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

Abstract

[1] 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.Citation: Sohbati, R., A. S. Murray, M. S. Chapot, M. Jain, and J. Pederson (2012), Optically stimulated luminescence (OSL) as a chronometer for surface exposure dating,

Age of a prehistoric “Rodedian” cult site constrained by sediment and rock surface luminescence dating techniques

“…The great advantage of this technique over the more conventional luminescence dating of buried sediments is that rock surfaces usually record their depositional history. Habermann et al (2000), Polikreti et al (2002), Liritzis et al (2010) and Sohbati et al (2011Sohbati et al ( , 2012a have all measured luminescence signal with depth into rock surfaces that were exposed to light immediately before measurement. However, by measuring such luminescence profiles into a buried rock surface, one can determine the degree to which the surface had been bleached prior to burial (Sohbati et al, 2011(Sohbati et al, , 2012aChapot et al, 2012) and even estimate, by using an appropriate calibration, how long the surface had been exposed before burial (Sohbati et al, 2012a;Freiesleben et al, 2015).…”

Age of Barrier Canyon-style rock art constrained by cross-cutting relations and luminescence dating techniques

“…The stimulation and release of trapped charge by sunlight that resets luminescence signals happens at the surface of rocks as well as sediment. Recent work takes advantage of how this "bleaching" of rock penetrates through time into the subsurface up to a few centimenters (16,17). The luminescence signal within the core of rocks is saturated over geologic time due to ionization from local radioactivity.…”

“…Full OSL results are presented in Tables S1 and S2 rockfall clast at the Great Gallery, with calibration to a local Navajo sandstone sample in an analogous position with respect to aspect and shielding and with independently known exposure duration (16).…”

“…Normalized OSL signal with depth into the buried face of the rockfall clast (blue data points and model-fit line) and the local calibration sample with known exposure age (red data points and model-fit line), reported with analytical error, modified from ref. 16. During burial and dosing, the OSL signal accumulated over time toward a saturated level set by crystallographic characteristics.…”

“…Among the latest applications of OSL are techniques dating the outer surfaces of rock clasts that have become shielded from light, including those with archaeological context (12)(13)(14)(15). Recent work has furthermore used the "bleaching" profile of decreasing luminescence signal toward the surface of rock to estimate exposure time to sunlight (16,17). Using these dating tools, we can constrain the age of rock art and gain new insight into past cultures and landscapes.…”