关键词: 湖相沉积物, 粒度分布模式, 环境意义, 柴达木盆地, 察尔汗

Abstract: The Qarhan playa located in the eastern-central Qaidam Basin，surrounded by Gobi and yardang fields in actively folding sedimentary strata，has been become a lacustrine deposition center of the Tarim basin since the Quaternary. As a huge pitfall，the palaeolake traps a mass of aeolian deposits，so the sediments of this area are a mixed volume，which contains multiple components （such as lacustrine component，aeolian component，fluvial component and so on） and its grain-size frequency distribution curves show polymodal pattern. However，so far，these multiple components and their significance are still one of the fundamental enigmas. This study aims to distinguish the above multiple components and demonstrate the environmental significance of each component. Here a 102.13-meter-long lacustrine sediment core （labeled ISL1A） is presented obtained from the Qarhan playa. Through observing the grain-size frequency distribution curves for samples of ISL1A core，it is found that the grain-size distribution patterns of most lacustrine samples are trimodal，but part samples of hyperhaline phase are bimodal，absenting the third peaks. Based on grain-size parameter analysis as well as the method which is comparing the grain-size frequency distribution curves for samples of ISL1A core with that for several classic sediments，it is found that the curves of 2-10 μm fraction for this core are similar as that of stable lacustrine sediment，suggesting they are mainly lacustrine component; the curves of 10-70 μm and 70-650 μm fraction are similar as the that of loess and dust storm sediment，suggesting they are both mainly aeolian component; Grain-size distribution patterns of different evolution phases （salt-forming phase，semiarid phase and humid phase） show the fraction of less then 2 μm may relate to the origin of microorganismal chemistry，because the fraction disappears in hyperhaline phase but presents in semiarid and humid phase. In order to testify the above conclusion，the multi-proxies records are introduced （halite，calcite，Fe content，A/C value for pollen and F value for grain-size） and found that the respective component of grain-size all relates to the change-process of palaeoclimate. Moreover，different components indicate the different paleoclimate significances: the fractions of less than 2 μm and 2-10 μm reflect salinity of paleolake and the runoff intensity of catchment; 10-70 μm fraction probably indicates variation of the East Asian winter monsoon; 70-650 μm fraction indicates the intensity of dust storms of in the past. Finally，the multi-proxies records well testify our previous conclusion and can reconstruct the paleoclimate evolution course since 92.9 ka in the Qaidam Basin: 92.8-83.1 ka （MIS 5b），the climate was somewhat droughty; 83.1-71.0ka （MIS 5a），the climate became rather humid; 71.0-64.0ka （MIS 4），the climate was relatively dry; 64.0-47.1ka （MIS 3c），the palaeoclimate was very humid，the lake became freshwater-lake; 47.2-34.3 ka （MIS 3b），the climate became pretty dry，the palaeolake turned into a saline lake，and the East Asian Winter Monsoon （EAWM） was rather strong; 34.3-22.2 ka （MIS 3a），the climate got humid again; 22.2-14.1 ka （MIS 2），the palaeoclimate appeared very degradational，the lake went into the second salt period，and the EAWM was strengthen sharply; 14.1-9.0 ka （MIS 1），it went into post glacial and the climate began to improve.

Key words: Lacustrine sediments, Grain-size distribution pattern, environmental significance, Qaidam Basin, Qarhan area