Loess Records - Mineral Dust: A Key Player in the Earth System - page 409

glacial times of loess deposition and interglacial times of soil formation has been
correlated with the deep-sea oxygen isotope record of glacial-interglacial cycles.
In general, this model has validity, but when loess records are examined in more
detail, the stratigraphy is rarely simple. Perhaps a more realistic way to view the
system is to consider loess sedimentation and soil formation as competing processes.
When loess sedimentation rates are high, pedogenic processes cannot keep up, and
relatively unaltered sediment accumulates (Verosub et al. 1993 ; Muhs et al. 2004 ).
In contrast, when loess sedimentation rates are low, soil-forming processes extend
deeper into previously deposited loess. Thus, whereas in deep-sea or lacustrine
sediments, a case can be made for more or less continuous sedimentation, loess-
paleosol sequences are more complex systems, and distinguishing between sediment
and soil is not always an easy task. Examples of loess-paleosol sequences are
provided below.

16.6 Loess Geochronology

Precisely because sedimentation is not continuous in loess-paleosol sequences,
numerical dating is essential in loess stratigraphy. A particularly illuminating
example of this is the study by Stevens et al. ( 2007 ), where an extensive suite of
luminescence ages demonstrates that many loess sequences in China have been
affected by nonconstant sedimentation rates, diagenesis, bioturbation, and erosion.
Muhs et al. ( 2003 ), using a combination of radiocarbon ages, tephrochronology,
and inventory 10 Be methods, show that loess sequences in Alaska contain numerous
unconformities. Today, the most commonly used methods in dating loess are
paleomagnetism, luminescence geochronology, radiocarbon dating, and magnetic
susceptibility (see review in Muhs ( 2013b )).

16.7 Paleoclimatic and Paleoenvironmental Interpretation

of Loess Deposits

Much valuable paleoenvironmental information can be obtained from loess-
paleosol sequences, particularly because loess covers large areas of most continents
(Figs. 16.3 , 16.4 , 16.5 ,and 16.6 ). Loess properties can change over a landscape, and
these variations can yield important clues about the paleowinds that deposited the
loess. Thickness of loess, particle size, and carbonate content, in general, decrease
away from sources (Liu 1985 ;Porter 2001 ; Bettis et al. 2003 ; Muhs and Bettis
2003 ; Muhs et al. 2004 , 2008 , Muhs 2013b ). Reduction in sediment load downwind
from a source is inferred from decreases in loess thickness. Paleowinds can also be
inferred from the decrease in mean particle size away from a source, reflecting a
winnowing of the coarse load. With decreases in loess deposition rate away from a