Optically Stimulated Luminescence

This page was contributed by Dr Georgina King from the Aberystwyth Luminescence Research Laboratory in the Institute for Geography and Earth Sciences.

Introduction | How do we measure the OSL signal? | How do we measure the radiation dose rate? | Calculating Age | Challenges for OSL | Case studies of OSL dating in glacial environments | References | Comments |

Introduction

Another way of dating glacial landforms is optically stimulated luminescence dating (OSL). OSL is used on glacial landforms that contain sand, such as sandur or sediments in glacial streams. 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 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.

How do we measure the OSL signal?

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”. 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 then give our sand sample a range of laboratory radiation doses and measure the luminescence that each dose produces to develop a calibration curve. From this curve we can calculate the dose that our sample must have received to produce the amount of light that we measured first. We call this measurement our “equivalent dose”, because it is equivalent to the dose that the sample received in nature. The equivalent dose value is measured in the SI unit “grays” (Gy).

How do we measure the radiation dose rate?

Once we have calculated our equivalent dose, we need to measure the environmental radiation dose rate. This can be measured either at the sample location using a portable gamma spectrometer, through measurement of alpha, beta and gamma counts in the laboratory, or through direct measurement of uranium, thorium and potassium  concentrations using inductively-coupled plasma mass spectrometry. The methods through which dose rates are calculated vary between different laboratories worldwide. The radiation dose rate is also measured in grays, but as we calculate the dose rate per thousand years, it is grays per thousand years (Gy/ka).

Other important factors that need to be considered when calculating the radiation dose rate are the water content of the sediment and how much sediment is on top of the sample site. This is because water attenuates (scatters) the radiation, reducing the total radiation dose that the sample has been exposed to. 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.

Calculating Age

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 circumstances¹ although the technique is more commonly applied to sediments up to 100,000 years old.

Challenges for OSL

The biggest challenge for OSL dating in glacial environments is partial bleaching (resetting) of the luminescence signal. This occurs if the grains of sand are not exposed to sufficient sunlight prior to deposition within a landform such as a glacial moraine. Sediment transport in glacial environments is often over short distances in turbid meltwater streams, which can limit the sunlight exposure that the grains of sand receive. If the OSL signal is not fully reset, it may result in an age overestimation. OSL specialists overcome these challenges through only sampling certain glacial landforms, where greater sunlight exposure is likely to have occurred prior to deposition e.g. glacial sandars (or outwash plains) and proglacial deltas are likely to have well bleached sediments. A second challenge for OSL dating in glacial environments is that the luminescence sensitivity (brightness) of the quartz is often very low. Recent advances in OSL dating techniques for feldspar ^2,3 may result in this becoming the preferred mineral for OSL dating of glacial sediments, although feldspars are often more severely affected by partial bleaching than quartz.

Case studies of OSL dating in glacial environments

OSL has been widely used to date glacial sediments, because organic material required for radiocarbon dating is often absent. ⁵Rowan et al. (2012) used OSL of quartz to date glaciofluvial sediments from the Canterbury Plains, South Island, New Zealand. ⁶Thrasher et al. (2009) used the OSL of quartz to date glaciofluvial sediments from the Devensian ice age around the British Isles. King et al. (In Prep) have sampled a suite of modern glaciofluvial sediments from Jostedalen, Southern Norway, to quantify the unbleached residuals of different glacial sediments and also to explore whether the luminescence signature of quartz and feldspars provides information about the different depositional pathways that sediments are transported along.

OSL dating can also be used effectively with other dating techniques such as cosmogenic nuclide dating. Glasser et al. (2006)⁷ used single-grain OSL dating of quartz together with cosmogenic nuclide dating to reconstruct the ice-extent in Patagonia at the Pleistocene-Holocene transition ~10,000 years ago. Owen et al. (2009)⁸ used cosmogenic nuclide dating and OSL together to constrain glacial advances in the Rongbuk Valley in the Mount Everest region.

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