Geoscience Reference
In-Depth Information
Increasing precipitation and vegetation cover
Increasing leaching and soil development
Prevailing wind
Remnant
primary loess
Colluvially
reworked
loess
Primary
loess
Desert source
of sand
and dust
Colluvially
reworked loess
1
2
3
Active
sand dunes
Bare
rock
Zone of no accumulation
Zone of largely colluvially reworked loess
Zone of primary loess with
some colluvial reworking
Hyper-arid
Sub-humid
Arid to semi-arid
Not to scale
Climate
fluctuations
1
2
3
Modern soil
Modern soil
Carbonate
horizon
Clayey
weathered
loess
Massive
loess
Massive
coarse
loess
Palaeosol
Thick
carbonate
horizon
Massive
loess
Figure 17.10
Schematic model of the distribution of peridesert loess in the landscape (modified after Pye and Sherwin, 1999).
episodic fluvial transport (McTainsh, 1987; Assallay
et al.
,
1998; Smith, Wright and Whalley, 2002).
The limited extent of low-latitude peridesert loess is
now sometimes attributed to the lack of suitably vegetated
dust-trapping surfaces in these areas; thus most of the dust
escapes into the atmosphere and is available for wider
global dispersal, including into oceans (Tsoar and Pye,
1987). It may also be due to a more prosaic reason: it
has not been properly identified in the field (McTainsh,
1987; Hesse and McTainsh, 2003), partially because it is
incorporated into soils as dust production and weathering
tends to occur at similar rates, limiting opportunities for
blanket deposits to develop (Vine, 1987).
Pye and Sherwin (1999) have produced a schematic
model of the characteristics of peridesert loess (Fig-
ure 17.11). They distinguish nondepositional zones on
to trap dust: primary dust accumulation settings in vege-
tated zones of higher relief and, due to its susceptibility to
reworking, areas of colluvially reworked loess on downs-
lope locations.
17.5
Dynamic aeolian landscapes in the
Quaternary period
Environmental change during the Quaternary period has
contributed greatly to the nature of today's deserts and
drylands (Chapter 3). Sarnthein's (1978) hypothesis of
expanded aridity during glacial times (Figure 17.11) was
based both on an analysis of aeolian sediments in marine
cores and on identifying the extent of currently 'fixed'
sand dunes on the margins of today's active sand seas. This
