Millennial-scale variations and centennial-scale events in the Southwest Asian monsoon: pollen evidence from Tibet

Using quantitative reconstructions of vegetation and climate based on 234 surface samples and four fossil pollen records, a systematic study of millennial-scale variations and centennial-scale events in the Southwest monsoon over the last 14 000 years in the Tibetan Plateau was conducted. The SW monsoon stayed weak between 14 000 and 11 000 cal. yr BP. A marked drop in July temperature during 12 800 –11 500 cal. yr BP may indicate the occurrence of the Younger Dryas cold event. The SW monsoon started to intensify at 11 000 cal. yr BP. However, it did not increase monotonically, but abruptly in three steps to reach its maximum. The three transitions from weak to strong monsoon occurred at 11 000, 10 000 and 8000 cal. yr BP, respectively. The last transition marked the onset of a 1200-yr period of monsoon maximum. After the monsoon maximum, the SW monsoon decreased to the present level through two strong-to-weak transitions, one starting at 6800 cal. yr BP, and the other at 3100 cal. yr BP. The millennial-scale variation seems to suggest that the monsoon system switched from one mode to another during different periods, probably triggered by variations in insolation and glacial boundary conditions. The pollen records have also revealed a clear pattern of abrupt centennial-scale monsoon weakening events. These events exhibit a distinct duration at centennial-scale and a pacing at millennial-scale. There are ten events, occurring at about 1100, 2100, 3000, 4500, 5800, 6500, 7700, 8200, 9200, and 10 100 cal. yr BP with a significant periodicity of 830-900 years. The mechanisms responsible for these events probably include external forcing such as solar activities and tidal cycles, and internal forcing such as non-linear feedbacks and threshold behavior in the climate system. A long pollen record from the Zoige Basin in the northeastern Tibetan Plateau provides information on vegetational and climatic changes during the last two glacial-interglacial cycles. Subalpine spruce-fir forests were widespread during the interglacial and interstadial times, suggesting warm and wet climatic conditions. Alpine periglacial or dry desert existed under cold and dry climatic conditions during the penultimate and the last glacial maxima. Alpine sedge meadows dominated during the stadial epochs

Holocene vegetational and climatic history of the Xuguo Co catchment in the central Tibetan Plateau

A 101-cm core was taken from a large lake in the central Tibetan Plateau. Its pollen and loss-on-ignition analyses provide a Holocene vegetational, climatic, and environmental history of the lake catchment. Pollen analysis shows that: dense steppe dominated regional vegetation in the early Holocene (9,200–8,000 cal. yr BP); regional vegetation coverage gradually decreased in the middle Holocene (8,000–4,100 cal. yr BP); and marsh meadow grew on the lake edge and sparse steppe occupied the lake catchment after 4,100 cal. yr BP. Our result also reveals that: 9,200–8,000 cal. yr BP witnessed summer temperature, monsoonal rainfall, and lake-level maxima, as well as few winter and spring aeolian activities and frequent wildfires; 8,000–4,100 cal. yr BP saw a nonlinear decline in temperature, rainfall, lake level, and wildfires; and modern climatic and environmental conditions were established after 4,100 cal. yr BP. Three major monsoon-weakening events at ca. 6,700, 5,800, and 4,100 cal. yr BP were detected by pollen signals and proxies of the climate and environment

Pollen-based reconstructions of Holocene vegetation and climatic change of Tibetan Plateau

A synthesis of Holocene pollen records from the Tibetan Plateau shows the history of vegetation and climatic changes during the Holocene. Palynological evidences from 24 cores/sections have been compiled and show that the vegetation shifted from subalpine/alpine conifer forest to subalpine/alpine evergreen sclerophyllous forest in the southeastern par of the plateau; from the alpine steppe to alpine desert in the central, western and northern part; and from alpine meadow to alpine steppe in the eastern and southern plateau regions during the Holocene. These records show that increases in precipitation began about 9 ka from the southeast, and a wide ranging level of increased humidity developed over the entire of the plateau around 8 - 7 ka, followed by aridity from 6 ka and a continuous drying over the plateau after 4 - 3 ka. The changes in Holocene climates of the plateau can be interpreted qualitatively as a response to orbital forcing and its secondary effects on the Indian Monsoon which expanded northwards during the early Holocene and retreated from the plateau since the mid-Holocene. Also, there is teleconnection between the Tibetan Plateau and North Atlantic

Table_1_Coupled and decoupled legumes and cereals in prehistoric northern and southern China.xlsx

Legumes and cereals, which provide different nutrients, are cultivated as coupled crops in most centers of plant domestication worldwide. However, as the only legume domesticated in China, the spatio-temporal distribution of soybeans and its status in the millet- and rice-based agricultural system of the Neolithic and Bronze Ages remains elusive. Here, archaeobotanical evidence of soybeans (n=254), millet (n=462), rice (n=482), and zooarchaeological evidence of fish (n=138) were synthesized to elucidate the phenomenon of coupled or decoupled cereals and legumes in prehistoric China. During the Neolithic and Bronze Ages, soybeans was mostly confined to northern China and rarely found in southern China, serving as a companion to millet. In contrast, fish remains have been widely found in southern China, indicating a continuous reliance on fish as a staple food besides rice. Thus, an antipodal pattern of millet-soybeans and rice-fish agricultural systems may have been established in northern and southern China since the late Yangshao period (6000–5000 cal BP) respectively. These two agricultural systems were not only complementary in terms of diet, but they also exhibited positive interactions and feedback in the coculture system. Consequently, these two systems enabled the sustainable intensification of agriculture and served as the basis for the emergence of complex societies and early states in the Yellow and Yangtze Rivers.</p