Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 61

aeolian removal of material, resulting in a streamlined, linear ridge (Fig. 3.1 d).
Yardangs can develop in bedrock or sediment, as long as there is sufficient particle-
to-particle binding that a remnant landform can persist after erosion. They are
thought to form primarily by aeolian abrasion from saltating sand, which explains
their common occurrence near (and particularly downwind of) dunefields.
Stone pavements, also known as “gobi” and “gibber” (see Laity 2011 ), are
concentrations of gravel-sized clasts on the surface of a landform that is composed
(usually) of unconsolidated materials. Traditionally, they have been interpreted to
be the lag deposits left behind after aeolian deflation has removed all the finer-
grained material. Thus, under this interpretation, stone pavements can be regarded
as evidence for a former dust source. We use the word “former” here as a modifier,
because after its creation, the pavement itself serves as an armor that prevents further
aeolian erosion. Some investigators have proposed that stone pavements actually
form by aeolian accretion , rather than deflation, thus making them dust sinks rather
than sources (McFadden et al. 1987 ). It is likely that pavements can form by more
than one process, and they are not mutually exclusive. Thus, stone pavements are
not always reliable indicators of dust sources.

3.4.2 Physical Properties of Dust Deposits

Smith ( 1942 ) developed the use of physical properties of loess to identify ancient
dust sources in North America. He studied glaciogenic loess that is found southeast
of the Mississippi and Illinois rivers in the North American mid-continent. This
sediment, now called Peoria Loess, is of last-glacial age and for many decades
was hypothesized to have its origins in fine-grained glacial outwash in river valleys
draining the last-glacial-age Laurentide Ice Sheet (e.g., the Mississippi and Illinois
rivers) during the last glacial period. In a series of transects to the southeast of
both rivers, Smith ( 1942 ) reported systematic decreases in loess thickness, which he
interpreted to be the result of a reduction in sediment load in a downwind direction.
He also reported that particle size changed along the same transects, with eastward-
decreasing coarse silt (50-30 m) content and increasing fine silt and clay contents,
reflecting a winnowing of coarse particles in a downwind direction, consistent
with the thickness trend. This pioneering study clearly identified these major river
systems as the source of dust that generated Peoria Loess and has served as the
foundation for many source-to-sink studies of loess around the world. Systematic
decreases in loess thickness away from hypothesized sources have been observed
elsewhere in North America, such as Iowa and Alaska, as well as in China (see
examples in Muhs 2013b ). In the cases of Iowa and Alaska, hypothesized loess
sources are, as in the Illinois case, major outwash-bearing river systems. In the case
of China, the hypothesized immediate sources are thought to be desert basins that
occur to the northwest of the Loess Plateau region.