Developing a mathematical model to describe multiple daylight exposure and burial events for rocks and developing a preparation method for rock samples for Optically Stimulated Luminescence Dating.

摘要

Interest in the optical luminescence dating (OSL) of rock surfaces has increased signifi- cantly over the last few years, as the potential of the method has been realised. In this study we investigate the information available in blue-stimulated luminescence depth pro- files into the surfaces of 4 quartz-rich cobbles from a Neanderthal site (Les Roches d’Abilly) in western France, and IR stimulated luminescence depth profile from a feldspar-rich gran- ite whetstone from an Iron Age villages near Aarhus in Denmark. These profiles show qualitative evidence for multiple daylight exposure and burial events. To quantify both burial and daylight exposure events a new model is developed. The existing model de- scribing the evolution of luminescence depth profiles is expanded to include burial before and after light exposure, and the possibility of repeated sequential daylight exposure and burial events. By determining the burial ages from the surface layers of the cobbles and by investigating the fitted luminescence profile, it is concluded that all cobbles were ap- parently well bleached before burial. This indicates that the estimated burial ages are reliable. In all cases the burial age of the most recent burial event is consistent with the expected age (quartz OSL on sediments from Les Roches d’Abilly and archaeological con- text for the Aarhus site). In addition, a recent known daylight exposure event provides an approximate calibration for daylight exposure events. This study thus confirms the suggestion that rock surfaces contain a record of daylight and burial history. Rock surfaces can therefore be dated with confidence, and it may be possible to determine a daylight exposure history using a known natural light exposure as calibration. Besides developing and applying the mathematical model, a preparation method for the samples used in this study has been developed. This development was based on knowledge of the chemical structure and properties of quartz and feldspar, together with X-ray fluorescence (XRF) measurements of full rock slices before and after chemical treatment. It is concluded that, in order to extract pure quartz grains from the French samples, hydrogen chloride (HCl) should be added before etching with hydroflouric acid (HF). If HCl is not used first to remove calcium carbonate from the rock slices, HF can not attack and remove feldspar grains. It is also shown that the absence of feldspar in XRF analysis does not necessarily indicate that there is no infrared luminescence sensitivity, indicating that feldspar has not been completely removed.