Geoscience Reference
In-Depth Information
have been possible (Henderson,
2006
), interpretation of those records will always
contain elements of uncertainty. For example, Maher and Thompson (
2012
)have
recently argued that the
18
O records from Chinese stalagmites do not in fact reveal
changes in the amount of summer rainfall but rather reflect changes in the source of the
moisture. Because changes in air mass precipitation sources will in any event result
in changes in the amount of rainfall, it may be that the differences in interpretations
will turn out to be more apparent than real.
14.3.5 Speleothem studies from semi-arid North America
13
C record from two stalagmites from a cave in central Missouri show positive
excursions at 3.5 ka and 1.2-0.9 ka. These episodes are broadly coincident with
dry intervals inferred from other lines of evidence across the semi-arid Great Plains
of North America (Denniston et al.,
2007
). Possible factors controlling the
The
13
C
excursions include a change in the abundance of C
4
plants above the cave and/or
a greater input from bedrock carbon, both of which could lead to a reduction in
effective moisture. Given that the
18
O record showed no change during the times of
13
C, it would appear that there was no change inmean annual temperature
or in rainfall seasonality during these episodes of inferred aridity.
A particular form of speleothem known as 'cave mammillaries' has been used to
determine past elevations of the watertable at nine sites in the Grand Canyon (Polyak
et al.,
2008
). Using uranium-lead dating of the speleothems and equating rates of
groundwater-table decline with concomitant rates of incision, these authors found
that in the western Grand Canyon, the rates amounted to 55-123 m/Ma during the
past 17 Ma. In the eastern Grand Canyon, the corresponding rates were much faster,
amounting to 166-411 m/Ma. The overall conclusion was that the Grand Canyon has
evolved through headward erosion from west to east, accompanied by accelerated
incision in the eastern sector during the past 3.7 Ma or so.
anomalous
14.4 Extracting climatic signals from tufas
Tufas form in much the same way as speleothems. Carbon dioxide comes out of
solution in river, lakes or spring water, often as a result of an increase in water
temperature, and the dissolved calcium carbonate is precipitated (
Equation 14.3
).
This tends to occur at the outlet of springs, on waterfalls and along lake, swamp or
even flood-plain margins. Tufas occur in a wide variety of locations within desert
landscapes. They may form a series of roughly horizontal benches along valley sides,
especially those associated with former river terraces and valley fills (Butzer and
Hansen,
1968
; Butzer,
1984
; Pentecost,
2005
;Pedley,
2009
). Tufa dams are common
in ephemeral stream channels, particularly near waterfalls and knick points, as in
central Afghanistan (Bouyx and Pias,
1971
) and the seasonally wet Kimberley region
