Climatic frameworks: legacies from the past - Arid Zone Geomorphology - page 41

many areas, including the western and southern interior,
showed marked aridity between 25 000 BP and 16 000 BP,
through to 12 000 BP in the arid core (Bowler and Was-
son, 1984). The picture is, however, far from clear-cut
and is much more complex than was previously posited.
Dune activation occurred in punctuated phases through-
out the last glacial, often separated by periods of stability
and soil formation (Nanson et al. , 1988, 1992; Nanson,
Chen and Price, 1992; Fitzsimmons et al. , 2007). OSL
and radiocarbon-dated regional wetlands (Williams et al. ,
2001; Williams, Nitschke and Chor, 2006; Williams and
Nitschke, 2005) and flood deposits (Haberlah et al. , 2010)
in the now semi-arid Flinders Range occurred during the
period

ern hemispheres. Their calculations suggest that, during
the late glacial, climatic belts in both hemispheres could
have been displaced southwards. This lends support to
the views of Tricart (1956), Newell (1973) and Lancaster
(1979), who suggested that, over Africa, the meteorologi-
cal equator (the intertropical convergence zone) may have
been displaced south of the equator during the last glacial.
As well as latitudinal shifts in the hemispheric circulation
systems, longitudinal covariance with ice volume has also
been proposed. For example, in southern Africa, present
day arid zone expansions are closely linked to expanded
westerly circulation associated with intensified blocking
anticyclone conditions inhibiting the penetration of moist
easterly air from the Indian Ocean to the southern African
interior (Tyson, 1986; Thomas and Shaw, 2002).
Increased sophistication of climatic models have al-
lowed predictions of the theoretical configuration of late
glacial circulation (Nicholson and Flohn, 1980) to be
tested by climate simulations that can include complex
interactions of forcings and feedbacks within the envi-
ronment. The assumed mirroring of conditions in the
tropics north and south of the equator (Figure 3.3) was
based primarily on the assumption that this would be
achieved through the reduction of interhemisphere tem-
perature contrasts at times of maximum ice expansion.
In contrast, some model outputs predict a hemispheri-
cally asynchronous pattern of aridity in response to or-
bital controls on insolation, which in turn controls the
intensity and penetration of monsoon rain-bearing sys-
tems. A growing body of empirical evidence supports
this (e.g. Partridge et al. , 1997). However, the potential
for large-scale reorganisation of circulation systems and
differing regional responses to persistent suborbital scale
events such as Dansgaard-Oeschger and Heinrich events
has also been demonstrated using coupled-ocean atmo-
sphere GCMs (e.g. Thomas et al. , 2009).

35-16 ka, reflecting the complex response of the
environment to climatic change.
The severity and extent of India's arid regions have
been variable over time and heavily influenced by precip-
itation from both the southwest monsoon during summer
and the penetration of westerly disturbances in the winter
(Pant and Kumar, 1997). OSL ages on fossil dunes (e.g.
Chawla, Dhir and Singhvi, 1992; Singhvi et al. , 1994)
and fluvial systems (e.g. Kar et al. , 2001) as well as the
lacustrine system response (Enzel et al. , 1995) have made
a significant contribution to the reconstruction of fluctu-
ating environmental conditions through the last glacial.
Luminescence dates from relict and active dunes within
the Thar Desert have been used to interpret major periods
of dune construction at 115-100 kyr,

∼

∼ 75 kyr, ∼ 55 kyr,

∼ 30 kyr and 11-13 kyr (Singhvi and Kar, 2004; Singhvi

and Porat, 2008). Dune-building activity in this region has
been associated with increased windiness preceding the
SW monsoon (Wasson, 1983). Singhvi and Porat argue
that when the monsoon is weakened, as is hypothesised
for the LGM, winds were weaker and there was minimum
aeolian transport. Aeolian activity only resumed with in-
creased intensity of the monsoon and its associated winds
at around 14 kyr. At some point shortly after this, in-
creased moisture from an established monsoon decreased
the erodibility of sediment, so reducing dune accretion.
Initial evidence from north Africa and Australia
(Williams, 1975, 1985; Rognon and Williams, 1977) or
from oceanic cores (Sarnthein, 1978) suggested that trop-
ical aridity was the norm during the late glacial. For ex-
ample, Sarnthein (1978) has written that: 'Today about
10 % of the land area between 30 ◦ N and 30 ◦ Siscov-
ered by active sand deserts

3.5 Conclusions

There are many issues that remain unresolved in the re-
construction of the extent of past arid zones. Some of these
concern the evaluation of morphological and sedimento-
logical evidence. Bowler and Wasson (1984) illustrated
how unusual aspects of the relict arid zone landscapes of
Australia could be better understood when explanations
incorporated an awareness of the environmental condi-
tions preceding a climatic change. This concept of 'geo-
morphological inheritance' (Bowler and Wasson, 1984)
also illustrates that the direction of an environmental
change influences the geomorphological response. Knox

... [These were] much more

widespread 18 000 years ago [characterising] almost

50 % of the land area between 30 ◦ N and 30 ◦ S, forming
two vast belts.'
The analysis of palaeotemperature data by Harrison
et al. (1984) has indicated that there were considerable

...