Magee, John William
Description
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...[Show more] 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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