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authors describe the significance of the Lanzhou loess and paleosol sequence ... In the Lanzhou area the loess sequence attains 400 m thickness, in which five.
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© 1991 (June) by Kluwer Academic Publishers

Loess Stratigraphy of the Lanzhou Profile and its Comparison with Deep-Sea Sediment and Ice Core Record Chen Fahu, Prof; Li Jijun, Prof.; Zhang Weixin, Prof.; Lanzhou University, Department of Geography, Lanzhou, China ABSTRACT: After summarizing the results of Quaternary climatic fluctuations in China, authors describe the significance of the Lanzhou loess and paleosol sequence in Quaternary studies. In the Lanzhou area the loess sequence attains 400 m thickness, in which five stratigraphic units are identified with at least 21 intercalated paleosols, TL, paleomagnetic and fission track dating techniques have been applied to determine the age of the loess horizons. Correlations are made with the terrace of the Yellow River and oxygen isotope curves of the Pacific deep-sea cores V28-238 and V28-239 and the Vostok ice core isotope curves. The comparative study proves that the Lanzhou loess is a very sensitive recorder of climatic fluctuation.

Introduction The studies of the history of Quaternary climatic fluctuations have made a great progress since world war II. The new break-through resulted from deep-sea oxygen isotope chronology founded by C. Emiliani (1955), while the oxygen isotope curves and divisions of the well-known Equatorial Pacific deep-sea cores V 28-238 and V28-239 have been generally recognized as the standard curves of climatic fluctuations for the Late Quaternary (Shackleton, Opdyke 1973). Therefore, the typical Alpine glacial model of the Quaternary proposed by A. Penck and E. Bruckner was completely abandoned. The importance of ice core climatic records in polar regions is worldwidely arousing a great attention among Quaternary researchers, while it has been proved by J. Kukla und others that loess, especially the loess of China, is another excellent record of Quaternary climatic fluctuations on continents. The details and accuracy of the record can be compared to the ice core and deep-sea records or even have advantage over them in some aspects (Kukla 1975). Loess of China is widely spread in North China in a belt N of the Kunlun- Qinling mountains and south of the ArtainDaxinganling mountains (Liu et al. 1985). However, it mainly occurs in the loess plateau which is also the most

sensitive and violent area to climatic fluctuations and is called Monsoon Triangle of China (Fig 1) (Li, Feng, Tang 1988). This area shows the highest rate of dust deposition in China (Zhang 1984). The studies of Chinese loess have brought important results in the last decade. The Luochuan loess profile in Shaanxi is one of the standard loess profiles of Quaternary climatic records not only in China but also in the world. Many researchers have studied the profile (Liu et al. 1988; An, Lu 1984; Wei et al. 1981; Wang 1987). However, the importance of Lanzhou loess in reconstructing the history of Quaternary climatic fluctuations has not aroused proper attention by scientists. As one of Quaternary geological problems, the Lanzhou loess had been reported before 1970s (Yang 1957; Wang 1958; Zhang 1962; Shui 1965). The studies of Lanzhou foes have only made new progress since the end of 1970s. Wang Yongyan and his colleagues who first investigated the Lanzhou loess revealed some characteristics of the loess west of Liupanshan Mt. and measured the paleomagnetic data (Wang et al. 1978; Wang 1978; Wang et al. 1982). A number of geologists and geographers have been engaged in scientific research in many ways to Lanzhou loess. They have put forward many new opinions about loess-palesol sequence, sedimentation environment and date of Lanzhou loess (Burbank, Li 1985; Li, C h e n , Kang 1989; Cao et al. 1988; Chen et al. 1989; Bai

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Fig 1 Loess source area (A), depositional area (B), Monsoon Triangle area (C) and Loess Plateau (D) of China

1987; Derbyshire, Wang et al. 1987). Opinions differed greatly and attracted much attention to Lanzhou loess by scientists in China and abroad.The INQUA Commission on Loess initiated a special discussion of Lanzhou loess in Lanzhou in 1988. Many well-known Chinese and foreign scientists attended the discussion and pushed Lanzhou loess research forward greatly. This paper is just a summary of the background to Lanzhou loess research. Before our discussion, the importance of Lanzhou loess research should be explained. It is noted in Fig 1 that Lanzhou is the nearest place to the source area of loess in the loess plateau and the depositional rate is the highest. Therefore, the thickest eolian loess was formed in Lanzhou. Jiuzhoutai loess profile is 318 m thick and there is up to 409 m loess (including alluvial loess) revealed in the Xijinchun core at the south bank of the Yellow River in Lanzhou. Moreover, the Monsoon Triangle of China is just like a gate. It was open during glacials and closed during interglacials. Loess steppe in glacials and summer-green broad-leaved forest in interglacials alternately occupied the area to form loess and paleosol sequence. Lanzhou is just located at a key position in the Monsoon Triangle as Cape Hatteras for the North Atlantic Ocean during the Quaternary period (Ruddiman, Mclntyre 1976). Therefore, the alternations of Quaternary glacial or climatic cycles and the pulsation of global climatic changes are sensitively recorded. Lanzhou is also the present-day converging center of a number of rivers from NE Tibet Plateau. In the background of crustal uplift, the rivers cut down to form a series of terraces which were

called Lanzhou type terraces by Huang Jiqing (Yang, Bian 1937; Chen 1947; Huang 1958), while different grades of terraces were covered by loess of different age providing favourable conditions to loess research and comparison.

Paleosol Sequence Loess stratigraphy in the Lanzhou area identifies five units. Holocene loess is 0.5 m to 5.0 m thick. It contains up to four layers of paleosol. The paleosol, also called Holocene Black Loam, are dark grey in colour (10YR 6/3-6/2) and rich in organic matter. A few of standard profiles are shown in Fig 2. Malan loess which was formed in the last glaciation is 3 5 50 m thick and yellow red in colour (10YR 7/3). It is very coarse and contains a lot of holes. More than four layers of paleosol with the degree of development close to Black Loam were formed in it. The Beiyuan and Jiuzhoutai loess profiles can be regarded as its standard profiles. The upper part of Lishi loess, 75 m to 120 m thick and yellow brown in colour, is very hard. 12 to 15 layers ofpaleosol developed in it. Magnetic susceptibility between the paleosol and loess differs greatly, i.e. paleosol has very high peaks on the susceptibility curve. The lower part of Lishi loess, 150 m thick, is very hard and contains up to 16 layers of paleosol. The B / M boundary is located in the middle of layer Ls, while the Jaramillo event is between S10 and $1~.It contains two sand-loess marker layers. The lower one is its lower boundary. Wucheng loess in Lanzhou area is thin. In the

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Holocene loes-paleosol profiles and their comparison

Jiuzhoutai loess profile, the world's thickest loess profile, it is only 41 m thick including 21 m of loess formed under alternating aqueous and subaerial environments. In the Xijinchun core (Bai 1987), there is thicker Wucheng loess but most of it is alluvial loess. To be easily identified an compared, the Lanzhou loess horizons are marked as L0, L1, L2 etc. resp., while paleosol layers are as So, $I, $2 etc. resp. from the youngest to the oldest. The paleosol So, also called Holocece Black Loam Soil, consists of one to four layers in the Lanzhou loess. Among them the paleosol formed 7500 a BP-3500 a BPwas widely distributed. About three to four layers of paleosol were formed in Malan loess. The paleosol layers in the Beiyuan and Jiuzhoutai profiles are almost complete (Fig 3). Among them S.... S,I-b, Sm-cwere formed between 27 and 53 ka BP (according to 14C and TL ages). The three paleosols correspond to the fine grain layer in the Malan loess of North China (An, Lu 1984) orthe stronglyweathered layerin Malan loess at Luochuan and other loess profiles (Wang, Sasajima 1985). S~ is composed of only one layer in the Luochuan profile, while two to three layers in the Lanzhou loess (Fig 3). $2 and $3 resp. are composed of three sublayers

in the Lanzhou loess, while one or two layers in the Luochuan profile. $4 is a well-developed paleosol. It is 1.52.0 m thick and brown in colour (5YR 6/3). Its susceptibility peak is very obvious. It is also a turning layer in paleosols inasmuch as the paleosol and loess layers formed earlier do not differ significantly, but those formed subsequently show considerable variation, clearly reflected by the susceptibility curve. There is a thin layer of paleosol under $4 in the Duiwashan profile. $5 corresponding to the'Three Red Strip' paleosol in the loess horizons east of Liupanshan mountain is composed of three or four layers. The loess between two consecutive paleosol layers is 5 to 6 m thick. $6 in the Lanzhou loess is the same as in the Luochuan profile, a single layer. However, $7 is composed of two or three layers. In the Jiuzhoutai profile, there is a layer of 7 m thick loess between $7., and S7.b, of which ST-amay correspond to the calcicrete layer in L7 in the Luochuan profile. $8 is also a double layer, but Ss-a is quite poorly developed and its susceptibility is a little higher than that of loess. It has been pointed out before that the B/M boundary is situated in the middle of Ls, which is about the same as at Luochuan and other loess profiles. $9 is only one layer in the loess sequence

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The loess profiles, climatic records of the Late Pleistocene in Lanzhou area and comparison with Vostokice core record. L2=Jiuzhoutai Susceptibility in Jiuzhoutai; L4 = Susceptibility in the Beiyuan profile; Ls = Content in Beiyuan profile; L6=Isotope curve and stages of Antarctic Vostok ice core; Lt = Beiyuan profile.

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east of Liupanshan mountain, while it is composed of two layers in Lanzhou loess. L9 is the upper sand-loess marker layer. Its susceptibility, ratio of SiO2 to A1203 content and other indices are all very low. There are one or two paleosols formed in it, but they are quite poorly developed and are characterized by leaching and deposit of calcium sulphate. There are nine layers of close distributed paleosols under $9. In Jaramillo event, four layers ofpaleosol were formed: S~0_~, Sl0-b, $10_oand $11.The other three layers are $12, $13 and $14, respectively. L~5 is the lower sand-loess marker in loess. Relatively speaking, the paleosols between the upper and lower marker layers are very close, which is the general law in loess layers of China. Nine layers of paleosol distribute in the section 4-45 m high. They are S~5 to S2a respectively an correspond to Ws_l of the Luochuan profile, which is proven by the fact that one fission track date at the Jiuzhoutai profile bottom is 1.488 ___0.11 Ma BP. The paleosol sequence older than 1.49 Ma is not clear yet and is being proven. It is found during the studies of the Lanzhou loess in recent years that the Lanzhou loess-paleosol sequence is the

same in different loess profiles, but quite different from the loess east of Liupanshan mountain (Fig 4). Generally speaking, the paleosol layers in the Lanzhou loess tend to increase, indicating Lanzhou loess is very sensitive to climatic fluctuations (Li et al. 1989).

~4C-Datings of Holocene Loess There are many 14C-datings on the Holocene loess at Lanzhou (Fig 2). The data are mainly in the following three periods: 10000-8500 a BP, 7000-3500 a BP and 2700-2000 a BP, but the paleosol formed at 7000-3500 a BP is the widest distributed one representing the Holocene climatic optimum. There is a layer of still older paleosol generally distributed in alluvial loess of high riverside deposits. Its oldest 14C-date is 12920 -b 350 a BP (sample taken from base of paleosol). The period when the paleosol formed may correspond to the Aller6d Interstadial in Europe.

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Fig 4 The paleosol sequence of Lanzhou loess and its comparison with the Luochuan profile. 141.39-+14.1 Ka

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