Aridity in Australia: Pleistocene records of Palaeohydrological and Palaeoecological change from the perched lake sediments of Fraser Island, Queensland, Australia

A Quaternary sedimentary sequence (ca. 600 ka) from a perched lake (Old Lak e Coomboo Depression) on the World Heritage-listed coastal sandmass of Fras er Island has been analysed for dry bulk density, carbonised particles, pol len and chemistry. A chronology has been constructed for the organic sedime nts using a combination of radiocarbon and uranium/thorium disequilibrium a nalysis. The lake basin is small (ca. 9 ha) with a restricted groundwater c atchment, delimited by an aquitard, and minimal surface runoff. It therefor e acts as a sensitive raingauge with the perched groundwater-table, and hen ce the sediment facies deposited within the lake, reacting sensitively to a ny changes in the water budget. The sequence passes through a series of gla cial cycles, demonstrating hydrologic and vegetation change. The record ind icates a long-term, three-stage fall in the water-table from lake-full ca. 600 ka to an ephemeral lake in the Holocene, paralleled by a shift in the v egetation composition from predominantly rainforest to sclerophyllous compo nents. The evidence for fire is minimal at the beginning of the record, inc reases from >350 ka through the sequence culminating at or before the LGM, is low during the LGM and is relatively high during the Holocene. Successio n, fire and climatic change, along with the accumulative effect of a series of 100 ka cycles, are believed to have driven the hydrologic and vegetatio n change and a human factor is not required to explain the record. Within t he overall long-term increase in aridity recorded through about six glacial cycles, there appears to be a variation in the 'dryness' signal of glacial maxima, suggesting some form of 'supercycle' phenomenon. This record complements and extends the information available from some of the oldest pollen-analysed Quaternary lake sequences in Australasia, such a s Lynch's Crater in the Atherton Tableland of north Queensland and Lake Geo rge in the Southern Tablelands of New South Wales; and from some of the mos t sensitive lacustrine/ fluvial sedimentary records in the Lake Lyre basin. Signals for major environmental changes recorded in deep sea cores and the loess deposits of China may also be represented in the record. The implica tions of the Depression record, if further corroborated, are significant in relation to climatic modelling, rainforest survival and faunal extinction. (C) 1999 INQUA/Elsevier Science Ltd. All rights reserved.