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within landforms such as sand dunes, loess deposits and lake shorelines. The application of luminescence dating
has enabled a much greater understanding of environmental change in situations where the absence of organic
material (e.g. in former desert contexts) prevents the use of radiocarbon dating and also where sediments are older
than the age limits of radiocarbon dating.
What is luminescence dating?
Within all sediments the presence of naturally occurring radioisotopes of uranium-238, thorium-232 and potassium-
40 undergoing radioactive decay create a low-level background radiation field (alpha, beta and gamma radiation).
Cosmic rays from the Sun also provide a significant contribution of radiation to near-surface sediments. The specific
nature of quartz and feldspar minerals allows them to behave as dosimeters: recording the total sum of ionizing
radiation that they have been exposed to since they were buried (see Preusser et al. , 2009, for a review). This is
possible because structural defects and impurities within the minerals behave as traps in which unbound electrons
(freed from their parent atom by energy derived from the radiation field of the sediment) will accumulate over
time. The rate at which electrons are displaced depends on the strength of the level of radiation in the sediment
during its burial history and is typically known as the 'dose rate'. These displaced 'trapped' electrons act as a store
of potential energy that can be naturally, or artificially, dissipated in the form of photons (luminescence). Each
time the minerals are exposed to sunlight during transport (e.g. the sediment is blown by the wind or eroded and
washed downstream), the trapped electrons are able to obtain enough energy to be freed from this state and the
dosimetric clock is reset ('bleached'). The grains may at some point be deposited within an indicative landform
such as a river terrace or sand dune and again become shielded from sunlight. The accumulation of electrons in
defect sites is time-dependent and it is this property that can be utilised to obtain burial ages. A simple analogy that
is frequently used to explain this process is to imagine the quartz or feldspar grains as a bucket underneath a tap
into which water is dripping at a constant rate: If we know the rate of dripping (analogous to the dose rate) and we
can measure the total amount of water in the bucket (analogous to the total amount of trapped electrons), then it
is possible to calculate how long the bucket has been located under the dripping tap. Figure 17.14 summarises the
main principles in the accumulation of a measurable luminescence signal in a sediment grain.
How are luminescence ages produced?
To avoid mineral grains being exposed to light prior to measurement, sediment samples collected in the field
must be extracted and stored in lightproof containers. In the laboratory, under controlled wavelength low-light
conditions, the sediment undergoes a series of chemical treatments to isolate the quartz or feldspar component.
The total population of trapped charge within the minerals can then be measured by stimulating the material with
external energy in the form of light (optically stimulated luminescence: OSL) or heat (thermoluminescence: TL).
As the trapped electrons are released, energy is dissipated in the form of luminescence. Specific measurement
conditions are used that include pre-heating the sample and monitoring any change of sensitivity to laboratory
stimulation during measurement in order to isolate the luminescence signal from only those traps that are thermally
stable over the burial period. The natural luminescence signal of the quartz or feldspar can be measured using a
photomultiplier tube (cf. Bøtter-Jensen et al. , 2003) and calibrated against the luminescence emitted when the same
minerals are exposed to known laboratory doses of radiation. From this it is possible to establish the equivalent
laboratory radiation dose ( D e ) to that received during the burial period (the Palaeodose) (see Wintle and Murray,
2006, for a review of dating protocols).
To complete the age equation the rate of electron displacement, determined by the ionizing radiation of the burial
environment ( D ), must also be known. This can be estimated by either directly measuring the gamma radiation (in
the field or laboratory) or by laboratory determination of the radioisotope concentration within the sediment (see
Aitken, 1998).
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