Optically Stimulated Luminescence dating as a geochronological tool for the dating of Late Quaternary sediments in the Red Sea region
D. Sanderson, and T. Kinnaird. . International Workshop on 'The Red Sea, its Origin, Structure and Environment', Jeddah, Saudi Arabia, (February 2016)
Optically stimulated luminescence, and its related variants, have emerged in recent decades as important geochronometric tools for constraining sediment formation ages and post-depositional processes in many Quaternary palaeo-environments, including those of the Red Sea region. In this presentation the underlying principles and age limits of luminescence dating are outlined. Examples are discussed of work which has defined the age limits of relic dune systems in the Saudi peninsula, and studies conducted within the DISPERSE project on marine cores off the Farasan coast and on the littoral fringe of southwest Saudi Arabia. The results confirm the utility of luminescence dating to palaeo-environmental work in the Red Sea region, and testify to varied environmental conditions in the past, which make this area critical for human dispersal and occupation during the Palaeolithic. It is to be hoped that further research in coming decades will provide a dense regional chronology that can be used to define the environmental history of this important area.
Luminescence dating is part of a class of radiation damage dating methods, which include electron spin resonance (ESR) dating and methods based on stimulated luminescence of minerals, by heat, as in thermoluminescence (TL), or light, as in photostimulated luminescence (PSL) or optically stimulated luminescence (OSL). In these methods prolonged exposure to ionising radiation in the physical environment deposits energy, and information, through charge-trapping at defect centres in insulating minerals. By stimulating release of such radiation induced trapped charge, using heat (TL) or light (OSL), measureable light emission occurs that can be detected and quantified using photon counting methods. The strength of luminescence signals depends on the extent of prior exposure to natural radiation since an earlier zero condition, and on the sensitivity of the individual minerals, which has a complex dependence on formation and post formation conditions. In dating techniques, a series of calibration experiments are undertaken with laboratory sources to quantify the natural luminescence signal in terms of “equivalent dose”. This corresponds to the radiation dose (in units of Grays, which are a measure of energy deposition per unit mass) which would account for the natural signals observed from the mineral system under study. In parallel with the equivalent dose determination, a combination of low-level radiometric measurements, radionuclide concentration measurements, and dosimetric modelling is used to define palaeo-dose rates from the sample and its environment. The luminescence age is then determined as the quotient of equivalent dose to palaeo-dose rate.
For sediments which have (i) been effectively zeroed prior to last deposition (for example by light exposure, or by heat), (ii) which have not been subject to post-depositional alterations, (iii) whose luminescence signals are retained with stability, and (iv) whose dose response characteristics can be reconstructed in the laboratory without bias, luminescence ages correspond to the physical time interval since deposition. For systems with more complex behavior of origins, the system can deliver information on depositional and post-depositional conditions which may be of value in combination with other information to help constrain understanding of sedimentary records. The system is also capable of yielding information about past thermal histories, and of registering the effects of prolonged light exposure to lithic surfaces.