научная статья по теме CLIMATIC CHANGES INDICATED BY TRACE ELEMENTS IN CHAGELEBULU STRATIGRAPHIC SECTION, BADAIN JARAN DESERT, CHINA, SINCE 150 KYR B.P Геология

Текст научной статьи на тему «CLIMATIC CHANGES INDICATED BY TRACE ELEMENTS IN CHAGELEBULU STRATIGRAPHIC SECTION, BADAIN JARAN DESERT, CHINA, SINCE 150 KYR B.P»

ГЕОХИМИЯ, 2008, № 1, с. 104-111

КРАТКИЕ СООБЩЕНИЯ

CLIMATIC CHANGES INDICATED BY TRACE ELEMENTS IN CHAGELEBULU STRATIGRAPHIC SECTION, BADAIN JARAN DESERT,

CHINA, SINCE 150 KYR B.P.

© 2008 r. Yang Yi*, Li Bao-sheng*'**, Qiu Shi-fan*, Wu Zheng*, Gao Quan-zhou***, Li Yun-zhuo*

* School of Geography science, South China Normal University, Guangzhou, Guangdong 510631, China; **State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences,

Xi'an, Shaanxi 710061, China; ***School of Geography science, Zhongshan University, Guangzhou, Guangdong 510631, China.

E-mail: yyi1980@sohu.com Поступила в редакцию 25.01.2007 г.

1. Introduction

Being situated in the centre of Alxa Desert, western part of Inner Mongolia Plateau and the transitional zone between the eastern and western deserts in China, Badain Jaran Desert (39°30' ~ 41°30'N, 100° ~ 103°E) is the terminus of modern East Asia monsoon. The special location is of great significance on the study of environmental evolution and paleo-monsoon's advance-retreat in late Quaternary.

However, only a few scholars have made Quaternary investigations on the spot before 1980s [1-3], due to the huge mega dunes of Badain Jaran Desert and the traffic inconvenience. Until 1988, when Lanzhou Institute of Desert Research, Chinese Academy of Sciences, cooperating with German Universities Gottingen, Cologne and Free University of Berlin, made an aeolian geomorphological research on the southeastern part of Badain Jaran Desert (mainly from Saritai to Badain Jaran Temple), more and more research productions on its environmental evolution produced [4-6]. Yang Xiaoping, Yan Mancun and Gao Quan-zhou et al have made research on Badain Jaran Desert from different angles, among which Yang Xiaoping emphasized on the special landscapes, such as: mega dunes, perennial lakes stratified calcareous concretions and the its paleoen-vironment, especially that since 30 k years ago [7-12]. Yan Mancun et al's study shown that the formation of huge mega dunes in Badain Jaran Desert was induced by west and northwest winds [13, 14]. Gao Quanzhou et al studied the correlation between the climatic changes and the pale-omonsoons' advance-retreat on the southern fringe of Ba-dain Jaran Desert since late Pleistocene with a wide range of proxy climatic data [15-18].

In 1992, some authors of this paper made a Quaternary investigation on the southwestern and southeastern fringe of Badain Jaran Desert. During the course, they discovered that Chagelebulu Stratigraphic Section, formed from Upper Pleistocene to Holocene on the southeastern fringe of Badain Jaran Desert, was a relatively complete strati-graphic section with multi-layer aeolian dune sands and

fluvial-lacustrine faces alternately interbedded. Chagele-bulu Stratigraphic Section was also a representative section of Badain Jaran Desert's formation and evolution since 150 kyr B.P. because it had taken down the sedimentation and climatic vicissitude of Badain Jaran Desert since 150 kyr B.P. With the development of the research work, we gradually noticed that the subdivision of Chagelebulu Stratigraphic Section was rather general resulted from the ignorance of some environmental evolution details. Therefore, we examined Chagelebulu Strati-graphic Section once again in 2003, and dated some of the strata. Based on the investigation and the results of experiments, this paper discussed the distribution characteristics of 16 trace elements and the paleoclimate they reflected since 150 kyr B.P.

2. The physical environment and general situation of Chagelebulu Section

Chagelebulu Section (39°53'N, 103°18'E) is situated on the southern fringe of Badain Jaran Desert, 1.800 m in height above sea level with Yabulai Mountain on the southeast (Fig. 1). Being in the deep inland, it has obvious climatic characteristics of continental temperate arid desert, such as: a precipitation of 40 to 60 mm mainly concentrating in summertime, an average annual temperature of 8 to 10°C, an annual temperature range over 35°C, the northwest wind and west wind perennially prevailing with an annual average velocity of 4 m/s and maximum speed 9 m/s.

Chagelebulu Section is surrounded by a flat surface made up of loess. Due to the gulches 15 to 20 meters deep incised in the surface, the strata on both sides of the gulches are exposed abundantly. According to the field investigation, the Quaternary strata of Chagelebulu Section overlie on the Lishi Loess that discordantly contacts with the hypometamorphic gneisses of the Presinian system. There are 59 layers in Chagelebulu Section: 19 layers of aeolian paleo-mobile dune sands, 6 layers of aeolian paleo-mobile

Fig. 1. Wind-drift landscape of the southern Badain Jaran Desert and the position of Chagelebulu Section. 1 - Barchans and barchanic chains, 2 - Barchanic ridges (present dunes), 3 - Grid-liko dunes, 4 - Asterotdal dunes, 5 - Compound barchans and mega barchans, 6 - Dunes with shrubs, 7 - Flat sand, 8 - Gobi, 9 - Dry lake bed, 10 - Lanes in the desert, 11 - Altitude of dunes, 12 - Height above sea level, 13 - The position of Chagelebulu Section barchans and barcbanic chains.

and paleo-semi-mobile dune sands, 1 layer of modern mobile dune sands, 4 layers of loess, 2 layers of paleosols (immature soil), 19 layers of lacustrine facies and 8 layers of gravels combined with sandy soil. To refer them easily, they are successively designated as MD, D, FD, LS, AGS, S and L hereinafter in Fig. 2. We can see clearly the sedimentary rhythms of Chagelebulu Section from Upper Pleistocene and Holocene, composed of aeolian dune sands, lacustrine-swampy faces and alluvial and colluvial sandy-gravel layers. They overlapped with each other repeatedly and formed 25 sedimentary cycles. Every sedimentary facies are described as the following:

Aeolian paleo-mobile dune sands(D): brownish yellow or grayish yellow, brownish grey exceptionally, loose and slightly cemented, mainly fine sands, small parts of very fine sands and very little silt and clay, no pollen-spores contained.

Paleo-fixed-semi-fixed dune sands (FD): light brownish yellow or grayish yellow, mainly fine or very fine sands, partly silt and clay, dense and evenly sorted, no

stratification, Plant root system can be seen, small amount of pollen-spores, mainly of herbage and semi-shrub.

Lacustrine-swampy faces (LS): grayish yellow, light grey, greenish-grey, grayish green, mainly silty very fine sands, the rest clayey silt or silty fine sands, slightly cemented or semi-cemented, fossil mollusks and thallophytes can be seen in some of the strata.

Loess (L): grayish-yellow or brownish-yellow, mainly silt, partly very fine sands, evenly sorted, no stratification, loose, vertical joints developed.

Paleosol (S): immature soil, light brown, clayey silt, silty very fine sands with clay, with grayish white calcareous pseudo-hyphae and plant root system, massive, heavy texture. Some fossil terraneous snails can be seen.

Sandy gravel layers (AGS): grayish-yellow or dark grey, the talus consists mainly of granitgneiss and grano-diorite, sub-faceted, 1 to 3 cm in grain-size, semi-cemented, its cement is dark grayish-yellow silty sand.

Furthermore, Fig. 2 also shows the sedimentary sequence of Chagelebulu Section's strata and the age of the horizon with fossil mollusks. There are 14 horizon dates in

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Mid Pleistocene Riss Glacial Period

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12

3 13

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Fig. 2. Chagelebu stratigraphical section and the vertical distribution of its trace elements.

1 - recent mobile dune, 2 - paleo mobile dune sands (fine sands), 3 - paleo mobile dune sands (very fine sands), 4 - paleo fixed to semi-fixed dune sands (sity fine sands): 5 -paleo fixed 10 semi-fixed dune sands (silly very fine sands), 6 - silty fine sands-silty very fine sands, 7 - clayey silts, 8 - paleoso(immature, silty-clayey silts or silty very fine sands), 9 - peat horizon, 10 - sandy alluvial sand and gravel horixon, 11 - Holocebe Loess, 12 - Malan Loess: 13 - Lishi Loess, 14 - denudation surface, 15 - pre-Quaternary

base rick, 16 - congeliturbated fold: 17 - mollusk fossil, 18 - C age, 19 - TL age.

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this section in all. Among them, the dating matters of 2S, 4L, 5FD, 7L, 8FD and 9L are soil organic matters and plant relics. While the dating matters of 12LS, 14LS, 20LS, 22LS and 26LS are silt. They were measured by lab assistant Hu Zhiyu (14C laboratory of Arid and Cold Regions' Environmental and Engineering Research Institute, Chinese Academy of Sciences). The TL age of 58D was measured by lab assistant Lu Liangcai (TL lab of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences) with aeolian quartz particles.

According to the general characteristics and datings of the lithostrata, the boundaries of the strata in Chagelebulu Section are described as the following:

The age of the boundary between 59AGS and 60 L, also the boundary between middle Pleistocene and upper the Upper Pleistocene is approximately 150 kyr B.P. (its TL age is 147.8 ± 11.8 kyr B.P.).

The age of the boundary between 39FD and 40 AGS, also the boundary between lower the Upper Pleistocene and upper the Upper Pleistocene is approximately 75 kyr B.P. [19].

The age of the boundary between 9L and 10FD, also the boundary between the upper Pleistocene and Ho-locene is approximately 10 kyr B.P. (its 14C age is 9.431 ± ± 0.122 kyr B.P.).

3. The distribution characteristics of the trace elements in Chagelebulu Section since 150 kyr B.P.

Based on the experimental analysis of 16 trace elements in the 56 samples collected in Chagelebulu Section, they were determined as P, Mn, Sr, Zr, Ba, V, Cr, Co, Ni, Cu, Zn, Pb, As, Rb, Y, Nb. The experiment was carried out by lab assistant, Sun Zhong (central lab of Arid and Cold Regions' Environmental and Engineering Research Institute, Chinese Academy of Sciences) with a

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