Optically stimulated luminescence (OSL) as a chronometer for surface exposure dating

Abstract

We pioneer a technique of surface-exposure dating based upon the characteristic form of an optically stimulated luminescence (OSL) bleaching profile beneath a rock surface; this evolves as a function of depth and time. As a field illustration of this new method, the maximum age of a premier example of Barrier Canyon Style (BCS) rock art in Canyonlands National Park, Utah, USA, is constrained. The natural OSL signal from quartz grains is measured from the surface to a depth of > 10 mm in three different rock samples of the Jurassic Navajo Sandstone. Two samples are from talus with unknown daylight exposure histories; one of these samples was exposed at the time of sampling and one was buried and no longer light exposed. A third sample is known to have been first exposed 80 years ago and was still exposed at the time of sampling. First, the OSL-depth profile of the known-age sample is modeled to estimate material-dependent and environmental parameters. These parameters are then used to fit the model to the corresponding data for the samples of unknown exposure history. From these fits we calculate that the buried sample was light exposed for ∼ 700 years before burial and that the unburied sample has been exposed for ∼ 120 years. The shielded surface of the buried talus sample is decorated with rock art; this rock fell from the adjacent Great Gallery panel. Related research using conventional OSL dating suggests that this rockfall event occurred ∼ 900 years ago, and so we deduce that the rock art must have been created between ∼ 1600 and 900 years ago. Our results are the first credible estimates of exposure ages based on luminescence bleaching profiles. The strength of this novel OSL method is its ability to establish both ongoing and prior exposure times, at decadal to millennial timescales or perhaps longer (depending on the environmental dose rate) even for material subsequently buried. This has considerable potential in many archeological, geological and geo-hazard applications. © 2012. American Geophysical Union. All Rights Reserved.