Novel luminescence kinetic models for rock surface exposure dating

Luminescence dating of rock surfaces is an emerging absolute chronological technique that has the potential to determine how long a rock surface has been exposed to daylight and/or how long it has been buried. By measuring the OSL signal as a function of depth into the rock surface, it is possible to determine how long the rock surface has been exposed to daylight by fitting a luminescence kinetic model to the measured luminescence-depth profile. Despite increasing use of this technique, the possible dependence of derived ages on the chosen model has not been examined. Existing studies are based on the assumption that first-order kinetics apply and a first order model (F.O.M.) has been used. In this study, we use (i) a general-order kinetics model G.O.M., (McKeever and Chen, 1997) and (ii) a localised recombination model here referred to as the fading model, F.M., (Jain et al., 2015) to develop new analytical expressions for luminescence-depth profiles in rocks. These models are tested using both simulated and experimental data based on the IR50 signal. Based on these tests it is concluded that the derived apparent exposure ages are dependent on the chosen model. This model dependence occurs despite the fact that the speed of progression of the bleaching front into the rock surface is model independent. The reasons for this apparent discrepancy are discussed in details. Calibration with a single known-age sample (Sohbati et al., 2012b) gives inaccurate exposure ages for all investigated models unless the appropriate model is applied and/or the age of the calibration profile is of the same order of magnitude as the age of the unknown profile, but using the G.O.M. improves the accuracy of age estimates for IRSL profiles. An alternative approach to exposure dating is presented here as ‘‘Exposure Response Curve’’ calibration. We conclude that using inflection points, to avoid the difficulties caused by fitting parameter correlation, proves model-independent, and is a promising method for further investigation in Rock Surface Luminescence Exposure dating.