Badain Jaran Desert

The Badain Jaran desert (BJD) is located on the northwestern Alashan Plateau of Inner Mongolia, north China (Fig. 1), covering about 5×10⁴ km² (Zhu et al. 2010). The desert is surrounded by the Beida Mountains to the southeast, the Yabulai to the east, the alluvial fan formed by the Heihe River to the northwest, and Guaizihu Wetland to the north (Dong et al., 2004). There is not any surface runoff in the desert. About 100 lakes of various sizes (from hundreds square meters to ~1 km²) and salinities (0.01–345.0 g/L) are distributed across the lowland of megadunes in the southeastern part of the desert (Zhu et al., 1980; Dong et al., 2013). Climatically the area is an extremely arid continental type and located in the marginal area of East Asian Summer Monsoon (Gao et al., 2006). In winter, the desert frequently undergoes the affected by cold and dry continental air masses originating from mid-Siberia and Mongolia, while in the summertime the desert under influence of the East Asian monsoon which responsible for the most rainfall in desert. The annual mean temperature of BJD is 7.7°C in its southeastern edge, the mean annual precipitation decrease from 118 mm in the southeastern to 37 mm in the northwestern and most of precipitation concentrated in July and August (Yang et al., 2003). Contrast the low precipitation, the potential evaporation in BDJ is more than 1000 mm on the lake surface (Yang et al., 2010). The northwest wind and west wind are prevailed with an annual velocity from 2.8 m/s to 4.6 m/s and the strongest wind occurred in April and May (Dong et al., 2013).

Fig. 1

Study area and the location of the section ZZGD in BJD. The section ZZGD is in the lowland of the megadunes and beside the modern lake Zongzhegedan. The section is higher than the modern lake surface about 7 m.

The Zongzegedan (ZZGD) section and samples collection

The Zongzegedan (ZZGD) section (N 39°51’57”; E 102°17’55”) is located about 1 km to one modern Lake Zongzegedan (Fig. 1), which is also in the lower land of megadune in the southeast of BJD. The section is composed by 3.9 m thickness lacustrine/aeolian sand deposits sequences, the top of the section is higher than the modern lake level about 7 m. Three layers were roughly divided by field observation from top to bottom as follows and Fig. 2:

Fig. 2

The ZZGD section and the typical deposits in the section. The red dots indicated the optical dating sampling points.

Seven OSL dating samples were collected from the ZZGD section at the depth of 0.45 m, 1.20 m, 1.65 m, 1.95 m, 2.90 m, 3.00 m and 3.65 m respectively (Fig. 2, red dots). All samples were collected by hammering 4 cm diameter stainless steel tubes into cleaned vertical profiles. Then pull out the tube and quickly covered with lid and sealed inside black plastic bags with tape to ensure the sample kept out of light and retained its original water content.

Raw samples were also collected at about 3 cm interval throughout the section for grain size analyze.

Optical dating

The quartz and K-feldspar grains were separated from raw samples in the laboratory for optical dating. All processes were carried out under subdued red light condition, the 3–4 cm material from each end of tube scraped away considering it may have been exposed to light and used for water content and radioactive element measurements. Sample preparation followed the methods described by Aitken (1998). Raw samples were first treated with 10% HCl and 20% H₂O₂ to remove carbonate and organic matters. The specific grain sizes of 90–125 μm were extracted by sieving the sample in water, and then the series densities of 2.62, 2.73 and 2.58 g/cm³ heavy liquids were used to separate the quartz and K-feldspar fractions. After drying, the quartz grains were etched with 40% HF for 40 min to remove the outer layer irradiated by alpha particles and any remaining feldspar grains. The K-feldspar grains were etched with 10% HF for 20 min to remove the outer layer. Finally all samples were treated with 1 mol/L HCl for 10 min to remove fluorides created during the HF etching.

The quartz and feldspar grains were mounted on the center of 9.7 mm diameter aluminum discs using silicone oil for equivalent dose (D_e) determination. All the luminescence signals measurements were carried out in the Luminescence laboratory of the Cold and Arid Environment and Engineering Research Institute, CAS, using an automated Risø TL/OSL-DA-15 reader (Markey et al., 1997). The quartz OSL signal was detected through two 3 mm thick Hoya U-340 filters and the IRSL signal was detected through one BG-39 and one Coring-759 filters using a photomultiplier tube. Laboratory irradiation was carried out using ⁹⁰Sr/⁹⁰Y sources built into the reader, with dose rates of 0.080 Gy/s.

To confirm the equivalent dose (D_e) in the age calculation, both of the OSL dating for quartz and IRSL dating for K-feldspar grains were applied. The single-aliquot regenerative-dose (SAR) protocols (Murray and Wintle, 2000; Wintle and Murray, 2006) has been applied to quartz grains to determine their D_e values. The preheat temperature in the SAR measurements process was determined to one typical sample (ZZGD-365) by using the test of preheat plateau. Preheat temperature from 200°C to 280°C with an interval of 20°C were applied, the cut-heat was kept at 220°C, both preheat and cut-heat using the heating rate of 5°C/s. For each temperature point, three aliquots were measured and its average value was calculated. The results showed that a plateau could be observed for temperature from 220°C to 260°C (Fig. S1). At the temperature of 240°C, the sample equivalent dose showed relatively lower inter-aliquot scatter and smallest recuperation of about 0.6%, thus we selected a preheat temperature of 240°C for routine D_e determination.

The validity of modified SAR procedure with the pre-heat of 240°C for D_e determination was further checked with a dose recovery test (Murray and Wintle, 2003). This test was applied to 6 aliquots of sample ZZGD-290 that had been bleached under the sunlight for 6 days. A laboratory dose of 160 Gy was given to each of the bleached aliquot as an unknown dose. Same SAR procedure was applied to the 6 aliquots and the measured D_e was 160.7 ± 0.70 Gy. The ratio of the given dose to the measured dose was 0.96 ± 0.04, indicated that the modified SAR procedures is able to recover a given laboratory dose, the modified SAR protocol used in this study is suitable for D_e determination.

The two-step post-IR IRSL (pIRIR_50,290) (Thomsen et al., 2008) and the new multiple elevated temperature (MET) post-IRIR methods (Li and Li, 2011; 2012) were employed to K-feldspar grains, which could overcome the age underestimation from IRSL signals anomalous fading. Preheating of 320°C for 60 s was used in the origin of MET-pIRIR in this study. All protocols used in this study were summarized in the Table 1. Sensitivity changes corrected by the fixed test doses signals, the ratio of regeneration dose signals to test dose signals was used to construct sensitivity-corrected dose response curve and calculate the D_e value for all IRSL signals. For checking the bleaching situation of the IRSL signals from K-feldspar grains, at least 24 small aliquots (~1 mm diameter samples on one aliquot) of every samples was used in the measurements of the MET-pIRIR procedure. The D_e values could be drawn as function with the IR stimulation temperature. The D_e values increased with stimulation temperature, and a ‘plateau’ (shown as a dashed line in each plot of Fig. S2) is reached at higher temperatures (>200°C) for most of the samples, further confirming that a non-fading component is present at the elevated temperatures (Li and Li, 2011; 2012). We, therefore, calculated the final D_e values obtained from the signals measured at 250°C for all the K-feldspar samples.

Concentration of radioactive element U, Th, K that used for calculate the environment dose rate was determined by means of Neutron Activation Analysis (NAA). All results were converted to alpha, beta and gamma dose rates according to the conversion factor of Aitken (1998). The cosmic ray dose rate was calculated by the transition function of the sample burial depth and altitude (Prescott and Hutton, 1994). 12.5 ± 0.5% K content was adopted to calculate the internal dose rate of K-feldspar grain (Huntley and Baril, 1997; Zhao and Li, 2005). The observed water content in laboratory was 0.5–1.5% for all the samples, and the saturated moisture content of the sediment is about 30%, considering the lacustrine sediment water content may be changed strongly from saturate (in the lake period) to dry (after the lake disappeared) the water content of 10 ± 10% with large uncertainty was adopted to calculate dose rate for all samples in this study.

Grain size analysis

The grain size analysis was conducted in the Key Laboratory of Western China’s Environment Systems (Ministry of Education), Lanzhou University. The detailed sample pretreatment process was followed the method of Lu and An (1998). 0.5 g sample was firstly weighted and poured it into a 100 ml glass beaker. Then 10 ml of 30% H₂O₂ and 10 ml of 10% HCl were successively added to remove organic and carbonate at 105°C. The beaker was then filled with distilled water and left to deposit for 24 h. 10 ml of 0.05 g/L (NaPO₃)₆ was added to the sample after siphoning off the redundant water and dispersed for about 10 min in an ultrasonic vibrator. The Malvern Mastersizer 2000 laser grain-size analyzer with a measurement range of 0.02–2000 μm was used in this study. The prepared sample was transferred to the laser grain-size analyzer, producing measurements as a volume percentage.

OSL and IRSL dating

All the dosimetry and dating results are show in Table 2. It could be found that the OSL dating using SAR procedure from quartz show that the lacustrine sediments in the ZZGD section were deposit during about 34–65 ka. However, the IRSL dating results from K-feldspar using pIR₅₀IR₂₉₀ and MET pIRIR procedures show that the deposit age of lacustrine layers are during about 60–84 ka. This contradiction of dating results needs to check carefully.

The poor bleaching of OSL or IRSL signals may lead age overestimation of deposit events. Normally, the bleaching of the IRSL signals from K-feldspar grains should be slower than the OSL signals from quartz grains when the two minerals from same sediments (Atiken, 1998). The D_e values of 250°C for 6 samples from lacustrine layer, which may experience poor bleaching, are displayed as radial plots in Fig. 3 respectively. All of the samples, with 24–48 aliquots, have D_e values distributed around a central value (Fig. 3). This indicated that well bleaching of all the samples. We determined the weighted mean D_e of the individual single-aliquot values using the Central Age Model (CAM) (Galbrith et al., 1999; Galbrith and Roberts, 2012). The unbleachable high temperature IRSL signals in pIRIR protocol may produce residual dose of D_e values and lead to overestimation of the deposit ages (Sohbati et al., 2012). In this study, four aliquots from sample ZZGD-45 were bleached under sunlight for 28 h and then measured using same protocol of MET-pIRIR in Table 1. The obtained residual dose is 2.31 ± 0.12 Gy, which is relatively small compare with the sample D_e values of >200 Gy. Nevertheless, it is still subtracted in K-feldspar age calculation.

Fig. 3

Radial plot of the D_e values of 6 lacustrine sediments in MET-pIRIR protocol.

Thermal unstable OSL signal of fast components from quartz grains may lead to significant age underestimation (Steffen et al., 2009; Fan et al., 2011). Pulse-annealing experiments were carried out to the bleached (28 h under sunlight) three quartz aliquots from each of several samples: studied sample (ZZGD-365), sample ZZGD-365 but preheated 60 s at 500°C, the sample (DSGW275) that with thermal stable OSL signals (Zhao et al., 2016), and the standard quartz sample from Risø National Laboratory, Denmark, respectively. The detailed pulse-annealing experiment procedures are similar but modified from the SAR OSL protocol in Table 1. Each aliquot was given a regenerative dose of 32 Gy. The aliquot was then subject to a cut-heat temperature of T °C. And then the OSL signal was read 80 s at 125°C. Correction for sensitivity changes during the measurements was made using OSL created by a test dose of 8 Gy with a following cut-heat to 200°C. At the end of each run, the aliquot was bleached at 280°C to remove all potential signals. The measurement cycle was repeated with the cut-heat temperature T being increased from 200 to 400°C, in increments of 20°C. The OSL signal (in first channel ~0.5 sec) remaining after heating to each temperature was normalized to the initial value (at T=200°C), the average normalized remaining OSL signals from 3 aliquots of every samples were shown in Fig. 4. The normalized remaining OSL signals from ZZGD-365 showed an early decrease from relatively low temperature (240°C), but the aliquot endured a preheat 500°C showed much slower decrease after 320°C. The standard sample and sample DSGW 270 also showed obvious decrease after 320°C. This experiment indicated that the fast components natural OSL signals from quartz in BJD have thermal unstable and which may lead to significant age underestimation.

Fig. 4

Pulse-annealing curves for quartz aliquots from four samples. The remnant OSL signals are versus the annealing temperature.

The well bleached pIRIR signals from K-feldspar were used to establish the age framework of the ZZGD section.

MIS5 lacustrine deposits and implications

The dating results and the grain size analysis are shown in Fig. 5. It could be noticed that the pIR₅₀IR₂₉₀ and MET-pIRIR protocol yield consistence ages for most of the lacustrine deposits, except the bottom two samples older than 80 ka (Li et al., 2014). In this study, we used the MET-pIRIR results for discussion.

Fig. 5

The optical dating results of studied section ZZGD and the grain size analysis results. Over 80% of the sediments grains in the section are coarse grain larger than 63 μm as the bottom aeolian sand layer.

All the lacustrine layers were dated during about 85–60 ka, that means a lake existed in late MIS 5 till late MIS 4 (Martinson et al., 1987) and the bottom of the lake was higher than modern lake level about 7 m. The snail shells were appeared at about 65 ka and 60 ka indicated the shallow lake environments appeared at studied point during MIS4. The grain size analysis shows that the sediments from this section are uniform in both lacustrine and bottom aeolian sand layers. More than 80% sediments are coarse grains in most part of the section. This indicates that the sediments in the paleo-lake are mainly supplied by the surrounded aeolian dunes.

The IRSL dating results of the paleo-lake lacustrine relics beside the modern lake ZZGD in BJD indicate paleo-lake existing and also humid environment during 85–60 ka. This is similarity to the optical dating results for lacustrine deposit sequences indicated the lake existed during about 100–70 ka in the Tengger Desert (Long et al. 2012), Qinghai Lake (Liu et al., 2010) and Ejina Basin close to BJD (Li et al., 2015). Furthermore, the paleolake at late MIS5 appeared at a same position with the modern lake in BDJ have the geomorphological implications. The drilling core in the west of BDJ (Wang et al., 2015) showed that there was thick aeolian sand layer between the two MIS5 and Holocene lacustrine layers. That suggested the paleolake during MIS5 disappeared firstly, and then the aeolian sand layer indicated a long arid environment lasted in BDJ for about 60 ka. Till to the beginning of the Holocene, the modern lake was appeared at the same location with the paleolake during MIS5. There are two possibilities to explain this phenomenon, one is that the geomorphology in BJD were keep relatively stable that the wind field in the desert always eroded the same location of the desert, and form the lower sand land in the same position that could accumulate water to form lakes in humid periods. Another possibility is there were much larger/deep lakes in BJD in during MIS5 and these lakes distributed wildly in the desert. Section ZZGD is about 10 m higher than present lake level and other MIS5 lacustrine relics are also found beside modern lakes (Bai et al., 2011; Fan et al., 2014). These lakes left over lacustrine relics widely in the desert and some of them occasionally at same position with modern lakes. Further studies are still needed to explore the geomorphology process in BJD that the MIS5 lacustrine sediments could be found widely and beside the modern desert lakes.