Middle Holocene Seawater Intrusion And Human's Rapid Response In The Yaojiang-Ningbo Coastal Plain,East China

Coastal zones constitutes 2%of the world's land area but contains 10%of world's population.It is one of the most active and economically prosperous regions in the world today.With global warming and sea-level rising,coastal zones are experiencing increasing frequency of extreme weather,seawater intrusion,and coastal flooding,posing serious challenge to the sustainable development of ecosystem and human society in the region.The coastal zones are also the main site for the origin and development of the Neolithic cultures.For example,the Yaojiang-Ningbo plain in the southeast Yangtze coast is the core distribution area of the famous“Hemudu Culture”.Due to its low elevation and sensitivity to sea level fluctuations,the region has experienced complex land-sea interactions in the Middle Holocene,which has had a profound impact on the ancient human societies in the region.Therefore,the study of the human-environment relationship can not only understand the environmental constraints on the human activities and the their adaptation to the environmental change,but also provide detailed evidences for the environmental vulnerability and the urgency of risk prevention and control in coastal lowlands.In this study,four sedimentary profiles from the excavation trenches of area and the sediment cores at the Neolithic Yushan site?profile T0410W and core YS160?in the northeastern part of the Ningbo Plain,the Neolithic Lujiaqiao site in the southwest?core LJQ?and the Neolithic Fujiashan site?core FJS?the northeastern part of the Yaojiang Plain,respectively,were examined for AMS ¹⁴C and OSL dating,lithology,organic elemental geochemistry,sporopollen and charcoal.The changes of paleohydrologic and paleoecological environments and the history of human activities for these profiles during the Neolithic period were reconstructed.Together with the temporal and spatial migrations of the ancient human settlements during the period from the first phase of Hemudu Culture to Shang-Zhou dynasties,and the spatial comparison of stratigraphic records from the multiple sites in the region,we summarized the regional seawater intrusion,extreme storm and flooding events and analyzed their linkage to the cultural interruptions or discontinuities of cultural layers.Our aim is to explore the hydrologic and ecologic environmental changes in the coastal lowlands and the rapid response of ancient human activities in the Neolithic period.Main results were obtained as follow.?1?The results of sporopollen and organic geochemical elements in the four sedimentary profiles showed that marine dinoflagellate cysts increased significantly and the TOC content decreased sharply with a major marine source of the OC at c.6300 cal yr BP,reflecting the impact of seawater intrusion at all three Neolithic sites at that time.Aquatic plants and freshwater algae increased in profiles at Yushan and Fujiashan sites at c.5800 cal yr BP,while marine dinoflagellate cysts increased in profile at Lujiaqiao site.These phenomena indicate the rise of local water level and short-term marine transgression,which we speculate being the responses to the extreme weather events in different geomorphological environments.Aquatic plants and freshwater algae significantly reduced in cores YS160 and FJS at c.5000 cal yr BP,which may reflect the shrinkage of lakes and the decline of local water levels.The present study further revealed the sedimentary evidences of a storm event at c.4500cal yr BP and the subsequent marine transgression with age constrains at Yushan and Fujiashan sites.Seawater intrusion from 4500 cal yr BP possibly also occurred at Lujiaqiao site but with no direct age control.The above stratigraphic records of hydrologic events have a good correspondence with the natural sedimentation layers formed between cultural layers or the absence of cultural layers at other Neolithic sites in study area,suggesting that there are two regional marine transgressions,which occurred at c.6300 cal yr BP and 4500 cal yr BP,respectively.The flooding and short-term seawater intrusion induced by extreme weather at c.5800 cal yr BP were also recorded at other sites in the study area,but its temporal and spatial scales remain to be further studied.The results of sporopollen,charcoal,organic elemental compositions in the four profiles also showed that the prehistoric human activities highly agreed with the formation of coastal wetlands.When the marine transgression occurred,the wetland degraded and the human activities rapidly weakened.These phenomena indicate that the Hemudu cultural ancients were highly dependent on the coastal wetlands.?2?Although the regional marine transgression event of c.6300 cal yr BP did not lead to the end of Hemudu Culture,the event corresponded to the decline of Hemudu Culture from early to late period reported by the archaeologists.The decreases in contents of Poaceae pollen??38?m?and charcoal??100?m?versus recovery of trees and shrubs occurred in the late Hemudu cultural layers reflect the weakening of the human activities.In addition,the archeological sites during the period of the third and forth phases of Hemudu Culture migrated to the inland of the Yaojiang Plain.Together with the hydrologic event at 5800 cal yr BP,we speculate that the extreme weather and flooding disasters were frequent in study area in the late Hemudu period,which hindered the rice farming and restricted the development of prehistoric human society.?3?The decline in Poaceae??38?m?and charcoal??100?m?in core YS160 at the Yushan site and core FJS at the Fujiashan site during 5000–4600 cal yr BP indicate the weakening of human activities during late Liangzhu period.Meanwhile,the shrinkage of lakes and the reduction of local water levels occurred in cores YS160and FJS,which is supposed to be related to the global climate decline at that time.However,such climate and hydrologic change is difficult to explain the weakening of human activities in the study area;particularly,the Liangzhu Culture was extremely prosperous on the north bank of Hangzhou Bay in the same period.Therefore,this issue needs more researches on natural and human factors.