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towards Lake Eyre in the heart of the arid zone, losing most of its discharge en route.
Cohen et al. (
2010a
) concluded that enhanced flows were evident in the Cooper at
120-100 ka, 85-80 ka, 28-18 ka and during the early to mid-Holocene. The first
three of these phases were associated with the more or less synchronous formation
of source-bordering dunes. The dated stratigraphic profiles of the dunes indicate
formation by vertical accretion of sediment, with little sign of downwind accretion
and no evidence of long-distance transport of dune sand. Particularly interesting is
the strong evidence of enhanced river flows immediately before and during the Last
Glacial Maximum - a time widely considered as having been both very cold and very
dry throughout Australia.
Maroulis et al. (
2007
) had earlier obtained a long luminescence chronology of
alluvial sand transport along Cooper Creek from Marine Isotope Stages (MIS) 8 to 3,
showing a trend of progressively declining discharge during that time. Peak source-
bordering dune activity dated to late MIS 5 (around 85-80 ka) and mid-MIS 3
(50-40 ka), after which the flood-plains became mantled with mud and dunes became
islands in a sea of mud, much as in the Gezira alluvial plain of central Sudan. Nanson
et al. (
2008
) dated the alluvial deposits in the lower 500 km of Cooper Creek and
found multiple episodes of enhanced flow during the last quarter of a million years.
There was a progressive reduction in discharge until aridity became severe around
40-35 ka and source-bordering dunes ceased to form.
In northern Australia, the sediments associated with two relict plunge pools indicate
extreme floods just before the LGM and during the early to mid-Holocene, with flood
discharges up to five times greater than those of any floods in the last 40,000 years
(Nott et al.,
1996
; Nott and Price,
1999
). Extreme rainfall can therefore occur during
both cold and warm climatic phases.
In the upper Shoalhaven River Valley in south-east Australia, Nott et al. (
2002
)
dated four sets of paired alluvial terraces spanning about the last 500 ka. They found
a lack of fluvial activity during the cold, dry LGM and an overall decrease in fluvial
activity after 60 ka, consistent with an increase in regional aridity.
22.6.6 Lake fluctuations
The salt lakes of central and western Australia originated as a well-integrated Late
Cretaceous to early Cenozoic drainage system that had become disrupted by the
mid-Miocene as a result of successive tectonic movements (Bunting et al.,
1974
;
VandeGraaffetal.,
1977
; Salama,
1997
; Zheng et al.,
1998
; Zheng et al.,
2002
).
The resulting lakes dried out progressively, some attaining peak aridity by around
500 ka (Zheng et al.,
1998
), when gypsum began to be widely deposited. Optical
dating using green-light stimulated luminescence and electron spin resonance dating
of relic gypsum dunes associated with two such lakes in south-west Australia showed
intensified eolian deflation during the last glacial period, lunette construction during
