Hydroclimatic interpretation of Quaternary shorelines on South Australian playas

Abstract

The catchment of Lake Eyre is one of the world's largest internally drained basins. The playas near its depocentre, the driest region of Australia, contain a partial record of Quaternary climatic and hydrologic events for the last full glacial cycle, and probably beyond. Ancient beach-ridges marginal to lakes Eyre, Frome, Callabonna and Blanche have been dated using thermoluminescence (TL) to provide evidence for major changes in the hydrological regime of the basin. Beach ridges around Lake Eyre provide evidence of high-lake stands up to 27 m above the present lake floor during what probably corresponds to the middle to latter part of Marine Oxygen Isotope Stage 5. There is evidence also for even higher lake stands associated with earlier isotope stages. Three TL dates identify a period of aeolian activity during Stage 4 and a further 5 TL dates from lakes Eyre and Frome indicate that high lake stands occurred between about 55 and 40 ka, corresponding with Stage 3. The Stage 5 and Stage 3 high stands both relate to periods of enhanced fluvial activity previously identified in the Lake Eyre basin and elsewhere in Australia. In contradiction with other work, a few TL dates from some playas suggest a possible major episode of high lake levels immediately preceding or at the start of the Last Glacial Maximum (LGM) (26–22 ka). This may relate to a sharp temperature suppression and an increase in runoff by rivers fed from monsoons in the north. While a set of relatively low-elevation late Holocene beaches have been dated on two of the playas, a period of enhanced precipitation and stream flow in the early to mid-Holocene appears not to have formed higher beaches, possibly due to high temperatures and evaporation rates at that time. The filling of Lake Eyre during and since Stage 5 appears to have been to no more than to a level of ∼12 m Australian height datum (AHD), possibly due a spillway at about this elevation in the form of the Warrawoocara Channel connecting Lake Eyre with playas to the southeast (lakes Gregory, Blanche, Callabonna and Frome). Such overflows from one large basin to another would have had a major impact on the hydrology of the region. In addition to enhanced runoff, essential for the maintenance of high lake levels must have been local temperatures and evaporation rates significantly reduced from present day levels.