Mid-Holocene Geomorphologic And Hydrologic Environmental Change And Human Response In The Ningbo Plain,East China

The early human societies were highly dependent on the natural environment due to their low productivity.Coastal wetlands provided a favourable settling site for the Neolithic people because of their highly exploitable biomass.However,the hydrologic and ecological environment of the coastal lowland is extremely vulnerable to marine hazards such as coastal flooding.The adaptation of the prehistoric people to the coastal wetlands is a hot topic of current environmental archaeological studies.Since the middle Holocene,a deceleration of sea-level rise caused the near-synchronous development of these deltas,and encouraged the seaward migration of the prehistoric people.There are many Neolithic sites discovered in Ningbo Plain on the south coast of Hangzhou Bay,East China.It is reported that layers of natural deposits always sandwiched between cultural layers at the archaeological sites in this region.Stratigraphy at these sites thus may have recorded the catastrophic events of coastal flooding in the middle Holocene and the response of prehistoric people to the great changes of living environment.Studying these archaeological records can provide a good case study for understanding the internal relationship between sea level change,geomorphologic environment,extreme events and prehistoric people activities in the coastal areas of the middle Holocene,and a reference for the preparation for the global warming and sea level rise in the future.In this study,two sedimentary profiles from the excavation trenches were obtained from the Neolithic Wuguishan site,which is located in the northeastern part of the Ningbo Plain and is close to the present coastline,and the Neolithic Xiawangdu site,which is located in the southwest part of the Ningbo Plain.The two profiles were analyzed for sedimentology,chronology,organic elemental and isotopic compositions and alkaline earth(Ca,Sr,and Ba)elemental compositions.Stratigraphy,chronology data and cultural context of other typical Neolithic sites reported in the YaojiangNingbo Plain were also collected and compiled.Surficial sediments were collected from the present-day alluvial plain,tidal river,and saltmarsh/tidal flat environments in the Ningbo Plain to test the sensitivity,feasibility and reliability of using the alkalineearth metals in the HAc-leachates to identify the sea water intrusion in the coastal plain.The sedimentary environmental evolution including the extreme events during the Neolithic period were reconstructed in the two profiles,and the mechanism of the cultural interruptions was analyzed.Furthermore,this thesis discussed the midHolocene hydrologic and geomorphologic environmental change and the associated adaption strategies of prehistoric people in the Ningbo Plain.Main results were obtained as follows:1)Based on the comparison of three extraction methods,the results showed that alkaline-earth metals in the HAc-leachates were the most sensitive components to the seawater,followed by the HCl-leachates.No variation in Sr/Ba can be distinguished in the bulk sediments of the three types of surficial sediments.Consistent results of both surficial and profile samples were obtained by using different sample amount and measuring instrument when applying for the HAc method,further indicating that the HAc method is feasible and reliable to extract alkaline-earth metal elements linked to the sedimentary processes.At last,the comparison of the alkaline-earth metal elements from different types of surficial sediments demonstrated that significant variations of the alkaline-earth metals in the HAc-leachates occurred for the surficial sediments in the Ningbo Plain.Ca and Sr concentrations were the highest in the saltmarsh-tidal flat sediments and the lowest in the alluvial sediments,while Ba concentration was reversal.We thus suggest that end-member analyses of the alkaline-earth metals in the HAc leachates can be used to effectively identify the transgressions and regressions recorded in the sedimentary stratigraphy in the coastal zone.2)At the Wuguishan site,sediments of 6300-6000 cal.yr BP were characterized by high Ca and Sr concentrations and high Sr/Ba ratios in the HAc-leachates,and high total inorganic carbon contents,which coincided with the marine incursion recorded at the nearby Neolithic Yushan site.Sr and Ca concentrations and Sr/Ba ratios decreased after ~6,000 cal.yr BP,indicating that the site developed into a low-salinity marsh in the supratidal environment after the rapid accumulation by a storm event at ~6,020 cal.yr BP.In addition,the typhoon event at ~4510 cal.yr BP was also recorded at this site.The geomorphological setting at the Xiawangdu site was characterised by the tidally influenced river channel and wetland environment,which was formed along with the outbuilding of the Fenghuajiang alluvial plain as a response to the regression in the backdrop of relative stable or falling sea level from ~5600 cal.yr BP.Extreme typhoon events were recognised at ~5300,5230-5145,and 4575-4330 cal.yr BP in the buried tidally influenced river channel at the Xiawangdu site according to the coarsening grain size and high values of Sr concentrations and Sr/Ba ratios in the HAc leachates.3)A compilation of stratigraphic records and dating results at various Neolithic sites in the Yaojiang-Ningbo Plain indicated several seawater intrusion events during the middle Holocene.However,spatial variations occurred for these events.The transgression at ~6300 cal.yr BP was recorded in the northeast Ningbo Plain and the Yaojiang Valley and the sedimentary records of storm event at ~4500 cal.yr BP was found in the whole Ningbo Plain.By contrast,the transgression at ~5300 cal.yr BP only occurred in the southwest Ningbo Plain and the Yaojiang Valley.4)The stratigraphic records at Wuguishan and Xiawangdu sites also showed the shaping effect of extreme events on local morphology.For example,storm of ~6020 cal.yr BP formed a gentle ridge at Wuguishan site.Flooding of the Fenghuajiang River formed the natural levee,and the Hemudu people at Xiawangdu site lived in the marsh behind the levee.5)The extraordinarily high Ca and Sr concentrations in the Hemudu cultural layers at Wuguishan site reflect seafood consumption,indicating that prehistoric people made full use of the food resources provided by the coastal wetland;while widespread occurrence of coarse sands,gravels,burnt earth,and pottery fragments characterised by high Ba concentrations suggests that the the Hemudu people accumulated these materials intentionally so they could reach the saltmarsh and tidal flat more conveniently using hardened pathways.Besides,the artificial terrace of late Liangzhu culture appeared above the flood layer of ~4.5 cal kyr BP at Xiawangdu site,which reflected that prehistoric people tried to resist the threat of marine hazards by using slightly elevated man-made mounds.These phenomena reflected the survival strategies of prehistoric people for living in the vulnerable coastal lowlands during the Neolithic period.