Desert dunes - Climatic Change in Deserts - page 126

of the record had been preserved as a result of a change to wetter conditions in the
Sahara between 11.5 and 7 ka.
8.11 Desert dunes of the Sinai and Negev deserts
Efforts to extract information about past climatic changes from desert dunes can often
lead to unexpected and sometimes counter-intuitive results, as shown by recent studies
from the Negev Desert of Israel (Roskin et al., 2011a ; Roskin et al., 2011b ). Vegetated
and stabilised linear dunes occupy a dune field of 1,300 km 2 in area in the north-west
Negev Desert and form the eastern extremity of the northern Sinai sand sea. The
main phase of dune accumulation began as recently as 23 ka, although there had been
sporadic dune deposition since about 100 ka (Roskin et al., 2011a ). A detailed set of
97 OSL ages obtained from thirty-five dunes and inter-dune swales point to three main
episodes of dune mobility: 18-11.5 ka (post-LGM), 2-0.8 ka (very late Holocene) and
the last 150 years (modern). The post-LGM interval was the most widespread phase
of dune movement, and it involved dune damming of certain small valleys, which led
to impeded drainage and the formation of small lakes and ponds between the dunes.
Late Pleistocene lowering of the sea level by about 120 m led to the exposure of the
previously submerged sands in the Nile Delta. These sands were carried eastwards by
longshore drift and blown inland and eastwards to feed the sand dunes of the Sinai
Desert and northern Negev. Initial advance of the linear dunes from the Sinai into the
northern Negev was underway during the LGM (23-18 ka), but gained momentum
around 16-13.7 ka, with a later minor phase of advance at 12.4-11.6 ka (Roskin et al.,
2011b ). These two phases were synchronous, respectively, with Heinrich Event 1 in
the North Atlantic and with the Younger Dryas cold event in Greenland.
The orientation of the linear dunes indicates that the sand-moving winds were
blowing from the west. Roskin et al. ( 2011b ) concluded that movement of the linear
dunes during the late Pleistocene was associated with stormy winter cyclones from
the eastern Mediterranean. Such cyclones would have brought both violent winds and
more rainfall, indicated by the presence of lakes, paleosols and prehistoric occupation
sites in the swales between the linear dunes. Although the Holocene climate was
more arid than the climate that prevailed during the late Pleistocene, the decrease
in storminess led to dune stabilisation. Wind regime was thus more important than
aridity in controlling the movement and eventual stabilisation of the vegetated linear
dunes in the northern Negev Desert. We thus have the paradox of maximum dune
activity coinciding with a time of increased and not decreased rainfall in this region.
8.12 Desert dunes of Arabia, Pakistan and India
Two dominant wind systems control the distribution and orientation of the sand dunes
in the deserts of peninsular Arabia (Glennie et al., 2002 ). The Shamal (Arabic for