Ancient walled settlements are considered important symbols of human civilization (Redfield and Singer Reference Redfield and Singer1954; Sun Reference Sun1990; Wang Reference Wang1995; Yan Reference Yan1999; Qian Reference Qian2000; Xu Reference Xu2009; Gates Reference Gates2011). The research of ancient walled settlements has not only revealed information about military, religion, and the economy in the past (Zuiderhoek Reference Zuiderhoek2016), but also provided valuable knowledge of human–environmental interaction in human history (Dong et al. Reference Dong, Liu, Yang, Yang, Zhou, Wang, Ren and Chen2016a). The ancient walled settlements in China are of great scientific value as China is the locus of the first empire to be established in East Asia, which has undergone gradual sociopolitical transitions from conflicting territorial states to a prosperous empire. The investigation of the ancient walled settlements in China may help understand the social evolution, rising social complexity, and the emergence of early states in East Asia.
The earliest ancient walled settlement in China appeared in the north-central region during the late Yangshao Period (3500–3000 BC) (Zhang et al. Reference Zhang, Zhao and Qiao1999). During the Longshan Period (2600–2000 BC), numerous walled settlements emerged along the Yellow River and Yangtze River valleys (Qian Reference Qian2000), which indicate a dramatic transformation of social structure, possibly brought by a relatively cold and dry climate (Marcott et al. Reference Marcott, Shakun, Clark and Mix2013; Wanger et al. Reference Wanger, Tarasov, Hoisner, Fleck, Ehrich, Chen and Leipe2013; Chen et al. Reference Chen, Xu, Chen, Birks, Liu, Zhang, Jin, An, Telford, Cao, Wang, Zhang, Selvaraj, Lu, Li, Zheng, Wang, Zhou, Dong, Zhang, Huang, Bloemendal and Rao2015a; Hosner et al. Reference Hosner, Wagner, Tarasov, Chen and Leipe2016). This transformation preceded the emergence of ancient civilizations in the Xia and Shang Dynasties in the second Millenium BC. Later, the Zhou Dynasty (Western Zhou and Eastern Zhou) (1046–221 BC) witnessed a further significant change in politics, society, and scholarship. Also, conflicts became more frequent, while more walled settlements were built for the protection of inhabitants (Qian Reference Qian2000). Despite the considerable academic value of ancient walled settlements in China, they remain insufficiently explored, especially those in the Hexi Corridor in northwestern China.
The ancient Silk Road played a critical role in the transcontinental cultural and technological exchanges that took place from the second millennium BC. In Chinese history, the Hexi Corridor was a crucial military node and the key section of the ancient Silk Road connecting China and Central Asia (Dong et al. Reference Dong, Yang, Liu, Li, Cui, Wang, Chen, Dodson and Chen2018). Most ancient walled settlements in the Hexi Corridor are dated between Han (202 BC–AD 220) and Qing (AD 1644–1911) Dynasties, which are thousands of years later than those in the Yellow and Yangtze River valleys. The notable exception is the Sanjiao walled settlement in Jinchang, which has been dated between 1110–430 cal BC. Sanjiao is generally considered as “the earliest walled settlement” in the Hexi Corridor (Pu and Pang Reference Pu and Pang1990; Li Reference Li1997).
Nevertheless, there is uncertainty around the dated age of Sanjiao. So far, only two charcoal samples from the cultural units at Sanjiao have been collected for dating, with the liquid scintillation counting (LSC) dating method employed (IACASS 1981; Table 1). Although those samples give the approximate time range from the foundation to abandonment or destruction of Sanjiao, they may not be sufficient to determine the precise age of the initial construction of the site (Shao Reference Shao2012). Also, the age of the walls rather than that of potentially later contexts within those walls (i.e., cultural units) could give a more accurate estimate about the age of the settlement. Hence, there is a need to reassess the chronology of Sanjiao (Schiffer Reference Schiffer1986; Dong et al. Reference Dong, Wang, Ren, Matuzeviciute, Wang, Ren and Chen2014). Furthermore, other issues pertinent to Sanjiao such as how the inhabitants of Sanjiao adapted to local environmental conditions and why Sanjiao was built remain unexplored. Resolving the above issues is vital in supplementing the history of the Hexi Corridor and tracking the historical development of the ancient Silk Road. In this study, we employ accelerator mass spectrometry radiocarbon (AMS 14C) dating to remeasure samples from Sanjiao and reassess its chronology. Also, we explore the subsistence strategies of the site inhabitants according to the identification of charred plant seeds and charcoal. Finally, the purpose of building Sanjiao itself is explored.
Sanjiao is located near the modern city of Jinchang, in the east-central section of the Hexi Corridor, surrounded by the Qilian Mountains and Badain Jaran Desert (Figure 1). The altitude of Jinchang decreases gradually from the southwest to the northeast, with a mean altitude of 1500 m asl. The climate in the region is arid and windy, with a mean annual temperature of 9.5°C and mean annual precipitation of 122.3 mm (Huang et al. Reference Huang, Han and Liu2018). The Dongda and Xida Rivers flow from the Qilan Mountains through this region.
Sanjiao was first discovered and excavated in the 1980s. Pu and Pang (Reference Pu and Pang1990) considered Sanjiao to be an ancient city. However, in comparison to other ancient settlements in central China, Sanjiao appears to have been less developed and somewhat smaller, which is better to be considered as a walled settlement. Sanjiao is oriented toward the south, with an irregular triangle-shape. The site measures 154 m from north to south and 132 m from east to west. The walls of Sanjiao are made of piled earth and are 6–8 m thick at their base (Figure 2).
Sanjiao is classified as part of the Late Bronze Age Shajing Culture (1000–400 BC) based on the unique ceramics unearthed there (Pu and Pang Reference Pu and Pang1990). This places the site later than the Machang type of the Majiayao Culture (2300–2000 BC), Qijia Culture (2300–1500 BC), and Siba Culture (1900–1400 BC) in the Hexi Corridor.
MATERIALS AND METHODS
Fieldwork was conducted at Sanjiao in 2014 and 2017. During the 2014 fieldwork, we collected 10 soil samples from different cultural contexts inside the Sanjiao walled settlement. Three were obtained from ash pits and seven were derived from different cultural layers as shown in Figure 2A. During the 2017 fieldwork, we collected three soil samples from the base of the walls in the northwestern part of Sanjiao. One of them was taken from the surface of the wall (Figure 2D), and two of them were taken inside the wall (Figure 2C). All of the soil samples were processed by the flotation method. The soil was washed over in a bucket with a #80 mesh sieve (aperture size of 0.2 mm) to gather any carbonized remains. The carbonized remains were dried in the shade and then sorted. Charred plant seeds were identified by Professor Jingang Yang at the Paleoethnobotany Laboratory, Institute of Archaeology, Chinese Academy of Social Science. Charcoal was identified according to its cellular structure under a reflected light microscope in the MOE Key Laboratory of Western China’s Environmental Systems at the Lanzhou University.
Charcoal from the walls as well as charred barley seeds from three different contexts (including an ash pit and two cultural layers) were dated. The two types of material were dated to cross check and validate the resulting ages. All samples were prepared with the standard pretreatment (acid-alkali-acid) in the MOE Key Laboratory of Western China’s Environmental Systems at the Lanzhou University. The carbonated samples were treated in 2 mol/L hydrochloric acid (HCl) at 60ºC. The HCl solution was refreshed every two hours until no effervescence was observed. Then, samples were rinsed with distilled water, soaked in 0.5 mol/L NaOH at 60ºC. The NaOH solution was also refreshed every two hours until the liquid became clear. Third, those samples were treated under 1 mol/L hydrochloric acid (HCl) at 60ºC for 2 hr and rinsed again with distilled water. Lastly, the samples were dried for 48 hr for AMS 14C dating performed at Peking University. Ages were calibrated using the OxCal v4.3.2 online program (Bronk and Lee Reference Bronk and Lee2013) and the IntCal13 calibration curve (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Irka, Hatte, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and Van der Plicht2013). Also, phase modeling in the OxCal v4.3.2 program was employed to give a more robust estimate of Sanjiao’s initial construction date. All ages are reported as “cal. AD/BC.”
Archaeologically, the ages the walls of Sanjiao could give a better chronology of Sanjiao’s initial construction date. The three AMS 14C ages derived from the charcoal from the walls are relatively close and fall into the period 888–551 cal BC (Figure 3; Table 1), which coincides with the Western Zhou (1046–771 BC) and spring and autumn periods (770–476 BC). The result of the phase modeling further shows that the walls of Sanjiao were built in the period 855–797 cal BC, with the median age of 828 ± 92 cal BC (Figure 4). The 14C dates from the contexts inside Sanjiao more likely reflect the length of occupation. For the three AMS 14C dates derived from charred barley seeds in the Shajing contexts within Sanjiao, one in the ash pit is dated 380–206 cal BC. The two others in cultural contexts are dated at 358–116 and 384–204 cal BC, respectively (Figure 3; Table 1). Our AMS dates from the charred barley seeds fall between the mid-late Warring States Period (475–221 BC) and early Qin-Han Dynasties (221 BC–AD 9), indicating Sanjiao was inhabited during the mid-late Warring States and early Qin-Han Dynasties in China.
Through the flotation of soil samples from different contexts inside Sanjiao, 469 charred plant seeds of 17 taxa were identified (Figure 5; Table 2). Among these charred seeds, the crop seeds including broomcorn millet, barley, and foxtail millet are found in the largest proportion (Figure 5; Table 2). Charred grass seeds of 14 taxa were identified, including Squarrose Agriophyllum (Agriophyllum squarrosum (L.) Moq.), Lambsquarters (Chenopodium album L.), Common Russian Thistle (Salsola collina Pall.), Salsola ruthenica (Salsola ruthenica lljin), Green Bristlegrass (Setaria viridis (L.) Beauv.), Barnyardgrass (Echinochloa crus galli (L.) Beauv.), Garden Sorrel (Rumex acetosa Linn.), Dockleaved Knotweed (Polygonum lapathifolium L.), Sphaerophysa salsula, Alhagi sparsifolia, Nitraria sibirica Pall., Chinese Iris (Iris lacteal pall. var. chinensis Koidz), Vittaria flexuosa, and Carolina Geranium (Geranium carolinanum L.) (Figure 5; Table 2). Additionally, a total of 894 pieces of charcoal of seven taxa were identified, including Tamarix chinensis, Tamarix (Tamarix L.), willow (Salix L.), spruce (Picea), sea buckthorn (Hippophae), Poaceae, and birch (Betulaceae L.) (Figure 6; Table 3).
Sanjiao is the Earliest Walled Settlement in the Hexi Corridor
Previous studies, which were based on conventional 14C dates from Sanjiao and the nearby Hamodun and Xigang tombs, stated that Sanjiao was built during the Shajing Period (1000–400 BC; Table 1; Pu and Pang Reference Pu and Pang1990; Xie Reference Xie2002). Yet, those 14C dates may be inaccurate due to the “old wood effect” (Dean Reference Dean1978; Schiffer Reference Schiffer1986; Dong et al. Reference Dong, Wang, Ren, Matuzeviciute, Wang, Ren and Chen2014). The 14C dates obtained from charcoal or dead woods unearthed from archaeological sites may be much older than the actual age of those sites, as the dated material could have originated from long-lived trees that grew decades or centuries before their utilization by settlers (Dong et al. Reference Dong, Wang, Ren, Matuzeviciute, Wang, Ren and Chen2014). Additionally, the dates from Sanjiao were all measured through LSC. Compared to LSC, the advantage of AMS is that only tiny amount of carbon-containing material is required for dating, which decreases the chance of mixing carbon from different sources (Lanting and van der Plicht Reference Lanting and van der Plicht2015).
The AMS 14C dates of our charcoal obtained from the walls of Sanjiao match very well. Also, they correspond with the previous LSC 14C dates of charcoal obtained from the cultural contexts of Sanjiao (Figure 3), suggesting a minimal “old wood effect.” Their good match may be related to the absence of old trees or forests around the site. Taking northeastern Tibetan Plateau as an example, the farmers of the Majiayao Culture in the Loess Plateau started to settle in that area during the mid-late Holocene (Chen et al. Reference Chen, Dong, Zhang, Liu, Jia, An, Ma, Xie, Barton, Ren, Zhao, Wu and Jones2015b), a time when the forest was abundant according to the tree pollen record of the Qinghai Lake (Shen et al. Reference Shen, Liu, Wang and Matsumoto2005). At different Majiayao sites, there is a considerable disparity of 14C dates between charcoal and charred seeds, even though those materials are obtained from the same cultural contexts. This could be attributable to the utilization of old trees in those sites, subjecting the dating materials to the “old wood effect” (Dong et al. Reference Dong, Wang, Ren, Matuzeviciute, Wang, Ren and Chen2014).
The climate became cold and dry during 4200–2800 BP (Mischke and Zhang Reference Mischke and Zhang2010), and the reduction of tree pollen suggests that forest was gradually replaced by the shrubs and herbs (Shen et al. Reference Shen, Liu, Wang and Matsumoto2005). This coincides with the establishment of the agro-pastoral economy and the associated expansion of grazing on the northeast Tibetan Plateau (Chen et al. Reference Chen, Dong, Zhang, Liu, Jia, An, Ma, Xie, Barton, Ren, Zhao, Wu and Jones2015b; Dong et al. Reference Dong, Ren, Jia, Liu, Dong, Li, Wang, Xiao and Chen2016b).
Due to continuous cultural exchange across Eurasia during the Bronze Age, in the Hexi Corridor, the subsistence strategies were changed from millet agriculture to the combination of agro-pastoral activities and barley agriculture. Bronze-smelting activities also started to take place. This led to deforestation and the desertification in the Hexi Corridor (Li et al. Reference Li, Sun, Dodson, Zhao and Zhou2011; Zhou et al. Reference Zhou, Li, Dodson, Zhang, Atahan, Sun and Yang2012), and only drought-tolerant shrubs were found there in the Bronze Age and the historical period (Liu et al. Reference Liu, Yang, Shi, Storozum and Dong2018). Also, the agro-pastoral economy facilitated human mobility and hence, fewer permanent structures were built and fewer trees were consumed. The settlement at Sanjiao was primarily composed of portable yurts (Pu and Pang Reference Pu and Pang1990). Those portable yurts together with the piled-earth walls at Sanjiao may further evidence the dominance of the agro-pastoral economy during the time (Pu and Pang Reference Pu and Pang1990). Our AMS dates from charcoal and the previous LSC dates support the hypothesis that settlement was built during the Shajing Period (1000–400 BC), as indicated by our phase modeling suggesting that the walls of Sanjiao were built in the period of 855–797 cal BC, with the median age of 828 ± 92 cal BC.
Based on the historical records such as Shiji (The Records of the Scribe) and Hanshu (The Book of Han), the Hexi Corridor was conquered and controlled by various nomadic regimes, including Rouzhi,Wusun, and Xiongnu, before it was under the rule of the central government of China during the reign of Emperor Hanwu (141–87 BC). After the victory over Xiongnu, the Han Dynasty government set up four counties in the Hexi Corridor. In addition, several walled settlements were built there when the Silk Road was established. According to dozens of 14C dates from multiple ancient cities in eastern Qinghai (Dong et al. Reference Dong, Liu, Yang, Yang, Zhou, Wang, Ren and Chen2016a) and archaeological investigation in ancient Juyan Oasis (Hu and Li Reference Hu and Li2014), there are no walled settlements built earlier than Sanjiao in the Hexi Corridor. Based on these lines of evidence, Sanjiao could be seen as the earliest walled settlement in the Hexi Corridor.
Why Was the Sanjiao Walled Settlement Built?
Climate-induced shortage of subsistence resource could drive social instability or even conflicts among regions (Lee et al. Reference Lee, Zhang, Pei, Jia and Yue2017; Lee Reference Lee2018). This might eventually induce the need for walled settlements for the protection of settlers (Wu Reference Wu2002; Qiu and Yao Reference Qiu and Yao2003). The construction of walled settlements in northern China is coincident with frequent conflicts in the region (Qian Reference Qian2000; Qiu and Yao Reference Qiu and Yao2003). It is worth noting that the period of widespread construction of walled settlements in China corresponds with a cold and dry climate in East Asia (Cao Reference Cao1996; Ma Reference Ma1998; Qian Reference Qian2000; Marcott et al. Reference Marcott, Shakun, Clark and Mix2013; Chen et al. Reference Chen, Xu, Chen, Birks, Liu, Zhang, Jin, An, Telford, Cao, Wang, Zhang, Selvaraj, Lu, Li, Zheng, Wang, Zhou, Dong, Zhang, Huang, Bloemendal and Rao2015a), suggesting climate change to be an essential factor in facilitating the construction of walled settlements. A cold and dry climate lasting for decades or centuries might lead to the shrinkage of agricultural production, causing frequent conflict among different tribes/states and even the transformation and alternation of ancient cultures (Dong et al. Reference Dong, Liu and Chen2017b). Such a phenomenon is found all over the world, for example, the collapse of the Akkadian Empire in Mesopotamia (2334–2154 BC; Weiss et al. Reference Weiss, Courty, Wetterstrom, Guichard, Senior, Meadow and Curnow1993), the de-urbanization of Harappa Culture in Indus Valley (2600–1500 BC; Staubwasser et al. Reference Staubwasser, Sirocko, Grootes and Segl2003), and the decline of Maya Culture in Mesoamerica (AD 400–900; Haug et al. Reference Haug, Günther, Peterson, Sigman, Hughen and Aeschlimann2003; Medina-Elizalde and Rohling Reference Medina-Elizalde and Rohling2012).
The Hexi Corridor, located at the margin of the monsoon region, is a transitional zone between farming and pastoral activities. The climate there is much more arid than that in the east of China. This makes local precipitation and water availability critically important to people’s livelihoods and societal development in the Hexi Corridor. Increasing aridity during 4000–3500 BP is often emphasized as one of the proximate causes of socio-political chaos in this area (Zhou et al. Reference Zhou, Li, Dodson, Zhang, Atahan, Sun and Yang2012). To prove whether the construction of Sanjiao was related to climate deterioration, we compared the 14C dates from Sanjiao with various high-resolution paleo-climatic records in the Hexi Corridor and the nearby regions (Figure 7). Those paleo-climatic records indicate that the temperature and the precipitation dropped drastically at the beginning of the first millennium BC, and the change is a macro-regional one (Figure 7) (Dykoski et al. Reference Dykoski, Edwards, Cheng, Yuan, Cai, Zhang, Lin, Qing, An and Revenaugh2005; Yang et al. Reference Yang, Qin, Wang, He, Melvin, Osborn and Briffa2014; Marcott et al. Reference Marcott, Shakun, Clark and Mix2013; Lecavalier et al. Reference Lecavalier, Fisher, Milne, Vinther, Tarasov, Huybrechts, Lacelle, Main, Zheng, Bourgeois and Dyke2017), potentially leading to social upheaval and conflict, and consequently the emergence of settlement fortification (Li Reference Li1995). Given the coincidence between the 14C dates of Sanjiao and the aforementioned climate deterioration (Figure 7), the construction of Sanjiao walled settlement might be a typical example of the settlement fortification driven by climate change. Similar examples can be found in the Mediterranean in the same period (Drake Reference Drake2012).
What Was the Livelihood in Sanjiao City?
Archaeobotanical studies shed light on human plant-utilization strategies across time (Lee et al. Reference Lee, Crawford, Liu and Chen2007; Hunt et al. Reference Hunt, Linden, Liu, Matuzeviciute, Colledge and Jones2008; Jin et al. Reference Jin, Zheng, Liu, Wang and Gao2011; Yang et al. Reference Yang, Wan, Perry, Lu, Wang, Zhao, Li, Xie, Yu, Cui, Wang, Li and Ge2012; Zhang et al. Reference Zhang, Lu, Gu, Wu, Zhou, Hu, Xin and Wang2012; Chen et al. Reference Chen, Dong, Zhang, Liu, Jia, An, Ma, Xie, Barton, Ren, Zhao, Wu and Jones2015b; Long et al. Reference Long, Wagner, Demske, Leipe and Tarasov2017). According to archaeological and archaeobotanical studies, human settlements appeared in the Hexi Corridor in the Majiayao Period (3300–2000 BC) (Xie et al. 2002; Dong et al. Reference Dong, Yang, Liu, Li, Cui, Wang, Chen, Dodson and Chen2018). Ancient agriculture relied on foxtail millet and broomcorn millet (Zhou et al. Reference Zhou, Li, Dodson and Zhang2016), plants first domesticated in northern China around 8000 BC (Lu et al. Reference Lu, Zhang, Wu, Li, Zhou, Ye, Zhang, Zhang, Yang, Shen, Xu and Li2009; Zhao Reference Zhao2011; Zhang et al. Reference Zhang, Lu, Gu, Wu, Zhou, Hu, Xin and Wang2012). Barley and wheat, domesticated in southwestern Asia during the early Holocene (Lev-Yadun Reference Lev-Yadun, Gopher and Abbo2000), was introduced to the Hexi Corridor around 2000 BC (Dosdon et al. Reference Dosdon, Li, Zhou, Zhao, Sun and Atahan2013; Dong et al. Reference Dong, Yang, Han, Wang and Chen2017a). While millet crops were initially the dominant subsistence plants (Ma et al. Reference Ma, Dong, Liu, Lightfoot, Chen, Wang, Li and Jones2013, Reference Ma, Dong, Lightfoot, Wang, Liu, Jia, Zhang and Chen2014, Reference Ma, Dong, Jia, Wang, Cui and Chen2016), exotic barley and wheat became essential staples in the western part of the Gansu-Qinghai region after 1700 BC (Flad et al. Reference Flad, Li, Wu and Zhao2010; Chen et al. Reference Chen, Dong, Zhang, Liu, Jia, An, Ma, Xie, Barton, Ren, Zhao, Wu and Jones2015b; Zhou et al. Reference Zhou, Li, Dodson and Zhang2016).
Since the first millennium BC, the pastoral economy flourished in the Hexi Corridor. Kuz’mina (Reference Kuz’mina2008) argued that the development of pastoral subsistence strategies and eastward migration by nomads was caused by a cold and dry climate across the Eurasian steppe during the time. The origins of nomadic culture in East Asia can also be dated back to the first millennium BC, with rapid development in the eastern grasslands in northern China (Di Cosmo Reference Di Cosmo2010). During 600–400 BC, the nomadic culture diffused to northwestern China, including Qinghai, Gansu, and Ningxia (Watson Reference Watson1971; Di Cosmo Reference Di Cosmo2010). Some studies argue that Shajing Culture has its origins in Rouzhi, a typical nomadic people (Pu and Pang Reference Pu and Pang1990). The sheep and cattle bones unearthed from Sanjiao and the tombs nearby such as Hamodun and Xigang reveal that the pastoral economy was vital to people’s livelihoods in Shajing Culture during the Late Bronze Age.
The three AMS 14C dates of carbonized barley seeds from the ash pit and cultural layers in Sanjiao range between 384–116 cal BC, indicate that Sanjiao was still inhabited in the Late Bronze Age (1000–210 BC). Barley and broomcorn millet were the most important local cultivated crops and the ubiquity of these species reached 100% of the 10 flotation samples (Table 2). Foxtail millet was utilized at Sanjiao, but its ubiquity is only 20% (Figure 5; Table 2). The presence and ubiquity of barley and broomcorn millet indicates the Shajing Culture at Sanjiao engaged not only in agro-pastoralism but also agricultural production.
Charcoal obtained from archaeological sites is critical for understanding human wood-utilization practices in the past (Thiébault Reference Thiébault2002; Asouti and Austin 2005; Dufraisse Reference Dufraisse2006). Charcoal collected from certain cultural relics in archaeological sites could be considered as the proxy of human activities, and its concentration in cultural remains directly reflect the daily practices of people at the time. In general, humans collect wood for firewood, food, medicine, perfume, and building materials (Wang et al. Reference Wang, Wang, Wang, Liang, Qi and Ren2016). In the collected floatation samples, we found 894 pieces of charcoal. They originated from seven tree species, including Tamarix chinensis, Tamarix, Salix, Picea, Hippophae, Betulaceae, and Poaceae (Figure 6; Table 3).
Tamarix occurred with the highest frequency, and its ubiquity reached 100%. Tamarix is often considered a shrub or small tree, and it is mainly distributed in ecologically fragile areas, especially in arid zones such as northwestern China. It is one of the constructive/dominant species of desert and wilderness communities with the advantage of long life expectancy, rapid growth, and high tolerance to aridity and saline/alkaline soil (Yin Reference Yin2002). Tamarix was used by ancient people in the Hexi Corridor at least as long ago as 2000 BC. During the Siba Culture Period (1900–1400 BC), Tamarix was used as both building material and fuel at the Xichengyi site (Wang et al. Reference Wang, Li, Zhang, Chen, Wang and Zhao2014). At the Huoshiliang site, Tamarix was gathered as fuel for bronze smelting (Sun et al. Reference Sun, Li and Zhou2010). At Sanjiao, Tamarix was probably the primary source of fuel due to its local abundance.
Hippophae is a kind of shrub or small tree that grows in alpine shrub environments and stratified alluvial terraces. It is drought-resistant and able to impede wind-drift sands (Forest Stewardship Council 1997). Due to its highly nutritious fruit, Hippophae is a vital food and medicine source, and its archaeological presence is significant for revealing the livelihoods of ancient people. At Sanjiao, seven pieces of charcoal were identified as Hippophae, with relatively low ubiquity (30%). Despite its low quantity and ubiquity, the presence of Hippophae indicates that its fruit was consumed, and its wood was used as firewood by the inhabitants at Sanjiao. At the Xichengyi site, Hippophae was collected for food (Wang et al. Reference Wang, Li, Zhang, Chen, Wang and Zhao2014).
Tamarix chinensis, Salix, Picea, Betulaceae, and Poaceae comprise only a tiny portion of our collected charcoals. Salix has a high ubiquity, though its absolute count is small. Salix favors moisture and often grows in stratified alluvial terraces. Because of its ecological adaptability, it is widely distributed (Forest Stewardship Council 1997). Picea, which favors a cold and humid environment, is often found on mountain slopes in northwestern China, and together with Betulaceae, forms the theropencedrymion (Forest Stewardship Council 1997). Our charcoal identification results suggest that these five wood species were seldom utilized by the settlers at Sanjiao. Their remains could probably come from the broad-leaf and coniferous forests that were sporadically distributed around the settlement during the time, with locally available Tamarix as the dominant utilized species.
Briefly, most of the archaeo-botanical crop and wood remains found in Sanjiao are cold-dry resistant species, which corresponds to the climate deterioration in East Asia since 1600 BC (Dykoski et al. Reference Dykoski, Edwards, Cheng, Yuan, Cai, Zhang, Lin, Qing, An and Revenaugh2005; Marcott et al. Reference Marcott, Shakun, Clark and Mix2013; Chen et al. Reference Chen, Xu, Chen, Birks, Liu, Zhang, Jin, An, Telford, Cao, Wang, Zhang, Selvaraj, Lu, Li, Zheng, Wang, Zhou, Dong, Zhang, Huang, Bloemendal and Rao2015a). Barley is highly resistant to low temperature, while broomcorn millet has a stronger adaptive capacity to dry environments relative to foxtail millet (Zhang 1986; Guedes et al. Reference Guedes, Lu, Henin and Schmidt2015). Many barley and broomcorn millet remains were also found at the Nuomuhong site in the Qaidam Basin where the climate is dry and cold (Dong et al. Reference Dong, Ren, Jia, Liu, Dong, Li, Wang, Xiao and Chen2016b). The tree species utilized by the settlers at Sanjiao, such as Tamarix and Hippophae, are also cold-dry resistant. As revealed by the subsistence strategy at Sanjiao, the inhabitants of the settlement might have adapted to the harsh environment in the Hexi Corridor by utilizing cold-dry resistant plant species.
Sanjiao is affirmed by AMS 14C dates as the first walled settlement in the Hexi Corridor. It was built around 828 ± 92 cal BC. The settlement was probably constructed for defense purposes in relation to the geopolitics in the Hexi Corridor during the late West Zhou and spring-autumn periods. It was still inhabited in 384–116 cal BC. During that time, people relied on barley, broomcorn millet, and auxiliary foxtail millet for food and utilized trees such as Tamarix and Hippophae as fuel. These species are cold-dry resistant, suggesting that people adapted their subsistence strategy to the harsh environment in the Hexi Corridor. Subsequent construction of other ancient settlements around Sanjiao might have facilitated the early development of the Silk Road.
This research was supported by the National key R&D Program of China (Grant 2018YFA0606402), Strategic Priority Research Program of the Chinese Academy of Sciences, Pan-Third Pole Environment Study for a Green Silk Road (Plan-TPE) (Grant Nos. XDA2004010101), the National Natural Science Foundation of China (Grant Nos. 41620104007; 41825001), Research Grants Council of The Government of the Hong Kong Special Administrative Region of the People’s Republic of China (17610715), and the Improvement on Competitiveness in Hiring New Faculties Funding Scheme (4930900) of the Chinese University of Hong Kong. We thank Professor Zhijun Zhao and Mr. Jingang Yang from the Paleoethnobotany Laboratory, Institute of Archaeology, CASS, for identifying crop seeds.