GROWTH RATE OF ROCK VARNISH DETERMINED FROM HIGH LEAD CONCENTRATIONS ON ROCKS AT SOCORRO, NEW MEXICO

Rock varnish is a dark patina that develops on rocks in arid and semi-arid environments. It consists of Mn and Fe oxides, clays, silica, and traces of metal oxides in a layered stratigraphy. Lead is often found at ppm levels in rock varnish, believed to be the result of anthropogenic activities, but because varnish accretes very slowly, Pb is usually found in the outer-most layer of these coatings. We have found percent-level Pb in varnish coatings from several sites near Socorro, NM where an historic smelter operated in the late 19th century.

The Pb levels vary in depth from the surface, depending on the exposure environment and aspect of the surface, providing a means to determine the varnish growth rate bracketed by a distinct historical period. Varnish from rock surfaces that are occasionally wetted by runoff exhibited the highest lead concentration at the greatest distance under the surface; the distance from the edge of the varnish in one sample was 4 microns, yielding an equivalent growth rate of 36 microns per millennia. It is well known that varnish stratigraphy does not correlate exactly between sample sites. We have found that the growth rate and resultant stratigraphy appears to be the result of the amount of water that the individual varnish site receives: perennial dry areas exhibit lead directly at the surface whereas areas that receive water occasionally (e.g. ephemeral run off channels) contain lead under the surface of the varnish, indicating faster growth. Sloping or flat-lying (upward-facing) varnish contains lead while vertical surfaces do not. This confirms that lead accumulation results from accretion of particulate matter. Since vertical surfaces do not contain lead, manganese and iron components of varnish must come from aqueous solution. The role of water is consistent with our prior work showing significant involvement of microflora in varnish production. X-ray maps on several patchy varnish samples that may be newly formed varnish sites reveal that Pb distribution is heterogeneous within small submillimeter-sized patches. This indicates uneven growth of varnish, since several areas contained no Pb and have probably formed since the closure of the smelter 110 years ago.