Geoscience Reference
In-Depth Information
Figure 11.6. The Willandra Lakes viewed as a cascading system. (After Bowler
et al.,
2011
.)
The consequence of this system of basins forming a type of stairway relative to each
other was that each individual basin responded quite differently to the fluctuations
in discharge from the parent stream, with some basins filling faster and drying out
sooner than other basins. As a result, in some basins pelletal clay was being blown
from the seasonally exposed floor of increasingly saline lakes, whilst in others the
lakes were still deep and fresh and beach gravels were forming under high-energy
wave action. This diachronous response to regional changes in climate and river flow
is probably true of other reservoir lake systems, such as those in southern Africa fed
from the Okavango River.
During the LGM, when the lakes in seasonally wet, tropical northern Australia were
mostly dry (English et al.,
2001
), Lake Eyre in central Australia was totally dry, and its
dry bed was being actively lowered by wind erosion (Magee et al.,
1995
; Croke et al.,
1996
; Magee and Miller,
1998
; Magee et al.,
2004
). Deflation of desert lake floors
during arid intervals may lead to successively lower lake levels after each interval of
deflation, giving the impression of progressive desiccation. To eliminate this possible
source of confusion, other forms of proxy evidence are needed to test and refute or
validate the inferred lake level history. Lake Eyre provides some useful examples.
Johnson et al. (
1999
) analysed the carbon isotopes in fossil emu eggshell from around
Lake Eyre in central Australia. They found significant changes in the proportions of
C
4
to C
3
grasses over the last 65 ka (see
Chapter 7
). The data imply 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 is entirely
consistent with the reconstructed lake levels. Miller et al. (
1997
) used the temperature-
dependent amino acid racemisation reaction (see
Chapter 6
) in radiocarbon-dated
emu eggshells from the continental interior to reconstruct subtropical temperatures
at low elevations over the last 45 ka. They concluded that millennial-scale average
temperatures were at least 9
C lower between around 45 and 16 ka than they were after
16 ka. There was a sharp change at around 16 ka, followed by rapid warming. These
°
