The pluvial debate - Climatic Change in Deserts - page 221

Pleistocene intertropical aridity had been synchronous in both hemispheres (Willi-
ams, 1975 ; Williams, 1985 ). It now seems probable that during times of maximum
glaciation, the tropical deserts were even drier than they are today and during the
interglacial phases, they were somewhat wetter. For example, during the last glacial
maximum, many dunes were active well beyond their present limits ( Chapter 8 ),
and considerable volumes of desert dust ( Chapter 9 ) were deposited downwind of
the desert margins in central Asia and in China, India, Nigeria and Australia (Pye
and Tsoar 1990 ; Williams et al., 1998 ). These dust mantles are now vegetated and
relatively stable.
A similar pattern of glacial aridity is evident also in the Gulf of Aden and the
Red Sea (Deuser et al., 1976 ). The isotopic composition of planktonic foraminifera
from deep-sea cores in this region shows that during the last 250 ka, at least, glacial
maxima were times of extreme aridity, with increased sea-surface salinity reflecting
even higher rates of evaporation than those that prevail there today. We can therefore
conclude that in many of the world's hot deserts, the dominant climate during the
Last Glacial Maximum (21

2 ka) was drier, windier and colder than it is today,
although the summers may still have been very hot. In addition, the desert lakes that
had been full and fresh until about 23 ka now dried out or became hyper-saline,
previously perennial desert rivers became seasonal and rivers that had once been
seasonal became intermittent or ephemeral streams. With glacially lowered sea levels,
land areas were greater and so the aridity associated with enhanced continentality
was accentuated. Stronger Trade Winds associated with steeper pressure gradients
between the equator and the poles caused increased upwelling of cold water close
offshore, further accentuating the aridity of coastal deserts. Maximum concentrations
of desert dust in deep-sea cores from the equatorial Atlantic Ocean coincide with
glacial maxima during the last 0.6 Ma (Parmenter and Folger, 1974 ; Bowles, 1975 )
and probably for far longer. Such dust is easily recognised by its high degree of
sorting. In a marine sediment core collected off the coast of Mauritania in the western
Sahara, deMenocal et al. ( 2000 ) found high concentrations of desert dust until the
supply was abruptly curtailed at 14.5 ka with the onset of the so-called African Humid
Period. [The term is a misnomer because the climate of Africa was not uniformly wet
during this period, as Gasse et al. ( 2008 ) have demonstrated]. We have enlarged on
these topics in Chapters 8 and 9 .
In addition to the evidence of enhanced glacial aridity offered by desert dunes and
desert dust, many lakes in the intertropical zone were dry during the Last Glacial
Maximum, including Lake Bosumtwi, a crater lake in West Africa and thus an excel-
lent paleo-rain gauge (Russell et al., 2003 ), Lake Tana in the Ethiopian headwaters
of the Blue Nile (Lamb et al., 2007 ), Lake Victoria in the Ugandan headwaters of
the White Nile (Johnson et al., 1996 ; Talbot et al., 2000 ) and Lake Mungo in arid
Australia (Bowler and Price, 1998 ).
Recent analysis of two Atlantic marine cores, one located immediately west of the
Mauritanian Sahara, the other in the North Atlantic within the zone receiving periodic

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