Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 52

Critical to understanding the importance of dust in all aspects of the Earth's
physical and biological systems is identification of the sources of dust. Knowledge
of dust sources is the fundamental first step in recognizing the entire pathway of
dust entrainment, transport, and deposition. Because dust has importance in so
many scientific disciplines, a surprisingly diverse set of tools has been generated for
identification of dust sources. In this chapter, we review what is known about global
dust sources, but with a particular emphasis on methods of identifying dust sources,
both directly and indirectly. With this background, we review the geography of
present dust sources and in addition, we review methods of identifying dust sources
from the geologic record. Finally, we discuss how dust sources can change over
time, with an emphasis on such changes over geologic time scales.

3.2 Processes of Dust Particle Formation

In order to provide the context for understanding dust sources, it is important
to consider those processes that generate dust-sized particles (Kok et al. 2012 ).
Formation of dust-sized mineral particles can be classified broadly into four
groups of processes: (1) volcanogenic, (2) physical mechanisms of coarse particle
reduction, (3) chemical mechanisms of coarse particle reduction, and (4) inheritance
from fine-grained rocks. Much of what we observe as mineral dust in the atmosphere
is produced by coarse particle size reduction either by physical or chemical means.
Loess, which is sediment dominated by aeolian silt and clay, has long been
considered to be produced primarily by physical reduction of coarse particles
through glacial grinding. Prime examples of glaciogenic silt production can be
found today where glaciers still exist, such as in Svalbard, Iceland, Canada, and
Alaska (Bullard 2013 ). Satellite images often capture dust plumes emanating from
these sources (Crusius et al. 2011 ;Prosperoetal. 2012 ). Bullard ( 2013 ) gives an
excellent overview of modern glaciogenic dust-forming environments that are good
analogs for what was a much more important process in the last glacial period.
It has been observed that there is little loess that accumulates in non-glacial
settings, such as desert regions (Tsoar and Pye 1987 ). Thus, some investigators
have proposed that glacial grinding is the only efficient means by which silt-sized
particles can be produced (Smalley 1966 , 1995 ). Nevertheless, non-glacial loess has
been identified in many desert regions (Coudé-Gaussen 1987 ; Crouvi et al. 2010 ).
Furthermore, experimental work has shown that although glacial grinding is indeed
an efficient method of silt particle production, other processes, including fluvial
and aeolian abrasion, are surprisingly effective mechanisms; silt particles can also
be produced by frost and salt weathering (Smith et al. 2002 ; Wright and Smith
1993 ; Wright et al. 1998 ; Wright 2001 ). In many of the world's desert regions,
where there is no history of glacial erosion, fluvial comminution in stream valleys,
aeolian abrasion and ballistic impacts in dune fields, and salt weathering in playas
and on alluvial fans have all been shown to produce silt-sized particles (Fig. 3.1 ).