Historical settlement abandonment in the middle Hexi Corridor linked to human-induced desertification

Palaeogeography, Palaeoclimatology, Palaeoecology 545 (2020) 109634

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Historical settlement abandonment in the middle Hexi Corridor linked to human-induced desertification

T

Linhai Yanga,b,c, Hao Longb,d, , Hongyi Chenge, Guangyin Hua,c, Hanchen Duanc, Hui Zhaoc ⁎

a

School of Geography and Tourism, Shaanxi Normal University, Xi'an 710062, China State Key Laboratory of Lake Sciences and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China c Key Laboratory of Desert and Desertification, Northwest Institute of Eco-environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China d CAS Center for Excellence in Quaternary Science and Global Change, Xi'an 710061, China e College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China b

ARTICLE INFO

ABSTRACT

Editor: Thomas Algeo

Understanding past human-environment interactions over a long time-scale offers an analogue to predict, adapt and mitigate the environmental issues caused by global change and human activities in the future. The transient existence of historical cities from arid northern China can provide a valuable reference for this issue. In this study, we investigated a set of sand dunes which accumulated alongside the city wall of South Heishuiguo ancient city (SHC) from the middle Hexi Corridor in arid northern China. Dating these dune sands can provide the timing of both desertification and city abandonment. A series of sand samples (a total of 20) were collected, and their ages were determined by luminescence dating of K-feldspar fraction. The results suggest an abrupt sand dune accumulation between 0.40 ± 0.03 ka and 0.25 ± 0.02 ka, corresponding to 1590–1790 CE. This study confirms a phase of desertification between the late 16th century and the late 18th century in the Hexi Corridor, and the abandonment of the SHC at the ~17th century. In order to assess the anthropogenic influences on desertification, we also reconstructed the population history of the Hexi Corridor over the last 2000 years based on historical literature as a quantitative index of human activity intensity. By comparison of the timing of onset of desertification around the SHC with robust paleoclimate and historical population reconstructions, we conclude that (1) the desertification of 1590–1790 CE was likely the result of considerably enhanced human activities between the late Ming and the early Qing Dynasties, as this period was dominated by a relatively moist climate and could restrain the sand deflation; and (2) the desertification in turn caused abandonment of SHC as the impact of environmental changes on people.

Keywords: Luminescence dating Demography Historical period Climate change Northern China Anthropocene

1. Introduction Research on past human-environment interaction has increasingly received attention from researchers with different disciplines in recent years (Roberts et al., 2011; Turner and Sabloff, 2012; Verstraeten, 2014; Jia et al., 2016a; Wu et al., 2016; Dong, 2018, 2020; Ge et al., 2019; Xie et al., 2019). Deciphering human-environment interaction from a long-term perspective can enhance our understanding of human society development as well as its relationship with environment. This provides us references on prediction, adaptation and mitigation to environmental issues caused by global change in the future. As summarized by Dong (2018), the past human-environment interaction research mainly focuses on three aspects: (1) influence of climatic and environmental change on the evolution of humans and human societies (Weiss et al., 1993; Weiss and Bradley, 2001; Wang, 2005; Lee et al., ⁎

2008; Dong et al., 2012, 2017; Wang et al., 2014; Yang et al., 2015; Guo et al., 2018); (2) human adaption to different habitats and climate change (deMenocal, 2001; An et al., 2005; Mercuri et al., 2011; Wilmshurst et al., 2011; Chen et al., 2015a; Jia et al., 2016b; Zheng et al., 2018); and (3) anthropogenic impact on surrounding environments during the prehistoric and historic periods (Li et al., 2006; Lawrence et al., 2007; Ruddiman et al., 2008; Smith and Zeder, 2013; Feeser and Dörfler, 2014; Waters et al., 2016; González-Arqueros et al., 2017; McConnell et al., 2018; Tóth et al., 2019). Since the introduction of agriculture in the early Holocene, the human-environment relationship has changed progressively from a stage with rapidly growing human impacts on all ecosystems of our planet to a stage with further acceleration in the pace of environmental changes, resource use, and vulnerability for societies and economies (Messerli et al., 2000). The ongoing debate concerning the establishment of an Anthropocene

Corresponding author at: 73 East Beijing Road, Nanjing 210008, China. E-mail address: [email protected] (H. Long).

https://doi.org/10.1016/j.palaeo.2020.109634 Received 21 July 2019; Received in revised form 22 January 2020; Accepted 26 January 2020 Available online 10 February 2020 0031-0182/ © 2020 Elsevier B.V. All rights reserved.

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Epoch as well as its start date (Lewis and Maslin, 2015; Waters et al., 2016) reminds us that further research of human-environment interaction is still imperative. Arid and semi-arid regions account for one third of the Chinese landmass. These regions have been regarded as the ideal place for studying past human-environment interaction owing that: (1) these areas are influenced by three different atmospheric systems, i.e., the East Asian monsoon, Indian Ocean monsoon and Westerlies (Yang et al., 2011a; An et al., 2012), resulting in very sensitive environmental conditions; (2) arid and semi-arid northern China mainly includes extensive steppe, gobi desert, and sandy desert, and its ecosystems are very fragile (Wang et al., 2007); (3) Chinese dry-land agriculture originated from the area along the Yellow River in northern China (Zhao, 2011), and human occupation began as early as the Neolithic period and has been lasting since then. Extensive efforts have been made on several topics such as influence of climate change on the human evolution and agriculture origin (Wang et al., 2014; Chen et al., 2015a; Zhang et al., 2018), climatic and environmental impacts on prehistoric cultures and ancient Chinese civilization (An et al., 2004, 2005; Dong et al., 2012; Guo et al., 2018), climate context of society development and dynasty replacement (Zhang et al., 2005; Lee et al., 2008; Liu et al., 2009; Wu et al., 2016; Feng et al., 2019), as well as anthropogenic influences to climate, environment and their interplay (Li, 2005; Li et al., 2006, 2011; Zhou et al., 2012; Zhao et al., 2013; Yang et al., 2017; Liu et al., 2019). Hexi Corridor is a hot spot to investigate the climatic and anthropogenic factors responsible for the desertification, one of the most critical environmental issues in this region during the past 2000 years as well as nowadays (Li, 2003a; Gao, 2007; Cheng, 2007). However, diverse or even controversial viewpoints with regard to the relative role of natural and human factors in desertification have been proposed. One attributed the historical desertification in Hexi Corridor to unconscionable human activities in terms of extensive agricultural development (Jing, 2000; Li, 2003b). In contrast, some considered that desertification in this region represents a long-term process that has been occurring for the past 5000 to 6000 years and the impact of human activity on environmental change may thus have been overestimated in previous studies (Wang et al., 2006, 2008). Additionally, some realized that different periods of desertification may have different climatic background and anthropogenic impacts and the relative role of human activities may change with time (Wang et al., 2003). Furthermore, more recent studies found out that the causes of historical desertification in different places are likely to be disparate (Zhao et al., 2014). In general, most reconstructions of the past desertification process in the Hexi Corridor were mainly inferred from written historical documents and archaeological findings. However, on one hand, historical records in ancient China is primarily not for geographical study and hence likely to give imprecise information on desertification (Huang et al., 2009); on the other hand, archaeological materials could only provide sporadic and indirect clues to environmental change. Thus, most inferences about historical desertification based on historical documents and archaeological findings are speculative. Furthermore, the geological/sedimentological evidences of desertification from stratigraphic record are usually lacking, owing to the progressively enhanced anthropogenic disturbance on the upmost sedimentary unit during the historical period. Thus, the historical desertification as well as its relationship with climate change and human activity in Hexi Corridor still need to be confirmed and clarified by further geological/ sedimentological evidences. The South Heishuiguo ancient city (SHC; 39°06′18″ N, 100°20′36″ E; 1412 m a.s.l., Fig. 1), located in the middle reaches of Heihe River, is one of the historical cities (Fig. 1) with important historical significance in the Hexi Corridor (Li, 2003b). In this study, the eolian sands, which accumulated beside the SHC city wall, were extracted for age constraint of the desertification phase by using luminescence dating techniques. The intensity of human activity was defined by the population data in

the Hexi Corridor. Combining with human activity data and climate records, we aim to clarify the relative roles of climatic and human factors in desertification in this region, as well as the relationship between desertification and the abandonment of SHC. 2. Study area The Hexi Corridor is a typical agriculture region fed by inland rivers from arid area of northern China (Fig. 1); it is immediately adjacent to the northeastern margin of the Tibetan Plateau, and surrounded by the Beishan, Helishan, Longshoushan and Qilianshan mountains and bordered by three deserts (Kumtag, Badain Jaran and Tengger) to its west, north and east, respectively (Fig. 1) (Li et al., 2011; Nottebaum et al., 2015). Three inland river systems (Shiyanghe, Heihe and Shulehe rivers) flow from the Qilianshan Mountains to the deserts and bring a large amount of detritus to the piedmont forming massive alluvial fans (Wang, 2011). Climatically, Hexi Corridor lies in the northern margin of the Asian monsoon region and is affected by both the monsoon and the Westerlies (Li et al., 2012; Wang et al., 2013). The mean annual temperature of Hexi Corridor is approximately 0—10 °C, with mean January temperature -12—10 °C and mean July temperature 11—25 °C (Yang et al., 2009). The annual precipitation of Hexi Corridor declines from 300 mm in the east to 60 mm in the west, with mean annual precipitation in its central area less than 150 mm (Yang et al., 2009). The Hexi Corridor is generally vegetated by low shrubs and perennial herbs, including Tamarix, Reaumuria, Hexinia, Hippophae, Nitraria, Alhagi, Calligonum and Chenopodiaceae. The steppe is mainly distributed over the oasis across the corridor and its southeast, e.g., Stipa, Artemisia, Cleistogenes, Allium and Achnatherum; Populus euphratica and Elaeagnus oxcarpa forest occur along the Heihe and Shulehe rivers owing to sufficient water sources there (Huang, 1997). Dunes in Hexi Corridor are distributed primarily in three areas (Wu, 2009): (1) the west of the modern oasis of the lower reaches of Shiyanghe River, where dunes are mainly represented by nebkhas, barchans and barchan chains; (2) the middle reaches of the Heihe River, dominated by barchans, barchan chains and sand sheets; and the lower reaches of the Danghe River (main tributary of Shulehe River), mainly consisting of large barchan chains, pyramid dunes and complex sand hills. As the key section of ancient Silk Roads connecting the Asia and Europe continents, Hexi Corridor has a long history of human occupation and subsistence, and acts as the bridge of Chinese and Western transportation, communication, cultural exchanges and national amalgamation (Li, 2003a). Various types of Neolithic and Bronze-Age Cultures have flourished in the Hexi Corridor since several thousand years, including the Majiayao, Qijia, Siba and Shajing cultures; and multiple rises and falls of the farming and husbandry occurred in the pre-historic period (Gao, 2007). Since the Hexi Corridor was taken over and administrated by the central government of Western Han Dynasty at about 2000 years ago, a set of significant projects were conducted to develop the social and economic conditions of this region, such as people immigration, land reclamation, establishment of water conservancy system, frontier defence build-up and stationing of troops (Cheng, 2007). The SHC is nearly square-shaped with its remnant city wall as height as 7–8 m and enclosing an area of 5800 m2. Archaeological investigation discovered abundant remains of the activities of ancient people inside and outside the city, including building ruins, pottery, fresco, cereals and tombs (Wang, 1990; Wu, 2008). According to these archaeological findings and historical literature, it is inferred that the SHC was constructed in Tang Dynasty (618–907 CE) as courier station (named “Gongtun”) and then successively used in Yuan Dynasty (1271–1368 CE) and Ming Dynasty (1368–1644 CE, named “Xicheng” and “Xiaoshahe”, respectively), and finally abandoned in the period of late Ming Dynasty and early Qing Dynasty (1644–1911 CE, Wang, 1990; Li, 2003b; Wu, 2008). At present, the SHC is surrounded by cultivated cropland. It is partly covered by a sand belt extending from 2

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Fig. 1. Location of the Hexi Corridor (green shadow) and its surrounding area. White and yellow filled circles indicate the location of the referred moisture records, i.e., Sugan Lake (Chen et al., 2009) and Badain Jaran Desert (Ma and Edmunds, 2006; Gates et al., 2008), respectively. Red and black filled squares indicate the South Heishuiguo ancient city (SHC) and other historical cities in Hexi Corridor. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

northwest to southeast, and the city wall was almost completely buried by the eolian sand.

long with 7 cm internal diameter. The probe was always fully filled with sediments when we pulled it out for sampling, and immediately we hammered a steel tube (with length of 12 cm and diameter of 4 cm) into the probe to collect the sediments for luminescence dating. Then, the two ends of sampling tube were sealed with lightproof tape and the residuum in the probe was gathered for dose rate estimation.

3. Materials and methods 3.1. Sampling

3.2. Luminescence dating

Field work was carried out in April 2018 in the SHC. In order to sample eolian sands covering the city wall for luminescence dating, we conducted drillings in four positions (HS-1, HS-2, HS-3, and HS-4 in Fig. 2a) using a hand drill in the dunes beside the city wall of SHC. The sediments from these drilled cores are mainly composed of yellowish fine sand in the upper part and whitish clayey silt at the bottom (Fig. 2c). A total of 20 samples for luminescence dating were taken from the drilled cores at intervals of 0.1–1.0 m. The drilling probe is 20 cm

Sample preparation and luminescence measurements were carried out in the Luminescence Dating Laboratory of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (Nanjing, China). Due to very dim signals of quartz mineral in young sediments, we select K-feldspar as dosimeter. The materials for equivalent dose (De) measurement were first wet sieved to obtain the coarse grain (CG)

Fig. 2. (a) Aerial image of SHC and the position of drilling cores. (b) Field photograph of SHC showing the eolian sand accumulation alongside the city wall. (c) Lithology of the drilling cores together with 20 luminescence ages. 3

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fraction (100–200 μm), and then the K-feldspar fraction (< 2.58 g/cm3) was extracted for measurement with routine procedure (Long et al., 2017). Luminescence measurements were made on an automated luminescence reader (Risø TL/OSL DA-20) equipped with stimulation units of blue light (470 ± 20 nm) and infrared light (IR; 870 ± 40 nm). Irradiation was carried out using a 90Sr/90Y beta source built into the reader. The IR-stimulated luminescence (IRSL) signal of Kfeldspar was detected through a combination of Schott BG-39 and BG-3 filters in the blue light spectrum between 320 and 450 nm. De measurement of K- feldspar was determined with the post-IR IRSL protocol which determines the IRSL signal observed at an elevated temperature following a prior IR stimulation at 50 °C (Thomsen et al., 2008). We adopted the so called pIRIR150 protocol (Table 1) which has been successfully applied in many kinds of young sediments (Madsen et al., 2011; Reimann and Tsukamoto, 2012; Long et al., 2017), to measure a standard IRSL signal at 50 °C (IRSL50), as well as an IR signal at 150 °C (pIRIR150) after an IRSL50. For both IRSL50 and pIRIR150, the integrated signal was calculated from the first 10 s minus a background from the last 10 s. For saving machine time, we used both standardised growth curve (SGC) method (Roberts and Duller, 2004) and single aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000) to determine the De. Three discs were measured with SAR protocol to build up the SGC of each sample, and then nine discs were used for the determination of natural signal corrected by test dose (i.e., Ln/Tn). In total, we obtained 12 De values for each sample. Dose-recovery and residual dose tests were carried out for four representative samples (one for each of four cores, i.e., NL-1593, 1598, 1601, and 1604). Anomalous fading rate of feldspar luminescence signal is usually quantified by the g-value to assess the signal loss per decade of normalized storage time (Huntley and Lamothe, 2001). It is presumed that pIRIR signal could be used to minimize fading rate to a negligible level. To verify this, the gvalues of these four samples (NL-1593, 1598, 1601, and 1604) were measured for both IRSL50 and pIRIR150 signals using the method suggested by Auclair et al. (2003). For dose rate calculation, the concentration of uranium (U) and thorium (Th) was measured using inductively coupled plasma mass spectrometry (ICP-MS), and potassium (K) concentration was measured using inductively coupled plasma atomic emission spectrometry (ICPAES) at the Chang'an University (Xi'an, China). Considering the semiarid study area, the water content of 5 ± 5% was estimated for all samples. For the internal dose rate calculation of K-feldspar, a K concentration of 12.5 ± 0.5% and a rubidium (Rb) content of 400 μg/g (Huntley and Hancock, 2001) were assumed. The cosmic ray dose rate was estimated for each sample as a function of depth, altitude, and geomagnetic latitude according to Prescott and Hutton (1994). The total dose rates were calculated using the conversion factors of Guérin et al. (2011).

Table 1 pIRIR dating protocol. Step

Treatment

1 2 3 4 5 6 7 8 9

Dose Preheat (180 °C for 60 s) IRSL, 100 s at 50 °C IRSL, 100 s at 150 °C Test dose Preheat (180 °C for 60 s) IRSL, 100 s at 50 °C IRSL, 100 s at 150 °C Return to step 1

Observe

Lx1 Lx2 Tx1 Tx2

different kinds of documents have different types of terminology and written style. So, we translated these documents into a uniform and lucid version. (3) Estimate the historical population in different time windows using the approaches of human demography, and calculate the population of a specific year or period if documents available and reliable. In short, the population calculation processes was always based on document records, estimation, and then modification. For some important time windows that population records are absent, we estimated the population based on the population growth rate, cultivated land area as well as the population of adjacent regions. (4) Verify and calibrate the statistical results using the demographic theories and principles. For instance, the population in a region during a certain period would grow steadily if there were no wars, natural disasters and major population migration at that time. Then, the historical population of the Hexi Corridor as a whole and the three sub-regions (i.e., the drainage basins of Shiyanghe, Heihe and Shulehe rivers) were calculated using the historical population of individual counties. For some periods, because of the limited materials, we only estimated the historical population of the Hexi Corridor as a whole while the population of each sub-regions were absent. The main literature used for the statistics of historical population are listed in the note of Table 3. 4. Results 4.1. Luminescence characteristics and ages An example of decay curves for one aliquot of sample NL-1592 shows that the natural IRSL50 signals are obviously more intensive than that of pIRIR150 signals (Fig. 3). Based on the two luminescence signals, the determined IRSL50 De is slightly smaller compared with the De derived from the corresponding pIRIR150 signals (inset of Fig. 3). Dose recovery tests show that most measured-to-given dose ratios for pIRIR150 signals range from 0.9 to 1.1 after subtracting the residual dose values (Fig. 4a); the mean dose-recovery ratio is 1.03 (n = 12). Most residual doses are smaller than 0.1 Gy (Fig. 4b), approximately equivalent to an age of 30 years. Fading rates of IRSL50 (Fig. 4c) and pIRIR150 (Fig. 4d) signals for these four samples (six aliquots for each) were compared; the average g-values are 2.6%/decade (n = 24) and 0.5%/decade (n = 24), respectively. According to the dating results of all samples (Table 2 and Fig. 2c), except sample NL-1599 which yielded an age of 2.67 ± 0.13 ka,1 other 19 sandy samples have an age range between 0.40 ± 0.03 ka and 0.25 ± 0.02 ka, equivalent to 1590–1790 CE if dating uncertainties are taken into account.

3.3. Historical demography In addition to climatic context, human activity is another factor that could impact the historical desertification. As the indicator of the intensity of human activities, population data is very straight and convictive, especially commonly used in the historical period with low productivity (Li, 2005). In order to obtain quantitative data of human activity intensity, firstly, historical population of individual counties in the Hexi Corridor was reconstructed using following procedures: (1) Collect the history records written officially, archival files, local chronicles, travel notes, census registers, account books and other documents with relation to the historical population. These documents were collected from libraries, achieves, museums of different levels of administrative division (province, city, county, etc.) and then a copy was made only for the parts related to historical population of Hexi Corridor for further analysis. (2) Analyse these documents and interpret the terminology and description concerning historical population. Most of the historical documents were written in ancient Chinese and

4.2. Population history The reconstructed historical population in the Hexi Corridor over the past 2000 years shows a general increasing trend with fluctuations 1 In this paper, luminescence dates are reported in ka (millennia), which is the absolute age relative to the sampling time (the year 2018).

4

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20 samples (Fig. 5), suggesting the suitability of the used SGC method. In addition, the dose recovery results suggested that the measurement protocol is able to accurately measure doses given in the laboratory before heating and optical treatment. The residual doses are negligible compared with the De values of all sediments, thus the residual doses were not subtracted from the measured De values for age calculation. The fading is ubiquitous for most of feldspars when the IRSL signal is measured at low temperature of 50 °C (Huntley and Lamothe, 2001). In contrast, pIRIR signal can be used to minimize fading rate to a negligible level (Reimann and Tsukamoto, 2012). The general fading behaviour for the investigated sandy materials was demonstrated by the fitting of sensitivity corrected luminescence signals over the different time delays for all aliquots (Fig. S1), indicating an obvious fading of IRSL50 signals with a g-value of ~2.5%/decade. Although fading rates (~0.7%/decade) of the pIRIR150 signals apparently suggest anomalous fading in a certain degree in this study, we did not correct the pIRIR150 ages for anomalous fading. This is because the laboratory-fading experiments should not be considered as convincing evidence for loss of trapped charge during storage, especially for such low fading rates. All these laboratory tests indicate that the overall behaviour of the measurement protocol applied to the pIRIR signals from the samples seems to be satisfactory. Although, in the wind-borne sedimentary environment, resetting of

Fig. 3. Typical IRSL50 and pIRIR150 decay curves of K-feldspar fraction and corresponding growth curves (inset) from a representative sample (NL-1592).

Fig. 4. (a) Dose-recovery tests and (b) residual tests on pIRIR150 signal for four samples (three discs were used for every test for each sample). Fading rate (g-value) determination of IRSL50 (c) and pIRIR150 (d) signals for these four samples.

from the West Han Dynasty to the beginning of 21st century. The past population of the Hexi Corridor had not exceeded the level of Western Han Dynasty before the late Ming Dynasty, but surged explosively thereafter. In addition, population history of individual sub-regions, i.e., the drainage basins of Shiyanghe, Heihe and Shulehe rivers, appears to show a same pattern with the whole Hexi Corridor, although the Shulehe River drainage basin has a relatively smaller population (Table 3).

the latent luminescence signal to a sufficient near-zero residual value occurs almost always, the bleaching condition should be investigated for the studied samples as any insufficient resetting could cause completely inaccurate age estimation for such young sediments (Wallinga, 2002). The resulting De distributions for four representative samples (Fig. S2) show very small differences between the aliquots for each sample, suggesting sufficient reset of luminescence signals before deposition. To further test the reliability of the pIRIR150 ages, the fading corrected IRSL50 ages were also compared with them. Although the apparent dates obtained from IRSL signals are obviously underestimated (Fig. 6a) compared with pIRIR signals, the fading corrected IRSL50 ages are in good agreement with the corresponding pIRIR150 ages (Fig. 6b). This line of evidence further supports that the dated samples were well bleached before deposition, since K-feldspar pIRIR signals were bleached more slowly than the IRSL signals. The consistency of the K-feldspar ages derived from both signals provides a cross-check of the reliability of these dates.

5. Discussion 5.1. Evaluation of the chronology of dune accumulation First, the validity of the SGC based Des was tested although this method has been applied to many kinds of sediments (Lai, 2006; Long et al., 2010; Yang et al., 2011b). The straightest approach is to compare the SGC Des with the De values derived from SAR protocol. The De values obtained from two methods are within 10% of unity for most of 5

Palaeogeography, Palaeoclimatology, Palaeoecology 545 (2020) 109634

0.38 0.38 0.39 0.42 0.40 0.36 0.39 2.74 0.27 0.30 0.31 0.30 0.33 0.31 0.31 0.33 0.34 0.35 0.30 0.31

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.24 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.37 0.36 0.36 0.39 0.37 0.34 0.40 2.67 0.25 0.26 0.27 0.31 0.32 0.30 0.30 0.35 0.33 0.34 0.33 0.34

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.13 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.02

Table 3 The reconstructed historical population in the Hexi Corridor (thousands of people).

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.11 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.03 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.15 0.15 0.15 0.15 0.16 0.15 0.15 0.16 0.16 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.16 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 3.05 3.00 3.06 2.97 3.14 2.96 3.10 3.37 3.10 3.06 2.96 3.01 2.84 3.11 2.99 2.87 2.93 2.84 3.14 3.54 0.02 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± HS-4

HS-3

HS-2

NL-1592 NL-1593 NL-1594 NL-1595 NL-1596 NL-1507 NL-1598 NL-1599 NL-1600 NL-1601 NL-1602 NL-1603 NL-1604 NL-1605 NL-1606 NL-1607 NL-1608 NL-1609 NL-1610 NL-1611 HS-1

1.48 1.48 1.49 1.52 1.55 1.41 1.44 1.59 1.51 1.52 1.44 1.52 1.43 1.58 1.51 1.49 1.50 1.42 1.60 1.73

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.02 0.03 0.04 0.04 0.01 0.03 0.02 0.06 0.04 0.04 0.04 0.02 0.03 0.02 0.01 0.04 0.04 0.03 0.01

6.03 5.84 6.39 4.63 7.04 5.95 7.16 8.31 6.23 6.07 6.14 5.42 4.80 6.65 6.13 5.22 5.75 5.60 6.89 8.82

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.04 0.34 0.19 0.22 0.27 0.08 0.15 0.19 0.13 0.14 0.29 0.05 0.11 0.21 0.15 0.19 0.16 0.08 0.40

1.38 1.34 1.51 1.63 1.62 1.31 1.53 1.84 1.39 1.38 1.37 1.24 1.18 1.29 1.33 1.19 1.33 1.32 1.50 2.14

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.03 0.10 0.09 0.06 0.04 0.03 0.08 0.05 0.04 0.07 0.05 0.03 0.04 0.07 0.01 0.06 0.05 0.04 0.08

0.24 0.21 0.19 0.16 0.15 0.24 0.21 0.19 0.24 0.21 0.19 0.24 0.21 0.19 0.16 0.15 0.14 0.13 0.13 0.12

Total dose rate (Gy/ka) Cosmic dose rate (Gy/ka) U (ppm) Th (ppm) K (%) Sample code Drilling cores

Table 2 Luminescence dating results.

Note: For each sample, 12 aliquots were measured for De calculation. The water content was estimated at 5 ± 5% for all samples of the current study.

0.31 0.31 0.32 0.35 0.33 0.30 0.32 2.19 0.23 0.25 0.26 0.25 0.27 0.26 0.26 0.27 0.28 0.29 0.25 0.26 1.12 1.09 1.11 1.17 1.16 1.00 1.24 9.02 0.78 0.80 0.81 0.93 0.92 0.94 0.90 0.99 0.97 0.95 1.03 1.21 0.94 0.94 0.97 1.03 1.05 0.89 0.99 7.38 0.72 0.76 0.76 0.76 0.77 0.79 0.79 0.78 0.82 0.81 0.79 0.92

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

pIRIR De (Gy) IRSL De (Gy)

0.02 0.04 0.05 0.04 0.04 0.02 0.08 0.12 0.03 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.04 0.03 0.08 0.04

IRSL age (ka)

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.11 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01

Corrected IRSL age (ka)

pIRIR age (ka)

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Dynasty (time)

Hexi corridor

Shiyanghe drainage basin

Heihe drainage basin

Shulehe drainage basin

West Han (135 BCE) West Han (2 CE) East Han (140 CE) Wei (220–232 CE) Xi Jin (280 CE) North Wei (532–534 CE) Sui (609 CE) Tang (639 CE) Tang (742 CE) Tang (752 CE) Yuan (1290 CE) Ming (1368–1399 CE) Ming (1522–1566 CE) Qing (1776 CE) Qing (1820 CE) Qing (1851 CE) Qing (1880 CE) Qing (1910 CE) 1928 CE 1936 CE 1940 CE 1945 CE 1950 CE 1966 CE 1980 CE 2004 CE

70–80 373.3 249.3 50 198.8 16 182.4 159.6 275.5 303.7 90–120 129 306 2640 2945 3181 806 1244 1002.8 1130.3 1184 1127 1952.6 2732.4 3580.6 4863.9

– 128 55.4 – 47.5 – 83.4 89.1 177.1 205.3 30–50 54.8 137 1348 1504 1625 458 716 512.7 551.6 519.2 508.5 1000.2 1394.9 1751.8 2417.8

– 151 115.1 – 86.4 – 43.6 33.7 46.1 54.6 50–60 64.2 159 1223 1363 1472 312 483 437.5 507.1 586.8 534.4 835.7 1037.3 1459.7 1964.2

– 94.3 78.8 – 64.9 – 55.4 36.8 52.3 43.8 10 10 10 69 78 84 36 45 52.6 71.6 78 84.1 116.7 300.2 369.1 481.9

Note: the unit of the population is thousands of people; “–” means no data at that time. Main source literatures used for the statistics of historical population: (1)History of the Former Han Dynasty, Records of Geography (Han Shu Di Li Zhi), compiled by Gu Ban in 100 CE. (2)History of the Later Han Dynasty, Records of the Prefectures and States (Hou Han Shu Jun Guo Zhi), written by Ye Fan in 445 CE. (3)History of the Northern Wei Dynasty, Records of Topography (Jin Shu Di Xing Zhi), compiled by Shou Wei in 554 CE. (4)History of the Jin Dynasty, Records of Geography (Jin Shu Di Li Zhi), compiled by Xuanling Fang et al. in 648 CE. (5)History of the Sui Dynasty, Records of Geography (Sui Shu Di Li Zhi), compiled by Defen Linghu et al. in 656 CE. (6)Tong Dian, an encyclopedia of past economy, politics, laws, annals, military affairs, national affairs etc. before the middle Tang dynasty, compiled by You Du in 801 CE. (7) Old History of the Tang Dynasty, Records of Geography (Jiu Tang Shu Di Li Zhi), compiled by Xu Liu in 945 CE. (8)New History of the Tang Dynasty, Records of Geography (Xin Tang Shu Di Li Zhi), compiled by Xiu Ouyang et al. in 1060 CE. (9)History of the Yuan Dynasty, Records of Geography (Yuan Shi Di Li Zhi), compiled by Lian Song et al. in 1370 CE. (10)General Chronicle of Shaanxi Province (Shan Xi Tong Zhi), compiled by Tingrui Zhao et al. in 1593 CE. (11)Chronicle of Suzhou Fort, (Su Zhen Zhi), compiled by Yingkui Li in 1610 CE. (12)Revised Chronicle of Ganzhou Fort (Chong Kan Gan Zhen Zhi), compiled by Maochun Yang in 1657 CE. (13)Qin Bian Ji Lue, a geography book about the northwest China of the Qing Dynasty base on on-the-spot investigation, compiled by Fen Liang in 1694 CE. (14)Revised Chronicle of Suzhou Prefecture (Chong Xiu Su Zhou Xin Zhi), compiled by Wenwei Huang in 1737 CE. (15)Chronicle of Ganzhou Prefecture (Gan Zhou Fu Zhi), compiled by Gengqi Zhong in 1779 CE. (16)Chronicle of Yongchang County (Yong Chang Xian Zhi), compiled by Dengying Li et al. in 1785 CE. (17)Chronicle of Yongchang County (Yong Chang Xian Zhi), compiled by Jihan Nan in 1816 CE. (18)Chronicle of Zhenfan County (Zhen Fan Xian Zhi), compiled by Xiexiu Xu et al. in 1825 CE. (19)Chronicle of Dunhuang County (Dun Huang Xian Zhi), compiled by Cheng Zeng in 1831 CE. (20)Chronicle of Shandan County (Shan Dan Xian Zhi), compiled by Jing Huang et al. in 1831 CE. (21)Revised General Chronicle of the Qing Dynasty during the Jiaqing Reign Period (Jia Qing Chong Xiu Da Qing Yi Tong Zhi), compiled by Xien Fan in 1842 CE. (22)Continued Chronicle of Yongchang County (Xu Xiu Yong Chang Xian Zhi), compiled by Quan Yan et al. in 1918 CE. (23)Chronicle of Gaotai County (Gao Tai Xian Zhi), compiled by Jiarui Xu in 1921 CE. (24)Draft Chronicle of Gansu Province (Gan Su Tong Zhi Gao), compiled by Yufen Liu et al. in 1936 CE. (25)Revised Chronicle of Gulang County (Chong Xiu Gu Lang Xian Zhi), compiled by Peiqing Li in 1939 CE. (26)Population reports of the Counties, Towns and Villages in 6

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desertification from the late 16th century to the late 18th century, which can be supported by some historical documents-based studies (Jing, 2000; Li, 2003a; Wang et al., 2003). On the other hand, given that sands inside or in the vicinity of a city have been cleaned up when people still lived in the city, the appearance of sand deposition likely indicates that the city was largely abandoned (Cheng, 2007). Thus, we infer that the SHC was abandoned at around 17th century, which might have been caused by severe desertification around the city during the period of late 16th century to the late 18th century. Previous studies based on archaeological findings and historical literature also suggested that the SHC was ultimately abandoned within the time period between the late Ming Dynasty (1368–1644 CE) and the early Qing Dynasty (1644–1911 CE, Wang, 1990; Li, 2003b). In arid and semi-arid regions of northern China, rich sand sources and frequent wind are the basis for the occurrence of desertification, while climatic variations and human activities are also considered to be the main influencing factors for the formation and expansion of desertification (Wang, 2011). For example, Huang et al. (2009) showed that two phases of serious desertification occurred during the last two millennia in the Mu Us Sandy Land of northern China, the former phase of desertification is closely related to abrupt climate change in the mid8th century while human activities contributed to the development of the later one during the 16th–17th century. In the Horqin Sandy Land to the northeast, Yang et al. (2017) recognized four phases of desertification during the last two millennia, and their temporal influencing factors in climatic change and human activity.

Gansu Province, compiled by the Government of Gansu Province in 1940 CE. (27)New Chronicle of Zhangye County (Xin Xiu Zhang Ye Xian Zhi), compiled by Cehou Bai et al. in 1949 CE. (28)Compilation of the Population Statistics of Gansu Province during 1949–1987, compiled by the Statistical Bureau of Gansu Province and Provincial Public Security Department of Gansu Province in 1988 CE. (29)Gansu Yearbook 2005, compiled by the Editorial Committee of Gansu Yearbook in 2005 CE.

5.3. Climate context of the desertification In order to understand the impact of climatic changes on the desertification during the period of late 16th century and late 18th century, it is necessary to examine the climate variations history over a relatively long time interval. Although substantial differences existed in different regions, Northern Hemisphere climate evolution over the last millennium can roughly be divided into three major episodes, i.e., “Medieval Warm Period” (MWP), “Little Ice Age” (LIA), and post-industrial warming (Fig. 7a; Chen et al., 2010), with generally colder conditions during the LIA than that during the MWP (Mann et al., 2009; Ljungqvist, 2010; Christiansen and Ljungqvist, 2012). The temperature variations in different climatic regions of China are broadly in phase

Fig. 5. Comparison of Des derived from SGC and SAR methods for all 20 samples.

5.2. Desertification and its relationship with the abandonment of the SHC According to the luminescence dating results, eolian sand accumulation alongside the city wall of SHC date to the time period between 1590 CE and 1790 CE, mainly around 1700 CE when sand dunes of more than 6 m thickness formed. This may indicate a phase of

Fig. 6. Comparison of the pIRIR150 ages with the apparent IR50 ages (a) and the fading corrected IR50 ages (b). 7

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Fig. 7. (a) Temperature anomaly of northern hemisphere (Moberg et al., 2005). (b) Temperature anomaly of China (Ge et al., 2013). (c) Lake salinity time series from the Sugan Lake (Chen et al., 2009). (d) Water recharge reconstruction for the Badain Jaran Desert (Ma and Edmunds, 2006; Gates et al., 2008). (e) Historical population reconstruction of Hexi Corridor in this study. (f) Historical population reconstruction of Heihe River drainage basin in this study. (g) Age cluster of sand accumulation of SHC, and its range indicated by the yellow band. (h) Sketching of Chinese dynasties. The shadow bands show three age ranges, i.e., 1000–1400 CE, 1400–1850 CE, and after 1850 CE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

effective moisture and rainfall changes of the northeastern Tibetan Plateau including Hexi Corridor (Chen et al., 2010). According to these reconstruction results, the climate conditions of the last millennium in Hexi Corridor can generally be divided into three stages: warm-dry climate between 1000 and 1400 CE, and relatively cold and moist climate between 1400 and 1850 CE, followed by a warming and drying climate after 1850 CE (Fig. 7). In this study, a phase of desertification is dated to the late 16th century and late 18th century, which exactly lies in the generally cold but moist stage between 1400 and 1850 CE (Fig. 7), despite of obvious fluctuations in moisture at centennial timescale. In addition, several review studies on the moisture changes also pointed out that the arid northern China was wet or moderately wet during the LIA (Chen et al., 2015b; Chen et al., 2019a; Chen et al., 2019b). It is well known that in a natural state, desertification is mainly triggered by arid climate (Zhu and Chen, 1994), though low

with those of the Northern Hemisphere during the last two millennia (Yang et al., 2002; Ge et al., 2003, 2013; Fig. 7b). However, the spatial pattern of moisture or precipitation was probably asynchronous or diverse in different regions, because of the heterogeneous nature of hydroclimatic variations under different atmosphere circulation patterns (Chen et al., 2019a). For example, in mid-altitude Asia north of 30° N, monsoonal northern China was generally wetter, while arid central Asia was generally drier during the MWP than in the LIA (Chen et al., 2015b). With regard to precipitation variation, we selected two reconstructed records, covering the last millennium, with explicit climatic indication and relatively high resolution, i.e., the salinity reconstruction of Sugan Lake (Fig. 7c; Chen et al., 2009) and the groundwater recharge rate of Badain Jaran Desert (Fig. 7d; Ma and Edmunds, 2006; Gates et al., 2008). Both records have been considered to indicate 8

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Fig. 8. A conceptual model of human-environment interaction proposed based on the case study of SHC.

temperature may also contribute to the decrease of vegetation cover and then increase the erosion of land surface to some extent. Therefore, it is possible that there is no direct causality between climatic context and the desertification between late 16th century and late 18th century.

century, corresponding well with the desertification stage indicated by the sand accumulation of SHC during the same period (Fig. 7g). A number of other studies have also attributed the desertification in the Ming and Qing Dynasty of Hexi Corridor to irrational or excessive human activities (Jing, 2000; Li, 2003a; Wang et al., 2003). Chen et al. (1999) also suggested that during the past 300 years, significant increase in population and corresponding farmland acreage and use for irrigation water has caused the shrinkage and disintegration of a megalake in the Shiyanghe River drainage basin of the Hexi Corridor. Accordingly, we propose that the desertification during the period of late 16th century and late 18th century identified in this study may be related to the significantly enhanced human activities. In fact, human impacts on environment of Hexi Corridor as early as Bronze Age has already been recognized by Zhou et al. (2012), who suggested that agricultural activity during the Bronze Age caused an increase in farmland and a decrease in grassland. Li et al. (2011) also reported a correlation between reduced vegetation cover and the deforestation resulted from early smelting activity in the Hexi Corridor. Recently, compilation of lake records nearby the Hexi Corridor by Mischke et al. (2019) revealed that man-made impact on surface waters must already have reached an order of magnitude during the Han Dynasty through the water withdrawal from the tributaries of the lakes for irrigation farming. In contrast to these macroscopic analysis of human impacts on the environment of Hexi Corridor, our study provides a realistic scenario of human-environment interaction at a local scale but with regional significance, which could be illustrated intuitively by a conceptual model shown in Fig. 8. Based on this conceptual model, we may come to a preliminary conclusion that the SHC was forced to be abandoned at around 17th century by a phase of desertification, which was in turn caused by extensive human activities during the period of late Ming and early Qing Dynasty. In fact, the influence of climatic condition and human activity on the desertification is very complex, and it is difficult to distinguish their relative roles unequivocally. Considering the continuing reinforcement of human activity since historical period, it is widely accepted that the contribution of human impacts on desertification have enhanced significantly during the last three centuries (Jing, 2000; Li, 2003a; Wang et al., 2003; Cheng, 2007). Nevertheless, for the sustainable development in economy and society of the Hexi Corridor with fragile ecosystem, it is necessary to keep the population as well as the human activity intensity within a reasonable extent to match the environmental bearing capacity of this region.

5.4. Human impacts on the desertification Since the Hexi Corridor was taken over back from the northern minority (mainly the Huns) by the central government of Han Dynasty at about 2000 years ago, human activities in this region have been extensive and continuous up to the present (Gao, 2007). A number of significant approaches were introduced to promote the socioeconomic development by successive dynasties, which could be summarized as following aspects: (1) People immigration and land reclamation are the most important measures to develop the agriculture. Extensive agricultural exploitation was conducted at three phases, i.e., Han, early Tang and Ming-Qing Dynasty, which have changed the natural landscape of Hexi Corridor dramatically (Gao, 2007; Cheng, 2007). (2) Construction of water conservancy and application of advanced farming technique were carried out by government to increase agricultural efficiency, and some of the irrigation channel systems are still in use nowadays (Cheng, 2007; Shi, 2014). (3) Frontier defence and stationing of troops were built up. A large number of defensive facilities such as barriers, forts, beacon towers and the Great Wall were set up to keep the Hexi Corridor from the aggression of northern nomads. Accompanied by the construction of frontier defence, masses of troops were stationed in the Hexi Corridor. For example, there were more than one hundred thousand military forces in Hexi Corridor in the Ming Dynasty (Li, 2003a, 2003b; Cheng, 2007; Shi, 2014). However, it is difficult to quantify human activity precisely. Previous studies have suggested that population data is a direct and convincing proxy of the intensity of human activities during the historical period (Li, 2005; Cheng, 2007). Furthermore, compared with other human activity indices such as agricultural acreage or rate of the water resource utilization, historical population is relatively easy to recover from historical documents in despite of general discontinuity. In this study, the reconstructed two population datasets from Hexi Corridor and the Heihe River drainage basin, where the SHC is located (Fig. 7e and f), appear to show similar tendency with the population development of China (Ge, 1991). In addition, we found that the population of the two regions remained small prior to the late Ming Dynasty, but exploded in the period of late Ming Dynasty and middle Qing Dynasty. This sudden rise of population since 1600 CE was also observed by Song (1997) in the adjacent Shiyanghe River drainage basin. This suggests that the human activity intensity took a leap by roughly an order of magnitude in the period of late 16th century and late 18th

6. Conclusions (i) Accumulation time of eolian sands covering the city wall of the South Heishuiguo ancient city dated to the interval between 9

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1590 CE and 1790 CE, confirming a phase of desertification in that time in Hexi Corridor. (ii) This desertification is likely attributed to extensive human activities during the period of the late Ming and early Qing Dynasties, and subsequently resulted in the abandonment of South Heishuiguo ancient city. (iii) This study provides a realistic scenario of historical human-environment interaction in the Hexi Corridor of arid-semiarid northern China.

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