Geoscience Reference
In-Depth Information
south by the salt marshes of the Rann of Kutch and to the north by the presently dry
valley of the Ghaggar (see Chapter 10 ). The Thar Desert forms the eastern terminus
of the wide stretch of tropical deserts that extend from the Sahara across Arabia,
Iraq, Iran, Afghanistan and Pakistan to north-west India, a distance of 8,000 km,
spanning 110
of longitude. It therefore lies within the tropical northern Erem-
ian vegetation zone, comprising the Saharo-Sindian flora and the Irano-Touranian
flora.
Much of the better-watered eastern desert is well-vegetated, and the flora is remark-
ably reminiscent of that found along the southern Sahara and across Arabia, including
such familiar trees as Acacia senegal , Salvadora spp., Tamar i x spp., Calotropis pro-
cera , Ziziphus spp. and Leptadenia pyrotechnica, as well as the well-known dune
stabilising grasses Panicum turgidum , Cenchrus biflorus and Eragrostis spp., with the
ubiquitous bitter melon Citrullus colocynthis trailing along the ground.
We saw in Section 8.2 that in the Thar Desert, vegetated dunes extend well to
the east of the present desert into areas with a mean annual rainfall of 450 mm or
more. Leaving aside the question of human disturbance, active dunes in this desert
are today mainly confined to areas with less than 250 mm annual rainfall. Goudie
et al. ( 1973 ) considered that this implied that the climate had been drier and the desert
more extensive during the very late Pleistocene. Wasson et al. ( 1983 ) questioned this
interpretation on the grounds that even vegetated dunes could be mobile and, in any
event, dune sands continued to accumulate well into the Holocene.
We noted earlier in this section that the Thar Desert dunes reflect the influence of
the south-west summer monsoon winds, with the linear dunes in the drier western
half of the desert oriented parallel to the dominant sand-transporting wind direction.
In favourable circumstances, such as in the lee of ranges of low hills aligned roughly
perpendicular to the dominant sand-moving winds, a long, nearly continuous record
of dune accretion may be preserved. One such dune, near Didwana in the north of the
desert, with the unromantic site name of 16R, contains a remarkably well-preserved
sequence of twelve cycles of dune accretion, soil formation, calcrete development and
subsequent erosion spanning the last 190 ka ( Figure 8.11 ) (Singhvi et al., 2010 ). It
also contains a stratified sequence of prehistoric stone artefacts ranging from Lower
through Middle and Upper Palaeolithic to Mesolithic in age, synchronous with more
humid climatic interludes in this region (Misra, 1983 ;Dhiretal., 1992 ;Dhiretal.,
2010 ; Singhvi et al., 2012 ). The calculated time interval between successive phases of
dune sand accumulation ranged from 22.2 ka to 15.8 ka, with a mean of 19.0 ka. These
values are consistent with a precessional influence (see Chapter 3 ) on dune activity.
Initial dune accretion was associated with the onset of early monsoonal activity in this
region. Carbon isotopes measured on organic matter within the sand profiles show
consistent values close to
°
, pointing to deposition during a transitional
climatic regime characterised by a change from open C 3 grassland to C 4 woodland or
forest (Singhvi et al., 2010 ).
21.6
±
1
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