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Luminescence dating of baked earth and sediments from the Qujialing archaeological site, China
Quaternary Geochronology 5 (2010) 353–359
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Research Paper
Luminescence dating of baked earth and sediments from the Qujialing archaeological site, China Xiao Fu a, Jia-Fu Zhang a, *, Duo-Wen Mo a, Chen-Xi Shi a, Hui Liu b, Yi-Yin Li a, Li-Ping Zhou a a b
Laboratory for Earth Surface Processes, Department of Geography, Peking University, Beijing 100871, China Hubei Provincial Institute of Cultural Relics and Archaeology, Wuhan 430077, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2008 Received in revised form 4 June 2009 Accepted 22 June 2009 Available online 1 July 2009
The Qujialing site is a representative Neolithic archaeological site in the middle reaches of the Yangtze River, China. Absence of suitable material for radiocarbon dating in this region makes the timing of the similar sites difficult. Here we applied optically stimulated luminescence (OSL–SAR) and thermoluminescence (TL–SAR) techniques to date the archaeological and natural deposits from the Qujialing site with known age, testing the techniques on samples at archaeological sites in this region. The results showed that the luminescence properties of quartz from sediment and baked earth samples are very similar. The quartz OSL ages obtained for a sediment sample and a baked earth sample from the cultural layer are 5.4 0.3 and 5.1 0.3 ka, respectively. The quartz TL age of the baked earth sample is 5.6 0.5 ka. These dates are consistent with the calibrated radiocarbon ages (4.9 0.1 and 5.1 0.1 ka cal BP (1s)) of the two charcoal samples from the cultural layer at a nearby locality, and are also in agreement with the age of Qujialing culture period. The results indicate that the OSL dating techniques can be applied to date similar archaeological sites in the middle reaches of the Yangtze River, China. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Luminescence dating OSL–SAR TL–SAR Baked earth and sediments Qujialing archaeological site
1. Introduction In southern China, many archaeological sites have been excavated. However, many of them have never been accurately dated because of the absence of suitable material for radiocarbon dating which is the most widely used technique for this period. Optical dating techniques, especially the single-aliquot regenerative-dose (OSL–SAR) protocol (Murray and Wintle, 2000a), have been applied successfully to date many archaeological deposits elsewhere (Roberts, 1997; Feathers, 2003; Wintle, 2008). Meanwhile, the single-aliquot regenerative-dose (TL–SAR) procedures applied to TL signals have also been developed (e.g. Bailiff and Petrov, 1999; Fattahi and Stokes, 2000; Murray and Wintle, 2000b; Bassinet et al., 2006) and are generally used for dating heated materials such as ancient bricks (e.g. Bailiff and Holland, 2000; Chruscinska et al., 2008). For example, Hong et al. (2006) obtained consistent equivalent doses (De) using OSL–SAR and TL-SAR procedures on three heated quartz samples from brick and pottery, and Chruscinska et al. (2008) obtained Des with both good accuracy and precision using the TL–SAR procedure on five brick samples. Thus, these luminescence techniques may provide a possible dating tool to
* Corresponding author. Tel./fax: þ86 10 62754411. E-mail address: [email protected] (J.-F. Zhang). 1871-1014/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2009.06.006
establish the timing of burial of the archaeological sites in southern China. In this paper, the OSL–SAR and TL–SAR protocols were tested on sediment samples and a known age baked earth sample from the Qujialing archaeological site in southern China. 2. Archaeology and stratigraphy The middle reaches of the Yangtze River in Hubei and Hunan provinces, China, is considered as one of the places of origin of the Chinese civilization. Many Paleolithic to Neolithic archaeological sites, such as Yinjialing, Zhongjialing and Zhongziba, have been found there (He, 2004). The Qujialing site (30 500 15.700 N, 112 54012.300 E) is one of them, and belongs to the Qujialing culture estimated to be 4.5–5.1 ka cal BP by radiocarbon dating (He, 2004). The site is located at the Qujialing village, w30 km from the Jinghsan County, Hubei province, China, and surrounded by the Qingmu and Qingmudang rivers (Fig. 1). The locality for OSL sampling is situated in the northwest of the area (filled star in Fig. 1). A pit was dug for sampling. The sediments in the section can be divided into four stratigraphic units (Fig. 2a). The top unit (Unit 1) is a 0.22-m-thick cultivated soil layer. Unit 2 from 0.22 to 2.29 m in depth consists of gray clayey silt. The sediments are slightly mottled by Fe and Mn oxides in the depth of 1.68–2.29 m, the oxides are only identified by color in field. Unit 3 is a 0.71-m-thick cultural layer, which is mostly composed of reddish brown fragments of baked earth (Fig. S1), the
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Fig. 1. Map showing the location of the Qujialing archaeological site.
size of the fragments generally varies from 2 to 10 cm in diameter. These fragments are typical archaeological remains widely distributed in this area, and believed to be made of local materials and heated by fire (Zhang, 1965). The voids between fragments are filled with fine materials (silt). Below the cultural layer is Unit 4, it consists of grayish yellow clayey silt. The contacts between the cultural layer and Units 2 or 4 are clear and sharp. The silts at this site are massive, and no bedding is visible. The grain-size distribution of the sediments from the section was measured using a Malvern Mastersizer 2000 laser grain-size analyzer with the procedure reported by Sun et al. (2002). The results are shown in Fig. 2b. It can be seen that the natural deposits mainly consist of silt (4–63 mm) with the mean grain-size of 5.3–6.3 mm in diameter. There is little change in grain-size from top to bottom, suggesting that all the sediments have similar depositional environment. Additionally, one clayey silt sample from the cultural layer was also analyzed, its distribution pattern is very similar to those of the samples from the overlying and underlying units. This suggests that the fine materials in the void between the fragments of baked earth in the cultural layer should be natural deposits (clayey silt). Samples were taken by cutting out entire large blocks from the pit walls, and tightly wrapped with aluminum foil and tape. A total of seven samples were collected in the field (Fig. 2a). Sample QJL-OSL07 was taken from the cultural layer (Unit 3), it is composed of fragments of baked earth and silt sediment. In the
laboratory, it was separated into two subsamples. The portion of the fragments was numbered as sample QJL-OSL07-B, the silt as QJL-OSL07-S. Additionally, two charcoal samples for 14C dating were collected from the cultural layer at a nearby locality (open star in Fig. 1), about 0.5 km from the OSL sampling locality. No difference in archaeological deposits between the two localities was observed, except that no charcoals were found at the OSL sampling locality. 3. Dose-rate determination The contents of U, Th and K in samples were analyzed using laser ablation inductively coupled plasma mass spectrometry (laser ICP– MS). Based on analyses for standard samples, uncertainties on ICP– MS analyses of U, Th and K are taken to be 11%, 9% and 2.5%, respectively. Water contents were evaluated by weighing samples before and after drying in the laboratory. Uncertainties of the water contents are assigned to 10%. The alpha efficiency factor (a-value) of 0.04 0.01 was used for quartz OSL (Rees-Jones, 1995). The factor for quartz TL was not measured in this study, and assigned to 0.10 0.02. Using the revised dose-rate conversion factors (Adamiec and Aitken, 1998), the elemental concentrations were converted into effective dose rate with the ‘AGE’ program of Gru¨n (2003), in which cosmic ray contributions to dose rate is involved. The dose rate of samples QJL-OSL07-B and QJL-OSL07-S from the cultural layer will be discussed in more detail in Section 5.1.
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Optical age, ka
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Fig. 2. (a) Stratigraphic section showing the position of OSL samples. (b) Grain-size distribution, only one fine sediment sample from Unit 3 (the cultural layer) was measured, its mean size is 5.9 mm. (c) Plot of luminescence age versus depth. For comparison, the calibrated radiocarbon ages of two charcoal samples from the cultural layer at nearby locality (open star in Fig. 1) are also displayed.
4. De determination 4.1. Sample treatment Fine-grain (4–11 mm) quartz was extracted for De determination, as suggested by Zhang and Zhou (2007) and Roberts (2007). Under subdued red light in the dark room, 2-cm-thick outer layers of the block samples were first removed, and 30% H2O2 and 10% HCL were used to remove organic matters and carbonates, respectively. For the baked earth sample (QJL-OSL07-B), the fragments were carefully ground in a mortar filled with water. After that, polymineral fine-grain fractions were first isolated using settlement techniques, and purified quartz fractions were then obtained by treating the polymineral fine-grain fractions with silica-saturated H2SiF6, followed by etching in 10% HCl and washing in deionized water. The IR stimulation (Hu¨tt et al., 1988; Duller, 2003) for the quartz fractions indicated that feldspar contaminant was completely removed. 4.2. Luminescence measurements All luminescence measurements, beta irradiation and preheat treatments were performed in an automated Risø TL/OSL reader equipped with a 90Sr/90Y beta source (Bøtter-Jensen et al., 2000). Blue light (470 30 nm) LED stimulation was used for quartz OSL measurements. Luminescence was detected by an EMI 9235QA photomultiplier tube with a 7.5 mm Hoya U-340 filter in front of it. The OSL–SAR procedure (Table S1) (Murray and Wintle, 2000a) was applied to De determination of quartz fractions. The preheat temperature was determined by preheat plateau and dose recovery tests. The first 0.64 s integral of the initial OSL signal minus a background estimated from the last 3.2 s integral of the OSL signal was used for De calculation. De values were obtained using the Analyst 3.22b program, and the error on individual De values was calculated using the counting statistics and an instrumental uncertainty of 1.0 % (Duller, 2007). De for samples QJL-OSL07-B and QJL-OSL07-S from the cultural layer were also determined using a TL–SAR procedure. The TL–SAR
procedure used here (Table S1) is the same as the OSL–SAR method, except that the OSL measurements (steps 3 and 6) in the OSL–SAR procedure were replaced by TL measurements. In order to remove the contribution of 110 C TL peak, a preheat of 160 C for 10 s was performed before the TL measurements. The sensitivity-corrected TL signals were obtained by dividing the natural or regeneration TL signals by test-dose TL signals. The integrated TL signals over a defined temperature range were used for the construction of a dose-response curve. The temperature range is determined by plateau tests (Aitken, 1985): the ratios of natural to artificial TL signals must be constant over an interval of temperature corresponding to the main peak. 5. Results and discussion 5.1. Dose rate The dose rates obtained for these samples are listed in Table S2. It should be noted that the dose rates of samples QJL-OSL07-B and QJLOSL07-S from the cultural layer may be a priori inaccurate because they are deduced from the U, Th and K contents determinated with the ICP–MS method: this technique based on very small sample does not take into account the eventual heterogeneities of the archaeological layer, while the travel range of gamma rays is over 30 cm in sediments. However, Table S2 shows that the contents of U, Th and K of the baked earth and the silt samples are consistent within error limits, suggesting that the mixture (the archaeological deposits) of the fragments and the silt from the layer is, to some extent, homogenous with respect to radioactivity. Based on the element contents and water contents, their dose rates were calculated, respectively, to be 3.01 0.18 and 2.95 0.18 Gy/ka for OSL (a-value ¼ 0.04 0.01) and 3.70 0.30 and 3.60 0.29 Gy/ka for TL (a-value ¼ 0.10 0.02). It should be noted that here the avalue of 0.10 0.02 for TL in quartz is assumed. If the a-values are determined, the accuracy of the dose rates would be improved. This is because the a-value for TL varies from sample to sample. For examples, for polymineral fine grains, it ranges from 0.02 to 0.22
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(Zink and Porto, 2005), 0.06 to 0.23 (Richter et al., 2000), 0.05 to 0.12 (Novothny et al., 2002), 0.08 to 0.16 (Frechen et al., 1997), 0.08 to 0.14 (Bergmann et al., 2008) and 0.11 to 0.14 (Zacharias et al., 2007). The a-value for TL in quartz is rarely reported in the literature. According to Valladas and Valladas (1982), the a-value for quartz should be smaller than that for polymineral fine grains. If the a-value of 0.04 0.01 is used for calculating TL dose rate, the calculated TL ages will be increased by w22%. Additionally, for the dose rate of sample QJL-OSL04 taken from a depth of 2.02 m, the accuracy of the actual measured value may be influenced by the formation of Fe and Mn oxides in the layer. This requires further investigation. 5.2. Luminescence properties 5.2.1. OSL properties Some luminescence properties of the fine-grain quartz extracts are shown in Fig. 3. It can be seen from Fig. 3a that these samples have very similar TL glow curves with a dominant w325 C TL peak (the easy-to-bleach TL signal) and a minor w210 C TL peak. Their natural LM-OSL curves (Fig. 3b) and decay curves (Fig. 3c) show that their OSL signals are dominated by fast component, suggesting that the SAR procedure should be applicable. Examples of the regenerated OSL dose-response curves obtained using the OSL–SAR procedure are demonstrated in Fig. 3d. For all the samples measured, the recycling ratios are between 0.95 and 1.05, and the recuperation (regeneration dose ¼ 0 Gy) is small, generally one percent of the natural, in the preheat temperature range of 180 w 300 C. This implies that the OSL–SAR procedure should be suitable for the heated and unheated quartz from these samples. Preheat plateau tests were also carried out on these samples. Four representative samples from Units 2, 3 and 4 were measured
b 1200
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5.2.2. TL properties Fig. 5 shows the TL properties of the heated quartz from the baked earth sample (QJL-OSL07-B). The natural TL signal presents a main TL peak at 325 C and a minor peak at 210 C, and the artificial signal exhibits a relatively strong TL peak centered on 210 C (life time at room temperature of 0.75–2.0 ka only (Bailiff and Petrov, 1999)), 110 C TL peak was removed by preheating (Fig. 5a). The ratio of natural to artificial TL is expected to be independent of the temperature on most of the 325 C TL peak. This plateau test was performed on all aliquots measured (20 aliquots measured for sample QJL-OSL07-B; 10 aliquots for QJL-OSL07-S).
QJL-OSL03 QJL-OSL05 QJL-OSL07-B QJL-OSL07-S
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at different preheat temperatures within the range 160–300 C using the OSL–SAR procedure. It can be seen in Fig. 4a that the De values are independent of the preheat temperature in the ranges from 160 to 270 C for sample QJL-OSL03 and from 160 to 290 C for samples QJL-OSL05, QJL-OSL07-B and QJL-OSL07-S, respectively. Dose recovery tests were also performed on these samples. The natural samples were first exposed to blue light within the reader for 1000 s to remove natural signals, and then irradiated with known beta doses approximately equal to their Des. The given dose was assumed to be the natural, and the OSL–SAR procedure was performed using different preheat temperatures in the range of 160–300 C at an interval of 20 C. The results presented in Fig. 4b show that the recovered doses are in good agreement with the given doses in the temperature ranges 160–260 C for sample QJL-OSL03, 180–280 C for sample QJL-OSL05, and 160–300 C for samples QJL-OSL07-B and QJL-OSL07-S. These results are consistent with those of the preheat plateau tests discussed above. Based on the results of the preheat plateau and dose recovery tests, a preheat of 200 C for 10 s was employed for De measurements for all samples.
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Fig. 3. Luminescence properties of the fine-grain quartz fractions of the samples from the section in Fig. 2a. (a) Natural TL glow curves, TL was measured using a heating rate of 5 C/s. (b) Natural LM-OSL curves, the LM-OSL signals were measured at 125 C for 1000 s (from 0 to100% power) after a preheat at 200 C for 10 s. For comparison, the data were normalized to the maximum point of each curve. (c) Natural OSL decay curves. The OSL signals were measured at 125 C after a preheat at 200 C for 10 s. The data were normalized to the first point of each curve. (d) Growth curves for the quartz fractions of samples QJL-OSL07-B and QJL-OSL07-S, the data points were exponentially fitted.
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a 8
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Fig. 4. (a) Plots of OSL De as a function of preheat temperature; (b) dose recovery test results for the OSL–SAR protocol. Each data point represents a result from one aliquot.
Fig. 5. TL properties of the heated quartz extracted from the baked earth fragments (sample QJL-OSL07-B). (a) Natural (NTL) and artificial TL (ATL) glow curves measured after a preheat of 160 C for 10 s, using a heating rate of 5 C/s. (b) Plateau test for one aliquot showing a plateau between 260 and 330 C. (c) Relationship between the regenerative-dose and test-dose TL signals (integrated TL signal of 300–330 C was used), the regression is forced through the origin. (d) A TL growth curve obtained using the TL–SAR procedure (see text). The open circle represents the sensitivity-corrected natural TL signal, and the open square is the TL signal induced by a repeated regenerative-dose.
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Based on the plateaus obtained for all aliquots, the integral TL signals within the temperature range 300–330 C were used for De calculations (Fig. 5b). Fig. 5c shows that there is a good linear relationship between the regenerated TL and the subsequent test-dose TL signals. However, the recycling ratios for the measured aliquots are in the range 1.05–1.19, with an average of 1.13, implying that the sensitivity correction in the TL–SAR procedure is not satisfied compared to that in the OSL–SAR procedure. Dose recovery tests were also performed on ten aliquots of sample QJL-OSL07-B using the TL–SAR procedure. Aliquots were first heated to 500 C to erase natural TL signals, and then irradiated by a beta dose of 20.7 Gy (given dose), followed by the application of the TL–SAR procedure. The average ratio of the measured to given dose for the ten aliquots measured is 0.92 0.02 only, this is very similar to the results of the dose recovery tests by Bassinet et al. (2006) in which the TL–SAR protocol underestimates the three applied doses by around 10%. The result of the tests by Richter and Temming (2006) was also less satisfied. The underestimation of the measured dose may be attributed to the sensitivity correction in the procedure discussed above. The residual TL dose after bleaching for both samples QJL-OSL07-B and QJL-OSL07-S from the cultural layer was also estimated. Ten aliquots for each sample were exposed to sunlight for 20 h in October in Beijing. The aliquots were then measured using the TL–SAR procedure. The average residual dose of the ten aliquots of the sediment sample (QJL-OSL07-S) after the exposure was determined as 2.95 0.25 Gy. Interestingly, for the heated sample (QJL-OSL07-B), the residual signal cannot be distinguished from the background. 5.2.3. Comparison of heated and unheated quartz Figs. 3–5 also demonstrate that the luminescence properties of the heated quartz from the fragments of baked earth (sample QJL-OSL07-B) are very similar to those of the unheated quartz from the sediments, especially for sample QJL-OSL07-S, except that the proportion of the fast component in the OSL signals in the heated quartz is relatively higher than that in unheated quartz. It is inferred that, except for erasing the previous TL signal, the baking did not greatly change the luminescence properties of the quartz in the baked earth. Additionally, the similarity in luminescence properties and grain-size distributions of the sediments from the different units may imply that these sediments are of the same origin. 5.3. OSL and radiocarbon ages The average De values with one standard error obtained using the OSL–SAR procedure, dose rates and OSL ages are listed in Table S2. The ages are also shown in Fig. 2c. Samples QJL-OSL01, 02, 03 and 04 from Unit 2 were dated to 0.7 0.1, 1.0 0.1, 2.4 0.2 and 5.2 0.4 ka, respectively. They are in stratigraphical order. The OSL ages of the baked earth sample (QJL-OSL07-B) and the sediment sample (QJL-OSL07-S) from the cultural layer (Unit 3) are 5.1 0.3 and 5.4 0.3 ka, respectively, and are thus consistent within errors. The OSL age (5.2 0.4 ka) of sample QJL-OSL04 from Unit 2 indicates that this site should immediately receive sediments after cultural deposition. The ages of the samples (QJL-OSL05 and QJL-OSL06) from Unit 4 were determined as 15.2 1.0 and 17.5 1.2 ka, respectively. They are much older than the overlying sediments, implying that Unit 4 is the base of the Qujialing archaeological site. Two charcoal samples collected from a cultural layer at the nearby locality were dated using a new AMS system at Peking University. The radiocarbon ages obtained were 4290 60 and 4475 40 a BP, respectively. These data were calibrated using the
Fairbanks0107 calibration program (Fairbanks et al., 2005). The calendar ages are 4.9 0.1 and 5.1 0.1 ka cal BP (1s), respectively, in good agreement with the OSL ages of the samples from the cultural layer. Moreover these ages are consistent with the expected ages for the Qujialing culture (4.5–5.1 ka). This suggests that the OSL ages obtained for these samples are reliable. The consistency between the OSL ages of the baked earth fragments (sample QJLOSL07-B), the silt (QJL-OSL07-S) and the calibrated radiocarbon ages of the charcoal samples indicates that the sediments were completely bleached prior to burial. 5.4. TL ages Although the recycling ratios and dose recovery tests using the TL–SAR procedure on samples QJL-OSL07-B and QJL-OSL07-S are not very satisfied, the TL ages of the two samples are still calculated with this method in order to test it, by comparing the TL ages with those obtained using other methods. The dose used for age calculation was obtained by subtracting the residual dose from the measured De value. For the heated sample (QJL07-OSL07-B), zero dose was subtracted because its residual TL signal had been erased by heating. The results are listed in Table S2, and the TL ages are also displayed in Fig. 2c for comparison. Despite the modest results for the recycling ratio and dose recovery test, the TL age (5.6 0.5 ka) of the baked earth sample (QJL-OSL07-B) is consistent with its OSL age (5.1 0.3 ka) within error limits. The TL age (7.9 1.6 ka) of the sediment sample (QJL-OSL07-S) is larger than its OSL age (5.4 0.3 ka) and the TL or OSL ages of sample QJL-OSL07-B. This could be attributed to the incomplete zeroing of the bleachable TL signals in the sediment at the time of deposition (Godfrey-Smith et al., 1988). This subtraction of the residual unbleachable signal only is thus insufficient. This could also be partly due to the problems of sensitivity correction. The above results demonstrate that both the OSL–SAR and TL–SAR procedures can be applied to heated/ fired materials. By comparison, the OSL–SAR procedure has more advantages such as a less scatter in OSL–SAR De between aliquots and higher luminescence sensitivity to radiation and better sensitivity correction. Therefore, the OSL–SAR protocol is more practicable for these materials, as suggested by Thomas et al. (2008). 6. Conclusions The quartz extracted from the baked earth and sediment samples form the Qujialing archaeological site fulfills the basic conditions (recycling ratio, dose recovery, recuperation) for the OSL–SAR to be successful. The OSL–SAR ages obtained for sediment and baked earth samples from the cultural layer are statistically identical, suggesting the signal zeroing at deposition was completed. Moreover, they are in agreement with the calibrated radiocarbon ages of the charcoal samples from a nearby identical layer, and they are also consistent with the age of the Qujialing culture period. The TL–SAR procedure was also attempted on the sediment and baked samples from the cultural layer. The results for the basic tests were however less satisfying than for the OSL–SAR protocol, and only for the baked sample the TL and OSL results were consistent. While further investigations are needed to improve the TL–SAR procedure, the results obtained are encouraging for applying the OSL to date similar archaeological sites in the middle reaches of the Yangtze River. Acknowledgments The work was supported by MOST (No.: 2006BAK21B02). We thank Long-Jiang Mao and Ke Hu for help in sampling. We are also grateful to the anonymous reviewer, her/his valuable comments
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and constructive suggestions significantly improved the quality of the manuscript. Editorial handling by: R. Gru¨n Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quageo.2009.06.006. References Adamiec, G., Aitken, M.J., 1998. Dose rate conversion factors: update. Ancient TL 16, 37–49. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Bailiff, I.K., Petrov, S.A., 1999. The use of the 210 C TL peak in quartz for retrospective dosimetry. Radiation Protection Dosimetry 84, 551–554. Bailiff, I.K., Holland, N., 2000. Dating bricks of the last two millennia from Newcastle upon Tyne: a preliminary study. Radiation Measurements 32, 615–619. Bassinet, C., Mercier, N., Miallier, D., Pilleyre, T., Sanzelle, S., Valladas, H., 2006. Thermoluminescence of heated quartz grains: intercomparisons between SAR and multiple-aliquot additive dose techniques. Radiation Measurements 41, 803–808. Bergmann, R., Hajek, M., Fugger, N., Vana, N., 2008. Comparative study of infraredstimulated luminescent and thermoluminescent dating of archaeological artefacts. Radiation Measurements 43, 781–785. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 523–528. Chruscinska, A., Jesionowski, B., Oczkowski, H.L., Przegietka, K.R., 2008. Using TL single-aliquot regenerative-dose protocol for the verification of the chronology of the Teutonic Order Castle in Malbork. Geochronometria 30, 61–67. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiation Measurements 37, 161–165. Duller, G.A.T., 2007. Assessing the error on equivalent dose estimates derived from single aliquot regenerative dose measurements. Ancient TL 25, 15–24. Fairbanks, R.G., Mortlock, R.A., Chiu, T.-C., Cao, L., Kaplan, A., Guilderson, T.P., Fairbanks, T.W., Bloom, A.L., 2005. Marine radiocarbon calibration curve spanning 10,000 to 50,000 years B.P. based on paired 230Th/234U/238U and 14C dates on Pristine corals. Quaternary Science Reviews 24, 1781–1796. Fattahi, M., Stokes, S., 2000. Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz. Radiation Measurements 32, 479–485. Feathers, J.K., 2003. Use of luminescence dating in archaeology. Measurement Science and Technology 14, 1493–1509. Frechen, M., Horva´th, E., Ga´bris., G., 1997. Geochronology of Middle and Upper Pleistocene loess sections in Hungary. Quaternary Research 48, 291–312. Godfrey-Smith, D.I., Huntley, D.J., Chen, W.H., 1988. Optical dating studies of quartz and feldspar sediment extracts. Quaternary Science Reviews 7, 373–380.
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Gru¨n, R., 2003. Age.exe. Computer program for the calculation of luminescence dates. Unpublished Computer Program. RSES, Canberra. He, J.J., 2004. The Neolithic Civilization in Middle Reaches of the Yangtze River. Hubei Educational Press, Wuhan (in Chinese). Hong, D.G., Kim, M.J., Choi, J.H., El-Faramawy, N.A., Go¨ksu, H.Y., 2006. Equivalent dose determination of single aliquot regenerative-dose (SAR) protocol using thermoluminescence on heated quartz. Nuclear Instruments and Methods in Physics Research B 243, 174–178. Hu¨tt, G., Jaek, I., Tchonka, J., 1988. Optical dating: K-feldspar optical response stimulation spectra. Quaternary Science Reviews 7, 381–385. Murray, A.S., Wintle, A.G., 2000a. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73. Murray, A.S., Wintle, A.G., 2000b. Application of the single-aliquot regenerative-dose protocol to the 375 C quartz TL signal. Radiation Measurements 32, 579–583. Novothny, A., Horvath, E., Frechen, M., 2002. The loess profile at Albertirsa, Hungary – improvements in loess stratigraphy by luminescence dating. Quaternary International 95–96, 155–163. Rees-Jones, J., 1995. Optical dating of young sediments using fine-grain quartz. Ancient TL 13, 9–14. Richter, D., Temming, H., 2006. Testing heated flint palaeodose protocols using dose recovery procedure. Radiation Measurement 41, 819–825. Richter, D., Waiblinger, J., Rink, W.J., Wagner, G.A., 2000. Thermoluminescence, electron spin resonance and 14C-dating of the late Middle and early Upper Palaeolithic site of Geißenklo¨sterle Cave in southern Germany. Journal of Archaeological Science 27, 71–89. Roberts, H.M., 2007. Assessing the effectiveness of the double-SAR protocol in isolating a luminescence signal dominated by quartz. Radiation Measurements 42, 1627–1636. Roberts, R.G., 1997. Luminescence dating of archaeology. Radiation Measurements 27, 819–892. Sun, D.H., Bloemendal, J., Rea, D.K., Vandenberghe, J., Jiang, F.C., An, Z.S., Su, R.X., 2002. Grain-size distribution function of polymodal sediments in hydraulic and aeolian environments, and numerical partitioning of the sedimentary components. Sedimentary Geology 152, 263–277. Thomas, P.J., Nagabhushanam, P., Reddy, D.V., 2008. Optically stimulated luminescence dating of heated materials using single-aliquot regenerative-dose procedure: a feasibility study using archaeological artefacts from India. Journal of Archaeological Science 35, 781–790. Valladas, G., Valladas, H., 1982. Effet de l’irradiation alpha sur des grains de quartz. A specialist seminar on thermoluminescence dating. PACT 6, 171–178. Wintle, A.G., 2008. Fifty years of luminescence dating. Archaeometry 50, 276–312. Zacharias, N., Schwedt, A., Garrigo´s, J.B.I., Michael, C.T., Mommsen, H., Kilikoglou, V., 2007. A contribution to the study of post-depositional alterations of pottery using TL dating analysis. Journal of Archaeological Science 34, 1804–1809. Zhang, J.F., Zhou, L.P., 2007. Optimization of the ‘double SAR’ procedure for polymineral fine grains. Radiation Measurements 42, 1475–1482. Zhang, Y.P., 1965. Qujialing, Jinghsan. The Science Press, Beijing (in Chinese). Zink, A., Porto, E., 2005. Luminescence dating of the Tanagra terracottas of The Louvre collections. Geochronometria 24, 21–26.
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Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo
Research Paper
Luminescence dating of baked earth and sediments from the Qujialing archaeological site, China Xiao Fu a, Jia-Fu Zhang a, *, Duo-Wen Mo a, Chen-Xi Shi a, Hui Liu b, Yi-Yin Li a, Li-Ping Zhou a a b
Laboratory for Earth Surface Processes, Department of Geography, Peking University, Beijing 100871, China Hubei Provincial Institute of Cultural Relics and Archaeology, Wuhan 430077, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2008 Received in revised form 4 June 2009 Accepted 22 June 2009 Available online 1 July 2009
The Qujialing site is a representative Neolithic archaeological site in the middle reaches of the Yangtze River, China. Absence of suitable material for radiocarbon dating in this region makes the timing of the similar sites difficult. Here we applied optically stimulated luminescence (OSL–SAR) and thermoluminescence (TL–SAR) techniques to date the archaeological and natural deposits from the Qujialing site with known age, testing the techniques on samples at archaeological sites in this region. The results showed that the luminescence properties of quartz from sediment and baked earth samples are very similar. The quartz OSL ages obtained for a sediment sample and a baked earth sample from the cultural layer are 5.4 0.3 and 5.1 0.3 ka, respectively. The quartz TL age of the baked earth sample is 5.6 0.5 ka. These dates are consistent with the calibrated radiocarbon ages (4.9 0.1 and 5.1 0.1 ka cal BP (1s)) of the two charcoal samples from the cultural layer at a nearby locality, and are also in agreement with the age of Qujialing culture period. The results indicate that the OSL dating techniques can be applied to date similar archaeological sites in the middle reaches of the Yangtze River, China. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Luminescence dating OSL–SAR TL–SAR Baked earth and sediments Qujialing archaeological site
1. Introduction In southern China, many archaeological sites have been excavated. However, many of them have never been accurately dated because of the absence of suitable material for radiocarbon dating which is the most widely used technique for this period. Optical dating techniques, especially the single-aliquot regenerative-dose (OSL–SAR) protocol (Murray and Wintle, 2000a), have been applied successfully to date many archaeological deposits elsewhere (Roberts, 1997; Feathers, 2003; Wintle, 2008). Meanwhile, the single-aliquot regenerative-dose (TL–SAR) procedures applied to TL signals have also been developed (e.g. Bailiff and Petrov, 1999; Fattahi and Stokes, 2000; Murray and Wintle, 2000b; Bassinet et al., 2006) and are generally used for dating heated materials such as ancient bricks (e.g. Bailiff and Holland, 2000; Chruscinska et al., 2008). For example, Hong et al. (2006) obtained consistent equivalent doses (De) using OSL–SAR and TL-SAR procedures on three heated quartz samples from brick and pottery, and Chruscinska et al. (2008) obtained Des with both good accuracy and precision using the TL–SAR procedure on five brick samples. Thus, these luminescence techniques may provide a possible dating tool to
* Corresponding author. Tel./fax: þ86 10 62754411. E-mail address: [email protected] (J.-F. Zhang). 1871-1014/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2009.06.006
establish the timing of burial of the archaeological sites in southern China. In this paper, the OSL–SAR and TL–SAR protocols were tested on sediment samples and a known age baked earth sample from the Qujialing archaeological site in southern China. 2. Archaeology and stratigraphy The middle reaches of the Yangtze River in Hubei and Hunan provinces, China, is considered as one of the places of origin of the Chinese civilization. Many Paleolithic to Neolithic archaeological sites, such as Yinjialing, Zhongjialing and Zhongziba, have been found there (He, 2004). The Qujialing site (30 500 15.700 N, 112 54012.300 E) is one of them, and belongs to the Qujialing culture estimated to be 4.5–5.1 ka cal BP by radiocarbon dating (He, 2004). The site is located at the Qujialing village, w30 km from the Jinghsan County, Hubei province, China, and surrounded by the Qingmu and Qingmudang rivers (Fig. 1). The locality for OSL sampling is situated in the northwest of the area (filled star in Fig. 1). A pit was dug for sampling. The sediments in the section can be divided into four stratigraphic units (Fig. 2a). The top unit (Unit 1) is a 0.22-m-thick cultivated soil layer. Unit 2 from 0.22 to 2.29 m in depth consists of gray clayey silt. The sediments are slightly mottled by Fe and Mn oxides in the depth of 1.68–2.29 m, the oxides are only identified by color in field. Unit 3 is a 0.71-m-thick cultural layer, which is mostly composed of reddish brown fragments of baked earth (Fig. S1), the
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Fig. 1. Map showing the location of the Qujialing archaeological site.
size of the fragments generally varies from 2 to 10 cm in diameter. These fragments are typical archaeological remains widely distributed in this area, and believed to be made of local materials and heated by fire (Zhang, 1965). The voids between fragments are filled with fine materials (silt). Below the cultural layer is Unit 4, it consists of grayish yellow clayey silt. The contacts between the cultural layer and Units 2 or 4 are clear and sharp. The silts at this site are massive, and no bedding is visible. The grain-size distribution of the sediments from the section was measured using a Malvern Mastersizer 2000 laser grain-size analyzer with the procedure reported by Sun et al. (2002). The results are shown in Fig. 2b. It can be seen that the natural deposits mainly consist of silt (4–63 mm) with the mean grain-size of 5.3–6.3 mm in diameter. There is little change in grain-size from top to bottom, suggesting that all the sediments have similar depositional environment. Additionally, one clayey silt sample from the cultural layer was also analyzed, its distribution pattern is very similar to those of the samples from the overlying and underlying units. This suggests that the fine materials in the void between the fragments of baked earth in the cultural layer should be natural deposits (clayey silt). Samples were taken by cutting out entire large blocks from the pit walls, and tightly wrapped with aluminum foil and tape. A total of seven samples were collected in the field (Fig. 2a). Sample QJL-OSL07 was taken from the cultural layer (Unit 3), it is composed of fragments of baked earth and silt sediment. In the
laboratory, it was separated into two subsamples. The portion of the fragments was numbered as sample QJL-OSL07-B, the silt as QJL-OSL07-S. Additionally, two charcoal samples for 14C dating were collected from the cultural layer at a nearby locality (open star in Fig. 1), about 0.5 km from the OSL sampling locality. No difference in archaeological deposits between the two localities was observed, except that no charcoals were found at the OSL sampling locality. 3. Dose-rate determination The contents of U, Th and K in samples were analyzed using laser ablation inductively coupled plasma mass spectrometry (laser ICP– MS). Based on analyses for standard samples, uncertainties on ICP– MS analyses of U, Th and K are taken to be 11%, 9% and 2.5%, respectively. Water contents were evaluated by weighing samples before and after drying in the laboratory. Uncertainties of the water contents are assigned to 10%. The alpha efficiency factor (a-value) of 0.04 0.01 was used for quartz OSL (Rees-Jones, 1995). The factor for quartz TL was not measured in this study, and assigned to 0.10 0.02. Using the revised dose-rate conversion factors (Adamiec and Aitken, 1998), the elemental concentrations were converted into effective dose rate with the ‘AGE’ program of Gru¨n (2003), in which cosmic ray contributions to dose rate is involved. The dose rate of samples QJL-OSL07-B and QJL-OSL07-S from the cultural layer will be discussed in more detail in Section 5.1.
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Fig. 2. (a) Stratigraphic section showing the position of OSL samples. (b) Grain-size distribution, only one fine sediment sample from Unit 3 (the cultural layer) was measured, its mean size is 5.9 mm. (c) Plot of luminescence age versus depth. For comparison, the calibrated radiocarbon ages of two charcoal samples from the cultural layer at nearby locality (open star in Fig. 1) are also displayed.
4. De determination 4.1. Sample treatment Fine-grain (4–11 mm) quartz was extracted for De determination, as suggested by Zhang and Zhou (2007) and Roberts (2007). Under subdued red light in the dark room, 2-cm-thick outer layers of the block samples were first removed, and 30% H2O2 and 10% HCL were used to remove organic matters and carbonates, respectively. For the baked earth sample (QJL-OSL07-B), the fragments were carefully ground in a mortar filled with water. After that, polymineral fine-grain fractions were first isolated using settlement techniques, and purified quartz fractions were then obtained by treating the polymineral fine-grain fractions with silica-saturated H2SiF6, followed by etching in 10% HCl and washing in deionized water. The IR stimulation (Hu¨tt et al., 1988; Duller, 2003) for the quartz fractions indicated that feldspar contaminant was completely removed. 4.2. Luminescence measurements All luminescence measurements, beta irradiation and preheat treatments were performed in an automated Risø TL/OSL reader equipped with a 90Sr/90Y beta source (Bøtter-Jensen et al., 2000). Blue light (470 30 nm) LED stimulation was used for quartz OSL measurements. Luminescence was detected by an EMI 9235QA photomultiplier tube with a 7.5 mm Hoya U-340 filter in front of it. The OSL–SAR procedure (Table S1) (Murray and Wintle, 2000a) was applied to De determination of quartz fractions. The preheat temperature was determined by preheat plateau and dose recovery tests. The first 0.64 s integral of the initial OSL signal minus a background estimated from the last 3.2 s integral of the OSL signal was used for De calculation. De values were obtained using the Analyst 3.22b program, and the error on individual De values was calculated using the counting statistics and an instrumental uncertainty of 1.0 % (Duller, 2007). De for samples QJL-OSL07-B and QJL-OSL07-S from the cultural layer were also determined using a TL–SAR procedure. The TL–SAR
procedure used here (Table S1) is the same as the OSL–SAR method, except that the OSL measurements (steps 3 and 6) in the OSL–SAR procedure were replaced by TL measurements. In order to remove the contribution of 110 C TL peak, a preheat of 160 C for 10 s was performed before the TL measurements. The sensitivity-corrected TL signals were obtained by dividing the natural or regeneration TL signals by test-dose TL signals. The integrated TL signals over a defined temperature range were used for the construction of a dose-response curve. The temperature range is determined by plateau tests (Aitken, 1985): the ratios of natural to artificial TL signals must be constant over an interval of temperature corresponding to the main peak. 5. Results and discussion 5.1. Dose rate The dose rates obtained for these samples are listed in Table S2. It should be noted that the dose rates of samples QJL-OSL07-B and QJLOSL07-S from the cultural layer may be a priori inaccurate because they are deduced from the U, Th and K contents determinated with the ICP–MS method: this technique based on very small sample does not take into account the eventual heterogeneities of the archaeological layer, while the travel range of gamma rays is over 30 cm in sediments. However, Table S2 shows that the contents of U, Th and K of the baked earth and the silt samples are consistent within error limits, suggesting that the mixture (the archaeological deposits) of the fragments and the silt from the layer is, to some extent, homogenous with respect to radioactivity. Based on the element contents and water contents, their dose rates were calculated, respectively, to be 3.01 0.18 and 2.95 0.18 Gy/ka for OSL (a-value ¼ 0.04 0.01) and 3.70 0.30 and 3.60 0.29 Gy/ka for TL (a-value ¼ 0.10 0.02). It should be noted that here the avalue of 0.10 0.02 for TL in quartz is assumed. If the a-values are determined, the accuracy of the dose rates would be improved. This is because the a-value for TL varies from sample to sample. For examples, for polymineral fine grains, it ranges from 0.02 to 0.22
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(Zink and Porto, 2005), 0.06 to 0.23 (Richter et al., 2000), 0.05 to 0.12 (Novothny et al., 2002), 0.08 to 0.16 (Frechen et al., 1997), 0.08 to 0.14 (Bergmann et al., 2008) and 0.11 to 0.14 (Zacharias et al., 2007). The a-value for TL in quartz is rarely reported in the literature. According to Valladas and Valladas (1982), the a-value for quartz should be smaller than that for polymineral fine grains. If the a-value of 0.04 0.01 is used for calculating TL dose rate, the calculated TL ages will be increased by w22%. Additionally, for the dose rate of sample QJL-OSL04 taken from a depth of 2.02 m, the accuracy of the actual measured value may be influenced by the formation of Fe and Mn oxides in the layer. This requires further investigation. 5.2. Luminescence properties 5.2.1. OSL properties Some luminescence properties of the fine-grain quartz extracts are shown in Fig. 3. It can be seen from Fig. 3a that these samples have very similar TL glow curves with a dominant w325 C TL peak (the easy-to-bleach TL signal) and a minor w210 C TL peak. Their natural LM-OSL curves (Fig. 3b) and decay curves (Fig. 3c) show that their OSL signals are dominated by fast component, suggesting that the SAR procedure should be applicable. Examples of the regenerated OSL dose-response curves obtained using the OSL–SAR procedure are demonstrated in Fig. 3d. For all the samples measured, the recycling ratios are between 0.95 and 1.05, and the recuperation (regeneration dose ¼ 0 Gy) is small, generally one percent of the natural, in the preheat temperature range of 180 w 300 C. This implies that the OSL–SAR procedure should be suitable for the heated and unheated quartz from these samples. Preheat plateau tests were also carried out on these samples. Four representative samples from Units 2, 3 and 4 were measured
b 1200
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5.2.2. TL properties Fig. 5 shows the TL properties of the heated quartz from the baked earth sample (QJL-OSL07-B). The natural TL signal presents a main TL peak at 325 C and a minor peak at 210 C, and the artificial signal exhibits a relatively strong TL peak centered on 210 C (life time at room temperature of 0.75–2.0 ka only (Bailiff and Petrov, 1999)), 110 C TL peak was removed by preheating (Fig. 5a). The ratio of natural to artificial TL is expected to be independent of the temperature on most of the 325 C TL peak. This plateau test was performed on all aliquots measured (20 aliquots measured for sample QJL-OSL07-B; 10 aliquots for QJL-OSL07-S).
QJL-OSL03 QJL-OSL05 QJL-OSL07-B QJL-OSL07-S
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at different preheat temperatures within the range 160–300 C using the OSL–SAR procedure. It can be seen in Fig. 4a that the De values are independent of the preheat temperature in the ranges from 160 to 270 C for sample QJL-OSL03 and from 160 to 290 C for samples QJL-OSL05, QJL-OSL07-B and QJL-OSL07-S, respectively. Dose recovery tests were also performed on these samples. The natural samples were first exposed to blue light within the reader for 1000 s to remove natural signals, and then irradiated with known beta doses approximately equal to their Des. The given dose was assumed to be the natural, and the OSL–SAR procedure was performed using different preheat temperatures in the range of 160–300 C at an interval of 20 C. The results presented in Fig. 4b show that the recovered doses are in good agreement with the given doses in the temperature ranges 160–260 C for sample QJL-OSL03, 180–280 C for sample QJL-OSL05, and 160–300 C for samples QJL-OSL07-B and QJL-OSL07-S. These results are consistent with those of the preheat plateau tests discussed above. Based on the results of the preheat plateau and dose recovery tests, a preheat of 200 C for 10 s was employed for De measurements for all samples.
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Fig. 3. Luminescence properties of the fine-grain quartz fractions of the samples from the section in Fig. 2a. (a) Natural TL glow curves, TL was measured using a heating rate of 5 C/s. (b) Natural LM-OSL curves, the LM-OSL signals were measured at 125 C for 1000 s (from 0 to100% power) after a preheat at 200 C for 10 s. For comparison, the data were normalized to the maximum point of each curve. (c) Natural OSL decay curves. The OSL signals were measured at 125 C after a preheat at 200 C for 10 s. The data were normalized to the first point of each curve. (d) Growth curves for the quartz fractions of samples QJL-OSL07-B and QJL-OSL07-S, the data points were exponentially fitted.
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Fig. 4. (a) Plots of OSL De as a function of preheat temperature; (b) dose recovery test results for the OSL–SAR protocol. Each data point represents a result from one aliquot.
Fig. 5. TL properties of the heated quartz extracted from the baked earth fragments (sample QJL-OSL07-B). (a) Natural (NTL) and artificial TL (ATL) glow curves measured after a preheat of 160 C for 10 s, using a heating rate of 5 C/s. (b) Plateau test for one aliquot showing a plateau between 260 and 330 C. (c) Relationship between the regenerative-dose and test-dose TL signals (integrated TL signal of 300–330 C was used), the regression is forced through the origin. (d) A TL growth curve obtained using the TL–SAR procedure (see text). The open circle represents the sensitivity-corrected natural TL signal, and the open square is the TL signal induced by a repeated regenerative-dose.
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Based on the plateaus obtained for all aliquots, the integral TL signals within the temperature range 300–330 C were used for De calculations (Fig. 5b). Fig. 5c shows that there is a good linear relationship between the regenerated TL and the subsequent test-dose TL signals. However, the recycling ratios for the measured aliquots are in the range 1.05–1.19, with an average of 1.13, implying that the sensitivity correction in the TL–SAR procedure is not satisfied compared to that in the OSL–SAR procedure. Dose recovery tests were also performed on ten aliquots of sample QJL-OSL07-B using the TL–SAR procedure. Aliquots were first heated to 500 C to erase natural TL signals, and then irradiated by a beta dose of 20.7 Gy (given dose), followed by the application of the TL–SAR procedure. The average ratio of the measured to given dose for the ten aliquots measured is 0.92 0.02 only, this is very similar to the results of the dose recovery tests by Bassinet et al. (2006) in which the TL–SAR protocol underestimates the three applied doses by around 10%. The result of the tests by Richter and Temming (2006) was also less satisfied. The underestimation of the measured dose may be attributed to the sensitivity correction in the procedure discussed above. The residual TL dose after bleaching for both samples QJL-OSL07-B and QJL-OSL07-S from the cultural layer was also estimated. Ten aliquots for each sample were exposed to sunlight for 20 h in October in Beijing. The aliquots were then measured using the TL–SAR procedure. The average residual dose of the ten aliquots of the sediment sample (QJL-OSL07-S) after the exposure was determined as 2.95 0.25 Gy. Interestingly, for the heated sample (QJL-OSL07-B), the residual signal cannot be distinguished from the background. 5.2.3. Comparison of heated and unheated quartz Figs. 3–5 also demonstrate that the luminescence properties of the heated quartz from the fragments of baked earth (sample QJL-OSL07-B) are very similar to those of the unheated quartz from the sediments, especially for sample QJL-OSL07-S, except that the proportion of the fast component in the OSL signals in the heated quartz is relatively higher than that in unheated quartz. It is inferred that, except for erasing the previous TL signal, the baking did not greatly change the luminescence properties of the quartz in the baked earth. Additionally, the similarity in luminescence properties and grain-size distributions of the sediments from the different units may imply that these sediments are of the same origin. 5.3. OSL and radiocarbon ages The average De values with one standard error obtained using the OSL–SAR procedure, dose rates and OSL ages are listed in Table S2. The ages are also shown in Fig. 2c. Samples QJL-OSL01, 02, 03 and 04 from Unit 2 were dated to 0.7 0.1, 1.0 0.1, 2.4 0.2 and 5.2 0.4 ka, respectively. They are in stratigraphical order. The OSL ages of the baked earth sample (QJL-OSL07-B) and the sediment sample (QJL-OSL07-S) from the cultural layer (Unit 3) are 5.1 0.3 and 5.4 0.3 ka, respectively, and are thus consistent within errors. The OSL age (5.2 0.4 ka) of sample QJL-OSL04 from Unit 2 indicates that this site should immediately receive sediments after cultural deposition. The ages of the samples (QJL-OSL05 and QJL-OSL06) from Unit 4 were determined as 15.2 1.0 and 17.5 1.2 ka, respectively. They are much older than the overlying sediments, implying that Unit 4 is the base of the Qujialing archaeological site. Two charcoal samples collected from a cultural layer at the nearby locality were dated using a new AMS system at Peking University. The radiocarbon ages obtained were 4290 60 and 4475 40 a BP, respectively. These data were calibrated using the
Fairbanks0107 calibration program (Fairbanks et al., 2005). The calendar ages are 4.9 0.1 and 5.1 0.1 ka cal BP (1s), respectively, in good agreement with the OSL ages of the samples from the cultural layer. Moreover these ages are consistent with the expected ages for the Qujialing culture (4.5–5.1 ka). This suggests that the OSL ages obtained for these samples are reliable. The consistency between the OSL ages of the baked earth fragments (sample QJLOSL07-B), the silt (QJL-OSL07-S) and the calibrated radiocarbon ages of the charcoal samples indicates that the sediments were completely bleached prior to burial. 5.4. TL ages Although the recycling ratios and dose recovery tests using the TL–SAR procedure on samples QJL-OSL07-B and QJL-OSL07-S are not very satisfied, the TL ages of the two samples are still calculated with this method in order to test it, by comparing the TL ages with those obtained using other methods. The dose used for age calculation was obtained by subtracting the residual dose from the measured De value. For the heated sample (QJL07-OSL07-B), zero dose was subtracted because its residual TL signal had been erased by heating. The results are listed in Table S2, and the TL ages are also displayed in Fig. 2c for comparison. Despite the modest results for the recycling ratio and dose recovery test, the TL age (5.6 0.5 ka) of the baked earth sample (QJL-OSL07-B) is consistent with its OSL age (5.1 0.3 ka) within error limits. The TL age (7.9 1.6 ka) of the sediment sample (QJL-OSL07-S) is larger than its OSL age (5.4 0.3 ka) and the TL or OSL ages of sample QJL-OSL07-B. This could be attributed to the incomplete zeroing of the bleachable TL signals in the sediment at the time of deposition (Godfrey-Smith et al., 1988). This subtraction of the residual unbleachable signal only is thus insufficient. This could also be partly due to the problems of sensitivity correction. The above results demonstrate that both the OSL–SAR and TL–SAR procedures can be applied to heated/ fired materials. By comparison, the OSL–SAR procedure has more advantages such as a less scatter in OSL–SAR De between aliquots and higher luminescence sensitivity to radiation and better sensitivity correction. Therefore, the OSL–SAR protocol is more practicable for these materials, as suggested by Thomas et al. (2008). 6. Conclusions The quartz extracted from the baked earth and sediment samples form the Qujialing archaeological site fulfills the basic conditions (recycling ratio, dose recovery, recuperation) for the OSL–SAR to be successful. The OSL–SAR ages obtained for sediment and baked earth samples from the cultural layer are statistically identical, suggesting the signal zeroing at deposition was completed. Moreover, they are in agreement with the calibrated radiocarbon ages of the charcoal samples from a nearby identical layer, and they are also consistent with the age of the Qujialing culture period. The TL–SAR procedure was also attempted on the sediment and baked samples from the cultural layer. The results for the basic tests were however less satisfying than for the OSL–SAR protocol, and only for the baked sample the TL and OSL results were consistent. While further investigations are needed to improve the TL–SAR procedure, the results obtained are encouraging for applying the OSL to date similar archaeological sites in the middle reaches of the Yangtze River. Acknowledgments The work was supported by MOST (No.: 2006BAK21B02). We thank Long-Jiang Mao and Ke Hu for help in sampling. We are also grateful to the anonymous reviewer, her/his valuable comments
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