Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 53

Fig. 3.1 Geomorphic settings in the Sahara Desert that are common dust sources: ( a )wadiand
alluvial fan of the Oued Foum el Khanga, draining the Atlas Mountains of Tunisia and Algeria; ( b )
dunes of northernmost part of Grand Erg Oriental adjacent to southeastern side of Chott (playa) el
Jerid, Tunisia; ( c ) fine-grained sediments in Chott el Gharsa, derived from sandy clays of Neogene
age, northeast of Tozeur, Tunisia; ( d ) yardangs eroded into Neogene deltaic sands and clays,
southern Tunisia (Photographs by D.R. Muhs)

For example, in the northern part of the Saharan Desert in southern Tunisia, fluvial
comminution can take place in large wadis or dry washes that drain major upland
terrain such as the Atlas Mountains (Fig. 3.1 a).
Despite the importance of physical processes of silt production and the domi-
nance of silt in many source regions, much long-range transport (LRT) dust consists
of clay-sized particles with diameters less than 2 m. Such fine-grained particles are
difficult to generate by the physical processes described above. LRT dust derived
from Africa, carried across the Atlantic, and collected on Barbados and Miami,
Florida, consists almost entirely of particles less than 20 m (Prospero et al. 1970 ).
About half of this mass comprises particles with diameters less than 2-3 m
(Li-Jones and Prospero 1998 ;Prosperoetal. 2001 ;Reidetal. 2003a , b ).
Many (though not all) clay-sized particles, including those found in dust, have
a very different mineralogy compared to that of coarser particles (Glaccum and
Prospero 1980 ). Coarse particles, greater than 2 m, consist largely of primary,