Uranium-series dating of lake and dune deposits in southeastern Australia: a reconnaissance

Palaeogeography, Palaeoclimatology, Palaeoecology, 84 (1991): 285-298

285

Elsevier Science Publishers B.V., Amsterdam

Uranium-series dating of lake and dune deposits in southeastern Australia: a reconnaissance A n d r e w L. H e r c z e g a' 1 a n d A u d r e y C h a p m a n a

aEnvironmental Geochemistry Group, Research School of Earth Sciences, The Australian National University, GPO Box 4, Canberra, ACT 2601, Australia (Received September 12, 1988; revised and accepted January 25, 1990)

ABSTRACT Herczeg, A.L. and Chapman, A., 1991. Uranium-series dating of lake and dune deposits in southeastern Australia: a reconnaissance. Palaeogeogr., Palaeoclimatol., Palaeoecol., 84: 285-298. Changes in the hydrologic cycle throughout the late Quaternary in Australia are evident from raised lake shorelines in the interior salt-lake basins, evaporites recovered from cores within these lakes, and carbonate pedogenesis within aeolian dune sequences. Most of these features are fraught with poor absolute chronologic control especially beyond the range of radiocarbon dating (> 35,000 yr B.P.). Uranium-series methods can potentially extend the chronology to about 350,000 yr B.P. provided that the minerals remain closed to uranium and thorium exchange after deposition and that corrections to detrital contamination can be adequately made. Samples of carbonates, gypsum and halite were collected from a variety of sites within the semi-arid and arid regions of southeastern Australia in an attempt to assess the feasibility of the U-series dating technique. The U-series method shows some promise for placing constraints on the timing of palaeoclimatic changes in Australia. Contamination with non-radiogenic 23°Th can be overcome in most instances using an isochron correction scheme for a sequential acid-leach procedure. Uraniumseries methods can provide the most reliable dates from samples in the arid regions of the continent where post-depositional exchange with groundwater U can be assumed to be minimal. Where "'reliable" 14C dates have already been obtained, the U-series dates are in general accord except at Lake Mungo, N.S.W., where U-series dates are considerably younger. Two major high lake-stands were identified at Lake Frome, South Australia at ~ 21,500 and 140,000 yr B.P. Dune stabilisation (i.e. humid conditions) inferred from dates of pedogenic CaCO3 occurred within the Strzelecki dunefield of northern South Australia at around 22,000, 68,000 and 145,000 yr B.P. These dates fall between most TL dates for dune-building episodes within the Strzelecki desert and therefore are consistent with palaeoclimatic reconstructions for the arid core of Australia. U-series dates on upper salt horizons of Lake Eyre and Lake Frome suggest that at least two periods of hyper-aridity occurred within the Holocene (< 10,000 yr B.P.).

Introduction P a l a e o c l i m a t i c r e c o n s t r u c t i o n s o f the late Q u a t e r na ry in A u s t r a l i a h a v e relied on r a d i o c a r b o n , p a l a e o m a g n e t i s m an d m o r e recently t h e r m o l u m i nescence a n d a m i n o - a c i d r a c e m i z a t i o n to p r o v i d e c h r o n o l o g i c a l c o n t r o l (e.g. Bowler, 1976; Bowler et al., 1976; Singh et al., 1981; G a r d n e r et al., 1Present address: Centre for Groundwater Studies, CSIRO, Division of Water Resources Private Bag No. 2, Glen Osmond, South Australia 5064, Australia. 0031-0182/91/$03.50

1987; M u r r a y - W a l l a c e et al., 1987). U r a n i u m - s e r i e s d a t i n g techniques have been applied to c o n t i n e n t a l c a r b o n a t e and salt deposits (e.g. K a u f m a n and Broecker, 1965; Peng et al., 1978; K u et al., 1979: Bischoff et al., 1985; H i l l a i r e - M a r c e l et al., 1986, L a o and Benson, 1988) but have n o t h e r e t o f o r e been applied to c o n t i n e n t a l deposits in Australia. All o f these m e t h o d s are f r au g h t with uncertainties because o f c o n t a m i n a t i o n p r o b l e m s , a s s u m p t i o n s related to dose rate or t e m p e r a t u r e history. We therefore need as m a n y m e t h o d s as possible to p r o v i d e a c c u r a t e c h r o n o l o g i c a l c o n t r o l s to inter-

© 1991 - - Elsevier Science Publishers B.V.

286

A.L. H E R C Z E G A N D A. C H A P M A N

pret past changes in continental water-balance and relate them to global changes in parameters such as CO2 concentration and astronomical forcing. In this paper, we evaluate the uranium-series disequilibrium method for dating carbonates (inorganic and biogenic) and stratigraphically preserved salts in reconnaissance samples from southeast Australia (Fig.l). The aim is to provide a better chronology for palaeoclimatic reconstruction of the Australian continent up to 350,000 yr B.P. 23°Th/234U age estimates in continental carbonates and salts

Uranium-series disequilibrium dating is based on the ingrowth of 23°Th as a result of incorporation of 23sU and Z34U into the CaCO3 lattice (or other salt structure) at the time of formation. 23°Th is a daughter product of 234U with a halflife of 77,000 yr. Because thorium is essentially insoluble in natural waters, most or all of the 23°Th measured in pure CaCO3 will have been derived from decay of 234U and its age calculated from the ratio of 2 3 ° T h / Z 3 4 u and 234U/238U(see Ivanovich and Harmon, 1982). Carbonates in continental environments present several complications to the dating scheme such as incorporation

.... s~:~-

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Australia

: i Coo,~

~ F ~ : ~

,i

eensland

IC~'~

,

--~

..............

....

South

f~

Canberra•

x..

i

I

Fig. 1. Location map of sites investigated as part of the SLEADS program from which samples were collected for uranium-series dating. U and Th results from each site are given in Table 1.

of detrital particles into the carbonate lattice, the poorly crystalline nature of some pedogenic carbonate (which facilitates post-depositional U and Th exchange) and mobility of thorium due to inorganic or organic complexing. Previous studies of the uranium-series dating technique in continental environments (Kaufman and Broecker, 1965; Szabo and Rosholt, 1969; Kaufman, 1971; Peng et al., 1978; Schwarcz, 1980; Ku and Liang, 1984) give a variety of correction schemes for 23°Th and 234U derived from non-carbonate detrital material. Kaufman and Broecker (1965), Kaufman (1971), and Lao and Benson (1988) use various isochron approaches by making multiple analysis of contemporaneous deposits with different amounts of detrital material to determine "true" 23°Th/234Uratios. Dating samples at a single locality or for several samples with similar detrital and uranium content, the U and Th isotopes in both the acid-soluble and insoluble portions must be determined. Two separate correction schemes can be applied (described in detail in Ku and Liang, 1984). The first correction scheme assumes secular equilibrium amongst U and Th isotopes in the detrital minerals while allowing for differential extraction of U relative to Th after a dilute nitric acid leach. In this case, several samples are analysed and the leachate fractions plotted as isochrons. This "isochron" method, is recommended by Schwarcz and Latham (1989) because the detrital components of a mixture of calcite and "dirt" may be a mixture of easily leachable and more resistant phases. The method allows for differential isotopic fractionation for each pair of samples. However, this method has two major disadvantages in that two or more coeval samples must be analysed and it assumes secular equilibrium amongst the U and Th isotopes within the detrital minerals and that the detritus in different coeval samples has the same isotopic ratio. However, the 2 3 ° T h / 2 3 4 U ratio in some detritus components exceed unity (Szabo and Rosholt, 1982) which would make this scheme unworkable. Alternatively, we need not assume secular equilibrium in the detritai phase but assume that U and Th leached from these minerals is unfractionated. The isotope ratios for the "pure" carbonate

URANIUM-SERIES

D A T I N G O F L A K E A N D D U N E D E P O S I T S IN S O U T H E A S T E R N

AUSTRALIA

287

NYAHWESTLAYER3

fraction are given by the following equations (Ku and Liang, 1984): 23°Tho

(23°ThL/232ThL)--(23°ThR/232ThR)

234U° - (234uL/E32ThL)_(234UR/232ThR) 234Uo 238U °

18 , 3-

1.2q1441'6q

slope=l2 8 ~

(1) 08

(234UL/232ThL)--(234UR/232ThR) --

(238UL/232ThL)

__

(238UR/232ThR)

slope~

06

(2)

From Eqs. 1 and 2 it can be seen that the slopes of 23°Th/232Th vs 234U/232Th and 234U/232Th VS 238U/232Th give the (23°Th/234U)o and (234U/ 238U)o ratios, respectively, of the pure carbonate. Almost all the samples analysed for our investigation contained more than 5% detrital material. We applied the second of Ku and Liang's correction schemes to those samples which showed internally consistent U and Th isotope ratios. That is, the Th/U ratio of the detrital fractions should not be too different from average shale (i.e., between 1 and 3) and 2 3 4 U / z 3 8 u and 23°Th/234U ratios of the detrital material not be more than _+50% from unity. Furthermore, the plots of 234U/ 232Th vs 238u/E32Th and 23°Th/ 232Th vs 234U/ 232Th (for the second correction scheme) should have positive slopes and an intercept close to zero. Two examples of such plots are given in Fig.2. These samples are from one locality (Nyah West - see below) in which coeval samples dated by the L/R method gave ages that were essentially identical. It is possible that "old" continents such as Australia may have detrital components that are so weathered that they only have an effective single leachable detrital isotopic signature during a dilute acid leach, but have undergone isotopic fractionation during the process. Therefore, the L/R method may be more appropriate for samples from highly weathered environments such as arid-zone Australia.

Analytical methods Samples of carbonate (calcite with variable Mg content) and halite were cleaned of surficial detritus with a brush then ground with a mortar and pestle. Gypsum samples were similarly brushed but then placed in a ultrasonic bath with a small amount of distilled water for half an hour before being ground.

1

04

0 R ~ i i r 0 0.5 1 15 2 25 3 3.5 238U/232Th

234U/232Th

LARK PIT LAYER2 4.5 43.5-

8

7-

6 i- 5-

slope=

~_

3-

2.5~- 2 1.5-

2 1 0

0.5 i

J

i

i

i

i

i

238U/232Th

i

i

0

/

0 1 2 3 4 5 6 7 8 234U/232Th

Fig.2. Examples of 234U-238U and 23°Th-234Uplots for calculation of corrected U Th ages. These plots demonstrate how the appropriate activity ratios for "pure" carbonates were calculated from the slope of plots of 234U/232Thvs 238U/232Th (to get initial 2 3 4 U / 2 3 8 U ratios) and 23°Th/232Thvs 234U/232Th (to get initial 23°Th/234U ratios). The intercepts of each of these plots are close to zero and all other requirements for internal consistency were met.

For carbonates, we closely followed the dilute nitric acid leach method described by Ku and Liang (1984) except that organic matter was removed first by boiling with H202 rather than combusted in a muffle furnace at 800°C. Purification and separation of U and Th isotopes was accomplished using the method similar to that given by Anderson and Fleer (1982). After completely dissolving the carbonate and non-carbonate fractions, a 232U/228Th spike was added and left 1-2 days for equilibration. Then the pH was raised to 6-7 using CO32- -free NH4OH causing precipitation of iron oxyhydroxide which co-precipitated all hydrolysable metals. The precipitates were taken up in 9N HCI and passed through an anion exchange resin which adsorbs U and Fe but not Th and Pa. The same column was then converted to 8M HNO 3 using only ~10 ml (1 column volume) of the nitric acid to separate some of the

288

U and Fe. U and remaining Fe was eluted with 0.1N HC1. The U was further purified by solvent extraction with isopropyl ether and a second, 5 ml 8N HNOa column. U was taken up in 2N HC1 and extracted into thenoyltrifluoroacetone (TTA) and evaporated onto stainless steel planchets. Th was separated from Pa and other metals on a 10 ml 8N HNO3 column, extracted into TTA, and deposited onto stainless steel planchets. Insoluble residues were dissolved using successive H F and HC104 digestions and taken up with HC1 and purified as above. Alpha spectrometry was performed on Ortec ruggedized Si surface barrier detectors attached to a Tracor-Northern pulse-height analyser where peak resolution was 32 keV FWHM at 35.8% efficiency. One sigma quoted errors are based on square-root counting statistics and include uncertainties in yield tracer recovery.

Results

Table 1 gives the relevant isotope ratios, U and Th concentrations and corrected 23°Th/234U ages for samples analysed in this study. 234u/2a8u activity ratios (AR's) in leachate fractions are mostly between 1.0 and 1.7. These AR's are well within the range of typical surficial continental waters and are caused by preferential solubilization of 234U from mineral grains due to alpha recoil effects (Kigoshi, 1971; Osmond and Cowart, 1982). Carbonates that precipitate directly from continental or marine waters would reflect this ratio. Two samples, 224L and 229L, had AR's significantly less than unity though AR's of this magnitude are known in some groundwater systems (Osmond and Cowart, 1982). Residues had slightly lower AR's perhaps reflecting preferential leaching of 234Uduring weathering due to alpha recoil damage to mineral grains (Rosholt, 1982). 23°Th/232Th ratios of leachate samples were generally quite low (1-5) which may indicate low quantities of ingrown 23°Th relative to detrital derived 23°Th and 2a2Th, or that thorium was present in solution and incorporated into authigenic phases as they precipitated from solution. In either case, these high apparent initial 232Th

A.L. HERCZEG AND A. CHAPMAN

contents are the primary cause for rather large uncertainties in many of the corrected U/Th ages given in Table 1. In general, 23°Th/232Th ratios that exceed 10 in the leachate fractions cause rather small corrections to the derived 2a°Th/234U ages. Where 23°Th/232Th ratios in the leachate are _<1, the age uncertainties become quite large. Lao and Benson (1988) measured U and Th concentrations in rivers entering Pyramid and Walker Lake in Nevada and found much higher Th concentrations than that predicted if Th concentration was controlled by carbonate complexes (e.g. Simpson et al., 1982; LaFlamme and Murray, 1987). They hypothesised that Th was complexed by dissolved organic matter or attached to sub-micron particles. Some of the salt samples collected in this study which were apparently completely devoid of particulate matter also showed evidence for enhanced 232Th concentrations in the brines from which the salts had precipitated (e.g. Lake Eyre: halite, Lake Amadeus: gypsum). Although it has never been demonstrated unequivocally, there is some evidence for enhanced Th-mobility in environments which are high in chloride or sulphate (Herczeg et al., 1988; B. Dickson, pers. comm.) although organic thorium complexes might be more important in most natural waters (Langmuir and Herman, 1980). Therefore we could only assign tentative maximum ages based on measured dissolved U and Th concentrations for salt samples which had low 23°Th activities. Two sets of dates are given in our sample suite in Table 1. The first is the age calculated directly from the measured 23°Th/234U and 234U/238U ratios of the leachate. The second is the corrected isochron age using Method II of Ku and Liang (1984). In most cases, rather large corrections for non-radiogenic 23°Th were required because of the low 23°Zh/232Th ratios in the leachate fractions (i.e., small amounts of radiogenic 23°Th relative to common 23°Th). Many residue analyses show rather large deviations of 23°Th/234U ratios from unity, which may indicate some fractionation artifact from the leaching procedure or mobility of thorium within the soil or lake sediment. In either case, 23°Th/Ea4U ratios less than 0.7 or greater than 1.5 may give unreliable dates.

URAN|UM-SERIESDATINGOF LAKEAND DUNEDEPOSITSIN SOUTHEASTERNAUSTRALIA

289

TABLE 1 U and Th activity ratios for leachates (L) and residues (R) for some lake and dune systems in southeast and central Australia. The uncorrected ages are calculated from the "face-value" 23°Th/234U activity ratios of the leachates. The corrected ages are calculated from the determination of U and Th isotopes in the residue fractions and applying correction scheme II from Ku and Liang (1984). Sample description is given in the appendix and localities are in Fig.l Sample number

Site number sample ID

234"U/23Su

23°Th/232U

23°Th/23'~'U

U (ppm)

Th (ppm)

Age uncorrected (kyr)

Age corrected (kyr)

169

I TAN 2161

1.23+0.06

3.27+0]16

0.20___+0.01

0.19

0.04

25+2

20+2

183L 183R

1 TAN 2 1 6 0

1.27+0.04 1.00+0.06

1.065+0.05 0.80+0.06

0.26+0.02 0.84+0.03

0.40 0.236

0.38 0.754

33+3

182L 182R

2 MUN I

1.33+0.06 0.88+0.05

0.74+0.04 0.59+0.04

0.52+0.03 1.18+0.08

0.18 0.21

0.51 1.16

79+7

19+2

186

2 MUN 2

1.09_0.05

7.3+0.02

0.07+0.02

0.53

0.02

10+2

8+2

231L 231R

2 MUN 4

1.08__0.05 0.60+0.02

3.61+0.06 0.65+0.03

0.25__+0.04 0.45+0.06

0.64 1.07

0.14 1.34

32+3

28+2

227

2 MUN 8

1.17+0.02

8.10+0.1

0.13+0.08

1.15

0.06

16+2

16+2

165L 165R

3 NW 1

1.10+0.06 1.00+0.05

1.44+0.05 0.56+0.02

0.48+0.02 1.31 +0.04

0.38 0.13

0.42 0.95

72+6

45+4

205L 205R

3 NW 2

1.12+0.04 0.73+0.02

1.17___0.05 0.55+0.04

0.51 +0.02 0.92+0.04

0.46 0.31

0.67 1.13

79+7

51 +4

184L 184R

3 NW 3

1.21+0.04 0.93-t-0.06

1.75+0.04 0.56+0.05

0.468+0.03 0.929+0.05

0.66 0.24

0.64 1.15

69+6

51+5

185L 185R

3 NW 4

1.17-t-0.03 0.92-+0.05

2.10_+0.05 1.04-+0.04

0.54_+0.03 1.43-+0.05

0.58 0.22

0.53 0.84

85+7

44___4

188

4 FR 1

1.47_+0.05

0.79_+0.07

1.79

0.30

155+ 14/- 12

148+ 14/- 12

189L 189R

4 FR 2

1.13_+0.05 1.19+0.06

5.30___0.07 2.40+0.06

0.34_+0.03 2.11-+0.06

0.61 0.22

0.14 0.69

47_+5

25+3

170L 170R

4 FR 4

1.69_+0.10 0.96_+0.05

1.31___0.06 0.99+0.04

0.35-+0.02 0.51 _+0.01

0.38 0.44

0.52 0.66

48_+5

22_+3

158

4-FR82/2 55-57 cm

1.18_+0.07

1.06+0.10

0.59_+0.03

0.50

1.01

98_+ 10

out of range

159

4-FR82/2 350-351 cm

1.14___0.04

1.12-+0.03

1.09+0.03

0.49

1.66

>350

out of range

160L 160R

4-FR82/2 435-436 cm

1.14+0.03 0.89___0.11

1.26+0.03 1.01+0.12

0.37_+0.03 4.27+0.08

1.65 0.59

1.70 6.80

53_+4

out of range

200L 200R

5-STRZ 1 Lark Pit 1

1.19+0.07 1.12_+0.04

1.67+0.10 0.83+0.06

0.26+0.02 0.31+0.01

0.22

0.12 0.66

33+3

22+3

201L 201R

5-STRZ 2 Lark Pit 2

1.12+0.04 1.12+0.04

4.32+0.08 1.00+0.09

0.46+0.03

0.72

0.26

74+7

68-+8

202L 202R

5-STRZ 3 Della 4

1.00_+0.02 0.76-+0.04

10.5-+0.1 1.9+0.1

0.76-+0.03 0.93-+0.01

0.72 0.16

0.16 0.19

1 6 2 + 2 2 / - 18

142+25/-22

223L 223R

5-STRZ 4 Site 4

1.36+0.04 0.86__+0.03

0.28_+0.03 1.20+0.03

0.69 0.29

0.18 0.56

37_+3

24_+3

21.0_+0.1

4.55+0.07 1.64+0.07

9.5+ 1.5

290

A.L. HERCZEG AND A. CHAPMAN

TABLE 1

(continued)

Sample number

Site number sampleID

234"U/23su

23°Th/232U

23OTh/234U

U (ppm)

Th Age (ppm) uncorrected (kyr)

224L 224R

5-STRZ 5 YG I

0.69__+0.04 0.91+0.04

4.36 ___0.02 1.27+0.07

1.32 __+0.04 0.46+0.03

0.65 0.94

0.41 0.93

out of range

225L 225R

5-STRZ 6 YG 2

1.04__+0.05 0.97+0.06

2.16+0.05 0.72+0.03

0.98+0.01 0.98+0.02

0.11 0.33

0.19 1.32

>350

out of range

229L 229R

5-STRZ 7 Gypsum

0.82+0.05 0 . 8 6 _ 0.04

1.13+0.06 1.09 _ 0.06

0.65+0.01 0.33 _ 0.01

0.09 0.17

0.13 0.13

120+ 15

out of range

238

6-LE 82/2 62-70 cm

0.96+0.04

1.27+0.06

0.29+0.02

0.02

0.01

38+4

<10

Discussion Site 1 - - Lake Tandou (Menindee Lakes system)

Two samples from Lake Tandou of the Darling River system (Fig.3) that had been previously dated by radiocarbon were obtained for intercomparison of the two methods. The first (TAN 2161) is that of mussel shells that were composed of aragonite (with some calcite overgrowths that were shaved off), collected from the dry lake floor, many still in their upright growth position. These shells are presumed to date the last retreat of water from the lake. The shells contained a small amount of 232Th so we subtracted an appropriate amount of 234U and 23°Th based on analyses of detrital material in another sample (TAN 2160) from Lake Tandou. The corrected age (20,000 __ 2000 yr B.P.) is in excellent agreement with the radiocarbon age (18,000 _+ 1200 yr B.P.). A sample of pedogenic carbonate taken from one of the lunettes (TAN 2160) gave a corrected age of 9500 __ 1500 yr B.P. which is in reasonable agreement with the radiocarbon age of 11,500 __ 600 yr B.P. Site 2 - - Lake Mungo (Willandra Lakes system)

The Willandra Lakes system in southwestern New South Wales, which includes Lake Mungo (Fig.3), is characterised by large relict lakes and sand dunes which represent times much wetter and drier respectively than the present regime (Bowler et al., 1986a). The chain of lakes oriented roughly NE-SW were fed by the now dry Willandra Creek.

Age corrected (kyr)

No surface water occurs within the region except for isolated surface soaks emanating from the feet of some of the dunes (Bowler et al., 1986b). Evidence for past climatic changes at the Lake Mungo site are represented by remnants of various aquatic fauna, sediment facies typical of lake full to dry conditions, aeolian units and palaeosols delineated by carbonate nodules that are also representative of a return to more humid conditions. The best exposed stratigraphic site within the Willandra Lakes is known as "Walls of China" at the southwestern edge of Lake Mungo. Figure 4 displays a cross-section of the sampling site which has been delineated into four main units (Bowler et al., 1986b). The Golgol Unit forms the underlying core of the ridge. Carbonate nodules within this unit (MUN 4) gave a U/Th age of 28,000 +_ 2000 yr B.P. 14C ages on similar nodules in soils developed within this unit gave ages around 35,000 yr B.P. which are reasonably close to the U/Th age but are close to the limit for 14C dating in this environment and therefore the latter are unreliable. The Golgol Unit contains well sorted beach sands associated with shelly fauna indicating freshwater conditions. During this stage the lake supported extensive Unio (a species of freshwater mussel) populations. U/Th dates on shells retrieved from their in situ position from the centre of Lake Mungo were 16,000 +_ 2000 yr B.P. while radiocarbon dates are consistently in excess of 40,000 yr. Because the U/Th and 14C dates are from different samples, it is possible that the 14C ages represent the main Mungo lacustral phase while the U/Th

URANIUM-SERIES

DATING

OF LAKE AND DUNE

-34°

~

DEPOSITS

IN S O U T H E A S T E R N

291

AUSTRALIA

~ 0

(3

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% %

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i

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9

0

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Mildura E~ Perennialake l (~ Intermittentormainlydrylake

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I:~

io i

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~

0 I

20 i

40 I

60 I

80 I

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142" I

Fig.3. Map of a portion of the Mallee region of Australia showing the Tandou Lakes system and the Willandra Lakes system(after Bowler, 1986b). ages are from the last remnants o f surface water within Lake Mungo. The reason(s) for the discrepant U/Th dates and 14C dates is not known. Accumulation of uranium after deposition would give rise to younger U/Th ages, as has been demonstrated for molluscs in marine environments (Kaufman et al., 1969). However, these samples would have been exposed to water intermittently after deposition so this seems unlikely. Incorporation of " d e a d " carbon at the time of deposition of CaCO 3 and organic carbon results in old appar-

ent 14C ages. A detailed survey of U-series and t4C systematics is urgently needed in this type of continental environment to properly resolve the discrepancy. Above this unit is the Lower Mungo member, characterised by a dark, humic calcareous soil developed on quartz sand. Carbonate nodules within this unit ( M U N 1) gave a U/Th age of 19,000 ___ 2000 yr B.P. which is much younger than that expected by interpolation of radiocarbon ages of over- and underlying units. This unit is

292

A.L.

HERCZEG

AND

A. CHAPMAN

100

Zanci soil ~-~

Carbonatecap Zanci

90

~

UpperMungo

~

ower Mungo

~

80

Golgol 8000

Quartz sand

Zanci gravels 7O

~

Mungoclaysands

19000yr, J

27 500 vr~

60

I

0

I

I

100

I

I

200

i

I

Metres

300

i

I

400

i

I

500

Fig.4. Lake Mungo ages determined by this study and stratigraphy (Bowler et al., 1986b) from the site known as the "Walls of China".

abruptly overlain by thick ( ~ 5 m) grey, clayeysands designated the Upper Mungo member. There are numerous fish remains spread around a single bedding plane within this unit. Bowler et al. (1986b) suggest that the salinity of the lake increased to levels where the fish could no longer survive and after they floated to the surface, they were collected and transported above the take shore by the early humans. U/Th dates on some of the remnant fish ear bones (otoliths) (MUN 2) were 8000 ___2000 yr B.P. which are substantially younger than 14C ages of charcoal remains for the associated fireplaces (32,000-34,000 yr B.P.). Again, accumulation of uranium since being deposited, which is a well known phenomenon in bone phosphates (Schwarcz, 1982), may have occurred but as the bones were deposited above the water table we cannot identify an appropriate mechanism. Site 3 - - Nyah West dune sequence

The Nyah West dune sequence in northwest Victoria (Fig.l) contains five or more distinct layers of pedogenic carbonate which delineate an-

cient soil horizons separating dune-building episodes (Fig.5). Bowler et al. (1986b) suggest that each soil unit represents a period of relative stability with respect to dune formation (i.e. humid episodes). The intervening carbonate-free layers represent unstable conditions during which dunebuilding episodes indicate times of relative aridity. Accurate dating of the timing of pedogenesis may show some evidence for periodicity related to Milankovitch cycles or a relationship between the time of maximum ice-volume established from deep-sea sediment records and continental aridity. It should be stressed here that such a relationship has not yet been unequivocally confirmed for the Australian continent or for other mid-latitude regions such as Lake Lahontan in the Great Basin of the western United States (Peng et al., 1978; Bischoff et al., 1985; Lao and Benson, 1988). Our U-series results gave apparently very good internally consistent dates having satisfied the requirements for dating suitability. However, the upper four carbonate layers give U-series ages ranging from 43,500 to 51,000 yr B.P. and almost overlap when the age estimate uncertainties

293

URANIUM-SERIES DATING OF LAKE A N D D U N E DEPOSITS IN SOUTHEASTERN AUSTRALIA

w

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carbonate

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5 Fig.5. Schematic diagram of Nyah West dune-pedogenic sequence and U-series ages determined by this study for each respective layer.

(+ 5000 yr) are taken into account. Each of the samples is porous, Mg-poor nodular CaCO3 though well cemented. A uniform addition of uranium at about 45,000-50,000 yr ago would not give ages observed for each of the carbonate horizons as it would require initial [U]=0. Measured U concentrations range from 0.38 to 0.66 ppm and AR's between 1.10 and 1.21 for the acid-soluble fractions indicating that such a uniform overprint did not occur. The data at least indicate that the different carbonate horizons did not precipitate from a single groundwater source. None of the Nyah West samples displays particularly unusual 23°Th/Z32Th o r 23°Th/234U ratios in the residuum. However, the 2a°Th/232Th ratios are

quite low in layers 1, 2 and 3 (Table 1) thus requiring large corrections for the U/Th ages and possibly by coincidence causing erroneous results that are all nearly the same. We must conclude then that U/Th dating cannot be applied to the Nyah West sequence without more detailed work on the U/Th systematics of carbonates and detrital material within each carbonate layer. Site 4 - - L a k e Frome high lake stands

Two high lake-stands at Lake Frome, South Australia have been identified at two sites on the western and northwestern shore. The first of these is at about 1-1.5 m above the present shore and

294

is associated with nodular pedogenic carbonate (sample FR 4). U-series dates on these nodules are 21,500 + 1200 yr B.P. indicating that the shoreline must be at least that old. There is however, no way of telling how long after shoreline development that soil carbonate formed. A second high stand at about 6-8 m above the present lake level is indicated by a sediment layer containing freshwater shells (FR 1). The shells are aragonitic freshwater molluscs with a coating of calcite (which was shaved off) and give a U-series age of 148,000 + 15,000/-12,000 yr. Because this age is well beyond the limit of radiocarbon dating there is no way to confirm its accuracy. It is the first time a "wet" phase has been recognised in the arid region of Australia at about 150 kyr B.P. Site 5 - - Strzelecki Dunefields

The Strzelecki Dunefield forms part of the continental anti-clockwise whorl of linear dunes that dominate the Australian dunefields (Wasson and Bowler, 1986). The dunes to the east of Strzelecki Creek (Fig.6) are quartzose and red-brown in colour while the dunes on the west side of the creek are pale brown and consist of quartz particles and clay pellets (or aggregates) (Wasson, 1983a,b). Gullies and pits which intersect the pale-brown dunes reveal some older aeolian features underneath the present dunes separated by calcareous palaeosols which were sampled for U-series dating. These carbonate nodules are supposed to represent more humid periods when dunes were stable and soils could develop. Two distinct palaeosols within the pale brown dunes, which contain calcium carbonate nodules (STRZ 1 and 2), separate 1-1.5 m sand/clay layers at the Lark Pit site (Fig.6). They gave U-series ages of 22,000 ___ 3000 and 68,000 ___ 8000 yr B.P., respectively (Table 1) indicating periods of soil development and dune stability at those times. The correction for detrita123°Th contamination in layer 1 samples was substantial and therefore the age uncertainty is quite large. The corrected age for layer 2 is about 10% less than the uncorrected age. Carbonate nodules from a nearby stratigraphic site (STRZ 4) (Fig.6) were dated at 24,000 + 3000 yr B.P. corresponding reasonably well

A.L. HERCZEG AND A, CHAPMAN

with Lark Pit layer 1 and with a period of soil development in one of the alluvial shoreline facies at Lake Frome recorded at about 25,000 yr B.P. Carbonate nodules from the Della 4 site (STRZ 3) gave a U-series age of 142,000 +25,000/-22,000 yr B.P. with a relatively small (<15%) correction factor employed. This age, inferred to be a period of dune stabilisation, corresponds closely in age to the high-lake stand recorded at Lake Frome (see above). Two palaeosol layers from a site near the Yaningurie waterhole (YG1 and YG2) gave apparent infinite U-series ages on the carbonate nodules (>350,000 yr) although the detrital 23°Th was large and these apparent ages are unreliable. Thermoluminescence age determinations indicate dune-building episodes at about 160 kyr, 90 kyr, 10-30 kyr, 4-2 kyr, and the present within the central Australian dunefield (Wasson and Bowler, 1986; Gardner et al., 1987). Our U-series dates indicate dune stabilisation at ~ 15-25 kyr, 65-72 kyr and ~ 140-150 kyr. The U-series results (which indicate times of relative dune stability) straddle the above-mentioned TL dates (which indicate dune mobility) and may be considered roughly consistent with the TL dates. Salt chronology: Lakes Frome, Eyre and Amadeus

Considerable effort has been made to establish the chronology and palaeoenvironments of Lakes Frome and Eyre (Bowler et al., 1986a; Ullman and McLeod, 1986; De Deckker et al., 1988; Magee et al., 1988; Singh and Luly, 1991). These lakes lie close to the winter/summer rainfall boundary (Singh, 1981) and should therefore be sensitive to changes in precipitation. We have investigated the possibility of U/Th dating authigenic salts from SLEADS cores with a view to extending the radiocarbon chronology. Samples of salts (halite, gypsum) removed from archived SLEADS cores retrieved from Lake Frome and Lake Eyre were analysed for U and Th isotopes. Three samples from the Lake Frome core (LF82/2) one sample from Lake Eyre (LE82/2) were undatable by U-series because in almost every case, the 232Th concentration was too high relative to 23°Th, and the ratios of

URANIUM-SERIES DATING OF LAKE AND DUNE DEPOSITS IN SOUTHEASTERN AUSTRALIA

295

Fig.6. Major dunefield orientation in the Strzelecki Dunefield and stratigraphic sites sampled for U-series dating (after Wasson and Bowler, 1986).

23°Zh/232Th and 23°Th/234U in

the salt components were too low to make firm conclusions regarding absolute ages. Assuming some reasonable regional values for the detrital or initial 23°Th activity of the salts, we could only assign m a x i m u m ages of 10,000 yr for salts in the upper 2 m or so in Lake F r o m e and Lake Eyre. G y p s u m samples from salt layers in an island dune at Lake Amadeus were obtained for the purpose of dating times when salt was removed from the lake surface

(hyper-aridity) and deposited on the dunes. U and Th activities in the Lake Amadeus dune-gypsum samples were far too low to permit any estimate of their age and are not reported here.

Comparison of U-series dates with the deep-sea 6180 record Figure 7 compares the U-series dates from the SE Australian sites with the ice-volume inferred

296

A.L. HERCZEG AND A. CHAPMAN

rapid and irregular changes in continental climates while the ice-volume response as recorded in deepsea sediments will be much slower. U-series dates on continental systems such as those presented here can only give a record of past hydrologic changes on the continent, and then only provide roughly the ratio of precipitation to evaporation.

U-SERES DATES Nt~

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>350

V28-239

Fig.7. Time intervalsof aridityand humidityderived from the U-serieschronologycomparedwiththe deep-searecord(Shackleton and Opdyke, 1973) and other continentalpalaeoclimatic records from Australiaand USA. Althoughthe response time of each of these records to climate forcing is significantly different and therefore not directly comparable, there is no clear correlationbetweenrelativelywetterperiodson the southeast Australiancontinentand minimumice-volume. from the deep-sea 6180 record (Shackleton and Opdyke, 1973). We might expect high lake, stands and periods of dune stabilisation to coincide with relatively small ice-volumes (i.e. interglacial maxima) and intervals of dune building and salt deposition (continental aridity) with glacial maxima. There is no relationship between maximum ice volume and continental aridity derived from any of these records as might be expected. High lake-stands at ,-~15,000-25,000 yr B.P. and ,-~ 140-160,000 yr B.P. coincide with times of relatively large ice-volume though they could represent times when the amount of ice was decreasing. Some evidence for more humid conditions around 60,000-70,000 yr B.P. might also be related to a decrease in ice volume recorded in core V28-238 (Shackleton and Opdyke, 1973). There is unequivocal evidence for aridity throughout most of interior Australia during most of the Holocene and this probably extends beyond the last maximum interglacial at ~ 11,000 yr B.P. There is no a priori reason for continental climates to follow those of the marine record because the internal adjustment times might be completely out of phase. For example, changes in atmospheric circulation patterns driven by the variations in the Earth's orbital cycle a n d amplified by ocean circulation patterns and atmospheric pC02 may cause

About half of a suite of 24 samples from southeast Australia appear to satisfy the necessary requirements for valid U-series dates in that they (1) contain sufficient uranium for alpha spectrometric measurement, (2) have 23°Th/232Th ratios that are high enough to minimise corrections for the effect of "common" thorium, and (3) exhibit enough evidence to establish a minimal amount of post-depositional mobility of U or Th. Further work on the behaviour of U and Th in arid hydrologic regimes is necessary to develop the U-series technique for application to Pleistocene deposits and palaeoclimates of semi-arid and arid continental environments. U-series ages appear to compare reasonably well with x4C ages at Lake Tandou in the Darling River system but are significantly younger in the Lake Mungo sample suite. The dates indicate a more humid environment in southwestern New South Wales around 8-10 kyr and 15-12 kyr B.P. Dates from Lake Frome and the Strzelecki Dunefield indicate more humid conditions in northern South Australia at ~20-25 kyr; 65-72 kyr and 140-160 kyr B.P. These dates coincide reasonably well with times of glacial/interglacial transition but there is no correlation with icevolume inferred from the deep-sea 180 record nor with the dust record in Antarctic ice cores.

Acknowledgements We are indebted to Jim Bowler, John Magee and Bob Wasson for guidance during and after fieldwork and interpretation of U-series ages. Reviews of the manuscript by H. P. Schwarcz and T. Torgersen are greatly appreciated.

URANIUM-SERIES DATING OF LAKE AND DUNE DEPOSITS IN SOUTHEASTERN AUSTRALIA

Appendix - - Sample locations and descriptions Site 1 - - Tandou L a k e (Darling River s y s t e m )

TAN 2161 TAN 2160

Aragonite mussel shells from lake floor Pedogenic carbonate nodules from aeolian unit at lake shore

Site 2 - - L a k e M u n g o ( W i l l a n d r a L a k e s )

MUN1

Carbonate nodules in midden (Walls of China) Lower Mungo Unit Otoliths from fireplace (Walls of China) Upper Mungo Unit CaCO 3 nodules (Golgol Unit) Aragonite mussel shells from lake floor

MUN 2 MUN 4 MUN 8

Site 3 - - N y a h West

NWI, 2,3,4

Carbonate nodules from palaeosols separating four dune successive sequences 100 m W of Nyah W Post Office at railway cutting

Site 4 - - L a k e F r o m e

FR 1

Curbiculina shells from Millyera Fm ~ 6 m above lake floor from section at Lake Putnamutna Carbonate nodules - - west shore of Lake Frome alluvial facies Gypsum from core LF82/2 at 55-57, 350-351 and 435-436 cm depth

FR 4

FR82/2

Site 5 - - S t r z e l e c k i Dunefield

STRZ STRZ STRZ STRZ

1 2 3 4

Carbonate nodules, Lark Pit layer 1 Carbonate nodules, Lark Pit layer 2 Pedogenic carbonate Della 4 dune Strzelecki dunefield, stratigraphic site 4, layer

1 STRZ 5 STRZ 6 STRZ 7 Site 6

Yaningurie site, layer 1 Yaningurie site, carbonate nodules, layer 2 Gypsum at Yaningurie site Lake Eyre

LE82/2 (62-70) Halite crust at 62-70 cm depth

References Anderson, R.F. and Fleer, A.P., 1982. Determination of natural actinides and plutonium in marine particulate material. Anal. Chem., 54:1142-1147.

297

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A.L. HERCZEGAND A. CHAPMAN paleomagnetic stratigraphy of equatorial Pacific core V28238: Oxygen isotope temperatures and ice-volumes on a 105 year and 106 year scale. Quat. Res., 3:39-55. Simpson, H.J., Trier, R. M., Toggweiler, J. R., Mathieu, G., Deck, B. L., Olsen, C. R., Hammond, D. E., Fuller, C. and Ku, T.-L., 1982. Radionuclides in Mono Lake, California. Science, 216: 512-514. Singh, G., 1981. Late Quaternary pollen records and seasonal paleoclimates of Lake Frome, South Australia. Hydrobiologia, 82: 419-430. Singh, G., Opdyke, N.D. and Bowler, J.M., 1981. Late Cainozoic stratigraphy, palaeomagnetic chronology and vegetational history from Lake George, N. S. W. J. Geol. Soc. Aust., 28: 435-452. Singh, G. and Luly, J., 1991. Changes in vegetation and seasonal climate since the last full glacial at Lake Frome, South Australia. Palaeogeogr., Palaeoclimatol., Palaeoecol., 84: 75-86. Szabo, B.J. and Rosholt, J.N., 1969. Uranium-series dating of Pleistocene molluscan shells from southern California - An open system model. J. Geophys. Res., 74: 3253-3260. Szabo, B. J. and Rosholt, J.N., 1982. Surficial continental sediments. In: M. Ivanovich and R.S. Harmon (Editors), Uranium-Series Disequilibrium: Application to Environmental Problems. Oxford Univ. Press, Oxford, pp. 246-267. Ullman,W. J. and McLeod, L. C., 1986. The Late-Quaternary salinity record of Lake frome, South Australia: Evidence from Na ÷ in stratigraphically preserved gypsum. Palaeogeogr., Palaeoclimatol., Palaeoecol., 54: 153-169. Wasson, R.J., 1983a. The Cainozoic history of the Strzelecki and Simpson dunefields (Australia), and the origin of the desert dunes. Z. Geomorphol. Suppl., 45:85-115. Wasson, R. J., 1983b. Dune sediment types, sand colour, sediment provenance and hydrology in the Strzelecki-Simpson Dunefield, Australia. In: M.E. Brookfield and T.S. Ahlbrandt (Editors), Eolian Sediments and Processes. Elsevier, Amsterdam, pp. 165-195. Wasson, R.J. and Bowler, J.M., 1986. Semi-arid and arid zones of southeastern Australia. Part 2: Lake Frome and Strzelecki Desert, In: Field notes for Excursion 20B, 12th Int. Sedimentol. Congr., Canberra, 36 pp.