The Research On Holocene Climate Changes And Geomorphic Evolution In The Mu Us Desert,China

Since the end of the last glacial period,the Holocene has been the closest geological period to modern times,which is called the modern interglacial period.There is a generally warm climate in the Holocene,but there are also periodic climate fluctuations,reflecting the spatial and temporal instability.Located at the monsoonal boundary areas in China,Mu Us desert is sensitive to climate change.And there are rich and intact sedimentary sequences in the southeast edge of the desert,keeping track of climate change and geomorphic evolution in the geological history.Mu Us desert is the ideal place to study the Holocene climate changes and can provide the historically similar type for climate change and future climate prediction.The Holocene climate change is complicated in Mu Us desert on which a lot of researchers have conducted many studies.But the research on the aeolian geomorphic evolution and the evolution process of rivers and lakes in the Mu Us desert is relatively few,and the coupling process between climate changes and geomorphic evolution is still unclear.Therefore,firstly,this study discusses the process of Holocene climate change in the Mu Us desert and provides new geological evidence for the Holocene climate change in the Mu Us desert by employing the substitute indicators such as grain size,organic matter and geochemical elements,based on the stratigraphic profiles sampled in the field survey and previous research results.Secondly,the thesis reveals the geomorphic evolution process through calculating the probability density of four sedimentary strata in the Holocene of the Mu Us desert,including fluvio-lacustrine facies,peat layer,aeolian sand and paleosol with the analytical method of probability density based on the changes of sedimentary facies and the results of profile dating.Thirdly,the thesis also discusses the driving mechanism of climate change and geomorphic evolution based on other paleoenvironmental and climatic records in the monsoonal region of the Holocene in the east Asian.Hejiagou(HJG)profile is located in the southern Mu Us desert,alternating deposit with lacustrine-swamp facies,aeolian sand and paleosol,the sedimentary and age sequences reveal that the climate of the Mu Us in the Holocene has experienced several changes.Before 13.5 ka BP,the aeolian sand was accumulated with prevailing sand activity.And it was as dry and cold as the environment in post-glacial period.When it was between 13.5 and10.4 ka BP,there was a lacustrine-swamp facies sediment indicating that there existed a shallow lake and the climate was relatively warm and humid.Between 10.4 and 8.9 ka BP in developed swamp,the water declined but the climate was still humid;Between 8.9 and 8.4 ka BP,the precipitation continued to decrease and aeolian activity becomes gradually active but vegetation growth was still being kept.Besides,the climate featured as dryness and coldness.Between 8.4 and 6.7 ka BP,there was a thick layer of aeolian sand,with active sandstorm activity and dry and cold climate.Between 6.7 and 4.1 ka BP,sandy paleosol was strengthened and a warm and humid climate increased vegetation coverage and pedogenic process.After4.1 ka,aeolian sand began to develop again,and the climate turned to dry and cold once more.The accumulation of secondary loess at the top of the section was the product of late flowing water,which represented the periodical increase of precipitation and the humid climate.The statistical analysis of probability density for age data extracted from different sedimentary sequence of Mu Us desert in the Holocene shows that Mu Us desert has experienced three times of climate geomorphic evolution in the Holocene.In the early Holocene lacustrine facies before 8.4 ka BP,regional precipitation was increasing but at the moment,aeolian sand accumulation was also recorded,reflecting the active aeolian activity that coexisted with lakes and sand landforms.In the middle Holocene,lacustrine strata in the low-lying valley area of the southern margin of the Mu Us desert continued to develop between 8.4 and 4.2 ka BP,and the aeolian sand accumulation decreased.The sandy paleosol in the desert/loess boundary of the southeastern margin of the Mu Us desert developed extensively,and thesand dunes were basically fixed.Although there was a short period of aeolian activity,it was still a landscape dominated by rivers and lakes.Since 4.2 ka BP in the late Holocene,the amount of aeolian sand accumulation gradually increased while the paleosol decreased.As a result,the aeolian activity began to strengthen and the aeolian landform developed again.There were three geomorphic evolution in the Holocene of the Mu Us desert,each of which was a shifting process between eolian sand accumulation and the development of rivers and lakes.The evolution processes in spatial and temporal are as follows.The lake expands,the lake surface rises,and the fluvial-lacustrine deposits overlap the aeolian sands.Instead,the lake shrinks,the surface of the lake falls,and aeolian activity increases.These phenomena make the original lake sediment a source of aeolian sand that overlays the fluvial-lacustrine sediments in a circulating process.The geomorphic evolution of the Mu Us desert was not unidirectional.The evolution between aeolian landform and river/lake landscape was driven by the interaction of wind and water by geomorphologic forces,which was an evolutionary process dominated by climate change,revealing the fluctuation of the whole geomorphic landscape.The climate and environment changes of the Mu Us desert in the Holocene showed that it is related to the solar radiation in summer which is caused by orbital or suborbital scale changes and takes effect through the strength of the east Asian summer monsoon and the regional precipitation.And it is synchronize with the monsoon margin area such as Daihai,Gonghai lake and Otindag sand land and Horqin sand land.When the east Asian summer monsoon intensifies,monsoon precipitation increases,vegetation coverage increases,aeolian activity decreases,and rivers and lakes develop.When the east Asian summer monsoon weakens,the precipitation decreases,the lake area shrinks and the vegetation degrades,the aeolian activity increases,and the aeolian landform develops.The climate change and geomorphic evolution of the Mu Us desert are the feedback of strength change of the east Asian monsoon margin area of China.The north Atlanticice raft event “8.2 event” and “4.2 event” also had an important impact on the climate and environment change in the Mu Us desert and other areas in the monsoon margin area of China.