Geoscience Reference
In-Depth Information
timescales over which this occurs can be studied using
several dating methods. Continuing accretion in carbon-
ate horizons can result in their upper surface progres-
sively approaching the ground surface, over substantial
fractions of Quaternary time. These time-related develop-
ments have been studied with a view to using the depth
and thickness of carbonate layers to estimate the age of
desert surfaces within which such accumulations occur
(e.g. Marion, 1989). The voluminous carbon storage in
soil carbonates of the drylands is a significant component
of the global carbon cycle (Kraimer, Monger and Steiner,
2005), and this provides yet another example of the way
in which desert soils exert an influence that extends well
beyond the drylands themselves.
The detailed history of most desert soils remains un-
known, and is likely to be complex. However, the present-
day morphology of such soils has been studied in greater
detail. We will turn now to consider some of the key
morphologic features found in many desert soils that are
important in setting their place in the hydrologic and
geomorphic functioning of the landscape. The features
present often constitute a mixture of ages, some being
rather young and some quite old. Desert soils are truly
features that are
polygenetic
, forming a palimpsest of past
events.
(a)
(b)
7.4
Stone-mantled surfaces and
desert pavements
(c)
Many desert soils carry a surface veneer of stones, often
coated with a desert varnish containing at least some al-
lochthonous (foreign) components (Figure 7.1). The stone
veneer often overlies a stone-free subsoil. Dregne (1976,
p. 42) argued that stone mantles were usually the result of
the removal of fines by wind or water, leaving the gravel
as a
lag
deposit. It appears, however, that the stones can be
concentrated at the surface by other means. A mechanism
now widely envisaged is that windblown dusts, settling
on to a desert surface, are washed down between surface
stones, perhaps passing into the regolith along desicca-
tion cracks (see Chapter 19). Weathered rock debris is
thus kept exposed and continually rides to the top of the
accumulating soil materials, being itself too large to be
washed into soil crevices. Thus, far from signifying wind
erosion, desert stone mantles may reflect quite the oppo-
site, a local
accumulation
of significant amounts of mate-
rial; the soils may thus deepen with time in a process that
has been termed 'cumulic pedogensis' (McFadden
et al.
,
1998; Gustavson and Holliday, 1999; Ugolini
et al.
, 2008).
Figure 7.1
Desert pavement capping soils in arid northern
South Australia: (a) pavement of well-sorted stones with sig-
nificant exposure of silts; (b) pavement of poorly sorted stones
resulting in very high areal coverage and little exposed fine
soil material; (c) view across arid desert pavement to residual
hills in the distance. Scattered patches of grass can be seen,
marking locations where the pavement is less complete, and
where water infiltration is possible.
