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ones most commonly used to determine former environmental fluctuations in the arid
lands and, indeed, elsewhere.
Changes in the stable isotopic composition of carbon, expressed as the ratio of
12 Cto 13 C(or
13 C) can shed light on past changes in vegetation and in human and
animal diet. Changes in the ratio of 16 Oto 18 O(or
18 O) in organic and inorganic
carbonates and in ice have been used to infer past changes in temperature, global ice
volume and salinity. Finally, changes in the ratio of light to heavy hydrogen D/H (or
D) in, for example, ice cores, reflect local temperature changes above the ice caps.
In fluid inclusions within speleothems, the D/H ratio reflects the ambient temperature
within the cave.
Another potentially significant advance is the use of 'clumped isotopes' to determ-
ine past temperature changes, although this technique is still very much in its infancy
and will need considerable independent testing and calibration (Ghosh et al., 2006a ).
Clumped isotopes are those rare isotopes in which the concentration of 13 C- 18 O bonds
in reactant carbonate is a function of temperature at the time of carbonate growth.
The method has also been used to determine the uplift rates of the Altiplano plateau
of the Bolivian Andes by analysing the concentration of 13 C- 18 O bonds in paleosol
carbonates (Ghosh et al., 2006b). Inferred rates amounted to 1.03
0.12 mm/year
between approximately 10.3 and approximately 6.7 Ma. Uplift rates over the past two
decades measured from space geodetic observations amount to around10 mm/year,
indicating an order of magnitude increase in uplift rate since the late Miocene (Fialko
and Pearse, 2012 ). Clumped isotopes have also been used to determine speleothem
temperature in Soreq Cave (Affek et al., 2008 ) and can be used to determine the
temperature to within
±
±
C in any carbonate precipitated in equilibrium with its sur-
rounding environment. This would preclude speleothems in caves where evaporation
rates were high and variable.
Another set of isotopes that have seen increasing use are strontium isotopes; the
strontium isotope ratio 87 Sr/ 86 Sr preserved in alluvial clays and aquatic snail shells
is used to reconstruct past changes in river discharge, as well as to fingerprint indi-
vidual layers of volcanic ash. In combination with the neodymium isotope ratios
143 Nd/ 144 Nd, the 87 Sr/ 86 Sr ratios found in wind-blown dust, as well as in reworked
desert loess and associated soil carbonate nodules, have been used to identify dust
source areas. Similarly, the isotopic ratio of carbon to nitrogen (C/N) provides an
indication of the diet of prehistoric (and modern) mammals, including humans.
Examples of each of these applications are given in the subsequent sections. In
addition to the use of stable isotopes, there have been significant advances in the use
of trace element geochemistry to determine past changes in temperature and salin-
ity in lakes, as well as likely sources of desert dust. Trace element geochemistry is
also routinely used to fingerprint volcanic ash beds and thus to determine the parent
volcanic source of the ash (Shane et al., 1995 ; Shane et al., 1996 ; Westgate et al.,
1998 ).
2
°
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