Geoscience Reference
In-Depth Information
in part from the former headwaters region of the Yamuna, Sutlej and Beas rivers
Misra suggested that tectonism rather than climatic desiccation caused the demise of
these settlements, with tectonic movements disrupting the headwaters of the ancestral
Yamuna, which was diverted eastwards to join the Ganga at Allahabad in north-central
India, depriving the ancient Ghaggar-Hakra of much of its flow. Attractive though this
hypotheses may be, it is not supported by more recent work which has shown that
capture of the Yamuna to the east and its loss to the Indus took place between 49 and
10 ka, that is, well before the final collapse of the Harappan centres (Clift et al.,
2012
).
(In Hindu tradition, the ancestral Ghaggar-Hakra was the mythical Sarasvati River,
which is reputed to flow underground to the present-day Ganga-Yamuna confluence
at Allahabad - today a sacred pilgrimage site for devout Hindus from all over India).
Geochemical analysis by Wasson et al. (
1984
) of sediments from Lake Didwana
in the eastern Thar Desert and sedimentological analysis by Enzel et al. (
1999
)at
Lake Lunkaransar in the now arid western Thar Desert confirmed that the climate
was wetter during the early Holocene, indicating a stronger summer monsoon at that
time. At present, the south-west summer monsoon provides more than 80 per cent
of the annual rainfall in the Thar Desert, and the weaker north-east winter monsoon
provides less than 20 per cent (Sikka,
1997
; Enzel et al.,
1999
).
Other evidence of previouslywetter climates in the Thar Desert comes from the very
extensive deposits of calcrete and the ubiquitous calcareous paleosols in this region,
discussed in
Chapter 15
. In one polygenic dune in the eastern Thar Desert near Lake
Didwana, which was excavated to a depth of 18.4 m, there were twelve calcrete layers
separated by wind-blown sands, indicating twelve phases of soil development and
carbonate precipitation during relatively wet phases, with eleven episodes of dune
accretion during relatively dry intervening phases, all within the past 190 ka (Singhvi
et al.,
2010
).
This brief survey suggests that in the deserts of north-west China and north-west
India, the last pluvial phase was in the early to mid-Holocene but that the last glacial
climate in these areas was in general arid rather than wet. We elaborate on these
preliminary conclusions in Chapters
18
to
22
in which we show that the late glacial
climates were not uniformly dry.
12.7 Pluvial lakes in Australia
If a lake has remained stable for any length of time and its shorelines have not been
subject to tectonic or isostatic displacement, we can define a simple water balance
equation (see
Chapter 11
, Equations 11.1 and 11.2) in which water inputs are in
balance with water losses. The water inputs represent direct precipitation onto the
lake and run-off into the lake from its total catchment area. The water losses are those
from evaporation from the lake surface, seepage from the lake floor and any losses
from overflow by the lake. Provided seepage losses are negligible, the level of the lake
