Desert Varnish

Desert varnish, or rock varnish, is a thin paint-like surface coating found on rocks everywhere, but more commonly in arid regions of the world. Often, this red-brown to nearly black coating covers light colored rock. This manganese-rich coating is also known to incorporate other elements. Dr. Nowinski's research-directing hypotheses were 1) "could this coating hold a record of recent air pollution, in spite of its very slow growth? and 2) "could one just pick up varnished rocks and use them to identify an area impacted by point sources of air pollution"? To answer these questions, samples of varnished rocks were collected upwind and downwind of two coal-fired power plants. Several methods were employed, including destructive laser-ablation inductively coupled plasma mass spectrometry, and, non-destructive portable X-ray fluorescence spectroscopy, to analyze these coatings for up to 26 chemical elements. The results are remarkable. They show, unequivocally, that these slow-growing coatings do hold records of recent air pollution. It is quite clear that collecting and analyzing varnished rocks can be used to assess the impact of environmental pollution from point sources.