Geoscience Reference
In-Depth Information
the arid LGM and a return to high lake levels during the early to mid-Holocene (Zheng
et al.,
2002
).
A 300 ka record from the Gregory Lakes in north-west Australia shows very high
lake levels at around 300, around 200 and around 100 ka, or broadly coeval with
MIS 9, 7 and 5 (Bowler et al.,
2001
). Dunes formed on the lake floors during drier
phases dated to around 230 and around 70 ka. The largest lake (6,500 km
2
) was at
around 300 ka, and later lakes were smaller, indicating increasing aridity over time,
much as in the Sahara. These lakes were fed from the summer monsoon. An increase
in the frequency of high-magnitude rainfall events would have helped generate these
Quaternary mega-lakes. Pack et al. (
2003
) analysed the carbon isotopic composition
of bulk soil organic matter from two sediment units in the Lake Gregory region
spanning roughly the last 200 ka. They found a shift from mainly C
3
to mainly C
4
plant communities over the last 120 ka. For woodland to decrease and grassland to
increase requires a decrease in rainfall and an increase in rainfall seasonality. They
concluded that there had been a late Quaternary increase in aridity, consistent with
the observations of Bowler et al. (
2001
).
Lake Eyre has the most detailed record of Upper Pleistocene fluctuations of any
desert lake in Australia (Magee et al.,
1995
; Croke et al.,
1996
; Magee,
1998
; Magee
and Miller,
1998
;DeVogeletal.,
2004
). The Lake Eyre Basin occupies one-sixth
(1.3 million km
2
) of the present land area of Australia and thus contains a good
portion of Australia's late Quaternary environmental record. It is fed mainly by the
Cooper and Diamantina rivers, which flow from the Eastern Highlands and are fed
primarily by the summer monsoon (Bonython and Mason,
1953
;Kotwicki,
1986
). At
present, it is a vast saltpan and at its lowest point is 15 m below sea level. In 1974, it
attained its highest historical level of
9.5m.DuringLaNina years, when the seas
east and north of Australia are warmer than average at the surface, precipitation is
above average in northern and eastern Australia, and run-off into Lake Eyre leads to
temporary flooding (Kotwicki and Isdale,
1991
; Kotwicki and Allan,
1998
).
The chronology that underpins the history of high and low lake stands over the past
130,000 years is based on three independent dating methods (radiocarbon, uranium-
series disequilibrium and luminescence - both TL and OSL) supplemented by amino
acid racemisation (AAR) dating of the egg-shells of emus and of their near relative,
the now extinct giant ratite
Genyornis
found around the former lake margins (Magee
and Miller,
1998
). Lake Eyre attained its maximum level around 130-110 ka, with
progressively lower levels thereafter, at 95-80 and 65-62 ka (Magee,
1998
). Forty
AMS radiocarbon dates show that the lake was dry between 35 and 10 ka (Magee and
Miller,
1998
), but these ages are in conflict with two TL ages obtained by Nanson et al.
(
1998
) that suggest high beach levels during this time. Furthermore, the AAR ages
obtained by Magee and Miller (
1998
) from these same beach ridges do not support the
TL ages. Pending future work, this issue is best left open. During the last interglacial
−
