OFR 151.pdf - CRC LEME

mountainous regions of Tasmania. Data from individual sites that preserve vegetation
histories covering all or part of Late Miocene to Late Pliocene time flesh-out the
palaeoclimatic inferences. A particularly detailed record of climatic change in south-west
Western Australia during the mid Pliocene warm period is preserved in a probable meteor
impact crater site at Yallalie north of Perth (Dodson and Ramrath in press, Dodson and
Macphail in press).
1. Rainfall
The survival of Araucaria, Agathis, Dacrycarpus, Dacrydium and Nothofagus (Brassospora)
spp. in south-west Western Australia (Yallalie), southwestern New South Wales (Lachlan
Valley), southwestern Victoria (Grange Burn), southeastern Victoria (Hapuku-1), and western
Tasmania (Linda Valley) is compelling evidence that conditions during the mid Pliocene were
effectively wetter than at present although at Yallalie and possibly Grange Burn groundwater
discharge may have compensated for any moisture deficit during summer. Transient upsurges
in the relative abundance of semiarid plants and halophytic diatoms at Yallalie demonstrate
that warm (upper mesotherm) and seasonally wet conditions within and beyond the crater
were interrupted by three distinct episodes of aridification at about 2.90 Ma, 2.59 Ma and 2.56
Ma. In contrast, if stands of Araucariaceae dry rainforest at Grange Burn were wholly
supported by rainwater, then mean annual rainfall will have been between 1300-2000 mm
versus 680 mm at present. Trends in relative pollen abundance in the Murray and Gippsland
Basins imply that conditions in south-eastern Australia remained relatively wet (humid)
during the Early Pliocene, and rainfall became weakly to strongly seasonal during the Late
Pliocene. The apparent increase in the mean annual precipitation on the Southeastern
Highlands of New South Wales (Lachlan Valley, Lake George) is likely to be local, due to
orographic cloudiness reducing evapo-transpiration losses.
2. Temperature
The survival or re-establishment of Araucariaceae at Grange Burn before 4.46 Ma and
Yallalie between 2.5-3.6 Ma indicates that mean temperatures in south-west and central
southern Australia during the Early to Middle Pliocene were within the upper mesotherm
range. If the climatic envelope for the southern-most living species of Agathis (A. bidwillii) is
used as a guide, then the mean minimum temperature of the coldest month will have been
about 5-10 0 C and mean maximum temperature of the hottest month will have been about 28-
32 0 C. Slightly lower mean values are obtained if the climatic data for living Araucaria are
used.
Because of site-specific factors, palaeotemperatures in southeastern Australia are more
difficult to estimate in absolute terms. However geographic and temporal changes in the
relative abundance of Araucariaceae indicate: (1) Mean values (lower mesotherm) were lower
than in southwestern and central southern Australia. Cool-cold (microtherm) conditions
occurred at high elevations on the Southeastern Highland and Tasmania. (2) Mean
temperatures during the Early Pliocene were generally warmer (mesotherm range) than in the
Late Pliocene (microtherm range).
3. Late Neogene climatic instability
Oxygen isotope data indicate that global climates became increasingly variable (unstable)
during the Late Neogene. Two sites that appear to link the aridification of Australia with
increasingly variable climates are diatomites preserved in Palaeolake Bunyan on the
Southeastern Highlands of New South Wales, and Yallalie (ibid). Taylor et al. (1990)
propose that cyclic bedding of the Bunyan diatomite is evidence for climatic cycles with a
periodicity less than or equal to 8000 years during the Late Miocene in southeastern Australia.
The arid vegetation response seen at Yallalie is similar to that seen in southeastern Australia
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