1. School of Geography and Environmental Science, Monash
University, Clayton, Vic 3800
2. Research School of Earth Sciences, The Australian National
University, Canberra, ACT 020
3. Department of Earth Sciences, Syracuse University, 204 Heroy
Geology Laboratory, Syracuse, NY 13244-1070 USA
Preliminary results are provided of palynological analysis of 40 metres of organic-rich silt-clay sediment recovered from a small (~300 metre diameter) palaeo-lake, the Stony Creek Basin, at 600 metres a.s.l. near Daylesford, in the Western Uplands of Victoria.
Based on palynostratigraphic considerations, preliminary analysis of magnetic polarity (reversed polarity in the uppermost 21 metres analysed to date), and a fission track date (1.74 Ma) from detrital zircons buried in the lake sediments, the sequence apparently accumulated during the late Pliocene or Early Pleistocene, probably over some 300-500 kyr.
Pollen analysis of the uppermost ~8 metres reveals repetitive, distinctive changes in pollen dominance, evidently reflecting dramatic vegetation change. Several complete vegetation cycles are presented, which appear to reflect rapid climatic oscillations during the late Pliocene or early Quaternary. These cycles generally involve a repetition of floristic associations and patterns of taxon replacement, but there are also some differences between cycles in the presence/absence of certain taxa. Broadly, the record shows alternation between periods during which diverse cool temperate (microtherm) rainforest taxa, in particular various Podocarpaceae, played an important role in the upland vegetation, and periods when this vegetation apparently disappeared, at least locally, to be replaced by open forest/sclerophyll trees with some herbaceous and shrub taxa. Cupressaceae (probably Callitris), (Allo)Casuarina, and Myrtaceae (mostly Eucalyptus) pollen alternately dominate these open forest periods.
Organic matter content in the sediment corresponds to these changes in pollen abundance: highest organic matter is found during phases of greatest rainforest representation, and lowest organic matter during phases of Casuarina + herb/shrub dominance. Although only the uppermost few cycles have so far been analysed for pollen, the organic matter curve suggests the sediment pile records approximately 10-14 climatic cycles.
Many of the rainforest taxa represented are now extinct, either from western Victoria (Podocarpus, Symplocos, Quintinia, Rapanea, Myrsine, Phyllocladus, Cunoniaceae, Araucariaceae) or from most or all of Australia (Ilex, Dacrydium, Dacrycarpus, Beauprea, and Tertiary form-taxa related to Microcachrys).
The rapid alternation between rainforest vegetation with
characteristically "Tertiary" composition, and more familiar
"Quaternary" open forests, is biogeographically important for
understanding the nature and timing of the development of the
overwhelmingly sclerophyllous mid and late Quaternary vegetation of
southeastern Australia. Moreover, the record suggests that much more
diverse cool temperate rainforests than any existing in Australia
today were capable of repeated range expansion and contraction during
alternating favourable and unfavourable climates around the
Tertiary/Quaternary boundary.