A review of the history of dating rock varnishes

Abstract

The history of rock varnish dating spans almost 20 years from the initial suggestion to use uranium series isotopes through the use of a geochemical ratio (cation-ratio dating) to the current application of accelerator mass spectrometry radiocarbon dating (AMS 14C). Uranium isotopes were ignored while other dating methods were tested exhaustively on several continents. Evidence of the unreliable dating of the formation of varnishes using cation-ratios was demonstrated by examples of environmental and textural observations reflecting localised leaching, and by chemical analyses that contradicted the fundamental assumptions of the method. The radiocarbon dating of substances embedded in varnishes is also problematic where carbon compounds and sources are not identified. The incompatible coexistence of different carbonaceous components, a recent controversial issue, is explained with reference to different environments of formation. The future of AMS 14C dating of rock varnishes centres around overcoming the practical limitations of sampling and analysing extremely small quantities of carbon-bearing substances. Future directions for dating varnished rock surfaces may include innovative refinements to the U-series, cosmogenic isotope and luminescence methods.