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

• Published in: Palaeogeography, palaeoclimatology, palaeoecology palaeoclimatology, palaeoecology, 1998-12, Vol.144 (3), p.307-329
• Main Authors: Magee, John W, Miller, Gifford H
• Format: Article
• Language: English
• Subjects: amino acid racemization; aridity; Aves; beach ridges; eggshell; Lake Eyre; palaeohydrology; Quaternary
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/S0031-0182(98)00124-2

Lake Eyre is presently an ephemeral playa-lake in an extremely large (1.3 million km 2 internal drainage basin), with most of its inflow derived from monsoon-watered northern Australia. The hydrologic state of the lake has varied 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 of 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 stranded high beach ridges. A major deflation episode between 60 and 50 ka excavated the present Lake Eyre basin and deposited a gypsum- and clay-rich aeolian phase (the Williams Point aeolian unit) at a number of sites around the 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 suite of amino acid racemization (AAR) analyses of mollusc shell and bird eggshell samples from beach ridges at +5 and +10 m Australian Height Datum (AHD) suggest that they are oxygen-isotope stage 5 in age. Sedimentologic evidence suggests that it is unlikely that the shells are reworked from older deposits. These AAR 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 calibration of the AAR is still at a preliminary stage and only one site was sampled for both techniques. Further stratigraphic and chronologic work is required to fully assess the apparent discrepancy. Between about 30,000 and 12,000 yr B.P. Lake Eyre was at least as dry as it is today. At many sites around Madigan Gulf a lunette-like, playa-marginal, aeolian unit (the Shelly Island unit) was deposited during this period, formed by material deflated from the playa floor. Forty AMS radiocarbon dates span the period 35,000 to 10,000 yr B.P., from the Shelly Island unit (11) and from aeolian sediment close to playa level (29), indicating that the lake was dry during this period. This evidence conflicts strongly with 2 TL dates from latest oxygen-isotope stage 3 and oxygen-isotope stage 2 from the +5 and +10 m AHD beach ridges (Nanson et al., this volume). Additionally, the AAR results from the high beach ridges cannot