| With Chaohu Lake as its study object,which is located in the areas along the middle and lower reaches of the Yangtze River,and taking the methods of field investigation, comparison among the sedimentary piles,drilling lacustrine deposits core in the centre of lake,laboratory analysis of sediments,textual research of documents as well as an analysis of natural disasters in the historical periods et al,this doctoral dissertation focuses on exploring such issues as the environmental events and the environmental evolution process whose records were kept in the deposits,natural disasters and their sequences reflected in historical documents,interaction process between human activity and drainage area environmental change against the background of global environment and climate evolution in Holocene.It is concluded that:1.Four remarkable characteristics have been shown in the environmental evolution of Chaohu Lake since the Holocene metaphase:There has been a frequent phase transition process both temporally and spatially in the fluvial-lacustrine deposits.The area of Chaohu Lake has frequently expanded and contracted in accordance with the ascent and decent of lake level.Dry and wet climate indicated in the lake sediments have changed frequently.The influence of human activity has gradually increased.2.According to an analysis of the age data,granularity and magnetics of deposits in Chaohu lake,sample core CH-1 could be divided into 4 sedimentary chronological sections,to wit:①1 177 aA.D.to 1 271 aA.D.is proven to be a wet period,matched along with the Medieval Warm Period(MWP ).②1 271 aA.D.to 1 747 aA.D.is proven to be a wet / dry alternative action period, with a total of 5 extreme drought events,including the drought maximum in 1 750 aA.D.or so,which indicates the climatic characteristics of The Little Ice Age(LIA ) in East China.③1 747 aA.D.to 1 865 aA.D.is proven to be a continuous milder stage after The Little Ice Age Maximum.④1 865 aA.D.to Present is proven to be a colding and drying climatic process,whose maximum happened in the 1930's A.D.3.According to a 15-indicator analysis of the chemical elements and mineral nutrient of deposits in core CH-1,it is shown that lacustrine deposits in Chaohu Lake basically indicate the MWP,LIA and the cold period in the early 20th century.After taking normalization of the chemical elements and mineral nutrients content,it is seen that the contents of Cu and Zn have increased with unnatural levels since 1860s,while the content of mineral nutrients has enriched with unnatural levels since 1660s.4.According to an analysis of the clay mineral of deposits in core CH-1,it is shown that there was an extreme cold and dry event in 1 912 aA.D,with other cold and dry events separately in 1 865 aA.D.,1 536 aA.D.to 1 583 aA.D.and 1 442 aA.D..It is also proven by the Hw,IJ and Ns values of Illite that the general trend of climatic environment is to be much warmer and wetter for the past approximate millennium.5.The sporopollen spectrum of deposits in core CH-1 could be divided to 3 pollen zones according to the sedimentary chronology,to wit:①a wetter climatical environment from 1 177 aA.D.to 1 477 aA.D..②a moist climatical environment from 1 477 aA.D.to 1 771 aA.D..③a moist climatical environment from 1 477 aA.D.to 1 771 aA.D.,but the humidity level was a little lower than before.The wet-dry weather change circle can be quantificationally divided by the Wood Index,to wit:①1 177 aA.D.to 1 477 aA.D.was a moist and warm period in which the forest area was absolutely dominant on the earth,and also just stepped over the Medieval Warm Period.②1 477 aA.D.to 1 853 aA.D.was a period in which the forest in surrounding areas of Chaohu Lake degraded seriously,just matching with the whole The Little Ice Age.The Wood Index offers a new approach to quantitative description of vegetation environment with spore and pollen analysis materials.6.According to the exploratory research of Environmental Change Events indicated by the deposits in core CH-1,the history of the environmental change in surrounding areas of Chaohu Lake for the past approximate millennium could be divided into 6 periods according to the sedimentary chronology.①1 177 aA.D.to 1 300 aA.D.was a moist and warm period in the mass,but short-term extreme drought events also happened at that time.②1 300 aA.D.to 1 480 aA.D.was mainly a moist and moderate period so that the forest flourished very much.③1 480 aA.D.to 1 600 aA.D.was a dry and moderate period in the mass so that the forest started to degrade.④1 600 aA.D.to 1 760 aA.D.was a dry and cold period in the mass so that the forest degraded seriously.⑤1 760 aA.D.to 1 880 aA.D.was a wet and moderate period in the mass.⑥1 880 aA.D.to present has been a wet and warm period in the mass so that the forest has flourished again.7.Through selection of the principal component analysis,8 target variables that closely reflect the information of environmental change are confirmed.Thereby,the cluster analysis of deposits is conducted,and the deposits are further classified into 3 types and 4 subtypes.Based on the classification,integrative sequence of Chaohu Lake Area for the past approximate millennium can be rebuilt:1 177 aA.D.to 1 241 aA.D.was a dry and moderate period;1 241 aA.D.to 1 488 aA.D.was a wet and moderate period;1 488 aA.D.to 1 635 aA.D.was a dry and moderate period;1 635 aA.D.to 1 753 aA.D.was a dry and cold period;1 753 aA.D.to 1 818 aA.D.was a dry and warm period;1 818 aA.D.to present has been a wet and warm period; 8.1 750 aA.D.or so is an important boundary of Chaohu Lake environmental change history.On the one hand,there are remarkably different sedimentary characteristics between its before and after reflected by parameters of magnetics and granularity in deposits,which may indicate an adjustment that occurred during this period;on the other hand,there are notable different characteristics of the environmental change between its before and after,demonstrating a cold climate beforel 750 aA.D.and a warm climate afterl 750 aA.D.in the mass.9.Generally speaking,the Chaohu lake sedimentary records for the past approximate millennium respond to the global environment change well:on the one hand,they indicate the global MWP / LIA clearly and definitely;on the other hand,they are almost as same as the result of other work.10.After taking normalization of the chemical elements and mineral nutrients content, the Enrichment indices for Cu,Zn are the highest,correlating well with the clay constituents and magnetic parameters,so it is thought that the contents of Cu and Zn have increased with unnatural levels since 1860s,which could be considered as metal pollution impacted by human activity and is the sedimentary response to the origin of industry in Anhui province(Anqing Inner Armory,founded in 1861).An analysis of the contents of mineral nutrients indicates one mark of the element P's enrichment with unnatural levels during the 1660s which explains why the amount of population and arable land had greatly increased during that time.Another mark of the enrichment of TOC,TN,TP with unnatural levels during the 1880s explains why the amount of population and arable land greatly increased again.Sediment records prove that Lake eutrophication had started in Chaohu area since that time.Normalization of the chemical elements and mineral nutrients content could be a new method to discriminate the enrichment as the unusually natural reason. 11.Natural disasters have been frequent for the past approximate 500a in Chaohu drainage area,and occurred in Qing Dynasty more frequently than in Ming Dynasty. Floods are more serious than droughts.The formation discipline of these floods and droughts is in close agreement with the variation of wet/dry climate.It is thought that floods and droughts for the past approximate 500a in Chaohu area result from the drive of climatic environment change,whose mainly formative reason amounts to the natural factor.At the same time,the attenuation of climatic stability is also a formative reason of the frequent and serious disasters.12.The conditions of floods and droughts for the past approximate 500a in Chaohu area illustrate that human adaptation to disasters still stayed in the passive mode in Ming and Qing Dynasty.Positive adaptation to environmental change needs that human beings accurately master the law of Environmental Change,which depends on the further research of Global Change.In this thesis there is still room for improvement in areas such as analysis of the sedimentary chronology,acquisition of High Resolution Deposit Sample,explanation to the difference among the proxy indicators of environmental evolution,lacustrine deposits response to human activity et al.,which is expected to be explored with the further study in future.As is shown in the thesis,three aspects,namely the sediment records analysis of the fluvial-lacustrine-interacted lake in the areas along the Yangtze River in Holocene(especially since Late Holocene),the sediment records responses to the human activity of the lake in the areas along the Yangtze River in historical periods,and the human adaptation to environmental evolution,should be the primary issues in future research.
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