A dual-aliquot regenerative-dose protocol (DAP) for thermoluminescence (TL) dating of quartz sediments using the light-sensitive and isothermally stimulated red emissions

ARTICLE IN PRESS

Quaternary Science Reviews 25 (2006) 2513–2528

A dual-aliquot regenerative-dose protocol (DAP) for thermoluminescence (TL) dating of quartz sediments using the light-sensitive and isothermally stimulated red emissions Kira E. Westaway, Richard G. Roberts GeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia Received 9 February 2005; accepted 1 June 2005

Abstract Quartz sediments, collected from a cave deposit in eastern Indonesia, display very weak optically stimulated luminescence (OSL) ultraviolet emissions, which we attribute to their volcanic provenance. They do, however, emit at longer (‘red’) wavelengths. Here we provide details of a new method of using a light-sensitive red thermoluminescence (TL) signal to date the last time of exposure of quartz grains to natural sunlight, which we have used previously to constrain the burial age of Homo floresiensis remains found on the Indonesian island of Flores. The samples examined typically contained a rapidly bleaching (‘bleachable’) signal, a slowly bleaching signal and a light-insensitive (heat-reset) signal. We isolated the bleachable TL signal from the other (‘unbleachable’) TL signals by means of a dual-aliquot regenerative-dose protocol (DAP), and the bleachable dose was estimated by subtracting the unbleachable dose from the total dose, taking into account the dose–response differences between these signals. The bleachable TL signal was found to be stable over geological timescales (
17.5 million years), to display minimal sensitivity change during successive cycles of dosing, bleaching and heating, and to exhibit reproducible dose-response behaviour. Red TL measurements are commonly plagued by poor signal-to-noise ratios due to incandescence, and possibly thermal quenching, at high temperatures. Such difficulties have been minimised in this study by measuring the red TL isothermally (at 260 1C) for an extended duration (1000 s). Red TL dating results are presented for eight samples of quartz from diverse sedimentary environments, to illustrate the potential of this dating procedure, in particular but not exclusively, for quartz that has been heated in the past (e.g., due to volcanic activity). r 2006 Elsevier Ltd. All rights reserved.

1. Introduction Optical dating protocols for quartz have traditionally concentrated on measuring the ultraviolet (UV) emissions associated with the ‘fast’ optically stimulated luminescence (OSL) component to estimate the equivalent dose or ‘palaeodose’ (Huntley et al., 1985; Aitken, 1998; BøtterJensen et al., 2003). These protocols, however, have proven troublesome to implement when applied to quartz of volcanic origin (Bonde et al., 2001; Nakagawa and Hashimoto, 2003; Morwood et al., 2004), requiring substantial modifications to single-aliquot regenerativedose protocols (Tsukamoto et al., 2003). Corresponding author. Tel.: +61 602 4221 4688; fax: +61 602 4221 4250. E-mail address: [email protected] (K.E. Westaway).

0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2005.06.010

Hashimoto et al. (1986, 1987) published the first reports of an orange-red thermo luminescence (TL) peak for quartz from volcanic ash layers, using a TL colour-imaging technique, and noted the potential of this signal as a geochronometer for volcanic sediments. The blue and red emissions from different laboratory-irradiated grains were attributed to their sedimentary and volcanic origins, respectively (Hashimoto et al., 1986). Conventional bialkali photomultiplier tubes (PMTs), developed primarily to detect UV–blue wavelengths, typically yield acceptable red TL signal-to-noise ratios for only the most intense redemitting quartz grains (Miallier et al., 1991; Scholefield and Prescott, 1999). Recent investigations, however, have demonstrated the potential of using sensitive trialkali PMTs, in tandem with a cooling system, to efficiently measure weaker red TL emissions from quartz (Fattahi and Stokes, 2000a,b; Hashimoto et al., 2002).

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Red TL emissions are commonly concentrated on a peak at around 620 nm and are visible in synthetic and natural quartz of various origins, but quartz that has been heated in the past (e.g., quartz from volcanic areas and archaeological pottery) most often gives rise to the strongest emissions (Hashimoto et al., 1987; Miallier et al., 1991; Krbetschek et al., 1997; Scholefield and Prescott, 1999; Fattahi and Stokes, 2003). This is thought to be due to impurities and defects associated with high-temperature annealing and rapid cooling, which enhance the red emissions (Hashimoto et al., 1986, 1996; Miallier et al., 1991; Rendell et al., 1994). Success in dating heat-reset quartz has been achieved by measuring the total red TL signal (e.g., Pilleyre et al., 1992; Miallier et al., 1994a, 2004; Fattahi and Stokes, 2000a), but few studies have explored the dating potential of the bleachable red TL signal. The latter have revealed that, depending on the source area of the quartz, some samples show clear evidence of bleaching (Hashimoto et al., 1989 as cited in Miallier et al., 1994b; Scholefield and Prescott, 1999; Franklin et al., 2000), whereas others do not (Miallier et al., 1991, 1994b). Most studies have reported a rapidly bleaching peak displaying first-order kinetics, with a more complex, slowly bleaching or ‘steadfastly unbleachable’ peak at a higher glow curve temperature (Scholefield and Prescott, 1999; Franklin et al., 2000; Spooner and Franklin, 2002). Scholefield and Prescott (1999) reported a bleachable red TL signal at a glow curve temperature of 305 1C, using a heating rate of 5 K s1 and a 30 min bleach by 4470 nm wavelengths of sunlight, which is similar to the readout temperature of the rapidly bleaching blue TL signal in quartz (Spooner et al., 1988), implying a common set of source traps. A luminescence dating protocol that exploits the lightreset red TL signal to establish an age for the last sediment transport event will provide a means of dating quartz grains that have been reworked since initial deposition (e.g., cave and river terrace sediments). By contrast, use of the heat-reset signal is appropriate only for deposits in primary deposition (e.g., in situ tephras). In this paper, we characterise some of the red TL properties of Indonesian quartz (the first such observations, to our knowledge) and describe a dual-aliquot regenerative-dose protocol (DAP) that isolates the bleachable signal as a means of estimating the last time of exposure to sunlight; a brief account of DAP has been given previously in connection with the dating of burial sediments containing the type specimen of Homo floresiensis (Morwood et al., 2004: Supplementary Information). As with single-aliquot methods, the minimum requirement of two aliquots facilitates the dating of samples containing only small amounts of quartz and, although results are presented for quartz samples of volcanic origin, DAP is not restricted to volcanic materials.

palaeoanthropological and archaeological cave site of Liang Bua, which is located in a karst region of western Flores (Brown et al., 2004; Morwood et al., 2004). Five samples were collected from the cave deposit, a further two from a conglomerate deposit situated at the rear of the cave, and one each from separate river terraces located close to the cave entrance (Table 1). The cave and riverterrace sediments were collected by hammering light-tight tubes into freshly cleaned faces of excavations and exposures, respectively, while the concreted nature of the conglomerate deposit required night-time sampling of a freshly exposed section using subdued red light for illumination. Samples were prepared in dim red light using standard procedures for extraction and purification of 90–125 mmdiameter quartz grains, including an etch in 40% hydrofluoric acid for 45 min to remove the external a-irradiated portion of each grain (Aitken, 1998). Grains were mounted on stainless-steel discs (
5000 grains per disc or ‘aliquot’), which were then loaded into a Risø TL-DA-12 automated reader equipped with an Electron Tubes Ltd bialkaline 9235QA photomultiplier tube fitted with one Schott BG-39 (2 mm thick) long-cut and heat-rejection filter, and one Kopp 2-63 (3.5 mm thick) long-pass filter to restrict wavelengths shorter than 600 nm. This filter arrangement has a peak transmission in the 600–620 nm range; minimum and maximum transmitted wavelengths of 580 and 670 nm, respectively; and a high signal-to-noise ratio at glow curve temperatures of 300–430 1C (Fattahi and Stokes, 2000b). Use of a conventional ‘blue’ PMT was adequate to detect the red TL emissions from the Liang Bua quartz samples. Throughout the following investigations, unless otherwise indicated, TL glow curves were obtained by heating to 400 1C at 5 K s1 and the incandescence measured during a subsequent heating to 400 1C was subtracted; the term ‘preheat’ indicates that aliquots were heated to 260 1C but not held at this temperature (i.e., a cut-heat); and corrections for changes in TL sensitivity were made using the signal induced by a test-dose of 100 Gy (followed by a preheat). For purposes of bleaching, we used a 300 W mercury discharge lamp (most intense emissions at 365 and 435 nm; see Readhead, 1988), separated from the sample by a distance of 43 cm and filtered to transmit selected wavelengths. For most of the experiments described below, including DAP (Section 9), we illuminated samples for 1 h through a Lee 226 filter. This filter transmits only wavelengths longer than 380 nm (Smith et al., 2002) and the lamp does not emit at wavelengths of 380–400 nm, so the resulting bleach is deficient in UV.

2. Sample collection and instrumentation details

The dose rate is composed of g and b contributions from the radioactive decay of 238U, 235U, 232Th (and their progeny) and 40K in the sediment samples, together with lesser ionising radiation contributions from cosmic rays

Quartz samples were collected from two different depositional environments in the vicinity of the important

3. Dose rate determinations

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Table 1 Quartz samples from Indonesia investigated in this study using the dual-aliquot regenerative-dose protocol (DAP) Sample code

Sample location Depositional context / Depth (cm)a

Grain Dose rate TL signalc Palaeodose diameter (mm) (Gy ka1)b (Gy)b

LBS7-40 VII/485–490

Cave sediment

90–125

LBS7-42 VII/593–598

Cave sediment

90–125

LBC-36

Con/285–290

Cave sediment

90–125

LBC-37

Con/158–163

Cave sediment

125–212

LBS3-2

III/740–753

Cave sediment

90–125

LBS3-4

III/496–491

Cave sediment

90–125

LBS4-32 IV/748–753

Cave sediment

90–125

WR1

T5/117–122

River terrace

90–125

WR9

T3/170–175

River terrace

125–180

2.1670.08 U B 2.4870.09 U B 1.2670.05 U B 1.3870.05 U B 0.8470.03 U B 1.4170.05 U B 1.4370.06 U B 1.3870.05 U B 1.3370.05 U

19777 81716 23576 86710 325715 235740 295710 172775 349713 86725 372719 96717 232710 136718 387710 32710 451712

Recycling ratiod

Thermal transfer (%, Gy)e

D0 (Gy)f

TL age (ka)b

1.0070.04 1.2270.04 1.0270.03 1.0770.05 0.9970.05 0.9770.06 1.0870.05 1.0970.06 0.9670.02 1.0170.03 1.0170.01 0.9970.01 1.0970.03 1.0370.01 1.0370.03 0.9570.03 0.9670.02

1.5, 2.9

410,000 410,000 700 2300 2300 410,000 1600 9000 1800 75 2000 75 1500 700 1300 95 2100

9175 3878 9575 3574 259715 187732 214710 125755 414722 102730 263716 68712 163710 95713 245711 2377 340717

0.8, 1.9 2.8, 9.1 0.3, 0.8 0.5, 1.9 0.2, 0.9 1.1, 2.6 0.9, 3.4 0.4, 2.0

a

III, IV and VII, excavated sectors of cave floor deposit; Con, conglomerate deposit at rear of cave; T3 and T5, numbered terrace deposits located close to the cave entrance. b Uncertainties at 68% confidence interval. Palaeodose values include a systematic uncertainty of 72% associated with laboratory b-source calibration. c U, unbleachable red TL signal; B, bleachable red TL signal. d Determined from sensitivity-corrected TL values for duplicate regenerative-dose points used in the construction of the TL dose–response curves. e Percentage value determined from the sensitivity-corrected TL intensity of the zero-dose point divided by the sensitivity-corrected TL intensity of the natural dose point, multiplied by 100. The corresponding value in Gy was calculated as the product of the percentage value (divided by 100) and the unbleachable palaeodose. f Characteristic saturation dose derived from fitting a saturating-exponential-plus-linear function to the sensitivity-corrected TL dose–response data.

and the a and b emissions from radioactive inclusions internal to the quartz grains. g-dose rates were derived from field measurements of U, Th and K concentrations made using a portable g-ray spectrometer (to accommodate any in situ heterogeneity in the g-ray flux) and from highresolution g-ray spectrometry analyses of powdered samples (to determine the state of disequilibrium in the 238U and 232Th decay chains). The b dose rates were estimated from the high-resolution g-ray spectrometry measurements, allowing for b-dose attenuation (Mejdahl, 1979). The g- and b- dose rates were both corrected for the measured (field) water contents of 3–11% and calculated from radionuclide concentrations using the conversion factors of Olley et al. (1996). Cosmic-ray dose rates were estimated from published relationships (Prescott and Hutton, 1994), making allowance for the 7.4 m thickness of limestone roof above the cave deposit, the sediment overburden at each sample locality, and the altitude (
500 m above sea level) and geomagnetic latitude (
201) of the sampling sites. An effective internal a- and b-dose rate of 0.03 Gy ka1 (1 ka ¼ 1000 years) was assumed for each sample. 4. Characterising the red TL signals To identify and isolate the main red TL signals present in the naturally and laboratory-irradiated samples, glow curves were obtained for samples WR1 (terrace deposit)

and LBS7-42 (cave deposit) using aliquots that had received different pretreatments. We note that although these samples are from different depositional settings and may contain quartz grains derived from different sources, their red TL properties are similar. Aliquots of samples WR1 (Fig. 1(a)) and LBS7-42 (Fig. 1(b)) that had been heated to 400 1C to erase the natural TL, cooled to room temperature and then given a b dose of 500 Gy, display low-temperature peaks at 140 and 210 1C. These were erased by preheating at 260 1C. Neither peak was present in the natural aliquot of sample WR1, so we consider both peaks to be unstable over geological time periods and, therefore, unsuitable for purposes of TL dating. Both samples have a dominant TL peak at
380 1C, which is unaffected by the UV-deficient bleach. But bleaching did result in the loss of TL signal in the 260–305 1C interval, on the low-temperature ‘shoulder’ of the main TL peak. The inset plots show the bleachable red TL peak, as determined from the difference between the bleached and unbleached glow curves. The lowtemperature shoulder and the main peak of sample LBS7-42 were reduced significantly by bleaching with UV wavelengths (i.e., without any Lee 226 filter interposed between lamp and sample), as shown by the marked decreases in TL peak intensity after 1 and 6 h of unfiltered illumination.

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(a)

4

3 2 1 0 200

0 0 0

300 250 Temperature (°C)

350

200

100

300

400

500 Gy + no preheat 500 Gy + 260°C preheat 500 Gy + 1 hr bleach + 260°C preheat 500 Gy + 1 hr UV bleach + 260°C preheat 500 Gy + 6 hr UV bleach + 260°C preheat

20

10 8 6 4 2 0 200

TL (x100 c/5°C)

TL intensity (kc/5°C)

4

2

30

10

250 300 Temperature (°C)

The existence of rapidly bleaching red TL signals at
270 and 305 1C has previously been reported for nonAustralian quartz (Scholefield and Prescott, 1999), and Miallier et al. (1994b) had remarked that sunlight rapidly reduced the red TL measured at
340 1C, on the ‘shoulder’ on the main peak at 385–390 1C. Also, Franklin et al. (2000) had observed a similar temperature range (
260–350 1C) for the rapidly bleaching red TL signal in a sample of Australian quartz. Our data are broadly consistent with these observations. We present below further bleaching tests on the RBS, which we also refer to as the ‘bleachable’ red TL signal.

350

5. Selecting a suitable preheat

0 (b)

dating of sediments that had been exposed to direct sunlight for an extended duration prior to deposition; and (3) a rapidly bleaching ‘shoulder’ (RBS) at 260–305 1C, which is sensitive to wavelengths longer than 400 nm (i.e., visible spectrum only). This signal is the most appropriate to estimate the last time of sediment exposure to sunlight, as it does not require that the quartz grains were exposed directly to UV wavelengths.

Natural 500 Gy + no preheat 500 Gy + 260°C preheat 500 Gy + 1 hr bleach + 260°C preheat TL (x100 c/5°C)

TL intensity (kc/5°C)

6

0

100

200

300

400

Temperature (°C)

Fig. 1. Red TL glow curves for (a) naturally and laboratory-irradiated aliquots of sample WR1, and (b) laboratory-irradiated aliquots of sample LBS7-42. Sample treatments are shown in the legend, where ‘bleach’ denotes bleaching only by visible wavelengths (i.e., deficient in UV) and ‘UV bleach’ denotes bleaching by visible and UV wavelengths (see text for details). The inset plots show the difference in TL counts between the glow curve measured after preheating and that measured after UV-deficient bleaching and preheating. The resultant plots, therefore, show the temperature region of the glow curve associated with the bleachable red TL signal, which is most intense at 260–305 1C.

We thus consider the samples examined to consist of three main TL signals, all of which are thermally stable over geological timespans: (1) a dominant peak at
380 1C, which is insensitive to UV or longer wavelengths. This optically inert signal was reset by the last significant heating event (e.g., volcanic eruption, in the present context) and it accounts for approximately half of the total TL counts recorded in the high-temperature (300–400 1C) region of the glow curve; (2) a smaller TL signal, also centred at
380 1C, which is sensitive to UV but not to longer wavelengths. We refer to this signal as the slowly bleaching peak (SBP). This signal would only be suitable for TL

Annealing tests were implemented to establish a suitable preheat regime, our aim being to determine which preheat resulted in a glow curve most similar to that of the natural sample. Using natural and laboratory dosed aliquots of sample WR1, we applied increasingly stringent preheats to compare the shape and intensities of the resultant glow curves. No preheat was given to the natural sample in the first cycle of measurement. In the second cycle, a beta dose of 340 Gy was given, to simulate the expected natural dose in this sample, and was then preheated at 260 1C before measurement. The dosing and preheating cycle was then repeated, with the preheat temperature increasing by 10 1C with each cycle, up to 300 1C. For clarity of presentation, only the data obtained for preheats of 260, 280 and 300 1C are shown in Fig. 2(a), with the glow curves truncated at 350 1C. The data show that the natural signal is matched most closely in shape and intensity by the glow curve resulting from the 260 1C preheat, especially in the 260–305 1C interval where the RBS is most pronounced. Preheats higher than 260 1C appear to remove signal in this critical region. Similar results were obtained for aliquots of sample WR1 that had been given a more substantial laboratory dose (
500 Gy) before applying increasingly severe preheats. The data obtained for the 260, 300 and 340 1C preheats are shown in Fig. 2(b). As with the natural sample, the glow curve measured after the 260 1C preheat provides the closest match to that obtained without preheating, whereas higher temperature preheats resulted in the systematic erosion of the main red TL peak.

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1600 Natural 340 Gy + 260°C preheat

TL intensity (c / 5°C)

1200

340 Gy + 280°C preheat 340 Gy + 300°C preheat

800

400

0 (a)

150

250

300

350

300

400

500 Gy + no preheat 500 Gy + 260°C preheat 500 Gy + 300°C preheat 500 Gy + 340°C preheat

6000 TL intensity (c/5°C)

200

4000

2000

0 0 (b)

100

200 Temperature (°C)

Fig. 2. Red TL glow curves for (a) naturally and laboratory-irradiated (340 Gy) aliquots of sample WR1 subjected to various preheats, showing thermal erosion of the 260–305 1C region by 280 and 300 1C preheats, and (b) laboratory-irradiated (500 Gy) aliquots of sample WR1 given increasingly stringent preheats, showing thermal erosion of the main red TL peak by 300 and 340 1C preheats.

6. Isothermal TL measurements Red TL emissions have traditionally been measured using glow curves, in which samples are heated at a constant rate to a high temperature, such as 450 1C (Pilleyre et al., 1992; Miallier et al., 1994a; Fattahi and Stokes, 2000a; Yawata and Hashimoto, 2004). One of the drawbacks of this approach is that the red TL signal can be overwhelmed by the red-hot glow due to incandescence. An alternative approach, not previously used for dating, is to measure the red TL signal isothermally; that is, to hold the sample at a fixed, moderate temperature (e.g., 260 1C) for an extended duration (e.g., 1000 s). The resulting isothermal TL decay curves can then be used in place of the traditional glow curves to estimate sample palaeodoses.

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Isothermal measurements offer certain practical benefits that were pointed out to us by R.M. Bailey (pers.comm., 2003). First, incandescence is not a complication at temperatures below
300 1C, so the lower thermal background allows the existence of weak red TL signals to be identified more easily. Second, heating to lower temperatures for TL measurement reduces any loss of signal due to thermal quenching (which may affect some red TL peaks; Spooner and Franklin, 2002), and minimises any sensitivity changes induced by heating to high temperatures. Owing to these advantages, we adopted isothermal measurement of red TL for purposes of dating using DAP. Isothermal measurements were made at 260 1C to stimulate the slow release of charge over 1000 s, by which time the TL signal had decayed to background. Each aliquot was preheated to 260 1C immediately prior to these measurements, resulting in the thermal erosion of some of the 260 1C isothermal signal and thermal transfer to deeper traps. Accordingly, we observed peak TL intensities a short time (20 s) after the start of isothermal measurement. For our analyses, we integrated the TL signal over the 20–30 s interval of isothermal decay to capture the region of peak TL intensity, and estimated the background count rate from the final 160 s of decay (Fig. 3(b)). The isothermal temperature of 260 1C was chosen on the basis of preheat tests conducted on natural and laboratoryirradiated aliquots of sample WR1 (as discussed above) and the pulse-anneal isothermal experiments described in the next section. These analyses show that the RBS may be reliably sampled by making isothermal measurements at 260 1C, while simultaneously excluding the unwanted TL signal from the two, thermally unstable, peaks at 140 and 210 1C. The preheat tests also indicated that isothermal measurements conducted at higher temperatures (e.g., 300 1C) would be compromised by substantial erosion of the bleachable red TL in the RBS region (Figs. 2(a and b)). In addition, a ‘dose recovery’ test using a laboratoryirradiated sample showed that the correct (known) dose was recovered using the 20–30 s interval of isothermal decay at 260 1C (see Section 10). Direct comparisons of the red TL emissions from conventional glow curve measurements and from isothermal measurements of sample LBS7-40 are shown in Figs. 3(a) and (b), respectively. The TL signal corresponding to the natural dose (shown as filled circles in both plots) increases from
240 counts per 5 1C above background at a glow curve temperature of 260 1C (i.e., the RBS) to a maximum of
10,500 counts s1 above background when held at 260 1C for 1000 s. The important feature of this analysis is the substantial improvement in the signal-tonoise ratio, which is critical when dealing with dim samples using a conventional ‘blue’ photomultiplier tube. 7. The bleachable red TL signal Comprehensive resetting of the bleachable red TL signal by visible wavelengths (4400 nm) would lend confidence to

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16

TL intensity (kc / 5°C)

Natural 1000 Gy

12

1500 Gy Test dose 8

4

0 0

100

(a)

300 200 Temperature (°C)

400

TL intensity (kc/s)

25 Signal: 20–30 s integral

20

Natural 400 Gy Test dose Zero dose

15 10

Background: 840–1000 s integral

5 0 (b)

0

200

400

600

800

1000

Time (s)

Fig. 3. Comparison of (a) red TL glow curves for naturally and laboratory-irradiated aliquots of sample LBS7-40, and (b) red TL isothermal decay curves for naturally and laboratory-irradiated aliquots of the same sample (held at 260 1C for 1000 s). The latter also shows, as grey bands, the intervals of isothermal decay used to calculate the TL signal (20–30 s) and background (final 160 s).

the use of this signal as a means of estimating the time since the quartz grains were last exposed to direct or scattered sunlight. To investigate the bleaching potential of the red TL signal, we illuminated aliquots by the following means: (1) unfiltered daylight. Tests conducted in Wollongong during November 2003, between the hours of 9 a.m. and 5 p.m. and in the absence of any cloud cover; (2) unfiltered mercury discharge lamp (i.e., including the UV component); (3) mercury discharge lamp fitted with Lee 226 filter to transmit only visible wavelengths; (4) mercury discharge lamp fitted with Lee 226 (UVblocking) filter sandwiched between two Lee 101 filters to transmit only wavelengths 4470 nm. In each case, the effect of illumination on the red TL signal was determined for four different aliquots of sample

LBS7-40 using six bleaching durations: 15, 30 min, 1, 3, 6 and 12 h. Prior to these bleaching cycles, the aliquots had been repeatedly heated (to 260 1C) and dosed, with the final dose (of 200 Gy) being the starting point for the bleaching tests. The TL signals after bleaching were measured isothermally and the corresponding doses obtained by constructing dose–response curves (from a series of regenerative-dose cycles) and interpolating the ‘bleached’ TL intensities. The sensitivity-corrected and normalised doses are plotted in Fig. 4(a). Unfiltered daylight and the unfiltered lamp (see Fig. 4(b) also) bleach the red TL signal at similar rates and to a similar residual level of
50% initial TL intensity after 12 h exposure. A similar percentage relationship between the red TL emissions before and after 6 h of UV bleach was observed for the main TL peak in the glow curves of Fig. 1(b) (compare  symbols with filled rectangles). We interpret the remaining 50% of the total TL signal as arising from less bleachable and non-bleachable source traps, the latter being reset by heating to high temperatures (e.g., during volcanic events). Removing the UV component resulted in the TL signal declining rapidly to
80% of the initial TL intensity after
1 h illumination, but remaining constant thereafter. From this, we infer that UV wavelengths are partly responsible for bleaching, but that visible wavelengths contribute also. The latter are restricted to violet and blue photons (400–470 nm), given the lack of significant bleaching by 4470 nm light. Scattered sunlight is dominated by shorter wavelengths, so the use of violet and blue light for laboratory bleaching is considered appropriate for sediments deposited in or near the shaded cave mouth at Liang Bua, which may not have been exposed to direct sunlight when last transported. To confirm that the bleachable red TL signal originates from the 260–305 1C temperature interval, pulse annealing experiments were conducted on an aliquot of sample WR1 from which the natural signal had been thermally erased. The aliquot was given a regenerative dose of 200 Gy and preheated to 260 1C before isothermal measurement, and these steps were repeated with the preheat temperature increasing by 10 1C for each regenerative-dose cycle (up to 460 1C). The sensitivity-corrected pulse–anneal curve is shown in Fig. 5 (open circles). This experiment was repeated with a bleach given after the regenerative-dose step in each cycle to leave the unbleachable red TL signal (Fig. 5, filled circles). The difference between the two curves represents the bleachable signal. Most of this originates at 260–305 1C, with a small amount remaining up to 350 1C. Across this temperature range, the test-dose signal experienced negligible sensitivity change (Fig. 5, inset plot). 8. Thermal stability of the red TL signal To estimate the trap depth and lifetime at ambient temperature of the source traps giving rise to the initial (20–30 s) portion of isothermal decay (i.e., the bleachable

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0.8

0.8

0.6

0.6 0.4

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4

6

8

10

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12

1.2

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1.0

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UV lamp + Lee 226

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UV lamp + Lee 101

0.4 0

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(a)

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8

RBS

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Bleaching time (hr)

Test-dose TL (kc)

1.0

2.5

UV lamp

Sun

Sensitivity-corrected TL (a.u.)

Normalised dose

1.2

2519

2.0

1.5

6 4 2 0 250

1.0

300

350

400

450

Temperature (°C)

0.5

Total red TL Unbleachable red TL

0.0 260

310

360

410

460

Temperature (°C) 25

Fig. 5. Pulse-anneal curves for a laboratory-irradiated aliquot of sample WR1 measured before (open circles) and after (filled circles) a UVdeficient bleach for 1 h. The difference between the two curves indicates the temperature region associated with the bleachable red TL signal, denoted here as the rapidly bleaching shoulder (RBS) of the main TL peak. The inset plot shows the test-dose TL for each measurement cycle. Measurement uncertainties are smaller than the size of the symbols.

No bleach 15 min UV bleach 20

6 hr UV bleach

TL intensity (kc /s)

12 hr UV bleach 15

10

5

0 0

(b)

125

250

375

500

Time (s)

Fig. 4. Results of bleaching experiments conducted on aliquots of sample LBS7-40 that had been heated and irradiated before bleaching. (a) Plots showing reduction in dose with duration of bleaching by unfiltered daylight (‘Sun’), an unfiltered mercury discharge lamp (‘UV lamp’), the same lamp but with the UV component removed (‘UV lamp+Lee 226’), and the lamp filtered to transmit only 4470 nm light (‘UV lamp+Lee 101’). In each plot, values are normalised to the dose measured after a bleach of zero duration, and measurement uncertainties are smaller than the size of the symbols. (b) Corresponding isothermal TL signals for the ‘UV lamp’ bleach, truncated at 500 s stimulation time for clarity of presentation.

red TL signal), measurements were made on aliquots of sample WR1 that had each received a b-dose of 500 Gy, followed by periods of storage at elevated temperatures. This ‘isothermal decay’ approach consisted of various isothermal storage treatments, followed by isothermal measurement of the remaining TL signal. The irradiated quartz was held at temperatures of 160 and 200 1C for periods of up to 24 h, and at 240, 260, 280 and 300 1C for periods of up to 1440 s (24 min) before measurement, and an initial determination (L0) was also made for zero storage time. Our analysis assumes that the red TL signal obeys firstorder kinetics, and several workers have argued that this is

so for the bleachable red TL signal (Franklin et al., 2000; Spooner and Franklin, 2002). We also note that the 20–30 s interval of isothermal decay includes red TL from both unbleachable and bleachable source traps, so our data on thermal stability do not pertain solely to the bleachable signal—unlike Franklin et al. (2000) and Spooner and Franklin (2002), who separated the rapidly and slowly bleaching TL peaks for kinetic analysis. For each of the six storage temperatures, the sensitivity-corrected values (Ln) were normalised by L0 to obtain the ratios Ln/L0, which were then converted to natural logarithms and plotted against storage time (Figs. 6(a and b)). The trap lifetime at each of the storage temperatures was estimated by reading off the corresponding storage time at ln ðLn =L0 Þ ¼ 21. These lifetime estimates were plotted against the reciprocal of the storage temperatures and fitted by linear least-squares to estimate the trap depth (E, 1.59 eV) and the escape frequency factor (s, 4.64  1012 s1) (Fig. 6(c)), which were then entered into the following equation for trap lifetime (t, in s): t ¼ ð1=sÞ expðE=kTÞ, where k is Boltzmann’s constant and T is absolute temperature (Aitken, 1985). A lifetime of
17.5 million years was determined for an ambient temperature of 20 1C, which is similar to the present-day temperature of 21 1C at Liang Bua in the dry season. This lifetime is several ordersof-magnitudes shorter than that calculated by Fattahi and Stokes (2000a) for one of three red TL components in a sample of New Zealand quartz (5.9  1011 years), but our estimates of trap depth and lifetime are similar to the values of 1.65 eV and 57 million years obtained for the rapidly bleaching red TL peak in an Australian quartz

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2520

Time (s) 0

500

Time (ks)

1000

1500 240°C 260°C 280°C 300°C

ln (Ln /L 0 )

-0.4 -0.8

°

-1.2

0

ln (Ln /L 0 )

0

0

20

40

60

80

-0.4

160°C 200°C

-0.8

-1.6

(a)

-2.0

(b)

0s 20 s 70 s 320 s

8 TL intensity (kc /s)

16

ln (lifetime, s)

-1.2

12 8 4

6 4 2

y = 18502x - 29.166 0 0.0015

(c)

0 0.0017

0.0019

0.0021

0.0023

0.0025

1 / absolute temperature (K)

0

(d)

100

200

300

400

500

Time (s)

Fig. 6. Thermal stability data obtained from isothermal lifetime experiments using aliquots of sample WR1. Laboratory-irradiated aliquots were stored at six different temperatures (in the range 160–300 1C) for various durations, and then measured isothermally (at 260 1C for 1000 s). (a) and (b) show the log ratios of TL intensity (Ln) divided by the initial measurement at zero storage time (L0) plotted against storage time at the elevated temperature denoted in the legend; dashed lines denote ratios of –1. (c) Arrhenius plot based on the data from (a) and (b), showing the equation for the line of best fit from which E and s were obtained. (d) Isothermal TL signals measured after storage times of 0, 20, 70 and 320 s at 260 1C.

sample (Franklin et al., 2000; Spooner and Franklin, 2002). We attribute the small differences to sample-to-sample variability in the red TL behaviour of quartz. To avoid underestimation of the palaeodose, the trap lifetime should be at least 20 times the burial age of the sample (Aitken, 1998). A lifetime of
17.5 million years satisfies this requirement for sediments that were deposited up to 850 ka ago. In practice, however, the long-range dating potential of the red TL signal is limited less by the lifetime value than by the saturation dose. There also remain uncertainties about the shape of the dose–response curve and the extent of sensitivity change in samples older than 1 million years (Miallier et al., 1991; Pilleyre et al., 1992), although red TL ages of up to
1.3 million years, compatible with independent age control, have been reported for volcanically heated quartz (Fattahi and Stokes, 2000a). 9. Palaeodose determination using a dual-aliquot protocol (DAP) To estimate the sample palaeodose from the bleachable red TL signal, we developed a method that required as few aliquots as possible, owing to the paucity of quartz in the Liang Bua deposits. Because the bleachable red TL signal cannot be measured directly, but by subtraction of the unbleachable signal from the total signal, a minimum of two aliquots—hereafter termed ‘Aliquot A’ and ‘Aliquot B’—is required; hence, the DAP. DAP may be thought of as a modified single-aliquot regenerative-dose OSL protocol (Murray and Wintle,

2000a) combined with the philosophy of the ‘selective bleach’ TL technique (Prescott and Mojarrabi, 1993). This hybrid procedure is summarised in Fig. 7. After each of the preheats and before the TL measurements, each aliquot was exposed to infrared radiation (100 s at 50 1C) to drain any IR-sensitive signal from possible feldspar inclusions (following Olley et al., 2004). Dose–response curves for Aliquots A and B were constructed from a minimum of five regenerative-dose points, including a zero-dose point (to check that a negligible TL signal was obtained) and a duplicate regenerative-dose point (to check on the efficacy of the sensitivity-correction procedure; the ‘recycling ratio’). These points were fitted by a saturating-exponential-plus-linear function, the palaeodoses obtained by interpolation of the natural TL intensities, and the uncertainties estimated by Monte Carlo simulation (Yoshida et al., 2003). Aliquot A data were used to estimate the palaeodose associated with the unbleachable TL signal (Pu), and this was achieved by initially bleaching the aliquot to remove the light-sensitive TL signal. Hence, in Fig. 7, the natural signal (Nu) measured for Aliquot A represents the unbleachable natural TL. The same bleaching treatment was applied before measuring the corresponding test-dose TL signal (Tu), and in each of the subsequent regenerativedose (Ru) and test-dose cycles. Aliquot B data were used to determine the dose– response of the total TL signal—that is, bleachable plus unbleachable TL. In the first cycle, the total natural signal (Nt) was measured isothermally and normalised by its test-dose signal (Tu). The latter was bleached before

ARTICLE IN PRESS K.E. Westaway, R.G. Roberts / Quaternary Science Reviews 25 (2006) 2513–2528

TL

Aliquot B Total red TL signal

Aliquot A Unbleachable red TL signal B1

A1

Preheat & measure isothermally = total (unbleachable + bleachable) natural TL

Nt

Test-dose (100 Gy), bleach, preheat & measure isothermally = unbleachable test-dose TL

Tu

Bleach (1 hr) & preheat (260°C)

2521

N t / Tu

TL u+

Nu / Tu

TLb

Total TL dose-response Bleachable TL dose-response

B2 A2

Nu

Heat to 260°C & hold for 1000 s (isothermal measurement) = unbleachable natural TL

TLu

B3 A3

Tu

Test dose (100 Gy), bleach, preheat, measure isothermally = unbleachable test-dose TL

A4

Ru

Regenerative dose, bleach, preheat & measure isothermally = unbleachable regenerated TL

A5

Repeat steps A3 and A4, construct dose-response curve from Ru / Tu values, & interpolate Nu / Tu value

Dose Pu

Add dose equivalent to Pu (from aliquot A) & bleach

B4

Add regenerative dose, preheat, measure isothermally = (unbleachable + bleachable) regenerated TL

Rt

B5

Test-dose (100 Gy), bleach, preheat & measure isothermally = unbleachable test-dose TL

Fig. 8. Schematic diagram of total and bleachable TL dose-responses curves for Aliquot B, showing the unbleachable signal (TLu) arising from the addition of the unbleachable palaeodose (Pu, from Aliquot A) in step B3 of Fig. 7, the unbleachable signal induced by the regenerative doses (TLu+) in steps B4 and B5, and the bleachable signal deduced by subtraction (TLb) in step B6. The bleachable palaeodose (Pb) is estimated by interpolating the Nt/Tu value (determined from steps B1 and B2) on to the bleachable dose-response curve, which originates at the Nu/Tu value obtained from Aliquot A.

Tu

B6

Repeat steps B3–B5 to construct unbleachable + bleachable dose-response curve from Rt / Tu values

Pu Unbleachable palaeodose

Pb

B7

Subtract unbleachable signal from dose-response curve & interpolate Nt / Tu value

Pb Bleachable palaeodose

Fig. 7. Procedural steps for using the dual-aliquot regenerative-dose protocol (DAP) to determine the unbleachable palaeodose, Pu, from Aliquot A (left-hand column) and the bleachable palaeodose, Pb, from Aliquot B (right-hand column).

measurement to provide a standard unit of comparison for the sensitivity-corrected data obtained from Aliquots A and B, as required for the subtraction technique described below. Aliquot B then received a b-dose equal to the unbleachable palaeodose (Pu, derived from Aliquot A) and was bleached to empty the source traps responsible for the light-sensitive TL signal. (At this step in DAP, the unbleachable traps in Aliquot B are filled to the same extent as those in Aliquot A.) Aliquot B was then given a series of regenerative doses and the resultant total TL (Rt) measured isothermally without any further bleaching. The TL dose–response curve of Aliquot B originates at the dose where the unbleachable source traps are filled to their natural (Pu) state. At higher doses, the unbleachable and bleachable source traps both contribute to the

further growth in TL, so the unbleachable TL induced by the regenerative doses must be subtracted to isolate the dose–response and palaeodose (Pb) associated with the bleachable signal (Fig. 8). The unbleachable TL dose–response is known from Aliquot A, so we determined the fitting parameters of this curve (e.g., saturation TL intensity, Imax, and characteristic saturation dose, D0) and then subtracted its growth rate from that of Aliquot B, using Pu as the reference value common to both aliquots. The bleachable source traps are responsible for any remaining TL. For example, if Aliquot A has N u =T u ¼ 2:67 for Pu ¼ 200 Gy, and Ru =T u ¼ 3:00 for a regenerative dose of 225 Gy, then a value of 0.33 is subtracted from the Rt/Tu ratio of Aliquot B at a regenerative dose of 25 Gy (as the Aliquot B growth curve originates at Pu). The same process is repeated for each regenerative-dose point to determine the bleachable TL dose–response and estimate Pb. A detailed worked example is given in the Appendix. A significant benefit of DAP is that the correct palaeodose should be obtained even when the dose– response curves of the bleachable and unbleachable TL signals are dissimilar and/or non-linear. The latter flexibility is important, given the reported differences in growth curve shape between the rapidly and slowly bleaching blue TL signals in quartz (e.g., Wintle, 1997; Murray and Wintle, 2000b) and the possibility that their red TL counterparts may likewise differ in their dose–response behaviours. It might seem that the bleachable palaeodose could be calculated more easily from the difference between the total palaeodose (i.e., from bleachable and unbleachable source traps) from one aliquot and the unbleachable palaeodose from a second aliquot. But this ‘simple subtraction’ approach can be shown mathematically to yield incorrect palaeodose estimates for

ARTICLE IN PRESS K.E. Westaway, R.G. Roberts / Quaternary Science Reviews 25 (2006) 2513–2528

2522

samples with bleachable and unbleachable dose–response curves that differ in shape (see Appendix). One caveat of DAP (which applies equally to any multiple-aliquot method of palaeodose determination) is that both aliquots are assumed to exhibit similar TL properties, as the data from Aliquot A is input to Aliquot B. This assumption will almost certainly prove invalid for aliquots containing few grains, hence the need for both aliquots to be composed of several hundreds or thousands of grains. The latter requirement limits application of DAP to sediments that have been exposed to daylight for a prolonged period before final deposition.

(Table 1), which indicates the long-range dating potential of the heat-reset signal. A similarly large D0 value (6300 Gy) was obtained by Fattahi and Stokes (2000a) for the heat-reset red TL signal in a sample of New Zealand quartz. To validate the reliability of the TL ages obtained using DAP, we conducted several internal checks on protocol performance (e.g., recycling ratio and dose recovery tests) and, as an external measure of accuracy, we compared the ages obtained from the bleachable red TL signal with independent age constraints at Liang Bua (Morwood et al., 2004). The recycling ratios cluster around the desired value of unity, with 14 of the 17 ratios being consistent with unity at the 95% confidence interval, and a further two being consistent at the 99% confidence interval (Table 1). It is also reassuring that thermal transfer (represented by the zero-dose point of Aliquot A) is modest, ranging from 0.2% to 2.8% of the sensitivity-corrected natural TL signal, equivalent to a dose of 0.8–9.1 Gy (Table 1). Furthermore, the test-dose TL signals reveal only minor sensitivity changes over 7–11 measurements cycles (Fig. 10, inset plots), and are insensitive to variations in preheat (pulse–anneal) temperatures in the range 260–350 1C, whence the bleachable red TL signal originates (Fig. 5, inset plot). These findings confirm that the unbleachable and bleachable TL dose–response curves are robust reconstructions. A further demonstration of DAP validity is the favourable outcome of a dose recovery test. A dose of 35075 Gy was recovered from a fresh aliquot of sample LBS3-4 that had been isothermally stimulated, given a b-dose of 350 Gy (as a surrogate natural dose) and the unbleachable TL signal then measured using DAP (Fig. 9(d)).

10. Examples and validation of DAP data Fig. 9 displays four TL dose–response curves for sample LBS4-32. These relate to (a) the unbleachable signal, (b) the total signal, (c) the bleachable signal and (d) the unbleachable signal of a fresh aliquot subjected to a dose recovery test. The unbleachable and bleachable dose–response curves for samples LBS7-40 and LBS7-42 are shown in Fig. 10, together with inset plots of test-dose TL intensities versus measurement cycle. The general form of the unbleachable dose–response is similar to that reported previously for the red and blue TL emissions from quartz, becoming increasingly non-linear (saturating exponential or saturating-exponential-plus-linear) with added dose. The latter is most pronounced for the bleachable red TL signal, and is characteristic also of the rapidly bleached blue TL signal, whereas the total and unbleachable red TL signals continue growing at high doses, as has been observed previously (Miallier et al., 1991; Fattahi and Stokes, 2000a). The D0 values of the unbleachable signal range from 700 to 410,000 Gy with a median of
1800 Gy 10

10

8

8

6

6

Sensitivity-corrected TL (a.u.)

4

4 Nat

Nat

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2 P 0

0 0

250

500

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0

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(a)

(b)

6

10

125

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8 4 Nat 6 4 Nat

2

2 0

(c)

P 0

125

Recovered dose

0 250 Dose (Gy)

375

500

0

(d)

250

500 Dose (Gy)

750

500

Fig. 9. TL dose–response curves for aliquots of sample LBS4-32. Growth curves are shown for (a) the unbleachable signal and corresponding palaeodose (Pu) in Aliquot A, (b) the total signal in Aliquot B (where 0 Gy corresponds to Pu ¼ 232 Gy, as determined from Aliquot A), and (c) the bleachable signal and corresponding palaeodose (Pb) in Aliquot B (obtained after subtracting the unbleachable signal). The results of a dose recovery test on the unbleachable signal are shown in (d), where a dose of 35076 Gy was recovered from an aliquot that had received a known (surrogate natural) dose of 350 Gy. In each plot, ‘Nat’ denotes the TL intensity of the natural signal and measurement uncertainties are smaller than the size of the symbols.

ARTICLE IN PRESS K.E. Westaway, R.G. Roberts / Quaternary Science Reviews 25 (2006) 2513–2528

Test-dose TL (kc)

LBS7-40 5 4

2523

LBS7-42 40

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Nat

2

1.5

1 0 0 (c)

0 (b)

Pb

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100 150 Dose (Gy)

Pb

200

0

0 (d)

50

Fig. 10. Examples of DAP data for samples LBS4-40 and LBS4-42. Plots (a) and (b) show the dose-response curves for their respective unbleachable TL signals, while plots (c) and (d) show the corresponding bleachable TL dose-responses curves. The TL intensities of the natural signals are labelled ‘Nat’, and Pu and Pb denote the unbleachable and bleachable palaeodoses, respectively. Changes in TL sensitivity between measurement cycle are shown by the inset plots of the test-dose TL intensities. Measurement uncertainties are smaller than the size of the symbols in all plots.

An independent evaluation of the accuracy of the bleachable TL ages for the Liang Bua sediments can be made by comparison with the chronologies obtained for the cave deposit using radiocarbon (14C), thermal ionisation mass spectrometry (TIMS) uranium-series (230Th/234U), and coupled electron spin resonance (ESR)/ U-series dating methods (Morwood et al., 2004). A schematic diagram of the cave stratigraphy and age estimates (Fig. 11) illustrates that the bleachable signal returns TL ages in correct stratigraphic order and not inconsistent with the alternative chronologies. The red TL age uncertainties are, however, larger than those typically associated with blue TL age estimates, owing to the gentle slope of the bleachable dose–response curves (Figs. 9(c), 10(c) and 10(d)), which generates dose intercepts with large uncertainties. The Aliquot B uncertainties are enlarged further by the addition (in quadrature) of the palaeodose uncertainty determined for Aliquot A and the relative uncertainty of 72% associated with calibration of our laboratory b-source. Sample LBS7-42 (3574 ka) was collected from the same sedimentary layer as charcoal that yielded a calibrated 14C age of
18 ka (Morwood et al., 2004: Fig. 4). We do not know if this 17 ka age-offset applies equally to all samples, as different samples may incorporate grains that had been bleached at deposition to a greater or lesser extent. But a small, possibly negligible, age-offset at deposition is inferred for samples LBS4-32 and LBC-37, as their TL

ages are consistent with the independent age control when stratigraphic relations are taken into account. Sample LBS4-32 was collected from 3 m below a tooth dated by coupled ESR/U-series to
74 ka and yielded a TL age of
95 ka, while the TL age of
125 ka for sample LBC-37 is stratigraphically consistent with the TIMS U-series age of
102 ka for the flowstone precipitated 1.6 m higher up the sequence. Hence, some samples may have no TL age-offset at deposition, whereas sediments transported under less favourable bleaching conditions may have age-offsets of up to 20 ka. Additional independent age cross-checks are required to assess the extent to which the red TL signal is bleached in different geomorphic settings. Finally, it is instructive to compare the ages obtained using DAP with those determined using a ‘simple subtraction’ approach, in which the bleachable palaeodose is calculated as the difference between the total palaeodose (measured from one aliquot) and the unbleachable palaeodose (measured from another aliquot). The simple subtraction method was tested on samples WR1, LBC-37 and LBS7-42, but the total palaeodose were smaller than the unbleachable palaeodoses by 82–89 Gy. This result may seem counter-intuitive, but simulations show that incorrect estimates of bleachable palaeodose can arise when this method is applied to samples with bleachable and unbleachable dose-response curves that differ in shape (see Appendix). Hence, any dating protocol must allow for signal-to-signal variation in dose–response behaviour for

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2524 Independent ages (ka) 13.2 ± 0.2

Red TL ages (ka) 14

C 38 ± 8

18.1 ± 0.6

14

37.7 ± 0.1

U/Th

74 +14 /-12

ESR

C

35 ± 4

95 ± 13 102.4 ± 0.3

U/Th 125 ± 55

187 ± 32

Fig. 11. Schematic composite stratigraphy for Liang Bua, showing the red TL ages obtained from the bleachable signal (right-hand side) and the ages derived from alternative dating techniques (left-hand side). The latter include calibrated 14C ages for charcoal fragments, U-series (U/Th) ages for flowstones, and a coupled ESR/U-series age for a Stegodon tooth (Morwood et al., 2004). All ages are listed in ka, with uncertainties at the 68% confidence interval. The pictures of a human skull and a stone tool indicate the approximate stratigraphic positions of Homo floresiensis remains and associated artefacts, respectively (Brown et al., 2004; Morwood et al., 2004). The TL ages should conservatively be regarded as maximum ages for sediment deposition, because each aliquot contained a large number of grains and some of these would likely not have been bleached sufficiently during the last transport event.

the accurate determination of the bleachable palaeodose— DAP does this. 11. Discussion Our Indonesian quartz samples are characterised by a light-sensitive red TL signal that forms a rapidly bleaching ‘shoulder’ (at 260–305 1C) on the main TL peak (at
380 1C), which could not be bleached by visible wavelengths. The long-range dating potential of the unbleachable signal had been noted previously by Miallier et al. (1991), who had also remarked that the high-temperature peak resisted bleaching more firmly than the other red TL peaks. They went on to confirm the slowly bleaching character of the 380–390 1C peak, and observed that the ‘shoulder’ at
340 1C was rapidly lost on exposure to sunlight (Miallier et al., 1994a). These properties are broadly compatible with reports of a conspicuous, bleachable red TL peak at 260–350 1C in a sample of Australian quartz (Franklin et al., 2000) and with observations of a bleachable signal at
305 1C and a slowly bleaching signal

at
360 1C in quartz from Australia, North America, Japan and France (Scholefield and Prescott, 1999). The latter authors were able to bleach the red TL signal to
50% of initial intensity after 30 min exposure to wavelengths 4470 nm, but this combination had no effect on our Indonesian samples and illustrates the inherent variability in red TL behaviour of natural quartz. The use of DAP provides a means of isolating the bleachable signal for TL dating of reworked sediments. Use of the light-sensitive signal is mandatory for sediments collected from geomorphic settings in which the last heating event (e.g., volcanic eruption) is not contemporaneous with the ‘target’ depositional event. Such is the case at Liang Bua, where the quartz samples are not from in situ volcanic deposits. However, the unbleachable TL ages for the Liang Bua samples provide an internal check on the consistency of the bleachable TL ages, as the former should always be older than the latter. This is so for each of the samples tested (Table 1), which indicates at least some of the grains were illuminated by visible wavelengths before final deposition. But as the aliquots are composed of multiple grains and not all of these grains are likely to have been fully bleached when last transported, the bleachable TL ages should be viewed as maximum ages for sediment deposition. Some of our samples contained a small bleachable signal and a large unbleachable signal, causing the latter to dominate the total red TL signal. For sample WR9, the bleachable signal was too small relative to the unbleachable signal to determine the bleachable palaeodose. The problem of isolating the bleachable component is most acute for samples with large palaeodoses, because the unbleachable TL grows more rapidly than the bleachable TL with added dose, so the increase in total TL due to the growth in the bleachable signal cannot be determined with precision. In this study, the most precise bleachable TL ages were associated with samples that had among the smallest unbleachable palaeodoses (e.g., LBS7-42 and LBS4-32). DAP requires a minimum of two aliquots to determine the bleachable palaeodose. To minimise any complications arising from differences in material properties between Aliquots A and B, we used aliquots composed of many grains and compensated for differences in innate TL intensity by means of test-dose TL signals. But additional aliquots should be measured whenever possible, to account for aliquot-to-aliquot variability in absorbed dose, over and above that associated with luminescence measurement uncertainties (Galbraith et al., 2005). We were unable to do so because of the meagre amount of sand-sized quartz extracted from our Indonesian samples, which also precluded us from exploring alternative methods to isolate the bleachable red TL signal. If sufficient material were available to make a multiple-aliquot determination of the palaeodose, then the selective bleach technique of Prescott and Mojarrabi (1993) is a promising candidate, with the inclusion of test-dose TL signals as sensitivity monitors.

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Acknowledgements We thank Mike Morwood and associates for financial support from Australian Research Council (ARC) Discovery Project Grant DP0343334, Jose Abrantes for sample preparation, Hiro Yoshida and Richard Bailey for suggestions during development of DAP, Ed Rhodes for assistance with trap lifetime calculations, Jon Olley for high-resolution g spectrometry measurements, and Tip Lancaster, Wahyu Saptomo and Thomas Sutikna for field assistance. Westaway acknowledges the financial assistance of Pamela Westaway and the University of Wollongong (University Postgraduate Award and a Tuition Fee-Waiver Scholarship), and Roberts thanks the ARC for the support of a Senior Research Fellowship. The manuscript was greatly improved by the helpful comments of John Prescott and Didier Miallier.

A worked example of DAP is provided here, to help clarify the procedures involved in determining the un-

200

3000

150

2000 1000

100 50 0

0 0

(a)

Appendix

4000 TL (counts)

DAP displays good potential for establishing red TL age estimates for the light-sensitive signal in volcanic quartz and other heated quartz. This approach is particular beneficial when the blue TL emissions from quartz are non-existent; the feldspar luminescence signals suffer from fading; the heat-reset TL signal is an inappropriate means of determining the time of deposition of reworked sediments; and materials are unavailable or unsuitable for alternative dating methods. Such deposits are widespread across Indonesia. We have demonstrated that quartz from the Indonesian site of Liang Bua has a bleachable red TL signal that can be isolated from the unbleachable signal to yield ages compatible with independent age estimates. Thermal stability calculations show that the red TL source traps are sufficiently stable to permit dating of sediments deposited during the last several hundreds of millennia, and the long-range dating potential of the unbleachable signal appears particularly promising. Reproducible dose–response curves were constructed for both the unbleachable and bleachable signals, and the TL sensitivity of neither signal changed significantly between measurement cycles. DAP is especially useful for samples that contain too little quartz for conventional multiple-aliquot palaeodose determinations. Future work could usefully include an improved understanding and isolation of the bleachable red TL signal in quartz, the application of DAP to a broader range of sedimentary environments and heat-reset samples (e.g., fired pottery), and its extension to quartz with dim red emissions. The employment of more sensitive photondetection systems should assist in these endeavours.

bleachable and bleachable palaeodoses. We have assigned a linear dose–response curve to the unbleachable signal and a non-linear growth curve (approximating a saturating exponential) to the bleachable signal, to emulate the properties of our Indonesian quartz samples (Fig. A1). Also, to simulate our observation that the bleachable signal commonly represents only a ‘shoulder’ on the main unbleachable TL peak, we have set the TL intensity of the unbleachable signal at 10 times that of the bleachable signal for a dose of 75 Gy. The latter value is the assigned bleachable palaeodose (Pb), and the unbleachable palaeodose (Pu) has been fixed at 200 Gy. For simplicity, we have assumed that the unbleachable TL induced by a test-dose of 75 Gy (Tu) is constant from cycle to cycle, to reflect the lack of sensitivity change seen in our data, but the correct palaeodoses are obtained using DAP even in the presence of sensitivity changes. Table A1 contains the data used to construct the unbleachable and bleachable TL dose– response curves. Using DAP, the bleaching treatment administered to Aliquot A would result in measurement of the unbleachable TL intensities listed in the 3rd column of Table A1, and division of these values by the corresponding unbleachable test-dose TL signals (4th column) would generate the sensitivity-corrected values listed in the 5th column. For Aliquot B, the relevant values are listed in Table A2. Prior to the regenerative doses listed in the 2nd column, Aliquot B was given the dose equivalent to the unbleachable palaeodose as measured from Aliquot A (200 Gy). Hence, the total TL output from a 25 Gy regenerative dose to Aliquot B is: 3600 counts from the unbleachable signal (Table A1, 3rd column corresponding to a dose of 200+25 ¼ 225 Gy) plus an additional 50 counts from the bleachable signal (Table A1, 3rd column corresponding to a dose of 25 Gy), giving a total of 3650 counts (Table A2, 3rd column corresponding to a dose of 25 Gy). The bleachable dose-response in Aliquot B must now be isolated from the total (unbleachable+bleachable) measured signal by a series of subtractions, to remove the unbleachable TL induced by the regenerative doses. This is achieved using the unbleachable TL dose-response data obtained for Aliquot A. All calculations are performed on the sensitivity-corrected values listed in the 5th columns of

TL (counts)

12. Conclusions

2525

40

80 120 160 200 Dose (Gy)

0

(b)

40

80 120 Dose (Gy)

160

200

Fig. A1. A simulated unbleachable TL dose–response (a) and bleachabfle TL dose–response (b) for the worked example. These linear and saturating exponential dose-response curves closely emulate the properties of our Indonesian quartz samples.

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Table A1 The data used to construct the unbleachable and bleachable TL dose–response curves TL signal

Dose (Gy)

TL (counts)

Unbleachable test-dose TL (counts)

Sensitivity-corrected TL

Unbleachable

0 25 50 75 100 125 150 175 200 225 250 275 300 0 25 50 75 100

0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 0 50 90 100 140

1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200

0.00

Bleachable

Bleachable palaeodose

Table A2 The total TL intensities and corresponding unbleachable test-dose TL, used to generate the sensitivity corrected total TL values for Aliquot B TL signal

Total regenerated

Dose (Gy)

TL (counts)

Unbleachable testdose TL (counts)

Sensitivitycorrected total TL

Natural

3320 3200 3650 4090 4520 4940

1200 1200 1200 1200 1200 1200

2.77

0 25 50 75 100

2.67 3.04 3.41 3.77 4.12

Tables A1 and A2. Let us take, for example, the 25 Gy regenerative dose point in Aliquot B. In Aliquot A, the values for the unbleachable signal at doses of 200 Gy (2.67) and 225 Gy (3.00) differ by 0.33. This amount is subtracted from the total sensitivity-corrected value of 3.04 measured for Aliquot B at a regenerative dose of 25 Gy, resulting in a value for the bleachable signal of 2.71 (Table A3). This procedure is repeated for all regenerative-dose points to determine the TL dose–response of the bleachable signal. The latter is shown below, and the data used to construct it are listed in the 4th column of Table A3. This sample has a natural total TL intensity (Nt) of 3320 counts (i.e., the sum of 3200 counts from the unbleachable signal, Nu, and 120 counts from the bleachable signal), which corresponds to a sensitivity-corrected natural value of 2.77 (Table A2, 5th column). Interpolation of this value on to the bleachable dose-response curve results in a dose estimate of 75 Gy, which is the correct value for the bleachable palaeodose (Fig. A2 and Table A3).

Table A3 The sensitivity-corrected total TL intensities and corresponding sensitivity-corrected bleachable TL intensities for Aliquot B, determined by the subtraction technique TL signal

Dose (Gy)

Bleachable palaeodose

Sensitivity-corrected total TL

Sensitivity-corrected bleachable TL

0 25 50

2.67 3.04 3.41

2.67 2.71 2.74

75 100

3.77 4.12

2.77 2.78

2.80 Sensitivity-corrected TL

Unbleachable palaeodose

0.33 0.67 1.00 1.33 1.67 2.00 2.33 2.67 3.00 3.33 3.67 4.00 0 0.04 0.06 0.10 0.12

Natural 2.75

2.70

2.65 0

25

50

75

100

Dose (Gy) Fig. A2. The sensitivity-corrected TL dose–response curve for the bleachable signal. Note the restricted range on the y-axis.

Let us now apply a ‘simple subtraction’ technique, whereby the bleachable palaeodose is calculated from the difference between the total palaeodose and the unbleachable palaeodose, each measured using a separate aliquot.

ARTICLE IN PRESS K.E. Westaway, R.G. Roberts / Quaternary Science Reviews 25 (2006) 2513–2528

3.0

TL signal

Total regenerated

Dose (Gy)

0 25 50 75 100 125 150 175 200

TL (counts)

0 450 890 1320 1740 2150 2555 2958 3360

Total test-dose TL (counts)

1320 1320 1320 1320 1320 1320 1320 1320 1320

Sensitivitycorrected total TL

0.00 0.34 0.67 1.00 1.32 1.63 1.94 2.24 2.55

Using the same data sets as above, we obtain 3200 counts for the unbleachable natural signal corresponding to a palaeodose of 200 Gy (Table A1), and this would be divided by a test-dose value of 1320 counts (representing 1200 counts for the unbleachable test-dose TL, plus 120 counts (i.e., 10 times smaller) for the bleachable test-dose TL) to produce a sensitivity-corrected unbleachable natural value of 2.42. (We have taken the simplest approach and employed the total test-dose TL for sensitivity correction, rather than the unbleachable test-dose TL as used in DAP, which requires a bleach before TL measurement.) The total (unbleachable+bleachable) natural signal would give rise to 3320 counts (i.e., 3200+120 counts from the unbleachable and bleachable signals, respectively). When divided by the test-dose TL of 1320 counts, this yields a sensitivity-corrected total natural value of 2.52. The TL dose–response values for the total regenerated signals are listed in Table A4, the values in the 3rd column being the sum of the unbleachable and bleachable figures listed in Table A1. The sensitivity-corrected values in the 5th column can be fitted by a second-order polynomial to obtain the equation for the total TL dose–response curve (Fig. A3). Interpolation of the sensitivity-corrected total and unbleachable natural values of 2.52 and 2.42, respectively, produces corresponding total and unbleachable palaeodose estimates of 196 and 188 Gy. The latter is incorrect, as is the bleachable palaeodose (8 Gy) obtained from the difference between these two values. The bleachable palaeodose, in particular, greatly underestimates the true value (75 Gy) because the differing TL dose–responses of the unbleachable and bleachable signals have not been taken into account. If the TL intensities of the unbleachable and bleachable signals are set to the same value at 75 Gy (rather than differing by an order-of-magnitude), then the estimate obtained for the bleachable palaeodose moves closer to the true value while that of the unbleachable palaeodose moves farther away, and neither is correct.

Sensitivity-corrected total TL

Table A4 The TL dose–response values for the total regenerated signals

2527

y = -4E-06x2 + 0.0136x + 0.0032

2.5 2.0 1.5 1.0 0.5 0.0 0

50

100

150

200

Dose (Gy) Fig. A3. The sensitivity-corrected total TL dose–response curve, obtained using the total test-dose TL values and fitted by a second-order polynomial.

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