NEW APPROACH TO THE RADIOCARBON DATING OF ROCK VARNISH, WITH EXAMPLES FROM DRYLANDS

New electron microscope observations reveal that organic inclusions are often trapped in pockets under rock varnish. Accelerator radiocarbon dating of subvarnish organic detritus provides a new method of constraining when a rock was exposed by cultural or geomorphic processes. Applications are exemplified for human artifacts and landforms found in drylands. Three radiocarbon dates from 30,000 to 36,000 yr B.P. indicate rock engravings in South Australia are among the oldest art yet found in the world and show that humans migrated into what is now the "arid zone" of Australia at least 10,000 years earlier than previously thought. The first direct age control on Nazca geoglyphs and subterranean irrigation aqueducts in southern Peru indicate manufacture around 1400 to 2100 yr B.P. Geoglyphs along the Colorado River were made before 1100 yr B.P. Artifacts from a quarry site in the Mojave Desert, California, yield minimum C-14 ages from 3700 to 26,000 yr B.P.