Guanghui Dong

East Asian summer monsoon precipitation variability since the last deglaciation

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the “present” is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7–7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8–5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5–8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.

Agriculture facilitated permanent human occupation of the Tibetan Plateau after 3600 B.P.

Colonizing the roof of the world Humans only settled permanently on the Tibetan plateau about 3600 years ago. Chen et al. examined archaeological crop remains unearthed in northeastern Tibet, which elucidate the timing of agricultural settlement. Although much earlier traces of humans in Tibet have been dated to 20,000 years ago, year-round presence at the highest altitudes appears to have been impossible until the advent of suitable crops, such as barley. Surprisingly, these prehistoric farming communities expanded onto the plateau at the same time as climate was cooling. Science, this issue p. 248 Archaeobotanical data sets from the Tibetan Plateau reveal the Holocene prehistory of high-altitude human settlement. Our understanding of when and how humans adapted to living on the Tibetan Plateau at altitudes above 2000 to 3000 meters has been constrained by a paucity of archaeological data. Here we report data sets from the northeastern Tibetan Plateau indicating that the first villages were established only by 5200 calendar years before the present (cal yr B.P.). Using these data, we tested the hypothesis that a novel agropastoral economy facilitated year-round living at higher altitudes since 3600 cal yr B.P. This successful subsistence strategy facilitated the adaptation of farmers-herders to the challenges of global temperature decline during the late Holocene.

The development of agriculture and its impact on cultural expansion during the late Neolithic in the Western Loess Plateau, China:

Based on radiocarbon dating and our analysis of plant and animal remains from Buziping, a Majiayao (5300–4300 BP) and Qijia (4200–3800 BP) period site located in Dingxi, Gansu Province, China, and our review of archaeobotanical studies in the Western Loess Plateau and adjacent areas, we discuss subsistence strategies during the Majiayao and Qijia periods. We also discuss the development of agriculture in the Western Loess Plateau and its influence on cultural expansion during the late Neolithic period. Humans settled at Buziping for the first time during the Majiayao period (4890–4710 cal. yr BP by 14C dating). Charred seeds from the site indicate that people engaged in millet-based agricultural production. People continued this type of agriculture during a second phase of occupation (4130–3880 cal. yr BP by 14C dating) during the Qijia period, but the proportion of foxtail millet to broomcorn millet increased from the Majiayao to Qijia period. Raising domestic animals was another aspect of subsistence during the Qijia period. The main domestic animals were likely pigs and dogs, although hunting of wild animals also took place. Subsistence at Buziping site was affected by the rapid development of intensive agriculture that diffused across eastern Gansu Province during the late Neolithic. Our work suggests that millet-based agriculture spread from east to west across the Western Loess Plateau and likely promoted the expansions of those two cultures in the area during the Majiayao period and early–mid Qijia period. Climate change might have also promoted Majiayao and Qijia expansions and probably facilitated the adoption of rain-fed agriculture in this region.

On the timing of the East Asian summer monsoon maximum during the Holocene—Does the speleothem oxygen isotope record reflect monsoon rainfall variability?

The evolution of the East Asian summer monsoon (EASM) during the Holocene has long been of significant interest. Knowledge of past EASM variability not only increases our understanding of monsoon dynamics on a long timescale, but it also provides an environmental and climatic background for research into Chinese cultural development. However, the timing of the EASM maximum remains controversial. The popular concept of an “early Holocene maximum” is mainly based on speleothem δ18O (δ18Oc) records from caves in southern China; however, the interpretation of δ18Oc as a reliable proxy for EASM intensity is being increasingly challenged. The present paper is a critical review of the climatic significance of the δ18Oc record from China. Firstly, we suggest that precipitation in northern China is an appropriate index of EASM intensity, the variation of which clearly indicates a mid-Holocene monsoon maximum. Secondly, an interregional comparison demonstrates that the precipitation record in northern China is quite different from that in southern China on a range of timescales, and is inconsistent with the spatial similarity exhibited by speleothem oxygen isotope records. Furthermore, both modeling and observational data show that the δ18Oc records from southern China indeed reflect changes in precipitation δ18O (δ18Op) rather than precipitation amount, and therefore that their use as an EASM proxy is inappropriate. Finally, we address several significant monsoon-related issues—including the driving mechanism of the EASM on an orbital timescale, the climatic significance of speleothem oxygen isotopes, and the relationship between atmospheric circulation and precipitation in monsoonal regions.

Journey to the east: Diverse routes and variable flowering times for wheat and barley en route to prehistoric China

Today, farmers in many regions of eastern Asia sow their barley grains in the spring and harvest them in the autumn of the same year (spring barley). However, when it was first domesticated in southwest Asia, barley was grown between the autumn and subsequent spring (winter barley), to complete their life cycles before the summer drought. The question of when the eastern barley shifted from the original winter habit to flexible growing schedules is of significance in terms of understanding its spread. This article investigates when barley cultivation dispersed from southwest Asia to regions of eastern Asia and how the eastern spring barley evolved in this context. We report 70 new radiocarbon measurements obtained directly from barley grains recovered from archaeological sites in eastern Eurasia. Our results indicate that the eastern dispersals of wheat and barley were distinct in both space and time. We infer that barley had been cultivated in a range of markedly contrasting environments by the second millennium BC. In this context, we consider the distribution of known haplotypes of a flowering-time gene in barley, Ppd-H1, and infer that the distributions of those haplotypes may reflect the early dispersal of barley. These patterns of dispersal resonate with the second and first millennia BC textual records documenting sowing and harvesting times for barley in central/eastern China.

Exploring the history of cultural exchange in prehistoric Eurasia from the perspectives of crop diffusion and consumption

The history of cultural exchange in prehistoric Eurasia (CEPE) has been widely investigated. Based on archaeological evidence, this process is thought to date back to at least the early Bronze Age, although details about timings and routes remain unclear. It is likely that CEPE promoted the spread and exchange of crops that originated in different parts of Eurasia; since these remains can be definitely identified and directly dated, they provide ideal research materials to explore the history of CEPE. In this paper, we review the available archaeobotanical evidence and direct radiocarbon dates for crop remains, alongside carbon isotopic data from human bones unearthed from prehistoric sites in Eurasia, in order to investigate the history of the spread of millet crops, and wheat and barley, that were first domesticated in the eastern and western parts of Eurasia during prehistoric times. In combination with other archaeological evidences, we discuss the history of CEPE. Our results suggest that wheat and barley were domesticated in western Asia around 10500 a BP, spread into Europe and western Central Asia before 8000 a BP, and reaching eastern Central Asia and northwestern China between 4500 and 4000 a BP. Data show that both broomcorn and foxtail millet were domesticated in eastern Asia before 7700 a BP, spread into eastern Central Asia between 4500 and 4000 a BP, and into western Asia and Europe prior to 3500 a BP. Wheat, barley, and millet crops were first utilized together in eastern Kazakhstan within Central Asia around 4400 a BP, the region where earliest CEPE is likely to have taken place. These crops were mixedly used mainly in eastern central Asia and northwest China between 4500 and 3500 a BP, and then across the Eurasia before 2200 a BP. The results of this study suggest that transcontinental CEPE might have been initiated during the fifth millennium, before intensifying during the Bronze Age to lay the foundations for the creation of the ancient Silk Road during the Han Dynasty (between 202BC and 220AD).

History and possible mechanisms of prehistoric human migration to the Tibetan Plateau

Prehistoric human history on the Tibetan Plateau is a hotly debated topic. Archaeological research on the plateau during the past few decades has enormously improved our understanding of the topic and makes it possible for us to consider the processes and mechanisms of prehistoric human migration to the region. By reviewing the published archaeological research on the Tibetan Plateau, we propose that the first people on the plateau initially spread into the He-Huang region from the Chinese Loess Plateau, and then moved to the low elevation Northeastern Tibetan Plateau and perhaps subsequently to the entire plateau. This process consisted of four stages. (1) During the climatic amelioration of the Last Deglacial period (15–11.6 ka BP), Upper Paleolithic hunter-gatherers with a developed microlithic technology first spread into the Northeastern Tibetan Plateau. (2) In the early-mid Holocene (11.6–6 ka BP), Epipaleolithic microlithic hunter-gatherers were widely distributed on the northeastern plateau and spread southwards to the interior plateau, possibly with millet agriculture developed in the neighboring low elevation regions. (3) In the mid-late Holocene (6–4 ka BP), Neolithic millet farmers spread into low elevation river valleys in the northeastern and southeastern plateau areas. (4) In the late Holocene (4–2.3 ka BP), Bronze Age barley and wheat farmers further settled on the high elevation regions of the Tibetan Plateau, especially after 3.6 ka BP. Finally, we suggest that all of the reported Paleolithic sites earlier than the LGM on the Tibetan Plateau need further examination.

Early human impacts on vegetation on the northeastern Qinghai-Tibetan Plateau during the middle to late Holocene

The ecosystems of the Qinghai-Tibetan Plateau are regarded as being primarily regulated by climate because of the harsh environment of the region and the resulting sparse human population. Recent studies have revealed that Neolithic farmers and nomads extensively settled in the northeastern Qinghai-Tibetan Plateau from 5.2 ka (ka = cal ka BP); however, it is unclear how and to what extent human activity has affected its vegetation. Here we combine the results of the pollen analysis of a sediment core from Genggahai Lake, a shallow lake in Gonghe Basin on the northeastern Qinghai-Tibetan Plateau, with archaeological evidence and use them to assess the extent and nature of human impacts on the regional vegetation during the middle and late Holocene. The pollen record indicates that Stellera, an indicator of the extent of grazing-induced grassland degradation, first appeared at 4.7 ka, expanded during 3.6–3.0 ka, and finally increased significantly after 1.6 ka. In support of this finding, archaeological data indicate that the agro-pastoral Majiayao people arrived at ∼5 ka and groups of Kayue people, who practiced pastoralism, intensively colonized the Gonghe Basin and nearby Qinghai Lake basin during 3.6–3.0 ka. After ∼1.6 ka, from the Tang Dynasty onwards, human settlement and grazing activity intensified on the northeastern Qinghai-Tibetan Plateau, and this is in accord with the observed high percentages of Stellera in the pollen record. Based on comparison with other records, we conclude that the sediments of Genggahai Lake provide a record of anthropogenic impacts on vegetation, and that human activity may have contributed to regional forest decline during the middle Holocene, and to grassland degradation in the late Holocene. Grassland degradation caused by human activity may be an indicator of the start of the Anthropocene and potentially may have contributed to global climate change via increased dust emission to the atmosphere.

Reliability of radiocarbon dating on various fractions of loess-soil sequence for Dadiwan section in the western Chinese Loess Plateau

The accurate radiocarbon dating of loess-soil sequences plays an essential role in the reconstruction of the environmental and climatic changes in continental settings during the last glaciation and Holocene. However, our knowledge about the reliability of radiocarbon ages of various fractions of soil and loess samples is still insufficient. Here, we present our study results on radiocarbon ages based on bulk organic matter, humin fraction, and carbonate of samples collected from a loess-paleosol section in the western Chinese Loess Plateau. We compare these observations with the optically stimulated luminescence ages and charcoal radiocarbon ages to evaluate the reliability of these fractions. We observed that the radiocarbon ages of humin fraction are very close to those of charcoal and are consistent with the optically stimulated luminescence ages within the experimental errors. We observed a significant deviation in the radiocarbon ages of carbonate and bulk organic matter from those of charcoal and optically stimulated luminescence ages, likely due to the dilution of these fractions during the pedogenetic process. Our results reveal that, except for charcoal, the humin fraction may yield reliable 14C ages for the Chinese loess-soil sequence.

Prehistoric agriculture development in the Yunnan-Guizhou Plateau, southwest China: Archaeobotanical evidence

The origin, development and expansion of prehistoric agriculture in East Asia have been widely investigated over the past two decades using archaeobotanical analysis from excavated Neolithic and Bronze Age sites. Research on prehistoric agriculture has predominantly focused in the valleys of the Yellow River and the Yangtze River. Agricultural development during the Neolithic and Bronze Age periods in the Yunnan-Guizhou Plateau of southwest China, an important passageway for human migration into Southeast Asia, still remains unclear. In this paper, based on macrofossil and microfossil analysis and radiocarbon dating at the Shilinggang site, we investigate plant subsistence strategies in the Nujiang River valley during the Bronze Age period. Combined with previous archaeobotanical studies in the Yunnan-Guizhou Plateau, we explore agricultural development processes in this area during the Neolithic and Bronze Age. Our results indicate that rice and foxtail millet were cultivated in Shilinggang around 2500 cal a BP. Three phases of prehistoric agricultural development in the Yunnan-Guizhou Plateau can be identified: rice cultivation from 4800–3900 cal a BP, mixed rice and millet crop (foxtail millet and broomcorn millet) cultivation from 3900–3400 cal a BP, and mixed rice, millet crop and wheat cultivation from 3400–2300 cal a BP. The development of agriculture in the Yunnan-Guizhou Plateau during the Neolithic and Bronze Age periods was primarily promoted by prehistoric agriculture expansion across Eurasia, agricultural expansion which was also affected by the topographic and hydrological characteristics of the area.