| Our study area of northern Qaidam Basin, on the northeast margin of the Tibetan Plateau, is covered by large areas of gobi, sand desert and playa, composed of Quaternary gravel and sand deposits, most of the Basin is sparsely populated and vegetation there is scarce. Thus, typical arid climate conditions dominate this region which presents an arid landscape with intensively eolian processes, where Lake Sugan is an enclosed shallow lake mainly fed by groundwater, in which annually laminated varves are well preserved since mid-and late Holocene, therefore, Sediments from the lake provide ideal documents in reconstructing high-resolution history of atmospheric dust and aerosols.In order to identify the atmospheric dust input from the lake sediments, in the thesis, we detailed examined grain-size distribution of eolian deposits from different sedimentary environments on surface of northern Qaidam Basin, the results show that basin surficial deposits ubiquitously bear at least three Weibull components, including ultrafine, fine and coarse components, and only a few samples contain sand component. On the whole, those sediments are characterized by well-sorted coarse component with high content, as well as those poorly-sorted fine and ultrafine components with low content.Dustfall during dust storm bears 3 to 4 Weibull components, showing negatively skewed unimodal distribution characterized by well-sorted, high content and narrow kurtosis of coarse component, while fine and ultrafine components show similar contents that is far below that of coarse component. Annual and seasonal dustfall show almost the same grain size distribution with that in a single dust storm.The grain-size distribution of eolian dust trapped in the lake ice during the winter time, bearing three to four Weibull components, shows positively skewed distribution pattern dominated by fine component. Grain size of beach sand from the input short river, characterized by negatively skewed unimodal distribution characterized by high content of coarse component, bearing four Weibull components. Almost all the modern surface sediments of Lake Sugan bear four Weibull components, showing either negatively skewed distribution dominated by coarse component or positively skewed distribution dominated by fine component. Detailed analyses of the enrichment and dispersion behaviors of modern dust in varied sediments, we propose a concept model of grain size components of eolian deposition in Lake Sugan. And we consider that eolian dustfall during dust storm and that trapped in lake ice during the winter time, is the principal contributor to grain size component of sediments in Lake Suagan, and their combination well reflects grain-size distribution of lake sediments.Based on the above, we consider that sand component in Lake Sugan sediments is related to strong near-surface dust activity in dry land around the lake, possibly also indicating flood events caused by sudden increase of water from the input river. And coarse component documents regional dust storm events, and fine component mainly reflects wind conditions of winter time (November to March) in the study area, while ultrafine component can be used to indicate background of atmospheric dust and strength of westerly circulation to some extent.To further understand the climatic implication of the Weibull components of grain size of Lake Sugan sediments, using annul meteorological records by Lenghu meteorological station in the study area during 1957-2007, we compare the contents of three components (i.e., ultrafine, fine, coarse) and/or peak grain size (mode grain size) with regional westerly index, average wind speed of winter time (November to March) and days of dust storm, respectively. And we find a good positive correlation between surrogates and meteorological observation.On time scale of century to millennium, comparing the content of ultrafine component with Ca2+ion content of Greenland ice core, a representative of westerly intensity of northern hemisphere, we discovered close relationship between variations of those two. When comparing the content of fine component with winter monsoon strength, documented by Ti content and magnetic susceptibility from Maar Huguangyan, and Siberian High strength recorded by K+ion content of Greenland ice core, we found quite consistent variations between content of fine component and Siberian High strength. While comparing the content of coarse component and mode grain size with the dust rain frequency from historical archives in China and insoluble microparticle content in Dunde ice core, we considered a remarkable correlation between their variations. As to the intense dust activity and/or flood events indicated by sand component, we consider that they can be distinguished by reliable dust storm records (coarse component) and temperature indicated by oxygen isotope of Dunde ice core.Based on the climate change framework in Lake Sugan region previously established by former researcher, we synthesize climate change of the past 2684 years (i.e.,681B.C.-2002A.D.), especially the dust activity, winter monsoon strength and westerly evolution, we divide the dust evolution in the study area into 7 stages:682B.C.-200B.C., strong westerly with much water vapor input and very intense winter monsoon (during 300B.C.-200B.C. an obvious event of weakening winter monsoon wind with low frequent dust storm, afterwards sudden strengthening with warm-wet climate, stable hydrological conditions and great input water in the study area). 200B.C.-800A.D., westerly strength of intense level during 200B.C.-200A.D. and afterwards obvious weakening, weakening intensity of winter monsoon, quick increase of dust storm frequency, dry climate and lowering lake level with lowering input water.800A.D.-1200A.D., lower westerly intensity and winter monsoon with obvious decrease of dust storm frequency, warm-dry climate on the whole, lowering lake level with lowering input water. 1200A.D.-1400A.D., strong intensity of both westerly and winter monsoon, low frequent intense dust storm, climate characterized by cold-wet condition.1400A.D.-1550A.D., obvious decrease of westerly intensity and weakening winter monsoon to some extent, increase frequency of dust storm of not intensity, cold-dry climate of lower temperature.1550A.D.-1830A.D., strong westerly and major variation of winter monsoon, dramatic variation of dust storm frequency and intensity, cold-wet climate.1830A.D.-2002A.D., intense westerly, extremely unstable winter monsoon, gradual increase of dust storm frequency and intensity, drying in the drainage basin and lowering lake water level with decreasing input water.Finally, we have the driving mechanisms of intensity of westerly and winter monsoon and dust storm activity simply discussed, and consider that variation of solar activity could result in surface temperature increase which leads to increase of westerly intensity in north hemisphere, while increase of Siberian High is the direct cause of increase of winter monsoon strength, intensity of dust storm activity is directly related to surface temperature and controlled by sunspot numbers on time scale of century. |
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