Model of Grain-size Distribution of the Eolian Deposits in Lanzhou Area and its Paleoclimatic Significances

DAI Xue rong; LI Ji jun; YU Li zhong; SHI Yu xin; WANG Jia cheng

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

Model of Grain-size Distribution of the Eolian Deposits in Lanzhou Area and its Paleoclimatic Significances

1. Department of Geography,East China Normal University,Shanghai 200062;

2. Department of Geography,Lanzhou University,Lanzhou 730000

Received Date: 1995-05-25

Publish Date: 2000-03-10

Key words:

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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Reference (15)

[1]	刘东生.黄土与环境[M].北京：科学出版社，1985.191～207
[2]	Burbank D W,Li J J.Age and palaoclimatic significance of the loess of Lanzhou,north China[J].Nature,1985,316：429～431
[3]	Li J J,Feng Z D.Late Quaternary monsoon patterns on the Loess Plateau of China[J].Earth Surface Process and Landforms,1988,13(1)：125～135
[4]	李吉均，朱俊杰，康建成等.末次冰期旋回兰州黄土剖面与南极东方站冰岩芯的对比[J].中国科学(B辑)，1990，20(10)：1 086～1 094
[5]	Zhang L Y,Dai X R,Shi Z T,The Sources of Loess material and the Formation of the Loess Plateau in China[A].In CATENA SUPPLEMTNT 20(Cremlingen)[C].1991.1～14
[6]	陈发虎，张维信.甘青地区的黄土地层学与第四纪冰川问题[M].北京：科学出版社，1993.47～54
[7]	方小敏，戴雪荣，李吉均等，亚洲季风演化的突发性与不稳定性—以末次间冰期土壤发生为例[J].中国科学(D)，1996，26(2)：154～160
[8]	刘东生.中国的黄土堆积[M].北京：科学出版社，1965.141～172
[9]	王永焱，吴在宝，岳乐平.兰州黄土的生成时代及结构特征[J].西北大学学报，1978，(2)：1～27
[10]	康建成，李吉均.甘肃临夏黄土剖面：15万年环境演变的良好记录[J].地质论评，1993，39(2)：165～175
[11]	任明达，王乃梁.现代沉积环境概论[M].北京：科学出版社，1985.8～26
[12]	Kenneth P,Aeolian dust and dust deposits[M] Academic Press Inc.(London)Ltd.1987
[13]	丁仲礼，余志伟，刘东生.中国黄土研究新进展(三)时间标尺[J].第四纪研究，1991，(4)：336～347
[14]	戴雪荣，师育新，薛滨.兰州现代特大尘暴沉积物粒度特征及其意义[J].兰州大学学报(自)，1995，31(4)：130～134
[15]	师育新，戴雪荣，李节通等.末次间冰期兰州黄土记录中的粘土矿物及其环境意义探讨[J].海洋地质与第四纪地质，1997，17(1)：87～94

Model of Grain-size Distribution of the Eolian Deposits in Lanzhou Area and its Paleoclimatic Significances

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