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*Corresponding author (email: [email protected]) †Corresponding author (email: [email protected])

• 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, tradition-ally 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 lake basins from the Tibetan Plateau (TP) and adjacent are-as has been extensively studied to reconstruct past environ-mental 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 14C dating of lake shorelines and lacustrine re-

mains, nearly all studies suggested that the high lake level stands occurred at 40–25 ka, corresponding to the late ma-rine 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)), approxi-mately 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.

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2 Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.?

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 de-noted 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 denot-ed 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 of a profile from this bar dated back to 28650±670, 35100± 900, and 38600±680 a BP by the conventional 14C 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 spec-trometry (AMS) method, and obtained similar age ranges. The 14C-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), Ban-gongco 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 high-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 region, remains of lacustrine sediments and palaeoshore-lines 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 da-ting of bulk organic matter or mollusk shells. Their results showed that the highest water level formed at ~35–30 ka BP. The 14C chronologies of lacustrine beaches also suggested high lake levels during the MIS 3a in the Juyan Lake (Fig-ure 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 14C 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 high-stands ~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 high-stand 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 14C dating (Pachur et al., 1995; Zhang et al., 2004). In addi-tion, 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., 14C 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 re-sult, direct comparison of radiocarbon and luminescence age estimates for the same sediments is necessary. Here we present age estimates on the basis of 14C 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 as-signed 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 km2 and a mean elevation of 2800 m a.s.l., and contains a series of concentrated salt lakes with a total area of 460 km2, and with the Qarhan Salt Lake in the depocenter of the basin (Figure 1(b)). The av-erage annual precipitation in this region is 25–50 mm, the annual mean temperature is 2–4°C and the annual evapora-tion exceeds 3000 mm. By using a rotational drilling system with 3-m-long metal tubes with 90-mm diameters, a 100-m- long 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 evap-orate 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). Consid-ering the dating limitation of the 14C technique, we collected radiocarbon samples from the upper part (0–55 m) of this core only. Because the sediments from the core ISL1A con-tain little organic carbon and are devoid of plant macrofos-sils we sampled bulk organic matter (11 samples) for 14C 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 km2, with brackish water occur-ring 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 loca-tions) were selected for the study of comparing OSL and 14C dating techniques. Section BJ-S1 is from the highest


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., 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 pre-vious MG quartz age estimates. Two shell samples were collected from sections BJ-S1 and BJ-S2 (Figure 4 for sam-pling locations) for 14C dating and then comparison with OSL ages.

2 Methods

2.1 14C 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 col-lect undisturbed fossil shells for 14C dating. Full mollusk fossils from the shell-rich sedimentary layer in sections BJ- S1 and BJ-S2 were collected for 14C 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% H2O2 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 14C 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 CO2 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 14C 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


Figure 3 Stratigraphy and 14C-based chronology of core ISL1A. Filled triangles denote the locations of 14C samples.

ages (Long et al., 2012). These samples were first treated with HCl and H2O2 to remove carbonates and organics, fol-lowed by heavy liquid density separation with lithium-heter- opolytungstate to separate the quartz from any heavy min-erals (>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 im-portant 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 se-quence 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 re-generative-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 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).


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 frac-tions 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 de-termined 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 shrink-ing, 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 14C ages from core ISL1A and profiles BJ-S1 and BJ-S2 are listed in Table 1, along with the dating mate-rials. 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 repre-sentative 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, includ-ing a zero-dose for the measurement of recuperation and a recycling point for assessing the sensitivity change correc-tion. Figure 5(b) summarizes the recycling ratios, where the sensitivity-corrected luminescence intensity observed from the first regenerative dose is divided by the corrected ob-served 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 labora-tory 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 be-low 5%. These summary statistics suggest the applicability of the SAR protocol to these samples.

Figure 6 presents the Des distributions for the three sam-ples. Dose rate data determined by NAA and gamma spec-trometry techniques are shown in Tables 2 and 3. The over-dispersion of De 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 stratigra-phy are shown in Figure 4.

4 Discussion

4.1 Reliability of 14C and OSL dating in late Pleisto-cene 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 14C age (14C a BP) Calibrated age (a BP)

Core ISL1A

BA091109 ISL1A09-1 4.65

Bulk organic

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

BA091114 ISL1A09-6 38.35 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 Shells

31605±110 36982±181

Section BJ-S2 BA090371 BJ-S2-C1 0.5 5760±40 6562±97


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 con-sistency, with sequences generally yielding ages in strati-graphic 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 indi-cates 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 14C age pattern of core ISL1A, this could

Figure 6 Radial plots showing the distribution of the De values of sample BJ-S1-5 (a), BJ-S1-6 (b), and BJ-S2-4 (c). The resultant De 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


Table 3 Radionuclide concentration of sediments determined by gamma spectrometry method

Sample U from 234Th (ppm) U from 214Pb, 214Bi (ppm) U from 210Pb (ppm) Th from 208Tl,

212Pb, 228Ac (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 Dea) (Gy) CAM De

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

Figure 7 14C ages against depth for core ISL1A. The dashed line is the fitting and extrapolation of three 14C 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 14C 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 14C 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 14C age from BJ-S1 was obtained in the current study, a set of 14C 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 14C 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 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 be-tween 32–46 14C ka (36–50 ka BP), whereas a much older MIS 5e age (>117 ka) is suggested by both quartz and feld-spar OSL dating. Early dating of human occupation of the Australian continent also suggested the significant differ-ence between 14C and OSL ages (Bird et al., 1999). 14C de-terminations 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). Simi-larly, the discrepancy between luminescence and 14C ages was also noted by Zhang et al. (2006) based on OSL and AMS 14C 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 sec-tion Qingtuhu (QTL) (Figure 1(c) for its location), Long et al. (2011) found a good agreement between OSL and 14C dating back to ca. 13 ka BP (Figure 8(b)), indicating not only the negligible hard water reservoir effect of 14C sam-ples but also the consistency between OSL and 14C ages, at least for the Holocene lacustrine sediments in the study area.


Figure 8 Ages comparison. (a) Comparison between OSL and 14C ages (with errors) for two strata from profiles BJ-S1 and BJ-S2. (b) Comparison of OSL and 14C 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 14C ages (thin lines) compared with the 1:1 or un-contaminated line (thickest line). After Pigati et al. (2007).

Therefore, the comparison of OSL and 14C dating of the late Quaternary lacustrine sediments from the Tengger De-sert indicates that the two dating techniques agree well for the Holocene samples (younger than ca. 13 ka BP) but disa-gree for older samples (e.g., beyond ca. 30 ka BP). The sig-nificant discrepancy between the two techniques beyond 30 ka BP cannot be attributed to sampling different aged mate-rials (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 un-disturbed 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 overes-timation or dose rate underestimation. Partial bleaching has been identified as a potential problem in fluvial or lake en-vironments (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 minimum De age and 14C 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 disequi-librium 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 234Th, 214Pb, and 214Bi, and 210Pb 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 con-firm 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


ages older than 30 ka BP. The Holocene 14C date appears to be generally reliable.

4.2 Possible cause for 14C age underestimation beyond 30 ka BP

As discussed above, chronological comparison studies from the Tengger Desert seem to indicate that, while OSL and 14C 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 radiocar-bon dating underestimation. A possible reason is that con-tamination with modern carbon may occur after deposition, or during sampling and preparation for dating. In reality, younger carbon produced during soil formation may perco-late 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 14C ages, we suggest that extreme care should be taken, espe-cially with older samples, to avoid contamination and ex-clude reworked material.

4.3 Lake highstands over the TP and adjacent areas

In general, in the arid areas such as the Qaidam Basin, la-custrine clasts are deposited in freshwater conditions, indi-cating rising lake levels, and halite precipitation occurs un-der 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 di-vided 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 14C 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 in-dicates that the timing of freshwater conditions and high lake levels in the Qaidam Basin was much older than pre-viously 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 14C dating by Pa-chur et al. (1995) and Zhang et al. (2004).

For samples with De beyond ~200 Gy, however, quartz

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 expo-nential 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 Des for both samples, although close to the one calculated by the expo-nential plus linear fitting, are generally near or beyond the values of 2D0. 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 14C 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 14C-based chronology of late Pleistocene high lake-level events on the TP.

5 Conclusions

This paper compared 14C and OSL dating results of late Quaternary lacustrine sediments from Tengger Desert. Alt-hough OSL dating and 14C dating agree for the Holocene samples, a significant offset exists between these two kinds of dating techniques beyond ca. 30 ka BP, which likely re-sults 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 radio-carbon 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 14C 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


comments, which led to significant improvement of this manuscript. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41271002, 41430530), the State Key Laboratory of Loess and Quaternary Geology (Grant No. SKLLQG1101), the NIGLAS 1-3-5 Project (Grant No. NIGLAS2012135004), the State Key Laboratory of Lake Science and Environment (Grant No. 2012SKL002), and the China Postdoctoral Science Foundation (Grant Nos. 2012M520061, 2013T60567).

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