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Abstract

Rock varnish is a thin, black-to-brown coating that develops on exposed rock surfaces in arid and semiarid regions. It is formed by the biogeochemical transformation of manganese (Mn) and iron (Fe) leached from aerially deposited dust into a mixture of Mn and Fe oxyhydroxides, clay particles, and other dust residues. In the varnish, Mn is enriched by about two orders of magnitude relative to the major crustal elements and thus can serve as a proxy for the areal density of the varnish. We have developed an in-situ technique for the measurement of the areal density of Mn and Fe on rock surfaces using portable X-ray fluorescence (pXRF). When applied to surfaces of known age, such as dated lava flows, inscriptions, or petroglyphs, the accumulation rate of Mn can be obtained and used as a proxy for the varnish accumulation rate. The Mn/Fe ratio serves as a criterion to geochemically distinguish rock varnish sensu strictu from other rock coatings. We will present results from numerous sites across the Arabian Desert and western North America and discuss the observed variability at all scales from microscopic to global.

Once accumulation rates have been obtained, they can be used to determine the exposure ages of surfaces of unknown age. Age estimates are obtained by measuring the Mn areal densities on these surfaces and dividing them by the Mn accumulation rate. This approach has been successfully applied to obtain age estimates for petroglyph rock art in North America and Arabia. While these estimates are still subject to significant uncertainty due to the inherent variability of the accumulation rates, they are extremely valuable in an archaeological context, where little or no alternative exist for dating these important prehistoric artifacts. Applications of this technique to other geomorphological features are being explored.