Loess Records - Mineral Dust: A Key Player in the Earth System - page 408

America are due to higher amounts of carbonate minerals (dolomite and calcite),
derived from dolostones and limestones that were traversed by the Laurentide ice
sheet (mid-continent) and valley glaciers (Alaska).

16.4 Genesis of Loess Deposits

Although few (if any) modern investigators doubt the aeolian origin of loess, there
is still considerable debate about the specific processes whereby silt particles are
generated before they are entrained by the wind. The issue is whether silt particles
in loess can be produced only by glacial grinding or if they can be produced
in sufficient quantities by other processes. Thus, the debate has been between
advocates of “glacial loess” versus those of “desert loess” (see reviews in Pye 1995 ;
Tsoar and Pye 1987 ; Smalley 1995 ; Livingstone and Warren 1996 ; Wright 2001a ;
Muhs and Bettis 2003 ; Muhs 2007 , 2013b ).
The “glacial loess model” proposes that silt-sized particles are produced mostly
by glacial grinding of bedrock surfaces, deposition as till, reworking by fluvial
processes as outwash, and finally entrainment, transportation, and deposition by
wind. Glaciers are efficient producers of silt, and the close geographic association
of loess with glacial terrain supports this model.
“Desert” loess is a term that has been applied to aeolian silt generated in and/or
derived from arid or semiarid regions that were not glaciated. Silt-sized particles
found in what has been called desert loess may be derived from a variety of non-
glacial processes. These processes include frost shattering, chemical weathering,
salt weathering, fluvial comminution, and aeolian abrasion and ballistic impacts
(Wright 2001b ). Examples of desert loess formed by these processes are given
below.
An often overlooked source of silt is particle inheritance from sedimentary rocks,
such as siltstones and shales. Silt is abundant in the geologic record. In fact, Blatt
( 1987 ) estimates that fully half of the detrital quartz in the world's sedimentary rocks
is comprised of silt-sized particles. Furthermore, silt-sized volcanic ash particles
form the major or at least a significant source for many of the loess deposits in South
America, Iceland, Alaska, and New Zealand. We stress that the multiple pathways
of loess particle origins are not mutually exclusive. It is likely that loess in many
regions has origins from both glacial and non-glacial processes, as well as some
silt-particle inheritance.

16.5 Loess Stratigraphy

Paleosols are common in loess sections (Figs. 16.1 and 16.2 ). In most regions,
glacial periods are dominantly times of loess deposition, whereas interglacial and
interstadial periods are dominantly times of soil formation. Thus, this alternation of