Lake Eyre palaeohydrology from 60 ka to the present: beach ridges and glacial maximum aridity

Lake Lyre is presently an ephemeral playa-lake in an extremely large (1.3 m illion km(2) internal drainage basin), with most of its inflow derived from monsoon-watered northern Australia. The hydrologic state of the lake has v aried in the past, in response to climate change, from a perennial lake up to 25 metres deep to a groundwater-controlled playa, marked by substantial sediment deflation. This paper is concerned with the stratigraphic record o f the last 60 ka of that hydrologic history, particularly the character and age of a playa-marginal unit formed by deflation from the playa and of str anded high beach ridges. A major deflation episode between 60 and 50 ka exc avated the present Lake Eyre basin and deposited a gypsum- and clay-rich ae olian phase (the Williams Point aeolian unit) at a number of sites around t he lake. After deflation ceased a thick secondary gypsum profile developed on the dune early in oxygen-isotope stage 3; evidence for the state of Lake Eyre at this time is equivocal. Preliminary results from a substantial sui te of amino acid racemization (AAR) analyses of mollusc shell and bird eggs hell samples from beach ridges at +5 and +10 m Australian Height Datum (AHD ) suggest that they are oxygen-isotope stage 5 in age. Sedimentologic evide nce suggests that it is unlikely that the shells are reworked from older de posits. These results apparently conflict with early oxygen-isotope stage 3 thermoluminescence (TL) dates from the +5 m AHD beach ridge (Nanson et al. , this volume). However, the age difference is not substantial, the calibra tion of the AAR is still at a preliminary stage and only one site was sampl ed for both techniques. Further stratigraphic and chronologic work is requi red to fully assess the apparent discrepancy. Between about 30,000 and 12,0 00 yr B.P. Lake Lyre was at least as dry as it is today. At many sites arou nd Madigan Gulf a lunette-like, playa-marginal, aeolian unit (the Shelly Is land unit) was deposited during this period, formed by material deflated fr om the playa floor. Forty AMS radiocarbon dates span the period 35,000 to 1 0,000 yr B.P., from the Shelly Island unit (11) and from aeolian sediment c lose to playa level (29), indicating that the lake was dry during this peri od. This evidence conflicts strongly with 2 TL dates from latest oxygen-iso tope stage 3 and oxygen-isotope stage 2 from the +5 and +10 m AHD beach rid ges (Nanson et al., this volume). Additionally, the AAR results from the hi gh beach ridges cannot be reconciled with these TL dates. After 10,000 yr B .P. a minor lacustral phase occurred until the modern ephemeral playa regim e became established at 3000-4000 yr B.P. (C) 1998 Elsevier Science B.V. Al l rights reserved.