| The Yangtze River has a length of more than 6300 km and ranks as the world’s third longest river.Known as "The Mother River" of the Chinese nation,it flows through more than ten provinces and many cities throughout China.The Yangtze River Basin is an important economic zone and the birthplace and development area of Chinese civilization.The Chengdu Plain is one of the world’s Neolithic cultural centres and is located in the upper reaches of the Yangtze River,where the highly developed ancient Shu Culture developed.While it bred a variety of outstanding cultures,the Chengdu Plain also experienced frequent palaeofloods,drought,rapid cooling and other natural disasters in the mid-late Holocene.The archaeological excavations at the Baodun site,Sanxingdui site and Jinsha site,which are located in the Chengdu Plain,show that all these sites have likely experienced palaeofloods and that the palaeofloods produced notable impacts on the ancient Shu Culture.However,there is still a lack of environmental archaeological studies on the palaeoflood alluvial layers at these sites.It is vitally important to conduct Holocene environmental archaeological research in the Chengdu Plain to determine the links between environment change and human-earth relationships during the Shu Cultural period without documentation in the form of written records.The Hongqiao Village Site and the Jinsha Site are typical archaeological sites with palaeoflood information and abundant dating materials of the mid-late Holocene and ideal sites for environmental archaeological research.Based on comprehensive research such as chronology,geochemistry,grain size,zircon shape characteristics and pollen at these two typical sites,in combination with comparative analysis for sedimentary characteristics of modern rivers,quantity change and spatial-temporal distribution of archaeological sites,geographic location and settlement changes in the study area,environmental archaeological research was conducted for mid-late Holocene palaeoflood events in the Chengdu Plain and interactive response relationship with the evolution of the ancient Shu Culture.Furthermore,the time,characteristic processes and environmental background of the occurrence of mid-late Holocene palaeoflood events in the research area were revealed,and the impact of mid-late Holocene palaeoflood events on the evolution of the ancient Shu Culture is discussed.Research results indicate that:① Rainfall triggered debris flows under cold and dry conditions in the western mountainous areas of the Chengdu Plain,which likely caused environmental disasters in the lower reaches of the Chengdu Plain.The intense rainfall likely resulted in sluggish flow velocities in the many water networks on the Chengdu Plain,consequently resulting in the pluvial events.AMS14C and OSL dating shows that during the period between 4.6-3.7 ka BP and about 3.2 ka BP,palaeofloods were frequent in the Chengdu Plain.The rules of occurrence and climatic background to the mechanism of the mid-late Holocene palaeoflood disaster in the Chengdu Plain are further analyzed.The results show that in the period between 4.6-3.7 ka BP,while climatic fluctuations intensified the principal manifestation of climate was cold and dry with frequent palaeoflood occurrences in the Chengdu Plain.Around 4 ka BP,climatic variability peaked,cold and dry conditions were the most severe,and the scale of palaeoflood manifestation gradually increased with time.During the period between 3.7-2.6 ka BP,the climatic fluctuations gradually decreased and the cold and dry conditions moderated,but about 3.2 ka BP climatic conditions were particularly unstable and this was the period of frequent large-scale palaeofloods.Between 4.6-3.7 ka BP and about 3.2 ka BP,the frequent palaeoflooding in Chengdu Plain corresponds to a gradual deterioration in the late Holocene Megathermal,consistent with a period of abnormal climatic fluctuation.② Ancestors of the ancient Shu lived in upstream regions of the Minjiang River.At about 4.7 ka BP,the climate turned cold and dry and the upstream regions of the Minjiang River were no longer a suitable environment for humans.The ancestors of the ancient Shu gradually migrated towards the Chengdu Plain and established the Baodun Culture.Early sites of the Baodun Culture are rare,and relics are mainly distributed in the western and southwest regions of the Chengdu Plain,in higher edge zones closed to mountainous regions.In the mid-late period of the Baodun Culture,an increase in population and the development of rice agriculture stimulated human activities to extend from the central region of the Chengdu Plain.Sites of that time increased greatly and are distributed all over the Chengdu Plain.During the period of the Sanxingdui Culture,the number of sites declined dramatically and cannot be found in the alluvial fan of the Minjiang River basin,possibly because frequent large-scale palaeofloods in the late Baodun period forced the ancestors of the ancient Shu to abandon their settlements in the Minjiang River basin.Meanwhile,competition for resources among humans intensified and wars broke out constantly because of geographic limitations caused by frequent palaeofloods.As a result,people of the Baodun Culture were overcome by people of the Erlitou Culture,and large numbers of people migrated out of the Chengdu Plain leading to a dramatic decline in the population during period of the Sanxingdui Culture;consequently sites are rare and mainly distributed in the alluvial fan of the water system of the Tuojiang River,and one major reason for the Sanxingdui Culture turning into the Shi’erqiao Culture may be the political struggle caused by frequent flooding.After the Sanxingdui Site was repeatedly struck by floods,Ba Culture immigrating from the outside took over the Sanxingdui Site.They took a series of water control measures and made the alluvial fan of the Minjiang River basin a livable place for humans before moving to the Jinsha Site,at which time the Shi’erqiao Culture emerged.③Sediment from palaeofloods in the Chengdu Plain displays the following features:The palaeoflood sediments tend to be composed of coarse particles,with a relatively high percentage of traction load components and saltation load components.Various samples had relatively great differences in particle size distribution;the change in the percentage content of coarse sand particles being very obvious,and the sedimentary particles exhibiting upward coarsening within the same layer.When the hydrodynamic force was relatively weak,the Zr/Rb ratios are relatively low and the Ba/Nb ratios relatively high;whereas when the hydrodynamic force was strengthened,the Zr/Rb ratios increase and the Ba/Nb ratios decrease.The shapes of zircon are mainly oval and nearly spheral which indicates that the zircon was rounded after being carried for a long distance by powerful currents.Ti and Rb/Sr ratios contents are higher than those of sediments from cultural layers. |
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