Desert dust - Climatic Change in Deserts - page 149

probable that enhanced LGM aridity in the Bodele Depression and adjoining region
within the Chad Basin would have significantly increased the potential supply of dust
to the Harmattan winds of that time.
The present is also a useful guide to the past.Work over the past fifty years has shown
that years of drought in the Sahel zone of West Africa are followed by an increase in
dust flux from the southern Sahara across the Atlantic (McTainsh, 1985 ; Middleton,
1987 ;Pye, 1987 ; Williams and Balling, 1996 ; Goudie and Middleton, 2001 ; Prospero
and Lamb, 2003 ). This work indicates that both aridity and the concomitant reduction
in annual plant cover associated with drought conditions have a major influence on
the volume of dust blown from the source area. The observations of McTainsh in
northern Nigeria have indicated that although the maximum particle size of dust
carried by the Harmattan winds diminishes slowly downwind, it can diminish very
rapidly laterally, perpendicular to the long axis of dust transport (McTainsh, 1980 ;
McTainsh and Walker, 1982 ; McTainsh, 1984 ). As noted in Chapter 8 , there was also
increased LGM aridity in the Lake Eyre Basin in Australia, another major source of
dust in both the past and present.
It thus seems safe to conclude that aridity, suitable surface soils and a sparse plant
cover are necessary preconditions for dust entrainment and that given such conditions,
strong and gusty winds will initiate particle movement. Once in suspension, strong
unidirectional winds will ensure transport of the dust particles well-beyond the source
area.
9.5 Dust deposition and accumulation
Figure 9.3 shows the distribution of loess deposits across the globe. Dust in suspension
in the atmosphere can fall back to earth as dry dust fall, operating under Stokes' Law,
as described in Section 9.3 . Alternatively, it can be removed from the air during
rainfall events. Some of this dust may already have been present as nuclei for water or
ice droplets in clouds (Arimoto, 2001 ; Prenni et al., 2009 ),butmostofitisscavenged
from the atmosphere during rainstorms that frequently follow on the heels of major
dust-storms. Once the dust has reached the ground, it may be remobilised by wind
gusts, as in northern Nigeria during the Harmattan season (McTainsh, 1987 )or,if
suitable dust traps are available, it will remain on the surface and ultimately form
a dust mantle or loess deposit. Many authors have stressed the importance of dust
traps for desert loess formation (Coude-Gaussen and Rognon, 1983 ; Tsoar and Pye,
1987 ; Coude-Gaussen et al., 1987 ; Williamson et al., 2004 ). In fact, Coude-Gaussen
and Rognon ( 1983 ) have argued that a 'pluvial' climate and a dense cover of grasses
and shrubs were necessary for loess accumulation in the Matmata limestone uplands
of southern Tunisia during the late Pleistocene. In the Loess Plateau of north-central
China, the reverse pattern seems to have prevailed, with maximum rates of loess
accumulation during the colder, drier, windier glacial phases of the Quaternary and