Geoscience Reference
In-Depth Information
on past changes in precipitation (amount and origin), temperature and vegetation in
desert regions during the last half-million years.
14.2 Extracting climatic signals from speleothems
McDermott (
2004
) and Fairchild et al. (
2006
) have provided comprehensive reviews of
the technical aspects of speleothem analysis, including the precautions that should be
observed when attempting to interpret past environmental fluctuations from isotope
analysis. During the past few decades, technical improvements in uranium-series
dating (
230
Th/
234
Uand
234
U/
238
U) involving the use of inductively coupled plasma
mass spectrometers (see
Chapter 6
) have greatly improved the precision and accuracy
of age determinations of stalactites and stalagmites, giving precise ages for when such
speleothems were actively forming and when they were not forming in a variety of
caves throughout the desert world, dating back to at least 500 ka.
Three fundamental processes are involved in speleothem formation. First, ground-
water or water percolating from the surface is enriched in dissolved carbon dioxide
to form carbonic acid (
Equation 14.1
). The carbonic acid attacks the cave limestone,
which then forms calcium bicarbonate in solution (
Equation 14.2
). Finally, the dis-
solved carbon dioxide comes out of solution within the cave, causing the dissolved
calcium carbonate to be precipitated as some form of dripstone, including stalactites
and stalagmites (
Equation 14.3
).
H
2
O
+
CO
2
=
>
H
2
CO
3
(14.1)
H
2
CO
3
+
CaCO
3
=
>
(
HCO
3
)
2
Ca
(14.2)
Ca
(
HCO
3
)
2
=
>
CaCO
3
+
CO
2
+
H
2
O
(14.3)
Initial work on speleothems in desert caves where they were no longer actively form-
ing was confined to establishing the times when they were actually formed, because
this implied that conditions were more humid then. A major technical breakthrough,
pioneered by Hendy (
1971
) in New Zealand, involved the use of
18
O/
16
O isotopic
ratios measured in speleothem samples to determine past fluctuations in cave air
temperature. Hendy also provided a useful set of criteria, still in use today, to test
for any significant evaporative effects, because these would affect the temperature
interpretation. Later work enlarged the analytical repertoire and included the analysis
of
12
C/
13
C fluctuations as a measure of changes in biological activity and plant cover.
Schwarcz et al. (
1976
) went on to show that analysis of the deuterium/hydrogen
(D/H) ratios in fluid inclusions within the dripstone calcite could be used
to provide precise measures of past temperature rather than just relative fluctu-
ations from colder to warmer or warmer to colder. These three sets of analyses
(
18
O/
16
O,
12
C/
13
C and D/H ratios) have been at the forefront of speleothem studies
ever since. We illustrate this work using a few selected examples drawn from
