Geoscience Reference
In-Depth Information
2010b
; Fujioka and Chappell,
2011
). A technically more difficult application is in
dating alluvial deposits and beach ridges, such as the 386 m shoreline of the White
Nile. Here allowance must be made for bulk density changes and for later burial
events, requiring more complex modelling of the primary analytical data (Barrows
et al.,
2014
). Cosmogenic nuclides currently used in exposure dating (with age range
in brackets, after Walker,
2005
, table 3.2) include the stable nuclides
3
He (1 ka to
approximately 3 Ma) and
21
Ne (7 ka to approximately 10 Ma) and the unstable nuc-
lides
10
Be (3 ka to 4 Ma),
26
Al (5 ka to 2 Ma),
36
Cl (5 ka to 1 Ma) and, of course,
14
C,
discussed in some detail earlier in this chapter.
Cosmogenic
10
Be has been used with success to date late Quaternary glaciations in
the Tian Shan ranges in central Asia (Kong et al.,
2009
) and to date glacial and perigla-
cial deposits in the semi-arid SnowyMountains of south-east Australia (Barrows et al.,
2001
; Barrows et al.,
2002
; Barrows et al.,
2004
). Ages of Sierra Nevada cave sedi-
ments based on the ratio of aluminium-26 to beryllium-10 (
26
Al/
10
Be) have been used
to determine rates of late Pliocene and Quaternary river incision and are consistent
with tectonic uplift of the Sierra Nevada during the past 10 million years (Stock et al.,
2004
). One possible problem with the use of
10
Be, however, concerns variations in
its production rate. Raisbeck et al. (
1985
) found that during a geomagnetic reversal,
there was an increase in cosmogenic
10
Be. During the Last Glacial Maximum, the
earth's magnetic field was weaker than it is today, and this was reflected in an increase
in the production of cosmogenic
10
Be (Lao et al.,
1992
).
6.9 Chemical methods: amino acid racemisation dating
In one sense, this method may be regarded as a means of obtaining relative ages and so
might best be classed as a Category I correlation method. However, technical advances
have effected such substantial improvements in both accuracy and precision that it
deserves separate treatment. Amino acids are protein molecules with asymmetric
carbon bonds and are characterised by alternative molecular patterns, or
isomers
(somewhat akin to mirror images), termed
stereoisomers
. These stereoisomers consist
of two distinct types, the left, or L, type and the right, or D, type. Once an organism
dies, there is a progressive change from the L type dominant in living molecules to a
balancedmixture of L andD types, a process termed
racemisation
for isomers with one
asymmetric carbon atom and
epimerisation
for isomers with two such atoms. The rate
of change is a linear function of time, but it varies in regard to temperature (Blackwell,
2001
). Low temperatures slow down the racemisation rate, while higher temperatures
speed it up. Age of the fossil organism, whether shell, wood or bone, is estimated
from the relative proportions of L and D isomers, but because the racemisation rate
is highly sensitive to temperature, an independent method of assessing temperature is
needed if this technique is to provide credible age estimates. Alternatively, the sample
in question, for instance, ostrich or emu eggshell, can be dated independently using
