Early Human Occupation at Devil's Lair, Southwestern Australia 50,000 Years Ago

Quaternary Research 55, 3–13 (2001) doi:10.1006/qres.2000.2195, available online at http://www.idealibrary.com on

Early Human Occupation at Devil’s Lair, Southwestern Australia 50,000 Years Ago Chris S. M. Turney1 and Michael I. Bird Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

L. Keith Fifield Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia

Richard G. Roberts School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia

Mike Smith People and Environment Section, National Museum of Australia, Canberra, ACT 2601, Australia

Charles E. Dortch Anthropology Department, Western Australian Museum, Francis Street, Perth, Western Australia 6000, Australia

Rainer Gr¨un Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Ewan Lawson Physics Division, Australian Nuclear Science and Technology Organisation, PMB1, Menai, New South Wales 2234, Australia

Linda K. Ayliffe2 Laboratoire des Sciences du Climat et de l’Environnement, 91198 Gif-sur-Yvette Cedex, France

Gifford H. Miller Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0450

Joe Dortch Centre for Archaeology, University of Western Australia, Nedlands, Western Australia 6907, Australia

and Richard G. Cresswell3 Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia Received March 10, 2000 1 Present address: Centre for Quaternary Research, Geography Department, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK. 2 Present address: Department of Geology & Geophysics, The University of Utah, 135 South, 1460 East, Salt Lake City, UT 84112. 3 Present address: Land and Water Sciences, Bureau of Rural Sciences, P.O. Box E11, ACT 2604, Australia.

New dating confirms that people occupied the Australian continent before the earliest time inferred from conventional radiocarbon analysis. Many of the new ages were obtained by accelerator mass spectrometry 14 C dating after an acid–base–acid pretreatment with bulk combustion (ABA-BC) or after a newly developed 3

0033-5894/01 $35.00 C 2001 by the University of Washington. Copyright ° All rights of reproduction in any form reserved.

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TURNEY ET AL.

acid–base–wet oxidation pretreatment with stepped combustion (ABOX-SC). The samples (charcoal) came from the earliest occupation levels of the Devil’s Lair site in southwestern Western Australia. Initial occupation of this site was previously dated 35,000 14 C yr B.P. Whereas the ABA-BC ages are indistinguishable from background beyond 42,000 14 C yr B.P., the ABOX-SC ages are in stratigraphic order to ∼55,000 14 C yr B.P. The ABOX-SC chronology suggests that people were in the area by 48,000 cal yr B.P. Optically stimulated luminescence (OSL), electron spin resonance (ESR) ages, U-series dating of flowstones, and 14 C dating of emu eggshell carbonate are in agreement with the ABOX-SC 14 C chronology. These results, based on four independent techniques, reinforce arguments for early colonization of the Australian continent. ° 2001 University of Washington. Key Words: ABOX-SC; radiocarbon dating; luminescence dating; electron spin resonance dating; U-series dating; Australian archaeology. C

INTRODUCTION

Radiocarbon determinations over the last three decades show that the peopling of Australia began no earlier than ∼40,000 14 C

yr B.P. (Jones, 1998; O’Connell and Allen, 1998). Radiocarbon dates of about this age have been reported from, for example, the earliest cultural horizons of the Upper Swan, Carpenter’s Gap, and Ngarrabullgan sites (Fig. 1) (Pearce and Barbetti, 1981; O’Connor, 1995; David et al., 1997). However, alternative dating techniques (mainly thermoluminescence and optical dating), applied to the Malakunanja II, Nauwalabila I, and Lake Mungo sites (Fig. 1), imply human arrival in Australia by 50,000 yr B.P. or earlier (Roberts et al., 1990, 1994a, 1998b; Bowler and Price, 1998; Thorne et al., 1999). These differences are hard to reconcile with known variations in atmospheric 14 C concentrations (Kitagawa and van der Plicht, 1998; Voelker et al., 1998). In some cases the discrepancy may be due to contamination of archaeological samples by younger carbon close to the detection limit for radiocarbon dating, creating a “radiocarbon barrier” (Roberts et al., 1994b), but this explanation has been challenged on the grounds that “geological” deposits in Australia have yielded several 14 C ages of >40,000 14 C yr B.P., which would not be expected if contamination were pervasive (Allen and Holdaway, 1995). This challenge has in turn been contested (Chappell et al., 1996; Roberts and Jones, 2000). In hope of

FIG. 1. Map of Australia showing locations of sites discussed in text.

EARLY HUMAN OCCUPATION, SOUTHWESTERN AUSTRALIA

resolving the debate, we redated Devil’s Lair, an extensively studied site of human habitation (Dortch, 1979a, 1979b; Dortch, 1984; Dortch and Dortch, 1996). SITE DESCRIPTION

Devil’s Lair (30◦ 90 S, 115◦ 40 E) is a single-chamber cave (floor area 200 m2 ) formed in the Quaternary dune limestone of the Leeuwin–Naturaliste Ridge, 5 km from the modern coastline (Fig. 1 and 2). The stratigraphic sequence in the cavefloor deposit consists of 660 cm of sandy sediments, with >100 distinct layers, intercalated with flowstone and other indurated deposits (Fig. 3A). The sandy sediments have been interpreted as having been washed into the cave from the brown sandy soils and dunes outside the entrance, probably during periods of exceptionally heavy rainfall (Dortch and Merrilees, 1973; Shackley, 1978), whereas the flowstone layers probably represent periods of negligible sedimentation (Dortch, 1984). Excavations have been made in several areas of the cave floor. Since 1973, excavations have been concentrated in the central area (approximately on a north-west, south-east axis) of the cave (Fig. 2A), where 10 trenches have been dug (Fig. 2B). Most samples for dating were obtained from Trench 9. Where samples were obtained from different trenches, individual layers were easily traced between different locations, and the depths have been “corrected” to that of Trench 9 for comparison. Archaeological evidence for intermittent human occupation extends down to layer 30 (∼350 cm depth), with hearths, bone, and stone artifacts found throughout. The lower part of layer 30

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represents a fan of redeposited topsoil that accumulated rapidly after widening of the cave mouth, and it contains the earliest evidence for occupation of the cave. Below layer 30, only a half-dozen stone artifacts have been identified, including a single specimen each from layers 32–35, 37, and 38. No artifacts have been found below layer 38. DATING METHODS

Radiocarbon Dating The original chronology (Dortch, 1979a, 1979b, 1984; Dortch and Dortch, 1996) relied on liquid scintillation 14 C analysis of acid-washed or ABA pretreated charcoal fragments. The redating program reported here included radiocarbon dating of handpicked charcoal fragments and emu eggshell. The carbonate fraction of emu (Dromaius novaehollandiae) eggshell at several levels was dated by AMS 14 C using standard procedures (Miller et al., 1999), and it were measured at the NSF Arizona AMS facility (laboratory code AA-). Accelerator mass spectrometry (AMS) was used to measure the 14 C activity of charcoal using both the conventional ABA-BC technique and a newly developed ABOX-SC technique (Table 1). ABA-BC samples were prepared and measured at the Australian Nuclear Science and Technology Organisation AMS facility (laboratory code OZD-). ABOX-SC involves the sequential pretreatment of handpicked charcoal samples with HCI, HF, and NaOH followed by a K2 Cr2 O7 /H2 SO4 oxidation at 60◦ C for 14 h. (Bird and Gr¨ocke, 1997; Bird et al., 1999). Stepped combustion of the

FIG. 2. (A) Cross section of Devil’s Lair (modified after Williamson et al., 1976). (B) Trench plan of the main excavation.

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TURNEY ET AL.

FIG. 3. (A) Stratigraphic section of Devil’s Lair Trench 9, east face, from 100–660 cm depth below cave datum. Numbers on immediate sides of sequence refer to recognized stratigraphic layers. Shaded units indicate occupation of cave and/or immediate area. (B) Comparison among all data available for the Devil’s Lair sequence over 100–660 cm. Radiocarbon ages are not corrected for atmospheric 14 C variations. (C) Comparison among 14 C, optical, and ESR-EU age estimates for critical occupation levels in the Devil’s Lair sequence. Dashed envelope represents 1σ range of ABOX-SC 14 C ages.

ABOX-pretreated samples was undertaken at 330◦ , 630◦ , and 850◦ C on a vacuum line that was insulated from the atmosphere by a second backing vacuum to eliminate the risk of atmospheric leakage into the line at any stage of the procedure. Graphite targets were produced from the CO2 evolved at each temperature step. Samples were then measured using the 14UD Australian National University accelerator (laboratory code ANUA-). Both the ABOX pretreatment and stepped combustion proved necessary to ensure reliable 14 C age determinations beyond 40,000 14 C yr B.P. Most contaminants were removed by the ABOX pretreatment, during the 330◦ C combustion step, or both. In most cases only the 850◦ C target was measured, but all three targets were measured for charcoal samples obtained from 365 and 496 cm to verify the removal of contaminants. Similar ages for the 630◦ and 850◦ C fractions obtained from these samples suggests that all contamination was removed at the 330◦ C combustion step, permitting that a high degree of confidence be placed in the final ages (Fig. 4). The total procedural background determined from infinite-age natural charcoals using the

ABOX-SC technique is 0.10 ± 0.02% modern carbon, equivalent to ∼55,000 14 C yr B.P. (Bird et al., 1999), significantly “older” than with conventional pretreatment and graphitization systems applied to charcoal. Optically Stimulated Luminescence (OSL) Optical dating (Huntley et al., 1985; Aitken, 1998) was used to estimate the time since the quartz sediments deposited in the cave were last exposed to sunlight. Buried grains are exposed to the radiation flux (the dose rate) from the radioactive decay of 238 U, 235 U, 232 Th (and their daughter products), and 40 K in the deposit and from cosmic rays. The burial dose (paleodose) can be measured using optically stimulated luminescence (OSL), and the optical age is calculated by dividing the paleodose by the dose rate. Sediment samples were collected for optical dating from the cleaned faces of the original excavations. Samples DL4, DL7, DL19, and DL17 came from the eastern face of Trench 10 (from

EARLY HUMAN OCCUPATION, SOUTHWESTERN AUSTRALIA

TABLE 1 Radiocarbon Ages Obtained from Devils’s Lair of Emu Egg Carbonate (Laboratory Code AA-) and Charcoal Prepared Using the ABA-BC (Laboratory Code OZD-) and ABOX-SC (Laboratory Code ANUA-) Techniques Laboratory code

Layer

Depth below datum (cm)

AA-19695 AA-19691 AA-19690 AA-19689 OZD-320 OZD-321 OZD-323 OZD-322 OZD-324 OZD-325 OZD-326 OZD-327 OZD-330 OZD-328 OZD-329 OZD-331 OZD-332 OZD-333 OZD-334 ANUA-10002a ANUA-11709 ANUA-11502 ANUA-11512 ANUA-11511 ANUA-11507 ANUA-11510 ANUA-13008

G Pit 6 9 10 G I Hearth 2 Hearth 2 9 10d 11b 28 28 28 28 39 39 39 42 11a 28 30 33 39 39 44 51

82 184 239 247 86 89 96 102 237 266 276 339 335 345 345 468–470 480–488 495–501 546 276 345 365 397–405 465 496 562–605 660

Note. δ 13 C values assumed to be −24‰. All sample combusted at 330◦ C stage. b Not distinguishable from background. c Asymmetric errors.

a

14 C

yr B.P.

13,580 19,840 24,930 25,500 12,500 13,050 12,950 13,300 23,050 21,850 25,900 38,800 40,500 NDFBb 41,500 40,400 NDFBb NDFBb NDFBb 26,590 41,460 45,470 46,730 48,130 >52,000 >55,000 >54,000

1σ 110 75 335 275 100 90 110 120 250 210 300 1750 1750 2000 1900

+370–350c +1400–1190c +1420–1210c +2190–1720c +2590–1960c

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layers 9, 28, 32, and 39, respectively), and sample DL8 was taken from a hearth in the northern face of Trench 8 (layer 28). Quartz grains of 90–125 µm diameter were extracted from the sediment samples and etched in 40% HF acid for 45 min to remove the alpha-dosed rinds. OSL signals were measured using an automated Risø TL reader fitted with a green-plus-blue (420– 550 nm) light source. Equipment details are given in Galbraith et al. (1999). Paleodoses were determined using a regenerative-dose protocol (Murray and Roberts, 1998; Roberts et al., 1998a; Galbraith et al., 1999; Murray and Wintle, 2000) applied to 24 single aliquots of each sample, each aliquot consisting of ∼80 grains. The number of grains per aliquot was kept low to obtain OSL data that would allow an assessment of whether or not the grains had been adequately bleached before burial (Olley et al., 1999; Roberts et al., 1999). Each aliquot was optically stimulated for 100 s at 125◦ C. The OSL signals were integrated over the first 20 s of illumination, and the signals integrated over the final 20 s were subtracted as background. Paleodoses were calculated from the net OSL signals arising from the natural (burial) dose, a subsequent test dose (TN ), a regenerative dose that closely matched the burial dose, and a second test dose (TR ). The OSL signals induced by the 1–2 Gy test doses were used to correct for any changes in OSL sensitivity between the natural and regenerative dose cycles. The test doses were cut-heated to 160◦ C before optical stimulation, and the regenerative doses were preheated for 10 s at 160◦ –300◦ C to check that the paleodoses showed no dependency on preheat temperature. Samples DL7 and DL8 were also given a second regenerative dose, identical in size to the first, to ensure that the correct dose was calculated for aliquots that had received a known dose. Gamma-ray dose rates were calculated from field spectrometry measurements and the dose rate conversion factors of Adamiec and Aitken (1998). Cosmic-ray dose rates were

FIG. 4. Comparison between radiocarbon ages (uncorrected for background) of CO2 evolved at 330◦ , 630◦ , and 850◦ C stages during step combustion of ABOX pretreated charcoal extracted from depths of 365 and 496 cm. Weighted mean ages of ABOX-pretreated samples are also shown. Graphite weights are indicated in parentheses.

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TABLE 2 Dose Rates, Paleodoses, and Optical Ages for Devil’s Lair Sediment Samples

Sample code

Layer/depth below datum (cm)

238 U

226 Ra

210 Pb

228 Ra

228 Th

DL4 DL8 DL7 DL19 DL17

9/249 28/335 28/344 32/392 39/495

8.5 ± 2.1 5.3 ± 1.7 6.0 ± 1.4 7.3 ± 1.5 7.3 ± 1.3

8.5 ± 0.4 5.9 ± 0.3 7.9 ± 0.3 11.7 ± 0.3 9.2 ± 0.2

8.2 ± 2.7 4.5 ± 1.8 3.6 ± 1.7 9.5 ± 1.7 7.9 ± 1.5

17.1 ± 0.9 12.7 ± 0.8 19.6 ± 0.6 28.1 ± 0.6 20.3 ± 0.5

18.6 ± 0.4 12.4 ± 0.4 18.4 ± 0.4 28.3 ± 0.4 21.0 ± 0.3

40 K

Gamma dose rateb (mGy yr−1 )

Total dose ratec (mGy yr−1 )

Paleodosed (Gy)

Optical agec (103 yr)

240 ± 8 248 ± 8 240 ± 6 349 ± 6 290 ± 5

0.44 0.35 0.37 0.54 0.41

1.33 ± 0.06 1.18 ± 0.05 1.22 ± 0.05 1.71 ± 0.07 1.35 ± 0.06

34.0 ± 1.1 51.1 ± 1.4 54.1 ± 1.4 80.3 ± 3.0 69.0 ± 2.0

25.5 ± 1.4 43.4 ± 2.2 44.4 ± 2.1 47.1 ± 2.6 51.1 ± 2.6

Radionuclide activitiesa (Bq kg−1 )

a Measured by high-resolution gamma spectrometry. Concentrations of 1 ppm 238 U, 1 ppm 232 Th, and 1% 40 K correspond to activities of 12.4, 4.1, and 316 Bq kg−1 , respectively. b In situ measurements at field water content. Relative standard errors are estimated at 5%. c Mean ± total (1σ ) uncertainty, calculated as the quadratic sum of the random and systematic uncertainties. d Mean ± standard error of 24 single-aliquot estimates for each sample. The error term includes a systematic uncertainty of 2% associated with laboratory beta-source calibration.

estimated from published relationships (Prescott and Hutton, 1994), and an internal alpha dose rate of 0.03 mGy yr−1 was assumed for each of the acid-etched quartz samples. Beta dose rates were calculated from high-resolution gamma-spectrometry analyses of dried and powdered samples, using a beta-attenuation factor of 0.93 ± 0.03 (Mejdahl, 1979) and the dose rate conversion factors of Olley et al. (1996). We assumed that long-term average water contents were slightly greater than the measured field contents (1.4–6.9%) to allow for drying out of the pit-wall sediments since the original excavations. Ages increase by ∼1% for each 1% increase in water content. A condition of secular equilibrium presently exists in the 232 Th decay chain for all five samples, but the 238 U chain is in disequilibrium (Table 2). We attribute 210 Pb/226 Ra ratios of <1 to loss of radon gas to the atmosphere, and we assume that the measured ratios prevailed throughout the period of sample burial. Four of the samples have 238 U/226 Ra ratios of 0.6–0.9, which we attribute to preferential leaching of uranium in association with carbonate complexes derived from the dune calcarenite in which the cave has formed. However, uranium deficits of this magnitude have an insignificant (<2%) influence on the timeintegrated total dose rate for samples buried for <75,000 yr, when (as here) the 238 U chain accounts for a minor fraction (<15%) of the radiation flux (Olley et al., 1996, 1997). The OSL decay curves for three single aliquots of sample DL17 are shown in Fig. 5 for three different preheat temperatures, 180◦ (A), 240◦ (B), and 300◦ C (C). These data, typical of the Devil’s Lair samples, illustrate that the TN (solid line) and T R (dashed line) test dose OSL signals mimic the sensitivity changes in the natural (filled circles) and regenerative dose (open circles) OSL signals at different preheats. Figure 6A shows the TR /TN ratios obtained for all 24 aliquots of samples DL4, DL8, and DL17 using preheats of 160◦ –300◦ C, where the three aliquots of sample DL17 shown in Figure 6A are indicated by filled triangles. Sensitivity changes are indicated by the progressive increase in the TR /TN ratio with preheat temperature, but the corresponding paleodose estimates show no such temper-

ature dependency (Fig. 6B). Paleodose estimates for samples DL7 and DL19 are also constant for preheats of 160◦ –300◦ C. We conclude that use of the test dose OSL signals corrects satisfactorily for changes in OSL sensitivity between the natural and regenerative dose cycles, and that there is negligible thermal transfer from traps deeper than 160◦ C. The protocol is further validated by the correct (known) dose being calculated for the 24 aliquots of samples DL7 (55.2 ± 1.1 Gy) and DL8 (53.7 ± 1.4 Gy) that had each received a second regenerative dose of 55 Gy. Similar findings have been reported for other samples using variants of this protocol (e.g., Murray and Roberts, 1998; Roberts et al., 1998a, 1998b, 1999; Murray and Wintle, 2000). None of the paleodose estimates are much higher than the average value (Fig. 6B), suggesting that the quartz grains received sufficient exposure to sunlight before burial for complete bleaching. Had any of the grains been partially bleached, some of the palaeodose estimates should have been two or more times greater than the typical value obtained from measurement of small aliquots (<100 grains) or single grains (cf. Olley et al., 1999; Roberts et al., 1998a, 1999). The optical age of each sample (Table 2) was determined from the average paleodose for all 24 aliquots, which we calculated using the “central age” model of Galbraith et al. (1999). The optical ages are in correct stratigraphic order and the two samples from layer 28 give concordant ages. Electron Spin Resonance (ESR) ESR analysis was carried out on six teeth from large macropods (Macropus fuliginosus) following routine procedures (Gr¨un, 1989). As the teeth were relatively small, single aliquots (Gr¨un, 1995) were used to construct the dose-response curves. Representative sediment samples were analyzed by ICP-MS (U and Th) and flame photometry (K) to calculate the external beta dose rates, and gamma dose rates were measured in situ using a portable spectrometer. ESR ages (Table 3) were calculated assuming both early U-uptake (EU) and linear U-uptake (LU).

EARLY HUMAN OCCUPATION, SOUTHWESTERN AUSTRALIA

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FIG. 6. Test dose OSL (TR /TN ) ratios (A) and paleodose estimates (B) obtained at preheat temperatures of 160–300◦ C using a regenerative-dose protocol applied to 24 single aliquots of quartz from samples DL4, DL8, and DL17. The filled triangles denote the three aliquots shown in Figure 5. The DL8 and DL17 points have been shifted to the right for clarity. Three aliquots were held at each preheat temperature for 10 s. The 1σ error bars are shown when larger than the size of the symbol. The test dose OSL ratio corresponding to nil sensitivity change between the natural (TN ) and regenerative (TR ) cycles (TR /TN = 1) is indicated by the dashed line (A). In (B), the average paleodoses for samples DL4, DL8, and DL17 are marked by the lower, middle, and upper dashed lines, respectively.

U-Series Dating FIG. 5. OSL decay curves for three single aliquots of sample DL17 preheated for 10 s at 180◦ (A), 240◦ (B), and 300◦ C (C). The OSL signals arising from the natural dose (filled circles), a subsequent test dose (TN , solid line), a regenerative dose (70 Gy, open circles), and a second test dose (TR , dashed line) are shown for the first 25 s of optical stimulation, by which time all four signals had decayed to background. The TR /TN ratio is listed for each aliquot. For the 180◦ C preheat, the natural and TN signals are larger than the corresponding signals measured after regeneration. The reverse is true using a 300◦ C preheat, while a 240◦ C preheat results in a close match between the natural and regenerative dose signals and between the TN and TR signals.

TIMS U/Th ages were measured for three subsamples of one crystalline calcite flowstone near the base of the deposit. These samples were cleaned by agitation in an ultrasonic bath in alternate solutions of Milli-QTM water and AR-grade acetone. The samples were dissolved by the stepwise addition of HNO3 , and when they were dissolved, a mixed 233,236 U/229 Th spike was added. This sample-spike mixture was refluxed overnight in the presence of 1–2 ml of H2 O2 to ensure complete equilibration of the samples with spike U and Th isotopes and the oxidation of

H2/108–134 9.5 ± 0.5 O/134–137 9.7 ± 0.5 4/202–210 20.9 ± 0.6 21.8 ± 3.4 11/278–280 33.1 ± 4.9 27/333 43.3 ± 1.3 33.9 ± 0.6 39/493–508 54.6 ± 3.6 56.3 ± 5.1

DE (Gy) 0.94 0.20 0.93 0.66 1.10 2.00 1.20 0.67 1.50

1.00 1.30 0.25 0.25 1.04 1.50 1.50 2.31 2.31

1000 1000 800 800 1000 900 900 1000 1000

50 50 50 50 50 50 50 50 50

0.54 0.51 0.36 0.36 0.50 0.53 0.53 0.18 0.18

6.2 2.8 5.4 5.4 3.2 2.6 2.6 1.7 1.7

0.57 1.10 1.06 1.06 1.08 0.90 0.90 0.64 0.64

U(EN) U(DE) TT S1/S2 U Th K (ppm) (ppm) (µm) (µm) (ppm) (ppm) (%)

Sediment

425 ± 34 414 ± 34 542 ± 47 542 ± 47 519 ± 45 418 ± 36 418 ± 36 462 ± 41 462 ± 41

γ -D (µGy/a) 80 ± 13 115 ± 15 144 ± 17 144 ± 17 114 ± 16 108 ± 16 108 ± 16 65 ± 8 0±0

β-D (µGy/a) 152 ± 21 34 ± 4 167 ± 24 121 ± 18 229 ± 31 435 ± 56 257 ± 35 178 ± 25 375 ± 48

8±2 10 ± 1 2±0 2±1 9±1 15 ± 2 15 ± 2 25 ± 3 24 ± 3

Age (103 yr)

665 ± 42 14.3 ± 1.2 573 ± 38 16.9 ± 1.4 855 ± 55 24 ± 2 809 ± 53 27 ± 5 871 ± 57 38 ± 6 976 ± 69 44 ± 3 798 ± 53 42 ± 3 730 ± 49 75 ± 7 886 ± 64 64 ± 7

Int.D DE -D Total DC (µGy/a) (µGy/a) (µGy/a)

Early U-Uptake

72 ± 11 16 ± 1 78 ± 11 56 ± 8 106 ± 15 207 ± 28 120 ± 16 80 ± 12 178 ± 24

4±1 5±1 1±0 1±0 4±1 7±1 7±0 11 ± 2 11 ± 2

Age (103 yr) 58 ± 38 16.4 ± 1.4 550 ± 38 17.6 ± 1.5 765 ± 51 27 ± 2 743 ± 50 29 ± 5 743 ± 50 45 ± 7 740 ± 48 59 ± 4 653 ± 42 52 ± 4 618 ± 44 88 ± 9 663 ± 48 85 ± 10

Int.D DE -D Total D (µGy/a) (µGy/a) (µGy/a)

Linear U-Uptake

Note. H2, Hearth 2; EN, enamel; DE, dentine; TT, total enamel thickness; S1/S2, surface layer removed from each side of the enamel samples. Error in DE after Gr¨un and Brumby (1994); dose rates after Adamiec and Aitken (1998); beta dose attenuation after Monte Carlo experiments by Brennan et al. (1997); alpha efficiency: 0.13 ± 0.02 (Gr¨un and Katzenberger-Apel, 1994); initial 234 U/238 U = 1.4 ± 0.2 in enamel and dentine; water in dentine and sediment 7 ± 3 wt% (for beta dose rate); ICP-MS errors: (U,Th), 5% (relative); K, 0.05% (absolute). Gamma dose rate (measured in situ) includes cosmic dose rate derived from sediment thickness and average roof thickness of 5 ± 1 m. Western Australian Museum acquisition numbers: sample number 1447 (W.A.M. 77.6.314); sample number 1444 (W.A.M. 77.6.141); sample number 1449 (W.A.M. 75.5.13); sample number 1445 (W.A.M. 77.4.958); sample number 1437 (W.A.M. 76.9.201); sample number 14478 (W.A.M. 79.2.103).

1447 1444 1449A 1449B 1445 1437A 1437B 1448A 1448B

Layer/depth Sample below datum no. (cm)

TABLE 3 Chemical Analyses and ESR Early U-Uptake (EU) and Linear U-Uptake (LU) Age Estimates for Marsupial Teeth from Devil’s Lair

10 TURNEY ET AL.

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TABLE 4 U Concentrations and U and Th Activity Ratios of Three Sample Splits from the Same Bulk Flowstone Specimen Sample

U (ppm)

(230/232)act

±2σ

(234/232)act

±2σ

(238/232)act

±2σ

δ 234 U(0) a

±2σ

Agea (103 yr)

±2σ

DL (Tr8/9)-644cm-a/1 DL (Tr8/9)-644cm-a/2 Devil’s Lair-det.-a (acid leach)

0.013 0.013 —

3.2008 2.6679 0.3900

0.0264 0.0196 0.0070

6.4879 5.2354 0.1569

0.0733 0.0548 0.0034

5.9507 4.8548 0.1694

0.0704 0.0538 0.0031

114

16

63.2

1.6

Note. The same procedure was used for the analysis of each sample split (see Methods). Detrital corrected 230 Th/238 U and 234 U/238 U ratios were obtained from the slopes of the 230 Th/232 Th vs 238 U/232 Th and 234 U/232 Th vs 238 U/232 Th plots. Errors in the corrected age estimate were derived from the fit of each of these regressions. The half lives of 234 U and 230 Th used in the age calculation are 244,600 ± 490 and 75,381 ± 590 yr, respectively. a Data corrected for detrital contamination.

any organic compounds present in the samples. Chemical procedures for U and Th involve coprecipitation of U and Th with Fe(OH)2 at pH 7.0, followed by separation and purification of U and Th by standard anion exchange techniques (Stirling et al., 1995). Isotopes of U and Th were measured using a Finnigan MAT 262 solid source mass spectrometer equipped with an ion counting device. U and Th separates were loaded onto zone refined Re filaments between two layers of colloidal graphite. The respective U or Th isotopes were then measured by cycling each mass into the ion counter. The spike 233 U/236 U (=1) was used to correct for machine mass fractionation effects during U measurement. No attempt was made to correct Th isotopes for mass fraction. The ratio of 233,236 U to 229 Th of the mixed spike solution used in the analytical procedure was determined by calibration against the uraninite standard HU-1 assumed to be in secular equilibrium, and against standard solutions of U and Th. The details of the U and Th isotopic results are given in Table 4. Age estimates are given in years before present (yr B.P.) and all uncertainties quoted are ±2σ . RESULTS AND DISCUSSION

The original radiometric 14 C results on charcoal (Dortch, 1979a, 1979b, 1984) provide a coherent chronology for the upper part of the Devil’s Lair sequence. Many of them are consistent with the new ages for the upper part of the sequence based on AMS-14 C, OSL, and ESR (Fig. 3B and Tables 1–3). The early occupation of Devil’s Lair is bracketed by the ages obtained from layer 28 (345 cm) and layer 39 (496 cm). Below layer 28 the original conventional ages do not increase systematically with depth and only one date is older than 35,000 14 C yr B.P. (Fig. 3C). By contrast, both the ABA-BC and ABOX-SC AMS-14 C ages from units 28 and below are all older than 35,000 14 C yr B.P. However, whereas the ABOX-SC 14 C ages continue to increase systematically to the base of the sequence, the ABABC 14 C ages plateau at 40,000–42,000 14 C yr B.P., suggesting that the latter samples have reached a “radiocarbon barrier.” The ABOX-SC radiocarbon results and the optical ages consistently overlap at 1σ . In particular, beyond 40,000 14 C yr B.P. all results are in correct stratigraphic order. Moreover, they suggest a rapid rate of sedimentation between layers 30 and 38,

consistent with the presence of cut-and-fill structures and multiple, partly superimposed, channels that have convoluted or angular sections (Dortch, 1979a). By contrast, the discord between the other charcoal 14 C data sets and the new ABOX-SC 14 C chronology provides strong evidence that conventional charcoal pretreatment strategies are inadequate for removing younger carbon contaminants from these samples. Comparison of the ESR ages with the ABOX-SC 14 C and OSL chronologies favors the EU model and indicates that the teeth have not preferentially leached uranium. The ESR-EU ages for layer 39 (75,000 ± 7000 and 64,000 ± 7000 yr B.P.) are older than the optical age for this layer (51,000 ± 2600 yr B.P.). Additional U-series analysis of the dental material would be required to refine further the ESR age estimates for this layer (Gr¨un et al., 1999). The ABOX-SC 14 C ages for this and deeper levels (∼48,000 14 C yr B.P. and older) overlap with background values, but a maximum age for the cave fill is given by the detritally corrected uranium-series age for the basal flowstone at 660 cm (63,200 ± 800 yr B.P.), which is ∼1.6 m below layer 39. The new chronology indicates that the base of the stratigraphic sequence at Devil’s Lair is in excess of 55,000 14 C yr B.P. The earliest evidence of people in the vicinity of the cave, represented by the artifacts between layers 32 and 38, dates to ∼47,000– 48,000 14 C yr B.P., whereas the earliest evidence for actual occupation of the cave dates to 45,500 14 C yr B.P. Available 14 C calibration curves for the 40,000–50,000 yr B.P. time interval (Kitagawa and van der Plicht, 1998; Voelker et al., 1998) indicate that the offset between 14 C years and sidereal years in this range is 1000–2000 years, in which case people reached the Devil’s Lair area no later than ∼48,000 cal yr B.P. CONCLUSIONS

ABOX-SC 14 C, optical, ESR, and U-series ages from Devil’s Lair all support claims of pre-40,000 14 C yr B.P. arrival on the Australian continent (Roberts et al., 1990, 1994a, 1994b, 1998b; Thorne et al., 1999; Roberts and Jones, 2000). People had reached the extreme southwestern tip of the continent by ∼48,000 cal yr B.P. These findings were made possible, in part, by rigorous decontamination procedures that push back the “radiocarbon barrier” for charcoal.

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TURNEY ET AL.

ACKNOWLEDGMENTS This project was initiated by M.A.S. and R.G.R. in 1995. The Gnuraren Progress Association and the Bibbulmun Mia allowed C.E.D., J.D., M.A.S., and R.G.R. to undertake the fieldwork for this study in 1995. A. Baynes and the W.A. Museum provided samples for ESR dating. The original fieldwork was supported by the ANU Faculty Research Fund, ABA-BC 14 C dates were supported by an AINSE grant to R.G.R., and ABOX-SC 14 C dates were supported by a grant from the Institute of Aboriginal and Torres Strait Islander Studies to M.I.B. and L.K.F. The Australian Research Council provided fellowships to C.S.M.T., M.I.B., and R.G.R. H. Yoshida assisted with the optical dating, J. Olley provided the high-resolution gamma-spectrometry analyses, Q. Hua and U. Zoppi assisted with the ABA-BC 14 C dating, and R. Jones provided advice on the identification of artifacts. J. Allen read an earlier draft of this manuscript, and R. Gillespie, S. Forman, and an anonymous reviewer provided helpful comments.

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