SCIENCE CHINA Earth Sciences

SCIENCE CHINA Earth Sciences RESEARCH PAPER February 2015 Vol.58 No.2: 183 194 doi: 10.1007/s11430-014-4993-2 Underestimated 14 C-based chronology of late Pleistocene high lake-level events over the Tibetan Plateau and adjacent areas: Evidence from the Qaidam Basin and Tengger Desert LONG Hao 1,2* & SHEN Ji 1 1 State Key Laboratory of Lake Sciences and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China; 2 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi an 710075, China Received April 23, 2014; accepted September 25, 2014; published online November 19, 2014 The palaeolake evolution across the Tibetan Plateau and adjacent areas has been extensively studied, but the timing of late Pleistocene lake highstands remains controversial. Robust dating of lacustrine deposits is of importance in resolving this issue. This paper presents 14 C or optically stimulated luminescence (OSL) age estimates from two sets of late Quaternary lacustrine sequences in the Qaidam Basin and Tengger Desert (northeastern Tibetan Plateau). The updated dating results show: (1) the radiocarbon dating technique apparently underestimated the age of the strata of >30 ka BP in Qaidam Basin; (2) although OSL and 14 C dating agreed with each other for Holocene age samples in the Tengger Desert area, there was a significant offset in dating results of sediments older than ~30 ka BP, largely resulting from radiocarbon dating underestimation; (3) both cases imply that most of the published radiocarbon ages (e.g., older than ~30 ka BP) should be treated with caution and perhaps its geological implication should be revaluated; and (4) the high lake events on the Tibetan Plateau and adjacent areas, traditionally assigned to MIS 3a based on 14 C dating, are likely older than ~80 ka based on OSL chronology. Tibetan Plateau, lake highstand, lacustrine sediments, 14 C dating, OSL dating Citation: Long H, Shen J. 2015. Underestimated 14 C-based chronology of late Pleistocene high lake-level events over the Tibetan Plateau and adjacent areas: Evidence from the Qaidam Basin and Tengger Desert. Science China: Earth Sciences, 58: 183 194, doi: 10.1007/s11430-014-4993-2 *Corresponding author (email: longhao@niglas.ac.cn) Corresponding author (email: jishen@niglas.ac.cn) Since the 1980s, the late Quaternary evolution of closed lake basins from the Tibetan Plateau (TP) and adjacent areas has been extensively studied to reconstruct past environmental and climatic conditions (e.g., An et al., 2000; Lehmkuhl and Haselein, 2000; Shi et al., 2001; Yang et al., 2004; Herzschuh, 2006; Chen et al., 2008; Mischke et al., 2008; Daut et al., 2010; Long et al., 2010; Mügler et al., 2010; Yang and Scuderi, 2010; Wischnewski et al., 2011; Yang et al., 2011; Shen, 2013). Based on 14 C dating of lake shorelines and lacustrine remains, nearly all studies suggested that the high lake level stands occurred at 40 25 ka, corresponding to the late marine isotope stage 3 (i.e., MIS 3a; Martinson et al., 1987). These study sites (circled ones in Figure 1(a)) are distributed over the TP, as well as the foreland areas in the deserts (e.g., the Tengger Desert, Badain Jaran Desert, and Taklamakan Desert) (Lehmkuhl and Haselein, 2000; Shi et al., 2001; Yang et al., 2004; Yang et al., 2011). For instance, in the Qaidam Basin from the northeastern TP, Chen and Bowler (1986) found a palaeolake shell bar (Figure 1(b)), approximately 29 m above the modern level of the Qarhan Salt Lake; this shell bar consists of abundant mollusk fossils and mussels, reflecting fresh to slightly saline water conditions. Science China Press and Springer-Verlag Berlin Heidelberg 2014 earth.scichina.com link.springer.com

184 Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 Figure 1 Location of the study region. (a) Map showing the locations of lake highstand sites on the TP and adjacent areas. At the sites denoted by filled circles, the lake highstands dated back to the MIS 3a based on 14C dating (see Figure 2 for the radiocarbon dates of lake highstand timings). At the sites denoted by filled squares, the lake highstands dated back to MIS 5 based on OSL or U/Th ages. The dashed rectangles denote the Qaidam Basin and Tengger Desert, respectively. (b) Map showing the Qaidam Basin. The Qarhan Salt Lake is shown by the dashed line. The filled circle denotes the location of the shell bar studied by Chen and Bowler (1986), Chen et al. (1990), and Zhang et al. (2008). (c) Map showing the Tengger Desert. The Zhuyeze Lake is denoted by the rectangle. The filled circles denote the locations of the three lacustrine profiles sections BJ-S1, BJ-S2, and QTL (Long et al., 2011).

Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 185 Three shell samples from the upper, middle and lower parts of a profile from this bar dated back to 28650±670, 35100± 900, and 38600±680 a BP by the conventional 14 C method, which suggests that this lake had a high water level at ca. 39 28 ka BP (Chen et al., 1990). Zhang et al. (2008) further dated the same shell bar using the accelerator mass spectrometry (AMS) method, and obtained similar age ranges. The 14 C-dated high lake levels during the late MIS 3 seemed to occur not only in the Qaidam Basin but also in the western and central part of the TP (Figure 1(a)), e.g., Tianshuihai Lake (Li et al., 1991), Longmuco Lake (Li, 2000), Bangongco Lake (Zheng et al., 1989; Li et al., 1991), Zabuye Lake (Zheng et al., 1996), and Selinco Lake (Li, 2000). Similarly, there is good evidence of the MIS 3a highstands from the adjacent areas of the TP (Figure 1(a)). Taking the Tengger Desert (Figure 1(c)) for example, while there are still many lakes in the inter-dune basins in this region, remains of lacustrine sediments and palaeoshorelines indicate the more extensive occurrence of lakes and swamps in the past. Pachur et al. (1995) and Zhang et al. (2004) investigated in detail the palaeobeaches around the Zhuyeze Lake in the Tengger Desert using radiocarbon dating of bulk organic matter or mollusk shells. Their results showed that the highest water level formed at ~35 30 ka BP. The 14 C chronologies of lacustrine beaches also suggested high lake levels during the MIS 3a in the Juyan Lake (Figure 1(a)) on the northern margin of the Badain Jaran desert (Wünnemann et al., 1998). Radiocarbon dates for lacustrine remains from the Manas Lake (Rhodes et al., 1996), Barkol Lake (Yu et al., 2001) and Aiding Lake (Li et al., 1989) showed the MIS 3a highstand as well (Figure 1(a)). However, a set of recent studies on lake shorelines from the northeastern margin of the TP found that the highstands apparently dated back to MIS 3a by 14 C dating actually date back to the period beyond ~70 ka by optically stimulated luminescence (OSL) dating method (Madsen et al., 2008, 2014; Liu et al., 2010; Rhode et al., 2010; Long et al., 2012). The timing of late Pleistocene lake highstands from the TP and its adjacent areas remains undetermined. For instance, OSL chronology of early shorelines around the Qinghai Lake (Figure 1(a)) showed that the maximum highstands ~20 66 m above present-day lake levels occurred approximately during 100 90 ka (Madsen et al., 2008), not in association with MIS 3a as found in the Qaidam Basin. In the Lop Nor Lake (Figure 1(a)) the lake highstand dated back to 130 85 ka or even older (Wang et al., 2008) by OSL method. Our recent dating study found that the highstand around the Zhuyeze Lake from the Tengger Desert dated back to ca. 100 70 ka based on OSL dating (Long et al., 2012), instead of 35 30 ka as previously derived from 14 C dating (Pachur et al., 1995; Zhang et al., 2004). In addition, by using U/Th series dating techniques, the high lake level event in Nam Co (Figure 1(a)) was estimated at 130 75 ka (Zhu et al., 2004). The dates constraining the highstand timing are plotted together (Figure 2), and showing obvious differences in ages between the short (i.e., 14 C dating) and the long (e.g., luminescence dating) chronologies. Resolution of this issue is important because a large number of global climate models use lake sequences to assess the strength of Asian monsoons and hemispheric westerlies. It appears that such a resolution will involve a reconciliation of the dating problem; as a result, direct comparison of radiocarbon and luminescence age estimates for the same sediments is necessary. Here we present age estimates on the basis of 14 C or OSL method for two sets of late Quaternary lacustrine sequences from the Qaidam Basin and the Tengger Desert, respectively, and try to revisit the geochronology of highstands which were assigned to be developed during MIS 3. 1 Study area and materials The Qaidam Basin (36.6 37.2 N, 93.7 96.3 E), situated in the northeastern TP (Figure 1(a)), is bounded by the Kunlun Mountains to the south and the Aerjin Mountains and Qilian Mountain to the north (Figure 1(b)). This basin is a large playa with an area of 5850 km 2 and a mean elevation of 2800 m a.s.l., and contains a series of concentrated salt lakes with a total area of 460 km 2, and with the Qarhan Salt Lake in the depocenter of the basin (Figure 1(b)). The average annual precipitation in this region is 25 50 mm, the annual mean temperature is 2 4 C and the annual evaporation exceeds 3000 mm. By using a rotational drilling system with 3-m-long metal tubes with 90-mm diameters, a 100-mlong sediment core (ISL1A Core, 37 03 50 N, 94 43 41 E) was obtained from the central part of Qarhan Salt Lake (Figure 1(b)). The stratigraphy of ISL1A Core shows evaporate halite layers (mainly crystal salt) with some lacustrine clastic layers (i.e., silt-clay or clayey silt sediment) from ~52 m in depth to top, and that lacustrine clastic clay to silt was deposited from the base to ~52-m (Figure 3). Considering the dating limitation of the 14 C technique, we collected radiocarbon samples from the upper part (0 55 m) of this core only. Because the sediments from the core ISL1A contain little organic carbon and are devoid of plant macrofossils we sampled bulk organic matter (11 samples) for 14 C dating (Figure 3 for the sampling locations). In the Tengger Desert, there are numerous lakes in the inter-dune basins. The Zhuyeze Lake is one of those lakes, and is located at the terminal of the Shiyang River in the northern piedmont of the eastern Qilian Mountain (Figure 1(c)). It is now a salt marsh at an elevation of ~1281 m and has a surface area of ~42 km 2, with brackish water occurring 1 m below the surface. Annual mean temperature in the region is 7 C, annual precipitation is 48 mm and annual evaporation is 2600 mm. Two lake sedimentary sequences (sections BJ-S1 and BJ-S2, see Figure 1(c) for their locations) were selected for the study of comparing OSL and 14 C dating techniques. Section BJ-S1 is from the highest

186 Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 Figure 2 Timing of lake highstands in different lakes from the TP and adjacent areas. Filled circles with error bars denote 14 C ages, and filled squares with error bars denote OSL or U/Th ages. All 14 C ages fall in the range of 40 25 ka BP (left grey shading), and most OSL or U/Th ages are older than 70 ka (right grey shading). lake terrace in this area. Our previous study (i.e., Long et al., 2012) obtained the ages of the two sections by OSL dating of medium grained (MG, 38 63 m) quartz (Figure 4). In the present study, coarse grained (CG, 90 150 m) quartz, extracted from three representative samples (BJ-S1-5, BJ- S1-6 and BJ-S2-4) from the two sections, was used with the small aliquot technique for OSL dating to validate the previous MG quartz age estimates. Two shell samples were collected from sections BJ-S1 and BJ-S2 (Figure 4 for sampling locations) for 14 C dating and then comparison with OSL ages. 2 Methods 2.1 14 C dating For radiocarbon dating of mollusk fossils from lake deposits, a possible source of inaccuracy is from sampling reworked material from older deposits. To avoid this, we tried to collect undisturbed fossil shells for 14 C dating. Full mollusk fossils from the shell-rich sedimentary layer in sections BJ- S1 and BJ-S2 were collected for 14 C dating (Figure 4). We used clean tweezers to sample shells that were placed in plastic bags and then stored in the refrigerator until sending them out for analysis. In the radiocarbon dating laboratory, fossil shells were cleaned with 30% H 2 O 2 in an ultrasonic bath to remove the organic surface coating and adhering dust as well as detrital carbonate. Eleven bulk organic samples from the ISL1A core were collected for 14 C dating. Through a conventional treatment, the bulk sediments were treated with HCl (2N), NaOH (2%) and HCl (2N), and the humic acid fraction was obtained for combustion. The combustion to CO 2 of the organic fractions was performed in a closed quartz tube together with CuO and silver wool. All samples were prepared to graphite and AMS radiocarbon measurements were undertaken at Peking University. The 14 C ages were calibrated to calendar year (a BP) using the CALIB 6.1.0 program (http://calib.qub.ac.uk/ calib/) with the IntCal09 dataset (Reimer et al., 2009), which allows a direct comparison with OSL ages (ka). 2.2 OSL dating For the three samples (BJ-S1-5, BJ-S1-6 and BJ-S2-4) from the Tengger Desert, the CG fraction was extracted for OSL dating for comparison with the previously determined MG

Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 187 Figure 3 Stratigraphy and 14 C-based chronology of core ISL1A. Filled triangles denote the locations of 14 C samples. ages (Long et al., 2012). These samples were first treated with HCl and H 2 O 2 to remove carbonates and organics, followed by heavy liquid density separation with lithium-heteropolytungstate to separate the quartz from any heavy minerals (>2.75 g/cm 3 ) and feldspars (<2.62 g/cm 3 ). In the final step, the 2.62 2.75 g/cm 3 fractions were etched with 40% HF for 60 min (followed by an HCl rinse) to remove the outer (alpha-irradiated) surface of the quartz grains and also to eliminate any potential feldspar contamination. It is important to ensure that the feldspar contamination has been efficiently removed to avoid age underestimation (Roberts, 2007). The purity of the isolated quartz was checked by the IR depletion ratio method (Duller, 2003), and also by measuring the 110 C TL peak (Li et al., 2002) for the SAR sequence for each aliquot. The separated quartz grains were then mounted as mono-layers onto 10-mm-diameter aluminium cups using silicone oil adhesive (sample diameter 2 mm). OSL measurements were made on the automated Risø TL/OSL-15 reader at the University of Bayreuth, Germany. Stimulation was carried out by a blue LED ( =470±20 nm) stimulation source for 40 s at 130 C. Irradiation was carried out using a 90 Sr/ 90 Y beta source built into the reader. The OSL signal was detected by a 9235QA photomultiplier tube through a 7.5-mm-thick U-340 filter. OSL signals from the first 0.64 s of stimulation were integrated out of 40 s for growth curve construction after background subtraction for the last 8 s. For each sample 15 18 aliquots were measured to obtain equivalent dose (D e ) using the single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000). Long et al. (2012) carried out preheat plateau tests and dose-recovery tests at different preheat temperatures, and chose preheat of 260 C and cut-heat of 220 C for the D e Figure 4 Stratigraphy and chronology of two profiles (BJ-S1 and BJ-S2). The three OSL ages in single quotation were derived from CG quartz in this study, and the other OSL ages were derived from MG quartz (Long et al., 2012).

188 Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 measurement of MG quartz. These preheat conditions were also used for the OSL dating of the CG quartz from the two profiles BJ-S1 and BJ-S2 because both MG and CG fractions likely have the same sources (Long et al., 2007) and then similar luminescence characteristics. The concentrations of uranium (U), thorium (Th) and potassium (K) were measured by neutron activation analysis (NAA) for dose rate calculation. For the two samples (BJ-S1-5 and BJ-S1-6) from profile BJ-S1, the radionuclide concentrations of the surrounding sediments were also determined by high-resolution gamma spectrometry (Murray et al., 1987). The elemental concentrations were converted into annual dose rates (Aitken, 1998). The cosmic ray dose rate was estimated for each sample as a function of depth, altitude, and geomagnetic latitude (Prescott and Hutton, 1994). The two sequences (BJ-S1 and BJ-S2) were from the palaeoshorelines or terraces that formed during lake shrinking, and the water content of shoreline sediments changed after the lake level retreated. Considering the variability of the water content of shoreline sediments, we assumed water content of 5±2.5% for the dose rate calculation. 3 Results 3.1 Radiocarbon ages All 13 AMS 14 C ages from core ISL1A and profiles BJ-S1 and BJ-S2 are listed in Table 1, along with the dating materials. In Figures 3 and 4, these radiocarbon dates (calibrated ages) are shown together with the corresponding stratum. 3.2 Luminescence characteristics and ages Figure 5(a) shows the natural OSL decay curve for sample BJ-S1-5; the OSL signal decreases very quickly during the first second of stimulation, suggesting that the decay curve is typical for quartz, and appears to be dominated by the fast component (Bailey et al., 1997; Jain et al., 2003). A representative growth curve is shown in the inset of Figure 5(a); this is well represented by exponential plus linear fitting (black solid line) with six regeneration dose points, including a zero-dose for the measurement of recuperation and a recycling point for assessing the sensitivity change correction. Figure 5(b) summarizes the recycling ratios, where the sensitivity-corrected luminescence intensity observed from the first regenerative dose is divided by the corrected observed one when the same dose is repeated at the end of the SAR measurement sequence (Murray and Wintle, 2000). The measurements following laboratory irradiations are reproducible; all ratios are in the range of 0.9 1.1 and the mean is 1.016±0.005. The inset in Figure 5(b) shows the recuperation values, that is, the response to a 0 Gy laboratory dose, measured after the SAR cycle containing the largest regenerative dose (Murray and Wintle, 2000). These signals are expressed as a percentage of the sensitivity-corrected natural luminescence; all recuperation values lie below 5%. These summary statistics suggest the applicability of the SAR protocol to these samples. Figure 6 presents the D e s distributions for the three samples. Dose rate data determined by NAA and gamma spectrometry techniques are shown in Tables 2 and 3. The overdispersion of D e distribution is calculated (Table 4), which suggests that these samples are normally distributed or only slightly skewed. Thus, we use the central age model (CAM) of Galbraith et al. (1999) for age calculation (Table 4). The OSL ages of the two sections together with their stratigraphy are shown in Figure 4. 4 Discussion 4.1 Reliability of 14 C and OSL dating in late Pleistocene sediments According to the relationship between age and depth for the Table 1 Radiocarbon dating results for this current study Sampling site Lab No. Sample Depth (m) Material 14 C age ( 14 C a BP) Calibrated age (a BP) BA091109 ISL1A09-1 4.65 10225±45 12005±51 BA091110 ISL1A09-2 13.01 18230±65 21736±200 BA091111 ISL1A09-3 22.18 31490±140 36863±200 BA091112 ISL1A09-4 30.29 32370±180 37764±235 BA091113 ISL1A09-5 34.43 21245±75 25598±109 Core ISL1A BA091114 ISL1A09-6 38.35 Bulk organic 30615±140 35982±180 BA091115 ISL1A09-7 40.27 32605±175 38002±232 BA091117 ISL1A09-9 47.07 28840±110 34244±182 BA091118 ISL1A09-10 49.45 27405±100 32737±191 BA091119 ISL1A09-11 52.04 27485±100 32822±191 BA091120 ISL1A09-12 54.44 27140±100 32456±188 Section BJ-S1 BA090372 BJ-S1-C1 0.3 31605±110 36982±181 Shells Section BJ-S2 BA090371 BJ-S2-C1 0.5 5760±40 6562±97

Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 189 Figure 5 Luminescence characteristics. (a) Typical natural OSL decay curve, and SAR growth curves (inset) for one aliquot of sample BJ-S1-5 using the exponential plus linear fitting (black) and single exponential saturation fitting (red), respectively. (b) Summary of all available recuperation and recycling data for the three samples (BJ-S1-5, BJ-S1-6, and BJ-S2-4). core ISL1A (Figure 7), it can be seen that the three 14 C ages from the upper part (0 25 m) show reasonable internal consistency, with sequences generally yielding ages in stratigraphic succession. The other dates, however, do not increase with depth but are scattered in a wide range between 25 and 38 ka BP from 25 to 55 m depth. This likely indicates an underestimation of radiocarbon dates for the strata at depths of 25 52 m. Although very rapid deposition of massive sediment beds or re-deposition may alternatively explain the current 14 C age pattern of core ISL1A, this could Figure 6 Radial plots showing the distribution of the D e values of sample BJ-S1-5 (a), BJ-S1-6 (b), and BJ-S2-4 (c). The resultant D e value of the central age model (Table 4) is shaded. Table 2 Dose rates of the surrounding sediments determined by the NAA method Sample Depth (m) Water content (%) U (ppm) Th (ppm) K (%) Dose rate (Gy/ka) BJ-S1-5 1.1 5±2.5 5.83±0.23 7.74±0.26 1.60±0.05 3.90±0.27 BJ-S1-6 0.7 5±2.5 4.24±0.20 7.11±0.24 1.46±0.05 3.28±0.23 BJ-S2-4 1.3 5±2.5 1.23±0.16 5.76±0.23 1.86±0.06 2.85±0.17

190 Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 Table 3 Radionuclide concentration of sediments determined by gamma spectrometry method Sample U from 234 Th (ppm) U from 214 Pb, 214 Bi (ppm) U from 210 Pb (ppm) Th from 208 Tl, 212 Pb, 228 Ac (ppm) K (%) Dose rate (Gy/ka) BJ-S1-5 6.29±0.37 6.48±0.25 5.73±0.29 8.57±0.11 1.71±0.04 4.24±0.29 BJ-S1-6 4.77±0.31 4.81±0.20 4.45±0.25 7.99±0.11 1.64±0.04 3.66±0.26 Table 4 OSL dating results in this study Sample Minimum D e a) (Gy) CAM D e b) (Gy) Overdispersion (%) Minimum age (ka) CAM age (ka) BJ-S1-5 265.2±16.5 310.7±7.5 8.0 68.0±6.3 79.6±5.8 BJ-S1-6 215.9±2.2 306.1±11.3 13.2 65.9±4.6 93.4±7.3 BJ-S2-4 25.1±1.0 28.7±0.7 10.0 8.8±0.7 10.1±0.7 a) Minimum D e values are derived from the lowest measured aliquot and the minimum age is calculated based on the minimum D e. b) The CAM D e values and overdispersion are derived from all accepted aliquots (Galbraith et al., 1999). Figure 7 14 C ages against depth for core ISL1A. The dashed line is the fitting and extrapolation of three 14 C ages from the upper 25 m. Grey band denotes the onset of halite formation. not cause such significant scattering radiocarbon data. More reasonable explanation is that 14 C dating underestimates the ages of sediments beyond ca. 30 ka BP. As shown in Figure 4, the OSL ages of section BJ-S1 fall into the range of 90 80 ka, but the 14 C dating of the shell sample BJ-S1-C1 yielded an age of 36982±181 cal a BP, which is much younger than the OSL age (86.5±6.6 ka, sample BJ-S1-8) of the same stratum. Although only single 14 C age from BJ-S1 was obtained in the current study, a set of 14 C dates of shells from the lacustrine strata at the same elevation around Zhuyeze Lake also fell in the range of ca. 30 40 ka BP (Pachur et al., 1995; Zhang et al., 2004), which agrees with our 14 C date (i.e., 36982±181 cal a BP from sample BJ-S1-C1) but significantly contrasts with our OSL results (i.e., 90 80 ka). The similar dating offsets have been reported from many studies which have compared luminescence and radiocarbon dating for Pleistocene sediments or archaeological sites. For instance, Briant and Bateman (2009) presented nine directly comparable paired OSL and AMS radiocarbon ages from multiple sites within Devensian fluvial sediments in lowland Britain and showed that the two techniques agree well for ages younger than ca. 35 ka BP but disagree beyond ca. 40 ka BP. Busschers et al. (2011) compared a set of marine shell AMS radiocarbon age estimates from boreholes in the Netherlands (southern North Sea area) with luminescence dating control, and most of the marine shells give ages between 32 46 14 C ka (36 50 ka BP), whereas a much older MIS 5e age (>117 ka) is suggested by both quartz and feldspar OSL dating. Early dating of human occupation of the Australian continent also suggested the significant difference between 14 C and OSL ages (Bird et al., 1999). 14 C determinations suggested that humans first arrived about 40 ka BP (Allen and Holdaway 1995; O Connell and Allen 1998), whereas luminescence techniques suggested that humans may have arrived at 54 60 ka (Roberts et al. 1994). Similarly, the discrepancy between luminescence and 14 C ages was also noted by Zhang et al. (2006) based on OSL and AMS 14 C dating for a core from the Lake Juyan. Differential contamination may explain the radiocarbon dates from the Juyan core, as they seem to be all over the place regardless of depth in the core and despite being run by two separate labs. The luminescence ages were also run by two separate laboratories, but are in order and consistently older with depth, suggesting that they may be the more valid. In contrast, another directly comparable paired OSL and radiocarbon age determination (6.5±0.4 ka and 6562±97 cal a BP for samples BJ-S2-1 and BJ-S2-C1, respectively) from Section BJ-S2 (Figures 4 and 8(a)) suggests consistency in the two methods for the Holocene strata. In addition, in section Qingtuhu (QTL) (Figure 1(c) for its location), Long et al. (2011) found a good agreement between OSL and 14 C dating back to ca. 13 ka BP (Figure 8(b)), indicating not only the negligible hard water reservoir effect of 14 C samples but also the consistency between OSL and 14 C ages, at least for the Holocene lacustrine sediments in the study area.

Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 191 Figure 8 Ages comparison. (a) Comparison between OSL and 14 C ages (with errors) for two strata from profiles BJ-S1 and BJ-S2. (b) Comparison of OSL and 14 C ages (with errors) from section QTL. Data from Long et al. (2011). The dashed lines in (a) and (b) show a 1:1 relationship between the two age techniques. (c) Impact of modern contamination (0.25% 2% by weight) on measured 14 C ages (thin lines) compared with the 1:1 or uncontaminated line (thickest line). After Pigati et al. (2007). Therefore, the comparison of OSL and 14 C dating of the late Quaternary lacustrine sediments from the Tengger Desert indicates that the two dating techniques agree well for the Holocene samples (younger than ca. 13 ka BP) but disagree for older samples (e.g., beyond ca. 30 ka BP). The significant discrepancy between the two techniques beyond 30 ka BP cannot be attributed to sampling different aged materials (e.g., resulting from deposition reworking), because the sedimentological settings suggest that the materials were deposited approximately contemporaneously. For instance, the shells for radiocarbon dating from BJ-S1 were collected from a sedimentary stratum with abundant original and undisturbed fossil shells, which suggests that the dated shells were not transported before deposition. Therefore the robustness of each dating technique needs to be established. First, the reliability of the OSL age should be estimated. OSL age overestimation can come through either D e overestimation or dose rate underestimation. Partial bleaching has been identified as a potential problem in fluvial or lake environments (e.g., Zhang et al., 2003), largely because of the increased attenuation of sunlight by water and suspended sediment. This possibility must therefore be considered in relation to these samples. Long et al. (2011) confirmed that the OSL signal of MG quartz from the QTL section was fully reset before burial. D e values derived from CG quartz for the three samples (BJ-S1-5, BJ-S1-6 and BJ-S2-4) also show an approximately normal distribution in this study (Figure 6 and Table 4), indicating full bleaching. For comparative purposes the lowest measured D e value for each sample (based on a single aliquot) from section BJ-S1 was used to calculate age (Table 4). Given that this minimum D e value is from the aliquot within a partially bleached sample that has the most well bleached grains, the obtained OSL age according to this minimum D e should be comparable to radiocarbon chronology. However, this difference between minimum D e age and 14 C date is still present (Table 4). Thus, an overestimation of D e due to partial bleaching can be excluded for these samples. An additional source of error in OSL age estimates might be from dose rate determination, but for both OSL dates from the section BJ-S1 to accord with the radiocarbon ages, dose rates should have to at least double. This is unlikely because gamma spectrometry analyses of samples BJ-S1-5 and BJ-S1-6 yielded similar U, Th, and K values and dose rates as the NNA method (Tables 2 and 3). Furthermore, a potential problem with water-lain sediments is the disequilibrium of the U decay chain, leading to time-dependent changes in dose rate (Olley et al., 1996; Li et al., 2008). To check for disequilibrium, the U content of the surrounding sediments derived from 234 Th, 214 Pb, and 214 Bi, and 210 Pb were compared (Table 3). No significant discrepancy of these values was observed, indicating equilibrium for the U decay chain. In addition, although water content variations throughout the time of burial may have a significant impact on dose rates and then on age estimations, water content is seldom greater than 5%, and therefore within the error range specified for water contents. Furthermore, the OSL ages of CG fraction broadly confirm that derived from MG fraction for samples BJ-S1-5, BJ-S1-6, and BJ-S2-4 (Figure 4). This estimation allows us to have greater confidence on the OSL ages and, therefore, that the age offset seen between them and the radiocarbon ages could be attributed to underestimation of radiocarbon

192 Long H, et al. Sci China Earth Sci February (2015) Vol.58 No.2 ages older than 30 ka BP. The Holocene 14 C date appears to be generally reliable. 4.2 Possible cause for 14 C age underestimation beyond 30 ka BP As discussed above, chronological comparison studies from the Tengger Desert seem to indicate that, while OSL and 14 C dating agree well for the Holocene samples, there may be a significant offset between the two dating techniques beyond ca. 30 ka BP, which largely results from radiocarbon dating underestimation. A possible reason is that contamination with modern carbon may occur after deposition, or during sampling and preparation for dating. In reality, younger carbon produced during soil formation may percolate through the sequence and coat the sample material. A study by Pigati et al. (2007) showed that older radiocarbon dating samples have an increased susceptibility to modern carbon contamination, which is shown in Figure 8(c). This is because levels of radioactive carbon are much lower in these samples. As Figure 8(c) shows, for example, a 2% contamination with modern carbon of a 15-ka-old sample could lead to only a minor age underestimation. However, the same contamination for a 70-ka-old sample could lead to age underestimations in excess of 30 ka, which could be the reason why there is a significant age difference between OSL and radiocarbon dating for section BJ-S1 and a small age offset for section BJ-S2. Thus, to obtain reliable 14 C ages, we suggest that extreme care should be taken, especially with older samples, to avoid contamination and exclude reworked material. OSL age estimates are thought to be likely underestimated, even though the growth curve is still not saturated (Buylaert et al., 2007; Chapot et al., 2012). A single saturating exponential function was also used to build up the dose-response curves for the current study (e.g., red solid line in the inset of Figure 5(a)). We found that the obtained D e s for both samples, although close to the one calculated by the exponential plus linear fitting, are generally near or beyond the values of 2D 0. This indicates that the apparent ages (ca. 90 80 ka) are very likely underestimated. On the basis of OSL dates, therefore, we propose that the high lake period from the Tengger Desert is estimated to be older than ~80 ka, which is similar to the Qaidam Basin. Additionally, we highlight that more OSL dating methods on feldspar minerals (e.g., Chen et al., 2013; Li et al., 2014; Long et al., 2014a, 2014b) should be applied in the future work to extend the dating limit. This finding has significant implications for timing of high lake-level events recognized in the TP and adjacent areas. The lake highstands in these areas were traditionally assigned to MIS 3a based on numerous radiocarbon dates in the range of ca. 40 25 ka BP. It would appear that most of these radiocarbon ages need to be more critically evaluated. A combination of proximity to the radiocarbon limit, which results in the indistinguishable 14 C activity of sample from the background, and/or contamination with very small amounts of modern carbon may explain why many sites return similar ages (as per curvature on Figure 8(c)). This study thus shows a likely underestimation in the 14 C-based chronology of late Pleistocene high lake-level events on the TP. 4.3 Lake highstands over the TP and adjacent areas In general, in the arid areas such as the Qaidam Basin, lacustrine clasts are deposited in freshwater conditions, indicating rising lake levels, and halite precipitation occurs under saline conditions, indicating falling lake level (Chen et al., 1986; Zheng et al., 1989; Shi et al., 2001). Lithological observations showed that core ISL1A can be roughly divided into two stratigraphic units, i.e., alternating deposits of lacustrine clasts and chemical salts in the upper part (depth of 52 0 m) and lacustrine clastic sediments in the lower part (depth of 100 52 m). By extrapolation of the upper three 14 C ages from the core, halite formation began around 80 ka (Figure 7). Thus, the lower unit of clastic sediments probably formed beyond 80 ka, which likely indicates that the timing of freshwater conditions and high lake levels in the Qaidam Basin was much older than previously proposed MIS 3a. In the Tengger Desert, the two OSL ages of CG quartz from section BJ-S1 suggested that the highest lake level in the Zhuyeze formed ca. 90 80 ka, instead of 40 30 ka BP as derived from 14 C dating by Pachur et al. (1995) and Zhang et al. (2004). For samples with D e beyond ~200 Gy, however, quartz 5 Conclusions This paper compared 14 C and OSL dating results of late Quaternary lacustrine sediments from Tengger Desert. Although OSL dating and 14 C dating agree for the Holocene samples, a significant offset exists between these two kinds of dating techniques beyond ca. 30 ka BP, which likely results from the underestimation of radiocarbon dating. A set of radiocarbon dates from a sequence in the Qaidam Basin also led to the assumption on age underestimation of radiocarbon dating for the sediments older than 30 ka BP. This study confirms that high lake levels are likely to be older than 80 ka based on OSL dates. These findings have great significance for the timing of the high lake levels on the TP and adjacent areas, which have traditionally been assigned to MIS 3a based on 14 C dating. Thus, we urge that most of the published radiocarbon ages older than 30 ka BP should be treated with caution. We thank M. Fuchs for gamma spectrometry analysis. We are grateful to R.M. Briant, D.B. Madsen and S. Mischke for helpful comments on the earlier version. Two anonymous reviewers are also thanked for constructive

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