Chronology of Holocene sediments from the archaeological Salawusu site in the Mu Us Desert in China and its palaeoenvironmental implications

Journal of Asian Earth Sciences 45 (2012) 247–255

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Chronology of Holocene sediments from the archaeological Salawusu site in the Mu Us Desert in China and its palaeoenvironmental implications Kai Liu a,b, ZhongPing Lai a,⇑ a b

Luminescence Dating Group, Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 5 July 2011 Received in revised form 28 October 2011 Accepted 2 November 2011 Available online 26 November 2011 Keywords: Luminescence dating of lacustrine deposits Palaeolithic Salawusu site Mu Us Desert in China Holocene climatic change Incision rate of Salawusu river

a b s t r a c t The archaeological Salawusu site is located at the southeast margin of the Mu Us Desert, and in the northern marginal area of the East Asian Monsoon. Therefore, its environment is sensitive to the changes of the East Asian Monsoon. At the palaeolithic Salawusu site, most of the previous studies are concerned with the age of the palaeoanthropic fossils (ages ranging from 30 to 120 ka) and the climate change in the last glaciation period, while studies on the chronology and climate change since the Late Glacial are very limited. In the current study, eight luminescence samples were collected from Dishaogouwan (DSG) section and dated using quartz optically stimulated luminescence (OSL). Radiocarbon samples were also collected, and the past environmental changes since the Late Glacial have been reconstructed based on stratigraphical and chronological data. The results show that: (1) the AMS age of modern weed living under water is about 1550 ± 35 a, which is the current reservoir effect age, and after reservoir effect subtraction the radiocarbon age of the shells is in agreement with the OSL age, while the radiocarbon age of the bulk sample is younger; (2) aeolian sand mobilization occurred in the studied region before 12 ka; (3) within the Holocene, the wettest climate occurred during the onset of the Holocene when an extensive palaolake existed in the study area, marked by the development of lacustrine sediments at around 12 ka; (4) after 12 ka, the climate showed a trend of increasing aridity, which led to a continuous shrinkage of the palaolake, and its ultimate desiccation between 1.8 and 1.0 ka evidenced by the shift from lacustrine sediments to peat, and finally to palaeosol; (5) the dating results also show an enormous incision of about 60 m in about 1.75 ka by the Salawusu River. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The palaeolithic Salawusu site is situated on the southeastern margin of the Mu Us Desert along the Salawusu River (Fig. 1) in semi-arid northwestern China. It is a key paleolithic site in northeastern Asia (Huang and Hou, 2003), and has been regarded as a standard site for fluviolacustrine stratigraphy in China for the Upper Pleistocene period (Jia, 1950, 1982; Wang, 1964; Kozlovski, 1971; Qi, 1975). More than 80 years ago, a set of Quaternary strata with fossil vertebrates and ancient human remains was found along the Salawusu River by a French group (Teilhard de Chardin and Licent, 1924). Since then, the site has attracted international attentions, and more and more archaeological evidences have been found, such as Salawusu Vertebrate, Ordos Culture and Ordos Fossil Man (Licent et al., 1927; Boul et al., 1928; Teilhard de Chardin and Young, 1930; Teilhard de Chardin, 1941; Huang and Hou, 2003; Shang, 2008; Tong et al., 2008). Intensive investigations have been carried out in the fields of stratigraphy, geochronology, ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Z.P. Lai). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.11.002

paleontology, paleo-anthropology, and paleolithic and neolithic cultures (Jia, 1950; Pei and Li, 1964; Wang, 1964; Qi, 1975; Yuan, 1978; Dong et al., 1982; Li et al., 1987, 2000, 2007; Zheng, 1989; Su and Dong, 1997; Huang and Hou, 2003; Sun et al., 1996; Yin and Huang, 2004; Liu et al., 2010; Fan et al., 2011). Most of the research efforts on the palaeoanthropology and palaeoclimatic changes in the Salawusu site have been focused on the stratigraphic units containing the human fossils, which are older than the Holocene, and the application of optically stimulated luminescence (OSL) dating to the Salawusu site is very limited. Different views existed regarding the pattern of Holocene monsoon climate change in the arid and semi-arid northwestern China. The trend of Holocene Asian Monsoon history has been related to variations in summer insolation. Strong summer insolation in the Northern Hemisphere (Berger and Loutre, 1991) induced strong land–ocean pressure and temperature gradients, and increased onshore moist air flow in summer, causing an enhanced Asian summer monsoon (COHMAP Members, 1988), which brought much more precipitation to sustain the higher lake level in the northwestern China around 12–15 ka (Pachur et al., 1995; Chen et al., 2003a; Zhang, H.C., et al., 2004; Zhang, J.R. et al., in press).

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Fig. 1. Map showing the location of the study area and other sites mentioned in the text (modified from Sun et al., 2006). The insert shows the geographical location and the climate system of the study region in China. The dotted line is the limit of modern monsoon influence. SLWS: Salawusu, the study area, BJ: Lake Baijian (Pachur et al., 1995), YHZ: Lake Yanhaizi (Chen et al., 2003a), HSR: Hongshui River (Zhang et al., 2004), DH: Daihai Lake (Xiao et al., 2006), HQH: Huangqihai lake (Zhang et al., in press), QT: Qingtu lake (Long et al., 2010, 2011).

However, this view is in contrast to that of a more humid midHolocene (9–5 ka) than in the Late Glacial and early Holocene in northwestern China (e.g., Dong et al., 1999; Lu et al., 2005; Xiao et al., 2006; Sun et al., 2006; Mason et al. 2009; Long et al., 2010). The present work aims to use quartz OSL and radiocarbon dating methods to establish the chronology for sediment units since the Late Glacial, and to further discuss its implications for palaeoclimatic changes. 2. Study area The site actually belongs to the desert–loess transition belt along the Salawusu River Valley (Fig. 2). The Loess Plateau is located to its

south and southeast. In the area, thick-bedded Quaternary sediments are widely distributed, and the bedrock is purple-red malmstone of the Cretaceous period (Yuan, 1978). The valley is formed by erosion of the Salawusu River, which originates from the Baiyu Mountains (ca. 1800 m in altitude). The Salawusu River is about 100 km in length, and is one of the branches of the Yellow River. In summer, this region is dominated by the summer monsoon, which brings most of the moisture for the whole year from the western Pacific Ocean. In the winter half year, the region is controlled by the cold and dry continental air mass, dominated by northerly and northwesterly winds that generate frequent dust storms. The present annual precipitation is about 297–370 mm, and the mean annual temperature is about 6.5–7.9 °C (Wu et al., 2002).

Fig. 2. The location of the Salawusu site and the Dishaogou (DSG) section.

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Modern aeolian sand discontinuously covers the landscape in the study region. Many stabilized or buried sand dunes are exposed, and the aeolian sand is intercalated with loess or palaeosol horizons (Sun et al., 1998). In the field, we observed that the modern vegetations are dominated by Populus, Salix, Bothriochloa ischaemum, Artemisia, and Hippophae. The area lies close to the position of the East Asian summer monsoon front, and belongs to the climatic transition area from the semi-humid to semi-arid regions. Thus, it is an ideal site for research on global changes due to its high sensitivity to the variation of the East Asian Monsoon (Zhou et al., 2001). For the past 150 ka, based on the studies of the primary chemical elements, fossil vertebrates, mollusks and pollen grains, and combined with the chronological data, Li et al., (2004, 2007) reported that the different lithostratigraphical units in the Salawusu site reflect difference in depositional environments, which can be correlated with the oxygen isotopes in the deep sea sediments, continental glaciers, the loess and paleosol units in the Chinese Loess Plateau. Based on the sedimentary facies, grain size, spore-pollen, fossil vertebrates, the primary chemical elements and clay mineral in the Salawusu area, some studies showed that the Quaternary aeolian sand deposited under a relative dry and cold condition, but the lacustrine facies were formed under the relative wet and warm climatic condition (Yuan, 1978; Gao et al., 1985; Lu, 1985; Shao, 1987; Li et al., 1987, 2000). The Dishaogou (DSG) section (37°430 21.800 N, 108°310 11.900 E, ca. 1306 m in altitude) is situated in the middle reaches of the Salawusu River (Figs. 1 and 2). It is about 6.3 m thick, and the top of which is about 60 m above the present river bed. From the bottom to the top, there are no obvious signs of the hiatus in the exposed

sediments. The section can be divided into eight stratigraphic units (Fig. 3). Table 1 shows sedimentary descriptions of the Dishaogouwan Section. In the field, we observed that the depositional sequence from lacustrine deposit to peat was widely distributed in this area (see Fig. 4). This layer has been treated as stratigraphic marker in this region, which covers a distance of at least 15 km from the Milanggouwan (the north) to the Chelugouwan (the south) (Yuan, 1978). 3. Materials and methods For this study, a total of eight luminescence samples and four radiocarbon samples were collected from DSG section. All luminescence samples were collected by hammering steel tubes (22 cm long cylinder with a diameter of 5 cm) into freshly cleaned vertical sections. The tubes were then covered with aluminium foil, sealed with opaque tape and wrapped using black plastic bag to avoid light exposure. Bulk samples were also collected in each location of the luminescence samples, and sealed immediately to ensure that the sediment retained its natural water content. In the laboratory, the bulk samples were used for dose rate measurements and water content determination. For radiocarbon dating, at the depth 2.7 m of the section, parallel to the luminescence sample DSG-5B, a bulk sample (DSG-5B-C1) and a sample of shells (DSG-5B-C2) were collected. The bulk sample was the whole things of the corresponding layer, including inorganic and organic materials. The genus of shell was gastropod. For the estimation of reservoir effect, two living weed samples were collected from the Salawusu River, one sample of emerged weed

Fig. 3. Lithologic histogram of the Dishaogou section and OSL and radiocarbon ages.

Table 1 Sedimentary desciptions of the Dishaogouwan Section. Unit number

Depth (m)

Description of the units

Thickness (m)

A

6.30–3.95.

2.35

B C

3.95–3.80 3.80–3.00

D E F G H

3.00–2.55 2.55–2.10 2.10–1.30 1.30–0.20 0.20–0.00

A thick layer of fine greyish-yellow sand. The lower part (depth of 4.30–6.30 m) is sand; the upper part (depth of 3.95–4.30 m) of this unit contains yellow ferro-rusty spots, possibly affected by waterlogging. Not bottomed A transitional layer from sand deposit to lacustrine, which contains very fine light green sand A unit consists of fine light green silt, which has abundant mollusk shells (gastropod), indicative of a deeper lake environment Another transitional layer from lacustrine deposit to peat, which consists of fine grey silt A layer of loose peat containing fine greyish-black silt A layer of light red sandy loam soil, and consists of fine silt A layer of light-yellowish reworked silt deposits A thin layer of greyish-brown silty modern soil

0.15 0.80 0.45 0.45 0.80 1.10 0.20

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Fig. 4. A picture of the Dishaogou site. (a) The widely distributed lacustrine deposit. (b) The depositional sequence, with in order the aeolian sand, the lacustrine sediment, the peat, and the palaeosol.

(living above the water, DSG-C3) and another sample of submerged weed (DSG-C4). These samples were dated in Beijing University (China) using AMS. All radiocarbon dates were calibrated to calendar year (cal a BP) using CALIB 5.1 program with IntCal04 model (Reimer et al., 2004) after 13C/12C adjustment. 3.1. Luminescence sample preparation and measurement techniques In the laboratory, the possibly light-exposed material of about 3–4 cm thickness at each end of the tube was scraped away, and the light-unexposed middle part was processed to extract quartz (38–63 lm) for equivalent dose (De) determination. The samples were treated firstly with 10% HCl and 30% H2O2 to remove carbonates and organics respectively. The grain size fraction of 38–63 lm was extracted by wet sieving, and then etched by 35% H2SiF6 for about 2 weeks to remove feldspars (Lai and Wintle, 2006; Lai et al., 2007a; Roberts, 2007). The resulting quartz grains were washed with 10% HCl to remove fluoride precipitates. The purity of quartz grains was checked by routine IR (830 nm) stimulation to monitor the presence of feldspar. Any samples with obvious infra-red stimulated luminescence (IRSL) signals were retreated with H2SiF6 to avoid De underestimation (Lai and Brückner, 2008). The pure quartz grains were then mounted on the centre part (with a diameter of 0.5 cm) of stainless steel discs (with a diameter of 1 cm) using silicone oil. Optically stimulated luminescence (OSL) measurements were made using an automated Risø TL/OSL-DA-20 reader equipped with the blue diode (k = 470 ± 20 nm) and the IR laser diode (k = 830 nm) in the Luminescence Dating Laboratory of Qinghai Institute of Salt Lakes, Chinese Academy of Sciences (CAS). The OSL was stimulated by blue LEDs (k = 470 ± 20 nm) at 130 °C for 40 s, and detected using a 7.5 mm thick U-340 filter (detection window 275–390 nm) in front of the photomultiplier tube. Irradiations were carried out using a 90Sr/90Y beta source built into the Risø reader. Preheat was at 260 °C for 10 s for natural and regenerative doses, and cut-heat was at 220 °C for 10 s for test doses. Signals of the first 0.64 s stimulation were integrated for growth curve construction after background subtraction. The concentrations of U, Th and K were measured by neutron activation analysis in the China Institute of Atomic Energy in Beijing. For the 38–63 lm grains, the alpha efficiency value was taken as 0.035 ± 0.003 (Lai et al., 2008), and the attenuation of alpha particles within the quartz grains was taken into account when calculating the dose rate. The cosmic-ray dose rate was estimated for each sample as a function of depth, altitude and geomagnetic latitude (Prescott and Hutton, 1994).

3.2. Equivalent dose (De) determination The validity of the single aliquot regenerative-dose (SAR) protocol was tested with a ‘dose recovery test’ (Murray and Wintle, 2003). This test was conducted for six aliquots of sample DSG-06. The given laboratory dose was 28.28 Gy, approximately equal to the natural De. The measured De was 26.35 ± 0.31 Gy. Thus, the ratio of the measured to the given dose was 0.93 ± 0.01, suggesting that the SAR protocol is suitable for De determination of OSL samples from the Salawusu Valley. In the current study, the SAR protocol (Murray and Wintle, 2000) and the standardized growth curve (SGC) method (Roberts and Duller, 2004; Lai, 2006; Lai et al., 2007b) were employed for De determination. For each sample, six aliquots were measured using the SAR protocol. These data were used to construct a SGC for this individual sample. Then, the natural signal (LN) and the OSL of the test dose (TN) were measured for additional aliquots (12–17), using the same measuring condition as used in the SAR protocol. The value of LN/TN of each of the individual aliquots was then matched in the SGC to obtain De. De results determined by the SGC are well in agreement with those by the SAR protocol, suggesting that the SGC could be used for De determination for samples of the Salawusu Valley. For all samples, the final De is the mean of SAR Des and SGC Des. Fig. 5a and c shows typical OSL decay curves of the samples DSG-04 and DSG-09. The regeneration dose of 0 Gy is used to measure recuperation, which was calculated by comparing the sensitivity-corrected OSL signal of the zero dose to the sensitivity-corrected natural signal. Recuperation was in all cases <1% for all samples. The ‘recycling ratio’ was introduced to check for sensitivity change correction (Murray and Wintle, 2000). For most of aliquots, the recycling ratios fall into the acceptance range of 0.9–1.1. Fig. 5b and d shows the growth curves for the two samples DSG-04 and DSG-09. The growth curves can be well fitted using the exponential plus linear function. The histograms of De distribution of the eight luminescence samples are shown in Fig. 6. De distributions are close to a tight, symmetrical Gaussian, and the mean equivalent doses are almost indistinguishable from their medians. In arid northern China, previous investigations into the sources of lake sediments have shown that some of lakes receive the fine sand or silt quartz grains from the surrounding dunes (Liu et al., 2009; Long et al., 2010). In the Qingtu Lake (QT, see Fig. 1 for location), Long et al. (2011) also suggested that the OSL signal was fully reset before burial, and the OSL dating can provide reliable ages for lacustrine sediments in similar depositional environments in arid northern China.

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Fig. 5. OSL decay curves and growth curve for sample DSG-04 (a and b) and DSG-09 (c and d).

Fig. 6. Histograms of De distribution for the eight luminescence samples.

4. Dating results and interpretation The OSL dating results are listed in Table 2, and the ages are also shown in Fig. 3. In the DSG section, four OSL samples (DSG-07–DSG10) were collected from the aeolian sand near the bottom of the section (unit A, Fig. 3). They provided ages from 13.9 ± 1.1 ka to

12.3 ± 0.9 ka, indicating that aeolian sand mobilization occurred in the studied region during this period. From unit C to unit E, the sediments shifted from lacustrine to peat deposits, indicating that the depth of the lake was decreasing. Sample DSG-06 (depth of 3.30 m) collected from the upper part of the lacustrine deposit (unit C) provided an age of 11.9 ± 0.8 ka, suggesting that the climate was

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Table 2 Environmental radioactivity and OSL dating results. Sample ID DSG-04 DSG-5A DSG-5B DSG-06 DSG-07 DSG-08 DSG-09 DSG-10 a b

Depth (m) 1.50 2.30 2.70 3.30 4.20 4.70 5.60 6.20

K (%) 1.62 ± 0.05 1.65 ± 0.05 1.52 ± 0.06 1.26 ± 0.05 1.46 ± 0.06 1.46 ± 0.05 1.56 ± 0.06 1.49 ± 0.05

Th (ppm) 3.99 ± 0.14 6.58 ± 0.24 4.88 ± 0.28 4.87 ± 0.17 4.05 ± 0.17 3.18 ± 0.14 3.99 ± 0.17 3.52 ± 0.16

U (ppm) 0.77 ± 0.11 1.94 ± 0.43 2.34 ± 0.70 3.68 ± 0.18 1.52 ± 0.18 3.52 ± 0.19 1.42 ± 0.15 1.07 ± 0.15

Water content (%) 15 ± 5 17 ± 5 17 ± 5 17 ± 5 10 ± 5 10 ± 5 10 ± 5 10 ± 5

Dose rate (Gy/ka) 2.00 ± 0.15 2.41 ± 0.19 2.27 ± 0.22 2.36 ± 0.17 2.13 ± 0.16 2.57 ± 0.19 2.16 ± 0.16 2.09 ± 0.16

Aliquot number a

b

6 + 12 6a + 12b 6a + 12b 6a + 12b 6a + 17b 6a + 12b 6a + 12b 6a + 12b

De (Gy)

OSL Age (ka)

1.99 ± 0.03 4.23 ± 0.06 7.23 ± 0.11 28.3 ± 0.6 26.1 ± 0.7 32.2 ± 1.0 32.2 ± 0.8 29.1 ± 0.6

0.99 ± 0.07 1.75 ± 0.14 3.2 ± 0.3 11.9 ± 0.8 12.3 ± 0.9 12.5 ± 0.9 14.9 ± 1.2 13.9 ± 1.1

Aliquot number used for SAR. Aliquot number used for SGC.

Table 3 Radiocarbon dating results and the OSL age of the sample at the same depth in the section. Sample DSG-C3 and DSG-C4 are modern weeds living in the Salawusu River. Sample ID

Depth in the section (m)

Materials

Uncalibrated radiocarbon age (a BP)

After subtraction of reservoir effect age (a BP)

Radiocarbon age (2r) (cal a BP)

OSL age (ka)

DSG-5B-C1 DSG-5B-C2 DSG-C3 DSG-C4

2.7 2.7

Bulk sample Shells Modern weed living above water Modern weed living under water

3525 ± 35 4390 ± 40 Modern 1550 ± 35 (reservoir effect age)

1975 ± 50 2840 ± 53 Modern 0

1937 ± 117 2961 ± 120 Modern 0

3.2 ± 0.3 (DSG-5B) 3.2 ± 0.3 (DSG-5B) – –

wetter at ca. 12 ka. Sample DSG-5B (depth of 2.70 m) was collected from the transitional layer (unit D) from the lacustrine deposit to the peat layer, giving an age of 3.2 ± 0.3 ka. It suggests that there was a general drying trend from 12 ka to 3 ka. It is also possible that some events occurred during this period as the sedimentation rate change significantly. However, in the field, we did not notice any sign of a hiatus in between units C and D. Yuan (1978) also reported that the sediments in the Dishaogouwan composed a continuous fluviolacustrine stratigraphy. In the peat layer (unit E), sample DSG-5A (depth of 2.30 m) has an age of 1.75 ± 0.14 ka. Sample DSG04 (depth of 1.50 m) was collected from the sandy loam soil (unit F), and provided an age of 0.99 ± 0.07 ka. The AMS radiocarbon dating results are shown in Table 3. Sample DSG-C3, the weed living above the water, has a zero age as expected for plants using atmospheric CO2. Sample DSG-C4, the submerged weed, yielded an age of 1550 ± 35 a BP, which represents the reservoir effect for the current river water. At the depth of 2.7 m in the section, two samples were collected. A bulk sample (DSG-5B-C1) has an age of 3525 ± 35 a BP, and another shell sample (DSG-5B-C2) has an age of 4390 ± 40 a BP. Assuming that the reservoir effect age for samples DSG-5B-C1 and DSG-5B-C2 is the same as that of today, the real ages for these two samples are 1975 ± 50 and 2840 ± 53 a BP, and the calibrated ages are 1937 ± 117 and 2961 ± 120 cal a BP, respectively. The age (2961 ± 120 cal a BP) of gastropod shells (sample DSG-5B-C2) is in agreement with that of the OSL sample (3.2 ± 0.3 ka) collected from the same depth, while the age (1937 ± 117 cal a BP) of bulk sample (DSG-5B-C1) is relatively younger. The reason for this underestimation could be that the bulk sample contained the roots of relatively younger vegetation, causing the possible carbon contamination. The results show that, after reservoir effect subtraction, the radiocarbon age using shells can better reflect the age of sediments than that of the bulk sample in our study. The dating results also show an enormous incision of about 60 m in about 1.75 ka by the Salawusu River. Unit E of peat has an OSL age of 1.75 ± 0.14 ka (sample DSG-5A) (Fig. 3). The depositional sequence from lacustrine deposit (unit C) to peat (unit E) was widely distributed in this area and has been treated as a stratigraphic marker (see Fig. 4) (Yuan, 1978). In the DSG section, the thickness from the unit E to the bottom of the river bed is about 60 m, which has been cut by the river since the last 1.75 ka. As this is not the main focus of the paper, we will discuss in detail the incision issue elsewhere.

5. Discussions 5.1. The pattern of climate changes since the Late Glacial in the Salawusu Valley Our results indicate that aeolian sand mobilization occurred in the studied region before 12 ka, marked by aeolian deposits. This period could be due to a weakened summer monsoon (An et al., 1991). However, the development of lacustrine deposit, indicative of a lake environment, implies relatively higher effective moisture related to stronger summer monsoon conditions around 12 ka., In general, then the effective humidity was decreasing between about 12 ka and 3 ka, evidenced by the shift from lacustrine deposit to the transitional layer (from lacustrine deposit to peat). After 3 ka, the palaeolake further shrank, marked by the peat layer. After 1.7 ka, the palaeolake continued to shrink and dried up, as inferred from the development of sandy soil (0.99 ka) above the peat layer. Our results indicate that the highest lake level occurred at the onset of the Holocene (ca. 12 ka); then there was a trend of increasing aridity, which led to a continuous shrinkage of the palaolake, and its ultimate desiccation. Based on spore-pollen content, water solute salt and granulometric composition, Yuan (1978) also suggested that there was a large lake in the Dishaogouwan during the early Holocene, due to the moderate and humid climate. However, Dong et al. (1999) proposed that in Salawusu the lacustrine deposit formed during 9–4 ka.

5.2. Comparison of the patterns of climate changes between the study area and the adjacent areas In the current study, lacustrine deposits show that the palaeolake appeared at the onset of the Holocene (around 12 ka). Synchronous high lake levels have also been found in the Tengger Desert, which is also situated at the margin of the East Asian Monsoon. At the Baijian Lake in the Tengger Desert (BJ, see Fig. 1 for location), Pachur et al. (1995) assumed that wet conditions occurred ca. 12.8 ka BP or 15.1 cal ka BP. In the Tengger Desert, the dates of the Hongshui River section (HSR, see Fig. 1 for location) show that the Holocene lacustrine deposits were deposited as early as around 12.0 ka BP or 13.9 cal ka BP (Zhang et al., 2004). At Lake Yanhaizi in the Hobq Desert (YHZ, see Fig. 1 for location), preliminary evidence from a sediment core indicates that a humid phase

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has occurred during the period between 13.4 and 8.0 ka BP (Chen et al., 2003a). In Huangqihai lake (HQH, see Fig. 1 for location) in eastern Inner Mongolia, the wettest period in the Holocene occurred in the early Holocene (11.4 ka) (Zhang et al., in press). In addition, palaeoclimate records from elsewhere revealed an increase in monsoonal precipitation from the onset of the Holocene, such as the marine sediment record from the South China Sea (Wang et al., 1999), two peat records from both Hongyuan peat bog in Southwestern China (Hong et al., 2003) and Hani peat bog in Northeastern China (Hong et al., 2005), and two stalagmite records from both Dongge Cave (Wang et al., 2005) and Shanbao Cave (Shao et al., 2006) in Southeastern China. Our results indicate a general trend towards aridity from the early Holocene (11.9 ± 0.8 ka) to the late Holocene (0.99 ± 0.07 ka), which is in agreement with some recent studies. Chen et al. (2003a) suggested that there was a dry mid-Holocene climate between 8 and 4.3 cal ka BP, based on the interpretations of multiple climatic proxies of the lake Yanhaizi in the Hobq Desert (YHZ, see Fig. 1 for location). In the Alashan Plateau, part of the Mongolian Plateau, Chen et al. (2003b) also observed a mid-Holocene (around 5–7 cal ka BP) drought interval. Additionally, Zhang et al. (in press) suggested that there was a general trend towards aridity since the early Holocene in the Huangqihai lake (HQH, see Fig. 1 for location), located eastern in Inner Mongolia, semi-arid northern China. In this study, we conclude that the effective humidity was generally decreasing from 12 ka to 3 ka. However, this result is in contrast to the view of a humid mid-Holocene in northwestern China, which show that the climate was dry in early-Holocene, wet in mid-Holocene, and drier in late-Holocene (e.g. Lu et al., 2005; Xiao et al., 2006; Sun et al., 2006; Mason et al., 2009; Long et al., 2010; Yang et al., 2010). In the Mu Us and Otindag dune fields, based on OSL dating, Lu et al. (2005) reported that climate was dry from 14 ka to 7–8 ka, and wet between about 7–8 ka and 2.4 ka. In north-central China, studies in Daihai Lake (DH, see Fig. 1 for location) also indicate that the climate was warm and dry during the early Holocene (ca. 11.5–8.1 cal ka BP), warm and wet during the middle Holocene (ca. 8.1–3.3 cal ka BP), and cooler and drier in the late Holocene (ca. 3.3–0 cal ka BP) (Xiao et al., 2006). In the Hobq and Mu Us deserts in the central Inner Mongolia, Sun et al. (2006) also observed that widespread aeolian sand mobilization occurred during the beginning of the early Holocene from 11.5 ka to 9.0 ka, the climate became warm and humid during the period between 9.0 ka and 5.6 ka, and the region became arid after 5.6 ka, based on the single-aliquot-quartz optical dating. At the desert margin in northern China, aeolian sand mobility is recorded extensively between 11.5 and 8.0 ka, followed by a widespread shift toward limited mobility and soil development after 8.0 ka, and widespread late Holocene reactivation (Mason et al., 2009). Long et al. (2010) used 14C and OSL dating to trace the history of the Qingtu Lake (QT, see Fig. 1 for location), and suggested that the climate was warm and dry in the early Holocene (9.5–7.0 cal ka BP), cool and humid in the mid-Holocene (7.0–4.8 cal ka BP), and increasingly drier in the late Holocene (since 4.8 cal ka BP). By far, it is still difficult to explain the discrepancy/mechanism between the views of a wet early Holocene and that of a dry early Holocene, due to the limited data. Qingtu Lake and Baijian Lake are both in the Tengger Desert, and only less than 100 km from each other, and the two lakes were once connected. However, Pachur et al. (1995) reported that the early Holocene was humid in Baijian Lake, while Long et al. (2010) suggested a dry early Holocene in the Qingtu Lake. Daihai Lake and Huangqihai Lake are only less than 70 km from each other. Xiao et al. (2006) reported a dry early Holocene in the Daihai Lake, while Zhang et al. (in press) suggested a wet early Holocene. Yanhaizi Lake (Chen et al., 2003a) is located

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to the north of the Salawusu, and the results from both of the sites showed a humid early Holocene. Similar to the lacustrine records discussed above, the aeolian records in the deserts in north China also showed different patterns. Based mainly on luminescence dating, most of the studies suggested a dry early Holocene with mobile dunes, a humid mid-Holocene with palaeosol formation, and a dry late Holocene again (Lu et al., 2005; Sun et al., 2006; Mason et al., 2009; Yang et al., 2010), while some others showed that humid condition lasted from the onset of the Holocene till the middle Holocene (Li et al., 2002; Li and Sun, 2006). It is possible that local differences in the catchments (e.g. snow and frozen ground in the catchments) may provide moisture sources for some sites but not for other sites. It is also possible that the uncertainty in the chronology may contribute to this discrepancy. This surely requires further work. 6. Conclusions In the current study, luminescence and radiocarbon samples were dated to establish the chronology for the sediments since the Late Glacial from the palaeolithic Salawusu site. The stratigraphical and chronological studies show that: (1) the AMS age of modern weed living under water is about 1550 ± 35 a, which is the current reservoir effect age, and after reservoir effect subtraction the calibrated radiocarbon age using shells is in agreement with OSL age for a sample from the same depth of the section, while radiocarbon age of bulk sample is relatively younger in this study; (2) aeolian sand mobilization occurred in the studied region before 12 ka; (3) within the Holocene, the wettest climate occurred during the onset of the Holocene, with an extensive palaolake existed in the study area, marked by the development of lacustrine sediments at ca. 12 ka. (4) between 12 ka and 3 ka, the climate showed a trend of increasing aridity, with the palaolake ceaselessly shrank, and then dried up between 1.8 and 1.0 ka, evidenced by the shift from the lacustrine sediments to the peat, and finally to palaeosol; (5) the dating results also show an enormous incision of about 60 m in about 1.75 ka by the Salawusu River. Acknowledgments We thank Profs Baosheng Li, Guangrong Dong, and Baoyin Yuan for suggestions and discussions in the field, Lupeng Yu for help in sample collection, and Hao Long for suggestions during drafting. We also thank two anonymous reviewers for very constructive comments and suggestions that have significantly improved the manuscript. The research was supported by a project of China Geological Survey (1212011120046), China NSF (41172168), and the One-Hundred Talent Project of CAS granted to ZPL (A0961). References An, Z.S., Kukla, G., Porter, S.C., Xiao, J.L., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of Central China during the last 130,000 years. Quaternary Research 36, 29–36. Berger, A., Loutre, M.F., 1991. Insolation values for the climate of the last 10,000,000 years. Quaternary Science Reviews 10, 297–317. Boul, U., Breuil, H.F., Licent, E., Teilhard de Chardin, P., 1928. Le paleolithique de la China (paleontologic). Achieves de L’Institut de paleolithique Humanenne, Paris 4. Chen, C.T., Lan, H.C., Lou, J.Y., Chen, Y.C., 2003a. The dry Holocene Megathermal in Inner Mongolia. Palaeogeography, Palaeoclimatology, Palaeoecology 193, 181– 200. Chen, F.H., Wu, W., Holmes, J.A., Madsen, D.B., Zhu, Y., Jin, M., Oviatt, C.G., 2003b. A mid-Holocene drought interval as evidenced by Lake Desiccation in the Alashan Plateau, Inner Mongolia, China. Chinese Science Bulletin 48, 1401–1410. COHMAP Members, 1988. Climate changes of the last 18,000 years: observations and model simulations. Science 241, 1043–1052. Dong, G.R., Gao, S.Y., Li, B.S., 1982. New discovery of the Fossil Ordos Man. Kexue Tongbao (Chinese Science Bulletin) 27, 754–758. Dong, G.R., Su, Z.Z., Jin, H.L., 1999. New views on age of the Salawusu Formation of Late Pleistocene in northern China. Chinese Science Bulletin 44, 646–650.

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