A clue regarding the origin of rock varnish

Cs-137 and excess Pb-210 have been found in rock varnish samples from several localities in the desert regions of the western United States. As Cs-137 was absent in the environment prior to the atmospheric testing of nuclear weapons, and as Pb-210 half life of only 22 years, these isotopes must have been added to this slow growing patina during the last half of the present century, demonstrating that, at least in part, the elements constituting varnish are supplied from the atmosphere. The discovery opens a window into the mechanism by which this mysterious substance forms. Not only radioisotopes but also anthropogenic metals can be harnessed to assess the ratio of rate of accumulation of a given element on varnish to its delivery rate from the atmosphere. Based on measurements reported here, for Pb-210 this ratio is about 0.1 and for Cs-137 about 0.01.