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of Bengal in the northern Indian Ocean, sampled at close intervals above and beneath
the 74 ka ash layer in the core (Williams et al.,
2009a
).
Two other instances of stable isotope analysis of pedogenic carbonate in semi-arid
India come from the Thar Desert of Rajasthan in north-west India (Andrews et al.,
1998
; Singhvi et al.,
2010
). Andrews et al. (
1998
) studied pedogenic carbonates within
a 70 ka sequence of eolian sands and found that
13
C values were highest when the
18
O values indicated the most arid conditions. They concluded that during glacial
periods when the atmospheric carbon dioxide concentration (
p
CO
2
) was lowered,
C
4
grasses expanded at the expense of C
3
plants. Singhvi et al. (
2010
) found that
carbon isotopes measured on organic matter within the sand profiles of a 200 ka
polygenic dune profile showed consistent values close to
, pointing to
deposition of the eolian sands during a transitional climatic regime characterised by
a change from open C
3
grassland to C
4
woodland or forest. The assumption that sand
deposition had occurred during glacial maxima, implicit in earlier studies, was thus
replaced with a more nuanced interpretation of maximum dune activity during the
transition from weak to stronger summer monsoon in immediate postglacial time. In
this case, wind strength outweighed that of aridity in promoting sand movement, as
discussed in
Chapter 8
.
An interesting question is whether or not changes in vegetation caused by changes
in
p
CO
2
can be discerned from analysis of
−
21.6
±
1
‰
13
C values in pedogenic
carbonates from arid areas. Cole and Monger (
1994
) analysed paleosol carbon isotope
ratios on an alluvial fan in the Chihuahuan Desert of New Mexico and found a shift
from mainly C
4
grasses to mainly C
3
shrubs from 9 to 7 ka. This shift coincided
with an increase in
p
CO
2
in Antarctic ice cores and increased local aridity evident
from packrat middens. Because the
18
Oand
18
O values, which depend on both moisture
and temperature, were constant from 9 to 7 ka, when the plant cover changed, they
concluded that atmospheric CO
2
rather than regional climate change was the main
cause. From this, they also concluded that carbon isotope ratios in ancient soils could
be used as a proxy for past changes in atmospheric CO
2
.
A variant on the theme of using carbon isotope ratios in pedogenic carbonate
to reconstruct past changes in desert ecosystems involves using land snail shells.
Analysis of the
13
C/
12
C ratios in the organic matrix of fossil land snail shells from
the Negev Desert was used to map the past distribution of C
4
shrubs in this region
(Goodfriend,
1988
). The C
4
shrubs are mainly restricted to the arid zone with less than
280 mm of mean annual rainfall, so shifts in rainfall distribution can be inferred from
shifts in vegetation, as inferred from the snail shells. The results indicated that the
northern limit of C
4
shrubs had moved south by 20-30 km from approximately 4.5 ka
to 2.9 ka, indicating that the northern Negev was far wetter at that time. A further
refinement in snail shell analysis involves using the
18
O values in dated snail shell
carbonate to infer past changes in rainfall and rainfall source area. Goodfriend (
1991
)
obtained seventy-six radiocarbon ages from the Holocene fossil land snail
Trochoidea
