Developing and testing models for rock surface dating using optically stimulated luminescence

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. The development of this technique into a robust dating method will give the opportunity to determine the ages of previously un-dateable stone structures/formations in both archaeology and geology, including megaliths, chambered burial mounds, cairns, cobble fans, ice-scoured bedrock, and many others.

When a rock surface is exposed to light, the latent optically stimulated luminescence (OSL) signal is reset to different degrees depending on the distance from the surface and the duration of daylight exposure. Thus, 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, and how long it was subsequently buried, by modelling the measured luminescence-depth profile.

The challenges involved in this procedure are addressed here, and in particular, the ability of a rock surface to record multiple sequential burial and exposure events is investigated experimentally. Existing models are examined and new, potentially more appropriate models introduced. These models are tested using both simulated and experimental data.

Based on these tests it is concluded that exposure ages are very dependent on the exact model assumptions and that fitting parameters previously assumed to be constant with depth are in fact not. It is shown that, although correct model assumptions improve the quality of exposure age estimates, a significant discrepancies between observed and expected fitting parameter values remain and these discrepancies leads to inaccurate age estimation.

This is particularly the case when post-IR signals from feldspar are used. The spectral dependency of luminescence signals is examined to better understand these problems. The demonstrated depth dependency of fitting parameters previously assumed to be constant with depth, also gives rise to discrepancies in parameter values.

The surprising observation that, in rocks, the IR50 signal is apparently more easily bleached than the quartz fast-component OSL signal is explained in terms of light attenuation effects other than absorption (e.g. scattering and refraction) increasing the effective path length for shorter wavelengths, and so changing the shape of the light spectrum with penetration depth.

This complicates parameter estimates in exposure dating even further. Alternative approaches (rather than parameter estimation) for estimating how long a rock surface has been exposed to light are considered, based on modelling the shape and position of the measured luminescence-depth profile. It is concluded that the most accurate exposure age is derived by interpolating the depth of an unknown profile onto a curve of profile depths from known age profiles (the Exposure Response Curve, or ERC, approach).

Generating ERCs by artificially illuminating surfaces at very high intensities to bracket the unknown profile, may provide calibration profiles of arbitrary ’age’, determined by the total number of incident photons. Such an approach is very likely to give more accurate and precise light-exposure ages than using parameters calculated from first principles, or than using a single natural calibration profile (as is current practice).

The model dependency of rock surface burial dating is also investigated, and encouragingly it is concluded that the accuracy of burial dating is not significantly affected by the application of inappropriate models to determine the exposure history of a buried surface (and thus the degree of bleaching before burial).

Dating rock surfaces accurately requires that the environmental dose rate is modelled, because the dose rate is also depth dependent and influenced by the size of the rock itself. A simple analytical model designed with this is mind is presented and applied. To investigate the accuracy and precision of rock surface dating, both rock surface and standard OSL dating are applied to two important archaeological sites in Central France.

These two different applications determine: 1) the timing of the changeover from Neanderthal to anatomically modern humans, and 2) that Neanderthals were capable of making symbolic engravings on cave walls. In the first case, rock surface dating is successful, but in the other, the signal of interest recorded by the rock surfaces appear to have been erased by a prolonged exposure to daylight prior to sampling, or removed by significant erosion of the surface, and only the sediments retain the chronological information.

These two application studies illustrate both the potential and some of the limitations of the method.