John W. Magee

Late Quaternary climates of the Australian arid zone: a review

Abstract Aridity in Australia has generally been associated with glacial intervals of the last few glacial cycles. The causes and nature of aridity varied from north to south over the continent. In northern and central Australia, reduced monsoon rain in global cold stages caused lakes and rivers to dry, vegetation to become more sparse, sand dunes to become active, and dust advection to increase. While the monsoon is correlated with warmer global temperatures and high sea levels overall, the exact timing of the greatest development of the monsoon in Stage 5 remains uncertain. In southeastern Australia, the proxy records give a less clear picture. While vegetation seems to have been more sparse in cold stages and sand dune activity and dust flux consequently greater, records of runoff in rivers and lakes have mixed or opposite behaviour to northern or central Australia. The nature of aridity in southeastern Australia is not analogous to conditions anywhere on the continent today. Greatly reduced temperatures over the central and southern interior, greater influence of snow and periglaciation on runoff, high groundwater tables, lower rainfall and reduced atmospheric carbon dioxide levels may all have contributed to and interacted to produce the reconstructed picture of a cold, sparsely vegetated arid landscape with large rivers and perennial lakes. The largest changes in circulation patterns over the glacial cycle probably occurred in the location and/or intensity of summer tropical convergence in northern Australia. Over southern Australia, changes to the temperature and humidity of the westerly circulation have been more significant than the small fluctuations in latitude of the sub-tropical high pressure ridge.

Continuous 150 k.y. monsoon record from Lake Eyre, Australia: Insolation-forcing implications and unexpected Holocene failure

Our reconstructed history of Lake Eyre provides the first continuous continental proxy record of Australian monsoon intensity over the past 150 k.y. This continental records broad correspondence to the marine isotope record demonstrates that this very large catchment, with its hydrology dependent on a planetary-scale climate element, responds to Milankovitch-scale climate forcing. Abrupt transitions from dry phases to wet phases (ca. 125 and 12 ka) coincide with Northern Hemisphere winter insolation minima rather than Southern Hemisphere summer insolation maxima, indicating that Northern Hemi- sphere insolation exerts a dominant control over the intensity of the Australian monsoon. Stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but, within the uncertainties, are consistent with Northern Hemisphere forcing. Regardless of the hemispheric forcing, the low intensity of the early Holocene Australian monsoon—by comparison with the last interglacial and particularly the last high-level lacustrine event at 65-60 ka when all forcing elements were modest— is an enigma that can be explained by a change in boundary conditions within Australia.

Stratigraphy, sedimentology, chronology and palaeohydrology of Quaternary lacustrine deposits at Madigan Gulf, Lake Eyre, south Australia

Abstract Madigan Gulf is a large bay at the southern end of Lake Eyre North, a major ephemerally flooded playa in arid central Australia at the southwestern margin of a vast (1,300,000 km2) internal drainage basin. The stratigraphy and chronology of the Quaternary sequence in the gulf is described from 8 cores and a cliff exposure at the gulf margin. A number of depositional environments are recognised and their distinctive sedimentary components are described. Facies recognised include deep- and shallow-water lacustrine environments, dominated by surface-water processes, and dry or ephemerally flooded playa environments dominated by groundwater-controlled processes. Sedimentary components include terrigenous clastics from river inflow and shoreline erosion, carbonates of detrital, inorganic or biological origin and gypsum and halite evaporites. Carbonates and gypsum evaporites, precipitated within the basin, are frequently reworked as clastic components. The establishment of a preliminary chronology for the sequence, by the application of thermoluminescence, uranium/thorium disequilibrium, amino acid racemization and radiocarbon dating techniques, has allowed a reconstruction of the last 130 ka of Lake Eyre palaeohydrology. The wettest phase occurred during the last interglacial (early in oxygen isotope stage 5) when an enlarged Lake Eyre was up to 25 m deep. Subsequently there has been a number of dry periods separating successively less effective wet phases culminating in the deposition of a substantial halite salt crust around the time of the glacial maximum. The dry interludes are characterised by deflation of salts and sediment from the basin, a process controlled by lowering of the watertable. The record from Madigan Gulf demonstrates the dramatic and repetitive impact of lake deflation on the Quaternary record of Lake Eyre. In the early Holocene a minor, but mostly perennial, lacustrine event was terminated at about 3–4 ka when the modern ephemeral playa regime was established. The major catchment of Lake Eyre is located in the monsoon-watered areas of northern Australia. As demonstrated by large floodings of the modern ephemeral regime, major lacustrine episodes must indicate enhanced monsoon precipitation in northern Australia. In the Holocene the lake has not risen to levels achieved during the early stage 5 lacustral phase, indicating a marked reduction in the effectiveness of the monsoon in the present interglacial by comparison with its predecessor.

Sensitivity of the Australian Monsoon to insolation and vegetation: Implications for human impact on continental moisture balance

General circulation model experiments test the geologically based correlation of high monsoon rainfall over interior Australia with Northern Hemisphere insolation and evaluate the sensitivity of the Australian Monsoon to ecosystem change. Our results suggest that Northern Hemisphere insolation control on the intensity of the Siberian High, rather than summer insolation over the Australian continent, determines the strength of the Australian Monsoon on millennial time scales, unlike a classic monsoon regime. Additional simulations show that the penetration of monsoon moisture into the interior is sensitive to biosphere-atmosphere feedbacks linked to vegetation type and soil properties. This sensitivity offers a resolution to the observed failure of the Australian Monsoon to penetrate the interior in the Holocene. Postulated regular burning practiced by early humans may have converted a tree-shrub-grassland mosaic across the semiarid zone to the modern desert scrub, thereby weakening biospheric feedbacks and resulting in long-term desertification of the continent.

Late quaternary lacustrine, groundwater, aeolian and pedogenic gypsum in the Prungle Lakes, southeastern Australia

Abstract Detailed petrographic studies of recent or sub-recent evaporites where many critical environmental parameters are defined can help in the analysis of ancient deposits and in the establishment of chemical and environmental models for evaporite formation. The Prungle Lakes are part of the now dry Willandra Lakes system in semi-arid southwestern New South Wales. The Willandra Lakes have been a site of cyclic Quaternary sedimentation in response to the well documented global climatic fluctuations associated with the glacial-interglacial cycle. At times of hydrologic stress, low lake levels are associated with high salt volumes, inherited from evaporative concentration in large shallow lakes upstream and the inflow of saline groundwaters. Thus, at Prungle, a complex array of concentric shorelines, lake floor terraces and lunettes has developed as basin-within-basin landforms which formed in response to filling and drying cycles. Sediments associated with these geomorphic units include: 1. (1) Freshwater littoral, nearshore and deep-water deposited laminated sands and muds, with biogenic carbonates chiefly of ostracodal and algal origin; 2. (2) Evaporites comprising interbedded clay and laminated gypsum-clay couplets, which consist of discrete thin laminae of sugary white euhedral prismatic gypsum separated by detrital clay layers with a high degree of optical orientation. Primary accessory celestite (SrSO4) occurs as discrete patches within the gypsum laminae; 3. (3) Groundwater pyramidal, discoidal or lenticular gypsum crystals, grown from supersaturation produced by evaporation at the capillary fringe of the water table. These have formed displacively within the sediments and caused considerable disruption of the sediment matrix; 4. (4) Aeolian deposition in irregular transverse ridges of sand-sized reworked groundwater gypsum and disrupted lacustrine clays. The gypsum crystals are predominantly discoidal pyramidal forms which are conspicuously oriented parallel to bedding, well sorted and extensively corroded. The clay pellets are well sorted and moderately well rounded. Higher in the profile, bedding is indistinct due to pedogenic processes, which include modification of detrital grains and secondary crystallization of gypsum and carbonate. A thin gypcrete horizon occurs at the surface. The last cycle of sedimentation in the Prungle Lakes is believed, from correlation with lakes higher in the Willandra System, to have occurred during the period 50,000–16,000 yr B.P. Since then groundwaters have dropped below the zone of interaction with the lake floor and surface water no longer reaches the lakes. Sedimentation has ceased and soil formation with secondary carbonate and gypsum has continued to the present. A range of sedimentary environments is identified which depend on strong seasonal oscillations. These environments are virtually absent from the Australian continent today, and their occurrence during the Pleistocene supports the contention of enhanced climatic seasonality at that time. The array of sediment types and mineralogies is almost identical to that characteristic of coastal sabkha sequences highlighting the difficulty of relying on unusual evaporative mineralogy to differentiate marine and non-marine environments. Detailed petrographic analyses of the Prungle sequence have been made in association with a review of theoretical, experimental and observational studies of the environmental controls of calcium sulfate mineralogy and crystal habit. This has enabled the correlation at Prungle of depositional environment with gypsum petrofacies and associated compositional, fabric or structural features. These observations are more generally applicable to other evaporite deposits. Many gypsum evaporites reported in the literature have been compared with this scheme developed at Prungle.

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

Lake Eyre is presently an ephemeral playa-lake in an extremely large (1.3 million km2 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 be reconciled with these TL dates. After 10,000 yr B.P. a minor lacustral phase occurred until the modern ephemeral playa regime became established at 3000–4000 yr B.P.

Beyond the radiocarbon limit in Australian archaeology and Quaternary research

Allen (1994) and Allen & Holdaway (1995), noticing the pattern in early radiocarbon dates from Australia, have advanced the notion their limit records the human settlement of the continent. A critical analysis of context and content in those carbon determinations leads to a different view. The results may be disconcerting for every region which builds its late Pleistocene chronologies on radiocarbon!

A GIS-based reconstruction of late Quaternary paleohydrology: Lake Eyre, arid central Australia

Lake Eyre is the terminal playa in a large closed basin which receives the majority of its moisture from the summer monsoon. Modern Lake Eyre receives intermittent floodings at times of increased monsoon intensity, generally associated with the La Nin ‹ a phase of the ENSO cycle. Lake Eyre was a perennial lake during several phases of increased monsoon activity during the late Quaternary, specifically about 125 ka, 80 ka, 65 ka, and 40 ka [Magee (1997) Late Quaternary Environments and Palaeohydrology of Lake Eyre, Arid Central Australia. Unpubl. Ph.D. Thesis, Australian National University, Canberra]. These lake highstands are reflected by elevated shorelines mapped around Lake Eyre and, to a lesser degree, around the Frome^Gregory system to the southeast of Lake Eyre. Comprehensive research in the Lake Eyre catchment over the last several decades has constrained the ages and elevations of these shoreline deposits, which now enable us to use digital elevation models to map these paleo-lakes. By incrementally filling each lake meter by meter, the areas and volumes of the paleo-lakes are calculated and the paleohydrologic processes, including the flow direction across spillovers, are reconstructed. At the peak 125-ka highstand, Lake Eyre stood at +10 m Australian Height Datum (AHD) and covered nearly 35 000 km 2 , more than three times the area of the current playa. Together with the Frome^Gregory system, it held 430 km 3 of water. By contrast, the deepest historic filling held 30 km 3 . These lake levels represent climatic conditions very different from the modern situation. Due to discontinuous preservation of the shoreline deposits around the lakes and due to the scale of the lake systems, mapping the former levels of Lake Eyre and the Frome^Gregory system has been impossible prior to the availability of a digital elevation model. Paramatizing the area/volume relationships of water required to fill the lake to varying levels will provide data for quantification of climate and precipitation changes over time. H 2003 Elsevier B.V. All rights reserved.

Major episodes of Quaternary activity in the lower Neales River, northwest of Lake Eyre, central Australia

Abstract The Lake Eyre Basin, Australias largest internal drainage system, represents a key site in unravelling the complexities of terrestrial climate change. The basin, covering one-sixth of the Australian continent, spans a number of climatic zones, including the tropical monsoon system to the north and the mid-latitude westerly circulation to the south. This study describes four major episodes of Quaternary fluvial, lacustrine and aeolian activity in the Neales River, a western catchment of Lake Eyre. The first phase is represented by coarse-grained fluvial aggradation which dates to at least 170 ka. The depocentre of Lake Eyre is believed to have been located further to the east during this phase. The second episode is a phase of high lake level lacustrine deposition which occurred before 103 ka. The deltaic margin of the lake at this time was approximately 20 km west of the present playa. The third phase was characterised by significant base level lowering and channel incision after 50 ka but before 31 ka. Lake level lowering induced fluvial incision of up to 9 m, scouring several metres into the basal silicified Miocene sediments some time after 50 ka. The final phase was a period of aeolian and ephemeral-fluvial deposition which peaked between 20 ka and 18 ka, coincident with the Last Glacial Maximum. These episodes are compared with chronostratigraphic data from the monsoon dominated catchments of the Cooper and Diamantina Rivers. The nature and record of fluvial and lacustrine deposition are correlated throughout the basin during the penultimate and last interglacial cycles. The late Quaternary record is more ambiguous and further studies are required to elucidate the precise nature of climate change in the basin over the last 30 ka.

AMS radiocarbon dating in the study of arid environments: Examples from Lake Eyre, South Australia

Abstract Sediments containing small amounts of carbon are difficult to date using traditional radiocarbon techniques. This has resulted in less than satisfactory attempts to establish reliable chronologies for sedimentation and environmental change in the Australian arid zone. We describe here the first application of the AMS technique to the radiocarbon dating of salt lake core samples, using a chemical pretreatment methodology based on pollen extraction techniques. These results indicate that fine-grained charcoal and pollen have a similar source and depositional mechanism. The data from Lake Eyre imply a last glacial maximum deposition for the major buried halite layer, an Early Holocene return to lacustrine conditions, with a Late Holocene reduction of net sediment inout to Madigan Gulf as the present playa conditions were established.