Optically Stimulated Thermoluminescence Dating (OSL)
When quartz grains are buried, they begin to accumulate a trapped-charge population that increases in a measurable and predictable way in response to the ionising radiation to which the grains are exposed. Exposure to sunlight releases the light-sensitive trapped charge, thereby resetting the OSL signal: this process is commonly referred to as ?bleaching’. The time elapsed since sediment grains were last exposed to sunlight can be determined by measuring the OSL signal from a sample of sediment, determining the De that this represents, and estimating the rate of exposure of the grains to ionising radiation since they were buried [1,2]. The latter parameter of interest is termed the dose rate (Dr) and the burial age of well-bleached grains may be obtained from the following equation:
Burial age (years) =
Dr (Gy year-1)
(Gy = gray, where 1 Gy = 1 J/kg)
When clean quartz grains are exposed directly to sunlight, the OSL signal is reduced to a negligible level within a few seconds [2,3]. However, incomplete or non-uniform bleaching is commonplace in many depositional environments , due to surface coating on grains and/or poor exposure to sunlight during sediment transport. This results in grains being deposited with a heterogeneous distribution of residual trapped charge and a correspondingly wide range of measured De values. For such sediments, Olley et al. [5,6] suggested that the population of grains with lowest measured De values provides the most accurate estimate of Db: the burial dose to which those grains that were well bleached at deposition have been exposed since the most recent transport event.
- Huntley D.J., Godfrey-Smith D.I., Thewalt M.L.W., 1985. Optical dating of sediments. Nature 313, 105-107.
- Aitken, M.J., 1998. An Introduction to Optical Dating: The Dating of Quaternary Sediments by the Use of Photon-stimulated Luminescence. Oxford University Press, Oxford.
- Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiation Measurements 27, 769-817.
- Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometrica 21, 1-16
- Olley, J.M., Caitcheon, G.G., Roberts, R.G., 1999. The origin of dose distributions in fluvial sediments, and the prospect of dating single grains of quartz from fluvial deposits using optically stimulated luminescence. Radiation Measurements 30, 207-217.
- Olley, J.M., Pietsch, T., Roberts, R.G., 2003. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Submitted to Geomorphology
Jon Olley, CSIRO Land and Water« Back to Glossary Index