Mid-to Late Holocene Hydrology Changes In The South Taihu Area Of The Yangtze Delta Plain,China,and Its Relationship To The Development Of Neolithic Cultures

The large river delta area is heavily threatened by natural hazards caused by accelerating sea-level rise and warming climate,such as salt water penetrations,floods and storm tides.It is urgent to understand past hydrological environmental changes in the coastal area and its relationship to the ancient civilizations,so as to supply theoretical bases for forecasting futural changes of them,in responding to sea-level rise and global warming.During the middle Holocene,the hydrological environments in the Taihu plain,Yangtze Delta,China,varied tremendously under the influence of sea-level and climate change.Simultaneously,several Neolithic cultures,such as,the Majiabang,Songze,and Liangzhu culture,developed in this region.Therefore,the Taihu Plain is an ideal place for studying relationship between human and hydrological environments during the Neolithic period.The East Tiaoxi River Plain,south Taihu Plain,was occupied by the Palaeo-Taihu incised valley or Palaeo-Taihu Esturay during the Holocene,and its topography was relatively lower than its surrondings,and hence,more sensitive to sea-level changes,making it a key area for hydrology research in the Taihu Plain.Therefore,this thesis focuses on studying hydrology changes in the East Tiaoxi River Plain,during the mid-to late-Holocene,including sea water penetration,formation of fresh-water environments and lake expansion,and their influences on the Neolithic cultures.Basing on AMS14C dating,diatom identification,measurements of C-N elements and their stable isotopes of sediments from core DTX4 and DTX10,obtained in the East Tiaoxi River Plain,the following results are achieved.From-7100 cal.yr BP to 6500 cal.yr BP,percentages of Aulacoseira granulata in core DTX4,which is an indicator of connections of rivers to lakes/estuary and is dominant in the Yangtze River freshwater,are high,indicating the input of the Yangzte River ftreshwater.Its percentages remain constant during this period even when percentages of brackish diatoms increase,reflecting that influnce of the Yangtze River discharge was indiffenret to that of tide water current from the Hangzhou bay.Therefore,it is suggested that the Yangzte River connected likely to the Palaeo-Taihu incised valley during this period,through the palaeo-river channels or low lands between Wuhu city in Anhui province and Gaochun town in Nanjing city,Jiangsu province and palaeo-river channels or low lands around Jintan city in Jiangsu province in the north-west of the Taihu Plain,and then flew further south into the Palaeo-Taihu Estuary and the Hangzhou Bay.Before?7500 cal.yr BP,the diatom assemblage in core DTX10 is dominated by marine and brackish species which are usually found in estuary and coastal environments,and organic matter is mainly from marine algae.Therefore,an estuary environment(the Palaeo-Taihu Estuary)is interpretated.High sedimentation rate shows that the Palaeo-Taihu Estuary was undergoing rapid infilling during this period.Meanwhile,Aulacoseira granulata is in low concentration,reflecting that the Yangtze freshwater had not been discharged into the Palaeo-Taihu Estuary yet.From?7500 cal.yr BP to 7000 cal.yr BP,pedogenesis and sedimentation hiatus occurred in core DTX10,while salt tolerant freshwater benthic diatoms and organic matter of C3 plants and freshwater algae dominate in core DTX4,reflecting a low salinity marsh environment or subaerial exposured land formed,and hence receding of sea-water.The receding of sea-water and development of low salinity marsh is likely attributed to abundant discharge of the Yangtze River freshwater.Between?7000 and 6500 cal.yr BP,increase in percentages of brackish benthic diatoms in core DTX4,which are accompanied by decrese in salt tolerant freshwater diatoms,indicates the development of salt marshes.This interpretation is supported by the increase in organic matter from marine algae.The formation of salt marsh during this period is attributable to the intensified sea-water penetration likely caused by an accelerated sea-level rise.This period of transgression also influenced other places in the Taihu Plain where no sea water influence occurred in previous stages,hence,it was possibly the largest one,basing on hydrological environmental results from collected cores studied by previous researchers.After?5600 cal.yr BP,freshwater benthic diatoms and C3 plants organic matter dominate in core DTX4 while subarial land continued in core DTX10,indicating stable freshwater or terrestrialized condition was formed.Meanwhile,percentages of A.granulata reduced remarkably,reflecting the weakening of the Yangtze freshwater input.The retreat of the Yantze freshwater is likely because the Yangtze freshwater flew east towords the central and eastern Taihu plain or/and the Yangtze freshwater discharging into the palaeo-channels in the west-north Taihu plain reduced,inresponding to gradul infilling of the palaeo-channels,the Palaeo-incised Taihu valley and the Palaeo-Taihu Estuary.After?3000 cal.yr BP,lake expansion was recorded both in core DTX4 and DTX10 indicated by variations of the C-N element and their stable isotopes,likely in responding to the formation of present saucer-like depression of the Taihu Plain and discharge direction change of the East Tiaoxi river from north-to-south to south-to-north.The author speculates that hydrological changes of the East Tiaoxi River Plain played an important role on the migration and development of the Neolithic cultures in the Taihu Plain.The low-salinity marsh started during?7500-7000 cal.yr BP attracted the early Majiabang people to live around the Palaeo-Taihu Estuary.The sea water penetration between?7000-6500 cal.yr BP matches the left of the late Majiabang and early Songze people lived in the east of the Palaeo-Taihu Estuary,to the north and east of the Taihu Plain.Especially,the freshening of the East Tiaoxi River Plain or the terrestrialization of it after?5600 cal.yr BP,likely promoted the rapid rise of the Liangzhu culture,by forcing rivers originated from the west uplands and the Yangtze river discharge into the Taihu Plain,supplying sufficient freshwater for rice farming expansion,by protecting the Taihu Plain from sea-water penetration from the Hangzhou bay,and by encouraging connection between Liangzhu people in the east Taihu Plain and those in the coastal plains at the head of and south to Hangzhou bay and their migration in these places.