Sedimentary documents and Optically Stimulated Luminescence (OSL ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

November 2010 Vol.53 No.11: 1675–1682 doi: 10.1007/s11430-010-3085-1

Sedimentary documents and Optically Stimulated Luminescence (OSL) dating for formation of the present landform of the northern Ulan Buh Desert, northern China FAN YuXin1,2, CHEN FaHu1*, FAN TianLai1, ZHAO Hui3,1 & YANG LiPing1 1

MOE Key Laboratory of Western China’s Environmental Systems, Lanzhou University, Lanzhou 730000, China; 2 College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China; 3 Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China Received May 15, 2009; accepted March 3, 2010; published online July 29, 2010

GPR reflecting sections and core profiles revealed that sand dunes of the northern Ulan Buh Desert are overlying shallow lacustrine or palustrine sediments. Optical dating results of sediments from three core profiles indicate that the area of the northern Ulan Buh Desert was still covered by the shallow lake or marsh during 8.4–6.4 ka, and eolian sand started to accumulate since around 2 ka. Such a result supports the idea that the present desert landform of the northern Ulan Buh Desert started to form since 2 ka, which was likely a response to the desertification caused by ruin of the Han Dynasty and the large-scale abandonment of farming of the Han nationality. Our research results are consistent with the previous archaeological studies, and support the idea that the Ulan Buh Desert is composed of two parts with different histories, i.e., the old southern Ulan Buh Desert and the young northern Ulan Buh Desert. Ulan Buh Desert landform, formation, OSL dating Citation:

Fan Y X, Chen F H, Fan T L, et al. Sedimentary documents and Optically Stimulated Luminescence (OSL) dating for formation of the present landform of the northern Ulan Buh Desert, northern China. Sci China Earth Sci, 2010, 53: 1675–1682, doi: 10.1007/s11430-010-3085-1

The Ulan Buh Desert is an ideal place to study the cause of desertification in northern China for two reasons. First, the Ulan Buh Desert is located on the margin between the present monsoon dominant region and the westlies, and thus is sensitive to climate changes. Second, archaeological research revealed that the northern Ulan Buh Desert was cultivated before Han Dynasty [1]. A few scientists studied the formation of the northern Ulan Buh Desert and advocated two opinions. The first is that formation of the Ulan Buh Desert was the result of widespread abandonment of farmland by the Han nationality caused by the fall of the Han Dynasty [2]. The second is that Ulan Buh Desert was al-

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2010

ready formed before the Han Dynasty because of climate change [3, 4]. In recent years, we extended our field investigation to the Ulan Buh Desert as we studied paleolake evolution in the Jilantai basin. The preliminary results show that there was once a megalake, “Jilantai-Hetao”, covering almost the entire Jilantai Basin and Hetao Plain [5] as well as the Ulan Buh Desert. Based on the optical dating results of sand dunes on southern lakeshores of Jilantai Salt Lake and the limited field investigation in the area of Ulan Buh Desert, Chun et al. [6] preliminarily proposed that the Ulan Buh Desert should be formed at about 7 ka. However, such a conclusion can hardly reveal the evolutionary history in different regions of the Ulan Buh Desert, for this is based mainly on a few profiles in the southern Ulan Buh Desert and lakeshore of Jilantai Salt Lake, lacking evidence of earth.scichina.com

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complete sedimentary documents from hinterland of the Ulan Buh Desert. The development of Ground Penetrating Radar (GPR) provides a new perspective for studying structure of sand dunes, which is successfully used to study sand deserts in arid regions in America and Africa [7]. In this paper, three sand dunes in the northern Ulan Buh Desert were measured by using GPR, and core sediments are obtained by using sand drill. Formational age of the present desert landform is determined by measuring the optical dating results of eolian sand at the bottom of these sand dunes.

1

Study areas

The Ulan Buh Desert is located at northeast of the Alashan Plateau, south of the Yinshan Mountains, north of the Helan Mountains [1] and west of the Yellow River, including the large area covered by drifting sand west to the Hetao Plain. The present Ulan Buh Desert extends its sand west to the surroundings of the Jartai Salt Lake and covers several shorelines of the megalake Jartai-Hetao [6]. The whole Ulan Buh Desert can be divided into two areas with different landforms as revealed by remote sensing images (Figure

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1). The southern Ulan Buh Desert, main body of the desert, is covered by mega dunes, whereas the northern Ulan Buh Desert is covered by zonal drift sands. Field investigation shows that the southern Ulan Buh Desert is generally composed of huge compound and complex transverse dunes, linear dune and star dunes, whereas the northern Ulan Buh Desert is generally composed of low and small fixed and half-fixed dunes [6]. Sediments of the dried lakebeds are preserved in interdune depressions. In some interdune depressions, covering on the surface is about 1 to 2 cm thick gray marl that was formed as a result of temporary ponds. Meteorological data show that the mean annual precipitation is 107.8 mm and mean annual evaporation is 2956.9 mm. The Yellow River flows north and then northeast across the eastern border of the Ulan Buh Desert. Seasonal rainfall in the Helan mountainous region flows as runoff or phreatic water into the southern Ulan Buh Desert.

2

Methods

To obtain formational age of the Ulan Buh Desert landform, three sand hills or dunes were studied in this paper. These

Figure 1 Remote sensing image of the study area. The filled circles are sites of cores in the northern Ulan Buh Desert and filled triangles are sites of the profiles reported in ref. [6].

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three sand hills or dunes are located west-east in the zonal drift sand area of the northern Ulan Buh Desert (Figure 1). A GPR of Pulse Ekko Pro type equipped with the 1000 V transmitter and 50 MHz receiving antenna was used to measure stratigraphic structure of these sand hills. At the same time, terrain measurement of these sand hills was made by using GPS and geological compass. The terrain-corrected GPR sections of these sand dunes were obtained by integrating topographic data into the radar wave reflecting sections, which are illustrated in Figure 2. On windward slopes of these sand hills, sedimentary cores (shown as gray vertical bars in Figure 2) were obtained by using a sand drill with SA5010SSC sand auger produced by Dormer Engineering Company. In order to collect samples under dim light condition suitable for OSL dating, a thick light-tight cloth was used as a shade at the end of the sand auger while the core sediments was collected into the iron cans, in which the samples were kept from light. Age of samples was measured by using optical dating method on a Risø DA-15 TL/OSL reader in Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences.

3 3.1

Results and discussions Core sediments

3.1.1 Core WLD07A Core WLD07A is located on the western slope (windward)


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of a north-south extended linear sand dune, very close to the eastern Jilantai Town with the coordinate of 39°46′02.7″N, 105°45′38.2″E. The sand dune is about 3–5 m high with an altitude of 1023 m asl on its surface, overlying the dry lakebeds composed of reddish gravelly sand layer. The GPR section (Figure 2(c)) illustrates that there are three horizontal reflectors at the bottom of the sand dune. The first reflector was deduced to be the interface between eolian sand of the sand dune and its underlying dried lakebed, and the second as the interface between the dried lakebed and its underlying sediments. This deduction is tested by sediments of the core and the exploratory pit (see details below). Core WLD07A is 270 cm long, composed almost all of fine-medium eolian sand. Underlying the eolian sand is brown-reddish gravelly beach sand (sediments composing lakebeds), which is flinty to be drilled through. Referred to the GPR section, an exploratory shaft (S54 in Figure 2) was excavated 14 m west to the core WLD07A. The sediments downward are as follows: brown-reddish gravelly beach sand with Radix xauricularia and Gyraulus convexiusculus preservation (25 cm thick), top 20 cm of which was reddened as a result of pedogenesis; well sorted shallow lacustrine facies fine sand (125 cm thick), among which the upper 20 cm is brown-yellow and the lower 85 cm is caesious; red clayey sediments (deduced to be more than 300 cm thick from the excavated profile in the adjacent area). The brown red gravelley sand at the end of the core WLD07A has the same characteristics as sediments on the top of the exploratory shaft profile S54. Therefore, we compound sedimentary documents from the core WLD07A and the

Figure 2 GPR sections of sand hills or dunes in the Ulan Buh Desert. The gray vertical bars stand for the sites of cores, above which are the codes of cores. The first dark line at the bottom of dunes represents the surface of the dried lacustrine beach in sections (b) and (c). V=0.15 m/ns.

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shaft S54 as a column, which is illustrated as profile WLD07A in Figure 3 GPR reflecting section indicates that oblique beddings developed generally parallel to the surface of the sand dunes as in Figure 2(c), with several secondary oblique crossbeddings on the eastern slope (leeward slope) of the sand dune, while horizontal beddings occurred at the base of the sand dune. Grain size data (Figure 4) show that mode size of sediments is 293–310 μm at the depth of 0–120 cm and 250–267 μm at 120–240 cm for the eolian sand, while 265–280 μm at 240–290 cm for the dried lakebed sediments and 169–220 μm at 290–420 cm for the lacustrine sand.

Figure 3


3.1.2 Core WLD07B Core WLD07B was located on the southeastern slope (windward slope) NE65°-extending linear sand dune in the southeastern part of the northern Ulan Buh Desert. This sand dune is 5.6 m high. The altitude is 1063 m on the surface of the drilling site with coordinate of 39°51′40.8″N, 106°23'20.4"E. A GPR section was measured along the NW320° direction. The terrain-corrected GPR section is illustrated in Figure 2(a). GPR reflecting signals diminished at both sides of the sand dune. An exploratory shaft was excavated in the interdune depression. Sedimentary documents from this exploratory shaft show that there is a

Lithologic profiles of the sand cores and exploratory shaft in the Ulan Buh Desert. Profile WS2 was from ref. [6].

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Figure 4 Plot showing the relationship between mode size of sediments and the depth of sample in the core WLD07A.

phreatic water table at the depth of 170 cm, at which the water is salty. This information indicates that it is salty groundwater that exhausted the radar wave. In the sand dune, there is also only one radar wave reflection level. Drilling results on the windward slope of the sand dune revealed yellowish eolian fine sand at the depth of 0–454 cm, gray clayey sand layer at depth of 454–524 cm and phreatic water aquifer at the end of the core. The mode size of sediments is 280–418 μm for sediments at depth of 0–242 cm, 132–218 μm at depth of 242–454 cm, and 147–174 µm at 454–524 cm, respectively. 3.1.3 Core WLD07D Core WLD07D is located on the western slope of a NW-SE linear compound sand hill about 8 km east to the Jilantai town. Coordinate of the sand hill on its crest is 39°51′03.2″N, 105°49′54.1″E. This sand hill is situated on the N-S drift sand belt in the northwestern part of the Ulan Buh Desert, in which sands were transported from the Badan Jiran Desert (Figure 1). Judged from outcrops in the adjacent interdune, the base of the sand hill should be a dried lacustrine beach or paleo-wetland. A GPR section was measured along the NE46° direction, and the terrain-corrected section is illustrated in Figure 2(b). Several radar wave reflectors roughly parallel to surface of the sand hill and one generally horizontal reflector is at the base of the sand hill, below which radar wave signal almost disappeared. Calculated from radar velocity of sand, the horizon-


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tal reflector is almost as high as the surface of the interdune. Therefore, the horizontal reflector is deduced as the interface between the sand dune and its underlying dried lakebeds. By using sand auger, we drilled through the interface between the sand hill and its underlying lacustrine beach. At the end of the core, clayey sand of swamp facies was obtained, indicating that the sand hill developed over paludal facies sediments. The complete documents of this core (Figure 3) are composed of five segments, among which eolian sand dominates sediments at the depth of 0–1224 cm, caesious sandy soil of 1224–1493 cm, light yellow medium sand of 1493–1622 cm, caesious medium sand (palustrine facies) of 1622–1672 cm, and nearly saturated clayey medium sand (palustrine facies) of 1672–1857 cm respectively. Grain size data indicate that sediments on the sand hill are uniform with the mode size of 225–283 µm. And the GPR section indicates that large-scale cross beddings developed at the windward slope of the sand hill, whereas the base of the sand hill is level. Radar reflecting sections and core sediments of these three sand dunes or hills suggest that sand dunes are overlying lacustrine or palustrine sediments in the northern Ulan Buh Desert. This information supports the idea that the present northern Ulan Buh Desert was accumulated on the bed of a paleolake. Therefore, ages of eolian sand at the bottom of these sand hills or dunes should provide the initiatory time of the northern Ulan Buh Desert. 3.2

OSL chronological evidence

The 90–300 μm sized grains of pure quartz isolated by using the improved sample preparation method [8] were used to obtain ages of sediments in these cores. Equivalent dose (De) was obtained from post IR-blue OSL signals, and was measured by applying Double SAR protocol [9] under 260°C preheating temperature. During data processing, only aliquots that satisfied the following criteria were used in the calculation of De: (1) relative to OSL, there is an invisible IRSL decay above background; (2) the recycling ratio is between 0.9 and 1.1; and (3) the recuperated OSL signal is less than 5% of its natural signal. Radial plots of De values for representative eolian sand samples (Figure 5(a)) and lacustrine or palustrine sand samples (Figure 5(b)) are illustrated in Figure 5. For eolian sand samples, De values of individual dose estimates are distributed intensively into a narrow band with gray shadows in Figure 5(a). For these eolian sand samples, De values are obtained by calculating weighted mean value of all individual dose estimates by following reference [10]. For lacustrine and palustrine sand samples, De values of individual dose estimates are distributed into two bands with different dominant De values, such as sample S54-35-40 in Figure 5(b). After outliers (those illustrated as solid triangles) were eliminated, De values of those lacustrine and palustrine sand

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samples (Table 1) are obtained by calculating weighted mean value of the other individual dose estimates. Dose rate originates from natural radioactive decays of elements U, Th and K in samples, and its immediate surroundings as well as contributions of cosmic rays radiation. In this research, content of U, Th and K was determined by using Instrumental Neutron Activation Analysis method. Dose rate is calculated according to conversion factors [11], in which contributions from cosmic ray are calculated based on the burial depth and the altitude of the sample [12]. Water in the interstices among sediment particles absorbs parts of the radiation that would otherwise reach the grains on which OSL measurement is made [11]. Water content of present day is calculated from sample weights measured before and after drying in an oven in the laboratory. The saturated water content was also measured. Due to the uncertainty in water-content history since the sediments were buried, an estimation of water content was made based on

Figure 5


the present-day water content and the saturated water content. The estimated water content is finally used to calculate dose rates and OSL ages. The results are shown in Table 1 and Figure 3. 3.3

Discussions

In core WLD07A (Figure 3), sediments overlying the shallow lake facies sand of 6.4 ka are gravelly beach sand and eolian sand of 1.98 ka in turn. Such a sedimentary sequence indicates that it was until 1.98 ka that eolian sand set out to accumulate at this site after the lake dried up. Sedimentary information of the core WLD07B (Figure 3) indicates that it was a lake or marsh environment at around 8.4 ka in the present eastern Ulan Buh Desert, while eolian sand started to accumulate since about 1.7 ka. Above two core documents support the idea that the area covered by zonal drift eolian sand in the northern Ulan Buh Desert was still a

Radial plots of De values for typical samples. (a) Eolian sand sample WLD07A265–270; (b) shallow lacustrine sand sample S54-35-40.

Table 1 OSL dating results of sediments in the Ulan Buh Deserta)

Core

Surface elevation (m)

WS2

1080

WLD07A

1023

WLD07D

1028

WLD07B

1063

Cosmic Total ray Grain dose De Age Sedimentary Sample Aliquots dose (Gy) type rate size (μm) (ka) rate (Gy/ka) (Gy/ka) 2.39± WLBH040827-30* 0.3 5±5 1.63±0.09 4.39±0.14 1.10±0.06 0.25 90–125 16.26±0.71 8 6.81±0.56 0.17 Eolian sand 2.57± WLD07A265-270 2.65 5±5 1.6±0.05 6.47±0.11 1.31±0.08 0.21 125–180 5.09±0.58 11 1.98±0.25 0.14 Shallow 2.32± S54-35-40 3.05 10±5 1.85±0.04 4.97±0.16 1.04±0.05 0.17 90–125 14.78±0.93 33 6.37±0.53 lacustrine 0.13 sand WLD07D15812.27± 15.81 5±5 1.75±0.04 5.21±0.17 1.28±0.06 0.06 250–300 0.26±0.05 5 0.11±0.02 1602 0.13 WLD07D18412.51± Eolian 18.45 5±5 2.06±0.29 3.77±0.09 1.31±0.08 0.04 125–180 2.16±0.29 8 0.82±0.14 1849 0.30 sand 2.00± WLD07B445-454 4.5 10±5 1.55±0.03 2.98±0.11 0.86±0.05 0.14 90–125 3.36±0.55 19 1.68±0.29 0.11 Lacustrine or 2.23± WLD07B510-524 5.2 15±5 1.74±0.05 6.14±0.11 1.36±0.08 0.13 125–180 18.82±0.76 19 8.42±0.68 palustrine 0.16 sand Water Depth content (m) (%)

a) Samples followed by “*” are from ref. [6]

K (%)

Th (ppm)

U (ppm)

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shallow lake or marsh during about 8.4–6.4 ka, and eolian sand started to accumulate since 2–1.7 ka. Stratigraphic documents of core WLD07D (Figure 3), a core on slope of a NW-SE sand belt in the northern Ulan Buh Desert, showed that this area was still a marsh until about 0.8 ka, while eolian sand began to accumulate from that time on. Distribution (Figure 1) and stratigraphical profiles (Figure 3) of these three cores indicate that eolian sand started to accumulate into a landform of sand dunes or hills since 2 ka, whereas the area of the present northern Ulan Buh Desert was still covered by a lake or marsh during 8.4–6.4 ka. Such a cognition is consistent with the idea that the Ulan Buh Desert formed as a result of widespread abandonment of cultivation after the Han Dynasty [2, 13], which was revealed from archaeological evidence. Preliminary reports of eolian sand, accumulated at about 7 ka on the southern bank of the Jilantai Salt Lake (sites BS2, BS4, BS6 and BS10 in Figure 1) and on the northern margin of the southern Ulan Buh Desert (WS2 in Figure 2) at around 6.8 ka [6], suggest that the Jilantai paleolake once shrank and then sand accumulated as early as around 7 ka. The sand accumulation in the southern Ulan Buh Desert at around 7 ka occurred simultaneously with the drought event documented in desert lake sediments on the Alxa Plateau [14] is at least 5 ka earlier than the formation of the desert landform in the northern Ulan Buh Desert. The reasonable interpretation is that with the shrinkage of the Jilantai megalake [6], the area of the present southern Ulan Buh Desert emerged from the lake and started to receive sand accumulation as a response to the regional climate change. Meanwhile, the area of the northern Ulan Buh Desert was still covered by a lake or a marsh. Around 2 ka, eolian sand started to accumulate into most region of the present northern Ulan Buh Desert, where some area is still covered by seasonal water nowadays as illustrated in the remote sensing image (Figure 1). Such a spatial difference in environment in the Ulan Buh Desert suggests that history of the northern Ulan Buh Desert is likely different from that of the southern Ulan Buh Desert, which deserves further studies. Stalagmite documents revealed the Asian Monsoon was strong at around 2.0–1.7 ka [15] as well as around 1.5 ka [16]. The northern Ulan Buh Desert, on the other hand, was still a developed agricultural area with large areas of farmland during the Western Han Dynasty, which is suggested by the historical records and environmental-archaeological findings [2, 13]. And at that time, a huge lake, namely the Tushenze Lake supplied by the Yellow River, existed in the northern Ulan Buh Desert as well [13]. These previous studies indicate that natural environment of areas in the present northern Ulan Buh Desert was nice at around 2.0 ka. If climate were the dominant control factor of the eolian sand activity during that time, there would have been no large-scale sand accumulation. After the Eastern Han Dynasty, a large number of residents emigrated from this area and the farming activity stopped [13]. From then on, the


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landform of sand desert started to form in the northern Ulan Buh Desert, indicating that the large-scale abandonment of farming [2] caused the formation of the desert landform of the northern Ulan Buh Desert. At around 0.8 ka, on the one hand, the Asian Monsoon was weak [16]. On the other hand, the human activity was strengthened once again with the emergence and rapid development of the West Xia Dynasty in the northern Ulan Buh Desert and its vicinities [13]. Therefore, the enhancement of sand accumulation at around 0.8 ka is a possible result of both human activity and climate changes. All in all, the landform of the north Ulan Buh Desert formed since 2.0–1.7 ka, and was likely a response to the large-scale abandonment of farming in that area and its vicinities.

4

Conclusions

GPR sections and stratigraphic profiles from three sand hills or dunes west-east-distributed in the northern Ulan Buh Desert support the idea that sand dunes are overlying lacustrine or palustrine sediments. Optical dating results from sediments of three core profiles suggest that eolian sand started to accumulate into the present northern Ulan Buh Desert at about 2 ka, among which the north-south linear drift sand belt possibly started to form since 0.8 ka. A lake or marsh still existed in this area during 8.4–6.4 ka. The consistency between archaeological studies [2, 13] and our research indicates that the Ulan Buh Desert is composed of two parts with different histories, the old southern Ulan Buh Desert and the young northern Ulan Buh Desert. And the comparison among our research, archaeological result, and the records of monsoon activity supports that the present desert landform of the north Ulan Buh Desert started to form since 2.0–1.7 ka as a response to desertification caused by the fall of the Han Dynasty and the large-scale abandonment of farming of the Han nationality in the area. We thank Wang Z. T. for his help in processing GPR data, and Lu J., Zhao S., Ren Y. P. and Song B. W. for their contributions in the field drilling. We thank two anonymous reviewers for their comments and Wang X.L. for his insightful suggestions to improving the earlier version of this manuscript. This research was jointly supported by the Innovative Research Team Project of National Natural Science Foundation of China (Grant No. 40721061), National Natural Science Foundation of China (Grant Nos. 40502016, 40972116) and the Fundamental Research Funds for the Central Universities (Grant No. LZUJBKY-2009-68). 1

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