Quaternary Paleoenvironment And Paleoclimate In Chengdu Basin

Chengdu Basin is located in the west of Sichuan Basin,which belongs to the alluvial plain in the central and southern section of Longmen Mountain.The Quaternary sediments are widely distributed in Chengdu Basin,which records the relics of the environmental change in many historical periods.Different plants require for different ecological environments.In certain ecological environment,the humidity and the temperature are the dominant factors for plants to grow.Therefore,the research on the sporopollen stored in the strata can roughly reflect the climate law at that period of time.The research of the paleoenvironment and paleoclimate of the selected sections within the research area as well as the exploration on the paleoenvironment and paleoclimate of the Chengdu Basin are significant for the reconstruction of the Chengdu Basin's paleoenvironment and paleoclimate.This thesis mainly selected different sections of different ages of the Chengdu Basin in the west of Sichuan Basin to conduct the actual measurement and sampling,and conducted the sporopollen test,the granularity test,and the magnetic susceptibility test on the collected samples.According to the analysis of the corresponding experimental results in combination with the previous researches on the paleoenvironment and sediment characteristics of the Quaternary in the Chengdu Basin,it draws the conclusions as follows:(1)The results of this statistical analysis show that the sporopollen species in the five sections are about 64 families per genera,including 24 families of woody plants;the sporopollen species of shrub plants are about 11 families per genera;the sporopollen species of herbaceous plants are about 19 families per genera;the spores of the pteridophyta and the phycophyta account for about 10 families per genera.On the basis of the variation of the sporopollen species,content and the assemblage of the Quaternary section in Chengdu Plain,it can be divided into the landscapes as follows: the vegetation landscape of coniferous forest or the coniferous broad-leaved mixed forest(late early Pleistocene),forest grassland landscape(early Medio-Pleistocene),sparse grassland landscape(Medio-Pleistocene),the vegetation landscape of the deciduous broadleaved forest(late Pleistocene),and the deciduous broadleaf landscape with a small number of herbs under the development(Holocene,during which meadow landscape may appear due to a cooling event).(2)According to the comprehensive analysis of the sporopollen assemblage,the magnetic susceptibility characteristics,and the granularity parameter characteristics of Chengdu Basin,it is concluded that all the Quaternary sediments in Chengdu Basin belong to the alluvial origin.(3)According to the sporopollen analysis,the magnetic susceptibility analysis,and the granularity analysis on the samples of the actual measurement sections of the Chengdu Basin,it shows that the Quaternary climate in Chengdu Basin mainly experienced five stages as follows: the cold and wet stage,the warm and humid stage,the warm and semi-humid stage,the warm and humid stage,and the humid and hot stage(two obvious cooling events occurred during this period).(4)According to the sequence of the sporopollen assemblages and the previous researches on the Chengdu Basin,it is concluded that the climate and environment in the Holocene were generally humid and hot,and a significant cooling event occurred in this stage,which has a good correspondence with the global cooling events about 3000 years ago.In the sporopollen samples taken from the upper part of the section of the Longquan Mountain,it was found that the concentration of the sporopollen decreased again.It is inferred that the time was before 1200 years,which could correspond to the cooling events 900 years ago.