Sedimentary Characteristics And Paleoenvironmental Significance Of Zhifu Section In Yantai City During The Last Interglacial Period

A series of aeolian sediments are widely distributed in Shandong area,which con tain rich information of paleoclimate environment,and are good carriers for studying t he change of Quaternary paleoclimate environment.Because of their special geograph ical location and the double influence of ocean and land,it has certain research value and fully excavates its paleoenvironmental information.It is of great practical signific ance to reveal the climate change and monsoon evolution in this region during the last interglacial period.On the basis of many comprehensive field investigations in the no rthern part of Shandong Peninsula,the Zhifu section located in the northern part of Ya ntai City is selected to work.By using the climatic substitute indexes such as grain siz e,Ca CO₃and Ph value,combined with the results of OSL dating,the sedimentary cha racteristics of the last interglacial period of the loess-paleosol sequence in the Zhifu se ction are analyzed and compared with the deep-sea oxygen isotope and the magnetic s usceptibility curve of Chaona loess.The process of climate fluctuation and monsoon e volution during the last interglacial period in this region is revealed.The main conclus ions are as follows:(1)The test age of aeolian sediments in Zhifu section is 62.85-124.9 ka,which b elongs to the last interglacial period,which can be divided into two stages:MIS4(62.85-70.18 ka),MIS5a(70.18-83.4 ka),MIS5b(83.4-94.7 ka),MIS5c(94.7-105.2 ka),MIS5d(105.2-118.5 ka)and MIS5e(118.5-124.9 ka).The loess layer corresponds to t he MIS4,MIS5b and MIS5d stages,and the paleosol layer corresponds to the MIS5a,MIS5c and MIS5e stages.(2)The particle size composition of Zhifu section is mainly composed of silt(74.95%)and clay(10.05%),with low content of very fine sand(9.57%)and fine sand-co arse sand(3.98%).The average particle size is 5.77?and the separation coefficient is2.05.The grain size frequency curve shows the characteristic of bimodal distribution.The environmental sensitive granules were extracted by particle size-standard deviati on method and principal component factor analysis.It was found that the sensitive gra nules reflecting the winter monsoon were 27.904-94.574?m,indicating the dry and c old climate,and the sensitive granules reflecting the summer monsoon were 196.714-462.281?m,indicating the warm and humid climate.(3)The results of CaCO₃ content and pH value show that the average content of Ca CO₃ is only 0.138%,the content is low,and the average value of p H is 6.927,whic h belongs to acidic soil.The Ca CO₃ content and p H value of loess layer are higher tha n that of paleosol layer,and their high values indicate cold and dry climatic conditions,indicating that MIS5d,MIS5b and MIS4 are cold periods,while low values indicate warm and humid climatic conditions,indicating that MIS5e,MIS5c and MIS5a are w arm periods.(4)CaCO₃ content,p H value and average particle size show alternating changes of peak and valley in the vertical direction of the profile.The warm stage is synchroni zed with the strong summer monsoon period recorded by deep-sea oxygen isotopes,re flecting warm and humid climatic conditions,strong weathering and soil formation,a nd the development of paleosol;the cold stage corresponds to the strong winter mons oon period recorded by deep-sea oxygen isotopes,reflecting dry and cold climatic con ditions.In this stage,the winter monsoon is enhanced,the weathering soil formation i s weak,and the loess is widely developed.