The Environmental Evolution And Climatic Changes Of The Lower Yellow River Area During The Last 1400a From Lacustrine Sedimentary Records

The ancient Lake Dayeze is located in East Asian Monsoon area.It is very sensitive to climatic change owing to its location.During the historical periods,it has ever been a reservoir of several large rivers,which have experienced a number of times of avulsion due to the overbank flooding and the breaches of natural levees of the Lower Yellow River(LYR),making it an ideal site for revealling the Yellow River diversion and the environmental evolution of the Lower Yellow River region.In this thesis,a multi-stratigraphical study(e.g.grain size,magnetic susceptibility,total organic carbon,chemical elements)in conjunction with high-resolution AMS-¹⁴C dating is conducted on a550cm drill core from the ancient Lake Dayeze depression to reconstruct the depoits environment,paleoenvironmental,paleoclimatic and erosion changes in recent the 1400years.The results are summarized as followings.1)Utilizing the method of grain size frequency distribution curve and cumulative probability curve of the sediments,combined the lithology characteristics,the sedimentary environment of the study area can be divided to several characteristic facies:550-510 cm,belong to shallow marsh sedimentary environment;510-385 cm,developed to deep water lacustrine environment;385-260 cm and 90-30 cm,converted to strong hydrodynamic fluvial or flood sedimentary environment;260-90 cm,belong to weak hydrodynamic fluvial or flood sedimentary environment.2)Integration of multi-stratigraphical study(e.g.grain size,magnetic susceptibility,total organic carbon,chemical elements)in conjunction with high-resolution AMS-¹⁴C dating,as well as historical documents,we infer the ancient Lake Dayeze has experienced five distinct stages of environment,climatic and erosional changes during the last 1400 years.The results show that a low lake-level stand with a swamp sedimentary facies during625?940 AD,under dry and cold climate conditions in the study area,and a weaker chemical weathering was experienced around the ancient Lake Dayeze region under dryer and colder climate conditions.Following,the lake expanded and the lake level rose due to the flooding of the lower Yellow River,under warm and wet climate conditions during940?1250 AD,corresponding to the Medieval Warm Epoch,which further aggravated the erosion of the drainage basin,as a result,the erosion the ancient Lake Dayeze region was pretty strong.During 1250?1540 AD,the river is characterized by strong hydrodynamic force with fluvial environment or flood sedimentary environment,under a relatively humid climate,however,the flooding of the Yellow River rarely affected the Lake Dayeze area,the erosion the ancient Lake Dayeze region was relatively strong under background of the humid climate.and then,the river hydrodynamic reduction,under the dry and cold climate,corresponding to Little Ice Age,resulting in weaker erosion of the ancent Lake Dayeze area.Till 1860 AD,the precipitation began to increasing again under the background of Global Climate Warming,meanwhile,the Yellow River was diverted to the north again,as a result,the ancient Lake Dayeze was once again affected by the flooding of the Yellow River,and the erosion of the Lake Dayeze watershed was strengthened again.However,Lake Dayeze only used as the sediment bearing area of the LYR,due to the study area has been elevated to land.3)The environmental evolution and the basin erosion of the ancient Lake Dayeze are influenced by climate change and the Yellow River diversion floodings.However,under the background of climatic change,the Yellow River diverted to overflowing is the main cause for the environmental evolution and erosional changes of the ancient Lake Dayeze region.The ancient Lake Deyeze lost the water source due to the Yellow River diverted away,as a result,the ancient Lake Dayeze silted up,and the erosion of the watershed basin was weaker.The above results systematically reconstruct the evolution process of the ancient Lake Dayeze in the past 1400 years and its relationship with the Yellow River,which is of great significance for further understanding the evolution of lacustrine wetlands in the Lower Yellow River and the relationship with the Yellow River.