DESERT VARNISH (rock varnish):

Typically the rock varnish layer is ca. 20 µm (2 - 500 µm) thick

Layers may reach ages of 10³, 10⁵ yr (?)

Arid - Semiarid environments are optimal for growth
Hyper-arid environments lack water for bacterial growth
Mesic environments: lichen growth destroys rock varnish

dark dull varnish = bacterial growth dominant
light shiny varnish = clay accumulation dominant

black: manganese-rich
red: iron-rich

AMS ¹⁴C dates of rock varnish bacterial organic matter and detritus permit dating of surface features: archeological features, end moraines in Sierras, lava flows (Dorn et al, 1989)
However, rocks accumulate ¹⁴C directly by cosmic rays reaching the earth's surface
amount depends on type of rock (molecular composition)
would give date of ca. 60,000 yr B.P. (detectable with AMS ?)

The cation ratio of K⁺ + Ca²⁺ / Ti⁴⁺ decreases with age (doubtful)
Produced pre-Clovis (> 12,000 yr) dates for human artifacts in Mojave Desert (Dorn et al., 1986).
Mechanism appears to be decreasing incorporation of rock substrate as varnish layer grows in thickness, because ratios of K:Si and Ca:Mn do not change with depth as would be expected if K and Ca were being leached from the varnish (Reneau and Raymond, 1991)

1. carbon isotope ratios (δ ¹³C)
   • Three photosynthetic pathways C3 C4 CAM
     
     "Dark Reactions" of Photosynthesis
     
     
     
     C3 -- CO₂ enters the Calvin Cycle to produce 3 - P - Glyceric Acid (with 3 carbons)
     
     C4 -- CO₂ combines with PEP Phosphoenol pyruvat (4 carbons) – before the Calvin Cycle.
     Produces less energy than Calvin Cycle, but can use lower CO₂ concentrations, and is faster at high temperatures.
     
     CAM – Crasulacian Acid Metabolism
     CO₂ taken in through stomata at night
     
     C3 plants cool & moist, δ ¹³C = -25 – -23 ‰
     C4 plants warm & dry, δ ¹³C = -15 – -12 ‰
     CAM plants warm & dry, δ ¹³C = -25 – -12 ‰
     
   • plant fragments incorporated in varnish
     
   • ¹³C values more negative at higher elevation
     
     
     
   • vs. Atmospheric CO₂ concentration [link]
   
   
   • Miocene varnish from Sonoita, Az (-16, -18 ‰) (today C4)
     (Dorn Dickinson 1989)
   [link]
   
   
   
2. Mn:Fe Ratio
   • alkalinity in environment inhibits bacterial growth (less Mn)
     interpluvials (> alkalinity), Mn:Fe = 0
     pluvial Mn:Fe = 2
     

• Bierman, P.R. ND. Viewed online at http://geology.uvm.edu/morphwww/classes/gradsem/2001/varnishbib.doc September 25, 2003.
• Dalton, R. 1998. Dating in doubt as researcher is probed. Nature 392: 218-219. Viewed online at http://www.nature.com/cgi-taf/DynaPage.taf?file=/NATURE/journal/v392/n6673/full/392218a0_fs.html September 25, 2003.
• Dalton, R. 1999. 'Faked' rock-dating data charge dismissed... Nature 401: 629. Viewed online at http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v401/n6754/full/401629a0_r.html September 25, 2003.
• Dalton, R. 1999. Geographer sues critics of his rock-dating methods. Nature 401: 419. Viewed online at http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v401/n6752/full/401419b0_r.html September 25, 2003.
• Malakoff, D. 1998. Rock Dates Thrown Into Doubt, Researcher Under Fire Science 280: 2041-2042. Viewed online at http://www.sciencemag.org/cgi/content/full/280/5372/2041 September 25, 2003.
• Malakoff, D. 1999. Cleared of Misconduct, Geoscientist Sues Critics. Science 286: 883-885. Viewed online at http://www.sciencemag.org/cgi/content/full/286/5441/883b?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&titleabstract=CLEARED+OF+MISCONDUCT%2C+GEOSCIENTIST+SUES+CRITICS&searchid=1064593292841_7308&stored_search=&FIRSTINDEX=0&fdate=10/1/1995&tdate=9/30/2003 September 25, 2003.