Model of Grain-size Distribution of the Eolian Deposits in Lanzhou Area and its Paleoclimatic Significances
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摘要: 通过对兰州不同气候背景下形成的风尘沉积 1 80个样品详细的粒度分析,建立了该地区风尘沉积的粒度分布基本模式。发现研究区风尘沉积具有以下特征:(1 )沉积物分选差,主要由粒径 >4Φ的颗粒组成,平均6.4Φ (2 )粒级构成为极细砂 2.6 %、粉砂 77.8%、粘土 1 9.6 %,属粉质重亚粘土或典型黄土 (3)频率曲线为单峰、极正偏类型,且具有粗头短促、中间宽大和细尾拉长的特点 (4)短期悬浮颗粒与长期悬浮颗粒大致呈对等配比。这些特征同时也为兰州现代尘暴沉积物所具备,因而可视为该地区风尘沉积的一个基本诊断模式搬运方式。颗粒粒径 <5.5 0Φ的含量指标 (GSP1)和 4.5 0～ 6.5 0Φ的含量指标 (GSP2 )可分别作为冰期—间冰期尺度和冰段—间冰段尺度东亚冬季风演化和沙尘暴强弱程度的基本判据。Abstract: The Quaternary eolian deposits in Lanzhou,west of the Loess Plateau,is the thickest area in China as aresult of the geographic position and the topographic conditions.Investigation show that Lanzhou is also a dust-storm frequent area in the present time.Both provides us possibilities to form a grain size distribution model of the eolian deposits,and to identify deposits formed in different climatic conditions through researches of their grain composition. 180 samples are taken from newly dug 45m-deep loess well section at Gaolanshan near Lanzhou at a depth of 3 100cm to 4 000 cm,including stages LOMSS-4,5a,5b,5c,5d,5e and LOMSS-6 according to the magnetic susceptibility curve (LOMSS means loess magnetic susceptibility stage,and is basically corresponding to MIS).To conduct a very careful laboratory work of grain size analysis,the measuring range of grain diameter is set as 4.00-10.00Φ with step of 0.25Φ on the SKC-2000 Grain Size Analytical System. Studies show that the eolian deposits formed in different climatic stages are much similar to each other in grain size composition,leading to an establishment of grain size distribution model.They are characterized by four features：(a) badly sorted with particles more than 4Φ in diameter(mean diameter 6.4Φ);(b) named as a silty heavy clayey or typical loess with fine sand 2.6%,silt 77.8% and clay 19.6%;(c) mono-peaked and positively skewed frequency curve;(d) nearly the same in quantity for both long-term and short-term suspended particles according to Pye's eolian deposition model.Fortunately,these four features can also be seen from the present dust-storm deposits collected in the spring of 1993 in Lanzhou area.It shows that the eolian deposits are transported matinly by manner of dust stroms.This model can be used as a diagnosis model of the eolian deposits within the area of Lanzhou. Differences of grain size composition among the eolian deposits are noted too with regard to climate changes though it is not so big.Different time-scales of climatic change may cause changes in content of differnt grain size.For example,the content of＜5.50Φ particles in the deposits formed under the glacial climate is obviously higher than that under the interglacial,and the content of 4.5-6.75Φ particles formed under the stadial higher than under the interstadial.It implies a complex and delicate relationship between grain size composition and the winter monsoon.But we still recognize that two grain-size parameters(GSP)from the Lanzhou eolian deposits are of significance to the paleoclimate.GSP1(content of＜5.75Φ,＞5.75Φ or ＜5.75Φ/ ＞5.75Φ)can be used to reveal changes of winter monsoon on the scale of glacial-interglacial climate.GSP2(content of 4.5-6.75Φ,＞6.75Φ or 4.5-6.75Φ/＞6.75Φ)tends to reveal changes on the scale of stadial-interstadial climate
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Key words:
- eolian deposits /
- diagnosis model /
- grain size parameters(GSP) /
- dust storm /
- winter monsoon /
- Lanzhou
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