The Process Of Transgression And Regression Of Yaojiang Valley In South Hangzhou Bay And Its Neolithic Implications During Early To Mid-Holocene

The Yangtze River Delta and the Hangzhou Bay has underwent rapid sea-level rise since the early Holocene and the decelerated regression processes after that.The coastal plain and delta began to form.Two relatively independent trajectories of Neolithic cultures occurred on both flanks of Hangzhou Bay during the Holocene.One is that of the Majiabang(～7000-6000 cal yr BP)-Songze(～6000-5200 cal yr BP)-Liangzhu(～5200-4000 cal yr BP)and the Maqiao(～3900-3200 cal yr BP)cultural sequence in the Taihu Lake basin of the northern Hanghzou Bay,and the other is the Shangshan(～11000-8600 cal yr BP)-Kuahuqiao(～8000-7200 cal yr BP)and the Hemudu(～7000-5000 cal yr BP)cultures on the southern Hangzhou Bay,respectively.As one of the typical representatives of the Neolithic civilization in eastern China,these prehistoric civilization witnessed a series of significant humanity event,such as origin of the primitive rice farming,formation of the original settlement,rise and fading of the City-state.Research on the linkage between coastal evolution and Neolithic occupation on both flanks of Hangzhou Bay therefore become an important window to decode the relationship between human and nature in estuarine and coastal areas in historical period.The Yaojiang Valley(YJV)located in the central part of the Ningshao Plain along the southern Hangzhou Bay of eastern China.The valley connects the Shaoxing Plain from the west and the Ningfeng Plain from the east,the landforms feature for flat in general and mountains and hills lie on the south and north sides of the valley.The Yaojiang Valley is well known for its fostering of the long-lasting Neolithic Hemudu culture of the eastern China coast,and witnessed the Holocene sea-level changes,landscape processes and Neolithic human adaptions.The present study focused on two sediment cores retrieved from the centre and the eastern entrance of the YJV.Foraminiferal analysis combined with lithology description,geochemical element analysis,environmental magnetism analysis was completed to decipher the marine transgression and regression process on the basis of radiocarbon chronology.With references to other sediment cores of previous studies in the region,brief environmental transitions in the Yaojiang Valley during the Early to Middle Holocene are shown.Furthermore,the possible impacts of both marine and land processes on the occupations of the Hemudu culture were reevaluated from a geo-archaeological perspective.The main findings and conslusions are as follows:1)Foraminiferal results from two cores(YJ1505.YJ1508)situated in the centre and eastern entrance of Yaojiang Valley clearly showed that:the eastern entrance of the YJV was subject to marine invasion no later than ca.9240 cal yr BP.however,the centre of the Yaojiang Valley was affected by marine invasion until ca.8900 cal yr BP.Foraminiferal proxies from both two cores show that the timing of the maximum relative sea level(RSL)was about ca.7900 cal yr BP,but no marine influence is apparent in the relatively higher hilly margins of the western YJV.Afterwards,marine regression and coastal propagation happened.The YJV started to transition from a marine-influenced setting to a limnetic environment ca.7600 cal yr BP in the valley centre while there was no marine influence until ca.6250 cal yr BP in the east valley entrance.2)When the foraminiferal proxies of core YJ1505 indicating a shallow sea water or limnetic environment,environmental magnetism proxies χarm/χ and Xarm/SIRM showing a low value;While the foraminiferal proxies indicating a deep sea water,χarm/χ and χarm/SIRM showing a high value.This suggests that environmental magnetism proxies χarm/χ and χarm/SIRM can be used to indicate environmental transition in the valley.Meanwhile,when no foraminifera appearance in core YJ1508 that indicating a limnetic environment,geochemical element proxies Fe/Mn,Sr/Ba and Ca/Ca+Fe showing a low value;while the foraminiferal proxies indicating a marine transgression environment,geochemical element proxies Fe/Mn,Sr/Ba and Ca/Ca+Fe showing a high value.In this way,the foraminiferal proxies combined with the ratio of Fe/Mn,Sr/Ba and Ca/Ca+Fe can restore the environmental evolution process accurately in the study area.3)The rise,development and decline of the Neolithic civilization in the Yaojiang Valley are closely related to the Early to Middle Holocene marine transgression and regression processes.A relatively rapid sea level rise was indicated from ca.9200-7900 cal yr BP from marine transgression,the costline towards the westwards.A regression process happened afterwards,The YJV transitioned from a marine-influenced setting to a limnetic habitat and wetland environment after ca.7600-6500 cal yr BP from west to the east,which would have attracted early human occupation in the valley.The formation of a fluvial plain after ca.6500-5000 cal yr BP would have facilitated the development of the Hemudu culture and rice farming.The higher hilly margins of western Yaojiang Valley might have provided safer shelter for the innovative pioneers,and this provides new clues for the exploration of earlier human occupation comparable to the Kuahuqiao culture at the head of the southern Hangzhou Bay coast.The decline of the Hemudu culture in the YJV is still an open topic,and an integrated geological,geomorphological and archaeological study is needed to combine both natural and social factors.This would provide some insights into this key issue.