Geoscience Reference
In-Depth Information
10 m, covered about 35,000 km
2
and,
including the water in the Lake Frome complex, had a volume of about 430 km
3
,in
contrast to the historic maximum of only 30 km
3
(DeVogel et al.,
2004
).
The LGM in Australia was a time when the lakes in the seasonally wet tropics of
northernAustraliaweremostly dry (English et al.,
2001
) andwhen Lake Eyre in central
Australiawas dry and its bedwas being lowered by deflation (Magee et al.,
1995
;Croke
et al.,
1996
; Magee and Miller,
1998
; Magee et al.,
2004
). However, the interpretation
of the lake level fluctuations is not as straightforward as it might seem. At any one time,
the lakemay have been fed in part fromsurface run-off, in part fromgroundwater influx
and in part from direct precipitation, with the relative proportions of each varying over
time. Another complicating factor concerns the role of groundwater-limited deflation
in controlling lake basin shape, volume and subsequent shoreline levels during ensuing
lake transgressions. If the long-term export of sediment through deflation exceeded
the long-term replenishment of lake floor sediment, successive lake levels would be
lower, although the net input of water might not have diminished. During the course
of several glacial-interglacial cycles, with deflation especially active during the drier
phases, the cumulative deepening of the lake floor may result in progressively lower
lake shorelines, giving the possibly misleading impression that earlier interglacials
were hydrologically more effective than later ones (Williams,
2001a
). A way to test
this is to use independent evidence to reconstruct past changes in precipitation. To this
end, Johnson et al. (
1999
) analysed the carbon isotopes in fossil emu and
Genyornis
eggshells from around Lake Eyre. They found significant changes in the proportions
of C
4
to C
3
grasses over the last 65 ka. They concluded from their analyses that the
Australian monsoon was most effective between around 65 and around 45 ka, least
effective during the LGM and moderately effective during the Holocene, all of which
supports the reconstructed lake levels.
Bowler et al. (
2011
) have refined their earlier interpretation of the Willandra Lakes
in semi-arid western New South Wales. These lakes function as a cascading system
(
Chapter 11
,
Figure 11.6
) of lakes fed from Willandra Creek, a distributary channel
of the Lachlan River, which has its headwaters in the Eastern Highlands (
Chapter 11
,
Figure 11.3). Each lake basin reacted individually to fluctuations in discharge from
the parent river, with some filling faster and drying out sooner than others. Some
lakes were becoming saline at the same time that others were deep and fresh. Such
a time-transgressive response to regional climatic changes is reminiscent of the late
Pleistocene lakes fed by the Okavango River in Botswana.
Figure 22.9
is a synthesis of previous discussion and shows Australia and New
Guinea during the LGM, when the sea level was about 120 m lower than today,
temperatures were up to 8
(around 125 ka), Lake Eyre reached a level of
+
C cooler and ice was present on the highest peaks.
Figure
22.10
provides a vivid contrast, with warmer and wetter conditions prevalent across
the region.
°
