Geology Reference
In-Depth Information
235
U series
238
U series
232
Th series
238
U
234
U
235
U
2.48x10
5
7.13x10
8
4.49x10
9
β
228
Th
1.91y
231
Pa
232
Th
α
234
Pa
α
α
3.43x10
4
1.39x10
10
1.18m
β
β
β
234
Th
227
Th
228
Ac
230
Th
231
Th
α
α
6.13h
7.52x10
4
18.2d
24.1d
25.6h
β
β
227
Ac
228
Ra
α
22y
5.75y
226
Ra
1.60x10
3
Uranium
Decay Series
5α, 2β
α
5α, 2β
222
Rn
3.83d
Fig. 3.12
Uranium and thorium decay
chains.
Radioactive parents
238
U,
235
U, and
232
Th decay
through
α
and
β
decay steps to stable
daughter isotopes of Pb. Half-lives shown in
boxes are in years except where noted.
4α, 4β
stable:
206
Pb
207
Pb
208
Pb
used to date the carbonate coatings on pebbles,
whose thickness has already been discussed as
a surrogate for soil age. The method entails
scraping off all but the innermost coating,
adjacent to the clast upon which it is growing, in
order to analyze this innermost rind. Although
this method works in some desert settings (e.g.,
Ku
et al.
, 1979), it fails in many, owing either to
leakage of one or another product from the
carbonate or to severe impurity of the coatings.
Such unpredictable success renders this approach
as a tool of low priority, to be used only if no
other can be identified. Thankfully, one can
determine if significant leakage has occurred
and, hence, whether the age obtained is worthy
of interpretation - see, for example, Muhs
et al.
(1994) for discussion of these tests.
Whereas both societal concerns and geological
efforts to understand well the most recent
tectonic events often require that we focus on
the past thousand years, this interval is difficult
to date accurately using
14
C (Fig. 3.11). Although
the measurement precision in the best labs
may be as little as 10-20 years, the calendar
uncertainty is commonly many decades due to
the fluctuations of atmospheric
14
C during the
past 1000 yr - see Atwater
et al.
(1991) and Sieh
et al.
(1989) for examples of high-precision
14
C
dating. High-precision U-Th dating of corals
that grew during this same interval, however,
can have calendar uncertainties of <10 yr, and
thus can provide a detailed time resolution that
was previously unattainable with most other
techniques (Edwards
et al.
, 1988, 1993).
The production rate of
14
C has been even
further constrained by yet another method, which
employs the high-resolution sediment core from
the Cariaco Basin, South America (Hughen
et al.
,
2004). High-precision
14
C dating of the sediment
core now extends more than 50 000 years
(Fig. 3.14) and reveals significant departure of
the
14
C clock from calender years that is greatest
during the last major glacial. The offset history is
best explained by appeal to variations in the
