
    author = "Dorn, R. I. et al",
    title = "Cation-ratio and accelerator radiocarbon dating of rock varnish on Mojave artifacts and landforms",
    year = "1986",
    howpublished = "Science, v. 231, p. 830- 833",
    note = "talkorigins\_source = {true}; raw\_reference = {Dorn, R. I. et al., 1986, Cation-ratio and accelerator radiocarbon dating of rock varnish on Mojave artifacts and landforms: Science, v. 231, p. 830- 833.}"
}

In the Antarctic cold desert (Ross Desert), the survival of the cryptoendolithic microorganisms that colonize the near-surface layer of porous sandstone rocks depends on a precarious equilibrium of biological and geological factors. An unfavorable shift of this equilibrium results in death, and this may be followed by formation of trace fossils that preserve the characteristic iron-leaching pattern caused by microbial activity. Similar microbial trace fossil may exist in the geological record. If life ever arose on early Mars, similar processes may have occurred there and left recognizable traces.

@article{doi101126science11536571,
    author = "Friedmann, E. Imre and Weed, R.M.",
    title = "Microbial Trace-Fossil Formation, Biogenous, and Abiotic Weathering in the Antarctic Cold Desert",
    year = "1987",
    journal = "Science",
    abstract = "In the Antarctic cold desert (Ross Desert), the survival of the cryptoendolithic microorganisms that colonize the near-surface layer of porous sandstone rocks depends on a precarious equilibrium of biological and geological factors. An unfavorable shift of this equilibrium results in death, and this may be followed by formation of trace fossils that preserve the characteristic iron-leaching pattern caused by microbial activity. Similar microbial trace fossil may exist in the geological record. If life ever arose on early Mars, similar processes may have occurred there and left recognizable traces.",
    url = "https://doi.org/10.1126/science.11536571",
    doi = "10.1126/science.11536571",
    openalex = "W2035849767",
    references = "doi101177030913338200600301"
}

A collection of 79 bacterial isolates was obtained by plating scrapings and swabs from five rock varnish samples of the Negev Desert, Israel. Of the bacterial isolates which were obtained, 74 were able to oxidize manganese (MnII→MnIV) under laboratory conditions. The manganese-oxidizing bacteria were characterized and identified, when possible, to genus. The predominant genera were Bacillus, Geodermatophilus, Arthrobacter, and Micrococcus. Examples of all of these genera were found from more than one rock. These same genera have been reported from manganese-rich rock varnish samples from the Sonoran and Mojave Deserts of North America; however, many of the strains (species?) from the Negev appear to be different from those of the American Southwest. A single manganese-oxidizing actinomycete was also isolated. The high percentage of manganese-oxidizing bacteria to total cultivable bacteria (83 to 100% depending upon sample) from the Negev suggests that they are involved in desert varnish formation.

@article{doi101139m87165,
    author = "Hungate, Bruce A. and Danin, Avinoam and Pellerin, Nancy B. and Stemmler, Jayson and Kjellander, Petter and Adams, J. B. and Staley, James T.",
    title = "Characterization of manganese-oxidizing (MnII→MnIV) bacteria from Negev Desert rock varnish: implications in desert varnish formation",
    year = "1987",
    journal = "Canadian Journal of Microbiology",
    abstract = "A collection of 79 bacterial isolates was obtained by plating scrapings and swabs from five rock varnish samples of the Negev Desert, Israel. Of the bacterial isolates which were obtained, 74 were able to oxidize manganese (MnII→MnIV) under laboratory conditions. The manganese-oxidizing bacteria were characterized and identified, when possible, to genus. The predominant genera were Bacillus, Geodermatophilus, Arthrobacter, and Micrococcus. Examples of all of these genera were found from more than one rock. These same genera have been reported from manganese-rich rock varnish samples from the Sonoran and Mojave Deserts of North America; however, many of the strains (species?) from the Negev appear to be different from those of the American Southwest. A single manganese-oxidizing actinomycete was also isolated. The high percentage of manganese-oxidizing bacteria to total cultivable bacteria (83 to 100\% depending upon sample) from the Negev suggests that they are involved in desert varnish formation.",
    url = "https://doi.org/10.1139/m87-165",
    doi = "10.1139/m87-165",
    openalex = "W2019054566",
    references = "doi101007bf00378791, doi101007bf02097739, doi101016c2009007233x, doi101038276489a0, doi10108001490458609385943, doi101086649741, doi10113000167606195869487cdodv20co2, doi101139m87166, doi1073260003481911253913, openalexw2287553938"
}

Abstract Numerical and calibrated age determinations of the late Quaternary alluvial fan deposits of the Soda Mountains piedmont in the Mojave Desert provide an opportunity to study the utility of the multiparameter relative-age (RA) method for distinguishing and mapping geomorphic surfaces on a desert piedmont. Most RA parameters could not discriminate between deposits of Holocene age, although pavements have formed over locally significant parts of surfaces as young as middle Holocene. Several parameters, including lithologic composition, particle size, soil development, and varnish cover, permit distinguishing between Holocene surfaces and late Pleistocene surfaces. Statistically significant differences in initial particle size and lithology of the deposits, inferred to be the result of complex interaction among hillslope, alluvial fan, and eolian processes and climatic change, create conditions unfavorable for use of most RA techniques. In contrast, soil-profile development and varnish cover data are successful in discrimination among deposits of Holocene and Pleistocene age. This is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan.

@article{doi101016003358948990094x,
    author = "McFadden, Leslie D. and Ritter, John B. and Wells, Stephen G.",
    title = "Use of Multiparameter Relative-Age Methods for Age Estimation and Correlation of Alluvial Fan Surfaces on a Desert Piedmont, Eastern Mojave Desert, California",
    year = "1989",
    journal = "Quaternary Research",
    abstract = "Abstract Numerical and calibrated age determinations of the late Quaternary alluvial fan deposits of the Soda Mountains piedmont in the Mojave Desert provide an opportunity to study the utility of the multiparameter relative-age (RA) method for distinguishing and mapping geomorphic surfaces on a desert piedmont. Most RA parameters could not discriminate between deposits of Holocene age, although pavements have formed over locally significant parts of surfaces as young as middle Holocene. Several parameters, including lithologic composition, particle size, soil development, and varnish cover, permit distinguishing between Holocene surfaces and late Pleistocene surfaces. Statistically significant differences in initial particle size and lithology of the deposits, inferred to be the result of complex interaction among hillslope, alluvial fan, and eolian processes and climatic change, create conditions unfavorable for use of most RA techniques. In contrast, soil-profile development and varnish cover data are successful in discrimination among deposits of Holocene and Pleistocene age. This is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan.",
    url = "https://doi.org/10.1016/0033-5894(89)90094-x",
    doi = "10.1016/0033-5894(89)90094-x",
    openalex = "W2087899885",
    references = "doi1010079781461261124, doi1010160016706182900374, doi101016003358948790072x, doi1010970001069419660500000001, doi1010970001069419730900000003, doi101126science21345131245, doi101130001676061985961518lcleol20co2, doi10113000917613198715504ioeapp20co2, doi102307141416"
}

Pecked figures – seeming to depict humans, animals, objects and ‘abstract’ shapes - are an important and recalcitrant aspect to the archaeology of the desert USA, in the Great Basin and the Southwest. Where they are covered by desert varnish, they provide an opportunity for an absolute dating by cation-ratio method. Here - as they did not for a similar study in South Australia reported in an earlier Antiquity - the cation-ratio dates do seem to run alongside the chronological pattern inferred by conventional means.

@article{doi101017s0003598x00079692,
    author = "Loendorf, Lawrence L.",
    title = "Cation-ratio varnish dating and petroglyph chronology in southeastern Colorado",
    year = "1991",
    journal = "Antiquity",
    abstract = "Pecked figures – seeming to depict humans, animals, objects and ‘abstract’ shapes - are an important and recalcitrant aspect to the archaeology of the desert USA, in the Great Basin and the Southwest. Where they are covered by desert varnish, they provide an opportunity for an absolute dating by cation-ratio method. Here - as they did not for a similar study in South Australia reported in an earlier Antiquity - the cation-ratio dates do seem to run alongside the chronological pattern inferred by conventional means.",
    url = "https://doi.org/10.1017/s0003598x00079692",
    doi = "10.1017/s0003598x00079692",
    openalex = "W2464647092",
    references = "doi1010160033589483900650"
}

ABSTRACT Rock varnish occurs in virtually all environments, most commonly in arid and semi‐arid climates, including Antarctica. Rock varnish consists of thin layers of intimately mixed aeolian and chemical sediments often showing botryoidal and more rarely stromatolite‐like morphologies. Typical rock varnish samples collected at Twin Peak Mountain Park, near Phoenix, Arizona, consist of abundant quartz, with plagioclase, illite and a mixed layer, Fe‐clay mineral, probably corrensite. EDS, SEM (BSE) and TEM analyses revealed that the typical Mn, Fe minerals occur as minute particles; some of these particles and other mineral grains are attached to filaments. XRD and electron diffraction showed that the Mn.Fe‐bearing particles are poorly crystalline. The filaments, based on morphological criteria, are virtually indistinguishable from fungal filaments. Most filaments are fragments, probably broken by scraping during sample collection. Coccoid and rod‐shaped forms, resembling cyanobacteria and other bacteria, respectively, are also present. Unlike definitive minerals, these filaments disintegrated in the concentrated energy of the SEM electron beam at the instrumental and experimental conditions used. In addition, no filamentous, rod‐shaped or coccoid forms were observed in samples hydrolysed with 6 N HCl for 24 h at 100°C. Bacteria and fungi in powdered rock varnish were cultured on four media, incubated aerobically in the dark at 25°C. The culture media yielded dense growths of spore‐forming bacteria and filamentous fungi. One fungus and two Bacillus isolates oxidized and concentrated manganese. Control experiments revealed that fungi and bacteria are present on and below the surfaces of rock varnish. Free and hydrolysed, peptide/protein‐bound amino acids were identified in the rock varnish. Amino acids showed virtually no racemization with the exception of D/L asp = 0.1. Relatively high molecular weight humic matter was also separated from the rock varnish. High‐resolution mass spectrometry revealed non‐hydrocarbon moieties, similar to a Suwannee River (FL) humic acid standard. Micro‐organisms and their original biochemical compounds do not seem to be preserved for long in the accreting varnish layer. The studies showed that the filaments helped to trap mineral particles of rock varnish, and that bacteria and fungi abetted Mn concentration. Some structures in the layers of rock varnish resemble stromatolites and present definitions would allow them to be termed as such.

@article{doi101111j136530911991tb00376x,
    author = "Nagy, Bartholomew and Nagy, Lois Anne and Rigali, Mark J. and Jones, W. D. and Krinsley, David and Sinclair, Norval A.",
    title = "Rock varnish in the Sonoran Desert: microbiologically mediated accumulation of manganiferous sediments",
    year = "1991",
    journal = "Sedimentology",
    abstract = "ABSTRACT Rock varnish occurs in virtually all environments, most commonly in arid and semi‐arid climates, including Antarctica. Rock varnish consists of thin layers of intimately mixed aeolian and chemical sediments often showing botryoidal and more rarely stromatolite‐like morphologies. Typical rock varnish samples collected at Twin Peak Mountain Park, near Phoenix, Arizona, consist of abundant quartz, with plagioclase, illite and a mixed layer, Fe‐clay mineral, probably corrensite. EDS, SEM (BSE) and TEM analyses revealed that the typical Mn, Fe minerals occur as minute particles; some of these particles and other mineral grains are attached to filaments. XRD and electron diffraction showed that the Mn.Fe‐bearing particles are poorly crystalline. The filaments, based on morphological criteria, are virtually indistinguishable from fungal filaments. Most filaments are fragments, probably broken by scraping during sample collection. Coccoid and rod‐shaped forms, resembling cyanobacteria and other bacteria, respectively, are also present. Unlike definitive minerals, these filaments disintegrated in the concentrated energy of the SEM electron beam at the instrumental and experimental conditions used. In addition, no filamentous, rod‐shaped or coccoid forms were observed in samples hydrolysed with 6 N HCl for 24 h at 100°C. Bacteria and fungi in powdered rock varnish were cultured on four media, incubated aerobically in the dark at 25°C. The culture media yielded dense growths of spore‐forming bacteria and filamentous fungi. One fungus and two Bacillus isolates oxidized and concentrated manganese. Control experiments revealed that fungi and bacteria are present on and below the surfaces of rock varnish. Free and hydrolysed, peptide/protein‐bound amino acids were identified in the rock varnish. Amino acids showed virtually no racemization with the exception of D/L asp = 0.1. Relatively high molecular weight humic matter was also separated from the rock varnish. High‐resolution mass spectrometry revealed non‐hydrocarbon moieties, similar to a Suwannee River (FL) humic acid standard. Micro‐organisms and their original biochemical compounds do not seem to be preserved for long in the accreting varnish layer. The studies showed that the filaments helped to trap mineral particles of rock varnish, and that bacteria and fungi abetted Mn concentration. Some structures in the layers of rock varnish resemble stromatolites and present definitions would allow them to be termed as such.",
    url = "https://doi.org/10.1111/j.1365-3091.1991.tb00376.x",
    doi = "10.1111/j.1365-3091.1991.tb00376.x",
    openalex = "W1976002167",
    references = "doi101007bf00378791, doi101007bf01611203, doi1010160012825281900465, doi1010160033589483900650, doi101017cbo9780511794261004, doi101038276489a0, doi101177030913337700100202, doi102307634889, openalexw590518688, potter1977desert"
}

Abstract Precipitation of manganese by fungi and chemoorganotrophic bacteria isolated from desert rock was quantitatively shown to be dependent on increasing NaCl concentrations in vitro. Many fungi were able to precipitate Mn(IV) under conditions suggested to be similar to those found on desert rock. However, Mn(IV) precipitation in bacterial cultures took place mostly at pH values above 8. Our results lend farther support to the hypothesis on the microbial genesis and stabilization of rock varnish and the patination of ancient petroglyphs.

@article{doi10108001490459209377903,
    author = "Grote, G. and Krumbein, Wolfgang E.",
    title = "Microbial precipitation of manganese by bacteria and fungi from desert rock and rock varnish",
    year = "1992",
    journal = "Geomicrobiology Journal",
    abstract = "Abstract Precipitation of manganese by fungi and chemoorganotrophic bacteria isolated from desert rock was quantitatively shown to be dependent on increasing NaCl concentrations in vitro. Many fungi were able to precipitate Mn(IV) under conditions suggested to be similar to those found on desert rock. However, Mn(IV) precipitation in bacterial cultures took place mostly at pH values above 8. Our results lend farther support to the hypothesis on the microbial genesis and stabilization of rock varnish and the patination of ancient petroglyphs.",
    url = "https://doi.org/10.1080/01490459209377903",
    doi = "10.1080/01490459209377903",
    openalex = "W1969006683",
    references = "doi101002jobm3630040506, doi101002jobm3630100502, doi101007bf00378791, doi101007bf01611203, doi101007bf01820710, doi101016b9780080114385500143, doi101071bi9560238, doi101126science21345131245, doi102216i003188841044111, openalexw3094324519"
}

Abstract Rock varnish coatings tend to become thicker, darker, and more continuous over time, leading to the use of changes in overall varnish color and the percentage of clast surfaces covered by varnish as relative-age indicators. Manganese is the most characteristic element of subaerial rock varnishes, and the progressive development of varnish coats can be quantified by measuring the amount of Mn accumulated on a given area of rock surface. Manganese oxides were dissolved off varnished clasts collected from alluvial surfaces on the Soda Mountains piedmont in the Mojave Desert, California, and the amount of Mn was measured using inductively coupled plasma emission spectroscopy. On the distal piedmont, maximum varnish development increases from a mid- to late-Holocene surface, typically containing up to 0.15 mg/cm 2 of accumulated Mn, to an early- to mid-Holocene surface with up to 0.21 mg/cm 2. However, varnish is less developed on a nearby late Pleistocene surface, suggesting extensive abrasion of clasts on the Pleistocene desert pavements or disturbance of the clasts. Varnish is better developed on the proximal piedmont, typically containing up to 0.30 mg/cm 2 of Mn, although varnish from a Pleistocene surface is again no better developed than from a nearby early- to mid-Holocene surface. These 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.

@article{doi101006qres19931084,
    author = "Reneau, Steven L.",
    title = "Manganese Accumulation in Rock Varnish on a Desert Piedmont, Mojave Desert, California, and Application to Evaluating Varnish Development",
    year = "1993",
    journal = "Quaternary Research",
    abstract = "Abstract Rock varnish coatings tend to become thicker, darker, and more continuous over time, leading to the use of changes in overall varnish color and the percentage of clast surfaces covered by varnish as relative-age indicators. Manganese is the most characteristic element of subaerial rock varnishes, and the progressive development of varnish coats can be quantified by measuring the amount of Mn accumulated on a given area of rock surface. Manganese oxides were dissolved off varnished clasts collected from alluvial surfaces on the Soda Mountains piedmont in the Mojave Desert, California, and the amount of Mn was measured using inductively coupled plasma emission spectroscopy. On the distal piedmont, maximum varnish development increases from a mid- to late-Holocene surface, typically containing up to 0.15 mg/cm 2 of accumulated Mn, to an early- to mid-Holocene surface with up to 0.21 mg/cm 2. However, varnish is less developed on a nearby late Pleistocene surface, suggesting extensive abrasion of clasts on the Pleistocene desert pavements or disturbance of the clasts. Varnish is better developed on the proximal piedmont, typically containing up to 0.30 mg/cm 2 of Mn, although varnish from a Pleistocene surface is again no better developed than from a nearby early- to mid-Holocene surface. These 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.",
    url = "https://doi.org/10.1006/qres.1993.1084",
    doi = "10.1006/qres.1993.1084",
    openalex = "W2055900020",
    references = "doi101007bf00378791, doi1010160009254179900858, doi1010160033589483900650, doi101016003358948790072x, doi101016003358948990094x, doi101038276489a0, doi101086627271, doi10113000167606195869487cdodv20co2, doi101177030913338200600301, potter1977desert"
}

Abstract A fundamental problem in the analysis of landforms and surface artifacts is how to correlate the ages of non-contiguous surfaces. One solution to this problem may lie within the varnish coatings on desert rocks. When viewed with a light microscope in ultra-thin cross-sections, rock varnish reveals orange and black layers that record drier and wetter climates, respectively. Consistent patterns of alternating orange and black microlaminae are evident in some 2900 rock-surface depressions in 420 ultra thin sections from 360 rocks in Death Valley and the surrounding region. Microlaminae are organized into distinct layering units that provide relative ages for geomorphic and archaeological surfaces. The largest uncertainty in developing calibrated chronologies for layering units is the inability to date specific layers; we resolve this problem by correlating layering units with independent numerical ages. Because rock varnishes are ubiquitous in deserts, their visual microlaminations have great potential as a tool to assess temporal and spatial variations in dryland environments. This potential is illustrated for alluvial-fan deposits in Death Valley, petroglyphs, and fault scarps. One of the most surprising, if speculative, findings is that the ages of black laminations (wetter periods) in Death Valley coincide with the timings of iceberg armadas in the North Atlantic (Heinrich Events).

@article{doi101111j146783061996tb01750x,
    author = "Liu, Tanzhuo and Dorn, Ronald I.",
    title = "Understanding the Spatial Variability of Environmental Change in Drylands with Rock Varnish Microlaminations",
    year = "1996",
    journal = "Annals of the Association of American Geographers",
    abstract = "Abstract A fundamental problem in the analysis of landforms and surface artifacts is how to correlate the ages of non-contiguous surfaces. One solution to this problem may lie within the varnish coatings on desert rocks. When viewed with a light microscope in ultra-thin cross-sections, rock varnish reveals orange and black layers that record drier and wetter climates, respectively. Consistent patterns of alternating orange and black microlaminae are evident in some 2900 rock-surface depressions in 420 ultra thin sections from 360 rocks in Death Valley and the surrounding region. Microlaminae are organized into distinct layering units that provide relative ages for geomorphic and archaeological surfaces. The largest uncertainty in developing calibrated chronologies for layering units is the inability to date specific layers; we resolve this problem by correlating layering units with independent numerical ages. Because rock varnishes are ubiquitous in deserts, their visual microlaminations have great potential as a tool to assess temporal and spatial variations in dryland environments. This potential is illustrated for alluvial-fan deposits in Death Valley, petroglyphs, and fault scarps. One of the most surprising, if speculative, findings is that the ages of black laminations (wetter periods) in Death Valley coincide with the timings of iceberg armadas in the North Atlantic (Heinrich Events).",
    url = "https://doi.org/10.1111/j.1467-8306.1996.tb01750.x",
    doi = "10.1111/j.1467-8306.1996.tb01750.x",
    openalex = "W2141125144",
    references = "doi101006qres19931084, doi101007bf00378791, doi101016003101829090217u, doi101016003101829290017y, doi101038352790a0, doi101038365143a0, doi101038372421a0, doi101126science24148691043, doi105860choice300305, openalexw2912219260, potter1977desert"
}

An attempt was made to date rock surfaces with accelerator mass spectrometry (AMS) radiocarbon measurements of rock varnishes or rock weathering rinds. In two case studies, samples pretreated in the laboratory of Dr. Ronald Dorn prior to AMS analysis have been found to contain significant quantities of carbon-rich materials of two distinct classes. Type I material resembles bituminous coal, whereas type II material resembles pyrolized wood charcoal fragments. In samples where these type I and type II materials were separated and AMS-radiocarbon dated, they were found to have widely differing radiocarbon ages. In these cases, the measurement of the radiocarbon age of the entire sample would yield results that are, at best, ambiguous. Neither type I nor type II materials were found in comparable samples that were independently prepared.

@article{doi101126science28053722132,
    author = "Beck, Warren and Donahue, D. J. and Jull, A. J. T. and Burr, George S. and Broecker, Wallace S. and Bonani, Georges and Hajdas, Irka and Malotki, Ekkehart",
    title = "Ambiguities in Direct Dating of Rock Surfaces Using Radiocarbon Measurements",
    year = "1998",
    journal = "Science",
    abstract = "An attempt was made to date rock surfaces with accelerator mass spectrometry (AMS) radiocarbon measurements of rock varnishes or rock weathering rinds. In two case studies, samples pretreated in the laboratory of Dr. Ronald Dorn prior to AMS analysis have been found to contain significant quantities of carbon-rich materials of two distinct classes. Type I material resembles bituminous coal, whereas type II material resembles pyrolized wood charcoal fragments. In samples where these type I and type II materials were separated and AMS-radiocarbon dated, they were found to have widely differing radiocarbon ages. In these cases, the measurement of the radiocarbon age of the entire sample would yield results that are, at best, ambiguous. Neither type I nor type II materials were found in comparable samples that were independently prepared.",
    url = "https://doi.org/10.1126/science.280.5372.2132",
    doi = "10.1126/science.280.5372.2132",
    openalex = "W1970811642"
}

The sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply, sediment availability and the transport capacity of the wind. The sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system. Numerous factors impact the susceptibility of grains on a surface to transport, but these are cumulatively manifested by the actual transport rate, which serves as a proxy for sediment availability. Transport capacity is the potential sediment transport rate of the wind. Because the three aspects of sediment state can be given as a volumetric rate, they are directly comparable. Plotted simultaneously against time, the generated curves define nine possible classes of sediment state. Sediment supply that is stored occurs because it is transport or availability limited, or generated at a rate greater than the potential or actual transport rates respectively. Contemporaneous or lagged influx to an aeolian system may be limited by sediment availability, but cannot exceed the transport capacity of the wind. For the Kelso dune field in the Mojave Desert of California, a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system. The sediment supply was generated during the latest Pleistocene and earliest Holocene during humid periods of enhanced discharge by the Mojave River to form the Lake Mojave fan delta or terminal fan, and has been calculated over time from the sedimentation rate and the frequency of floods. Estimation of transport capacity over time was based upon modern wind data, with an allowance for greater winds during the Pleistocene based upon climatic models. Sediment availability was approximated by calculation of a modern dune mobility index, with variation over time based upon climatic inferences. While quantifying the Kelso or any natural system is subject to numerous uncertainties, the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at Kelso, as well as illuminating questions for future research.

@article{doi101046j13653091199900227x,
    author = "Kocurek, Gary and Lancaster, Nicholas",
    title = "Aeolian system sediment state: theory and Mojave Desert Kelso dune field example",
    year = "1999",
    journal = "Sedimentology",
    abstract = "The sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply, sediment availability and the transport capacity of the wind. The sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system. Numerous factors impact the susceptibility of grains on a surface to transport, but these are cumulatively manifested by the actual transport rate, which serves as a proxy for sediment availability. Transport capacity is the potential sediment transport rate of the wind. Because the three aspects of sediment state can be given as a volumetric rate, they are directly comparable. Plotted simultaneously against time, the generated curves define nine possible classes of sediment state. Sediment supply that is stored occurs because it is transport or availability limited, or generated at a rate greater than the potential or actual transport rates respectively. Contemporaneous or lagged influx to an aeolian system may be limited by sediment availability, but cannot exceed the transport capacity of the wind. For the Kelso dune field in the Mojave Desert of California, a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system. The sediment supply was generated during the latest Pleistocene and earliest Holocene during humid periods of enhanced discharge by the Mojave River to form the Lake Mojave fan delta or terminal fan, and has been calculated over time from the sedimentation rate and the frequency of floods. Estimation of transport capacity over time was based upon modern wind data, with an allowance for greater winds during the Pleistocene based upon climatic models. Sediment availability was approximated by calculation of a modern dune mobility index, with variation over time based upon climatic inferences. While quantifying the Kelso or any natural system is subject to numerous uncertainties, the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at Kelso, as well as illuminating questions for future research.",
    url = "https://doi.org/10.1046/j.1365-3091.1999.00227.x",
    doi = "10.1046/j.1365-3091.1999.00227.x",
    openalex = "W2075280574",
    references = "doi101016003358948790072x"
}

@article{doi101016s0012825299000598,
    author = "Watchman, Alan",
    title = "A review of the history of dating rock varnishes",
    year = "2000",
    journal = "Earth-Science Reviews",
    url = "https://doi.org/10.1016/s0012-8252(99)00059-8",
    doi = "10.1016/s0012-8252(99)00059-8",
    openalex = "W2055236587",
    references = "doi1010160033589483900650"
}

@article{doi10113000917613200028183hfdrvg20co2,
    author = "Liu, Tanzhuo and Broecker, Wallace S.",
    title = "How fast does rock varnish grow?",
    year = "2000",
    journal = "Geology",
    url = "https://doi.org/10.1130/0091-7613(2000)28<183:hfdrvg>2.0.co;2",
    doi = "10.1130/0091-7613(2000)28<183:hfdrvg>2.0.co;2",
    openalex = "W2026979800",
    references = "doi101038276489a0, doi101111j146783061996tb01750x, doi10113000167606195869487cdodv20co2, doi101177030913338200600301, potter1977desert"
}

@article{doi1011300091761320010290855dpaarv20co2,
    author = "Quade, Jay",
    title = "Desert pavements and associated rock varnish in the Mojave Desert: How old can they be?",
    year = "2001",
    journal = "Geology",
    url = "https://doi.org/10.1130/0091-7613(2001)029<0855:dpaarv>2.0.co;2",
    doi = "10.1130/0091-7613(2001)029<0855:dpaarv>2.0.co;2",
    openalex = "W1973208973",
    references = "doi101006qres19931084, doi101006qres19971959, doi1010079781461254546, doi1010160033589483900650, doi101016s0169555x97000950, doi101038137179b0, doi101111j146783061970tb00741x, doi101126science28053722132, doi101130001676061985961518lcleol20co2, doi1011300091761319950230613chsedo23co2, doi10113000917613200028183hfdrvg20co2"
}

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.

@article{doi1011301052517320010110004rvrodw20co2,
    author = "Broecker, Wallace S. and Liu, Tanzhuo",
    title = "Rock Varnish: Recorder of Desert Wetness?",
    year = "2001",
    journal = "GSA Today",
    abstract = "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.",
    url = "https://doi.org/10.1130/1052-5173(2001)011<0004:rvrodw>2.0.co;2",
    doi = "10.1130/1052-5173(2001)011<0004:rvrodw>2.0.co;2",
    openalex = "W2081735009",
    references = "doi101006qres19931084, doi101016003101829090217u, doi1010160033589483900650, doi1010160033589487900469, doi101038276489a0, doi101111j136530911991tb00376x, doi101111j146783061996tb01750x, doi101126science2615118198, doi1011300091761319950230613chsedo23co2, doi10113000917613200028183hfdrvg20co2, openalexw1465883202, openalexw2912219260, potter1977desert"
}

X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.

@article{doi102138am20015611,
    author = "McKeown, David A. and Post, Jeffrey E.",
    title = "Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy",
    year = "2001",
    journal = "American Mineralogist",
    abstract = "X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.",
    url = "https://doi.org/10.2138/am-2001-5-611",
    doi = "10.2138/am-2001-5-611",
    openalex = "W2244524084",
    references = "doi101007bf00378791, doi1010160009254179900858, doi101016092145269400655f, doi101038276489a0, doi101107s0567740876007371, doi101107s0567740882004968, doi101111j136530911991tb00376x, doi101111j146783061996tb01750x, doi101346ccmn19860340503, openalexw1568216123, openalexw2146521523, openalexw2463361634, potter1977desert"
}

Slow erosion has characterized the Namib Desert, the Namibian escarpment, and the adjacent Namibian highlands over the Pleistocene. Paired analyses (n = 66) of in-situ-produced 10 Be and 26 Al in quartz-bearing samples of bedrock primarily from inselbergs, of sediment from dry river and stream channels, and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years. Bedrock samples (n = 47) collected in three transects from the coast, across the escarpment, and into the highlands, show no spatial pattern in elevation-normalized nuclide abundance despite a difference in mean annual precipitation (MAP) between sample sites at the coast (MAP −1) and those in the highlands (MAP >400 mm yr −1). Average model erosion rates inland of the escarpment (3.2 ± 1.5, n = 9) are indistinguishable from average rates seaward of the escarpment (3.6 ± 1.9, n = 38) indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands. Sediment samples (n = 3) from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster (16 m my −1) than either a basin in the highlands (5 m my −1) or a basin in the coastal plain (8 m my −1). Data from large rivers (n = 4) constrain erosion rates, averaged over 10 5 yrs and 10 4 to 10 5 km 2, between 3 and 9 m my −1. Small quartz clasts (n = 12) collected from four desert surfaces record extraordinarily long, variable, and in some cases complex exposure histories. Simple 10 Be model ages are as high as 1.8 my; some minimum total histories, considering both 10 Be and 26 Al and including both burial and exposure, exceed 2.7 my. As a group, the Namibian cosmogenic data do not support the model of significant and on-going escarpment retreat. The similarity of erosion rates calculated from 10 Be analysis of fluvial sediments and longer-term (10 7 yr), average mass removal rates estimated by others using fission track analysis of rock suggests that Namibian erosion rates have reached a steady state and are changing little over time. At outcrop scales, the concordance of 10 Be and 26 Al in most bedrock samples suggests that the model of steady, uniform bedrock erosion is valid; there is no indication of intermittent burial, shedding of thick rock slabs, or stripping of previous cover. At an intermediate scale, a transect of bedrock samples north of Gobabeb demonstrates that the northern boundary of the massive Namib Sand Sea has been steady and unshifting. Similarly low cosmogenically estimated erosion rates across west and central Namibia suggest that the landscape is in geomorphic steady state, its overall appearance changing only slowly through time.

@article{doi102475ajs30145326,
    author = "Bierman, Paul R. and Caffee, Marc",
    title = "Slow Rates of Rock Surface Erosion and Sediment Production across the Namib Desert and Escarpment, Southern Africa",
    year = "2001",
    journal = "American Journal of Science",
    abstract = "Slow erosion has characterized the Namib Desert, the Namibian escarpment, and the adjacent Namibian highlands over the Pleistocene. Paired analyses (n = 66) of in-situ-produced 10 Be and 26 Al in quartz-bearing samples of bedrock primarily from inselbergs, of sediment from dry river and stream channels, and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years. Bedrock samples (n = 47) collected in three transects from the coast, across the escarpment, and into the highlands, show no spatial pattern in elevation-normalized nuclide abundance despite a difference in mean annual precipitation (MAP) between sample sites at the coast (MAP −1) and those in the highlands (MAP >400 mm yr −1). Average model erosion rates inland of the escarpment (3.2 ± 1.5, n = 9) are indistinguishable from average rates seaward of the escarpment (3.6 ± 1.9, n = 38) indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands. Sediment samples (n = 3) from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster (16 m my −1) than either a basin in the highlands (5 m my −1) or a basin in the coastal plain (8 m my −1). Data from large rivers (n = 4) constrain erosion rates, averaged over 10 5 yrs and 10 4 to 10 5 km 2, between 3 and 9 m my −1. Small quartz clasts (n = 12) collected from four desert surfaces record extraordinarily long, variable, and in some cases complex exposure histories. Simple 10 Be model ages are as high as 1.8 my; some minimum total histories, considering both 10 Be and 26 Al and including both burial and exposure, exceed 2.7 my. As a group, the Namibian cosmogenic data do not support the model of significant and on-going escarpment retreat. The similarity of erosion rates calculated from 10 Be analysis of fluvial sediments and longer-term (10 7 yr), average mass removal rates estimated by others using fission track analysis of rock suggests that Namibian erosion rates have reached a steady state and are changing little over time. At outcrop scales, the concordance of 10 Be and 26 Al in most bedrock samples suggests that the model of steady, uniform bedrock erosion is valid; there is no indication of intermittent burial, shedding of thick rock slabs, or stripping of previous cover. At an intermediate scale, a transect of bedrock samples north of Gobabeb demonstrates that the northern boundary of the massive Namib Sand Sea has been steady and unshifting. Similarly low cosmogenically estimated erosion rates across west and central Namibia suggest that the landscape is in geomorphic steady state, its overall appearance changing only slowly through time.",
    url = "https://doi.org/10.2475/ajs.301.4-5.326",
    doi = "10.2475/ajs.301.4-5.326",
    openalex = "W2111996152",
    references = "doi1011300091761319950230613chsedo23co2"
}

@article{doi101016s0169555x02003318,
    author = "Liu, Tanzhuo",
    title = "Blind testing of rock varnish microstratigraphy as a chronometric indicator: results on late Quaternary lava flows in the Mojave Desert, California",
    year = "2003",
    journal = "Geomorphology",
    url = "https://doi.org/10.1016/s0169-555x(02)00331-8",
    doi = "10.1016/s0169-555x(02)00331-8",
    openalex = "W2110302823",
    references = "doi101006qres19931084, doi1010160033589487900469, doi101029gm112, doi101038365143a0, doi101038366552a0, doi101038371326a0, doi101038372421a0, doi101038372663a0, doi101111j146783061996tb01750x, doi101126science26752001005, doi101126science27853411257, doi1011301052517320010110004rvrodw20co2, doi105860choice300305"
}

Abstract In the Earth’s lithosphere, fungi are of fundamental importance as decomposer organisms, animal and plant pathogens and symbionts (e.g. lichens and mycorrhizas), being ubiquitous in sub-aerial and subsoil environments. The ability of fungi to interact with minerals, metals, metalloids and organic compounds through biomechanical and biochemical processes, makes them ideally suited as biological weathering agents of rock and building stone. They also play a fundamental role in biogeochemical cycling of nutrients, (e.g. C, N, P and S) and metals (e.g. Na, Mg, Ca, Mn, Fe, Cu, Zn, Co and Ni) essential for the growth of living organisms in the biosphere. In addition they play an integral role in the mobilization and immobilization of non-essential metals (e.g. Cs, Al, Cd, Hg and Pb). Most studies on mineral-microbe interactions and microbial involvement in geological processes have concentrated on bacteria and archaea (Prokaryota): fungi (Eukaryota) have, to a certain extent, been neglected. This article addresses the role of fungi in geomicrobiological processes, emphasizing their deteriorative potential on rock, building stone and mineral surfaces and involvement in the formation of secondary mycogenic minerals. Such roles of fungi are also of importance for the global carbon reservoir and have potential biotechnological applications, e.g. in the bioremediation of xenobiotic-, metal- and/or radionuclide-contaminated soils and wastes, and metal/radionuclide recovery.

@article{doi1011800026461036760154,
    author = "Burford, Euan P. and Fomina, Marina and Gadd, Geoffrey Michael",
    title = "Fungal involvement in bioweathering and biotransformation of rocks and minerals",
    year = "2003",
    journal = "Mineralogical Magazine",
    abstract = "Abstract In the Earth’s lithosphere, fungi are of fundamental importance as decomposer organisms, animal and plant pathogens and symbionts (e.g. lichens and mycorrhizas), being ubiquitous in sub-aerial and subsoil environments. The ability of fungi to interact with minerals, metals, metalloids and organic compounds through biomechanical and biochemical processes, makes them ideally suited as biological weathering agents of rock and building stone. They also play a fundamental role in biogeochemical cycling of nutrients, (e.g. C, N, P and S) and metals (e.g. Na, Mg, Ca, Mn, Fe, Cu, Zn, Co and Ni) essential for the growth of living organisms in the biosphere. In addition they play an integral role in the mobilization and immobilization of non-essential metals (e.g. Cs, Al, Cd, Hg and Pb). Most studies on mineral-microbe interactions and microbial involvement in geological processes have concentrated on bacteria and archaea (Prokaryota): fungi (Eukaryota) have, to a certain extent, been neglected. This article addresses the role of fungi in geomicrobiological processes, emphasizing their deteriorative potential on rock, building stone and mineral surfaces and involvement in the formation of secondary mycogenic minerals. Such roles of fungi are also of importance for the global carbon reservoir and have potential biotechnological applications, e.g. in the bioremediation of xenobiotic-, metal- and/or radionuclide-contaminated soils and wastes, and metal/radionuclide recovery.",
    url = "https://doi.org/10.1180/0026461036760154",
    doi = "10.1180/0026461036760154",
    openalex = "W2106222412",
    references = "doi10108001490459209377903, doi101126science2394836149"
}

@article{doi101016jepsl200404038,
    author = "Thiagarajan, Nivedita and Lee, Cin‐Ty A.",
    title = "Trace-element evidence for the origin of desert varnish by direct aqueous atmospheric deposition",
    year = "2004",
    journal = "Earth and Planetary Science Letters",
    url = "https://doi.org/10.1016/j.epsl.2004.04.038",
    doi = "10.1016/j.epsl.2004.04.038",
    openalex = "W2135445963",
    references = "doi101016s0169555x02003318, doi101111j136530911991tb00376x, doi1011301052517320010110004rvrodw20co2"
}

@article{doi101016jicarus200411022,
    author = "Kuhlman, K. R. and Allenbach, Lisa and Ball, Christopher L. and Fusco, William G. and Duc, Myron T. La and Kuhlman, G. M. and Anderson, Robert C. and Stuecker, Tara and Erickson, Issac K. and Benardini, James N. and Crawford, Ronald L.",
    title = "Enumeration, isolation, and characterization of ultraviolet (UV-C) resistant bacteria from rock varnish in the Whipple Mountains, California",
    year = "2005",
    journal = "Icarus",
    url = "https://doi.org/10.1016/j.icarus.2004.11.022",
    doi = "10.1016/j.icarus.2004.11.022",
    openalex = "W1976379176",
    references = "doi101007bf00388810, doi1010160009254179900858, doi101016s0169555x02003318, doi101016s0960076097807996, doi101038345063a0, doi101093bioinformatics178754, doi101093nar25173389, doi10109900207713444846, doi101128aem5637827871990, doi101128mmbr5911431691995, doi101128mr5911431691995, doi10113000167606195869487cdodv20co2, doi1011301052517320010110004rvrodw20co2, doi101139m87165, doi101177030913338200600301, openalexw1589603082"
}

The age onset of hyperaridity in the Atacama Desert, Chile, which is needed to validate geological and climatological concepts, has been heretofore uncertain. Measurement of cosmogenic

@article{doi101130g211841,
    author = "Dunai, Tibor J. and López, Gabriel A. González and Juez-Larré, J.",
    title = "Oligocene–Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms",
    year = "2005",
    journal = "Geology",
    abstract = "The age onset of hyperaridity in the Atacama Desert, Chile, which is needed to validate geological and climatological concepts, has been heretofore uncertain. Measurement of cosmogenic",
    url = "https://doi.org/10.1130/g21184.1",
    doi = "10.1130/g21184.1",
    openalex = "W2125388221"
}

@article{doi101016jgeomorph200606008,
    author = "Liu, Tanzhuo and Broecker, Wallace S.",
    title = "Holocene rock varnish microstratigraphy and its chronometric application in the drylands of western USA",
    year = "2006",
    journal = "Geomorphology",
    url = "https://doi.org/10.1016/j.geomorph.2006.06.008",
    doi = "10.1016/j.geomorph.2006.06.008",
    openalex = "W2085739620",
    references = "doi1010160033589483900650, doi101016s0169555x02003318, doi101111j146783061996tb01750x, doi1011301052517320010110004rvrodw20co2"
}

Rock varnish from Arizona's Whipple Mountains harbors a microbial community containing about 10(8) microorganisms g(-1) of varnish. Analyses of varnish phospholipid fatty acids and rRNA gene libraries reveal a community comprised of mostly Proteobacteria but also including Actinobacteria, eukaryota, and a few members of the Archaea. Rock varnish represents a significant niche for microbial colonization.

@article{doi101128aem722170817152006,
    author = "Kuhlman, K. R. and Fusco, William G. and Duc, Myron T. La and Allenbach, Lisa and Ball, Christopher L. and Kuhlman, G. M. and Anderson, Robert C. and Erickson, Issac K. and Stuecker, Tara and Benardini, James N. and Strap, Janice L. and Crawford, Ronald L.",
    title = "Diversity of Microorganisms within Rock Varnish in the Whipple Mountains, California",
    year = "2006",
    journal = "Applied and Environmental Microbiology",
    abstract = "Rock varnish from Arizona's Whipple Mountains harbors a microbial community containing about 10(8) microorganisms g(-1) of varnish. Analyses of varnish phospholipid fatty acids and rRNA gene libraries reveal a community comprised of mostly Proteobacteria but also including Actinobacteria, eukaryota, and a few members of the Archaea. Rock varnish represents a significant niche for microbial colonization.",
    url = "https://doi.org/10.1128/aem.72.2.1708-1715.2006",
    doi = "10.1128/aem.72.2.1708-1715.2006",
    openalex = "W2108211322",
    references = "doi101006qres19931084, doi101007bf00388810, doi101016016895259090186a, doi101016b9780123721808500421, doi101016jicarus200411022, doi101016s0022283605803602, doi101016s0169555x02003318, doi101093bioinformatics178754, doi101128aem5637827871990, doi101128aem713150115062005, doi101128mr5911431691995, doi1011301052517320010110004rvrodw20co2, openalexw1589603082, openalexw3217097258"
}

@article{doi101130g223521,
    author = "Perry, Randall S. and Lynne, Bridget Y. and Sephton, Mark A. and Kolb, Vera M. and Perry, Carole C. and Staley, James T.",
    title = "Baking black opal in the desert sun: The importance of silica in desert varnish",
    year = "2006",
    journal = "Geology",
    url = "https://doi.org/10.1130/g22352.1",
    doi = "10.1130/g22352.1",
    openalex = "W2036499309",
    references = "doi101007bf02097739, doi101016jsedgeo200505012, doi101016s0169555x02003318, doi101016s0927776501001436, doi101038276489a0, doi101111j136530911991tb00376x, doi101126science21345131245, doi101126science21545361093, doi101128aem722170817152006, doi101306011704740561, doi105962bhltitle132168, openalexw649466005, potter1977desert"
}

@article{doi101016jgeomorph200703015,
    author = "Liu, Tanzhuo and Broecker, Wallace S.",
    title = "Rock varnish microlamination dating of late Quaternary geomorphic features in the drylands of western USA",
    year = "2007",
    journal = "Geomorphology",
    url = "https://doi.org/10.1016/j.geomorph.2007.03.015",
    doi = "10.1016/j.geomorph.2007.03.015",
    openalex = "W2032247773",
    references = "doi101016s0169555x02003318, doi101111j146783061996tb01750x, doi1011300091761320010290855dpaarv20co2"
}

Biofilms are interface micro-habitats formed by microbes that differ markedly from those of the ambient environment. The term 'subaerial biofilm' (SAB) was coined for microbial communities that develop on solid mineral surfaces exposed to the atmosphere. Subaerial biofilms are ubiquitous, self-sufficient, miniature microbial ecosystems that are found on buildings, bare rocks in deserts, mountains, and at all latitudes where direct contact with the atmosphere and solar radiation occurs. Subaerial biofilms on exposed terrestrial surfaces are characterized by patchy growth that is dominated by associations of fungi, algae, cyanobacteria and heterotrophic bacteria. Inherent subaerial settlers include specialized actinobacteria (e.g. Geodermatophilus), cyanobacteria and microcolonial fungi. Individuals within SAB communities avoid sexual reproduction, but cooperate extensively with one another especially to avoid loss of energy and nutrients. Subaerial biofilm metabolic activity centres on retention of water, protecting the cells from fluctuating environmental conditions and solar radiation as well as prolonging their vegetative life. Atmospheric aerosols, gases and propagatory particles serve as sources of nutrients and inoculum for these open communities. Subaerial biofilms induce chemical and physical changes to rock materials, and they penetrate the mineral substrate contributing to rock and mineral decay, which manifests itself as bio-weathering of rock surfaces. Given their characteristic slow and sensitive growth, SAB may also serve as bioindicators of atmospheric and/or climate change.

@article{doi101111j14622920200701301x,
    author = "Gorbushina, Anna A.",
    title = "Life on the rocks",
    year = "2007",
    journal = "Environmental Microbiology",
    abstract = "Biofilms are interface micro-habitats formed by microbes that differ markedly from those of the ambient environment. The term 'subaerial biofilm' (SAB) was coined for microbial communities that develop on solid mineral surfaces exposed to the atmosphere. Subaerial biofilms are ubiquitous, self-sufficient, miniature microbial ecosystems that are found on buildings, bare rocks in deserts, mountains, and at all latitudes where direct contact with the atmosphere and solar radiation occurs. Subaerial biofilms on exposed terrestrial surfaces are characterized by patchy growth that is dominated by associations of fungi, algae, cyanobacteria and heterotrophic bacteria. Inherent subaerial settlers include specialized actinobacteria (e.g. Geodermatophilus), cyanobacteria and microcolonial fungi. Individuals within SAB communities avoid sexual reproduction, but cooperate extensively with one another especially to avoid loss of energy and nutrients. Subaerial biofilm metabolic activity centres on retention of water, protecting the cells from fluctuating environmental conditions and solar radiation as well as prolonging their vegetative life. Atmospheric aerosols, gases and propagatory particles serve as sources of nutrients and inoculum for these open communities. Subaerial biofilms induce chemical and physical changes to rock materials, and they penetrate the mineral substrate contributing to rock and mineral decay, which manifests itself as bio-weathering of rock surfaces. Given their characteristic slow and sensitive growth, SAB may also serve as bioindicators of atmospheric and/or climate change.",
    url = "https://doi.org/10.1111/j.1462-2920.2007.01301.x",
    doi = "10.1111/j.1462-2920.2007.01301.x",
    openalex = "W1897286416",
    references = "doi101007bf00378791, doi101017s0006323199005356, doi10108001490459209377903, doi101128aem722170817152006, doi101139m87165"
}

@article{doi101016jgeomorph200806010,
    author = "Zerboni, Andrea",
    title = "Holocene rock varnish on the Messak plateau (Libyan Sahara): Chronology of weathering processes",
    year = "2008",
    journal = "Geomorphology",
    url = "https://doi.org/10.1016/j.geomorph.2008.06.010",
    doi = "10.1016/j.geomorph.2008.06.010",
    openalex = "W2025110801",
    references = "doi101016s0169555x02003318, doi101038276489a0"
}

Rock varnish is a very slow‐growing nanostratigraphic coating consisting of approximately 70% fine‐grained clay and 30% iron and manganese oxides that forms on the surfaces of rocks in arid and semiarid climates. The microbial diversity associated with rock varnish collected from the hyperarid Yungay region of the Atacama Desert was investigated using culture‐independent biomolecular methods and an adenosine triphosphate (ATP) assay. The extraction of DNA from rock varnish collected at Yungay, a region in which little to no DNA has been extracted from the surface soil (<1 cm) to date, indicates that rock varnish may provide a niche habitat for microbial life where water is essentially absent. The clone library constructed suggests the presence of numerous phylogenetically distinct microorganisms, ranging in diversity from Cyanobacterial to á‐proteobacteria lineages. The findings also show that only a few micrometers of varnish material are enough to shelter microbes like Chroococcidiopsis spp. from the intense ultraviolet radiation present in the Atacama Desert. Whether or not microorganisms are involved in its nucleation and/or growth, rock varnish appears to provide a microhabitat resembling cryptoendolithic communities seen on a larger scale.

@article{doi1010292007jg000677,
    author = "Kuhlman, K. R. and Venkat, Parth and Duc, Myron T. La and Kuhlman, G. M. and McKay, Christopher P.",
    title = "Evidence of a microbial community associated with rock varnish at Yungay, Atacama Desert, Chile",
    year = "2008",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Rock varnish is a very slow‐growing nanostratigraphic coating consisting of approximately 70\% fine‐grained clay and 30\% iron and manganese oxides that forms on the surfaces of rocks in arid and semiarid climates. The microbial diversity associated with rock varnish collected from the hyperarid Yungay region of the Atacama Desert was investigated using culture‐independent biomolecular methods and an adenosine triphosphate (ATP) assay. The extraction of DNA from rock varnish collected at Yungay, a region in which little to no DNA has been extracted from the surface soil (<1 cm) to date, indicates that rock varnish may provide a niche habitat for microbial life where water is essentially absent. The clone library constructed suggests the presence of numerous phylogenetically distinct microorganisms, ranging in diversity from Cyanobacterial to á‐proteobacteria lineages. The findings also show that only a few micrometers of varnish material are enough to shelter microbes like Chroococcidiopsis spp. from the intense ultraviolet radiation present in the Atacama Desert. Whether or not microorganisms are involved in its nucleation and/or growth, rock varnish appears to provide a microhabitat resembling cryptoendolithic communities seen on a larger scale.",
    url = "https://doi.org/10.1029/2007jg000677",
    doi = "10.1029/2007jg000677",
    openalex = "W2027173010",
    references = "doi101038345063a0, doi101093nar22224673, doi101093nargkg039, doi101093nargki038, doi10109900207713444846, doi101111j157469761997tb00351x, doi101128aem5637827871990, doi101128aem713150115062005, doi101128mmbr5911431691995, doi101128mr5911431691995"
}

@article{doi101130g24573a1,
    author = "Liu, Tanzhuo and Broecker, Wallace S.",
    title = "Rock varnish evidence for latest Pleistocene millennial-scale wet events in the drylands of western United States",
    year = "2008",
    journal = "Geology",
    url = "https://doi.org/10.1130/g24573a.1",
    doi = "10.1130/g24573a.1",
    openalex = "W1968880712",
    references = "doi101016s0169555x02003318, doi101111j146783061996tb01750x, doi1011301052517320010110004rvrodw20co2"
}

@incollection{doi10100797814020571997,
    author = "Dorn, Ronald I.",
    title = "Desert Rock Coatings",
    year = "2009",
    url = "https://doi.org/10.1007/978-1-4020-5719-9\_7",
    doi = "10.1007/978-1-4020-5719-9\_7",
    openalex = "W42927399",
    references = "doi101006qres19931084, doi1010160016703779900590, doi101017s0370164600052627, doi101073pnas9673404, doi1010970001069419660500000001, doi101111j183478191979tb05810x, doi101126science7008198, doi101130spe203p1, doi105860choice300305, doi105962bhltitle51652, openalexw1564144063"
}

Life at the atmosphere-lithosphere boundary is an ancient terrestrial niche that is sparsely covered by thin subaerial biofilms. The microbial inhabitants of these biofilms (a) have adapted to all types of terrestrial/subaerial stresses (e.g., desiccation, extreme temperatures, low nutrient availability, intense solar radiation), (b) interact with minerals that serve as both a dwelling and a source of mineral nutrients, and (c) provoke weathering of rocks and soil formation. Subaerial communities comprise heterotrophic and phototrophic microorganisms that support each other's lifestyle. Major lineages of eubacteria associated with the early colonization of land (e.g., Actinobacteria, Cyanobacteria) are present in these habitats along with eukaryotes such as microscopic green algae and ascomycetous fungi. The subaerial biofilm inhabitants have adapted to desiccation, solar radiation, and other environmental challenges by developing protective, melanized cell walls, assuming microcolonial architectures and symbiotic lifestyles. How these changes occurred, their significance in soil formation, and their potential as markers of climate change are discussed below.

@article{doi101146annurevmicro091208073349,
    author = "Gorbushina, Anna A. and Broughton, W. J.",
    title = "Microbiology of the Atmosphere-Rock Interface: How Biological Interactions and Physical Stresses Modulate a Sophisticated Microbial Ecosystem",
    year = "2009",
    journal = "Annual Review of Microbiology",
    abstract = "Life at the atmosphere-lithosphere boundary is an ancient terrestrial niche that is sparsely covered by thin subaerial biofilms. The microbial inhabitants of these biofilms (a) have adapted to all types of terrestrial/subaerial stresses (e.g., desiccation, extreme temperatures, low nutrient availability, intense solar radiation), (b) interact with minerals that serve as both a dwelling and a source of mineral nutrients, and (c) provoke weathering of rocks and soil formation. Subaerial communities comprise heterotrophic and phototrophic microorganisms that support each other's lifestyle. Major lineages of eubacteria associated with the early colonization of land (e.g., Actinobacteria, Cyanobacteria) are present in these habitats along with eukaryotes such as microscopic green algae and ascomycetous fungi. The subaerial biofilm inhabitants have adapted to desiccation, solar radiation, and other environmental challenges by developing protective, melanized cell walls, assuming microcolonial architectures and symbiotic lifestyles. How these changes occurred, their significance in soil formation, and their potential as markers of climate change are discussed below.",
    url = "https://doi.org/10.1146/annurev.micro.091208.073349",
    doi = "10.1146/annurev.micro.091208.073349",
    openalex = "W2163580752",
    references = "doi1010292007jg000677"
}

Scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms. To investigate this enigma, we utilized a three‐pronged approach that combined (1) culture‐independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of Black Canyon, New Mexico, and rocks with no visible varnish; (2) culturing of varnish in media supplemented with reduced forms of manganese and/or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and/or manganese oxides; and (3) scanning electron microscopy (SEM) of varnish and nearby rock that lacks macroscopically visible varnish. Our culture‐independent studies revealed significant differences between varnish and nonvarnish communities. Chloroflexi and Ktedobacteria dominated one varnish site, while the other varnish site was dominated by Cyanobacteria. The nonvarnish sites were dominated by Actinobacteria and, to a lesser extent, Cyanobacteria and were the only samples to contain Deinococcus ‐ Thermus sequences. Approximately 65% of varnish cultures produced visible manganese precipitates. Most culture isolates were not closely related to known manganese oxidizers, with the exception of Bacillus spp. SEM revealed microbial morphologies and two types of varnish morphologies: (1) relatively smooth layers and (2) patches of botryoidal pinnacles, which were often associated with increased manganese concentrations. “Bare” rock showed evidence of incipient varnish. These results have important implications for the detection of life on extraterrestrial planets such as Mars, where putative varnish coatings have been observed, and represent some of the first culture‐independent characterizations of varnish communities.

@article{doi1010292009jg001107,
    author = "Northup, Diana E. and Snider, Jessica R. and Spilde, M. and Porter, Megan L. and van de Kamp, Jodie and Boston, Penelope J. and Nyberg, A. and Bargar, John",
    title = "Diversity of rock varnish bacterial communities from Black Canyon, New Mexico",
    year = "2010",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms. To investigate this enigma, we utilized a three‐pronged approach that combined (1) culture‐independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of Black Canyon, New Mexico, and rocks with no visible varnish; (2) culturing of varnish in media supplemented with reduced forms of manganese and/or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and/or manganese oxides; and (3) scanning electron microscopy (SEM) of varnish and nearby rock that lacks macroscopically visible varnish. Our culture‐independent studies revealed significant differences between varnish and nonvarnish communities. Chloroflexi and Ktedobacteria dominated one varnish site, while the other varnish site was dominated by Cyanobacteria. The nonvarnish sites were dominated by Actinobacteria and, to a lesser extent, Cyanobacteria and were the only samples to contain Deinococcus ‐ Thermus sequences. Approximately 65\% of varnish cultures produced visible manganese precipitates. Most culture isolates were not closely related to known manganese oxidizers, with the exception of Bacillus spp. SEM revealed microbial morphologies and two types of varnish morphologies: (1) relatively smooth layers and (2) patches of botryoidal pinnacles, which were often associated with increased manganese concentrations. “Bare” rock showed evidence of incipient varnish. These results have important implications for the detection of life on extraterrestrial planets such as Mars, where putative varnish coatings have been observed, and represent some of the first culture‐independent characterizations of varnish communities.",
    url = "https://doi.org/10.1029/2009jg001107",
    doi = "10.1029/2009jg001107",
    openalex = "W2100072849",
    references = "doi101016jicarus200411022, doi101016s0169555x02003318, doi1011301052517320010110004rvrodw20co2"
}

The scarcity of liquid water in the hyperarid core of the Atacama Desert makes this region one of the most challenging environments for life on Earth. The low numbers of microbial cells in the soils suggest that within the Atacama Desert lies the dry limit for life on our planet. Here, we show that the Ca-sulfate crusts of this hyperarid core are the habitats of lithobiontic micro-organisms. This microporous, translucent substrate is colonized by epilithic lichens, as well as endolithic free-living algae, fungal hyphae, cyanobacteria and non photosynthetic bacteria. We also report a novel type of endolithic community, "hypoendoliths", colonizing the undermost layer of the crusts. The colonization of gypsum crusts within the hyperarid core appears to be controlled by the moisture regime. Our data shows that the threshold for colonization is crossed within the dry core, with abundant colonization in gypsum crusts at one study site, while crusts at a drier site are virtually devoid of life. We show that the cumulative time in 1 year of relative humidity (RH) above 60% is the best parameter to explain the difference in colonization between both sites. This is supported by controlled humidity experiments, where we show that colonies of endolithic cyanobacteria in the Ca-sulfate crust undergo imbibition process at RH >60%. Assuming that life once arose on Mars, it is conceivable that Martian micro-organisms sought refuge in similar isolated evaporite microenvironments during their last struggle for life as their planet turned arid.

@article{doi101111j14724669201000254x,
    author = "Wierzchoś, Jacek and Cámara, Beatríz and de los Rı́os, Asunción and Dávila, Alfonso F. and Almazo, I. M. Sánchez and Artieda, Octavio and Wierzchoś, Kacper and Gómez‐Silva, Benito and McKay, Christopher P. and Ascaso, Carmen",
    title = "Microbial colonization of Ca-sulfate crusts in the hyperarid core of the Atacama Desert: implications for the search for life on Mars",
    year = "2010",
    journal = "Geobiology",
    abstract = {The scarcity of liquid water in the hyperarid core of the Atacama Desert makes this region one of the most challenging environments for life on Earth. The low numbers of microbial cells in the soils suggest that within the Atacama Desert lies the dry limit for life on our planet. Here, we show that the Ca-sulfate crusts of this hyperarid core are the habitats of lithobiontic micro-organisms. This microporous, translucent substrate is colonized by epilithic lichens, as well as endolithic free-living algae, fungal hyphae, cyanobacteria and non photosynthetic bacteria. We also report a novel type of endolithic community, "hypoendoliths", colonizing the undermost layer of the crusts. The colonization of gypsum crusts within the hyperarid core appears to be controlled by the moisture regime. Our data shows that the threshold for colonization is crossed within the dry core, with abundant colonization in gypsum crusts at one study site, while crusts at a drier site are virtually devoid of life. We show that the cumulative time in 1 year of relative humidity (RH) above 60\% is the best parameter to explain the difference in colonization between both sites. This is supported by controlled humidity experiments, where we show that colonies of endolithic cyanobacteria in the Ca-sulfate crust undergo imbibition process at RH >60\%. Assuming that life once arose on Mars, it is conceivable that Martian micro-organisms sought refuge in similar isolated evaporite microenvironments during their last struggle for life as their planet turned arid.},
    url = "https://doi.org/10.1111/j.1472-4669.2010.00254.x",
    doi = "10.1111/j.1472-4669.2010.00254.x",
    openalex = "W1859641572",
    references = "doi1010292007jg000677"
}

SUMMARY: Molecular biology and microscopy techniques were used to characterize the microbial communities inside halite evaporites from different parts of the Atacama Desert. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that the evaporite rocks harbor communities predominantly made up of cyanobacteria, along with heterotrophic bacteria and archaea. Different DGGE profiles were obtained for the different sites, with the exception of the cyanobacterial profile, in which only one phylotype was detected across the three sites examined. Chroococcidiopsis-like cells were the only cyanobacterial components of the rock samples, although the phylogenetic study revealed their closer genetic affinity to Halothece genera. Gene sequences of the heterotrophic bacteria and archaea indicated their proximity to microorganisms found in other hypersaline environments. Microorganisms colonizing these halites formed microbial aggregates in the pore spaces between halite crystals, where microbial interactions occur. In this exceptional, salty, porous halite rock habitat, microbial consortia with a community structure probably conditioned by the environmental conditions occupy special microhabitats with physical and chemical properties that promote their survival.

@article{doi10243620150101113,
    author = "de los Rı́os, Asunción and Valea, Sergio and Ascaso, Carmen and Dávila, Alfonso F. and Kaštovský, Jan and McKay, Christopher P. and Gómez‐Silva, Benito and Wierzchoś, Jacek",
    title = "Comparative analysis of the microbial communities inhabiting halite evaporites of the Atacama Desert.",
    year = "2010",
    journal = "PubMed",
    abstract = "SUMMARY: Molecular biology and microscopy techniques were used to characterize the microbial communities inside halite evaporites from different parts of the Atacama Desert. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that the evaporite rocks harbor communities predominantly made up of cyanobacteria, along with heterotrophic bacteria and archaea. Different DGGE profiles were obtained for the different sites, with the exception of the cyanobacterial profile, in which only one phylotype was detected across the three sites examined. Chroococcidiopsis-like cells were the only cyanobacterial components of the rock samples, although the phylogenetic study revealed their closer genetic affinity to Halothece genera. Gene sequences of the heterotrophic bacteria and archaea indicated their proximity to microorganisms found in other hypersaline environments. Microorganisms colonizing these halites formed microbial aggregates in the pore spaces between halite crystals, where microbial interactions occur. In this exceptional, salty, porous halite rock habitat, microbial consortia with a community structure probably conditioned by the environmental conditions occupy special microhabitats with physical and chemical properties that promote their survival.",
    url = "https://doi.org/10.2436/20.1501.01.113",
    doi = "10.2436/20.1501.01.113",
    openalex = "W1544673086",
    references = "doi1010292007jg000677"
}

The Atacama Desert, located in northern Chile, is the driest and oldest desert on Earth. Research aimed at the understanding of this unique habitat and its diverse microbial ecosystems begun only a few decades ago, mainly driven by NASA's astrobiology program. A milestone in these efforts was a paper published in 2003, when the Atacama was shown to be a proper model of Mars. From then on, studies have been focused to examine every possible niche suitable for microbial life in this extreme environment. Habitats as different as the underside of quartz rocks, fumaroles at the Andes Mountains, the inside of halite evaporates and caves of the Coastal Range, among others, have shown that life has found ingenious ways to adapt to extreme conditions such as low water availability, high salt concentration and intense UV radiation.

@article{doi101016jfebslet201207025,
    author = "Azúa-Bustos, Armando and Urrejola, Catalina and Vicuña, Rafael",
    title = "Life at the dry edge: Microorganisms of the Atacama Desert",
    year = "2012",
    journal = "FEBS Letters",
    abstract = "The Atacama Desert, located in northern Chile, is the driest and oldest desert on Earth. Research aimed at the understanding of this unique habitat and its diverse microbial ecosystems begun only a few decades ago, mainly driven by NASA's astrobiology program. A milestone in these efforts was a paper published in 2003, when the Atacama was shown to be a proper model of Mars. From then on, studies have been focused to examine every possible niche suitable for microbial life in this extreme environment. Habitats as different as the underside of quartz rocks, fumaroles at the Andes Mountains, the inside of halite evaporates and caves of the Coastal Range, among others, have shown that life has found ingenious ways to adapt to extreme conditions such as low water availability, high salt concentration and intense UV radiation.",
    url = "https://doi.org/10.1016/j.febslet.2012.07.025",
    doi = "10.1016/j.febslet.2012.07.025",
    openalex = "W2159219812",
    references = "doi1010292007jg000677"
}

Rock varnish is a darkly pigmented coating rich in manganese oxides. Though microbes inhabit varnish deposits, it is unclear whether they are involved in varnish formation. The fungal communities of rock varnish and adjacent rock sites with no visible varnish deposits were examined. Microcolonial fungi were identified at all sampling sites, and were associated with manganese oxides in patches of incipient varnish at non-varnish sites. Fungi were closely related to manganese-oxidizing genera and seventeen isolates oxidized manganese in culture, producing six distinct manganese-oxide morphologies. Our results indicate that microcolonial fungi may play a crucial role in rock varnish formation. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.

@article{doi101080014904512011619636,
    author = "Parchert, Kylea Joy and Spilde, M. and Porras‐Alfaro, Andrea and Nyberg, A. and Northup, Diana E.",
    title = "Fungal Communities Associated with Rock Varnish in Black Canyon, New Mexico: Casual Inhabitants or Essential Partners?",
    year = "2012",
    journal = "Geomicrobiology Journal",
    abstract = "Rock varnish is a darkly pigmented coating rich in manganese oxides. Though microbes inhabit varnish deposits, it is unclear whether they are involved in varnish formation. The fungal communities of rock varnish and adjacent rock sites with no visible varnish deposits were examined. Microcolonial fungi were identified at all sampling sites, and were associated with manganese oxides in patches of incipient varnish at non-varnish sites. Fungi were closely related to manganese-oxidizing genera and seventeen isolates oxidized manganese in culture, producing six distinct manganese-oxide morphologies. Our results indicate that microcolonial fungi may play a crucial role in rock varnish formation. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.",
    url = "https://doi.org/10.1080/01490451.2011.619636",
    doi = "10.1080/01490451.2011.619636",
    openalex = "W2023911326",
    references = "doi101006qres19931084, doi101016016895259090186a, doi101016b9780123721808500421, doi101016jmycres200612001, doi101038nature05110, doi101038nprot2008133, doi101093nar22224673, doi101093nar25173389, doi1011289781555815882, doi101146annurevearth32101802120213, openalexw3217097258"
}

This article reviews current knowledge on microbial communities inhabiting endolithic habitats in the arid and hyper-arid regions of our planet. In these extremely dry environments, the most common survival strategy is to colonize the interiors of rocks. This habitat provides thermal buffering, physical stability, and protection against incident UV radiation, excessive photosynthetically active radiation, and freeze-thaw events. Above all, through water retention in the rocks' network of pores and fissures, moisture is made available. Some authors have argued that dry environments pose the most extreme set of conditions faced by microorganisms. Microbial cells need to withstand the biochemical stresses created by the lack of water, along with temperature fluctuations and/or high salinity. In this review, we also address the variety of ways in which microorganisms deal with the lack of moisture in hyper-arid environments and point out the diversity of microorganisms that are able to cope with only the scarcest presence of water. Finally, we discuss the important clues to the history of life on Earth, and perhaps other places in our solar system, that have emerged from the study of extreme microbial ecosystems.

@misc{doi10243620150101170,
    author = "Wierzchoś, Jacek and de los Rı́os, Asunción and Ascaso, Carmen",
    title = "Microorganisms in desert rocks: the edge of life on Earth.",
    year = "2012",
    booktitle = "PubMed",
    abstract = "This article reviews current knowledge on microbial communities inhabiting endolithic habitats in the arid and hyper-arid regions of our planet. In these extremely dry environments, the most common survival strategy is to colonize the interiors of rocks. This habitat provides thermal buffering, physical stability, and protection against incident UV radiation, excessive photosynthetically active radiation, and freeze-thaw events. Above all, through water retention in the rocks' network of pores and fissures, moisture is made available. Some authors have argued that dry environments pose the most extreme set of conditions faced by microorganisms. Microbial cells need to withstand the biochemical stresses created by the lack of water, along with temperature fluctuations and/or high salinity. In this review, we also address the variety of ways in which microorganisms deal with the lack of moisture in hyper-arid environments and point out the diversity of microorganisms that are able to cope with only the scarcest presence of water. Finally, we discuss the important clues to the history of life on Earth, and perhaps other places in our solar system, that have emerged from the study of extreme microbial ecosystems.",
    url = "https://doi.org/10.2436/20.1501.01.170",
    doi = "10.2436/20.1501.01.170",
    openalex = "W2153855038",
    references = "doi1010292007jg000677"
}

@article{doi101016jaeolia201304005,
    author = "Dorn, Ronald I. and Krinsley, David and Langworthy, Kurt and Ditto, Jeffrey and Thompson, T. J.",
    title = "The influence of mineral detritus on rock varnish formation",
    year = "2013",
    journal = "Aeolian Research",
    url = "https://doi.org/10.1016/j.aeolia.2013.04.005",
    doi = "10.1016/j.aeolia.2013.04.005",
    openalex = "W2038853259",
    references = "doi10100797814020571997, doi101080014904512011619636"
}

@article{doi101016jgca201310040,
    author = "Goldsmith, Yonaton and Stein, Mordechai and Enzel, Yehouda",
    title = "From dust to varnish: Geochemical constraints on rock varnish formation in the Negev Desert, Israel",
    year = "2013",
    journal = "Geochimica et Cosmochimica Acta",
    url = "https://doi.org/10.1016/j.gca.2013.10.040",
    doi = "10.1016/j.gca.2013.10.040",
    openalex = "W1986052302",
    references = "doi1010079783642730931, doi101007bf00322470, doi101016001670379290334f, doi1010160021979772901749, doi101016jgca200406013, doi101016s0016703700005780, doi101023a1022973114090, doi101073pnas9673447, openalexw1624806571"
}

Abstract The surface of Mars has long been considered a relatively oxidizing environment, an idea supported by the abundance of ferric iron phases observed there. However, compared to iron, manganese is sensitive only to high redox potential oxidants, and when concentrated in rocks, it provides a more specific redox indicator of aqueous environments. Observations from the ChemCam instrument on the Curiosity rover indicate abundances of manganese in and on some rock targets that are 1–2 orders of magnitude higher than previously observed on Mars, suggesting the presence of an as‐yet unidentified manganese‐rich phase. These results show that the Martian surface has at some point in time hosted much more highly oxidizing conditions than has previously been recognized.

@article{doi1010022014gl060329,
    author = "Lanza, N. and Fischer, Woodward W. and Wiens, R. C. and Grotzinger, J. P. and Ollila, A. and Cousin, A. and Anderson, R. B. and Clark, B. C. and Gellert, R. and Mangold, N. and Maurice, S. and Mouëlic, Stéphane Le and Nachon, M. and Schmidt, M. E. and Berger, Jeffrey A. and Clegg, S. M. and Forni, O. and Hardgrove, C. and Melikechi, Noureddine and Newsom, H. E. and Sautter, V.",
    title = "High manganese concentrations in rocks at Gale crater, Mars",
    year = "2014",
    journal = "Geophysical Research Letters",
    abstract = "Abstract The surface of Mars has long been considered a relatively oxidizing environment, an idea supported by the abundance of ferric iron phases observed there. However, compared to iron, manganese is sensitive only to high redox potential oxidants, and when concentrated in rocks, it provides a more specific redox indicator of aqueous environments. Observations from the ChemCam instrument on the Curiosity rover indicate abundances of manganese in and on some rock targets that are 1–2 orders of magnitude higher than previously observed on Mars, suggesting the presence of an as‐yet unidentified manganese‐rich phase. These results show that the Martian surface has at some point in time hosted much more highly oxidizing conditions than has previously been recognized.",
    url = "https://doi.org/10.1002/2014gl060329",
    doi = "10.1002/2014gl060329",
    openalex = "W2021694292",
    references = "doi101016jicarus201405038"
}

A significant proportion of the Earth's surface is desert or in the process of desertification. The extreme environmental conditions that characterize these areas result in a surface that is essentially barren, with a limited range of higher plants and animals. Microbial communities are probably the dominant drivers of these systems, mediating key ecosystem processes. In this review, we examine the microbial communities of hot desert terrestrial biotopes (including soils, cryptic and refuge niches and plant-root-associated microbes) and the processes that govern their assembly. We also assess the possible effects of global climate change on hot desert microbial communities and the resulting feedback mechanisms. We conclude by discussing current gaps in our understanding of the microbiology of hot deserts and suggest fruitful avenues for future research.

@article{doi101093femsrefuu011,
    author = "Makhalanyane, Thulani P. and Valverde, Ángel and Gunnigle, Eoin and Frossard, Aline and Ramond, Jean‐Baptiste and Cowan, Don A.",
    title = "Microbial ecology of hot desert edaphic systems",
    year = "2015",
    journal = "FEMS Microbiology Reviews",
    abstract = "A significant proportion of the Earth's surface is desert or in the process of desertification. The extreme environmental conditions that characterize these areas result in a surface that is essentially barren, with a limited range of higher plants and animals. Microbial communities are probably the dominant drivers of these systems, mediating key ecosystem processes. In this review, we examine the microbial communities of hot desert terrestrial biotopes (including soils, cryptic and refuge niches and plant-root-associated microbes) and the processes that govern their assembly. We also assess the possible effects of global climate change on hot desert microbial communities and the resulting feedback mechanisms. We conclude by discussing current gaps in our understanding of the microbiology of hot deserts and suggest fruitful avenues for future research.",
    url = "https://doi.org/10.1093/femsre/fuu011",
    doi = "10.1093/femsre/fuu011",
    openalex = "W2156850034",
    references = "doi101007bf00378791"
}

We present new reference values for nineteen USGS, GSJ and GIT‐IWG rock reference materials that belong to the most accessed samples of the GeoReM database. The determination of the reference values and their uncertainties at the 95% confidence level follows as closely as possible ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. We used analytical data obtained by the state‐of‐the‐art techniques published mainly in the last 20 years and available in GeoReM. The data are grouped into four categories of different levels of metrological confidence, starting with isotope dilution mass spectrometry as a primary method. Data quality was checked by careful investigation of analytical procedures and by the application of the Horwitz function. As a result, we assign a new and more reliable set of reference values and respective uncertainties for major, minor and a large group of trace elements of the nineteen investigated rock reference materials.

@article{doi101111j1751908x201500392x,
    author = "Jochum, Klaus Peter and Weis, Ulrike and Schwager, Beate and Stoll, Brigitte and Wilson, Stephen A. and Haug, Gerald H. and Andreae, Meinrat O. and Enzweiler, Jacinta",
    title = "Reference Values Following ISO Guidelines for Frequently Requested Rock Reference Materials",
    year = "2015",
    journal = "Geostandards and Geoanalytical Research",
    abstract = "We present new reference values for nineteen USGS, GSJ and GIT‐IWG rock reference materials that belong to the most accessed samples of the GeoReM database. The determination of the reference values and their uncertainties at the 95\% confidence level follows as closely as possible ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. We used analytical data obtained by the state‐of‐the‐art techniques published mainly in the last 20 years and available in GeoReM. The data are grouped into four categories of different levels of metrological confidence, starting with isotope dilution mass spectrometry as a primary method. Data quality was checked by careful investigation of analytical procedures and by the application of the Horwitz function. As a result, we assign a new and more reliable set of reference values and respective uncertainties for major, minor and a large group of trace elements of the nineteen investigated rock reference materials.",
    url = "https://doi.org/10.1111/j.1751-908x.2015.00392.x",
    doi = "10.1111/j.1751-908x.2015.00392.x",
    openalex = "W2291506047"
}

@article{doi101016jchemgeo201704009,
    author = "Macholdt, Dorothea S. and Jochum, Klaus Peter and Pöhlker, Christopher and Arangio, Andrea M. and Förster, Jan‐David and Stoll, Brigitte and Weis, Ulrike and Weber, Bettina and Müller, Maren and Kappl, Michael and Shiraiwa, Manabu and Kilcoyne, A. L. D. and Weigand, Markus and Scholz, Denis and Haug, Gerald H. and Alamri, Abdullah and Andreae, Meinrat O.",
    title = "Characterization and differentiation of rock varnish types from different environments by microanalytical techniques",
    year = "2017",
    journal = "Chemical Geology",
    url = "https://doi.org/10.1016/j.chemgeo.2017.04.009",
    doi = "10.1016/j.chemgeo.2017.04.009",
    openalex = "W2605506400",
    references = "doi1010079781402057199, doi10100797814020571997, doi101016001670377890114x, doi101016jchemgeo201405004, doi101016jenvpol200605028, doi101016jgca201310040, doi101016jtim200507009, doi101016s0016703700005780, doi101016s0043135402005249, doi101107s0909049502017739, doi101146annurevearth32101802120213, doi102138am20015611"
}

Fossil remnants of bacteria involved in the enhancement of manganese and iron rarely occur within the microstratigraphy of rock varnishes collected from warm desert environments, because varnish formation processes ultimately destroy these microfossils through remobilization of Mn-Fe and reprecipitation in a clay-mineral matrix. In contrast, Mn-Fe encrustations on budding bacteria commonly occur within varnishes that formed within just a century along the Erie Barge Canal, New York. Nanoscale imagery and elemental analyses reveal that these budding bacterial forms greatly enhance Mn, Fe, or both in encrustations surrounding hyphae and cells. The Mn and Fe precipitates have a granular texture on the scale of <1 nm to ∼10 nm. The precipitates also have a stringy texture, where strings are typically only a few nanometers wide. These in situ observations are consistent with expectations from studies of budding-bacteria cultures and with the polygenetic model of varnish formation. Given that the Erie Canal site presents the fastest known rate of varnishing, with typical thicknesses around 15 μm formed in a century, only one or two budding bacteria encrusting Mn-Fe oxides each year would be sufficient to generate the observed Erie Canal varnish. This contrasts with one bacterium growing every ∼400 y being needed to generate observed rates of varnishing in typical warm desert settings.

@article{doi101086691147,
    author = "Krinsley, David and DiGregorio, Barry E. and Dorn, Ronald I. and Razink, Josh and Fisher, Robert G.",
    title = "Mn-Fe-Enhancing Budding Bacteria in Century-Old Rock Varnish, Erie Barge Canal, New York",
    year = "2017",
    journal = "The Journal of Geology",
    abstract = "Fossil remnants of bacteria involved in the enhancement of manganese and iron rarely occur within the microstratigraphy of rock varnishes collected from warm desert environments, because varnish formation processes ultimately destroy these microfossils through remobilization of Mn-Fe and reprecipitation in a clay-mineral matrix. In contrast, Mn-Fe encrustations on budding bacteria commonly occur within varnishes that formed within just a century along the Erie Barge Canal, New York. Nanoscale imagery and elemental analyses reveal that these budding bacterial forms greatly enhance Mn, Fe, or both in encrustations surrounding hyphae and cells. The Mn and Fe precipitates have a granular texture on the scale of <1 nm to ∼10 nm. The precipitates also have a stringy texture, where strings are typically only a few nanometers wide. These in situ observations are consistent with expectations from studies of budding-bacteria cultures and with the polygenetic model of varnish formation. Given that the Erie Canal site presents the fastest known rate of varnishing, with typical thicknesses around 15 μm formed in a century, only one or two budding bacteria encrusting Mn-Fe oxides each year would be sufficient to generate the observed Erie Canal varnish. This contrasts with one bacterium growing every ∼400 y being needed to generate observed rates of varnishing in typical warm desert settings.",
    url = "https://doi.org/10.1086/691147",
    doi = "10.1086/691147",
    openalex = "W2594477291",
    references = "doi1010029780470712917, doi101007bf00378791, doi1010160045873287900076, doi101016jgca201310040, doi101016jtim200507009, doi101016s0016703703002175, doi101073pnas0900086106, doi101146annurevearth32101802120213, doi101146annurevmi38100184002503, doi101371journalpone0012570, doi1023071505543"
}

Abstract The behavior of rare earth elements (REE s) during hot desert weathering of meteorites is investigated. Ordinary chondrites (OC s) from Atacama (Chile) and Lut (Iran) deserts show different variations in REE composition during this process. Inductively coupled plasma–mass spectrometry (ICP ‐ MS) data reveal that hot desert OC s tend to show elevated light REE concentrations, relative to OC falls. Chondrites from Atacama are by far the most enriched in REE s and this enrichment is not necessarily related to their degree of weathering. Positive Ce anomaly of fresh chondrites from Atacama and the successive formation of a negative Ce anomaly with the addition of trivalent REE s are similar to the process reported from Antarctic eucrites. In addition to REE s, Sr and Ba also show different concentrations when comparing OC s from different hot deserts. The stability of Atacama surfaces and the associated old terrestrial ages of meteorites from this region give the samples the necessary time to interact with the terrestrial environment and to be chemically modified. Higher REE contents and LREE ‐enriched composition are evidence of contamination by terrestrial soil. Despite their low degrees of weathering, special care must be taken into account while working on the REE composition of Atacama meteorites for cosmochemistry applications. In contrast, chondrites from the Lut desert show lower degrees of REE modification, despite significant weathering signed by Sr content. This is explained by the relatively rapid weathering rate of the meteorites occurring in the Lut desert, which hampers the penetration of terrestrial material by forming voluminous Fe oxide/oxyhydroxides shortly after the meteorite fall.

@article{doi101111maps12894,
    author = "Pourkhorsandi, Hamed and D’Orazio, Massimo and Rochette, P. and Valenzuela, Millarca and Gattacceca, J. and Mirnejad, Hassan and Sutter, Brad and Hützler, Aurore and Aboulahris, Maria",
    title = "Modification of REE distribution of ordinary chondrites from Atacama (Chile) and Lut (Iran) hot deserts: Insights into the chemical weathering of meteorites",
    year = "2017",
    journal = "Meteoritics and Planetary Science",
    abstract = "Abstract The behavior of rare earth elements (REE s) during hot desert weathering of meteorites is investigated. Ordinary chondrites (OC s) from Atacama (Chile) and Lut (Iran) deserts show different variations in REE composition during this process. Inductively coupled plasma–mass spectrometry (ICP ‐ MS) data reveal that hot desert OC s tend to show elevated light REE concentrations, relative to OC falls. Chondrites from Atacama are by far the most enriched in REE s and this enrichment is not necessarily related to their degree of weathering. Positive Ce anomaly of fresh chondrites from Atacama and the successive formation of a negative Ce anomaly with the addition of trivalent REE s are similar to the process reported from Antarctic eucrites. In addition to REE s, Sr and Ba also show different concentrations when comparing OC s from different hot deserts. The stability of Atacama surfaces and the associated old terrestrial ages of meteorites from this region give the samples the necessary time to interact with the terrestrial environment and to be chemically modified. Higher REE contents and LREE ‐enriched composition are evidence of contamination by terrestrial soil. Despite their low degrees of weathering, special care must be taken into account while working on the REE composition of Atacama meteorites for cosmochemistry applications. In contrast, chondrites from the Lut desert show lower degrees of REE modification, despite significant weathering signed by Sr content. This is explained by the relatively rapid weathering rate of the meteorites occurring in the Lut desert, which hampers the penetration of terrestrial material by forming voluminous Fe oxide/oxyhydroxides shortly after the meteorite fall.",
    url = "https://doi.org/10.1111/maps.12894",
    doi = "10.1111/maps.12894",
    openalex = "W2620831758",
    references = "doi101111maps12607"
}

Desert varnishes are dark rock coatings observed in arid environments and might resemble Mn-rich coatings found on Martian rocks. Their formation mechanism is not fully understood and the possible microbial involvement is under debate. In this study, we applied DNA metagenomic Shotgun sequencing of varnish and surrounding soil to evaluate the composition of the microbial community and its potential metabolic function. We found that the α diversity was lower in varnish compared to soil samples (p value < 0.05), suggesting distinct populations with significantly higher abundance of Actinobacteria, Proteobacteria and Cyanobacteria within the varnish. Additionally, we observed increased levels of transition metal metabolic processes in varnish compared to soil samples. Nevertheless, potentially relevant enzymes for varnish formation were detected at low to insignificant levels in both niches, indicating no current direct microbial involvement in Mn oxidation. This finding is supported by quantitative genomic analysis, elemental analysis, fluorescence imaging and scanning transmission X-ray microscopy. We thus conclude that the distinct microbial communities detected in desert varnish originate from settled Aeolian microbes, which colonized this nutrient-enriched niche, and discuss possible indirect contributions of microorganisms to the formation of desert varnish.

@article{doi1011111758222912634,
    author = "Lang‐Yona, Naama and Maier, Stefanie and Macholdt, Dorothea S. and Müller-Germann, I. and Yordanova, Petya and Rodríguez‐Caballero, Emilio and Jochum, Klaus Peter and Alamri, Abdullah and Andreae, Meinrat O. and Fröhlich‐Nowoisky, Janine and Weber, Bettina",
    title = "Insights into microbial involvement in desert varnish formation retrieved from metagenomic analysis",
    year = "2018",
    journal = "Environmental Microbiology Reports",
    abstract = "Desert varnishes are dark rock coatings observed in arid environments and might resemble Mn-rich coatings found on Martian rocks. Their formation mechanism is not fully understood and the possible microbial involvement is under debate. In this study, we applied DNA metagenomic Shotgun sequencing of varnish and surrounding soil to evaluate the composition of the microbial community and its potential metabolic function. We found that the α diversity was lower in varnish compared to soil samples (p value < 0.05), suggesting distinct populations with significantly higher abundance of Actinobacteria, Proteobacteria and Cyanobacteria within the varnish. Additionally, we observed increased levels of transition metal metabolic processes in varnish compared to soil samples. Nevertheless, potentially relevant enzymes for varnish formation were detected at low to insignificant levels in both niches, indicating no current direct microbial involvement in Mn oxidation. This finding is supported by quantitative genomic analysis, elemental analysis, fluorescence imaging and scanning transmission X-ray microscopy. We thus conclude that the distinct microbial communities detected in desert varnish originate from settled Aeolian microbes, which colonized this nutrient-enriched niche, and discuss possible indirect contributions of microorganisms to the formation of desert varnish.",
    url = "https://doi.org/10.1111/1758-2229.12634",
    doi = "10.1111/1758-2229.12634",
    openalex = "W2793400010",
    references = "doi101016jchemgeo201704009, doi101080014904512011619636, doi101126science1203109183"
}

Petroglyphs, engraved throughout the Holocene into rock varnish coatings on sandstone, were investigated in the Ha’il region of northwestern Saudi Arabia, at Jabal Yatib, Jubbah, and Shuwaymis. The rock art has been created by removing the black varnish coating and thereby exposing the light sandstone underneath. With time, the varnish, a natural manganese (Mn)-rich coating, grows back. To study the rate of regrowth, we made 234 measurements by portable X-ray fluorescence (pXRF) on intact varnish and engraved petroglyphs. Since many petroglyphs can be assigned to a specific time period, a relationship between their ages and the Mn surface densities (D Mn) of the regrown material could be derived. This relationship was improved by normalizing the D Mn in the petroglyphs with the D Mn of adjacent intact varnish. In turn, we used this relationship to assign a chronologic context to petroglyphs of unknown ages. Following the removal of the varnish by the artist and prior to the beginning of Mn oxyhydroxide regrowth, a thin Fe-rich film forms on the underlying rock. This initial Fe oxyhydroxide deposit may act as catalyst for subsequent fast Mn oxidation. After a few decades of relatively rapid growth, the regrowth of the Mn-rich varnish slows down to about 0.017 µg cm –2 a –1 Mn, corresponding to about 0.012% a –1 Mn of the intact varnish density, or about 1.2 nm a –1, presumably due to a change of the catalytic process. Our results suggest that petroglyphs were engraved almost continuously since the pre-Neolithic period, and that rock varnish growth seems to proceed roughly linear, without detectable influences of the regional Holocene climatic changes.

@article{doi1011770959683618777075,
    author = "Macholdt, Dorothea S. and Alamri, Abdullah and Tuffaha, Husam T and Jochum, Klaus Peter and Andreae, Meinrat O.",
    title = "Growth of desert varnish on petroglyphs from Jubbah and Shuwaymis, Ha’il region, Saudi Arabia",
    year = "2018",
    journal = "The Holocene",
    abstract = "Petroglyphs, engraved throughout the Holocene into rock varnish coatings on sandstone, were investigated in the Ha’il region of northwestern Saudi Arabia, at Jabal Yatib, Jubbah, and Shuwaymis. The rock art has been created by removing the black varnish coating and thereby exposing the light sandstone underneath. With time, the varnish, a natural manganese (Mn)-rich coating, grows back. To study the rate of regrowth, we made 234 measurements by portable X-ray fluorescence (pXRF) on intact varnish and engraved petroglyphs. Since many petroglyphs can be assigned to a specific time period, a relationship between their ages and the Mn surface densities (D Mn) of the regrown material could be derived. This relationship was improved by normalizing the D Mn in the petroglyphs with the D Mn of adjacent intact varnish. In turn, we used this relationship to assign a chronologic context to petroglyphs of unknown ages. Following the removal of the varnish by the artist and prior to the beginning of Mn oxyhydroxide regrowth, a thin Fe-rich film forms on the underlying rock. This initial Fe oxyhydroxide deposit may act as catalyst for subsequent fast Mn oxidation. After a few decades of relatively rapid growth, the regrowth of the Mn-rich varnish slows down to about 0.017 µg cm –2 a –1 Mn, corresponding to about 0.012\% a –1 Mn of the intact varnish density, or about 1.2 nm a –1, presumably due to a change of the catalytic process. Our results suggest that petroglyphs were engraved almost continuously since the pre-Neolithic period, and that rock varnish growth seems to proceed roughly linear, without detectable influences of the regional Holocene climatic changes.",
    url = "https://doi.org/10.1177/0959683618777075",
    doi = "10.1177/0959683618777075",
    openalex = "W2808411235",
    references = "doi101016jatmosenv201710022, doi101016jchemgeo201704009, doi101080014904512011619636"
}

@article{doi101016jchemgeo201905016,
    author = "Xu, Xiaoming and Li, Yan and Li, Yan and Li, Yanzhang and Li, Yanzhang and Lu, Anhuai and Qiao, Ruixi and Liu, Kaihui and Ding, Hongrui and Wang, Changqiu",
    title = "Characteristics of desert varnish from nanometer to micrometer scale: A photo-oxidation model on its formation",
    year = "2019",
    journal = "Chemical Geology",
    url = "https://doi.org/10.1016/j.chemgeo.2019.05.016",
    doi = "10.1016/j.chemgeo.2019.05.016",
    openalex = "W2945570555",
    references = "doi101002jrs1250070606, doi101002sici109745551997112811873aidjrs17730co2b, doi10100797814020571997, doi1010079783642730931, doi101016jchemgeo201704009, doi101016jgca201310040, doi101016s1386142503002798, doi10102988eo01136, doi101038nmat3697, doi101039c3nr01577g, doi101073pnas9673447, doi101086691147, doi101111j175348871981tb06752x, doi102138am20000416"
}

The evolution of the meteorite flux to Earth can be studied by determining the terrestrial ages of meteorites collected in hot deserts. We measured the terrestrial ages of 54 stony meteorites from the El Mdano area, in the Atacama Desert, Chile, using the cosmogenic nuclide 36 Cl. With an average age of 710 ka, this collection is the oldest collection of nonfossil meteorites at Earth's surface. This allows both determination of the average meteorite flux intensity over the past 2 m.y. (222 meteorites larger than 10 g per km 2 per m.y.) and discussion of its possible compositional variability over the Quaternary Period. A change in the flux composition, with more abundant H chondrites, occurred between 1 and 0.5 Ma, possibly due to the direct delivery to Earth of a meteoroid swarm from the asteroid belt.

@article{doi101130g458311,
    author = "Drouard, A. and Gattacceca, J. and Hutzler, A. and Rochette, P. and Braucher, R. and Bourlès, D. and Team, ASTER and Gounelle, M. and Morbidelli, A. and Debaille, V. and Ginneken, M. Van and Valenzuela, M. and Quesnel, Y. and Martinez, R.",
    title = "The meteorite flux of the past 2 m.y. recorded in the Atacama Desert",
    year = "2019",
    journal = "Geology",
    abstract = "The evolution of the meteorite flux to Earth can be studied by determining the terrestrial ages of meteorites collected in hot deserts. We measured the terrestrial ages of 54 stony meteorites from the El Mdano area, in the Atacama Desert, Chile, using the cosmogenic nuclide 36 Cl. With an average age of 710 ka, this collection is the oldest collection of nonfossil meteorites at Earth's surface. This allows both determination of the average meteorite flux intensity over the past 2 m.y. (222 meteorites larger than 10 g per km 2 per m.y.) and discussion of its possible compositional variability over the Quaternary Period. A change in the flux composition, with more abundant H chondrites, occurred between 1 and 0.5 Ma, possibly due to the direct delivery to Earth of a meteoroid swarm from the asteroid belt.",
    url = "https://doi.org/10.1130/g45831.1",
    doi = "10.1130/g45831.1",
    openalex = "W2954525562",
    references = "doi101111maps12607"
}

We investigated rock varnish formed on sandstone and petroglyphs in the Hima area, southwestern Saudi Arabia. To characterize the rock varnish, we made in-situ measurements by portable x-ray fluorescence (pXRF) and analyzed samples by femtosecond laser-ablation inductively coupled–plasma mass spectrometry (fs LA-ICP-MS). Detailed chemical analysis of the rock varnish samples and adjacent soil or aeolian dust yielded information about the varnish’s geochemical context and formation mechanism. Untypically low positive Ce anomalies in the rock varnish samples correlated with negative Ce anomalies in the dust, supporting the hypothesis that the dust is the source of the varnish material. To study the varnish development, we made use of the fact that engraving the petroglyphs exposes a fresh bare sandstone surface without varnish, on which varnish regrows subsequently. We determined by pXRF the areal density of manganese (Mn) and iron (Fe) that had been deposited as rock varnish since the creation of the rock art. The rates of Mn deposition in the newly formed varnish were then estimated by correlating the areal density of Mn in Ancient Arabian and Old Arabic inscriptions with their known age ranges. The observed deposition rates showed substantial variability resulting from differences in exposure conditions of the rock surface, but were in a range comparable with that of our previous measurements in northwestern Arabia. This variability could be reduced significantly by referencing the measurements to the intact varnish adjacent to the individual petroglyphs. This normalization provided a much clearer relationship between varnish deposition and age, and enabled tentative ages to be assigned to rock art motifs without previously known ages. These tentative ages spanned most of the Holocene period and were consistent with the culturally or ecologically derived ages of the animal and human figures depicted in the rock art and the styles of scripts used in different periods.

@article{doi1011770959683619846979,
    author = "Macholdt, Dorothea S. and Jochum, Klaus Peter and Alamri, Abdullah and Andreae, Meinrat O.",
    title = "Rock varnish on petroglyphs from the Hima region, southwestern Saudi Arabia: Chemical composition, growth rates, and tentative ages",
    year = "2019",
    journal = "The Holocene",
    abstract = "We investigated rock varnish formed on sandstone and petroglyphs in the Hima area, southwestern Saudi Arabia. To characterize the rock varnish, we made in-situ measurements by portable x-ray fluorescence (pXRF) and analyzed samples by femtosecond laser-ablation inductively coupled–plasma mass spectrometry (fs LA-ICP-MS). Detailed chemical analysis of the rock varnish samples and adjacent soil or aeolian dust yielded information about the varnish’s geochemical context and formation mechanism. Untypically low positive Ce anomalies in the rock varnish samples correlated with negative Ce anomalies in the dust, supporting the hypothesis that the dust is the source of the varnish material. To study the varnish development, we made use of the fact that engraving the petroglyphs exposes a fresh bare sandstone surface without varnish, on which varnish regrows subsequently. We determined by pXRF the areal density of manganese (Mn) and iron (Fe) that had been deposited as rock varnish since the creation of the rock art. The rates of Mn deposition in the newly formed varnish were then estimated by correlating the areal density of Mn in Ancient Arabian and Old Arabic inscriptions with their known age ranges. The observed deposition rates showed substantial variability resulting from differences in exposure conditions of the rock surface, but were in a range comparable with that of our previous measurements in northwestern Arabia. This variability could be reduced significantly by referencing the measurements to the intact varnish adjacent to the individual petroglyphs. This normalization provided a much clearer relationship between varnish deposition and age, and enabled tentative ages to be assigned to rock art motifs without previously known ages. These tentative ages spanned most of the Holocene period and were consistent with the culturally or ecologically derived ages of the animal and human figures depicted in the rock art and the styles of scripts used in different periods.",
    url = "https://doi.org/10.1177/0959683619846979",
    doi = "10.1177/0959683619846979",
    openalex = "W2946704488",
    references = "doi101016jatmosenv201710022, doi101086691147"
}

in 13 h, suggesting that birnessite helped deliver electrons instead of serving as an electron acceptor under light. Our study demonstrated that electroactive bacterial communities were positively correlated with Fe/Mn semiconducting minerals in varnish, and diversified EET process occurred on varnish under sunlight. Overall, these phenomena may influence bacterial-community structure in natural environments over time.

@article{doi103389fmicb201900293,
    author = "Ren, Guiping and Yan, Yingchun and Nie, Yong and Lu, Anhuai and Wu, Xiao‐Lei and Li, Yan and Wang, Changqiu and Ding, Hongrui",
    title = "Natural Extracellular Electron Transfer Between Semiconducting Minerals and Electroactive Bacterial Communities Occurred on the Rock Varnish",
    year = "2019",
    journal = "Frontiers in Microbiology",
    abstract = "in 13 h, suggesting that birnessite helped deliver electrons instead of serving as an electron acceptor under light. Our study demonstrated that electroactive bacterial communities were positively correlated with Fe/Mn semiconducting minerals in varnish, and diversified EET process occurred on varnish under sunlight. Overall, these phenomena may influence bacterial-community structure in natural environments over time.",
    url = "https://doi.org/10.3389/fmicb.2019.00293",
    doi = "10.3389/fmicb.2019.00293",
    openalex = "W2920316963",
    references = "doi101002sici109745551997112811873aidjrs17730co2b, doi101016jgca201310040, doi10103835104607, doi101038nrmicro1442, doi101038nrmicro1490, doi101038nrmicro201693, doi101073pnas9673447, doi101126science1146689, doi101126science1196526, doi101126scienceaad3317, doi102138am20000416"
}

Abstract. In drylands, microbes that colonize rock surfaces have been linked to erosion because water scarcity excludes traditional weathering mechanisms. We studied the origin and role of rock biofilms in geomorphic processes of hard lime and dolomitic rocks that feature comparable weathering morphologies, although these two rock types originate from arid and hyperarid environments, respectively. We hypothesized that weathering patterns are fashioned by salt erosion and mediated by the rock biofilms that originate from the adjacent soil and dust. We used a combination of microbial and geological techniques to characterize rock morphologies and the origin and diversity of their biofilms. Amplicon sequencing of the SSU rRNA gene suggested that bacterial diversity is low and dominated by Proteobacteria and Actinobacteria. These phyla only formed laminar biofilms on rock surfaces that were exposed to the atmosphere and burrowed up to 6 mm beneath the surface, protected by sedimentary deposits. Unexpectedly, the microbial composition of the biofilms differed between the two rock types and was also distinct from the communities identified in the adjacent soil and settled dust, showing a habitat-specific filtering effect. Moreover, the rock bacterial communities were shown to secrete extracellular polymeric substances (EPSs) that form an evaporation barrier, reducing water loss rates by 65 %–75 %. The reduced water transport rates through the rock also limit salt transport and its crystallization in surface pores, which is thought to be the main force for weathering. Concomitantly, the biofilm layer stabilizes the rock surface via coating and protects the weathered front. Our hypothesis contradicts common models, which typically consider biofilms to be agents that promote weathering. In contrast, we propose that the microbial colonization of mineral surfaces acts to mitigate geomorphic processes in hot, arid environments.

@article{doi105194bg1611332019,
    author = "Wieler, Nimrod and Ginat, Hanan and Gillor, Osnat and Angel, Roey",
    title = "The origin and role of biological rock crusts in rocky desert weathering",
    year = "2019",
    journal = "Biogeosciences",
    abstract = "Abstract. In drylands, microbes that colonize rock surfaces have been linked to erosion because water scarcity excludes traditional weathering mechanisms. We studied the origin and role of rock biofilms in geomorphic processes of hard lime and dolomitic rocks that feature comparable weathering morphologies, although these two rock types originate from arid and hyperarid environments, respectively. We hypothesized that weathering patterns are fashioned by salt erosion and mediated by the rock biofilms that originate from the adjacent soil and dust. We used a combination of microbial and geological techniques to characterize rock morphologies and the origin and diversity of their biofilms. Amplicon sequencing of the SSU rRNA gene suggested that bacterial diversity is low and dominated by Proteobacteria and Actinobacteria. These phyla only formed laminar biofilms on rock surfaces that were exposed to the atmosphere and burrowed up to 6 mm beneath the surface, protected by sedimentary deposits. Unexpectedly, the microbial composition of the biofilms differed between the two rock types and was also distinct from the communities identified in the adjacent soil and settled dust, showing a habitat-specific filtering effect. Moreover, the rock bacterial communities were shown to secrete extracellular polymeric substances (EPSs) that form an evaporation barrier, reducing water loss rates by 65 \%–75 \%. The reduced water transport rates through the rock also limit salt transport and its crystallization in surface pores, which is thought to be the main force for weathering. Concomitantly, the biofilm layer stabilizes the rock surface via coating and protects the weathered front. Our hypothesis contradicts common models, which typically consider biofilms to be agents that promote weathering. In contrast, we propose that the microbial colonization of mineral surfaces acts to mitigate geomorphic processes in hot, arid environments.",
    url = "https://doi.org/10.5194/bg-16-1133-2019",
    doi = "10.5194/bg-16-1133-2019",
    openalex = "W2891616223",
    references = "doi101128aem722170817152006"
}

@article{doi101016jchemgeo2020119775,
    author = "Otter, Laura M. and Macholdt, Dorothea S. and Jochum, Klaus Peter and Stoll, Brigitte and Weis, Ulrike and Weber, Bettina and Scholz, Denis and Haug, Gerald H. and Alamri, Abdullah and Andreae, Meinrat O.",
    title = "Geochemical insights into the relationship of rock varnish and adjacent mineral dust fractions",
    year = "2020",
    journal = "Chemical Geology",
    url = "https://doi.org/10.1016/j.chemgeo.2020.119775",
    doi = "10.1016/j.chemgeo.2020.119775",
    openalex = "W3039103596",
    references = "doi101016jatmosenv201710022"
}

Abstract We 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.

@article{doi101111aae12167,
    author = "Andreae, Meinrat O. and Alamri, Abdullah and Andreae, Claire M. and Guagnin, Maria and Jochum, Klaus Peter and Stoll, Brigitte and Weis, Ulrike",
    title = "Archaeometric studies on the petroglyphs and rock varnish at Kilwa and Sakaka, northern Saudi Arabia",
    year = "2020",
    journal = "Arabian archaeology and epigraphy",
    abstract = "Abstract We 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.",
    url = "https://doi.org/10.1111/aae.12167",
    doi = "10.1111/aae.12167",
    openalex = "W3092893687",
    references = "doi101016jatmosenv201710022, doi1018268bsgm2019v71n1a3"
}

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.

@article{doi101371journalpone0235421,
    author = "Andreae, Meinrat O. and Alamri, Abdullah and Andreae, T. W. and Garfinkel, Alan and Haug, Gerald H. and Jochum, Klaus Peter and Stoll, Brigitte and Weis, Ulrike",
    title = "Geochemical studies on rock varnish and petroglyphs in the Owens and Rose Valleys, California",
    year = "2020",
    journal = "PLoS ONE",
    abstract = "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.",
    url = "https://doi.org/10.1371/journal.pone.0235421",
    doi = "10.1371/journal.pone.0235421",
    openalex = "W3047255755",
    references = "doi101016jatmosenv201710022, doi101016jchemgeo201704009, doi101016jchemgeo201905016, doi101086691147, doi1018268bsgm2019v71n1a3"
}

Desert varnish is a dark rock coating that forms in arid environments worldwide. It is highly and selectively enriched in manganese, the mechanism for which has been a long-standing geological mystery. We collected varnish samples from diverse sites across the western United States, examined them in petrographic thin section using microscale chemical imaging techniques, and investigated the associated microbial communities using 16S amplicon and shotgun metagenomic DNA sequencing. Our analyses described a material governed by sunlight, water, and manganese redox cycling that hosts an unusually aerobic microbial ecosystem characterized by a remarkable abundance of photosynthetic Cyanobacteria in the genus Chroococcidiopsis as the major autotrophic constituent. We then showed that diverse Cyanobacteria, including the relevant Chroococcidiopsis taxon, accumulate extraordinary amounts of intracellular manganese-over two orders of magnitude higher manganese content than other cells. The speciation of this manganese determined by advanced paramagnetic resonance techniques suggested that the Cyanobacteria use it as a catalytic antioxidant-a valuable adaptation for coping with the substantial oxidative stress present in this environment. Taken together, these results indicated that the manganese enrichment in varnish is related to its specific uptake and use by likely founding members of varnish microbial communities.

@article{doi101073pnas2025188118,
    author = "Lingappa, Usha F and Yeager, Chris M and Sharma, Ajay and Lanza, Nina L and Morales, Demosthenes P and Xie, Gary and Atencio, Ashley D and Chadwick, Grayson L and Monteverde, Danielle R and Magyar, John S and Webb, Samuel M and Valentine, Joan Selverstone and Hoffman, Brian M and Fischer, Woodward W",
    title = "An ecophysiological explanation for manganese enrichment in rock varnish.",
    year = "2021",
    journal = "Proceedings of the National Academy of Sciences of the United States of America",
    abstract = "Desert varnish is a dark rock coating that forms in arid environments worldwide. It is highly and selectively enriched in manganese, the mechanism for which has been a long-standing geological mystery. We collected varnish samples from diverse sites across the western United States, examined them in petrographic thin section using microscale chemical imaging techniques, and investigated the associated microbial communities using 16S amplicon and shotgun metagenomic DNA sequencing. Our analyses described a material governed by sunlight, water, and manganese redox cycling that hosts an unusually aerobic microbial ecosystem characterized by a remarkable abundance of photosynthetic Cyanobacteria in the genus Chroococcidiopsis as the major autotrophic constituent. We then showed that diverse Cyanobacteria, including the relevant Chroococcidiopsis taxon, accumulate extraordinary amounts of intracellular manganese-over two orders of magnitude higher manganese content than other cells. The speciation of this manganese determined by advanced paramagnetic resonance techniques suggested that the Cyanobacteria use it as a catalytic antioxidant-a valuable adaptation for coping with the substantial oxidative stress present in this environment. Taken together, these results indicated that the manganese enrichment in varnish is related to its specific uptake and use by likely founding members of varnish microbial communities.",
    url = "https://pmc.ncbi.nlm.nih.gov/articles/PMC8237629/",
    doi = "10.1073/pnas.2025188118",
    openalex = "W3172329579",
    pmcid = "PMC8237629",
    pmid = "34161271",
    references = "doi1010160033589483900650, doi101016jchemgeo201704009, doi101016jchemgeo201905016, doi101016jgca201310040, doi101016jicarus201405038, doi101038nmeth3869, doi101038s4158701902099, doi101080014904512011619636, doi101093bioinformaticsbts174, doi101093bioinformaticsbts252, doi101093bioinformaticsbtu170, doi101093bioinformaticsbtv033, doi101093bioinformaticsbtz848, doi101093sysbiosyq010, doi101128aem0300605, doi10113000167606195869487cdodv20co2, doi101186147121059386"
}

Abstract Perseverance explored two geological units on the floor of Jezero Crater over the first 420 Martian days of the Mars2020 mission. These units, the Máaz and Séítah formations, are interpreted to be igneous in origin, with traces of alteration. We report the detection of carbonate phases along the rover traverse based on laser‐induced breakdown spectroscopy (LIBS), infrared reflectance spectroscopy (IRS), and time‐resolved Raman (TRR) spectroscopy by the SuperCam instrument. Carbonates are identified through direct detection of vibrational modes of CO 3 functional groups (IRS and TRR), major oxides content, and ratios of C and O signal intensities (LIBS). In Séítah, the carbonates are consistent with magnesite‐siderite solid solutions (Mg# of 0.42–0.70) with low calcium contents (<5 wt.% CaO). They are detected together with olivine in IRS and TRR spectra. LIBS and IRS also indicate a spatial association of the carbonates with clays. Carbonates in Máaz are detected in fewer points, as: (a) siderite (Mg# as low as 0.03); (b) carbonate‐containing coatings, enriched in Mg (Mg# ∼0.82) and spatially associated with different salts. Overall, using conservative criteria, carbonate detections are rare in LIBS (∼30/2,000 points), IRS (∼15/2,000 points), and TRR (1/150 points) data. This is best explained by (a) a low carbonate content overall, (b) small carbonate grains mixed with other phases, (c) intrinsic complexity of in situ measurements. This is consistent with orbital observations of Jezero crater, and similar to compositions of carbonates previously reported in Martian meteorites. This suggests a limited carbonation of Jezero rocks by locally equilibrated fluids.

@article{doi1010292022je007463,
    author = "Clavé, Elise and Benzerara, Karim and Meslin, Pierre‐Yves and Forni, O. and Royer, Clément and Mandon, Lucia and Beck, Pierre and Quantin‐Nataf, Cathy and Beyssac, O. and Cousin, A. and Bousquet, Bruno and Wiens, R. C. and Maurice, S. and Dehouck, E. and Schröder, Susanne and Gasnault, O. and Mangold, N. and Dromart, Gilles and Bosak, Tanja and Bernard, Sylvain and Udry, Arya and Anderson, R. B. and Arana, Gorka and Brown, A. J. and Castro, Kepa and Clegg, S. M. and Cloutis, E. A. and Fairén, Alberto González and Flannery, David and Gasda, P. J. and Johnson, J. R. and Lasue, J. and López-Reyes, G. and Madariaga, Juan Manuel and Manrique, J. A. and Mouëlic, Stéphane Le and Núñez, Jorge I. and Ollila, A. and Pilleri, P. and Pilorget, C. and Pinet, P. and Poulet, F. and Veneranda, Marco and Wolf, Z. U. and the SuperCam team",
    title = "Carbonate Detection With SuperCam in Igneous Rocks on the Floor of Jezero Crater, Mars",
    year = "2022",
    journal = "Journal of Geophysical Research Planets",
    abstract = "Abstract Perseverance explored two geological units on the floor of Jezero Crater over the first 420 Martian days of the Mars2020 mission. These units, the Máaz and Séítah formations, are interpreted to be igneous in origin, with traces of alteration. We report the detection of carbonate phases along the rover traverse based on laser‐induced breakdown spectroscopy (LIBS), infrared reflectance spectroscopy (IRS), and time‐resolved Raman (TRR) spectroscopy by the SuperCam instrument. Carbonates are identified through direct detection of vibrational modes of CO 3 functional groups (IRS and TRR), major oxides content, and ratios of C and O signal intensities (LIBS). In Séítah, the carbonates are consistent with magnesite‐siderite solid solutions (Mg\# of 0.42–0.70) with low calcium contents (<5 wt.\% CaO). They are detected together with olivine in IRS and TRR spectra. LIBS and IRS also indicate a spatial association of the carbonates with clays. Carbonates in Máaz are detected in fewer points, as: (a) siderite (Mg\# as low as 0.03); (b) carbonate‐containing coatings, enriched in Mg (Mg\# ∼0.82) and spatially associated with different salts. Overall, using conservative criteria, carbonate detections are rare in LIBS (∼30/2,000 points), IRS (∼15/2,000 points), and TRR (1/150 points) data. This is best explained by (a) a low carbonate content overall, (b) small carbonate grains mixed with other phases, (c) intrinsic complexity of in situ measurements. This is consistent with orbital observations of Jezero crater, and similar to compositions of carbonates previously reported in Martian meteorites. This suggests a limited carbonation of Jezero rocks by locally equilibrated fluids.",
    url = "https://doi.org/10.1029/2022je007463",
    doi = "10.1029/2022je007463",
    openalex = "W4311473001",
    references = "doi101016jicarus201405038"
}

Efficient water splitting by birnessite-clay minerals in natural rock varnish, paving the way for the synthesis of a new generation of cost-effective MnO 2 electrocatalysts inspired by nature.

@article{doi101039d2se00185c,
    author = "Chaddha, Amritpal Singh and Singh, Narendra Kumar and Malviya, Manisha and Sharma, Anupam",
    title = "Birnessite-clay mineral couple in the rock varnish: a nature's electrocatalyst",
    year = "2022",
    journal = "Sustainable Energy \& Fuels",
    abstract = "Efficient water splitting by birnessite-clay minerals in natural rock varnish, paving the way for the synthesis of a new generation of cost-effective MnO 2 electrocatalysts inspired by nature.",
    url = "https://doi.org/10.1039/d2se00185c",
    doi = "10.1039/d2se00185c",
    openalex = "W4226065928",
    references = "doi101016jchemgeo201905016, doi101016jmex2021101511"
}

Petroglyph sites exist all over the world. They are one of the earliest forms of mankind’s expression and a precursor to art. Despite their outstanding value, comprehensive research on conservation and preservation of rock art is minimal, especially as related to biodeterioration. For this reason, the main objective of this study was to explore the factors involved in the degradation of petroglyph sites in the Negev desert of Israel, with a focus on biodegradation processes. Through the use of culture-independent microbiological methods (metagenomics), we characterized the microbiomes of the samples, finding they were dominated by bacterial communities, in particular taxa of Actinobacteria and Cyanobacteria, with resistance to radiation and desiccation. By means of XRF and Raman spectroscopies, we defined the composition of the stone (calcite and quartz) and the dark crust (clay minerals with Mn and Fe oxides), unveiling the presence of carotenoids, indicative of biological colonization. Optical microscopy and SEM–EDX analyses on thin sections highlighted patterns of weathering, possibly connected to the presence of biodeteriorative microorganisms that leach the calcareous matrix from the bedrock and mobilize metal cations from the black varnish for metabolic processes, slowly weathering it.

@article{doi103390app12146936,
    author = "Laura, R. D. and Piñar, Guadalupe and Nir, Irit and Kushmaro, Ariel and Pavan, Mariela J. and Eitenberger, Elisabeth and Waldherr, Monika and Graf, Alexandra B. and Sterflinger, Katja",
    title = "A Multi-Analytical Approach to Infer Mineral–Microbial Interactions Applied to Petroglyph Sites in the Negev Desert of Israel",
    year = "2022",
    journal = "Applied Sciences",
    abstract = "Petroglyph sites exist all over the world. They are one of the earliest forms of mankind’s expression and a precursor to art. Despite their outstanding value, comprehensive research on conservation and preservation of rock art is minimal, especially as related to biodeterioration. For this reason, the main objective of this study was to explore the factors involved in the degradation of petroglyph sites in the Negev desert of Israel, with a focus on biodegradation processes. Through the use of culture-independent microbiological methods (metagenomics), we characterized the microbiomes of the samples, finding they were dominated by bacterial communities, in particular taxa of Actinobacteria and Cyanobacteria, with resistance to radiation and desiccation. By means of XRF and Raman spectroscopies, we defined the composition of the stone (calcite and quartz) and the dark crust (clay minerals with Mn and Fe oxides), unveiling the presence of carotenoids, indicative of biological colonization. Optical microscopy and SEM–EDX analyses on thin sections highlighted patterns of weathering, possibly connected to the presence of biodeteriorative microorganisms that leach the calcareous matrix from the bedrock and mobilize metal cations from the black varnish for metabolic processes, slowly weathering it.",
    url = "https://doi.org/10.3390/app12146936",
    doi = "10.3390/app12146936",
    openalex = "W4284959505",
    references = "doi101073pnas2025188118"
}

@article{doi101016jchemgeo2024121961,
    author = "Chaddha, Amritpal Singh and Sharma, Anupam and Singh, Narendra Kumar and Shamsad, Amreen and Banerjee, Monisha",
    title = "Biotic-abiotic mingle in rock varnish formation: A new perspective",
    year = "2024",
    journal = "Chemical Geology",
    url = "https://doi.org/10.1016/j.chemgeo.2024.121961",
    doi = "10.1016/j.chemgeo.2024.121961",
    openalex = "W4391288957",
    references = "doi101016jmex2021101511, doi101073pnas2025188118, doi101086691147"
}

Abstract In the last 15 years, more than 2700 meteorites have been recovered and officially classified from the Atacama Desert. Although the number of meteorites collected in the Atacama has risen, the physical and climatic properties of the dense collection areas (DCAs) have not been fully characterized. In this article, we compiled the published data of all classified meteorites found in the Atacama Desert to (i) describe the distribution by meteorite groups, (ii) compare the weathering degree of chondrites among different Atacama DCAs and other hot and cold deserts, and (iii) determine the preservation conditions of chondrites in the main Atacama DCAs in relation with the local climatic conditions. The 35 DCAs so far identified in the Atacama Desert are located in three main morphotectonic units: The Coastal Range (CR), Central Depression (CD), and Pre‐Andean Range/Basement. A comparison with reported weathering data from other cold and hot deserts indicates that the mean terrestrial weathering of Atacama chondrites (W1–2), displays less alteration than other hot deserts (W2–3) and resembles the weathering distribution of the Antarctic meteorites (W1–2). The highest abundance of Atacama chondrites with low weathering (≤W2) is localized in the CD (78.8%, N = 1435), which is protected from the coastal fog influence and seasonal rainfalls and displays the oldest surfaces in the Atacama Desert. The morphogenetic classification based on present‐day temperatures and precipitations of the main Atacama DCAs reveals similar regional/subregional climatic conditions in the most productive areas and a truly productive surface for meteorite recovery between 5% and 58% of the quadrangles formally defined for each Atacama DCA. Our morphogenetic classification lacks consideration of some meteorological parameters such as the coastal fog, so it cannot fully explain the differences in weathering patterns among CR chondrites. Future studies of chondrite preservation in the Atacama DCAs should consider other meteorological variables such as relative humidity, specific humidity, or dew point, in combination with exposure ages of meteorites and its surfaces.

@article{doi101111maps14125,
    author = "Pinto, G. and Tavernier, Adrien and Gattacceca, J. and Corgne, Alexandre and Valenzuela, Millarca and Luais, Béatrice and Flores, Laura and Olivares, F. and Marrocchi, Yves",
    title = "Dense collection areas and terrestrial alteration of meteorites in the Atacama Desert",
    year = "2024",
    journal = "Meteoritics and Planetary Science",
    abstract = "Abstract In the last 15 years, more than 2700 meteorites have been recovered and officially classified from the Atacama Desert. Although the number of meteorites collected in the Atacama has risen, the physical and climatic properties of the dense collection areas (DCAs) have not been fully characterized. In this article, we compiled the published data of all classified meteorites found in the Atacama Desert to (i) describe the distribution by meteorite groups, (ii) compare the weathering degree of chondrites among different Atacama DCAs and other hot and cold deserts, and (iii) determine the preservation conditions of chondrites in the main Atacama DCAs in relation with the local climatic conditions. The 35 DCAs so far identified in the Atacama Desert are located in three main morphotectonic units: The Coastal Range (CR), Central Depression (CD), and Pre‐Andean Range/Basement. A comparison with reported weathering data from other cold and hot deserts indicates that the mean terrestrial weathering of Atacama chondrites (W1–2), displays less alteration than other hot deserts (W2–3) and resembles the weathering distribution of the Antarctic meteorites (W1–2). The highest abundance of Atacama chondrites with low weathering (≤W2) is localized in the CD (78.8\%, N = 1435), which is protected from the coastal fog influence and seasonal rainfalls and displays the oldest surfaces in the Atacama Desert. The morphogenetic classification based on present‐day temperatures and precipitations of the main Atacama DCAs reveals similar regional/subregional climatic conditions in the most productive areas and a truly productive surface for meteorite recovery between 5\% and 58\% of the quadrangles formally defined for each Atacama DCA. Our morphogenetic classification lacks consideration of some meteorological parameters such as the coastal fog, so it cannot fully explain the differences in weathering patterns among CR chondrites. Future studies of chondrite preservation in the Atacama DCAs should consider other meteorological variables such as relative humidity, specific humidity, or dew point, in combination with exposure ages of meteorites and its surfaces.",
    url = "https://doi.org/10.1111/maps.14125",
    doi = "10.1111/maps.14125",
    openalex = "W4390588241",
    references = "doi101111maps12607"
}

Investigators seeking to solve the mystery of rock varnish formation have yet to embrace hypothesis testing. Thus, this paper presents nine tests that assess the validity of the eight hypotheses proposed over the last four decades to explain varnish formation. An almost singular focus on manganese (Mn)-enrichment by many in varnish research may have led to six of the eight hypotheses failing to explain iron (Fe)-enrichment, as well as why clay minerals dominate the composition of rock varnish. Many varnish researchers displayed a sampling bias by collecting samples only from hot and dry deserts; thus, it should be of no surprise that four hypotheses failed a test of explaining varnishes in different climates; five hypotheses failed to explain varnishes in subsurface locations; and seven hypotheses failed to explain differences in varnish growth rates in hot deserts versus wetter locations. In the end, seven of eight proposed hypotheses to explain varnish formation failed more than five tests, any one of which would falsify the hypothesis. Only one hypothesis “passed” all nine tests.

@article{doi10117703091333241248038,
    author = "Dorn, Ronald I.",
    title = "Rock varnish revisited",
    year = "2024",
    journal = "Progress in Physical Geography Earth and Environment",
    abstract = "Investigators seeking to solve the mystery of rock varnish formation have yet to embrace hypothesis testing. Thus, this paper presents nine tests that assess the validity of the eight hypotheses proposed over the last four decades to explain varnish formation. An almost singular focus on manganese (Mn)-enrichment by many in varnish research may have led to six of the eight hypotheses failing to explain iron (Fe)-enrichment, as well as why clay minerals dominate the composition of rock varnish. Many varnish researchers displayed a sampling bias by collecting samples only from hot and dry deserts; thus, it should be of no surprise that four hypotheses failed a test of explaining varnishes in different climates; five hypotheses failed to explain varnishes in subsurface locations; and seven hypotheses failed to explain differences in varnish growth rates in hot deserts versus wetter locations. In the end, seven of eight proposed hypotheses to explain varnish formation failed more than five tests, any one of which would falsify the hypothesis. Only one hypothesis “passed” all nine tests.",
    url = "https://doi.org/10.1177/03091333241248038",
    doi = "10.1177/03091333241248038",
    openalex = "W4395013421",
    references = "doi10100797814020571997, doi101016jmex2021101511, doi101086691147, doi1018268bsgm2019v71n1a3"
}

@article{doi101016jquascirev2024109146,
    author = "Liu, Tanzhuo and Broecker, Wallace S. and Hemming, Sidney R. and Roth, Helena and Dunseth, Zachary C. and Stiebel, Guy D. and Stein, Mordechai",
    title = "Holocene rock varnish microstratigraphy in the Dead Sea basin and Negev Desert: Chronometric application and climatic implication",
    year = "2025",
    journal = "Quaternary Science Reviews",
    url = "https://doi.org/10.1016/j.quascirev.2024.109146",
    doi = "10.1016/j.quascirev.2024.109146",
    openalex = "W4407030828",
    references = "doi101086691147, doi1018268bsgm2019v71n1a3"
}

Abstract Desert varnish, or rock varnish, is a thin reddish-black microlayer that covers rocks and consists mainly of clay minerals, as well as oxides of iron and manganese. In this paper, both terms—desert and rock varnish—are used interchangeably. Rock varnish has been an object of study and interest for more than two centuries. As it develops mainly in arid and desert areas, the interactions between solar radiation, wind, humidity, and microbial activity are determining factors in its formation. The growth rate of a few microns per millennium suggests that rock varnish can function as a tool for recording past environments. The current work presents the state of knowledge of rock varnish, focusing mainly on inorganic studies. The formation theories, geographical location, magnetic properties, lithodiversity, climatic variability, dating insights, pedofeatures, and some historical aspects are discussed. In addition, the different analytical techniques that have been used to study varnish chemical and mineralogical composition are grouped.

@article{doi101134s1064229325600447,
    author = "Martínez-Pabello, Pável U. and Mergelov, N. S. and Bronnikova, M. A. and Sedov, Sergey and Лебедева, М. П. and Golovanov, D. L. and Trejo-Martin, P.",
    title = "Desert Varnish—a “Microsoil” of the Arid Environments: Origin, Geographical Variability, and Paleoecological Significance (Review)",
    year = "2025",
    journal = "Eurasian Soil Science",
    abstract = "Abstract Desert varnish, or rock varnish, is a thin reddish-black microlayer that covers rocks and consists mainly of clay minerals, as well as oxides of iron and manganese. In this paper, both terms—desert and rock varnish—are used interchangeably. Rock varnish has been an object of study and interest for more than two centuries. As it develops mainly in arid and desert areas, the interactions between solar radiation, wind, humidity, and microbial activity are determining factors in its formation. The growth rate of a few microns per millennium suggests that rock varnish can function as a tool for recording past environments. The current work presents the state of knowledge of rock varnish, focusing mainly on inorganic studies. The formation theories, geographical location, magnetic properties, lithodiversity, climatic variability, dating insights, pedofeatures, and some historical aspects are discussed. In addition, the different analytical techniques that have been used to study varnish chemical and mineralogical composition are grouped.",
    url = "https://doi.org/10.1134/s1064229325600447",
    doi = "10.1134/s1064229325600447",
    openalex = "W4410548210",
    references = "doi101007bf00378791, doi101016jatmosenv201710022, doi101016jcatena200703007, doi101016jmex2021101511, doi101016s0166248108x70102, doi101029rf003p0189, doi101086691147, doi101089153110701750137413, doi101134s106422931512011x, doi101146annurevmicro541827, doi1018268bsgm2019v71n1a3, doi102138am20015611, doi1029003m41749785317072353, doi1058799m39, potter1977desert"
}