Rock Engraving Chronologies and Accelerator Mass Spectrometry Radiocarbon Age of Desert Varnish

Abstract

It has been proposed that the minimum age of rock engravings can be established from dating organic carbon associated with desert varnish, a naturally occurring coating on rocks in arid regions. In this study, five radiocarbon ages were obtained from organic carbon in previously undated varnishes from the Pilbara, Western Australia. Four varnish samples were scraped from dull black coatings and one from patchy, shiny black varnish. All radiocarbon ages were younger than 2800 yearsbp . Ages were younger at the rock/varnish interface than in overlying varnish, a reversal of expected stratigraphic relations. These results highlighted a number of difficulties associated with the interpretation of varnish radiocarbon ages, including the potential absence of a link between organic carbon age and varnish deposition. Age linkages are complicated by the time-transgressive nature of varnish formation, its weathering over time, and the probable mixed ages of dated organic carbon. Engraving chronologies based solely on radiocarbon ages of varnish should be treated with extreme caution.