Reconstructing the history of drought and aridity in Australia: Evidence from Lake George

Kathryn E. Fitzsimmons¹, Timothy T. Barrows¹, Geoff A.T. Duller² and Helen M. Roberts²

1 Research School of Earth Sciences, Australian National University
2 Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK

Australia is the driest inhabited continent on Earth. However, evidence preserved in the landscape suggests that arid and drought conditions have varied in intensity through time and space. Aridity during the late Quaternary presided over considerable expansion of desert dunefields across Australia (e.g. Fitzsimmons et al. 2007a, b), the drying of lake basins, and increased dust transport.

It is unclear how the present relatively arid, warm conditions relate to the cold, dry climate responsible for desert expansion during the late Quaternary, or to the current severe droughts. This knowledge gap is primarily due to the fact that there are few systematic records of regional aridity and drought in Australia. The key to understanding the development of aridity is to produce a chronological framework for terrestrial aridification. Recent and continuing work at the luminescence dating laboratory in the Research School of Earth Sciences aims to produce such a chronological framework, focusing on key sites which preserve landscape features responsive to aridity, such as dunes and lake shorelines.

Lake George is the largest freshwater lake in mainland Australia when full, and provides one of the most complete records of Quaternary sedimentation in the southeastern part of the continent. The lake is presently ephemeral in response to drought conditions, but sediments within the basin preserve evidence of multiple permanent and dry lake conditions in the past. Cross bedded gravels exposed in the southern part of the lake basin indicate the existence in the past of a substantial water body with water depth in excess of 20 m. Stratigraphic sequences and lake shorelines at the northern end of the lake record multiple periods of lake filling. Lacustrine sediments within the northern part of the lake basin are overlain by a thin veneer of alluvial material and aeolian dust representing shoreline retreat late in the Holocene.

Optically stimulated luminescence (OSL) dating has been applied to the various geomorphic features associated with the lake, with an examination of aeolian, fluvial and lacustrine sediments. The single aliquot regenerative dose (SAR) OSL dating protocol has been applied using both single aliquots and single grains for samples from the different geomorphic settings. We are presently seeking to extend the chronology beyond the limits of conventional SAR OSL dating through the use of thermally-transferred OSL (Tsukamoto et al. 2008), and are contributing to the development of this new technique.

We are working to reconstruct past lake levels based on the sedimentology of the dated units, and to relate these to environmental change in the region during the Late Quaternary. The preliminary OSL chronology shows a striking correlation between lake filling events at Lake George and permanent lake conditions/ high water tables across humid, semi-arid and arid Australia (Figure 1). The Holocene and oxygen-isotope stage 5 filling events at Lake George correspond to warm sea-surface temperatures (Barrows et al. 2007). Lake filling events also appear to correspond to relatively humid periods between major arid episodes in the desert dunefields of central Australia, also identified by OSL dating in the luminescence laboratory at the Research School of Earth Sciences (Fitzsimmons et al. 2007a).

 

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