Dating glacier layers

Another way of dating glacial landforms is optically stimulated luminescence dating (OSL).

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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.

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”).

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 0.018 – 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”.