Quaternary Research 66 (2006) 176–181www.elsevier.com/locate/yqres

Short Paper

Last glacial cycle hydrological change at Lake Tyrrell, southeast Australia

Tim Stone⁎

School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia

Received 15 June 2005Available online 5 May 2006

Abstract

Lake Tyrrell is the largest playa in the Murray Basin of southeast Australia. Optical dating of transverse dune (lunette) sediments extends thelake's radiocarbon chronology to the last interglacial period. The highest lake level was attained 131,000 ± 10,000 yr ago, forming LakeChillingollah, a megalake that persisted until around 77,000 ± 4000 yr ago. Pedogenesis of its sandy lunette continued until buried by a siltyclay lunette deflated from the lake floor 27,000 ± 2000 yr ago. The dated soil-stratigraphic units correlate with the upper Tyrrell Beds andcontain evidence that humans visited the lakeshore before 27,000 yr ago. The Lake Chillingollah megalake was synchronous with very highlake levels in monsoon-dominated Australia, yet it was not influenced by tropical monsoon systems. It was filled instead by increased winterrainfall from westerly low-pressure fronts. Greater effective precipitation across Australia is evident, the result of a weakened subtropical high-pressure zone.© 2006 University of Washington. All rights reserved.

Keywords: Lake Tyrrell; playa; lunette; pedogenesis; optically stimulated luminescence; hydrological change

Introduction

Lake Tyrrell is a saline groundwater discharge lake or playain the Murray Basin of southeast Australia (Fig. 1). It is thelargest playa in the region, with an area of ∼185 km2. Thepaleohydrological record of the playa was investigated in detailby the SLEADS Project (An et al., 1986; Bowler, 1986;Bowler and Teller, 1986). These studies focussed on the playa-floor sediments, which revealed a long history of oscillatinglake levels. Deep-water lacustral phases were inferred fromdetrital clastic deposits and drying from evaporites. Threehydrologic cycles were recognized and related to majorclimatic events of the past 700,000 yr. However, the durationof each cycle was unclear because chronological control>20,000 yr ago was poor.

Beach deposits and transverse source-bordering dunes(lunettes) adjoin the playa floor sediments (Fig. 1). TheLake Tyrrell shoreline sequence is composed of multiple siltyclay dune ridges, with one underlain by a unit of beach sand

⁎ Fax: +61 3 8344 7761.E-mail address: t.stone1@pgrad.unimelb.edu.au.

0033-5894/$ - see front matter © 2006 University of Washington. All rights reservdoi:10.1016/j.yqres.2006.03.007

that was deposited ∼13.5 m above the present lake floor.Macumber (1991) described the change from high lake levelsto desiccation of the lake floor (producing silty clay dunes) as“by far the greatest single hydrological change in northwesternVictoria over the last 50,000 years”.

The chronology proposed by Macumber (1991) for hydro-logical change at Lake Tyrrell was based on five charcoal 14Cages, four of which came from lunette sections exposed in BoxGully at the northern end of the lake (Fig. 1). However, theoldest ages obtained for the shoreline sequence are close to thelimit of the radiocarbon dating method. Strong soil profiledevelopment (a paleosol) in the lower part of the dated sequencesuggests that these underestimate the true age of the last highlake-level phase in the Lake Tyrrell basin and the timing ofhydrological change.

Optically stimulated luminescence (OSL) dating makespossible a revision of the Lake Tyrrell chronology. ThreeOSL ages trace the Lake Tyrrell shoreline sequence back to thelast interglacial period (marine oxygen isotope stage 5e). Thisnew chronology can be correlated with the record ofhydrological variation contained in the playa floor sediments.It shows that the most recent cycle of hydrological changecorresponds with the last full glacial cycle. The shoreline

ed.

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sequence also contains evidence that Pleistocene humansvisited the Lake Tyrrell shore.

Geomorphic setting

Hydrological change at Lake Tyrrell can be traced back tothe Pliocene by paleomagnetic correlation (An et al., 1986).Lake Tyrrell is inset into the Blanchetown Clay, a freshwaterdeposit of the Lake Bungunnia megalake. These sediments aremagnetically reversed, suggesting that Lake Bungunnia pre-dates the 780,000 yr Brunhes–Matuyama boundary. The LakeTyrrell playa-floor sediments (the Tyrrell Beds) are normallymagnetized. These form a ∼5-m-thick clastic and evaporitesequence, which records late Quaternary hydrological changes(Bowler and Teller, 1986; Teller and Last, 1990). This record isincomplete because at least half the sequence has been removedby successive episodes of playa floor deflation.

The drying of Lake Bungunnia was followed by emplacementof an east–west linear dunefield. The floor of Lake Tyrrell is∼30mbelow the level of the dunes. LakesWahpool andTimboram, east ofLake Tyrrell, are parts of the lake basin (Fig. 1). Macumber (1991)

Figure 1. Map of the Lake Tyrrell basin showing the distribution of silty clay dunes (luLakes, Evaporites and Aeolian Deposits) Project cored the playa floor sediments at

proposed that during the last high lake-level period, the three lakesjoined to form Lake Chillingollah. The elevation of beach sand∼13.5 m above the lake floor was used to reconstruct the irregularshoreline of the lake (Fig. 1). Its extent north of Lake Timboram isevident from a string of small groundwater discharge depressionsthat are relicts of the former lake (Macumber, 1991).

The shoreline deposits of Lake Chillingollah are exposed inthe walls of Box Gully (Fig. 1). Beach facies grade laterallyfrom medium to coarse quartz sand to sandy clay in the back-beach environment (Macumber, 1991). Coxiella mollusc shellsin the beach sediment suggest that the paleolake had a salinity ofbetween 30,000 and 50,000 mg l−1 in contrast to the presentlake water, which is well over 250,000 mg l−1 (Macumber,1991). The beach facies are conformably overlain by fine dunesand. This is the Lower Lunette (Macumber, 1991) of the LakeTyrrell shoreline sequence.

The Lower Lunette has a red calcareous soil profile (Fig. 2).Calcrete has formed in the lower part of the B horizon, withilluvial iron and clay concentrated above. The deepest profile inBox Gully has been scoured by recent stream flow, which hasremoved part of the B2k horizon and replaced it with

nettes) and the reconstructed shoreline of Lake Chillingollah. The SLEADS (SaltFolly Point.

Figure 2. Stratigraphic section of the Box Gully lunette sequence and position of the three OSL ages. The 14C ages are from farther up the gully and are projected ontothe stratigraphic section for comparison with the OSL ages. The radiocarbon barrier is clearly evident in the lower part of the Lower Lunette.

178 T. Stone / Quaternary Research 66 (2006) 176–181

redeposited sediments including beer cans. Clean, undisturbedbeach facies are present beneath the gully floor.

The A horizon of the Lower Lunette is composed of clayeyfine sand, with extensive evidence of burning on the paleosolsurface (Fig. 2). In one area of Box Gully, this surface containssmall chert artifacts, pieces of emu egg shell, and burnt clay,indicating that the lakeshore was visited by Pleistocene humans(Macumber, 1991). The older, humic-rich back beach depositscontain burnt animal bone and scattered fine charcoal unrelatedto the archaeological site.

Pedogenesis of the Lower Lunette ceased when buried by thegrey Upper Lunette, a silty clay dune deflated from the floor ofLake Tyrrell after the drying of Lake Chillingollah (Macumber,1991). Two charcoal 14C ages from the surface of the Lower

Table 1Summary optical age data for Box Gully, Lake Tyrrell

Sample Depth a Radionuclide contents b Water c αradiation d

(m) K (%) Th (ppm) U (ppm) (%) (Gy ka−1)

BGUL 1.0 1.65 ± 0.04 10.96 ± 0.16 1.52 ± 0.2 4.0 ± 2.0 0.03 ± 0.0BGLL 1.5 0.72 ± 0.02 4.22 ± 0.03 0.66 ± 0.05 3.0 ± 1.5 0.03 ± 0.0BGB 4.0 0.08 ± 0.01 1.7 ± 0.01 0.42 ± 0.03 1.5 ± 0.7 0.03 ± 0.0a Time-averaged to allow for changes in sample burial depth.b Derived from high-resolution Ge detector gamma spectrometry for BGUL usi

Analysis and field scintillometry for remainder.c Estimated time-averages based on measured field water contents.d Assumed internal alpha dose rate.e Derived from radionuclide contents, corrected for attenuation by water and betaf Derived from field spectrometry, corrected for attenuation by water.g Calculated using the equation of Prescott and Hutton (1994), based on sedimenh Including a ± 2% systematic uncertainty associated with calibration of the labor

Lunette place this deflationary event at ∼27,000 cal yr BP(Macumber, 1991). Compared to the Lower Lunette, soil profiledevelopment is minimal. Soft carbonate segregations arepresent throughout the profile but no calcrete.

The earlier high lake level period was placed between50,000 and 32,000 yr ago (Macumber, 1991). This period wasinferred by correlation with the Mungo Lacustral period(Bowler, 1983). Two charcoal 14C ages of >35,000 cal yr BPfrom the base of the Lower Lunette gave a minimum age forthe event. However, the Mungo-based chronology is untenablefor pedological reasons because it gives less time for thestrongly developed red calcareous soil of the Lower Lunette toform than the grey soil of the Upper Lunette, which hasminimal profile differentiation.

βradiation e

γ radiation f Cosmic raydose g

Dose rate De h Opticalage

(Gy ka−1) (Gy ka−1) (Gy ka−1) (Gy ka−1) (Gy) (103 yr)

1 1.70 ± 0.08 1.04 ± 0.05 0.19 ± 0.02 2.96 ± 0.13 81 ± 3 27 ± 21 0.73 ± 0.05 0.46 ± 0.02 0.18 ± 0.02 1.40 ± 0.06 107 ± 4 77 ± 41 0.16 ± 0.03 0.12 ± 0.01 0.12 ± 0.01 0.43 ± 0.02 55 ± 4 131 ± 10

ng the conversion factors of Adamiec and Aitken (1998). Neutron Activation

attenuation.

t density, time-averaged depth and site latitude and altitude.atory beta-source.

Figure 3. Frequency distributions of equivalent dose (De) estimates for singlegrains measured using the SAR protocol. The Box Gully Beach sample (a) ispositively skewed, which suggests some partial bleaching of grains prior toburial. In contrast, the aeolian Box Gully Lower Lunette (b) and Box GullyUpper Lunette (c) samples have relatively peaked De distributions.

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Methods

Three OSL samples were collected from the end of Box Gullyclosest to the lake, where the sedimentary units of the lunettesequence are well separated. Samples BGUL and BGLL, from theupper and lower lunettes respectively, were taken from a sheervertical face in a gully wall. Sample BGB, from underlyinglaminated beach sediments, was obtained by excavating a∼1.5-m-deep pit in the gully floor under the vertical face. The soil profiles ofthe sequence were then described.

The OSL samples were processed under subdued red lightusing standard procedures (Galbraith et al., 1999). The 90–125 μm quartz fraction was used for the BGUL and BGLLsamples and 180–212 μm quartz for BGB. Equivalent doses(De) were measured using a single aliquot regenerative dose(SAR) protocol (Murray and Roberts, 1998; Galbraith et al.,1999; Murray and Wintle, 2000). Single grains were loaded intoaluminium discs, each containing 100 chambers 300 μm wideand 300 μm deep. The single grains were preheated at 240°C for10 s and the OSL signals measured with a Risø TL-DA-15apparatus. A green (532 nm) laser was used for opticalstimulation (2 s at 125°C) and the ultraviolet luminescencerecorded by an Electron Tubes 9235QB15 photo multiplier tubewith a 7.5-mm Hoya U-340 filter. The bleached grains werethen dosed with a calibrated 90Sr/90Y beta-source andstimulated again to construct a dose–response curve with aminimum of five regenerative dose points. Test-doses of 10 Gywere given to monitor for OSL sensitivity changes between thenatural and regenerative cycles. Each analysis included aduplicate regeneration applied to test the reproducibility of theartificially-induced luminescence signals.

OSL data were processed using Analyst version 2.12software (Duller, 1999). Each OSL signal was measured for100 × 0.02 s channels with data integrated from channels 6–9and background subtracted using channels 90–100. Aftercorrecting the data for sensitivity changes, dose–responsecurves were constructed. The De was derived from the interceptof the regenerated dose–response curve with the naturalluminescence intensity. Central Des were calculated using thecentral age model of Galbraith et al. (1999).

Radionuclide concentrations in the sediment surroundingeach sample were measured to determine dose rates. Gammadose rates were measured with a field spectrometer andconverted to dry values by oven-drying sediment from thesample site. Internal alpha dose rates were assumed to be0.03 ± 0.01 mGy a−1. High-resolution gamma spectrometry,made available at CSIRO Land and Water, was used to analyzeK-40, U-238, R-226, Pb-210, Th-228 and Ra-228 activities forsample BGUL. Total K, Th and U contents were measured forthe other two samples by Neutron Activation Analysis (NAA) atBecquerel Laboratories. These results were converted to betadose rates using the conversion factors of Adamiec and Aitken(1998) with beta attenuation factors of 0.88 ± 0.03 for sampleBGB and 0.93 ± 0.03 for BGLL and BGUL (Mejdahl, 1979).Cosmic-ray dose rates were determined from establishedequations (Prescott and Hutton, 1994), allowing for changesin burial depth over time for BGB and BGLL. Field water

contents were considered to be representative of long-termaverages.

Results and discussion

The OSL ages from Box Gully extend the Lake Tyrrellshoreline sequence to the last interglacial period (Table 1, Fig.2). Clean, laminated beach sediments beneath the pedogenichorizons returned an OSL age of 131,000 ± 10,000 yr for thehigh lake-level shoreline of Lake Chillingollah. The De

distribution for this sample is positively skewed, suggestingpartial bleaching of some of the grains prior to burial (Fig. 3a).This may have produced a central age that is slightly older thanthe true age of the sample. Nonetheless, it is clear from the OSLdata that the formation of Lake Chillingollah was a marineoxygen isotope stage 5e (MIS 5e) event unrelated to the MungoLacustral period of MIS 3.

Table 2Radiocarbon and OSL chronology of Box Gully, Lake Tyrrell

Stratigraphic position Depth (m) Material Conventional 14C age (yr BP) Lab Number 14C age (cal yr BP) OSL age (yr)

Upper lunette 1.3 Quartz 27,000 ± 2000Lower lunette surface Charcoal 22,000 ± 730 SUA 588 25,500 ± 1610Lower lunette surface Charcoal 23,400 ± 340 SUA 1439 27,020 ± 740Lower lunette B2k horizon 2.7 Quartz 77,000 ± 4000Base of lower lunette dune Charcoal 31,700 ± 1140 SUA 559 35,520 ± 2210Lower lunette back beach Charcoal 37,500 ± 2900 SUA 1492 40,850 ± 5150Lower lunette beach 4.3 Quartz 131,000 ± 10,00014C ages were converted to calendar years using INTCAL98 (Stuiver et al., 1998) in the OxCal version 3.5 package (Bronk Ramsey, 2000). The Southern Hemisphereocean reservoir effect was adjusted for by subtracting 24 years (Stuiver et al., 1998). SUA ages and their stratigraphic positions from Macumber (1991), charcoalsamples from surface exposures.

180 T. Stone / Quaternary Research 66 (2006) 176–181

The shoreline of Lake Chillingollah appears to have been anactive depositional system for most of MIS 5. The fine dunesand overlying the beach deposit gave a MIS 5a age of77,000 ± 4000 yr (Table 1, Fig. 2), with only minor indicationsof partial bleaching or sediment mixing in the De distribution(Fig. 3b). This episode of dune sand mobilization was close tothe last from the Lake Chillingollah shore. Pedogenesis of theLower Lunette commenced with the drying of the lake in MIS 4.

Table 2 contrasts the OSL ages with the previous 14Cchronology. An OSL age of 27,000 ± 2000 yr for the base of theUpper Lunette (Table 1, Fig. 2) is the only one to corroborateany of the 14C ages. The De distribution for this sample ismarkedly peaked (Fig. 3c). An age of ∼27,000 yr ago(transitional between MIS 3 and 2) is clearly vindicated forthe deflationary event that formed the silty clay dune. The OSLand 14C ages also provide a minimum age of∼27,000 yr ago forthe cultural material stratified on the Lower Lunette surface.

This new chronology allows up to 50,000 yr for formation ofthe red calcareous soil profile in the Lower Lunette, almosttwice the amount of time available for soil formation in theUpper Lunette. Correlation of the dated soil-stratigraphic unitswith the playa floor sequence (Bowler and Teller, 1986: Fig. 7)now suggests that the Tyrrell Beds from 110 to 150 cm are MIS5 deposits of Lake Chillingollah. Earlier playa-floor depositsmay date back more than 500,000 yr, or most of the Brunheschron. However, it is difficult to estimate the timing of earlierhydrologic cycles because so much of the Tyrrell Beds sequencehas been removed by deflation.

The two uppermost pedogenic disconformities in the TyrrellBeds (Bowler and Teller, 1986: Figs 7 and 14), at depths of180 cm and 110 cm, now appear to correspond with MIS 6 andMIS 4, respectively. The paleosols are accompanied bycarbonate peaks, of which the youngest may correlate withthe red calcareous paleosol of the Lower Lunette. The intervalbetween these dry lake episodes brought deep-water detritalclastic deposition and construction of the Lower Lunette. TheMungo Lacustral period of MIS 3 is not preserved in the TyrrellBeds, although much lower lake levels with heightened salinitycan be inferred for this period.

Further reduction of the shallow lake ushered in a salineplaya phase (Bowler and Teller, 1986). Seasonal oscillations ofthe watertable formed pelletal clay aggregates on the playafloor. The clay pellets were then transported by wind to form the

Upper Lunette. The OSL and 14C ages for the Upper Lunetteshow that the saline playa phase commenced at the close of MIS3. With the approach of the last glacial maximum, the watertable fell farther, allowing vegetation to colonize the playa floor.Saline playa conditions returned at the onset of the Holocene.

The OSL ages from the Lake Tyrrell shoreline add LakeChillingollah to the growing list of MIS 5 megalakes in the aridAustralian interior (e.g., Herczeg and Chapman, 1991; Nansonet al., 1998; English et al., 2001; Magee et al., 2004). LakeChillingollah stands apart from other interior megalakesbecause it was not influenced by tropical monsoon systems.Instead, its water budget would have depended on increasedwinter rainfall from westerly low pressure fronts. Most of thewater fed to Lake Chillingollah came from Tyrrell Creek, adistributary of the Avoca River, which drains the temperateVictorian highlands.

Synchronous very high lake levels in monsoon-dominatedand temperate Australia have paleoclimatic implications.Clearly, MIS 5 was a period of greater effective precipitationacross the Australian landmass. Cupper (2003) proposed thatthe pluvial conditions resulted from a weakening or contractionof the subtropical high-pressure zone, which allowed greaterpenetration of both the summer monsoon and temperatetroughs. Certainly, persistent high winter rainfall would explainLake Chillingollah. As these winter rains steadily decreasedwith expansion of the high-pressure zone, the lake fragmentedinto a groundwater discharge system.

Acknowledgments

Bart Smith kindly obtained the high-resolution gammaspectrometry at CSIRO Land and Water. Matt Cupper, JacquiDuncan and Zvonka Stanin assisted in the field and Matt helpedprocess the OSL data. I am also indebted to Phil Macumber andJohn Magee for constructive criticism of the manuscript.

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