Geoscience Reference
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Given some of the problems inherent in radiocarbon dating, it is prudent to check
the validity of the ages obtained using other, independent methods of dating, such as
luminescence, uranium-series or amino acid racemisation (Prescott et al., 2007 ). In
addition, and wherever possible, different types of paired samples should be collected
for radiocarbon dating, including charcoal, shells and organic plant detritus in the case
of alluvial sediments (Williams et al., 2001 ; Haberlah et al., 2010a ; Haberlah et al.,
2010b ).
6.6 Radiometric methods using disequilibrium between parent and
daughter radioisotope
6.6.1 Uranium-series disequilibrium dating of speleothems, calcretes and corals
With analytical improvements in this method during recent decades leading to greater
precision and accuracy of ages obtained, uranium-series disequilibrium dating has
been used ever more widely. The method is based on initial decay of the three radio-
active isotopes 238 U, 235 Uand 232 Th, which have half-lives ranging across the entire
spectrum of geological time. The end members of each of the decay series are stable
isotopes of lead, but daughter isotopes formed during the decay process are themselves
unstable, so that a whole series of decay products are involved at different stages, as
shown in Equations 6.3 and 6.4 ,inwhich 231 Pa is the isotope protactinium-231. (Ga
is 10 9 years and ka is 10 3 years.) Samples suitable for dating using this technique
include corals, speleothems, evaporates, bones, shells, peat and even weathered allu-
vium, but not all attempts have yielded reliable results (see Gustavsson and Hogberg,
1972 ; Edwards et al., 1987 ; Nanson et al., 1991 ; Auler and Smart, 2001 ). Ages are
determined from the degree of disequilibrium between the higher members of the
uranium decay series (Williams et al., 1998 , p. 277).
238 U
234 U
230 Th
(
4
.
47 Ga
)
(
245 Ka
)
(
75
.
4Ka
) ...
(6.3)
235 U
231 Pa
(6.4)
If the sample contains no initial 230 Th, any produced during the radioactive decay
process will reflect aging during that particular decay pathway. However, if the sample
does contain an initial amount of 230 Th, certain corrections need to be made when
calculating its age. It is also necessary to correct for disequilibrium in the system. In
a closed system with no gains or losses of uranium to or from the system, equilibrium
occurs when the rate of parent decay and daughter formation are balanced. However,
chemical differences between parent and daughter can disrupt this balance, so that
the system is in a state of imbalance, or disequilibrium . If at any time the material
being dated has ceased being a closed system, errors accrue. For example, bone may
incorporate (or lose) uranium long after death and burial. Speleothems and tufas
may receive detrital thorium as wind-blown dust or in run-off. Corals and mollusc
(
0
.
7Ga
)
(
33 Ka
) ...
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