Geoscience Reference
In-Depth Information
9
Desert dust
The Lord shall make the rain of thy land powder and dust.
Deuteronomy 28.24
9.1 Introduction
Scientific interest in wind-blown dust has a respectable pedigree. In the late eighteenth
century, Dr Matthew Dobson (
1781
) had already described the dust-transporting role
of the Harmattanwind inWest Africa, a topic revisited two centuries later byMcTainsh
(
1980
) and McTainsh and Walker (
1982
). In January 1832, Charles Darwin collected
a sample of wind-blown dust using a gauze filter placed at the masthead of HMS
Beagle while that vessel was anchored at Porto Praya in the Cape Verde archipelago
off the west coast of the Sahara (Darwin,
1860
, pp. 6-7). He sent this sample (and
four other dust samples collected for him by the geologist Charles Lyell from a vessel
several hundred kilometres further north) to the eminent German naturalist Professor
Christian Gottfried Ehrenberg in Berlin. Ehrenberg identified no fewer than sixty-
seven species of diatoms (Darwin's 'infusoria') in the five samples, two of them
marine and the rest freshwater (Darwin,
1846
; Ehrenberg,
1851
).
Darwin correctly attributed the dust to transport by the Harmattan wind that in
winter blows from the Chad Basin across northern Nigeria and out across the Atlantic.
The siliceous diatom frustules observed by Darwin and by Ehrenberg arise from the
deflation of Holocene and older lake deposits in and around the Bodele Depression
in the Chad Basin (McTainsh,
1987
; Washington et al.,
2006
). The African origin of
wind-blown dust observed by vessels sailing across the Atlantic was thus recognised
two centuries ago and has been the focus of attention ever since (Morales,
1979
;
Schutz et al.,
1981
; Williams and Balling,
1996
, pp. 43-47; Goudie and Middleton,
2001
; Prospero and Lamb,
2003
; Goudie and Middleton,
2006
; Goudie,
2008
). This
dust reaches as far as the Amazon, where it is an important source of plant nutrients
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