DESERT VARNISH (rock varnish):

a biogenic deposit produced by mixotrophic bacteria living on rock surfaces, digesting organic debris. Rate of bacterial growth relative to clay accumulation produces appearance Fluctuations in clays and Fe and Mn oxides and hydroxides produce color
History of Study: biological origin proposed in 1921. Recent advances based on electron beam microprobe (Hooke et al., 1979), which demonstrated layering of varnish.

Technique: analyzed by X-ray diffraction (reveals clay mineralogy), infrared spectroscopy, electron microscopy, microprobe, bulk chemical analysis, TAMS dating, isotope analysis.

Dating: Paleoclimatic Reconstructions:
  1. carbon isotope ratios (δ 13C)

  2. Mn:Fe Ratio

Rock Varnish Readings