OFR 151.pdf - CRC LEME

maximum temperature of the hottest month between 28-32 0 C (Boland et al., 1994). Lower
values are obtained if modern distribution data for Araucaria is used.
Rainfall is less easily estimated since soil drainage, salt accumulation and fire are major
controls on plant community composition in drier regions of south-west Australia at present.
Deep fracturing due to the meteor impact raises the possibility that groundwater discharge has
contributed to the survival of rainforest taxa at Yallalie (Dodson and Macphail in press). This
is not the case on the Yilgarn Plateau where the survival of Nothofagus (Brassospora) spp.
and Lagarostrobos almost certainly will have been determined by summer rainfall. If correct,
then the combined data point to sub-humid climates characterised by relatively dry summers
and wet winters in the Yallalie district, and wetter (humid) and/or less strongly seasonal
conditions on the southern margin of the Yilgarn Craton. Spikes in the relative abundance of
semi-arid zone taxa, in particular Chenopodiaceae-Amaranthaceae and halophytic diatoms,
demonstrate that locally warm and seasonally wet conditions were interrupted by three
distinct episodes of aridification around 2.56, 2.59 and 2.90 Ma. The same data hint that the
frequency of phases of relative aridity increased around 2.6 Ma. How closely the Yallalie
sequence mirrors trends in continental climates elsewhere in southern Australia is unclear.
The vegetation response appears to be similar to that seen in southeastern Australia during
Late Pleistocene glacial arid maxima, and suggests that alternating cycles of high humidity
and aridity analogous to Late Pleistocene glacial-interglacial conditions were in evidence by
2.5 Ma. This observation ties in well with oxygen isotope data from marine cores. More
generally the period of record overlaps with initiation of continental glaciation and an
increase in the frequency and intensity of climatic fluctuations between 2.6-2.8 Ma in the
middle latitudes of the Northern Hemisphere.
5.2.5 Central southern Australia
As in southwestern Australia, the only microfloral evidence for Late Neogene climates comes
from palaeochannel fill deposits.
1. Eucla Basin
Benbow et al. (1995) have recorded Cupressaceae (frequent), Mimosaceae (Acacia) and
Euphorbiaceae (Micrantheum) in a palaeochannel deposit from the eastern side of Eucla
Basin, south-west South Australia. The absence of Nothofagus and other rainforest spp.
implies that the deposit is Late Pliocene or younger.
2. Eyre Peninsula
Truswell and Harris (1982) report trace numbers of Nothofagus (Brassospora) and
Podocarpaceae (Dacrydium, Lagarostrobos, Phyllocladus, Podocarpus-Prumnopitys) in a
possible Late Miocene-Early Pliocene deposit on the northern Eyre Peninsula. The
microfloras are co-dominated by Myrtaceae (including frequent to common Eucalyptus),
Asteraceae, Chenopodiaceae-Amarathaceae, Poaceae and Cyperaceae.
Alley (1983b) has reported a Late Pliocene or younger microflora at Tarcoola located ca. 200
km inland from Streaky Bay on the western side of Eyre Peninsula. Dominants are Poaceae,
Chenopodiaceae-Amaranthaceae and Asteraceae (Tubuliflorae). Rare taxa include Acacia
and rainforest elements are absent.
Inferred climate
Assuming the inferred ages are broadly correct, Late Neogene climates were strongly
seasonal with mean annual rainfall falling to semi-arid to arid values (below ~400 mm) during
the Pliocene.
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