The nature and early history of airborne dust from North Africa; in particular the Lake Chad basin

Africa is the great source of airborne dust, and a very large proportion of it blows out of the Lake Chad basin. There are various types of dust, an initial simple division might be into large dust and small dust, but the small dust category is itself divided into essentially monomineralic dust and clay mineral agglomerate (CMA) dust. The monomineralic dust populations are separated by Tanner gaps. Long weathering times allow small CMA dust particles to be produced (the PI process); the initial transportation processes (T1) have led to the vast accumulation of dust material in the Lake Chad basin. These early processes are important in the study of African dust. Ground material controls the nature of the dust cloud. Small dust is essentially old dust; large dust is young dust, and the two form distinct populations. CMA dust derived directly from old lake basins is basically controlled by the nature of the lake sediment. A simple Monte Carlo model shows how particle packing parameters control the size of small CMA dust, limiting it to the very fine silt fraction. (C) 2004 Published by Elsevier Ltd.