Geoscience Reference
In-Depth Information
in alkalinity (Dorn and Oberlander, 1982). In the cen-
tral Sahara, multilayer rock varnish occurs despite the
presently hyper-arid conditions that are not conducive to
the biogeochemical processes of Mn enrichment (i.e. low
alkalinity and limited fluctuations in Eh and Ph) necessary
for varnish formation (Cremaschi, 1996, 2002; Zerboni,
2008). Under these conditions the Mn-rich varnish is a
relict of differing conditions in the past and its notable
mineralogical differences within the microlayers reflects
a shift from humid to arid conditions during the Holocene
Period (Zerboni, 2008).
The micromorphology of rock varnish may also yield
information on past periods of aeolian activity (Dorn,
1986). When aeolian dust is abundant, the assimilation
of particles into the varnish results in a structure of par-
allel microplatelets, but, with less dust in the atmosphere,
accumulation occurs around distinct nuclei (Dorn and
Oberlander, 1982). The superimposition of different mi-
cromorphologies can therefore result from changes in ae-
olian activity through time. For example, in the southwest-
ern United States rock-varnish surfaces record periods of
dustiness alternating with less dusty episodes, the latter
coinciding with high lake-level stages (Dorn, 1986).
The stable isotope content of organic material incor-
porated in rock varnish is a further palaeoenvironmental
data source. Dorn and De Niro (1985) found a significant
correlation between
13
C content of varnish and moisture
in the environment, with Dorn, DeNiro and Ajse (1987)
using
13
C changes in varnish layers to deduce palaeocli-
matic impacts on alluvial fan development.
palaeoclimatic signals in fluvial sediments and forms (e.g.
Frostick and Reid, 1989; Reid, 1994; Tooth, 2005). Arid
conditions may favour channel aggradation. Reduced
rainfall, often accompanied by a marked seasonality,
means diminished stream power and the clogging up of
channels with relatively coarse sediment, derived from
slopes, because of reduced vegetation cover. Conversely,
wetter conditions favour incision and terrace develop-
ment. The terraces and sediments of the Nile have been
interpreted in such a way by Adamson, Gillespie and
Williams (1982), and the evidence used to indicate the
Late Pleistocene extension of arid conditions. In Ama-
zonia, Andean headwaters also aggraded during the Late
Pleistocene arid phase (Baker, 1978), but lowland rivers,
including the Amazon itself, incised their courses because
sea levels were lower (Tricart, 1975, 1984). One of the
biggest problems arrives because irregular flow in arid
regimes rarely allows equilibrium forms to develop (Nan-
son, 1986; Tooth and Nanson, 2000; Tooth and McCarthy,
2006) and therefore diagnostic forms are rare. Flow vari-
ability and the ability of rare high magnitude events to
conduct significant geomorphic work can also compro-
mise interpretation. Chapters 12 and 13 explore these
issues.
Under semi-arid and arid conditions, when vegetation
cover is sparse, sediment transported may be more active
on slopes than in channels. This may favour the accu-
mulation of thick, lower-slope colluvium aprons. Such
deposits have been identified in southeastern Africa and
interpreted as evidence of widespread Late Pleistocene
semi-arid conditions (Watson, Price Williams and Goudie,
1985; Thomas, 2011).
Alluvial fans may also act as buffers between arid zone
slope and fluvial systems, with their sediments providing a
valuable record of environmental changes in source areas
(see Chapter 14). The proposition that alluvial fans ag-
grade more favourably under conditions of reduced veg-
etation cover has been made by authors such as Dorn
(1988) and Blair, Clark and Wells (1990). However, the
palaeoenvironmental interpretation of fan and slope de-
posits may be as contentious as that of channel features
and sediments (Dorn, 1994), although systematic analy-
sis of sediment changes and inferred flow variations may
enhance interpretations (e.g. Harvey and Wells, 1994).
Most apparently diagnostic, in geomorphic terms, of
arid zone expansions are sand seas comprising now-
stabilised dune systems. Dune inactivity is inferred
through a range of criteria, with vegetation cover most
widely used, that lead to dunes being designated as relict
or fossil forms (cf. Flint and Bond, 1968). The identifica-
tion and mapping of inactive dune systems has played
3.2.7
Geomorphological evidence
of arid zone change
3.2.7.1
Arid zone extension
Despite caveats, Fairbridge (1970, p. 99) suggested that
'inherited landforms' are among the most diagnostic evi-
dence of past climates. Table 3.4 lists landforms that have
been used to indicate former extensions of arid and semi-
arid conditions. Fluvial systems provide some of the most
debatable and complex lines of morphological evidence
of former aridity, sometimes indicating dramatically os-
cillating wet-dry climate during the Quaternary (Nanson
et al.
, 2008). The complex controls on tropical water-
shed fluvial fluxes in the Late Quaternary have recently
been reviewed by Thomas (2011). Morphological and sed-
imentological evidence are strongly coupled, with channel
fills and terrace sequences providing the information of
past flow conditions. However, as channel flow responds
to a range of controls including, for example, climatic
