Quaternary Geochronology

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1 Quaternary Geochronology 1 (212) 15e155 Contents lists available at SciVerse ScienceDirect Quaternary Geochronology journal homepage: Research paper Optically stimulated luminescence and radiocarbon dating of sediments from Lop Nur (Lop Nor), China Jia-Fu Zhang a, *, Cheng-Lin Liu b, Xiao-Hong Wu c, Ke-Xin Liu d, Li-Ping Zhou a a MOE Laboratory for Earth Surface Processes, Department of Geography, College of Urban and Environmental Sciences, Peking University, Beijing 1871, China b Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 137, China c School of Archaeology and Museology, Peking University, Beijing 1871, China d School of Physics, Peking University, Beijing 1871, China article info abstract Article history: Received 4 October 211 Received in revised form 1 December 211 Accepted 5 December 211 Available online 8 December 211 Keywords: Lop Nur Playa sediment Radiocarbon dating Luminescence dating Radioactive disequilibrium Radiocarbon reservoir effect Lop Nur is a playa lake occupying the lowest part of the Tarim Basin, northwestern China, and is now a desolate and barren region. In the past decades, the ages of the lacustrine sediments from the lake were determined mainly by radiocarbon dating on bulk sediment. In this study, both optically stimulated luminescence (OSL) and radiocarbon methods were used to date the sediments from a pit in the central part of the lake. The OSL ages obtained for ten samples range from.5 to 9.4 ka, and are in stratigraphic order except for one sample. The 14 C ages obtained for twenty-two bulk sediment samples range from 5.8 to 3.2 cal ka BP with erratic distribution. Based on the comparison of 14 C with OSL ages and their agedepth models, we argue that the OSL ages are relatively reliable. The disequilibria in the U decay chain for some samples are deduced from the comparison of the NAA and TSAC results. We suggest that the OSL dating technique should preferably be applied to the playa sediments from Lop Nur, but the disequilibria in the U decay chain should be considered in evaluating dose rates. Additionally, radiocarbon reservoir effects in lakes in western China are reviewed. Ó 211 Elsevier B.V. All rights reserved. 1. Introduction Lop Nur (Lop Nor, Luobupo) is a playa lake in northwestern China. The well-known ruins of Loulan, a vanished oasis town on the Silk Road connecting China to Europe, are located on the northwest edge of the lake. Presently, Lop Nur is a desolate and barren region. The environmental evolution of the lake has attracted the attention of many geographers and geomorphologists (e.g., Huntington, 197; Zhao and Xia, 1984), and has been investigated by many researchers (e.g., Yan et al., 1983, 1998; Zhao and Xia, 1984; Wang et al., 2; Liu et al., 23; Xia, 27; Ma et al., 28; Luo et al., 29). Understanding the palaeoenvironmental change of the lake is important for both academic and practical reasons. This, for example, helps explain the disappearance of the Loulan civilization, the formation of potash deposits in the lake (Wang and Liu, 21). The sediments from the lake were previously dated mainly using the radiocarbon techniques. Some radiocarbon dates obtained for the lake sediments are considered problematic because significant age reversals were observed (F.-B. Wang, * Corresponding author. Tel./fax: þ address: jfzhang@pku.edu.cn (J.-F. Zhang). unpublished data). In this case, the sediments need to be dated using other techniques such as optically stimulated luminescence (OSL). The newly developed OSL dating technique has been widely used for dating playa sediments from other lakes (e.g., Bubenzer and Hilgers, 23; Cupper, 26; Owen et al., 27; Fuchs and Buerkert, 28; Telfer et al., 29). In this study, both the OSL and AMS 14 C dating techniques are applied to sediments from a pit in the lake. 2. Geographical settings and sampling Lop Nur (39 4 e41 2 N, 9 e91 3 E) is located in the eastern part of the Tarim Basin, Xinjiang, China (Fig. 1a). The Tarim Basin is one of the largest inland basins on Earth, and almost entire central part of the basin is covered by the Taklimakan Desert. The basin is surrounded to the north by the Tianshan Mountains, to the west by the Pamir Mountains, and to the south by the Kunlun Mountains. The Lop Nur is the lowest part (78e795 m asl) of the basin and the terminus of every river in the basin. At present, there is no water flowing into the lake, and it thus becomes the terminal playa of the basin. Lop Nur is a fault basin or depression, controlled mainly by NE-striking faults (Liu et al., 26). The depression is characterized by a relatively flat topography and a gently sloping /$ e see front matter Ó 211 Elsevier B.V. All rights reserved. doi:1.116/j.quageo

2 J.-F. Zhang et al. / Quaternary Geochronology 1 (212) 15e Fig. 1. (a) Google Earth satellite image showing the location of the Lop Nur lake in Tarim Basin; (b) Satellite image showing the location of the sample pit (YKD31) in the Lop Nur lake. ground surface, and occupied by an extensive salt crust with a thickness of.3e1. m. The ring-like shape of the salt crust on satellite image is very similar to a human ear, and the image is thus called Great Ear (Fig. 1b). The elevations of the localities on a Great Ear ring are identical, and the rings represent the shorelines of the lake basin before its subsequent drying up in about 193 (Li et al., 28). In 23, a pit ( N, E, numbered as YKD31) with a depth of 5.7 m in the center of the Lop Nur depression was dug for paleoenvironmental studies (Fig. 1b). A more detailed description of stratigraphy will be presented by Liu et al (in preparation). In the field, large block samples were retrieved from the pit section using PVC tubes of about 1 cm long and 1 cm in diameter. The block samples were tightly wrapped with tape and transported to the laboratory in Beijing where each was split for sampling for multidisciplinary paleoenvironmental analyses. The analytical results will be presented in a forthcoming publication. Ten samples for OSL dating and twenty-two bulk samples for radiocarbon dating were collected by removing small blocks from the PVC tubes in the laboratory. 3. Previous radiocarbon dating for Lop Nur and radiocarbon reservoir effect in lakes in western China The radiocarbon ages previously reported were obtained using both conventional and AMS methods on total organic carbon (TOC) from bulk sediments. Here these ages are converted into calendar years following Weninger and Jöris (28), and summarized in Table S1. They are also plotted against depth for comparison (Fig. S1). Because all the studied sections are situated in the central part of the lake with a relatively extensive flat bottom, similar agedepth models are expected for these sections. However, Fig. S1 shows that this is not the case. The implication is that the radiocarbon dating of Lop Nur may be inaccurate. It should be noted that these ages are not corrected for potential radiocarbon reservoir effects because of the lack of independent age controls. There are many lakes in western China, most of them are saline lakes, especially in the Tibetan Plateau (Williams, 1991). The lake sediments as archives of paleoclimate have been widely investigated, and their chronology relies mainly on radiocarbon dating. The dating has been carried out mainly on bulk sediment since terrestrial organic remains are scarce, and bulk sediment is the only readily available dated material for most of the lakes. The 14 C ages obtained are found to be significantly older than expected because of radiocarbon reservoir effects. The reservoir ages for 19 lakes in western China are summarized in Table S2. It shows that the reservoir effect exists in all the lakes, the maximum reservoir age is up to 667 years for shell material in Lake Bangong (Fontes et al., 1996). Table S2 also indicates that the reservoir ages for different organic components (humin, humic acid, total organic carbon) (e.g. Mischke et al., 21), or different dated materials (bulk sediment, shell, aquatic plant) (e.g. Mischke et al., 25; Henderson et al., 21) are different. The reservoir age also varies among lakes and temporally at individual lakes (e.g. Zhou et al., 29), similar to the sites in central North America (Grimm et al., 29). These show that the reservoir effects in lakes in western China are complicated. We thus infer that some 14 C ages without correction for reservoir effects obtained previously for Lop Nur may be problematic. 4. Methods 4.1. Optical dating Fine-grained (4e11 mm) quartz extracts for equivalent dose (D e ) measurements were obtained using the procedure at Peking University (Zhang and Zhou, 27). The purity of the extracts was checked by IR stimulation. Two samples were also examined by X- ray powder diffraction analysis using a Dmax 12 kw X-ray powder diffractometer (Table. S3). The table shows that the two raw samples are mainly composed of quartz (26% and 22%, respectively), feldspar (21% and 1%) and carbonates (23% and 3%). Other minerals include mica, chlorite, halite and gypsum. After etching with silica saturated fluorosilicic acid for three days, the remaining components of the fine-grained fractions are mainly quartz (81% and 74%, respectively) and muscovite (1% and 18%), carbonates and other salt minerals were completely removed. It is noted that a small amount (3% and 8%) of feldspar remained after etching, but IR stimulation did not show any IRSL signals from the extracts. We consider that the effect of the remaining feldspar on quartz OSL measurements is negligible for D e measurements (Zhang et al., 21). The samples were determined using the improved singlealiquot regenerative-dose procedure (SAR) (Murray and Wintle, 2), in which OSL signals were measured for 4 s at 125 C, and a cut-heat of 16 C was applied. Preheat plateau and dose recovery tests were carried out for determining suitable preheat conditions. The net initial OSL signals were derived from the decay curve, taking the first.64 s integral of the initial OSL signal, minus a background estimated from the last 3.2 s integral of a 4 s stimulation. All luminescence measurements, beta irradiation and preheat treatments were carried out in an automated Risø TL/ OSL-15 reader equipped with a 9 Sr/ 9 Y beta source. Blue light (47 3 nm) LED stimulation was used for OSL measurements. Luminescence was detected by an EMI 9235QA photomultiplier tube with three 2.5 mm Hoya U-34 filters in front of it.

3 152 J.-F. Zhang et al. / Quaternary Geochronology 1 (212) 15e155 Two or three subsamples of each sample were dried and ground to powder, and then used to determine their U, Th and K contents with neutron-activation-analysis (NAA) at the China Institute of Atomic Energy. The U and Th contents were also analyzed using thick-source alpha counting (TSAC) (a Littlemore Low Level Alpha Counter 7286 with 42-mm-diameter ZnS screens) for comparison. Saturated water content was measured in the laboratory and used for dose rate calculation, as the lake dried up completely in w193 (Li et al., 28) and the underground water table in this area is shallow (Ma et al., 21). Using the revised dose-rate conversion factors (Adamiec and Aitken, 1998) and an alpha efficiency factor (a-value) of.4.1 (Rees-Jones, 1995), the elemental concentrations were converted into effective dose rate with the AGE program of Grün (29), in which a cosmic ray contribution to dose rate is involved Radiocarbon dating The bulk samples were pre-treated by the standard acid-baseacid method in the laboratory at the School of Archaeology and Museology of Peking University. In this method, the samples were first treated with 2 N HCl to remove carbonates, other salts and acid-soluble organic compounds (fulvic acids), followed by 2% NaOH to remove humic acid, and finally using 2 N HCl to remove modern atmospheric CO 2, which could be dissolved in the NaOH solutions during the alkali step. After each step, the samples were rinsed repeatedly with high-purity deionized water. After drying, the remaining samples (humin fraction) were combusted in a vacuum line to obtain CO 2, which was subsequently converted to graphite using H 2 as reducing medium and iron power as the catalyst. The mixture of graphite and iron power obtained was finally pressed into target holders for AMS 14 C/ 12 C ratio measurements with the compact PCAMS facility at the Institute of Heavy Ion Physics of Peking University (Liu et al., 27). The radiocarbon ages obtained were calibrated into calendar years using the CalPal program with the calibration curve CalPal27_HULU (Weninger and Jöris, 28). Calibrated radiocarbon ages (cal ka BP) are used for comparison with OSL ages. 5. Results and discussion 5.1. OSL chronology Dose rate The NAA results, saturated water contents and the calculated dose rate (NAA-DR) are listed in Tables S4. It can be seen that for each sample the K or Th contents of its subsamples are statistically consistent. However, the U concentrations for some samples vary significantly between subsamples, and between samples. The average U content of the subsamples for each sample ranges from to ppm. Because NAA is performed on only a small amount of sample (1 mg at the China Institute of Atomic Energy), the large scatter in U content between subsamples and between samples may indicate the inhomogeneous distribution of the element in sediments. Table S5 gives the U and Th contents determined by TSAC and the corresponding dose rates (TSAC-DR). The two sets of dose rates (NAA-DR and TSAC-DR) are compared in Fig. 2a. It shows that the TSAC-DR values are relatively constant except for samples YKD31-OSL9 and 1 at depths of 4.4 and 5.7 m, whereas the NAA-DR values fluctuate intensively. For samples YKD31-OSL3, 5, 7 and 8, both their NAA-DR and TSAC-DR values are statistically consistent, while for the other samples the NAA-DR values are much larger than the TSAC-DR values, especially for sample YKD31-OSL9. Depth below surface, cm TSAC a NAA Dose rate, Gy/ka In the 238 U and 232 Th decay chains, NAA only measures the mass concentration of the parent nuclides, whereas TSAC records all the alpha particles in both chains, and the equivalent concentrations of U and Th were determined by pairs counting (Aitken, 1985). Therefore, the comparison of the concentrations of U and Th found from NAA and TSAC can be used for assessing the presence of the radioactive disequilibrium in the U and Th chains at the time of sample collection. It is noted that the comparison may be influenced by the precision of the U content found from TSAC due to relatively small number of pairs counts. The data in Tables S4 and S5 shows that the U concentrations found from NAA are significantly larger than the equivalent concentrations determined by TSAC, suggesting that the disequilibria in the U chain are significant for all the samples except for samples YKD31-OSL5, 7 and 8. This is also supported by the comparison of the dose rates in Fig. 2a, Table S4 and S5. The different geochemical properties of the various U-series isotopes can cause them to be fractionated and finally to produce radioactive disequilibria. Radioactive disequilibria in playa sediments are mainly due to solution and precipitation (Sonnenfeld, 1984), and carbonate formation in sediments can cause disequilibria in the U chain due to U mobilization (Olley et al., 1997). We deduce that groundwater movement, capillary action, evaporative concentration and mineral precipitation in the playa sediments (Ma et al., 21) are the main factors affecting U mobilization in sediments or the estimation of dose rate (Nathan and Mauz, 28). Similar significant U-series disequilibria were also found in other playa sediments (English et al., 21; Cupper, 26). For the Th chain, the disequilibrium is not important for sediments because of short half-lives of the 232 Th daughters (Prescott and Hutton, 1995; Olley et al., 1996, 1997). For some samples in this study, their Th contents found from NAA are not completely consistent with those determined using TSAC, this may be attributed to the precision of pairs counting technique. The disequilibrium in the U chains needs further investigation using high-resolution gamma spectrometry. The nuclides of the U decay chain contribute about 6e8% of the total dose rate in these samples, suggesting that the effect of the disequilibrium in the 238 U chain on dose rate is not negligible. TSAC b D e, Gy c NAA OSL age, ka TSAC Fig. 2. Plots of dose rate (a), D e (b) and OSL ages (c) versus depth. TSAC refers to the dose rates (or OSL ages) determined from thick-source alpha counting, and NAA to the dose rates (or OSL ages) determined from neutron activation analyses.

4 J.-F. Zhang et al. / Quaternary Geochronology 1 (212) 15e is performed on larger sample amounts, in contrast to a small amount of sample in case of NAA. TSAC is thus considered to provide more accurate dose estimates than NAA (Olley et al., 1996; Jacobs, 28). On the other hand, the stratigraphic consistency of the OSL ages (discussed below) obtained using the TSAC-DR (Fig. 2c) may suggest that the TSAC-DR values are more reliable than the NAA estimates Luminescence dating properties A typical natural OSL decay curve for these samples is illustrated in the inset to Fig. 3. Its shape indicates that the OSL signal is rapidly bleached by the blue light, implying that the signal is dominated by the fast component. As an example, the growth curve in Fig. 3 shows that (1) the recuperation is close to zero; (2) the recycling ratio is consistent with unity at 1s; (3) the OSL signal is not saturated at the level of the natural signal. These favorable luminescence characteristics suggest that these samples can be determined using the SAR protocol (Wintle and Murray, 26). Preheat plateau tests were carried out on samples YKD31- OSL4 and 9 to observe the effect of preheat temperature on D e. The samples were measured using the SAR protocol with various preheat temperatures. The individual D e values obtained as a function of preheat temperature are displayed in Fig. 4a. It shows no dependence of D e on preheat temperature in the ranges 18e29 C for sample YKD31-OSL9 and 16e29 C for sample YKD31-OSL4. The suitability of the SAR protocol for these samples was further evaluated by dose recovery tests. After bleaching with blue light at room temperature, the ten samples were irradiated with laboratory beta doses (given dose) approximately equal to their natural dose (D e ), they were then measured using the SAR protocol with a preheat of 2 C for 1 s. The ratios of measured to given doses are plotted in Fig. 4b. It shows that the measured doses are consistent with the given doses within error limits, indicating that the preheat condition of 2 C for 1 s are suitable for these samples. a D e, Gy b Measured dose, Gy YKD31-OSL9 YKD31-OSL Preheat temperature, C 1 : OSL ages At least 13 aliquots for each sample were measured using the SAR protocol with a preheat of 2 C for 1 s, and the average Given dose, Gy 8 Fig. 4. (a) Plot of equivalent dose against preheat temperature; (b) Dose recovery ratios for the studied samples (see text for details). Each point represents the result of an aliquot. Sensitivity-corrected OSL OSL, cts/.16s x Stimulated time, s Regeneration dose, Gy Fig. 3. Sensitivity-corrected OSL growth curve, constructed using the SAR protocol, for a typical aliquot from sample YKD31-OSL1. The open diamond is the repeated point at the first regeneration dose, and the open circle refers to the sensitivity-corrected natural OSL signals. The inset is a natural OSL decay curve for the aliquot D e of each sample are given in Table S6 and displayed in Fig. 2b. It is shown that D e increases with increasing depth except for samples YKD31-OSL6 and 1. Two OSL age values obtained for each sample correspond to NAA-DR in Tables S4 and TSAC-DR in Table S5, respectively, and are listed in Table S6 and compared in Fig. 2c. It can be seen that the OSL ages based on NAA-DR for the upper five samples are in stratigraphic order, whereas for the five samples below 2.2 m (samples YKD31-OSL5-1), the distribution pattern of the OSL ages becomes erratic. For the OSL ages based on TSAC-DR, except for sample YKD31-OSL6 at a depth of 2.76e2.82 m, all the samples yield stratigraphically consistent ages. The implication is that the OSL ages based on TSAC-DR appear to be more reasonable. In this case, the OSL ages mentioned below are referred to those calculated using TSAC-DR. It can be seen from Fig. 2c that the studied section can be divided into two units based on OSL ages. The upper unit (e2.2 m depth) is characterized by a steady increase in age with depth, and the age ranges from.5.1 ka for the uppermost sample

5 154 J.-F. Zhang et al. / Quaternary Geochronology 1 (212) 15e155 (YKD31-OSL1) at a depth of.34e.4 m to 9..9 ka for the sample (YKD31-OSL5) at a depth of 2.2e2.26 m. The lake dried up in w193 (Li et al., 28), implying that it had received sediments. The Great Ear rings on satellite image indicate that the salt crusts have not been eroded after drying. This means that the modern surface samples from the playa lake represent the surface sediment of the lake before drying, and should yield a modern age. Based on this, the OSL age of.5.1 ka for the subsurface sample (YKD31-OSL1) from the depth of.34e.4 m is reasonable. For the lower unit (2.26e5.7 m depth), the OSL ages (excluding sample YKD31-OSL6) are identical within error limits, and the mean is ka. The implication is that the sediments in the lake were rapidly deposited during this period, which is may relate to the 8.2 ka event (e.g. Liu et al., 23; Alley and Agustsdottir, 25). A more detailed explanation of the 8.2 ka event in Lop Nur will be given by Liu et al. (in preparation) Radiocarbon ages The results of radiocarbon dating are listed in Table S1 and displayed in Fig. 5 and Fig. S1. The subsurface sediment sample (sample Y11) at a depth of.16e.18 m was dated to cal a BP, and the bottom sample (Y448) at a depth of 4.66e4.67 m to cal a BP. The oldest age of cal a BP occurs at a depth of 3.54e3.55 m. The radiocarbon dates for the samples at depths of 1.6e1.12 m are in strict stratigraphic order. Below 1.12 m, there is no significant correlation between the dates and depth, especially for the samples below 2.6 m, in which the dates are commonly stratigraphically reversed. The age of cal a BP for the subsurface sample indicates that the radiocarbon ages could be overestimated, and the erratic distribution of ages below 1.12 m imply that these 14 C age estimates do not exactly reflect the true deposition age of the samples. In addition, most of the dates obtained in this study are much older than the previously obtained dates for the samples from similar depths in the area (Fig. S1), demonstrating that the establishment of a reliable radiocarbon chronology for the playa lake is complicated Comparison of radiocarbon and OSL ages The comparison of the 14 C and OSL ages in Fig. 5 shows that the 14 C ages are consistently older than the corresponding OSL ages, however, both sets of ages show the same broad trend of deposition. The difference between them from the same horizon generally becomes larger as increasing sediment depth, and the maximum is up to w2 ka. The 14 C age of 3229 years for the surface sediment was calculated by linear regression of calibrated age against depth for the upper five 14 C samples, and can be regarded as the size of the reservoir effect for the surface sample. However, it is difficult to explain the approximately 2 ka difference by reservoir effect alone. This is because although a large radiocarbon reservoir effect was observed in the lakes in Antarctica (Hall and Henderson, 21; Hendy and Hall, 26), the effect in lakes in western China is found to be less than 667 years (Table S2). It is well known that accurate radiocarbon dating of bulk sediment samples is somewhat problematic due to the presence of various organic carbon fractions, which have different 14 C activities indicating different ages (Martin and Johson, 1995; Abbot and Stafford, 1996; Eglinton et al., 1997; Barnekow et al., 1998; Brock et al., 21 and references therein). We infer that some organic components of the sediments from Lop Nur are from older deposits in the catchment, as proposed for Lake Michigan by Colman et al. (2). The variability in the constituents of the terrestrial materials with different 14 C activities results in the erratic distribution of the radiocarbon ages. Depth below surface, cm Conclusions The fine-grained quartz extracted from the sediments from the Lop Nur playa lake has favorable properties for luminescence dating. However, for some samples, radioactive disequilibrium in the U decay chain is deduced from the comparison of the NAA and TSAC results. The NAA results also showed that the distribution of U in the lake sediments is inhomogeneous. The NAA results lead to less accurate estimates of dose rates compared to the TSAC results. The OSL ages calculated from the TSAC results (U and Th) range from w.5 ka for the subsurface sample to w9.4 ka for the bottom sample. Except for one sample, all the OSL ages obtained from the TSAC results are internally and stratigraphically consistent. However, twenty-two radiocarbon dating samples yield ages between w5.8 and w3. cal ka BP with erratic pattern. The age of 5.8 cal ka BP for the subsurface sample and the comparison with OSL ages shows that the radiocarbon ages of the bulk sediment samples were overestimated, which may be attributed to both the reservoir effect and the input of relict terrestrial organic matter. We therefore consider that the OSL ages obtained are more reliable than the radiocarbons ages for these samples, and the palaeoenvironmental reconstruction of Lop Nur should be based on the OSL dating results. The radiocarbon dates including those obtained previously should be used with extreme caution. Acknowledgment Optical age This project was supported by NSFC (48342, 48719). We thank Gang Hu and Gue-Jie Guo for help with sample preparation and D e measurements. We appreciate the helpful comments provided by the anonymous reviewer. Editorial handling by: R. Grun Appendix. Supplementary material Age, ka Calibrated 14 C age Fig. 5. Relationship between calibrated radiocarbon and optical ages and depth. Supplementary data related to this article can be found online at doi:1.116/j.quageo

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