Loess Records - Mineral Dust: A Key Player in the Earth System - page 427

from the rivers and downwind of valley dust sources (Péwé 1975 ; Muhs et al.
2004 ). The transport and deposition of loess are processes that are still active
today in Alaska, particularly along the Delta, Knik, Matanuska, and Copper Rivers,
all of which drain mountain ranges with glaciers. Holocene loess is exposed in
these valleys. Although there is indirect evidence for glacial-age loess in Alaska,
there are surprisingly few ages that actually document last glacial-aged loess (see
review in Muhs et al. 2003 ). Paleosols are common in central Alaskan loess,
indicating that loess deposition here, like elsewhere, has been episodic.

16.9 Conclusion

Loess is aeolian sediment that is dominated by silt-sized particles. Unlike both
coarser dune sand and finer-grained LRT dust, loess is relatively poorly sorted,
reflecting a combination of transport processes, including saltation, low suspension,
and high suspension. Loess can be readily identified in the field and has thicknesses
ranging from a few centimeters to many tens of meters. It is found over large
areas of Europe, Asia, South America, and North America, and smaller areas of
New Zealand, Australia, Africa, and the Middle East. Most loess deposits have
compositions that are similar to and reflect derivation from the upper continental
crust. Loess can be derived from glacially ground rock and silt particles derived
from “desert” (non-glacial) processes of rock breakup or simply inherited from silt-
sized sedimentary rocks or silt-sized tephra.
Loess has distinct advantages over other Quaternary sediments for documenting
climate change in that it provides a direct record of atmospheric circulation and
can be dated directly by both luminescence and radiocarbon methods. Loess can be
used to reconstruct paleowinds using spatial trends of thickness and particle size.
Paleosols are common in loess and represent periods of little or no loess deposition.
Paleosols and snails in loess can be used to reconstruct paleoclimates.
In most of Europe and many parts of North America and South America, much
loess is clearly tied to glacial-sediment supplies. In both Europe and North America,
however, rates of loess deposition varied within glacial periods. In many areas, the
stratigraphic record is more complex than a simple correlation of loess with glacial
periods and paleosols with interglacial periods. In China, neither loess deposition
nor soil formation ever ceases completely, so sedimentation and pedogenesis are
best portrayed as competing processes that are modulated by climate change. Still,
glacial periods show higher rates of loess deposition than interglacial periods. In
the Great Plains of North America, non-glacial sources of loess were available
during both glacial and interglacial periods, but much greater rates of deposition
occurred during glacial periods, indicating that a glacial-type climate is one that is
most favorable to thick loess accumulation.