Pathways of dust input to the Chinese Loess Plateau during the last glacial and interglacial periods

Catena 40 Ž2000. 251–261 www.elsevier.comrlocatercatena

Pathways of dust input to the Chinese Loess Plateau during the last glacial and interglacial periods Huayu Lu ) , Donghuai Sun The State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences, Xiying Road 22-2, Xi’an 710054, People’s Republic of China Received 13 January 1999; received in revised form 22 February 2000; accepted 6 March 2000

Abstract It has been established that loess deposited on the Chinese Loess Plateau was derived from arid and semi-arid regions in north and northwest China, but the specific pathways of dust transport are still not proved. In this study, 32 loess–palaeosol profiles of the last glacial and interglacial periods were measured for magnetic susceptibility in order to investigate the dust transport pathways of dust input to the Loess Plateau. The magnetic susceptibility variations of the loess–palaeosol deposits within the last glacial cycle can be correlated with marine oxygen isotope variations to estimate the ages of boundaries of the loess units and intercalated palaeosols. The sedimentation rate of each depositional unit can be obtained by dividing the thickness by the time that it represents. Because sedimentation rates decrease downwind from the dust source, the spatial distribution of dust sedimentation rates shows that northwest and west winds were the two most important agents for transport of dust to the Loess Plateau during the last glacial cycle. The north and northeast winds played an almost negligible role. In addition, topography and dust from local sources strongly affected the dust accumulation rates at some measured sites. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Pathways of dust transport; Chinese Loess Plateau; The last glacial–interglacial period; Magnetic susceptibility

1. Introduction It is well known that loess and its intercalated palaeosols in the Chinese Loess Plateau are aeolian in origin ŽLiu, 1985; Kukla and An, 1989; An et al., 1990; Porter and )

Corresponding author. Tel.: q86-29-552-4746; fax: q86-29-552-2566. E-mail address: [email protected] ŽH. Lu..

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An, 1995.. Evidence from modern grain-size measurements and dust storm observations indicates that the dust has been transported mainly from north and northwest arid and semi-arid areas of the Loess Plateau. The East Asian winter monsoon circulation was an important agent for dust transport ŽAn et al., 1990, 1991; Zhang et al., 1994, 1999.. However, specific dust transport pathways and patterns are still not clear. The loess– palaeosol sequence is regarded as an excellent proxy record of the East Asia monsoon climate variations during the Quaternary period, and it can be used to reconstruct the palaeoclimatic regimes and patterns of the past atmospheric circulation. The palaeoclimatic changes derived from the loess–palaeosol sequence are globally significant because they can be correlated with other northern and southern hemispheric records ŽAn et al., 1990; Ding et al., 1994, 1995; Porter and An, 1995; Liu et al., 1996; Lu et al., 1996; An and Porter, 1997.. However, ambiguous pathways of dust transport may cause difficulty with palaeoclimatic interpretation because there may be local paleoclimatic signals that have been incorporated into the existing records. Here, we use magnetic susceptibility ŽMS. measurements from 32 loess–palaeosol profiles of the last glacial and interglacial periods to examine specific dust transport pathways.

2. Stratigraphy, MS measurements and correlation with the SPECMAP time series data Loess–palaeosol sequences in the Loess Plateau representing the last glacial and interglacial periods are quite well preserved, and stions can been found in many water-eroded gullies. These sections are composed of an upper loess unit and an underlying paleosol, both of aeolian origin. The loess is a dust deposit that formed during the glacial period. The palaeosol also represents accretion of dust, but during the interglacial period, under a warm, humid climate that favored pedogenesis ŽLiu, 1985; Kukla and An, 1989; An et al., 1991; Porter and An, 1995.. The loess and palaeosol units can be easily identified by the naked eye in the field. Weakly developed palaeosols within the loess units, for example the L1SS1, can also be distinguished by MS measurements. Numerous radiocarbon and thermoluminescence ŽTL. dates show that the upper loess unit Žnamely Malan loess, L1. was formed during the last glacial period; and the lower palaeosol unit was formed during the last interglacial period ŽLiu, 1985; Kukla, 1987; Kukla and An, 1989; Lu et al., 1987, 1988; Pye and Zhou, 1989; An and Porter, 1997; An et al., 1991; Forman, 1991; Porter and An, 1995; Zheng et al., 1995; Dong et al., 1997.. MS measurement is useful for the detailed discrimination of palaeosol and loess units in a sequence. It can also serve as a proxy index of palaeoclimatic variations in the Quaternary, although more work is needed to investigate the exact meaning of this index ŽKukla, 1987; Kukla and An, 1989; An et al., 1990, 1991; Zhou et al., 1990; Maher and Thompson, 1992; Meng et al., 1997.. MS was measured at 32 locations representing deposits of the last glacial and interglacial periods ŽFig. 1.. Of them, 22 locations are new Žsee Fig. 1, solid circle., while the other 10 are quoted from the work of others Žsee Fig. 1, open circle. ŽAn et al., 1991; Sun and Ding, 1997; Sun et al. 1995; Wu et al.,

H. Lu, D. Sun r Catena 40 (2000) 251–261

253

Fig. 1. Sampling sites within the Chinese Loess Plateau. Ž1. Xining. Ž2. Lanzhou. Ž3. Huining. Ž4. Tianshui. Ž5. Huanxian-1. Ž6. Huanxian-2. Ž7. Qingyang. Ž8. Xifeng-1. Ž9. Xifeng-2. Ž10. Ningxian. Ž11. Xunyi. Ž12. Chunhuan. Ž13. Yaoxian. Ž14. Qishan. Ž15. Jingyang. Ž16. Gaoling. Ž17. Xi’an. Ž18. Lantian. Ž19. Weinan. Ž20. Dali. Ž21. Heyang. Ž22. Huanglong. Ž23. Luochuan. Ž24. Fuxian. Ž25. Yanchang. Ž26. Wubu. Ž27. Jingbian-1. Ž28. Jingbian-2. Ž29. Yulin. Ž30. Ansai. Ž31. Dingcun. Ž32. Zhengzhou.

1995; Lu et al., 1996.. The loess deposits correspond to lower MS values, which indicate that the palaeoclimate was dry and cold, and the palaeosols correspond to higher MS values, which indicate that the palaeoclimate was humid and warm ŽFig. 2.. The different shapes of the MS curves in the last interglacial period are caused by spatial variations in sedimentation rates, where the higher sedimentation rates can offer high-resolution palaeoclimatic records, indicating three warm sub-stages and two cold sub-stages Žsee the Huanxian and Huining curves in Fig. 2.. In the other sections, MS peak and trough alternations during the last interglacial period are not obvious because lower dust sedimentation rates that do not produce high-resolution records, therefore the MS curves of this period are relatively uniform. Although there exist several different patterns of MS variations in the palaeosol deposits in the last interglaciation, all of them are synchronous with the deep-sea sediment record of stage 5 ŽLiu, 1985; Kukla, 1987; Kukla and An, 1989; Lu et al., 1987, 1988; Pye and Zhou, 1989; An and Porter, 1997; An et al., 1991; Forman, 1991; Porter and An, 1995; Zheng et al., 1995.. Previous research has demonstrated that the East Asian palaeomonsoon climate fluctuations coincide with global glacial–interglacial changes during the Quaternary ŽKukla, 1987; An and Porter, 1997; An et al., 1991, Ding et al., 1994; Porter and An, 1995.. Therefore, the MS variations of the loess–palaeosol sequences should correlate

254 H. Lu, D. Sun r Catena 40 (2000) 251–261 Fig. 2. Correlation between MS variations and the deep-sea oxygen isotope time series of Martinson et al. Ž1987.. The TL dates are cited from several references ŽLu et al., 1987, 1988; An et al., 1991; Forman, 1991; Dong et al., 1997.. The representative magnetic susceptibility curves and stratigraphy are presented in this figure.

H. Lu, D. Sun r Catena 40 (2000) 251–261

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with deep-sea oxygen isotope variations, and a strong correlation has been demonstrated ŽPorter and An, 1995; Lu et al., 1996; An and Porter, 1997.. In order to specifically date loessrpalaeosol boundaries, we have compared each MS curve with the oxygen isotope time series determined by Martinson et al. Ž1987. to obtain an age for each loess– palaeosol boundary ŽFig. 2. ŽAn and Porter, 1997; An et al., 1991; Porter and An, 1995; Zheng et al., 1995; Vandenberghe et al., 1997.. The points of rapid changes of the MS at the boundary of loess and paleosol are considered to coincide with oxygen isotope stage boundaries. In this way, the age of each loess and palaeosol boundary has been obtained. The detailed correlation is presented on Fig. 2. The loess and palaeosol boundaries are identified by rapid MS changes. The last glacial period loess deposit consists of two loess layers ŽL1LL1 and L1LL2. and one weakly developed palaeosol ŽL1SS1., which correspond to marine oxygen isotope stages 2, 3 and 4, respectively ŽKukla, 1987; Porter and An, 1995; Vandenberghe et al., 1997.. The last interglacial palaeosol is correlated with marine isotope stage 5 ŽKukla, 1987; An and Porter, 1997; Vandenberghe et al., 1997.. Through this correlation, the time represented by each sedimentation unit is obtained, and therefore the sedimentation rates can be calculated.

3. Discussion There are three fundamental requirements for loess deposition: Ž1. a sustained source of dust; Ž2. sufficient wind energy to transport the dust, and Ž3. a suitable accumulation site ŽPye, 1995.. Based upon previous investigations, it is widely accepted that the arid and semi-arid regions in north and northwest China are the sources of the loess–palaeosol sediments, and the Loess Plateau is a suitable accumulation place. But geological evidence for specific pathways of dust input the Loess Plateau during the last glacial cycle has not been reported. In areas where the surface conditions allow dust to accumulate and not be carried farther downwind, the dust sedimentation rate should decrease systematically with distance downwind from the dust source ŽHandy, 1976; Tsoar and Pye, 1987; McTanish et al., 1997; Mason et al., 1999.. The sedimentation rates synoptically decrease in almost every layer from the northwest to the southeast ŽTable 1 and Fig. 3.. In the northwest part of the investigated region, the dust sedimentation rate can be more than 50 mmrka, but in the southeast part of the region it decreases to around 10 mmrka. Fig. 3 indicates that sedimentation rates generally decrease from west to east. There is not a clear decrease in sedimentation rates from north to south and from northeast to the southwest. This indicates that the north and northeast wind played a negligible role in transporting dust. Our results clearly indicate that dust deposited in the Loess Plateau was mainly transported by northwest and west winds but not the north and northeast winds. Modern meteorological observation and investigation have proven that there are four pathways for the East Asian winter monsoon moving from its source area to north China. The average frequency of each pathway is: northwest Ž50%., east Ž33%., west Ž10%. and others Ž7%. ŽChen et al., 1991.. The sedimentation rates should gradually decrease downwind along the path of the winter monsoon ŽAn et al., 1990; Zhang et al., 1994; Porter and An, 1995.. However, our sedimentation rate investigation shows that

256

West to east Xining

Lanzhou

Huining

Tianshui

Qishan

Jingyang

Gaoling

Weinan 1

Weinan 2

Dali

Zhengzhou

570 800 490 690 2550

645 450 1050 710 2855

454 678 466 542 2140

235 190 235 370 1030

240 360 180 260 1040

246 446 200 298 1190

325 300 480 315 1420

236 304 194 238 972

170 250 210 200 830

140 360 220 290 1010

2980 1950 2150 740 7820

Nortwest to southeast Huanxian 1

Huanxian 2

Qingyang

Xifeng 1

Xifeng 2

Ningxian

Xunyi

Chunhua

Duanjiapo

L1LL1 L1SS1 L1LL2 S1 Total

450 490 630 530 2100

530 510 420 380 1310

250 475 345 315 1385

220 390 310 280 1200

230 380 320 260 1190

230 345 205 310 1090

160 240 180 210 790

110 200 90 150 550

L1LL1 L1SS1 L1LL2 S1 Total

740 590 575 400 2295

H. Lu, D. Sun r Catena 40 (2000) 251–261

Table 1 Thickness Žin cm. of each unit of the 32 loess and palaeosol profiles deposited in the last glacial and interglacial periods in the Chinese Loess Plateau

Table 1 Ž continued . North to south-1 Wubu

Yanchang

L1LL1 L1SS1 L1LL2 S1 Total

495 550 385 305 1735

238 407 477 252 1374

L1LL1 L1SS1 L1LL2 S1 Total

480 380 300 250 1410

Northeast to southeast Dingcun L1LL1 L1SS1 L1LL2 S1 Total

110 250 200 130 590

Luochuan 225 310 240 220 770

Heyang 160 280 190 110 740

Heyang

Dali

160 280 190 110 740

140 360 220 290 1010

Xunyi

Chunhua

Yaoxian

230 345 205 310 1090

160 240 180 210 790

145 236 200 218 799

Dali

Weinan 1

140 360 220 290 1010

236 304 194 238 972

120 75 160 165 520

Weinan 2 170 250 210 200 830

Weinan 1 236 304 194 238 972

Weinan 2 170 250 210 200 830

Xi’an 263 315 236 284 1118

Lantian

Xi’an

110 200 90 150 550

263 315 236 284 1118

H. Lu, D. Sun r Catena 40 (2000) 251–261

North to south-2 Fuxian

Huanglong

257

258

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Fig. 3. Variation of sedimentation rates with the winter monsoon pathways in the Loess Plateau.

the sedimentation rates decrease along the path of the northwest and west winds but not of the northeast, east and north winds. We explain these as there is sufficient material available for the northwest and west winds to erode as they move across arid and

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semi-arid regions toward the Loess Plateau. But the north and east winter monsoon pass over regions with dense vegetation in summer and ice and snow in winter; thus there is less material available to become aeolian dust. There is an unusually high sedimentation rate at Gaoling ŽFig. 3.. This could be caused by local dust input. Because this site is located at the terrace of the Weihe river, fine sediments from river bed have been moved to the terrace in the winter half year, when the water level is lower, the precipitation is low, and the wind is strong, a similar phenomenon to the high dust deposition rate at Zhengzhou ŽWu et al., 1995.. Similarly to the Gaoling and Zhengzhou sites, an anomalously high sedimentation rate occurred in Sanyao, for instance, 218.1 mmrka during stage 2, that is not in accord with its location. This phenomenon is also caused by a local environmental mechanism, because this profile is located close to the Yuhe river. The fluvial sediments in this wide river valley can be moved by the wind to the Sanyao site when it is dry in the winter half year. The sedimentation rates change rapidly between the west and east sides of the Liupan Mountains. These mountains have an average altitude close to 2000 m above sea level, and are more than 200 km in length, lying from northwest to southeast in Ningxia, Gansu and Shaanxi Provinces. The sedimentation rate at Huining, west of the Liupan Mountains, is 37.65 mmrka during the L1LL1 loess deposit, but east of the mountains, the rate sharply decreases to 20.73 mmrka during the same period. Our field investigations show that the loess deposit west of the Liupan Mountains has a loose structure, light color and coarse grain size, but east of the mountains, the loess has a hard structure, reddish color and fine grain size. Exposures of the loess profiles in the east form more vertical cliffs than those in the west, and the intercalated palaeosol is more strongly developed in the east than that in the west. This difference indicates that the Liupan Mountains play a significant role in influencing the sedimentation rates, and, it confirms that the dust contributed to the loess was transported by lower atmospheric circulation ŽPye and Zhou, 1989.. The area of aeolian loess deposit in Late Pleistocene in China was significantly expanded compared with the previous loess deposit, it even arrived in Yangtze river in south China. This expansion should indicate a more dry climate and strengthened winter monsoon in the late Pleistocene. The cause of the enlarged area of loess deposit is still not clear. One explanation is that the accelerated uplift of the Tibet Plateau may have strengthened the winter monsoon circulationŽKutzbach and Ruddiman, 1993., and therefore, forced the expansion of the aeolian deposit. However, more works are needed to investigate this mechanism.

4. Conclusion Dust input to the Chinese Loess Plateau during the last glacial cycle was mainly caused by northwest and west winds. The north, northeast and east winter monsoon played negligible roles in the dust transport. Local sediments could play an important role in providing dust for some loess– palaeosol sequences.

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Dust accumulation rate was significantly influenced by higher topography demonstrating that dust was transported by lower atmospheric circulation during the last glacial cycle.

Acknowledgements We thank Dr. J.A. Mason, Prof. M.J. Singer, Dr. J. Bloemendal and Mr. J. Head for significantly improving the early manuscript. This research work is under auspices of the National Key Project for Basic Research on Tibet Plateau ŽG199804800., the Key Research Project of Chinese Academy of Sciences ŽKZ951-A1-402. and the National Natural Sciences Foundation of China ŽNo. 49902009..

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