Problems of Dating Petroglyphs Using Measurements of Manganese Accumulation in Rock Varnish: New Research Findings and Their Implications

Recent attempts at petroglyph dating have focused on measuring the manganese accumulation in the rock varnish coating that develops in a motif groove after it was engraved, under the assumption that this amount is systematically related to age. These efforts reflect the basic need for chronological control for both heritage management and research purposes. Despite the wide availability of portable X-ray fluorescence devices and their ability to take non-destructive field measurements of manganese, these applications have not examined the nature or location of the manganese being analyzed. We employ a mixture of electron microscope methods on samples of petroglyphs obtained from prior investigations of petroglyphs to assess this problem. Results reveal that significant amounts of manganese associated with petroglyphs derive from sources other than the post-engraving rock varnish coating, including the weathering rinds underlying varnish, isolated patches of inherited varnish that were not completely removed during petroglyph engraving, and silica glazes. Significant amounts of manganese also may be removed by microcolonial fungi that, in our samples, dissolve between 22 and 60% of a rock varnish coating. Until the manganese accumulation petroglyph dating approach is able to distinguish manganese subtractions from acid-producing fungi and additions from non-varnish sources from varnish sources, it cannot provide reliable age estimates.