The accompanying thin section profile image of desert varnish strata shows a deep red varnish layer, and the contact line with the colorless quartz and feldspar grains and black iron oxides of the rock on the lower right. Image courtesy of California Institute of Technology Archaeologists have long been fascinated with desert varnish, as indigenous people around the world have often carved patterns and figures - designs called petroglyphs - into this thin brownish to black coating that forms on rock surfaces in deserts and other semi-arid locales. Although those petroglyphs tell one story, Laboratory scientist David Wayne of PIT Disposition Science and Technology (NMT-15) has discovered the rock varnish itself tells another - a story about the environment. Working in collaboration with earth scientists from the University of Nevada, Las Vegas and Eastern Washington University, Wayne has discovered that desert varnish may be an ideal environmental monitor for measuring levels of atmospherically-deposited heavy and potentially toxic metals. These heavy metals - lead, arsenic, zinc, cobalt, uranium, tungsten and others - are dispersed into Earth's atmosphere through natural phenomena, as well as through industrial activities like mining, smelting, oil refining, chemical processing and nuclear plant operations. Once airborne, the heavy metals are often captured in layers of desert varnish. In this way, rock varnish may serve as a passive indicator of airborne environmental pollutants, much as moss and lichens do in more humid regions. Desert varnish is actually a composite of alternating layers of materials, each having different compositions. Some of these layers are rich in windblown clays and dust-sized particles, while others are rich in iron and manganese oxides and hydroxides.
In research presented today at a meeting of the American Chemical Society in Anaheim, Calif., Wayne described a research project that analyzed desert varnish samples from the southwestern United States while looking for trace elements of heavy metals.
The impetus for Wayne's rock varnish study came when UNLV graduate student Tammy Diaz was looking for someone to do a trace element analysis of her thesis samples. Diaz, and her advisor Richard Orndorff (now at Eastern Washington University), had been unable to fine trace element data on desert varnish in the scientific literature and looked to Wayne to help create such data.
Diaz and Orndorff were interested in knowing if the desert varnish from near Fallon, Nev. showed any evidence of unusually high concentrations of certain trace elements - in particular, arsenic, thallium and tungsten. The Centers for Disease Control (CDC) had previously identified a cluster of a particular form of terminal childhood leukemia in Fallon and people in the community were concerned that emissions from local industries were somehow related to the cluster. The CDC report on the cluster identified unusually high levels of arsenic in the municipal water supply and reported high levels of tungsten in the blood of most of 204 local residents who had been tested.
Using a technique called Laser Ablation Inductively Coupled Plasma Mass Spectrometry, Wayne vaporized the top layer of desert varnish from samples provided by Diaz and Orndorff. This vaporized rock aerosol was placed in a mass spectrometer - a very sensitive instrument that indicates what chemical elements are in the desert varnish. Wayne's focus was on the trace elements - elements that are present at levels less than 1,000 parts per million - in the samples.
Testing the samples from a site near Fallon, as well as a location just outside of Las Vegas, Nev., and from the San Juan River in southeastern Utah, Wayne discovered that although trace element contents in desert varnish do seem to vary significantly with location, the desert varnish samples from near Fallon did have unusually high tungsten concentrations: 100 to 200 parts per million compared to one to 30 ppm from two other locations.
Wayne also discovered that while very high - greater than 2,000 ppm - surface concentrations of lead are ubiquitous in desert varnish throughout the southwestern United States; lead and arsenic concentrations are five to 10 times higher on the top-facing surfaces of varnish coated pebbles taken from the San Juan River. This suggests that lead, arsenic, and possibly other trace elements, are added and adsorbed preferentially on surfaces exposed to the atmosphere, Wayne said.
Wayne believes that if desert varnish is as good at capturing and preserving airborne heavy metals and other elements as it appears, scientists could use it to infer what sorts of activities have been going on nearby. For example, since the minerals in desert varnish are also known to adsorb and hold radionuclides such as uranium and plutonium, relatively quick and simple techniques also could be used to determine if any sort of nuclear materials had ever been released (intentionally or unintentionally) into the local environment. This makes the technique not only a good tool for earth science research, but also a potential environmental and nuclear nonproliferation analysis tool for understanding the recent past.