Rock varnish formation

The polygenetic model of rock varnish formation combines biotic enhancement of Mn with abiotic processes (Dorn, 1998). It builds on the mineralogy research of Potter and Rossman (1979a,b,c) and studies of bacteria fossil remains within varnish layers (Dorn and Meek, 1995 Krinsley et al., 1995 Dorn, 1998 Krinsley, 1998). In brief, weathered remains of Mn-rich bacterial casts (Figure 8.6) cement weathered clay minerals to rock surfaces. 

Table 8.4 Criteria that have been used to adjudicate competing models of rock varnish formation...

Krinsley, D. (1998) Models of rock varnish formation constrained by high resolution transmission electron microscopy. Sedimentology 45, 711-725. 

Analytical chemist Celeste Engel completed the first Masters thesis research on rock varnish and teamed up with geomorphologist Robert Sharp to write a seminal paper on rock varnish in the Mojave Desert (Engel and Sharp, 1958). The major elements are O, H, Si, Al, and Fe in approximately equal abundance with Mn. The key mystery of varnish formation (von Humboldt, 1812 Lucas, 1905 Engel and Sharp, 1958 Jones, 1991) is... 

Varnish formation probably starts with ubiquitous dust deposition on rock surfaces budding bacteria (Dorn and Oberlander, 1982) actively... 

Varnish formation, then, is a nanometre-scale marriage between clay minerals and the millennial accumulation and decay of Mn-Fe-encrusted bacterial casts. Nanometre-sized Mn-Fe remains of bacteria both feather and cement clay minerals to the rock and to prior varnish. 

Dorn, R.L Meek, N. (1995) Rapid formation of rock varnish and other rock coatings on slag deposits near Fontana. Earth Surface Processes and Landforms 20, 547-560. 

Hungate, B., Danin, A., Pellerin, N.B., Stemmier, J., Kjellander, P., Adams, J.B. Staley, J.T. (1987) Characterization of manganese-oxidizing (Mnll —> MnlV) bacteria from Negev Desert rock varnish implications in desert varnish formation. Canadian Journal of Microbiology 33, 939-943. 

Borns, D.J., Adams, J.B., Curtiss, B., Farr, B., Palmer, T., Staley, J. Taylor-George, S. (1980) The role of microorganisms in the formation of desert varnish and other rock coatings SEM study. Geological Society of America Abstracts with... 

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