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China Forestry Science Academe (2017)

Environmental Process of Sandy Loess Deposition in Southeastern Margin of Kumtagh Desert

丁峰;

Titre : Environmental Process of Sandy Loess Deposition in Southeastern Margin of Kumtagh Desert

Auteur : 丁峰;

Grade : Doctoral Dissertation 2017

Université : China Forestry Science Academe

Résumé
The loess is important Quaternary continental eolian sediments.There are different degrees of sedimentary sequences in north-central loess plateau,desert-loess transition zone,the desert edge in China.The characteristics of the loess deposit are long time series,abundant information of recording climate and so on,it occupies very impotant position in the quaternary research.In research the loess sediment particle size,geochemistry,magnetic susceptibility and the like information often are used to reconstruct reginal paleolimate information,especially the position of Asian monsoon and intensity evolution since quaternary period.According to the size of the loess sediment particle size and different regional position,loess zones from north to south in our country can be divided into sandy loess zone,loess zone,fine-grain loess zone,loess deposition in edge of the desert area is sandy loess deposition mostly,using the kind of sandy loess deposition to study the evolution of desert and local region of the paleoclimate change.In recent years,the Kumtagh desert research has gone to climax.Based on two consecutive national foundation special funds support,has been basically completed the geographical and geological environment investigation in desert zone,sand dune types and distribution,regional hydrology,animal and plant resources,and the formation of the desert evolution,et al.BL section is found in the desert investigating that is located in the southeast margin of the Kumtagh desert,it is a continuity sandy loess deposition stratum,the sedimentary of section records as the study area paleoclimate process provides the necessary materials.After analyzing the characteristics of grain size,mineral composition,geochemical elements and magnetic susceptibility of the BL section,the main conclusions are obtained as follows :(1)Sediment grain size analysis showed that 93.34% of BL section sediment particles is silty sand and coarse sand,it show that BL section is a continuous sandy loess sedimentary strata.BL sandy loess section deposition had started in the early Holocene,about 8.3ka or so.The process of deposition is not uniformity and rapid,the average deposition rate is 4.22cm/100 a,the initial deposition is faster,and the later speed has slowed down,the overall deposition is much higher than that of the loess strata in the central Loess Plateau.(2)The specific terrain and environment conditions in the BL section contributed to the deposition and accumulation of dust.There are very similar characteristics between the distribution mode of characteristic elements ratio of BL sediment section and the Kumtagh desert surface sand,different sediments of internal exposure strata,combined with the regional wind field and path,the provenance of BL section sediment can be determined from the Kumtag desert region,environmental information recorded in the sedimentary layer reflects the source region of ancient climate characteristics.(3)Through the discussion and analysis of various climatic proxies,pointed out that the >110μm grain size component content is a good index to winter monsoon climate(cold and dry),the ≤5μm grain size component content is index of summer monsoon(warm and dry)advance and retreat ;meanwhile,it is pointed out that frequency magnetic susceptibility can not apply to the extreme arid conditions.(4)The BL section sedimentary layer have different degrees of record about the rapid climate events of global 8.2ka,5.5ka,4.2ka,2.8ka,1.4ka,0.4ka in the Holocene.it is found in research that in the extreme arid zone,regional climate response to cold event more sensitive,to drought events are not sensitive ;also pointed out that in the early Holocene 7.5ka and 6.6ka,Kumtagh Desert Area also appeared two times stronger regional climatic cooling event.(5)BL section sediment information shows that since 8.3ka,there is a trend that the wind slows down and the climate warming in the Kumtagh desert area.The early Holocene(8.3 5.5ka),it is violent cold and warm period of change,its temperature is the lowest,its climate change fluctuation is more severe ;the mid Holocene(5.5 2.8ka),it is steady and warm period,its temperature is the highest,its climate change is the most stable ;the late Holocene(2.8ka to now),it is warm and fluctuant period,its temperature is more higher,its climate variation range is more lower.8.2-6.7ka and 3.4-1.4ka period is a multi wind period,the former wind strength is larger,the latter wind strength is low ;6.7-5.6ka and 1.4ka to now period is a little wind period,the former wind strength is more stronger,the latter wind strength is more weaker.In the 8.3-7.5ka the Kumtagh Desert Area appeared a certain degree of humid period,after a long time it is in the arid climate environment

Mots clés : Kumtagh desert; Sandy loess deposition; Provenance; the Holocene; Climate process;

Présentation (CNKI)

Page publiée le 20 janvier 2018