desert varnish

Desert varnish forms only on physically stable rock surfaces that are no longer subject to frequent precipitation, fracturing or wind abrasion. The varnish is primarily composed of particles of clay along with oxides of iron and manganese.{{cite journal |last1=Perry |first1=Randall Stewart |last2=Adams |first2=John B. |title=Desert varnish: evidence for cyclic deposition of manganese |journal=Nature |date=November 1978 |volume=276 |issue=5687 |pages=489–491 |doi=10.1038/276489a0|bibcode=1978Natur.276..489P |s2cid=4318328 }} There is also a host of trace elements and almost always some organic matter. The color of the varnish varies from shades of brown to black.{{cite book |last1=Chernicoff |first1=Stanley |last2=Whitney |first2=Donna |year=2007 |title=Geology: An Introduction to Physical Geology |edition=4th |publisher=Pearson Prentice Hall |page=585 |isbn=9780131751248}}

It has been suggested that desert varnish should be investigated as a potential candidate for a "shadow biosphere".{{cite journal |last1=Cleland |first1=Carol E. |title=Epistemological issues in the study of microbial life: alternative terran biospheres? |journal=Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences |date=December 2007 |volume=38 |issue=4 |pages=847–861 |doi=10.1016/j.shpsc.2007.09.007|pmid=18053938 }}{{cite news |last=McKie |first=Robie |title=Life on Earth... but not as we know it |url=https://www.theguardian.com/science/2013/apr/14/shadow-biosphere-alien-life-on-earth |work=The Guardian |date=13 April 2013 |access-date=2022-02-25}} However, a 2008 microscopy study posited that desert varnish has already been reproduced with chemistry not involving life in the lab, and that the main component is actually silica and not clay as previously thought. The study notes that desert varnish is an excellent fossilizer for microbes and indicator of water. Desert varnish appears to have been observed by rovers on Mars, and if examined may contain fossilized life from Mars's wet period.{{cite journal |last1=Perry |first1=Randall |last2=Sephton |first2=Mark |title=Solving the mystery of desert varnish with microscopy |journal=Infocus |date=6 September 2008 |issue=11 |pages=62–76 |doi=10.22443/rms.inf.1.36 |doi-access=free}}

Image:Desert Varnish on Bishop Tuff-750px.jpg.]]

Originally scientists thought that the varnish was made from substances drawn out of the rocks it coats.{{cite journal |last=Blake |first=W. P. |year=1905 |title=Superficial blackening and discoloration of rocks especially in desert regions |url=https://books.google.com/books?id=2lALAAAAYAAJ&dq=%22Superficial+blackening+and+discoloration+of+rocks+especially+in+desert+regions%22&pg=PA371 |journal=Transactions of the American Institute of Mining Engineers |volume=35 |pages=371–375}} Microscopic and microchemical observations, however, show that a major part of varnish is clay, which could only arrive by wind.{{cite journal |last1=Potter |first1=Russell M. |last2=Rossman |first2=George R. |title=Desert Varnish: The Importance of Clay Minerals |journal=Science |date=1977 |volume=196 |issue=4297 |pages=1446–1448 |doi=10.1126/science.196.4297.1446 |jstor=1744384|pmid=17776923 |bibcode=1977Sci...196.1446P |s2cid=31169889 }} Clay, then, acts as a substrate to catch additional substances that chemically react together when the rock reaches high temperatures in the desert sun. Wetting by dew is also important in the process.

An important characteristic of black desert varnish is that it has an unusually high concentration of manganese. Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight. In black desert varnish, however, manganese is 50 to 60 times more abundant. One proposal for a mechanism of desert varnish formation is that it is caused by manganese-oxidizing microbes (mixotrophs) which are common in environments poor in organic nutrients. A micro-environment pH above 7.5 is inhospitable for manganese-concentrating microbes. In such conditions, orange varnishes develop, poor in manganese (Mn) but rich in iron (Fe).{{cite journal |last1=Dorn |first1=R. I. |last2=Oberlander |first2=T. M. |title=Microbial Origin of Desert Varnish |journal=Science |date=11 September 1981 |volume=213 |issue=4513 |pages=1245–1247 |doi=10.1126/science.213.4513.1245|pmid=17744757 |bibcode=1981Sci...213.1245D |s2cid=39581858 }} An alternative hypothesis for Mn/Fe fluctuation has been proposed that considers Mn-rich and Fe-rich varnishes to be related to humid and arid climates, respectively.{{cite journal |last1=Liu |first1=Tanzhuo |last2=Dorn |first2=Ronald I. |title=Understanding the Spatial Variability of Environmental Change in Drylands with Rock Varnish Microlaminations |journal=Annals of the Association of American Geographers |date=June 1996 |volume=86 |issue=2 |pages=187–212 |doi=10.1111/j.1467-8306.1996.tb01750.x}}

File:Valley of Fire petroglyphs.jpg near Las Vegas, Nevada. Area shown is about one metre across.]]

Even though it contains high concentrations of iron and manganese, there are no significant modern uses of desert varnish. However, some Native American peoples created petroglyphs by scraping or chipping away the dark varnish to expose the lighter rock beneath.

Desert varnish often obscures the identity of the underlying rock, and different rocks have varying abilities to accept and retain varnish. Limestones, for example, typically do not have varnish because they are too water-soluble and therefore do not provide a stable surface for varnish to form. Shiny, dense and black varnishes form on basalt, fine quartzites and metamorphosed shales due to these rocks' relatively high resistance to weathering.

Its presence has been cited as a key factor in the preservation of a large number of petroglyphs dating back to the Iron Age and earlier in areas such as the Wadi Saham in the United Arab Emirates.

• {{annotated link|Aeolian processes}}
• {{annotated link|Desert pavement}}
• {{annotated link|Manganite}}
• {{annotated link|Shadow biosphere}}

File:Desert varnish at Capitol Reef National Park.jpg|Desert varnish at Capitol Reef National Park

File:Petroglyphs at Newspaper Rock, Canyonlands carved into desert varnish.jpg|Petroglyphs at Newspaper Rock State Historic Monument carved into desert varnish

File:Desert varnish - Valley of Fire State Park.JPG|Desert varnish at Valley of Fire State Park

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• [http://minerals.caltech.edu/FILES/VARNISH/ Desert Varnish]
• [https://web.archive.org/web/20080513151318/http://alliance.la.asu.edu/dorn/VarnishPages/VarnishPrimerIntro.html Rock Varnish (desert varnish): An Internet Primer for Rock Art Research] by Ronald I. Dorn, Professor of Geography Arizona State University
• [https://web.archive.org/web/20100409105520/http://www.geo.arizona.edu/palynology/geos462/14rockvarnish.html DESERT VARNISH (rock varnish) ]
• [http://www.space.com/scienceastronomy/060630_desert_varnish.html New Way Suggested to Search for Life on Mars] — Space.com
• [http://archives.cnn.com/2001/TECH/space/07/02/mars.rocks/index.html Researcher: Mars rock varnish hints of life July 2, 2001 By Richard Stenger CNN] {{Webarchive|url=https://web.archive.org/web/20060825232537/http://archives.cnn.com/2001/TECH/space/07/02/mars.rocks/index.html |date=August 25, 2006 }}
• [http://www.icamsr.org/docs/spie_4495-13_rock_varnish.pdf Rock Varnish As A Habitat For Extant Life On Mars]

Category:Deposition (geology)

Category:Sedimentology

Category:Deserts

Category:Rocks

Category:Hypothetical life forms