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Accueil du site → Doctorat → Australie → 1997 → Late quaternary environments and palaeohydrology of Lake Eyre, arid central Australia

Australian National University (1997)

Late quaternary environments and palaeohydrology of Lake Eyre, arid central Australia

Magee, John William

Titre : Late quaternary environments and palaeohydrology of Lake Eyre, arid central Australia

Auteur : Magee, John William

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 1997

This study has examined the Quaternary record of lacustrine, fluvial and aeolian sediments from the Lake Eyre region to determine the palaeoenvironmental and palaeohydrologic history of the basin, particularly over the past 130 ka (130,000 years). Detailed observations of the sedimentology, stratigraphy and geomorphology of the deposits are presented. These are organised on a regional basis and are used to infer the palaeoenvironmental history of the pronounced hydrologic response in Lake Eyre to climatic changes over the past 130 ka. Through much of the Quaternary, the lake’s depocentre has migrated towards the south and south-west. This process has been chiefly driven by groundwatercontrolled deflation processes and the asymmetry of sediment supply. This is related to the location of the major inflowing streams on the downwind, northern and north-eastern margins of the lake. These groundwater-controlled processes have excavated the modern Lake Eyre playa basin into sediments which were deposited during previous surface-water lacustrine episodes. The lake has at times been a vast perennial waterbody, with an area larger than the combination of the present Lake Eyre South and Lake Eyre North(> 10,000 km2) and a water depth of up to 25m above the present playa floor. Finely laminated clays, indicating deep water, are interbedded with evaporites, indicating high salinity, and the lake was at times salinity-stratified with anoxic bottom conditions. Large beach ridges and thick lateral accretion fluvial aggradation in the tributary valleys characterise these periods. At the other extreme, the basin has, at times, been drier than today, with no surface water and a falling watertable, resulting in deflation of material from the basin. Large quantities of sediment and salts have been removed from the basin during such periods and tributary streams incised into their former deposits in response to lowering of base level. Between these extremes the lake has existed as a smaller shallower saline lake, both perennially and ephemerally. It has also been a relatively stable dry playa with a constant watertable close to the playa surface, resulting in a salt crust. At such times, as is the case today, sediment influx, during rare ephemeral floods, is minor and minor deflation occurs during drought periods. As well as advancing our understanding of the nature of the sediment record in the basin, this study has, for the first time, enabled detailed correlation and chronology of that record by levelling sites to a common datum and by the application of numeric dating techniques. Sediment luminescence dating, chiefly using optically stimulated luminescence (OSL), was combined with a large data set (which has been made available from other related projects) of amino acid racemization analyses and AMS radiocarbon determinations, both mostly on bird eggshell. These various lines of research have converged in the development of a well-dated palaeohydrologic history of the Lake Eyre Basin, in the form of a lake-level curve, for the past 130 ka. This represents one of the first continuous lake-level curves, covering that time span, and based on multiple chronological techniques. In summary, the lake-level curve indicates that intervening dry periods separate five, successively less effective, lacustrine episodes through the past 130 ka. The highest lake levels, with perennial deep-water conditions and shorelines at +10 m AHD (Australian Height Datum), were recorded in the period 130 to 110 ka (early marine isotope (MI) stage 5). This was followed, after a dry period, by a mostly reduced-level lacustrine episode ( +5 m AHD), in the period 95 to 80 ka (later MI stage 5) which was characterised by wide variations in water depth and salinity. These two relatively prolonged wet episodes were followed, after another dry phase, by a lower ( -3 m AHD), and apparently shorter, lacustrine event with a pooled mean OSL age of 64.3 ± 2.5 ka. A major deflation event followed, between 60 and 50 ka, which excavated the Lake Eyre playa basin as we see it today. A low-level lacustrine period (-10m AHD) in the period 50 to 40 ka was followed by dry playa conditions with episodic minor deflation which continued through until about 12-10 ka and culminated in the deposition of a substantial halite salt crust. An early, low-level ( <-10 m AHD) Holocene lacustrine episode changed to the modern ephemerally flooded playa regime at about 3-4 ka. The catchment of Lake Eyre is dominantly in the northern Australian monsoon rainfall zone and the climate/ catchment-hydrology relationship indicates that major lacustrine episodes in Lake Eyre must represent an increase in the effectiveness of monsoon rainfall. This is amply illustrated by the association of major fillings of the modern ephemeral playa with periods of enhanced monsoonal circulation. The major lacustral phase at 130-110 ka suggests a marked enhancement of the Australian monsoon at that time, followed by a decline in the effectiveness of subsequent wet episodes and events. Lake-levels in the Holocene, comparatively much lower than in MI stage 5, indicate that the monsoon was less effective during the present interglacial, compared to the MI stage 5 interglacial and interstadials. However, the catchment is a vast (1.3 x 106 km2), low gradient region which spans a number of bio-climatic zones, and its hydrologic response to changes in precipitation is likely to be complex. The existence of beaches at Lake Eyre clustered at high levels ( +5 to + 10 m AHD) or low levels (around -10 m AHD), rather than in a continuum, strongly suggests that a threshold exists in the hydrological response of the catchment. Thus, some caution must be employed in attempting to infer the magnitude of climate change from the lake-level curve presented here.


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