Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 59

point is the determining factor in the length of run. When investigating transatlantic
dust advection to Barbados, Engelstaedter et al. ( 2009 ) used 12-day model runs,
calculating the transit time of air parcels and therefore dust from West Africa to
the West Indies to be 6-7 days. For back trajectories, the passage of an air parcel
is determined through time before its arrival at a location. Backward trajectory
analysis is typically used therefore in cases where an aerosol has been sampled
at some receptor location, and the provenance of the dust emission needs to be
estimated (e.g., Stuut et al. 2005 ,Fig. 3.4 ). In the Lake Eyre Basin of Australia,
Radhi et al. ( 2011 ) also used back-trajectory analysis to interpret the chemistry of
captured dust and provide corroborating evidence that Cl/Na correlations of samples
indeed indicated dry lake sources.

3.4 Identification of Past Dust Sources

Direct observation of dust storms and dust pathways using satellite imagery is
obviously not possible when studying dust records in the geologic past. Dust
archives in geologic records can take many forms, including those in deep-sea
sediments, lake sediments, ice cores, loess deposits, and soils. The nature of these
recordsisreviewedbyMuhs( 2013a ), but here we examine the methods geologists
have used in identifying sources of dust in such records.

3.4.1 Geomorphic Evidence of Past Dust Sources

There are some forms of geomorphic evidence that can be used to identify past (and
present) dust sources, landscape settings that would be classified as being of “high
potential for dust emissions,” by Bullard et al. ( 2011 ). These include pans, yardangs,
and stone pavements. Laity ( 1994 ) and Goudie ( 2008 ) provide good reviews of
landforms of aeolian erosion, some of which help identify dust sources.
Pans, also called “playas,” are relatively small (on the order of a few kilometers
or usually less in diameter), generally circular, closed depressions (Shaw and Bryant
2011 ). They often occur in clusters, or fields, which can number in the thousands.
Pans are found extensively in South America (Pampas and Patagonia), in the Great
Plains of North America, and in parts of Africa, Australia, Siberia, and China
(Gill 1996 ; Goudie 2008 ). Because fields of pans often occur in semiarid climates,
they are commonly thought to form primarily by aeolian excavation and therefore
can be considered dust sources. The association of many pans with lunette dunes
on the downwind sides provides good evidence that at least some pans may have
formed by aeolian erosion.
More direct forms of geomorphic evidence for dust sources are landforms called
yardangs (McCauley et al. 1977 ). Yardangs are erosional remnants resulting from