Geoscience Reference
In-Depth Information
The extreme nature of arid environments has further,
more complex dimensions that affect weathering. For ex-
ample, while moisture is highly limited in dryland envi-
ronments, evaporation and capillary rise are as important
as precipitation and runoff (especially within hot arid en-
vironments). This means that rocks and minerals at the
Earth's surface in arid environments are exposed to very
complex moisture movements in comparison, for exam-
ple, with those in humid tropical environments where
downward leaching of weathered material dominates. In
arid environments there is more commonly a two-way sys-
tem of water movements within soils and rock outcrops,
i.e. 'up and down' or 'in and out' - to put it more simply,
but rather crudely. Furthermore, moisture inputs to weath-
ering systems in arid environments do not only take the
form of rainfall, and thus quantifying moisture availabil-
ity can be much more complex than simply monitoring
annual rainfall amounts. For example, within coastal sub-
tropical deserts, such as the Atacama Desert in Peru and
Chile, South America, and the Namib Desert in Namibia,
Africa, fog can be a very important source of moisture
to many rock surfaces. In many desert environments con-
densation and dew may also be important, while widely
within arid environments groundwater contributes to the
moisture available for weathering. Quantifying these mul-
tiple sources of moisture and their transformations and
movements over time and space can be highly difficult,
especially given the sparse meteorological data available
on the ground in many arid environments. Within cold
arid environments, the presence of water in the form of
ice, and the possibility for transformations of water from
solid to liquid or gas phases, makes things even more
complicated.
The extreme nature of arid environments is also char-
acterised by nonequilibrium conditions in climate, vege-
tation and geomorphology. In terms of climate, nonequi-
librium conditions are visible in the high interannual and
interdecadal variability within arid zone climates (Viles
and Goudie, 2003). For vegetation, nonequilibrium condi-
tions have been recognised by ecologists, who have iden-
tified bi- or multiple-stable states in vegetation systems
as a response to changing climatic conditions. For exam-
ple, a range of studies have shown that arid and semi-
arid vegetation systems can oscillate between patchy (of-
ten patterned) and bare conditions depending on rainfall.
Such bi-stable or multiple-stability conditions can also be
identified in arid environment geomorphic systems, which
can, for example, flip between aeolian and fluvial process
regimes (Bullard and Livingstone, 2002). These nonequi-
librium conditions will have a strong effect on weathering
systems. For example, runs of dry and wet years will in-
fluence the dominant weathering processes and their rate
of operation at any one arid site. Furthermore, extrap-
olations of short-term monitoring studies of weathering
processes to the longer term might be quite misleading
if they were undertaken, for example, during extreme El
Nino conditions.
Arid environments also possess huge spatial diversity of
environmental conditions within and between areas. Thus,
some deserts are much drier than others, while within any
one desert area some individual sites may be much drier
than others. For example, across the Namib Desert from
west (coastal) to east (inland) annual rainfall amounts rise
from under 20 mm per year along the coast at Swakop-
mund to almost 50 mm per year at Ganab (over 100 km
inland), while fog receipt drops in reverse from around
35 mm a year at the coast to around zero 100 km inland
(Viles, 2005). Variability does not only relate to climatic
conditions, but also includes differences in environmen-
tal history, tectonic setting and geology. Some deserts are
of much greater antiquity than others, some have experi-
enced quite complex swings between arid and wetter en-
vironmental conditions and so on. Tectonically there are,
for example, huge differences between the Atacama and
Namib deserts, which, in broad climatic terms, are other-
wise quite similar. Most deserts exhibit a broad range of
rock types - with, for example, the Namib Desert having
exposures of basalt (in the north), marble and granites (in
the central areas) as well as a range of schists and other
rocks. Such spatial variability makes it very difficult to
make any meaningful generalisations about the nature,
tempo and geomorphological importance of weathering
systems within the arid zone. High spatial variability in
many deserts can make extrapolations from one place to
another only tens of km away difficult (Viles and Goudie,
2000).
6.3
Theoretical underpinnings of
weathering systems research
Despite the hostility of the desert environment, a large
number of studies have been carried out on weathering in
areas as diverse as the southwestern USA, the Sahara, the
Namib, the Atacama and the Negev ever since the earli-
est explorers visited. For many years weathering research,
in arid and other environments, fit broadly speaking into
the CLORPT framework, which identified the key con-
trols on weathering (and soil development) as climate,
organisms, relief, parent material and time (e.g. Bruns-
den, 1979). Quantification of those factors was thought to
