Climate-driven desertification and its implications for the ancient Silk Road trade

The ancient Silk Road played a crucial role in cultural exchange and commercial trade between western and eastern Eurasia during the historical period. However, the exchanges were interrupted in the early 16th century CE, during the Ming dynasty. Various causes for the decline of the ancient Silk Road have been suggested. Unlike social factors, natural factors have not been adequately addressed. In this study, we use evidence from a sedimentary site (Xishawo, XSW) in Dunhuang oasis, together with analysis of historical archives, to demonstrate the occurrence of extreme droughts and desertification events in the Dunhuang area post ∼ 1450 CE, which persisted at least for decades. The desertification may be closely associated with the accessibility of the ancient Silk Road in the area, which was responsible for a steep fall in the volume of trade, as well as political chaos and mass migrations. Therefore, besides socio-economic factors, climate change may have played an important role in trade exchange between the Ming government and the West and may have even influenced the rise and decline of the ancient Silk Road.


Introduction
The northern branch led to the northwest of Hami oasis and the oasis cities of Turfan, 142 Yanqi, Qiuci and Gumo, and then to Central Asia. The central branch passed through 143 Loulan city (300 km west of Dunhuang) which was abandoned in ~330 AD because of 144 eolian activity (Yuan and Zhao, 1999;James, 2007;Fontana et al., 2019). The southern 145 branch connected many cities along the southern edge of the Taklimakan Desert, such as 146 Ruoqiang, Qiemo, and Yutian, on the northern piedmont of the Altyn-Tagh and the 147 Kunlun Mountains (Fig. 1). As can be seen in Fig. 1, the location of Dunhuang oasis led 148 to its importance as the only logistical station between Hami oasis/Ruoqiang oasis and 149 Jiayuguan oasis during the Ming dynasty. 150 The XSW section in this study is located near the ancient city of Xishawo in the 151 modern Gobi Desert area of the Shule River Basin. The site is in the middle of the 152 Dunhuang and Guazhou oasis, ~50 km northeast of Dunhuang city (Fig. 1). The Xishawo 153 site was previously an ancient oasis with cultural sites, ancient cities and beacon towers. measurements of weight loss-on-ignition (LOI), grain size, and element contents. One 170 wood sample (at the depth of 10 cm, labeled XSW-10) and one charcoal sample (from the 171 cultural layer at 32 cm, labeled XSW-32) were collected for accelerator mass 172 spectrometry radiocarbon (AMS 14 C) dating. Two eolian samples for optically stimulated 173 luminescence (OSL) dating from the fine yellow sand layer (188 cm and 248 cm, labeled 174 XSW-188, XSW-248, respectively) were collected by hammering stainless-steel cylinders 175 into the section vertically, which were immediately sealed with opaque tape after removal. 176 Samples for the analysis of element contents were pretreated as follows. All samples 213

Methodology
were oven-dried for 24 hr and then pulverized into a powder. About 4 g of powder was 214 then pressed into a 4-6 mm-thick and 30 mm-diameter pellet under 30 t/m 2 of pressure. 215 The major, minor and trace element contents were measured with a Magix PW2403 216 Wavelength-Dispersive XRF Spectrometer. Elemental concentrations of 0.1 ppm to 100% 217 could be analyzed. Rb/Sr ratios were calculated for paleoenvironmental reconstruction. (1) Previous paleoclimatic records from the region 221 All available high-resolution paleoclimatic records for the study area and the adjacent 222 region were reviewed and compared. They include records of regional temperature, 223 precipitation, and river flow. In addition, documentary evidence of climate change in the 224 region during the Ming dynasty was investigated. 225  The Jiayuguan-Dunhuang area experienced frequent political turmoil and conflicts in 232 Ming dynasty (Zhang, 1974). Warfare in the northwestern part of the Ming domain was 233

Paleoclimatic record of the XSW section 290
The paleoclimatic record of the XSW section in the Dunhuang area was used to and 701±27 cal yr BP (1224-1278 cal AD), respectively. The two OSL samples from the 296 sand layer are dated to 2.6±0.2 ka (800±300 BC) and 2.8±0.2 ka (600±200 BC) ( Table 2). 297 Profiles of grain size, LOI and Rb/Sr ratio are shown in Fig. 3a, 3b, 3c and 3d, 298 respectively. The increase in median grain size and of the >63 μm fraction indicate an arid 299 environment and intense wind activity in the Dunhuang area during 800-600 BC and at 300 ~1450 AD. The LOI record reflects variation in organic matter content and the Rb/Sr ratio 301 of eolian sediments is positively correlated with weathering intensity. The Rb/Sr ratio of 302 the two sand layers is very low (Fig. 3), and therefore the effects of weathering are minor 303 the Dunhuang area and elsewhere, during ~800-600 BC and after ~1450 AD, which were 334 related to regional-scale climatic and environmental deterioration. Dunhuang oasis was located much closer to the center of the Gobi, and therefore it 347 experienced severe desertification which may have resulted in its abandonment.

Archaeological evidence of climate change in the Dunhuang area 349
The relatively dense distribution of prehistoric sites in the Hexi Corridor   However, there is a gap in radiocarbon dates during ~850-650 BC in the western Hexi 358 Corridor and eastern Xinjiang Province (Fig. A1), which suggests a hiatus in cultural 359 evolution and exchange during this period. This hiatus corresponds well to the 360 desertification event in in the Dunhuang area of the western Hexi Corridor during 361 ~800-600 BC (Fig. 3). However, even though the climate fluctuated substantially in 362 northwest China during the Bronze Age, human settlement was continuous in the eastern 363 Hexi Corridor at the same longitude (Fig. A1), which suggests that human occupation of 364 the Hexi Corridor was primarily determined by the environmental conditions. 365 The second desertification event occurred at ~1450 AD and is recorded both in the

Desertification events in the Ancient Silk Road area 388
Various indicators of climatic variations, wars and the tribute trade are plotted in 389 premise that at ~1450 AD the environment was characterized by a cold and dry climate 391 with intense sandstorms (Fig. 2e), low precipitation (Fig. 2f) and decreased streamflow 392 (Fig. 2g), there is a possibility that climate change played a role on the tribute trade of the 393 Ancient Silk Road. There was an abrupt decrease in the frequency of tribute trade at 394 ~1450 AD, but not during the two closures of the Jiayuguan Pass, in 1524 AD and 1539 395 AD ( Fig. 2b and 2d). In addition, at this time there was a lull in conflicts in the Dunhuang 396 area (Fig. 2a). Therefore, it is proposed that environmental deterioration likely contributed The influence of the desertification event in ~1450 AD on the streamflow of the 409 Qilian Mountains gradually decreased after the 1520s AD ( Fig. 2f and Fig. 2g). However, 410 the formation and evolution of an oasis is a long-term process (Stamp, 1961 activity (Zhang and Hu, 2002). Therefore, the regeneration of a degraded oasis would take 415 much longer than a change in the streamflow. Overall, we propose that the abrupt 416 decrease in tribute exchange and prosperity, or even the decline of the ancient Silk Road, 417 may have been affected by the deterioration of the environment along the routeway in the 418 Ming dynasty (Fig. 2b and Fig. 2c). Guazhou oasis were not functioning at ~1450 AD. This lengthened the distance from the 426 Jiayuguan oasis in the Hexi Corridor to the western oasis in Xinjiang (Fig. 1). According 427 to several researchers, camel caravans in deserts areas were able to travel a maximum 428 distance of ~30 km/day (Shui, 1990;Wang et al., 2000). In addition, the metabolism of a 429 domesticated camel will decrease within 20 days from the beginning of water deprivation 430 (Chen, 1982 climatic conditions, which are much more favorable than in ~1450 AD, and the maximum 434 distance was 24 km in one day (Blackmore, 2000). The maximum distance for a caravan 435 in the water-limited environment on the Silk Road was 30 km/day×15 days = 450 km. 436 The distances of Hami, Ruojiang and Jiayuguan to Dunhuang, where are Gobi Desert 437 without high mountains, are already close to or above this limit, which is barely sufficient 438 for camel travel (shown by the dots in Fig. 1 between Jiayuguan and Ruoqiang oases (the southern route) increased to ~600 km and 444 ~1,000 km, respectively. This is close to the maximum distance that a camel caravan can 445 achieve (~450 km) without a water supply, and would have substantially increased the 446 difficulty of travel across the region and was likely to be the physical cause of the decline 447 of the Ancient Silk road during the periods of desertification. 448