Geoscience Reference
In-Depth Information
considered to have blown in from the Sahara and so may have travelled thousands of
kilometres in suspension in the atmosphere. In contrast, the coarser particles are now
believed to be of more local origin, having formed as a result of sand-particle abrasion
during the eastward progression of the dunes in the Sinai Desert (Crouvi et al.,
2008
;
Crouvi et al.,
2010
; Enzel et al.,
2010
). The source of these latter dunes is the Nile,
and the sands are thought to have blown inland from deltaic sediments exposed during
times of lower glacial sea level (Amit et al.,
2009
). One unresolved issue with this
interpretation is why the coarse-grained loess is, according to present evidence, dated
by OSL as no older than about 180,000 years, particularly given that the Nile has
apparently been transporting sediments to the delta for more than 2 million years
(Williams and Talbot,
2009
; Davis et al.,
2012
).
The dominant component of most desert dust plumes is quartz, with lesser quantities
of other minerals, such as feldspar and variable amounts of goethite, haematite and
other iron minerals. The desert dusts of central Asia are enriched in illite, but illite
forms a very minor component of the dusts of the Near East and North Africa (Singer,
1988
). Dust blowing off continental shelves exposed to deflation as a result of glacially
lowered sea levels often contains relatively abundant particles of calcium carbonate,
which can be transported for hundreds or even thousands of kilometres from source
to be later incorporated into soils and sediments well inland or far-removed from the
source (Williamson et al.,
2004
;Dartetal.,
2007
;Amitetal.,
2010
).
After initial fallout of the coarser grains, the modal diameter of particles carried
long distances (i.e., in excess of 1,000 km) tends to remain fairly constant at 3.5-
2.5
μ
m. Deposition of the coarse grains is relatively rapid and is governed by Stokes'
Law:
9
gr
2
W
=
2
/
(
p
−
p
a
/
M
)
(9.1)
In this expression,
W
is the terminal velocity of a particle of radius,
r
, and density,
p
, falling through still air (density
p
a
and viscosity
M
), and
g
is the acceleration due
to gravity. This relation is also true of volcanic dust plumes in which only very fine
particles in the size range of 0.5 to 2
m are likely to persist in the stratosphere for
periods of several or more years (Lamb,
1972
, p. 411).
μ
9.4 Dust entrainment and transport
The detachment and entrainment of silt-sized dust particles is a function of wind
velocity and wind gustiness, and so is tightly controlled by local and regional synoptic
conditions. Among the other factors that control the potential availability of dust are
aridity, soil type, plant cover and surface roughness. As with desert dunes, we are again
faced with a multivariate problem, so that it is often hard to separate out which are the
more influential factors responsible for dust entrainment. Dry soils or sediments and
sparse or absent plant cover are obviously important prerequisites, but without strong
