Geoscience Reference
In-Depth Information
derived from charcoal and shells from these deposits (Pal et al.,
2004
; Williams et al.,
2006b
).
6.7.2 Electron-spin resonance dating of tooth enamel, mollusca and tephra
Electron-spin resonance (ESR) dating is very similar to TL and OSL dating in that
it depends on the trapping of electrons, but it is based on direct measurement of
the number of trapped electrons rather than the energy they release on stimulation
by heat or light (Blackwell,
2001
). This method has the advantage of being able to
date samples ranging from modern to more than a million years in age that are not
amenable to dating using TL or OSL. The precision of ESR dating is of the order of
10-20 per cent but can be as low as 100 per cent, depending on the type of sample.
One problem with ESR dating stems from the type of sample being dated, such as
tooth enamel or coral, and concerns the possible uptake or loss of uranium in the
sample, complicating the calculation of dose rate. However, this can be overcome in
part by using different models to estimate dose rate, one assuming a closed system
and one assuming an open system for the uranium in the tooth or coral sample under
analysis. As with all dating methods, independent verification of the age obtained is
important. A great advantage of ESR dating is that it provides a direct age for the
fossil being dated (Grun and Stringer,
1991
;Grun et al.,
2001
). Another advantage is
that samples are not altered by the application of heat or light, as with luminescence
dating, so replicate analyses are possible.
6.8 Dating with cosmogenic radioisotopes
Cosmogenic nuclide dating has really come of age in the last decade or so, and an
increasing number of research laboratories around the world are now specialising
in this powerful dating technique. When high-energy cosmic rays enter the atmo-
sphere they collide with the nuclei of atmospheric gas atoms, generating a flux of
high-energy neutrons and minor amounts of
muons
to the earth's surface. (Muons are
unstable elementary particles with a mass 207 times that of an electron, and are negat-
ively charged.) These and other subatomic particles react with certain rock minerals,
creating new nuclides. The concentration of these newly created nuclides decreases
from the surface down and is a direct function of the time elapsed since the rock sur-
face has been exposed to cosmic radiation, hence the alternative expression
surface
exposure dating
for this method of dating. One application of the method is to determ-
ine long-term rates of denudation. The assumption here is that over several million
years, the mean rate of surface lowering balances the rate of accumulation of cosmo-
genic nuclides in the near surface, which is a reasonable assumption for tectonically
stable uplands in arid areas, as well as for desert pavements on stable land surfaces
(Fujioka et al.,
2005
; Heimsath et al.,
2010
; Quigley et al.,
2010a
; Quigley et al.,
