Asian deserts - Climatic Change in Deserts - page 372

the wider region. A great deal of work is still needed using lake sediments and fossil
soils within the dune sands to determine past climatic fluctuations, while taking into
account the often large reservoir effects (up to a few thousand years) that occur
when using 14 C to date groundwater-fed lake deposits intercalated within the dunes
(Hofmann and Geyh, 1998 ;Yangetal., 2011b ).
19.5.3 River response to climate change
The big rivers of the drier parts of Asia contain a very partial record of Quaternary
events and one that is often hard to decipher. For example, the headwaters of the Indus,
Ganga, Yamuna and Huang Ho rivers have been subject to a variety of influences,
including tectonic events, river piracy linked to such events, the waxing and waning
of glacial activity and changes in monsoon intensity. As a result, it is difficult to derive
a clear climatic signal from the often fragmentary alluvial record preserved in their
middle and lower reaches (Srivastava et al., 2008 ;Srivastavaetal., 2009 ; Ray and
Srivastava, 2010 ). The sedimentary deposits of the Huang Ho in northern China also
reflect fluctuations in the input and reworking of desert dust, so changes in rates of
sedimentation may simply be a result of changes in dust inputs to the drainage basin.
In the lower reaches of these big rivers, the influence of sea level fluctuations linked
in particular to the growth and melting of the Northern Hemisphere ice caps adds
further complexity to unravelling the alluvial history of these rivers. What is needed
to obtain a climatic history is an alluvial record in valleys that are free from tectonic
and sea level influences and have minimal inputs of wind-blown dust.
The river valleys of northern India contain a remarkably well-preserved record of
alternating alluvial deposition and channel incision (Chamyal et al., 2002 ; Sridhar
and Chamyal, 2010 ; Joshi et al., 2013 ). However, the relatively well-studied record of
sedimentation in the Ganga (Srivastava et al., 2003 ) and Yamuna valleys reflects the
influence of both climatic and tectonic factors in both of their upstream reaches and
that of sea level fluctuations in the distal sector of the Ganga. South of the confluence
of the Ganga and Yamuna rivers at Allahabad are two river basins that appear to be
relatively free from both tectonic and sea level influences, namely the Son and Belan
rivers (Williams and Royce, 1982 ; Williams and Clarke, 1984 ; Williams and Clarke,
1995 ; Pal et al., 2004 ; Williams et al., 2006b ; Gibling et al., 2008 ). The alluvial
deposits laid down by both rivers also contain a wealth of prehistoric stone tools
ranging from Lower Palaeolithic to Neolithic, and these have helped to shed light on
the transition from hunting and gathering to early agriculture in this region (Sharma,
1973 ; Sharma et al., 1980 ; Sharma and Clark, 1983 ; Clark and Williams, 1986 ; Clark
and Williams, 1990 ). Both valleys contain four distinct alluvial formations, of which
those in the middle Son Valley are the best exposed and best dated ( Figure 19.4 ). From
oldest to youngest, these are the Sihawal, Patpara, Baghor and Khetaunhi formations,
with each type section named after a nearby village (Williams and Royce, 1982 ).