Geochronology And Paleoenvironmental Evolution Of Xujiayao Site Based On Regional Sedimentary Records

The rich paleohuman activities and detailed records of paleoenvironment evolution in Nihewan Basin make Nihewan Basin an important area to explore the relationship between ancient activities and paleoenvironment.Although some scholars have tried to explore the relationship between paleohuman activities and environment in Nihewan area,the relationship among the three major geological and historical events,namely,regional paleohuman activities,volcanic activities and lake retreat lake transgression,has not been clear.Xujiayao site,located in Nihewan Basin,is close to Datong Volcano Group on the west side in space and contains lake facies sedimentary layer in the stratigraphic section.Therefore,Xujiayao site has become an analytical sample area for exploring the relationship between ancient human activities and volcanic activities and lake retreat Lake intrusion events in Nihewan site group.This paper starts with regional sedimentary records,and constructs regional age framework by paleomagnetism and photoluminescence dating methods;analyzes the lake retreat Lake intrusion record by the data of magnetization and grain size;analyzes the volcanic activity record based on the data of magnetization and metal elements;and analyzes the paleoclimate environment evolution of the region by palynological data.Through the above analysis,the age and the characteristics of the ancient environment of Xujiayao site are determined,and the relationship between the site and the ancient environment is explored.Finally,based on the analysis of the regional Lake retreat lake transgression history,volcanic activity history and paleoclimate change history,the relationship between regional volcanic activity,Lake retreat Lake invasion and paleohumanity in Nihewan Basin is discussed comprehensively.The main work and conclusions are as follows:(1)The results of demagnetization of paleomagnetic samples from Xujiayao section 2 show that the age of Xujiayao 2 section is no more than 780000 years;the photoluminescence dating of the samples of Lake sand clay layer near the upper and lower layers of Xujiayao site section shows that the age of Xujiayao site is 138.1 ± 12.7ka.(2)Through the analysis of the magnetic susceptibility,particle size and metal elements of the typical section of Xujiayao site,it is shown that the Xujiayao site typically records the active period and the first lake retreat,and the volcanic active period and the lake retreat period are in the depth of 0-2m.(3)Based on the analysis of the sporopollen data of Xujiayao site section,the paleoenvironmental history recorded in the typical section of Xujiayao site is revealed.The Xujiayao site section can be divided into four sporopollen zones,showing the change of warm dry cold wet warm dry.The sporopollen assemblage of the corresponding layer in the cultural layer shows the coniferous forest vegetation type in the cold temperate zone,The paleoclimate during the period of Xujiayao site is analyzed quantitatively by using coexistence analysis method: the average annual temperature is 3.8 ℃-10.2 ℃ and the annual precipitation is 403.4 mm-1133 mm.(4)The relationship between the two sections is established by comparing the Xujiayao site profile with Xujiayao 2 section,that is,the top of the cultural layer of the site(2.8m)corresponds to the section 2 of Xujiayao at 2m.The results of photoluminescence and paleomagnetic dating show that the volcanic activity recorded in Xujiayao site profile is consistent with the time of the disappearance of the ancient lake,about 110000 years later.(5)Based on the analysis of the magnetic susceptibility,particle size,metal elements and sporopollen in the typical section of Xujiayao site,the relationship between Xujiayao site and the ancient environment is discussed.The ancient environment is closely related to the survival,distribution and development of Xujiayao people.Especially,paleoclimate change,crustal movement and volcanic activity,ancient lake and river evolution have deeply influenced Xujiayao people’s survival,distribution and development.When Xujiayao people lived in the environment of the disappearance of ancient lake and river development,the lakeside and river could provide water and food for Xujiayao people.After Xujiayao’s life,a volcanic activity occurred,which indicated the drastic movement of crust material at that time.This indicates that the violent movement and volcanic activity of the crust are not the direct cause of Xujiayao people’s death,but they will become an important factor restricting Xujiayao people’s survival here.Xujiayao has a cold and humid climate in his life.The cold climate makes Xujiayao people survive in this difficult place.The humid climate may breed a large number of animal and plant resources,which also provides Xujiayao people with food and some resources for survival.(6)Based on the study of Xujiayao site and the literature of Nihewan Basin,the relationship between ancient human sites and environment in Nihewan Basin is explored.The results show that the lake retreat Lake invasion and volcanic activities affect the distribution of ancient human sites in Nihewan Basin,especially after 0.90 Ma BP and 0.09 Ma BP.In the period earlier than 0.90 Ma BP(i.e.before the first phase Lake retreat),with the rise of the water in the ancient lake of Nihewan and the expansion of the area of the ancient lake,the ancient humans had to move far away from the ancient lake and higher places.In the period after 0.09 Ma BP,with the disappearance of ancient lake,the development level of Sanggan River is lower and lower,and the distribution of ancient human sites is also close to the center of ancient lake,and the distribution altitude is also lower and lower.