Loess Records - Mineral Dust: A Key Player in the Earth System - page 410

source, there is often syndepositional leaching, reflected in a decrease in carbonate
content. Thus, if a hypothesized loess source were a north-to-south-trending river
valley, a decrease in loess thickness and mean particle size to the east of the river
would imply northwesterly, westerly, or southwesterly paleowinds.
In contrast to unaltered loess itself, loess-hosted paleosols frequently yield
valuable information about interglacial or interstadial periods. Measurement of
magnetic susceptibility and other mineral magnetic properties has been one of the
most common approaches to interpreting paleoclimate from loess-hosted paleosols
(Verosub et al. 1993 ; Maher et al. 1994 ;Porter 2001 ; Porter et al. 2001 ; Singer and
Ve r o s u b 2007 ). Unfortunately, one problem that arises is that magnetic susceptibility
in modern, loess-derived soils in China is partly a function of particle size and
sediment accumulation rate (as a dilution effect), as well as climate. Porter et al.
( 2001 ) show that both of these factors are spatially variable but highly correlated
with one another (and climate) across the Chinese Loess Plateau. Another approach
is to examine the degree of chemical weathering in loess-derived paleosols, as this
reflects past climate and vegetation (Muhs 2007 ).
Loess lacks many of the Quaternary paleoecological indicators that are com-
monly used in lacustrine or marine sediments, such as pollen, diatoms, ostracodes,
radiolaria, or foraminifera. Furthermore, it is rare for mammalian fossils to be
preserved in loess, although Alaska (USA) is an important exception to this
generalization (Péwé 1975 ). Fortunately, it is common for the shells of land snails to
be preserved in loess, and they are abundant in China, Europe, and North America
(see Liu 1985 ; Rousseau 1991 ; and Rossignol et al. 2004 for examples). Most or all
of these snails are extant species, and their modern zoogeography is reasonably
well established. Thus, it is possible to infer past climates during the times of
loess deposition by identification of extralimital taxa, i.e., those species that do not
presently live at a locality where they are found as fossils.

16.8 Global Loess Deposits

16.8.1 Europe

Loess is extensive over much of Europe (Fig. 16.3 a; see also Haase et al. 2007 ;
Rousseau et al. 2007a ). Although there is no source for loess at present, during the
last glacial period, potential sources included glaciogenic silt from the ice sheets
on Ireland, the United Kingdom, and Scandinavia, plus smaller ice caps at lower
latitudes on mountain ranges such as the Alps, the Pyrenees, and the Vosges. In
addition, the continental shelf areas of the present English Channel, Celtic Sea, and
the North Sea were subaerially exposed due to lowered sea level and were potential
dust source areas.
European loess deposits are distributed over three main zones (Fig. 16.3 a).
Eastward to about longitude 15 ı east, loess is found mostly (and is thickest) in