c89a5c28_mical_Sediments_and_Landscapes__1
[0] Accretion Rate [2] If only a fraction of the reported organic remains or agencies precipitated Mn and Fe, formation would be more than 103 faster than observed rates. [3] The rare Mn enhancement by a few budding bacteria, observed in situ, would generate observed rates. [4] The silica binding process has no rate limiting step to slow growth. Under this model, there is no reason why decadal growth rates for silica glazes would not also apply to rock varnish. [1] 
There is no rate limiting step that would slow the accretion rate to fit observations. Even given the lowest dust deposition rates and fewest wetting - acidification events, this process would be 102 faster than observed rates.
Rock Varnish Chapter 8 in Geochemical Sediments and Landscapes
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