Geoscience Reference
In-Depth Information
within the paleosols has provided supporting evidence of the changing mosaic of
habitats in which the early hominids evolved (WoldeGabriel et al.,
2009
).
Amit and her co-workers have investigated soil-forming processes in the hyper-arid
southern Negev Desert and have used the insights gained from this work to analyse
seismic hazard and earthquake frequency in the Arava Valley section of the Dead Sea
Transform Fault (Amit et al.,
1999
). Her more recent work has made innovative use
of the desert Reg soils of the southern Negev to show that this region has remained
hyper-arid throughout much of the Quaternary, contrary to earlier speculations (Amit
et al.,
2006
). Finally, she and her co-workers have demonstrated that the coarser silt
particles in the primary loess deposits preserved on isolated ridge tops in the Negev
were most probably derived from a proximal dune source and fashioned by wind
abrasion (Crouvi et al.,
2008
).
Indeed, wind-blown dust plays a major role in the formation of desert soils, and it is
hard to overestimate its importance (Pye,
1987
; Liu et al.,
1985
;Liu,
1987
; Williams
et al.,
1991a
). Dust accumulates on the surface of stony deserts and especially in the
crevices between individual stones. Some of this dust is then washed down into the
underlying substrate, provided that the substrate is sufficiently porous. Over time,
a layer of silty clay develops above the bedrock surface and becomes progressively
thicker with age. If the desert dust is calcareous, as is often the case, successive
cycles of leaching, solution and precipitation of dissolved carbonate will result in the
formation of pedogenic carbonate within the subsoil. This layer is often protected
from wind and water erosion by a desert pavement, defined as 'armoured surfaces
composed of angular or rounded fragments, usually one or two stones thick, set on
or in matrices of finer material comprising varying mixtures of sand, silt or clay'
(Cooke et al.,
1993
, p. 68). Desert pavements can form in a variety of ways or through
a combination of processes. Winnowing of finer particles from the surface by wind
or running water can lead to the formation of a protective layer of surface stones
sometimes called a lag gravel. Another process involves the upward displacement of
stones as a result of expansion and contraction of the soil, usually caused by wetting
and drying, a process that gradually moves the larger particles to the surface. The
rocks forming the desert pavement will ultimately break down as a result of a variety
of chemical and physical processes, including frost shattering, salt weathering and
chemical attack from snails and lichens. Amit et al. (
1993
) have identified a series of
distinct stages in the shattering of the surface gravels of desert pavements as a result
of salt weathering.
Soils formed on alluvial terraces in deserts often contain a mixture of parent
materials. In the central Pilbara region of north-west Australia, the bedrock consists of
Precambrian ironstone that has undergone multiple cycles of weathering and erosion.
The Little Sandy Desert lies some 200 km to the east - too far to be a direct source
of wind-blown sand. However, during times when the prevailing easterly winds were
