Geoscience Reference
In-Depth Information
Flinders Ranges and a series of more northerly lakes were full, and overflowed into a
much larger Lake Eyre at intervals until a final major transgression at 50-47 ka. The
lake levels became progressively lower thereafter, with minor higher levels in Lake
Frome at around 30, 17, 13, 5 and 1 ka. Drawing on the evidence from speleothems
in caves located in the northern summer rainfall and southern winter rainfall zones,
Cohen et al. (
2010b
) deduced that southern sources of rainfall contributed to run-off
into Lake Frome during the 50, 30 and 17 ka lake transgressions. They also inferred
that there was a tropical contribution to Lake Eyre via the Cooper and Diamantina
rivers at 50-47 ka. The 13 ka and younger high levels of Lake Frome seem to be
a result of northerly inputs of run-off from tropical sources. More of this type of
work is needed to clarify the possible role of climatic change in the extinction of the
Australian megafauna.
22.8 Modelling changes in the Australian summer monsoon
There has long been a critical awareness of the influence of the Siberian high-pressure
cell on the summer monsoon in northern Australia. However, less well-appreciated
is the role that influxes of moist air from well south of the equator had on the
strength of the summer monsoon in north-west Australia in particular. In an effort to
resolve this issue, Wyrwoll et al. (
2007
,
2012
) used regional atmospheric circulation
models to explore the influence of changes in the earth's axial tilt and those of the
orbital precession cycle on changes in wind direction and strength associated with
synoptic conditions during the onset of the north-west Australian summer monsoon.
The outcome of the model experiments showed that when the axial tilt is greater than
it is today, there is a significant influx of air into the north-west of Australia from
southern latitudes offshore, leading to enhanced summer monsoonal precipitation.
The same is true of times of Southern Hemisphere perihelion.
22.9 Conclusion
Until about 120 million years ago Australia, Antarctica and Greater India formed
a single supercontinent. The long-term desiccation of Australia was caused by its
progressive separation fromGreater India and fromAntarctica. Greater India began to
move away fromAustralia in the early Cretaceous around 120 Ma ago. The associated
rifting and rift-margin uparching contributed to drainage disruption in the west of
Australia. Initial rifting between Australia and Antarctica began around 90 Ma ago,
and from about 45 Ma onwards, Australia moved north at a rate of 6-7 cm/year. As
the eastern margin of the Australia plate moved north, it passed over one or more
stationary hot spots in the mantle, resulting in volcanism and further uplift of the
Eastern Highlands. Movement of Australia into tropical latitudes associated with dry
subsiding air and semi-permanent high pressure led to accentuated aridity in central
