| Calculating Age | Challenges for OSL | Case studies of OSL dating in glacial environments | References | Comments | Another way of dating glacial landforms is optically stimulated luminescence dating (OSL). The OSL signal is reset by exposure to sunlight, so the signal is reset to zero while the sand is being transported (such as in a glacial meltwater stream). Once we have calculated our equivalent dose, we need to measure the environmental radiation dose rate.
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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 0.018 – 0.025 mm diameter), which we then measure in a specialised instrument.
In the Aberystwyth Luminescence Laboratory our instruments are made by a Danish manufacturer and are called “Risø OSL/TL readers”.
In addition to radiation from the surrounding sediment, OSL samples are affected by a cosmic dose rate, which reduces as the amount of sediment the sample is buried under increases.
The cosmic dose is useful in other situations, as it can be used to determine how long rocks, for example, have been exposed on the Earth’s surface using Cosmogenic Nuclide Dating.
This instrument stimulates the luminescence signal of the sand through shining the sample with blue or infrared light-emitting-didoes (LEDs), which give the electrons enough energy to escape their traps and recombine elsewhere, emitting a photon of light.
We measure this emitted light (the luminescence) and this is the first stage towards measuring the sample age. 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. Once the equivalent dose and dose rate have been measured, sample age can be calculated: Age (ka) = Equivalent dose (Gy) / dose rate (Gy/ka) OSL dating can be used to date sediments from decades up to 400,000 years in exceptional circumstances although the technique is more commonly applied to sediments up to 100,000 years old. Laboratory fading rates of various luminescence signals from feldspar-rich sediment extracts. The biggest challenge for OSL dating in glacial environments is partial bleaching (resetting) of the luminescence signal. Hollie Wynne (Aberystwyth University) stirs OSL samples being treated with acid in the preparation lab of the Aberystwyth Luminescence Research Laboratory. We make an approximation of the number of trapped electrons by measuring the light that they emit following stimulation by light (hence the name of the technique, “Optically stimulated luminescence”).