Alan, there have been numerous attempts to estimate the age of rock varnish (the term "desert varnish" is superseded) and similar deposits, most of them by Ron Dorn (numerous papers). However, he has withdrawn all his radiocarbon and cation ratio results in the mid-1990s. His CR method was calibrated by radiocarbon analysis near the sampling site. The problem with this is that all rock surface layers (ferromanganese or other accretions, weathering zones, etc.) effectively contain carbon from numerous sources, of random carbon "ages" not necessarily related to the age of the subsurface, which needs to be regarded as having an open carbon system. Therefore both the CR method and AMS analysis may yield results not reflecting the age of the varnish. The short answer is: there have been many such attempts, but none has provided non-controversial age estimates. There is no independent cross checking, the methods are destructive, and the analyst has to visit the site in the case of rock art.

Robert

Robert,

Thanks so much for taking the time to craft your thoughtful answer.  Much appreciated!

Some years ago we ran a couple of experiments on the CRD of stone artefacts. We published something but did not follow it up. I think that more experimental work on patinated artefacts from open sites, not rock art, might yield something interesting. The technology is now available for a detailed investigation of the nature of these accretions, ie, micro-tomography.

There is an interesting attempt by: Dietzel, Martin, Kolmer, Hans, Pölt, Peter & Simic, Sanja 2008. «Desert varnish and petroglyphs on sandstone - Geochemical composition and climate changes from Pleistocene to Holocene (Libya).» Chemie der Erde 68: 31-43.

In my previous comment I specifically mentioned experimenting on stone artefacts from suitable open sites as until the method has proven itself, especially the question of spatial variabilty, one should leave engravings alone.