Characteristics Of Fill And Evolution Since Holocene In The Taihu Lake

The evolution history of Taihu lake is short,however,it recoverds evolution of humen culture and several changes of Chinese climate since the last glacial epoch.Previous studies about Taihu lake pay much attention on sedimentation around the whole Quaternary which is too wide to be focused on,others prefer to 1 or 2 cores and miss many important information in the crosswise distribution of stratum.Our previous study find it out that there is a argillaceous stratum with iron-rich nodule around the Taihu lake,but the previous study pays more attention on nodule and less attention on stratum.Granularity is basic but also the most important one in the sedimentary characteristic,as it is easy to be tested and has a universality,granularity is always the first characteristic studied.The TOC(total organic carbon)content can reflect the number of animals and plants during the sedimentary age,so that it can reflect the tiny change of climate in sedimentary area,too,what make it be a key characteristic to study environment and rebuild paleoclimate of study area.Clayed soil is key to magnetic susceptibility of weathered stratum and soil,and it is related to climate as well as rainfall,so that we can analyze climate according to studying the magnetic susceptibility and comparing it with granularity and TOC content.At the last,the content of element can show content of mineral in sedimentation and weathering level of stratum,and help us to analyze the paleoenvironment.According to analyzing these characteristics,we can systematically rebuild paleoenvironment and paleoclimate in study area.According to analyzing granularity,TOC content,magnetic susceptibility and element content of core YLL1 and core JS1 in Xishan island,Taihu lake,we find it out that there is the similar trend in characteristics of two cores.Both of them have a middle part with fine particle and high clay content,what is more,in this middle part sedimentation,standard deviation of average diameter of particle is small which means the average diameter changes few as the depth increasing.Magnetic susceptibility of middle part sedimentation is the biggest in both of two cores,and TOC content of middle part sedimentation is the biggest in core YLL1.As for the element content,content of element K,Ca,Fe in middle part sedimentation is higher than other part's,but content of Si is much smaller than other part's.As it is,we can regard the middle part sedimentation as a special key bed which is far different from upper part and lower part.This special landmark layer widely occur in Taihu lake area containing abundant siderite concretions and iron-rich spherule concretions.14C data aging from the middle layer suggests that the middle layer formed in?7 ka BP.The sedimentary records of two cores of Taihu lake show three different lake basin environments.The lower layer is the hard loess layer of weathering origin after the last ice age.Taihu lake basin is not formed at that time yet.The middle landmark layer is corresponding to the mud deposit layer at?7 ka BP when the lake basin were formed and the whole basin were full of water for the first time.The upper layer represents the modern Taihu lake deposit layer when the whole basin were full of water continually since?5 ka BP ago.