Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 62

3.4.3 Mineralogy as a Guide to Dust Sources

Mineralogy is potentially one of the most fundamental methods of identifying past
dust sources, but unfortunately, it is one of the most difficult to apply. Much of the
difficulty in using mineralogy to identify sources is that most dust (Glaccum and
Prospero 1980 ;Reidetal. 2003a , b ; Trapp et al. 2010 ) and loess sediments have
a composition similar to average upper continental crust (Taylor and McLennan
1985 ; Gallet et al. 1996 ; Muhs et al. 2007 ). Thus, dust and loess have a mineralogy
similar to many of the rocks found on the Earth's surface. Nevertheless, detailed
mineralogical analyses combined with consideration of potential source areas can
often identify points of dust origin.
Mineralogy has been exploited in many studies simply to recognize that a
nonlocal dust component exists within a soil or a sediment body. Good examples
of this kind of approach include the recognition of exotic quartz in soils that occur
on basaltic or andesitic bedrock, lithologies that lack quartz. Such studies have
confirmed the existence of aeolian quartz in soils on mafic volcanic rocks of the
Hawaiian Islands (Jackson et al. 1971 ), Australia (Hesse and McTainsh 2003 ), the
California Channel Islands (Muhs et al. 2008 ), and the Canary Islands of Spain
(Muhs et al. 2010 ).
For the purpose of identifying dust sources, mineralogy of the clay (<2 m)
fraction is the approach that has been used most commonly. Special attention has
been paid to the clay mineral composition of dust sources in Africa, which have
a clear geographic trend. Scheuvens et al. ( 2013 ) provide an excellent review of
studies that have been done thus far on mineralogical, geochemical, and isotopic
composition of dust sources in Africa. One of the earliest observations was a
shipboard dust collection campaign in 1971 along the west coast of Africa, from
the Canary Islands to southern Africa (Chester et al. 1972 ). This study found that
the clay mineralogy of dust varied from north to south, with illite (mica) contents
high at extratropical latitudes, and kaolinite contents highest in tropical latitudes
(Fig. 3.4 ). This clay mineral zonation corresponds closely to the soil geography
of the continent, with Aridisols and Entisols supplying the illite (mica) in the
extratropical regions and Ultisols and Oxisols supplying the kaolinite from farther
south. Chiapello et al. ( 1997 ) and Caquineau et al. ( 1998 , 2002 ) used back-trajectory
analyses, geochemistry, and clay mineralogy to define three distinct dust source
areas in western Africa (Fig. 3.5 ). The northwestern sector of the Sahara, including
Western Sahara, Morocco, and western Algeria, has relatively low kaolinite and
relatively high illite. The central Sahara and southern Sahara, including Mauritania,
northern Mali, and southern Algeria, have somewhat lower illite content and higher
kaolinite content. Finally, the Sahel sector, including Senegal, southern Mauritania,
and southern Mali, has the lowest illite and highest kaolinite content.
Using these source areas with their characteristic mineralogy, it is possible to
infer the origins of dust at more distant localities. Dust collected on Barbados has
a mix of clay mineral types with values intermediate between the illite/kaolinite
ranges of the northwestern Sahara and the central and southern Sahara (Fig. 3.5 ).