OFR 151.pdf - CRC LEME

Similarly it is difficult to summarise bioclimates within most of the selected geographic
regions due to strengthening temperature and rainfall gradients between coastal, inland and
upland districts. Nonetheless changes in miospore dominance and the palaeodistribution of
some cryptogams provide unequivocal evidence of the impact of cooling followed by
temporary warming during the Oligocene-Early Miocene. Examples include the marked
impoverishment of Nothofagus forest in the onshore Gippsland Basin during the Eocene-
Oligocene transition (Upper Nothofagidites asperus Zone) and the re-establishment of
Lophosoria in southeastern Australia during the Early Oligocene (Macphail et al. 1994).
Fossil spores of Lophosoria (Cyatheacidites annulatus) indicate this large ground fern, which
is now restricted to cool-cold climates in South America, reached Tasmania via trans-oceanic
dispersal during or slightly before the Lemonthyme Glaciation (Macphail and Hill 1994). The
fern then spread rapidly northwards throughout southeastern Australia during the Early
Oligocene (Proteacidites tuberculatus Zone time) but is not recorded in southern Queensland
until the Early Miocene (Dettmann, 1986b), due to relatively warm conditions at lower
latitudes. Increasingly dry, seasonal climates explain the failure of Lophosoria to extend into
central and north-west Australia.
Palaeo-northern Australia
Sparse pollen evidence from the Argo abyssal plain off Port Hedland hints that continental
climates in north-west Western Australia were strongly seasonal (upper mesothermmegatherm,
semiarid-subhumid) although Oligocene to Early Miocene microclimates in the
Pilbara region were sufficiently humid during summer months to allow Nothofagus
(Brassospora) spp. and podocarps to survive along palaeochannels on the Hamersley Ranges
(M.K. Macphail unpubl. data). Conditions in inland northeastern Australia appear to have
been warm to hot (upper mesotherm) and seasonally very wet (perhumid) based on faunal
remains and leaf impressions of subtropical-tropical rainforest species preserved in karst
terrain at Riversleigh, northwestern Queensland (R.S. Hill pers. comm.).
Isotopic evidence from the Coral Sea indicates SSTs in the region increased from between
9.5-13.0 0 C (microtherm range) in the earliest Early Oligocene to 14.5-19.5 0 C (mesotherm
range) in the Early Oligocene and reached a maximum of 20.5 0 C (upper mesotherm) in the
Middle Miocene (Feary et al. 1991). Floristically complex Nothofagus communities
remained the dominant vegetation type on the western slopes of the Leichhardt Range inland
of Mackay, into Early-Middle Miocene (Canthiumidites bellus Zone Equivalent) time
(Beeston 1994). This rainforest included taxa with cool temperate NLRs, e.g. Dacrydium,
Lagarostrobos, Microcachrys and Nothofagus (Lophozonia), as well as taxa with warm
temperate to subtropical-tropical NLRs, e.g. Anacolosa, Archidendron-type, Cupanieae and
Ilex. How representative this flora and vegetation were of areas to the west in central
Queensland is unclear. Further to the south, uniformly wet and relatively cool (lower
mesotherm) climates allowed Nothofagus (Brassospora) spp. to become prominent during the
Late Oligocene in coastal southern Queensland. Conditions became increasingly warm
(upper mesotherm) and/or seasonally dry, resulting in the replacement of Nothofagus
communities by other rainforest types during the Early to Middle Miocene. The presence of
Nypa in Foram Zones N5-N7 marine sediments but not in Foram Zones N8-N9 marine
sediments in the Capricorn Basin implies maximum warmth in southern Queensland occurred
during the Early Miocene.
The combined data demonstrate that precipitation gradients across northern Australia were
parallel in direction but not as strong as those of the present-day. Because warm water flow
through the Indonesian archipelago was not severely constricted until the Early Pliocene
(Srinivasan and Sinha 1998), it is premature to explain these gradients in terms of modern
ocean current patterns or monsoonal climates associated with uplift of the Tibetan Plateau (cf.
Ramstein et al. 1997).
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