Loess Records - Mineral Dust: A Key Player in the Earth System - page 426

of Canada (Muhs et al. 2003 ). There are also large tracts of loess in the Palouse
area of eastern Washington and adjacent Oregon and the Snake River Plain and
adjacent uplands of Idaho. By far, however, the greatest extent of loess is found in
the mid-continent region, including the greater Mississippi River drainage basin and
the Great Plains (Fig. 16.4 a).
In the greater Mississippi River basin (including the Missouri and Ohio river
drainage basins), south of the Laurentide ice sheet, loess is dominantly glaciogenic,
although there are non-glacial contributions (Grimley 2000 ; Bettis et al. 2003 ).
Thus, loess units represent glacial periods and paleosols represent interglacial or
interstadial periods. To a great extent, therefore, loess deposition is a “turn-on/turn-
off” process that differs from the more complex pattern seen in China. In this region,
the three youngest loess units are the Loveland Loess of penultimate glacial age, the
Roxana Silt of early last glacial age, and the Peoria Loess of late last glacial age.
Dating of Peoria Loess to the last glacial period, about 25,000-12,000 radiocarbon
years ago, is based on studies conducted at dozens of sections (Bettis et al. 2003 ).
Peoria Loess can reach extraordinary thickness, up to 40 m in some sections in

western Iowa (Bettis et al. 2003 ).
Farther west, in the Great Plains region, studies by Aleinikoff et al. ( 2008 )
and Muhs et al. ( 2008 ) show that sedimentary rock (volcaniclastic siltstone) is the
most important source of silt-sized particles in the Peoria Loess of last glacial age
(Fig. 16.2 a). Despite the different modes of origin of Great Plains loess, radiocarbon
and OSL ages indicate that Peoria Loess in the Great Plains was deposited at
about the same time as Peoria Loess farther east (Roberts et al. 2003 ; Muhs et al.
2008 ). Thus, even without glaciers, the Great Plains region experienced favorable
conditions for loess formation during the last glacial period. Unlike the Mississippi
River basin, however, the Great Plains region also has a younger Holocene loess
called the Bignell Loess.
The loess deposits in the Columbia Plateau, or “Palouse” region (Fig. 16.4 a),
present a very different record than those of other parts of North America. In this
area, loess records may extend as far back as 1-2 Ma (Busacca 1991 ). The loess
deposits preserve many paleosols (Fig. 16.2 b), reflecting periods of nondeposition
and stability between times of more active loess deposition. The timing of the
deposition of loess in the Palouse region of eastern Washington differs from that
for other loess deposits of North America and, indeed, the world (Busacca et al.
2004 ). In this area, the principal source of the loess is believed to be flood sediments
of proglacial Lake Missoula, exposed during glacial maxima at least six times in
the Pleistocene (Busacca et al. 2004 ). Thus, the production of the loess deposits is
believed to peak late in the glacial cycle.
Loess is widely distributed in Alaska and adjacent Canada and is geographically
the most extensive surficial deposit in the region (Fig. 16.4 a). In central Alaska,
studies by Westgate et al. ( 1990 ) indicate that the Alaskan loess record may extend
back to 3.0 Ma. The thicknesses of the deposits range from a few centimeters in

some areas to more than 60 m near Fairbanks. Throughout Alaska, loess deposits
are thickest near rivers, with thicknesses decreasing rapidly with distance away