SCIENCE CHINA Underestimated 14C-Based Chronology of Late Pleistocene High Lake-Level Events Over the Tibetan Plateau and Adjace

SCIENCE CHINA Earth Sciences

• RESEARCH PAPER • doi: 10.1007/s11430-014-4993-2

Underestimated 14C-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 Hao1,2* & SHEN Ji1†

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

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 14C 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 14C 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 14C dating, are likely older than ~80 ka based on OSL chronology.

Tibetan Plateau, lake highstand, lacustrine sediments, 14C dating, OSL dating

Citation: Long H, Shen J. 2014. Underestimated 14C-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, doi: 10.1007/s11430-014-4993-2

Since the 1980s, the late Quaternary evolution of closed mains, nearly all studies suggested that the high lake level lake basins from the Tibetan Plateau (TP) and adjacent are- stands occurred at 40–25 ka, corresponding to the late ma- as has been extensively studied to reconstruct past environ- rine isotope stage 3 (i.e., MIS 3a; Martinson et al., 1987). mental and climatic conditions (e.g., An et al., 2000; These study sites (circled ones in Figure 1(a)) are distributed Lehmkuhl and Haselein, 2000; Shi et al., 2001; Yang et al., over the TP, as well as the foreland areas in the deserts (e.g., 2004; Herzschuh, 2006; Chen et al., 2008; Mischke et al., the Tengger Desert, Badain Jaran Desert, and Taklamakan 2008; Daut et al., 2010; Long et al., 2010; Mügler et al., Desert) (Lehmkuhl and Haselein, 2000; Shi et al., 2001; 2010; Yang and Scuderi, 2010; Wischnewski et al., 2011; Yang et al., 2004; Yang et al., 2011). For instance, in the Yang et al., 2011; Shen, 2013). Qaidam Basin from the northeastern TP, Chen and Bowler Based on 14C dating of lake shorelines and lacustrine re- (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 *Corresponding author (email: [email protected]) †Corresponding author (email: [email protected]) mussels, reflecting fresh to slightly saline water conditions.

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 January (2014) Vol.57 No.? 3

Three shell samples from the upper, middle and lower parts together (Figure 2), and showing obvious differences in of a profile from this bar dated back to 28650±670, 35100± ages between the short (i.e., 14C dating) and the long (e.g., 900, and 38600±680 a BP by the conventional 14C method, luminescence dating) chronologies. Resolution of this issue which suggests that this lake had a high water level at ca. is important because a large number of global climate models 39–28 ka BP (Chen et al., 1990). Zhang et al. (2008) further use lake sequences to assess the strength of Asian monsoons dated the same shell bar using the accelerator mass spec- and hemispheric westerlies. It appears that such a resolution trometry (AMS) method, and obtained similar age ranges. will involve a reconciliation of the dating problem; as a re- The 14C-dated high lake levels during the late MIS 3 seemed sult, direct comparison of radiocarbon and luminescence to occur not only in the Qaidam Basin but also in the western age estimates for the same sediments is necessary. Here we and central part of the TP (Figure 1(a)), e.g., Tianshuihai present age estimates on the basis of 14C or OSL method for Lake (Li et al., 1991), Longmuco Lake (Li, 2000), Ban- two sets of late Quaternary lacustrine sequences from the gongco Lake (Zheng et al., 1989; Li et al., 1991), Zabuye Qaidam Basin and the Tengger Desert, respectively, and try Lake (Zheng et al., 1996), and Selinco Lake (Li, 2000). to revisit the geochronology of highstands which were as- Similarly, there is good evidence of the MIS 3a high- signed to be developed during MIS 3. stands from the adjacent areas of the TP (Figure 1(a)). Tak- ing the Tengger Desert (Figure 1(c)) for example, while there are still many lakes in the inter-dune basins in this 1 Study area and materials region, remains of lacustrine sediments and palaeoshorelines indicate the more extensive occurrence of lakes and The Qaidam Basin (36.6°–37.2°N, 93.7°–96.3°E), situated swamps in the past. Pachur et al. (1995) and Zhang et al. in the northeastern TP (Figure 1(a)), is bounded by the (2004) investigated in detail the palaeobeaches around the Kunlun Mountains to the south and the Aerjin Mountains Zhuyeze Lake in the Tengger Desert using radiocarbon da- and Qilian Mountain to the north (Figure 1(b)). This basin is ting of bulk organic matter or mollusk shells. Their results a large playa with an area of 5850 km2 and a mean elevation showed that the highest water level formed at ~35–30 ka BP. of 2800 m a.s.l., and contains a series of concentrated salt The 14C chronologies of lacustrine beaches also suggested lakes with a total area of 460 km2, and with the Qarhan Salt high lake levels during the MIS 3a in the Juyan Lake (Fig- Lake in the depocenter of the basin (Figure 1(b)). The av- ure 1(a)) on the northern margin of the Badain Jaran desert erage annual precipitation in this region is 25–50 mm, the (Wünnemann et al., 1998). Radiocarbon dates for lacustrine annual mean temperature is 2–4°C and the annual evapora- remains from the Manas Lake (Rhodes et al., 1996), Barkol tion exceeds 3000 mm. By using a rotational drilling system Lake (Yu et al., 2001) and Aiding Lake (Li et al., 1989) with 3-m-long metal tubes with 90-mm diameters, a 100-m- showed the MIS 3a highstand as well (Figure 1(a)). long sediment core (ISL1A Core, 37°03′50″N, 94°43′41″E) However, a set of recent studies on lake shorelines from was obtained from the central part of Qarhan Salt Lake the northeastern margin of the TP found that the highstands (Figure 1(b)). The stratigraphy of ISL1A Core shows evap- apparently dated back to MIS 3a by 14C dating actually date orate halite layers (mainly crystal salt) with some lacustrine back to the period beyond ~70 ka by optically stimulated clastic layers (i.e., silt-clay or clayey silt sediment) from luminescence (OSL) dating method (Madsen et al., 2008, ~52 m in depth to top, and that lacustrine clastic clay to silt 2014; Liu et al., 2010; Rhode et al., 2010; Long et al., was deposited from the base to ~52-m (Figure 3). Consid- 2012). The timing of late Pleistocene lake highstands from ering the dating limitation of the 14C technique, we collected the TP and its adjacent areas remains undetermined. For radiocarbon samples from the upper part (0–55 m) of this instance, OSL chronology of early shorelines around the core only. Because the sediments from the core ISL1A con- Qinghai Lake (Figure 1(a)) showed that the maximum high- tain little organic carbon and are devoid of plant macrofos- stands ~20–66 m above present-day lake levels occurred sils we sampled bulk organic matter (11 samples) for 14C approximately during 100–90 ka (Madsen et al., 2008), not dating (Figure 3 for the sampling locations). in association with MIS 3a as found in the Qaidam Basin. In In the Tengger Desert, there are numerous lakes in the the Lop Nor Lake (Figure 1(a)) the lake highstand dated inter-dune basins. The Zhuyeze Lake is one of those lakes, back to 130–85 ka or even older (Wang et al., 2008) by and is located at the terminal of the Shiyang River in the OSL method. Our recent dating study found that the high- northern piedmont of the eastern Qilian Mountain (Figure stand around the Zhuyeze Lake from the Tengger Desert 1(c)). It is now a salt marsh at an elevation of ~1281 m and dated back to ca. 100–70 ka based on OSL dating (Long et has a surface area of ~42 km2, with brackish water occur- al., 2012), instead of 35–30 ka as previously derived from ring 1 m below the surface. Annual mean temperature in the 14C dating (Pachur et al., 1995; Zhang et al., 2004). In addi- region is 7°C, annual precipitation is 48 mm and annual tion, by using U/Th series dating techniques, the high lake evaporation is 2600 mm. Two lake sedimentary sequences level event in Nam Co (Figure 1(a)) was estimated at 130– (sections BJ-S1 and BJ-S2, see Figure 1(c) for their loca- 75 ka (Zhu et al., 2004). tions) were selected for the study of comparing OSL and The dates constraining the highstand timing are plotted 14C dating techniques. Section BJ-S1 is from the highest 4 Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.?

Figure 2 Timing of lake highstands in different lakes from the TP and adjacent areas. Filled circles with error bars denote 14C ages, and filled squares with error bars denote OSL or U/Th ages. All 14C 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., plastic bags and then stored in the refrigerator until sending 2012) obtained the ages of the two sections by OSL dating them out for analysis. In the radiocarbon dating laboratory, of medium grained (MG, 38–63 m) quartz (Figure 4). In fossil shells were cleaned with 30% H2O2 in an ultrasonic the present study, coarse grained (CG, 90–150 m) quartz, bath to remove the organic surface coating and adhering extracted from three representative samples (BJ-S1-5, BJ- dust as well as detrital carbonate. S1-6 and BJ-S2-4) from the two sections, was used with the Eleven bulk organic samples from the ISL1A core were 14 small aliquot technique for OSL dating to validate the pre- collected for C dating. Through a conventional treatment, vious MG quartz age estimates. Two shell samples were the bulk sediments were treated with HCl (2N), NaOH (2%) collected from sections BJ-S1 and BJ-S2 (Figure 4 for sam- and HCl (2N), and the humic acid fraction was obtained for 14 pling locations) for C dating and then comparison with combustion. The combustion to CO2 of the organic fractions OSL ages. 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 2 Methods University. The 14C ages were calibrated to calendar year (a 14 BP) using the CALIB 6.1.0 program (http://calib.qub.ac.uk/ 2.1 C dating calib/) with the IntCal09 dataset (Reimer et al., 2009), For radiocarbon dating of mollusk fossils from lake deposits, which allows a direct comparison with OSL ages (ka). a possible source of inaccuracy is from sampling reworked material from older deposits. To avoid this, we tried to col- 2.2 OSL dating lect undisturbed fossil shells for 14C dating. Full mollusk fossils from the shell-rich sedimentary layer in sections BJ- For the three samples (BJ-S1-5, BJ-S1-6 and BJ-S2-4) from S1 and BJ-S2 were collected for 14C dating (Figure 4). We the Tengger Desert, the CG fraction was extracted for OSL used clean tweezers to sample shells that were placed in dating for comparison with the previously determined MG Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? 5

ages (Long et al., 2012). These samples were first treated with HCl and H2O2 to remove carbonates and organics, followed by heavy liquid density separation with lithium-heter- opolytungstate to separate the quartz from any heavy minerals (>2.75 g/cm3) and feldspars (<2.62 g/cm3). In the final step, the 2.62–2.75 g/cm3 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 meas- uring 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 alumin- ium 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 90Sr/90Y 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 (De) using the single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000). Long et al. (2012) carried out preheat plateau tests and Figure 3 Stratigraphy and 14C-based chronology of core ISL1A. Filled dose-recovery tests at different preheat temperatures, and 14 triangles denote the locations of C samples. chose preheat of 260°C and cut-heat of 220°C for the De

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). 6 Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? measurement of MG quartz. These preheat conditions were component (Bailey et al., 1997; Jain et al., 2003). A repre- also used for the OSL dating of the CG quartz from the two sentative growth curve is shown in the inset of Figure 5(a); profiles BJ-S1 and BJ-S2 because both MG and CG frac- this is well represented by exponential plus linear fitting tions likely have the same sources (Long et al., 2007) and (black solid line) with six regeneration dose points, includ- then similar luminescence characteristics. ing a zero-dose for the measurement of recuperation and a The concentrations of uranium (U), thorium (Th) and recycling point for assessing the sensitivity change correc- potassium (K) were measured by neutron activation analysis tion. Figure 5(b) summarizes the recycling ratios, where the (NAA) for dose rate calculation. For the two samples sensitivity-corrected luminescence intensity observed from (BJ-S1-5 and BJ-S1-6) from profile BJ-S1, the radionuclide the first regenerative dose is divided by the corrected ob- concentrations of the surrounding sediments were also de- served one when the same dose is repeated at the end of the termined by high-resolution gamma spectrometry (Murray SAR measurement sequence (Murray and Wintle, 2000). et al., 1987). The elemental concentrations were converted The measurements following laboratory irradiations are into annual dose rates (Aitken, 1998). The cosmic ray dose reproducible; all ratios are in the range of 0.9–1.1 and the rate was estimated for each sample as a function of depth, mean is 1.016±0.005. The inset in Figure 5(b) shows the altitude, and geomagnetic latitude (Prescott and Hutton, recuperation values, that is, the response to a 0 Gy labora- 1994). The two sequences (BJ-S1 and BJ-S2) were from the tory dose, measured after the SAR cycle containing the palaeoshorelines or terraces that formed during lake shrink- largest regenerative dose (Murray and Wintle, 2000). These ing, and the water content of shoreline sediments changed signals are expressed as a percentage of the sensitivity-cor- after the lake level retreated. Considering the variability of rected natural luminescence; all recuperation values lie be- the water content of shoreline sediments, we assumed water low 5%. These summary statistics suggest the applicability content of 5±2.5% for the dose rate calculation. of the SAR protocol to these samples. Figure 6 presents the Des distributions for the three samples. Dose rate data determined by NAA and gamma spec- 3 Results trometry techniques are shown in Tables 2 and 3. The over- 3.1 Radiocarbon ages dispersion of De distribution is calculated (Table 4), which suggests that these samples are normally distributed or only All 13 AMS 14C ages from core ISL1A and profiles BJ-S1 slightly skewed. Thus, we use the central age model (CAM) and BJ-S2 are listed in Table 1, along with the dating mate- of Galbraith et al. (1999) for age calculation (Table 4). The rials. In Figures 3 and 4, these radiocarbon dates (calibrated OSL ages of the two sections together with their stratigra- ages) are shown together with the corresponding stratum. phy are shown in Figure 4.

3.2 Luminescence characteristics and ages 4 Discussion Figure 5(a) shows the natural OSL decay curve for sample 4.1 Reliability of 14C and OSL dating in late Pleisto- BJ-S1-5; the OSL signal decreases very quickly during the cene sediments first second of stimulation, suggesting that the decay curve is typical for quartz, and appears to be dominated by the fast 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 14C age (14C 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 January (2014) Vol.57 No.? 7

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 satura- tion 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 14C 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 in- crease 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 Figure 6 Radial plots showing the distribution of the De values of sample massive sediment beds or re-deposition may alternatively BJ-S1-5 (a), BJ-S1-6 (b), and BJ-S2-4 (c). The resultant De value of the explain the current 14C age pattern of core ISL1A, this could 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 8 Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.?

Table 3 Radionuclide concentration of sediments determined by gamma spectrometry method

Th from 208Tl, Sample U from 234Th (ppm) U from 214Pb, 214Bi (ppm) U from 210Pb (ppm) K (%) Dose rate (Gy/ka) 212Pb, 228Ac (ppm) 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

a) b) Sample Minimum De (Gy) CAM De (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 De values are derived from the lowest measured aliquot and the minimum age is calculated based on the minimum De. b) The CAM De values and overdispersion are derived from all accepted aliquots (Galbraith et al., 1999).

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 low- land 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 14C 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 14C and OSL ages (Bird et al., 1999). 14C 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 Figure 7 14C ages against depth for core ISL1A. The dashed line is the may have arrived at 54–60 ka (Roberts et al. 1994). Simi- 14 fitting and extrapolation of three C ages from the upper 25 m. Grey band larly, the discrepancy between luminescence and 14C ages denotes the onset of halite formation. was also noted by Zhang et al. (2006) based on OSL and AMS 14C dating for a core from the Lake Juyan. Differential not cause such significant scattering radiocarbon data. More contamination may explain the radiocarbon dates from the reasonable explanation is that 14C dating underestimates the Juyan core, as they seem to be all over the place regardless ages of sediments beyond ca. 30 ka BP. of depth in the core and despite being run by two separate As shown in Figure 4, the OSL ages of section BJ-S1 fall labs. The luminescence ages were also run by two separate into the range of 90–80 ka, but the 14C dating of the shell laboratories, but are in order and consistently older with sample BJ-S1-C1 yielded an age of 36982±181 cal a BP, depth, suggesting that they may be the more valid. which is much younger than the OSL age (86.5±6.6 ka, In contrast, another directly comparable paired OSL and sample BJ-S1-8) of the same stratum. Although only single radiocarbon age determination (6.5±0.4 ka and 6562±97 cal 14C age from BJ-S1 was obtained in the current study, a set a BP for samples BJ-S2-1 and BJ-S2-C1, respectively) from of 14C dates of shells from the lacustrine strata at the same Section BJ-S2 (Figures 4 and 8(a)) suggests consistency in elevation around Zhuyeze Lake also fell in the range of ca. the two methods for the Holocene strata. In addition, in sec- 30–40 ka BP (Pachur et al., 1995; Zhang et al., 2004), tion Qingtuhu (QTL) (Figure 1(c) for its location), Long et which agrees with our 14C date (i.e., 36982±181 cal a BP al. (2011) found a good agreement between OSL and 14C from sample BJ-S1-C1) but significantly contrasts with our dating back to ca. 13 ka BP (Figure 8(b)), indicating not OSL results (i.e., 90–80 ka). only the negligible hard water reservoir effect of 14C sam- The similar dating offsets have been reported from many ples but also the consistency between OSL and 14C ages, at studies which have compared luminescence and radiocarbon least for the Holocene lacustrine sediments in the study area. Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? 9

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 ro- bustness 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 De 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. De 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 com- parative purposes the lowest measured De value for each sample (based on a single aliquot) from section BJ-S1 was used to calculate age (Table 4). Given that this minimum De 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 De should be comparable to radiocarbon chronology. However, this difference between 14 minimum De age and C date is still present (Table 4). Thus, an overestimation of De due to partial bleaching can be ex- cluded 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

14 changes in dose rate (Olley et al., 1996; Li et al., 2008). To Figure 8 Ages comparison. (a) Comparison between OSL and C ages check for disequilibrium, the U content of the surrounding (with errors) for two strata from profiles BJ-S1 and BJ-S2. (b) Comparison 234 214 214 210 of OSL and 14C ages (with errors) from section QTL. Data from Long et al. sediments derived from Th, Pb, and Bi, and Pb (2011). The dashed lines in (a) and (b) show a 1:1 relationship between the were compared (Table 3). No significant discrepancy of two age techniques. (c) Impact of modern contamination (0.25%–2% by these values was observed, indicating equilibrium for the U weight) on measured 14C ages (thin lines) compared with the 1:1 or un- decay chain. In addition, although water content variations contaminated line (thickest line). After Pigati et al. (2007). throughout the time of burial may have a significant impact on dose rates and then on age estimations, water content is Therefore, the comparison of OSL and 14C dating of the seldom greater than 5%, and therefore within the error range late Quaternary lacustrine sediments from the Tengger De- specified for water contents. sert indicates that the two dating techniques agree well for Furthermore, the OSL ages of CG fraction broadly con- the Holocene samples (younger than ca. 13 ka BP) but disa- firm that derived from MG fraction for samples BJ-S1-5, gree for older samples (e.g., beyond ca. 30 ka BP). The sig- BJ-S1-6, and BJ-S2-4 (Figure 4). This estimation allows us nificant discrepancy between the two techniques beyond 30 to have greater confidence on the OSL ages and, therefore, ka BP cannot be attributed to sampling different aged mate- that the age offset seen between them and the radiocarbon rials (e.g., resulting from deposition reworking), because the ages could be attributed to underestimation of radiocarbon 10 Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? ages older than 30 ka BP. The Holocene 14C date appears to OSL age estimates are thought to be likely underestimated, be generally reliable. even though the growth curve is still not saturated (Buylaert et al., 2007; Chapot et al., 2012). A single saturating expo- 4.2 Possible cause for 14C age underestimation beyond nential function was also used to build up the dose-response 30 ka BP curves for the current study (e.g., red solid line in the inset of Figure 5(a)). We found that the obtained Des for both As discussed above, chronological comparison studies from samples, although close to the one calculated by the expo- the Tengger Desert seem to indicate that, while OSL and nential plus linear fitting, are generally near or beyond the 14 C dating agree well for the Holocene samples, there may values of 2D0. This indicates that the apparent ages (ca. be a significant offset between the two dating techniques 90–80 ka) are very likely underestimated. On the basis of beyond ca. 30 ka BP, which largely results from radiocar- OSL dates, therefore, we propose that the high lake period bon dating underestimation. A possible reason is that con- from the Tengger Desert is estimated to be older than ~80 tamination with modern carbon may occur after deposition, ka, which is similar to the Qaidam Basin. Additionally, we or during sampling and preparation for dating. In reality, highlight that more OSL dating methods on feldspar minerals younger carbon produced during soil formation may perco- (e.g., Chen et al., 2013; Li et al., 2014; Long et al., 2014a, late through the sequence and coat the sample material. A 2014b) should be applied in the future work to extend the study by Pigati et al. (2007) showed that older radiocarbon dating limit. dating samples have an increased susceptibility to modern This finding has significant implications for timing of carbon contamination, which is shown in Figure 8(c). This high lake-level events recognized in the TP and adjacent is because levels of radioactive carbon are much lower in areas. The lake highstands in these areas were traditionally these samples. As Figure 8(c) shows, for example, a 2% assigned to MIS 3a based on numerous radiocarbon dates in contamination with modern carbon of a 15-ka-old sample the range of ca. 40–25 ka BP. It would appear that most of could lead to only a minor age underestimation. However, these radiocarbon ages need to be more critically evaluated. the same contamination for a 70-ka-old sample could lead A combination of proximity to the radiocarbon limit, which to age underestimations in excess of 30 ka, which could be results in the indistinguishable 14C activity of sample from the reason why there is a significant age difference between the background, and/or contamination with very small OSL and radiocarbon dating for section BJ-S1 and a small amounts of modern carbon may explain why many sites age offset for section BJ-S2. Thus, to obtain reliable 14C return similar ages (as per curvature on Figure 8(c)). This ages, we suggest that extreme care should be taken, espe- study thus shows a likely underestimation in the 14C-based cially with older samples, to avoid contamination and ex- chronology of late Pleistocene high lake-level events on the clude reworked material. TP.

4.3 Lake highstands over the TP and adjacent areas 5 Conclusions In general, in the arid areas such as the Qaidam Basin, lacustrine clasts are deposited in freshwater conditions, indi- This paper compared 14C and OSL dating results of late cating rising lake levels, and halite precipitation occurs un- Quaternary lacustrine sediments from Tengger Desert. Alt- der saline conditions, indicating falling lake level (Chen et hough OSL dating and 14C dating agree for the Holocene al., 1986; Zheng et al., 1989; Shi et al., 2001). Lithological samples, a significant offset exists between these two kinds observations showed that core ISL1A can be roughly di- of dating techniques beyond ca. 30 ka BP, which likely re- vided into two stratigraphic units, i.e., alternating deposits sults from the underestimation of radiocarbon dating. A set of lacustrine clasts and chemical salts in the upper part of radiocarbon dates from a sequence in the Qaidam Basin (depth of 52–0 m) and lacustrine clastic sediments in the also led to the assumption on age underestimation of radio- lower part (depth of 100–52 m). By extrapolation of the carbon dating for the sediments older than 30 ka BP. This upper three 14C ages from the core, halite formation began study confirms that high lake levels are likely to be older around 80 ka (Figure 7). Thus, the lower unit of clastic than 80 ka based on OSL dates. These findings have great sediments probably formed beyond 80 ka, which likely in- significance for the timing of the high lake levels on the TP dicates that the timing of freshwater conditions and high and adjacent areas, which have traditionally been assigned lake levels in the Qaidam Basin was much older than pre- to MIS 3a based on 14C dating. Thus, we urge that most of viously proposed MIS 3a. In the Tengger Desert, the two the published radiocarbon ages older than 30 ka BP should OSL ages of CG quartz from section BJ-S1 suggested that be treated with caution. the highest lake level in the Zhuyeze formed ca. 90–80 ka, 14 instead of 40–30 ka BP as derived from C dating by Pa- We thank M. Fuchs for gamma spectrometry analysis. We are grateful to chur et al. (1995) and Zhang et al. (2004). R.M. Briant, D.B. Madsen and S. Mischke for helpful comments on the For samples with De beyond ~200 Gy, however, quartz earlier version. Two anonymous reviewers are also thanked for constructive Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? 11 comments, which led to significant improvement of this manuscript. This ecol, 393, 111–121 work was supported by the National Natural Science Foundation of China Li S H, Sun J M, Zhao H. 2002. Optical dating of dune sands in the north- (Grant Nos. 41271002, 41430530), the State Key Laboratory of Loess eastern deserts of China. 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