| With the deteriorating of climate and environment,the glbbal environmental change has become one of the most eye-catching social problems and scientific problems.Holocene has the closest relationship with mankind,and there are many prehistoric human cultures flourishing and declining during this period.This period have a pivotal role in climate change prediction and human development Therefore,research on climate change during this period is the focus of the PAGES researches.At present,the research about Holocene environmental change has been widely concerned all around the world.In the contemporary era,the geographical environment of Zhejiang Province is characterized by developed and densely populated.Especially after the G20 summit(G20)held in Hangzhou,Zhejiang has attracted more attention from all over the world.As a sensitive area of global environmental chrange,the future changes in the environment will undoubtedly affect the sustainability of Zhejiang's economic and social developmen.In this paper,the BHQ2 drilling hole(BHQ2),located in the Hangjiahu Plain on the north bank of Qiantang River,was selected as the main research object to study sediments sporopollen records.Combined with the Xinyicun drilling(XYC)in Xiaoshao Plain,in south bank of Qiantang River,the second-hole sediments were also studied to eplore the mid-late Holocene vegetation and climate evolution characteristics and to reconstruct the regional paleogeographic environment Through the study,the following conclusions were drawn:(1)The spectrum of Trees and Shrubs of core BHQ2 showed that in the study area,the main evergreen broad-heaved species in the mid-late Holocene vegetation were Quercus(Evergreen type),Cyclobalanopsis,Coriolis/Castanopsis,Sapindus,Euphorbiaceae,morus,etc.The main deciduous broadleaf species were Quercus(Deciduous type),Fagus,Elm,Celtis,Rosaceae,Corylus,Betula,Alnus,Salix,liquidambar,Pterocarya,Juglans and so on.The main coniferous species was Pinus.(2)Analysis results of pollen assemblages showed it was a climax of the Holocene warm period during 7700?5900 cal a BP,when subtropical evergreen broadleaf species were dominated ones in terms of vegetation composition.It reflected the strong East Asian monsoon and very warm and humid climate.During 7700?7610 cal a BP,the content of wet herbs were high and aquatic herbs gradually increased,which indicated that where were swamp development in the sampling area around the lake.And the emergence of Pseudomonas aeruginosa indicated that the study area might have had intermittent tidal invasion.During 7610?5900 cal a BP,Spiniferites Mantell suddenly broke out,and its content and concentration reached the highest value,which indicated that the study area might be affected by transgression It was inferred that this period was the highest sea level of the Holocene.(3)During 5900?5500 cal a BP,analysis results of pollen assemblages showed in terms of vegetation composition,the weather started getting cold,when the average proportion of subtropical evergreen trees decreased to 43%,temperate deciduous tree species rose to 34%(Quercus occupying the vast majority),the average percentage of coniferous trees rose to 23%(Pinus occupying the vast majority).In terms of vegetation composition,conifer species and deciduous species rose,with the proportion of evergreen trees decreased.Additionally,the pollen spectrum of Trees and Shrubs showed the concentration and content of temperate deciduous tree species rose with fluctuations,especially the genus Quercus,Celtis,Juglans.Subtropical evergreen tree species showed a downward trend with fluctuations indicating a decreased temperature.And the content of Spiniferites Mantell decreased rapidly and nearly approached 0.Aquatic herbaceous content followed by an upward trend.It reflected the reduction of sea level.The corollary was made that an environmental event(related to the global climate of 5.5 ka BP).(4)During 5500?3500 cal a BP,the sporopollen concentration of each family and genus further decreased especially the trees,shrubs,damp herbs raised in a damp environment.Among the trees and shrubs,Queus(E)and Cyclobalanopsis decreased most obviously,which reflects the climate further cooling.During 5500?4000 cal a BP,Spiniferites Mantell appeared relatively concentrated and had a high concentration.But the percentage content and concentration had small increasing amplitude.Therefore the corollary was made that there was a salt water intrusion.The decrease of density of damp herbs rose in damp environment and aquatic environment indicates a drying climate.During 4000?3500 cal a BP,the peak of percentage of coniferous species appeared once again(average percentage 42%).The percentage of evergreen trees suddenly abruptly temporarily reduced(averaging 32%).In terms of plant composition,coniferous trees became dominant.And all spore concentrations in this period decreased to the lowest stage,which reflects a cooling climate.The density of damp herbs raised in aquatic environment and Spiniferites Mantell decreased gradually.And the density of damp herbs raised in damp environment increased slightly,which indicates a falling sea level and that lakes had a tendency to develop to swamps.Therefore the corollary was made that the cooling event was related to the 4.2 ka BP event.However,the time of this cooling event greatly closes to the period when Liangzhu Culture disappeared.There is such a big coincidence between the climate of cooling event and cultural faults,which may implies that it was the 4.2 ka BP event that led to the decline of Liangzhu culture.About in 4000 cal a BP,the climate began to cool.Therefore the corollary was made that about 4000 cal a BP was the end time of Holocene warm period in studied area.(5)During 3500?1200 cal a BP,the evergreen broadleaved tree species began to rise with fluctuations from 3500 cal a BP.The proportion of evergreen trees reached 51%and the proportion of temperate trees was 31%while the percentages of coniferous trees were only 18%,indicating that the evergreen tree species became the dominated ones once again in terms of the vegetation composition and the temperature fluctuated rose with fluctuations.(6)The climate began to change again during the 1200?460 cal a BP period.In terms of pollens of trees and shrubs,the proportion of evergreen trees fell to 19%,and the coniferous species rose to 53%and became the dominated ones.In terms of percentage of pollens,damp herbs raised in damp environment occupied the main part.The spore-pollen concentration of each amily and genus was still in a low state,indicating that the temperature was still in the cold.However,the proportion of damp herbs raised in aquatic environment and algae rose sharply,which indicates that lakes began to develop again.The density of gramineae,whose grain size is greater than 37?m,appeared a gradual upward trend and reached a new higher density finally,which implies a strorger human activities.(7)In the BHQ2 drilling hole,the density of gramineae,whose grain size is greater than 37?m,suddenly dropped significantly in 4000 cal a BP.And the period is very close to the disappearance of Liangzhu culture period in terms of time.It suggests that it is possible that the 4.2 ka BP event interrupted all human activities.However,in the whole percentage map of spore pollen in BHQ2,the proportion of temperate deciduous trees has almost never changed.From the point of view of changes in woody plarts,subtropical evergreen species were replaced by coniferous coniferous species.But it is contradicted with the argument of human activities of deforestation and strengthening agriculture. |
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