Luminescence dating refers to a group of methods of determining how long ago mineral grains The uncertainty of an OSL date is typically % of the age of the sample. The traditional OSL method relies on optical stimulation and transfer of electrons from one trap, to holes located elsewhere in the lattice – necessarily. OSL has been widely used to date glacial Sampled for OSL dating in August . recognised as an important technique for establishing a time frame of the sample is not heated as in TL; thus, spurious luminescence is avoided and there is a.
Minerals[ edit ] The minerals that are measured are usually either quartz or potassium feldspar sand-sized grains, or unseparated silt-sized grains. There are advantages and disadvantages to using each. For quartz, blue or green excitation frequencies are normally used and the near ultra-violet emission is measured. For potassium feldspar or silt-sized grains, near infrared excitation IRSL is normally used and violet emissions are measured.
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Comparison to radiocarbon dating[ edit ] Unlike carbon datingluminescence dating methods do not require a contemporary organic component of the sediment to be dated; just quartz, potassium feldspar, or certain other mineral grains that have been fully bleached during the event being dated. In a study of the chronology of arid-zone lacustrine sediments from Lake Ulaan in southern MongoliaLee et al.
Westerly winds delivered an influx of 14 C-deficient carbon from adjacent soils and Paleozoic carbonate rocks, a process that is also active today. Once the sand grain has been buried and it is no longer exposed to sunlight, the OSL signal starts to accumulate.
OSL works because all sediments have some natural radioactivity, caused by the presence of uranium, thorium and potassium isotopes in heavy minerals such as zircons. We analyse the quartz or feldspar minerals in sand deposits. When these quartz or feldspar minerals are exposed to the ionising radiation emitted by the radioactive isotopes in zircons, electrons within the crystals migrate and become trapped in their crystal structure.
The number of trapped electrons depends on the total amount of radiation that the mineral has been exposed to. If we assume that the radiation dose rate of the sediment has remained constant over time, then if we measure that dose rate, we can calculate the sample age.
Optically Stimulated Luminescence
How do we measure the OSL signal? Photograph used with permission of Geoff Duller. The way that we do this is through sampling sand from the landforms in opaque plastic tubes and taking the sample back to a luminescence laboratory where only red light conditions are used.
We have to be very careful not to expose the sediments to sunlight when we do this! It is necessary to use red light conditions in the laboratory because the luminescence signal is light sensitive, and red light does not re-set it.
We prepare the sample through treating it with acids to remove any calcium carbonate or organic material, and sieve it to get a specific grain size usually between 0. This instrument stimulates the luminescence signal of the sand through shining the sample with blue or infrared light-emitting-didoes LEDswhich give the electrons enough energy to escape their traps and recombine elsewhere, emitting a photon of light.
- Optically Stimulated Luminescence
- Luminescence dating
We measure this emitted light the luminescence and this is the first stage towards measuring the sample age. Photographs taken by Geoff Duller Aberystwyth University.
Blue diodes are most commonly used to stimulate the OSL signal from quartz, which is then detected in the UV emission spectrum. The OSL signal of a potassium K- feldspar sandur sample and its laboratory calibration curve. Figure by Georgina King. We then give our sand sample a range of laboratory radiation doses and measure the luminescence that each dose produces to develop a calibration curve.
In the case of aeolian sediments, all of the analysed grains are assumed to be well bleached, and all the Dehave a similar value, which can be used to calculate the age of the sediment. However, partial or incomplete bleaching is common, especially if the transport history was short or the exposure to sunlight was insufficient, as can be the case for fluvial sediments.
This partial bleaching can be homogeneous all the grains being incompletely bleached in the same proportion or heterogeneous differential bleaching. In this latter case the Dedistribution shows a scattering fig.
Some aliquots can present a very high palaeodose, which greatly overestimate the age of the last transport event. This explains why the mean is not appropriate in estimating the accurate equivalent dose.
It is therefore necessary to use a statistical model. Several models have recently been developed. It will also overestimate the equivalent dose in the presence of a partially bleached sediment. As for the sampling strategy the choice of the model depends upon the kind of sediments and presupposes a discussion between the field and luminescence specialists Bailey and Arnold, Comparison with independent age control may also be very useful, as shown by H.
The relevance of these models increases with the number of aliquots. The number of 50 aliquots is sometimes considered as a minimal value to ensure a reliable equivalent dose determination Rodnight,but it is important to keep in mind that the number of aliquots to be measured depends on the sample and increases with the scattering. Applications and place of OSL in geomorphological research in France 15The physical principles of the optical dating method, and its reliability for quartz and for feldspars from silty to sandy sediments, have resulted in optical dating being applied to a diverse range of sedimentary environments, as described in several journal papers see for example special issue of Boreas 1, The aim of this section is to review the applied representative studies dealing with OSL in France.
As in other countries, the first dating of sediments was based on thermoluminescence Wintle et al. The first OSL applications tab. Loess deposits were successfully dated especially in NW France.
Several loess-palaeosol sequences Engelmann et al. Most of the research focused on the last interglacial-glacial cycle Antoine et al.Potassium-argon Dating
Coastal sands from the North Sea or Channel coastlines were also optically dated for more than one decade. The dating of raised beaches Balescu et al, ; Regnault et al, ; Coutard et al.