Geoscience Reference
In-Depth Information
the ages will only relate to sand that has been preserved at that site at that time,
and so is not necessarily indicative of major episodes of dune activity, as discussed
in
Section 8.14
.
8.9 Problems in using desert dunes to reconstruct climatic change
Because active dunes are a feature of all deserts, it is natural to assume that the presence
of now stable and vegetated dunes along the margins of many deserts indicates that
these regions were formed at a time when the arid zone was more extensive than it is
today. In addition, where cores have been collected from linear dunes for TL or OSL
dating, it is again often assumed somewhat uncritically that the ages obtained reflect
intervals of sand deposition and dune activity associated with greater aridity. Because
dune formation and movement is a complex function of effective precipitation, wind
velocity, sand supply and surface plant cover, it is instructive to examine a number of
case histories from different deserts to test whether or not dune activity is indeed a
good indicator of former aridity. The results of this exercise will prove surprising. Our
aim is not to provide a comprehensive review of dune studies on each continent but to
adopt a more selective approach and consider the scope and limitations of using desert
dunes to reconstruct past climates. We begin with the Northern Hemisphere deserts
of the Sahara, Negev and Sinai, Arabia, Pakistan, India and northern China before
moving to the Southern Hemisphere deserts of the Namib, Kalahari and Australia.
8.10 Desert dunes of the Sahara
During the Neogene, the central Sahara was drained by a series of major river systems
that flowed from the northern Chad Basin east of Tibesti across the Libyan Desert
to flow into the Mediterranean (Griffin,
1999
; Griffin,
2002
; Griffin,
2006
; Griffin,
2011
). These rivers dried out in the late Miocene to early Pliocene as the Sahara
became progressively more arid (Griffin,
2002
; Griffin,
2011
). A series of very large,
sinuous, dry river valleys is a legacy of this time and is clearly visible on satellite
images. Aridity was accentuated by tectonic uplift in East Africa that caused a major
change in atmospheric circulation and led to a reduction in rainfall over the Chad
Basin (Sepulchre et al.,
2006
). The change in rainfall regime over East Africa resulted
in a change from tropical forest to open grassland and woodland and was associated
with the proliferation of the Pliocene hominids unique to Africa (Williams et al.,
1998
; Sepulchre et al.,
2006
; Cerling et al.,
2011
).
Miocene uplift led to an acceleration of erosion in the Saharan uplands. Much of the
resulting sediment was carried to the sea by big rivers like the Nile, Niger and Senegal,
but a considerable amount began to accumulate in large subsiding sedimentary basins
such as the Kufra-Sirte Basin in Libya and the Chad Basin, providing the source
material for the Quaternary and possibly older desert dunes. In the Chad Basin,
