Geoscience Reference
In-Depth Information
at archaeologists but is of value to any non-specialist concerned with luminescence
dating. Luminescence dating has been in increasing use since the 1960s, when it was
widely used to date archaeological remains such as pottery and bricks. The method is
based on the fact that certain minerals, such as quartz, feldspar, calcite and zircon, can
store energy within their crystal lattice structure as trapped electrons (Aitken,
1985
;
Aitken,
1990
; Lian and Huntley,
2001
; Duller,
2004
; Duller,
2008
). Grains within a
sedimentary deposit receive energy from the emissions of minute amounts of radio-
active isotopes (mainly
40
K, U and Th) in the surrounding material and store it in this
way. The grains thus act as natural dosimeters, recording the amount of radioactivity
to which they have been exposed (Duller,
2004
; Duller,
2008
). The application of
controlled amounts of heat or light causes the grains to release the stored energy in
the form of light, a phenomenon termed
luminescence
. Precise measurement of the
brightness of the luminescence signal allows the total amount of radiation to which
the sample has been exposed since burial to be calculated. This is done by finding the
radiation dose applied in the laboratory that produces the same amount of lumines-
cence as the sample. This is called the
equivalent dose
. Dividing the equivalent dose
by the amount of radiation received by the sample each year (
dose rate
) will give the
age (Duller,
2008
). When heat is applied to release the stored energy, the method is
called thermoluminescence (TL) dating, and when light is used, it is called optically
stimulated luminescence (OSL) dating.
Huntley et al. (
1985
) invented OSL dating more than twenty-five years ago, and
it is now the most widely used method of luminescence dating (Aitken,
1998
), with
a possible age range from 10-20 years to more than 500,000 years (Huntley and
Prescott,
2001
). OSL has been especially successful in dating eolian sediments, such
as dunes (Huntley and Prescott,
2001
; Singhvi et al.,
2010
) and loess (Roberts et al.,
2003
), where the quartz grains have been well-exposed to sunlight before being
buried, so the grains have been fully bleached and the luminescence 'clock' reset to
zero. Results obtained from OSL dating of Nile alluvium are consistent with paired
radiocarbon ages from these deposits, inspiring cautious confidence in these dating
techniques (Williams et al.,
2003
; Williams et al.,
2010b
). However, there is always a
possibility that quartz grains laid down in alluvial or lacustrine settings may only be
partially bleached. This means that the luminescence energy acquired prior to burial
has not been removed, and the measured age will be too large. The use of aliquots
(sub-samples) containing only a small number of grains or single grain methods
pioneered by Duller (
1991
;
1995
) can enable detection of whether or not the grains
have been most fully bleached. Interesting applications of this technique include
using single grains to date rates of bioturbation in soils and rates of soil formation
(Pillans et al.,
1997
). There has also been some success in dating feldspars by using
infrared radiation to stimulate luminescence, a method termed IRSL dating. For
example, the results obtained on dating alluvial samples in the semi-arid Son Valley
of north-central India using IRSL appear consistent with the radiocarbon chronology
