OFR 151.pdf - CRC LEME

the adjacent highlands. Seasonal contrasts may have been weaker than in northwestern
Australia.
Palaeo-central Australia
The high degree of geographic variation in fossil floras is matched by disparities in climatic
reconstructions. Nevertheless general agreement exists that conditions were much wetter than
at present, and rainfall was adequate to support meandering river systems and permanent
freshwater lakes.
Macrofossil evidence indicates that the regional vegetation was a mosaic of mesotherm
rainforest and sclerophyll communities, consistent with very warm (upper mesotherm),
moderately wet (humid) but strongly seasonal (possibly monsoonal) conditions. In contrast,
microfossil data point to very wet (perhumid) and relatively cool (lower mesotherm)
conditions with only a weak seasonal bias in rainfall. One not unreasonable explanation is
that the two forms of palaeobotanical evidence represent different areas of vegetation, viz.
sclerophyll communities growing on drier, infertile (interfluve) sites with gallery rainforest
growing in wetter, more fertile (riverine) sites, and analogous moist habitats in the uplands. If
correct, then the regional temperatures are likely to have been at the upper end of the lower
mesotherm range whilst rainfall was geographically variable within the humid-perhumid
range. Since climatic change directly or indirectly will have underpinned the expansion of
Nothofagus in gallery rainforest in central Australia, the data are emphatic that conditions
during the Middle-Late Eocene were cooler and (riparian environments) effectively wetter
(more humid) than during the Early Eocene.
Palaeo-southern Australia
Assuming the microfloras are correctly dated, conditions at higher elevations in the Pilbara
region appear to have been relatively cool, and summer rainfall (or groundwater discharge)
was adequate to support Nothofagus (Brassospora) communities. Mean temperatures in the
southern Carnarvon Basin had fallen below the minimum value to support palms. The same
appears to have been the case elsewhere in southern Australia, except for the Polda and St.
Vincent Basins. Conditions remained wet to very wet although summer rainfall appears to
have been too low to support Nothofagus populations away from riverine habitats in southwest
Western Australia. For the first time in the Tertiary, macrofossils and microfossils are
preserved over a sufficiently wide region to allow reconstruction of climatic gradients within
some southern margin basins.
1. Palaeotemperature
Fossil Nypa pollen indicates that conditions in the eastern Bight Region (Polda and St.
Vincent Basins) were warmer (upper mesotherm) than in coastal south-west Western
Australia (southern Carnarvon and Eucla Basins) or southeastern Australia (Otway, Gippsland
and Bass Basins). This is best explained in terms of locally warm SSTs at the heads of
Spencer and St. Vincent Gulfs, analogous to the warm conditions that allow relict mangrove
to survive near Port Augusta at present. Elsewhere in the Bight region, temperate indicators
such as Nothofagus (Brassospora) spp. and podocarps such as Lagarostrobos are associated
with numerous angiosperm taxa whose NLRs are confined to subtropical rainforest. This
implies mean maximum temperatures were in the lower to low upper mesotherm range and, in
contrast to southeastern Australia, pollen of the cool climate taxon Nothofagus (Lophozonia)
are only sporadically recorded.
A more complicated pattern occurs to the east in the Gippsland and Murray Basins. Here
Lower Nothofagidites asperus Zone microfloras imply that conditions had cooled markedly
during earliest Middle Eocene time to lower mesotherm values, whilst Middle N. asperus
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