Loess Records - Mineral Dust: A Key Player in the Earth System - page 418

phyllosilicates, and feldspars, whereas the coarse mode is composed mostly of
quartz and feldspars (Crouvi et al. 2008 ). Recent OSL ages suggest that the upper
Pleistocene Negev loess started accumulating at 95 ka (Crouvi et al. 2008 , 2009 ).
Crouvi et al. ( 2008 , 2009 ) suggested that the main sources of the coarse silt grains
in the loess are the adjacent sand dunes in western Negev and Sinai that advanced
eastward during the late Pleistocene. These studies based their suggestion mainly
on the observation that the increase of the PSD fraction of coarse silt grains in the
loess (composed mainly of quartz) is temporally associated with the incursions of
adjacent sand dunes upwind from the loess. Elsewhere in the Middle East, loess is
found in northwestern Yemen (Fig. 16.6 ), on the volcanic plateau of Sana (Grolier
and Overstreet 1978 ; Nettleton and Chadwick 1996 ; Wilkinson 1997 ), and in the
United Arab Emirates (Goudie et al. 2000 ).
In a recent compilation, Crouvi et al. ( 2010 ) suggested that for all reported loess
in Africa and the Middle East, sand seas supply the coarse silt for desert loess,
based on the following observations: (1) all loess sites are spatially associated with
adjacent sand seas (Fig. 16.6 ) and are located only a few tens of kilometers from
sand dunes, within the range for aeolian transport of coarse silt grains (Tsoar and
Pye 1987 ); (2) there is a good agreement between the mineralogical composition
of the loess and that of the sand dunes; (3) according to the paleowind direction
interpreted from the orientation of the sand dunes, all loess sites are located
downwind of the adjacent sand seas; (4) there is evidence of decreases in grain size
downwind from the sand dunes toward the loess in a few sites; and (5) the estimated
ages of loess deposition and dune activity generally overlap, where, in most cases,
sand-dune activity preceded loess deposition, and no data indicate loess deposition
preceding sand-dune activity.
The identification of sand dunes as the source for coarse silt grains in downwind
loess deposits can support or rebut different hypotheses for the generation of silt
in deserts. Because, in most places, sand dunes are composed of well-sorted sand
grains, with limited amounts of coarse silt grains, this observation strengthens the
hypothesis raised in the past that aeolian abrasion of sand grains is one of the
important mechanisms for generating silt grains in deserts (e.g., Whalley et al. 1982 ;
Crouvi et al. 2010 ; Enzel et al. 2010 ). The reported angularity of desert loess grains
supports this hypothesis. Laboratory experiments show that aeolian abrasion of sand
grains produces both coarse and fine silt grains (e.g., Whalley et al. 1982 ; Bullard
et al. 2004 ). Given the absence of glacial activity near most low-latitude deserts,
even during the late Pleistocene, and the large sand seas there (Fig. 16.6 ), it is
reasonable that spalling might be the dominant source of silt, at least locally.

16.8.3 Asia

Loess mantles extensive regions in Asia, especially in the largest midlatitude arid-
semiarid zone in the Northern Hemisphere (Fig. 16.3 b). The most widespread loess
deposits in Asia occur in China, centered in the Loess Plateau, where the oldest loess