Geoscience Reference
In-Depth Information
provides a summary of the Cenozoic vegetation history of Australia and shows that
desiccation occurred in a series of steps, effectively starting in the mid-Miocene.
Combined with geochemical and sediment analysis, pollen studies in the Lake Eyre
Basin have revealed a complex history of deep weathering, formation of ferricrete
and silcrete, paleochannel sedimentation and changes in plant cover (Alley et al.,
1996
; Alley,
1998
; Alley et al.,
1999
). The earliest phase of deep weathering took
place before channel alluviation, perhaps in the early Mesozoic. Temperate rain-
forests flourished along what are now the southern margins of Australia during the
Palaeocene, and they gave way to open woodland during the colder, drier Oligocene
and Miocene. In central Queensland, there was intense chemical weathering under a
presumed warm and humid climate from late Oligocene to middle Miocene (Li and
Vasconcelos,
2002
). However, clay minerals in sites away from original sources are
not always a good guide to climate. John et al. (
2006
) attributed the high proportion of
middle Miocene kaolinite in three marine sediment cores off north-east Queensland to
the fluvial reworking of early Miocene lake deposits rich in kaolinite. They concluded
that the middle Miocene was a time of global cooling, evident in low glacio-eustatic
sea levels. In the central south of the continent, shallow alkaline lakes occupied some
of the valley bottoms. Pliocene and Pleistocene desiccation saw the proliferation of
open woodland and chenopod shrubland. In a novel approach, Byrne et al. (
2008
) used
evidence from phylogenetics and phylogeography and concluded that the first major
indications of aridity date back to the mid-Miocene around 15 Ma ago - a conclusion
in accord with the pollen evidence compiled by Martin (
2006
). The work of Byrne and
her colleagues, based on molecular phylogenies, demonstrated that certain taxa show
patterns of recent expansion and migration throughout the arid zone, while others
seem to have survived in many local refugia during cold, dry glacial times (Byrne,
2008a
; Byrne,
2008b
; Byrne et al.,
2008
).
22.6 Quaternary environmental fluctuations
22.6.1 Changes in plant cover
Lynch's Crater in the seasonally wet tropics of north-east Queensland has the longest
pollen record of any site in Australia and has provided important information about
past climatic fluctuations as well as possible human impacts on the biota (Kershaw,
1976
; Kershaw,
1994
; Kershaw,
1995
; Kershaw et al.,
2003a
; Kershaw et al.,
2003b
;
Turney et al
.,
2004
; Kershaw et al.,
2007
). Although it is at present outside the confines
of the dry subhumid tropics, as defined in
Chapter 1
, during cold, dry glacial intervals,
precipitation in this localitywas reduced to a fraction of that prevailing today (Kershaw
and Nanson,
1993
). During around 40-30 ka, the moist
Araucarian
rainforests were
progressively replaced by open sclerophyll woodlands dominated by
Casuarina
and
Eucalyptus
, reflecting both a drier climate and the impact of burning (Kershaw et al.,
