Uranium-series dating of lake and dune deposits in southeastern Australia: a reconnaissance

Changes in the hydrologic cycle throughout the late Quaternary in Australia are evident from raised lake shorelines in the interior salt-lake basins, evaporites recovered from cores within these lakes, and carbonate pedogenesis within aeolian dune sequences. Most of these features are fraught with poor absolute chronologic control especially beyond the range of radiocarbon dating (> 35,000 yr B.P.). Uranium-series methods can potentially extend the chronology to about 350,000 yr B.P. provided that the minerals remain closed to uranium and thorium exchange after deposition and that corrections to detrital contamination can be adequately made. Samples of carbonates, gypsum and halite were collected from a variety of sites within the semi-arid and arid regions of southeastern Australia in an attempt to assess the feasibility of the U-series dating technique. The U-series method shows some promise for placing constraints on the timing of palaeoclimatic changes in Australia. Contamination with non-radiogenic ²³⁰Th can be overcome in most instances using an isochron correction scheme for a sequential acid-leach procedure. Uranium-series methods can provide the most reliable dates from samples in the arid regions of the continent where post-depositional exchange with groundwater U can be assumed to be minimal. Where "reliable" ¹⁴C dates have already been obtained, the U-series dates are in general accord except at Lake Mungo, N.S.W., where U-series dates are considerably younger. Two major high lake-stands were identified at Lake Frome, South Australia at ∼21,500 and 140,000 yr B.P. Dune stabilisation (i.e. humid conditions) inferred from dates of pedogenic CaCO₃ occurred within the Strzelecki dunefield of northern South Australia at around 22,000, 68,000 and 145,000 yr B.P. These dates fall between most TL dates for dune-building episodes within the Strzelecki desert and therefore are consistent with palaeoclimatic reconstructions for the arid core of Australia. U-series dates on upper salt horizons of Lake Eyre and Lake Frome suggest that at least two periods of hyper-aridity occurred within the Holocene (< 10,000 yr B.P.).