Late Cainozoic history of vegetation, fire, lake levels and climate, at Lake George, New South Wales, Australia

Abstract

The results of pollen, spore, algal and charcoal particle analyses from an 18 m core sample, dating from ca . 730000-0 a before present (B.P.), from Lake George are described along with an account of a five year study of modern pollen-rain from the same site. Also, pollen analyses of two isolated samples, dating about 4-7 Ma B.P., in a separate core from the same location are reported for comparison. The sedimentary sequence is dated by means of magnetostratigraphy and radiocarbon. The microfossil record from Lake George provides the longest relatively continuous Quaternary continental sequence yet available from Australia and may document one of the world’s longest combined record of vegetation, bush-fires, lake levels and climates together with the record of accompanying plant migrations, redistributions and extinctions. It is so far the only chronologically secure Late Cainozoic palynological database available in Australia that spans the entire Brunhes Chron. The altitudinal shifts of vegetation belts inferred from the palynological sequence suggest significant past changes in terrestrial temperatures of the order of glacialinterglacial cycles. It is revealed that the upper treeline was depressed by 1200-1500 m and 300-600 m, respectively, during the glacial maxima and the cool-temperate intervals, and reverted during the interglacials. Assuming an average lapse rate of 0.7 °C per 100 m, the drop in mean temperature for the warmest month (January) with respect to the present during the glacial maxima and the cool-temperate periods respectively may have been about 8—10 °C and 2—4 °C. A series of about eight glacial-interglacial cycles (phases I-XIX ) are recognized during the Brunhes Chron at Lake George broadly corresponding to stages 1-19 of the deep sea 18 O palaeotemperature record. A correlation between the palaeotemperature sequence and the former lake levels at Lake George is presented for the relatively more continuous section, ca. 350000-0 a B.P., with a view to resolve past precipitation changes. It is inferred that periods of considerably lower precipitation than at present prevailed during the glacial maxima. Conversely, periods of higher precipitation than at present occurred for some considerable lengths of time during the interglacials. In general terms, the precipitation levels increased during both interglacials and interstadials with respect to glacial maxima. The plant microfossil evidence indicates that Eucalyptus -dominated, dry sclerophyll (low, open) forests, now growing in the lake catchment, and probably elsewhere in southeastern Australia are the result of a comparatively recent development. It is shown that the relatively ‘ fire-sensitive’ Casuarina -dominated forests, combined several equally or more ‘fire-sensitive’ rainforest taxa, dominated the vegetation for at least half a million years during all but the last two interglacials. The relatively ‘fire-tolerant’, Eucalyptus -dominated forests started to expand onwards from the last interglacial, some 130000 years ago, in conjunction with large increases in the amount of charcoal in the sediment. Since then, not only did the amount of charcoal remain at a generally high level but the overall dominance of open, eucalypt forest is maintained throughout during the warmer periods except for a cool-temperate interstadial interval (zone D) during the last glacial. The ‘fire-sensitive’ Casuarina (under 23 μm type) as well as all the rainforest taxa declined at the end of the last glacial and finally disappeared from the lake catchment during the Holocene, culminating in the total extinction of Casuarina type under 23 μm during the last few hundred years. Some of the changes in flora during the Brunhes Chron were undoubtedly the result of long-term climatic change but most appear to have been precipitated through increased fire-frequencies only during the last 130000 years (with the maximum impact occurring during the last 10000 years), probably on account of the bush-firing activities of early man in Australia. This presupposes the presence of the Aboriginal people some 90000 years earlier than the oldest available archaeological evidence for human occupation of the Australian continent, a proposition that remains to be tested by future archaeological investigations. In biogeographical terms, the studies reveal that a number of Gondwanic taxa, commonly seen during the late Tertiary in southeastern Australia, survived well into the Pleistocene and finally disappeared during the late Brunhes from Lake George.