NOTES Landscape evolution and precipitation changes in the Badain Jaran during the last 30 000 years

YANG Xiaoping Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Abstract On the basis of field expeditions into different parts of the Badain Jaran Desert, new evidence of landscape evolution and precipitation changes during the last 30 000 years has been found from the geomorphological, sedimentological and geochronological studies of the megadune. Different cemented surfaces of palaeo- have been observed in many localities, which are representatives of relatively moister environment. The chronological data show that four periods of more humid environment occurred in the past 30 000 years. The stabilization of dunes in a large scale and the existence of many palaeo-lakes and lacustrine terraces confirm the periodic fluctuations of climate. It implies that the intensity of westerlies and Eastasian monsoons varied in the region of Alashan, and the periods of higher precipitation took place even in the times close to the last glacial maximum. Keywords: desert, palaeo-, Lake, global change, Badain Jaran.

In the 1930s the Chinese-Swedish joint-expedition team investigating the northwestern China measured the terminal lakes in the lower reaches of ~uoshui"~.The working group of the Chinese Academy of Sciences for dune stabilization carried out comprehensive studies of physical geography and social economy of the Badain Jaran Desert in the 1950s and 1960s'~~'.During the last decade, the attention to Alashan was given by several researchers as Owing to the large dunes and traffic difficulties, the work concerning the megadunes and lakes in the interior of the Badain Jaran Desert is still very scarce. Located in the middle of the westerlies and near the outer boundary of the Asian summer monsoon, this desert could be a remarkable repository of palaeoclimatic information for the arid regions of northwestern China. On the basis of field expeditions into different areas and of detailed climatic-geomorphological and sedimentological studies of the dunes and lakes in the Badain Jaran Desert (fig. l), this note intends to give an insight into the precipitation changes in northwestern China during the last 30 000 years. 1 Essential characters of the landscape in the Badain Jaran Desert One peculiar property of the Badain Jaran Desert is the dense distribution of the megadunes that may reach a height of' almost 500 m and are presumably the highest ones in the world. Generally

Chinese Science Bulletin Vol. 45 No. 11 June 2000 NOTES

Fig. I. The over~ie\vof the Badain Jaran Ileset-t. 1, Dunes; 2, desert plain; 3, ancient lake bed; 4, pediment; 5. mountaini6, river; 7. location of observed calcareous cementation; 8, main expedition route and location of dune examinations; :%,. locution of fig. 3. speaking, the megadunes trend NE-SW and mainly occur in the southern part. The dunes in the north are relatively low, the highest dunes often with compound forms appear in the southeast. The materials on the ridge of the dunes are shifting sands, and the inclination on the top may reach a degree of 3 1.5" . The fact that basement rocks are found on the ridges of several dunes indicates some dunes originated from rock hills surmounted with windblown 102"27 ' sand. But the geometrical morphology of the I I initial landforms has been converted owing to G/!9 /'-, Pilin the massive deposition of aeolian sands. The \ _I 1- Ghaitu new moving dunes, normally with a height of 1.9 several meters, are often distributed in the lower ,;) I 1 @;+inaeertu 1, F: sections among the dunes, on the ancient lake bottom and on the old dunes. ~(~$5- - -> .!' C-@; f Compared with other of the world, '% another specially of the Radain Jaran Desert is (6.; the existence of numerous permanent lakes ./ E' /" @ among the dunes in its southeastern part. 39047'- e--, ,, Badanjilin-p( However, fieldwoik confirms that a great change ,D-',,j-; !@f?Zhalatu@; miao 'J1 of water surface has tagen place, some of the .a/ lakes have even dried up completely (fig. 2). At ,- --' I@ (@-huoertu - present, the largest one is Nuoertu, which is 1.5 ,q> '4 :@'; km2 in area and 16 m in depth. With various 0L2km (,' $, I/ .!a/ /If sizes and depths, these lakes show a very large , / salinity spectrum from <2 g/L to over 390 g/L. The relatively low elec,tric conductivity and low (I a2m3 salinity indicate that Ihe outflow lhe Fig. 2. The distribution of lakes in the SE part of the springs araund the lakes is mostly fresh waler. Badain Jaran Desert (after ref. [9],changed). 1, Present lake; Due to the decrease of water level, some large 2, palaeo-shoreline; 3, salina.

Chinese Science bullet^^? Vol. 45 No. 11 June 2000 1043 lakes were divided into small separate parts, which disappeared step by step. Deflation landforms have already come into appearance in the dried lake bottom, but old lacustrine terraces are still partly preserved. Under present climatic regimen, the shifting sand not only slides into lakes from the shorelines, but also falls down to the lake bed from the water surface. Therefore, there are aeolian sands in the sequences of lake deposits. A I-m-long core at the depth of 6.5 m in the lake south to Badanjilinmiao (see fig. 2) shows light-yellow aeolian sands at upper section, and brown gyttja appears only in the lower part. 2 Formation of megadunes Based on the bedded structures of cemented surfaces of palaeo-dunes and their vast spreading, it is reasonable to deduce that the megadunes are developed by the means of overlapping of the old and young dunes. The stratigraphy of different sand SE NW layers indicates that four main cemented surfaces % 1 exist on the megadunes (fig.. 3), their cementation material is mainly calcium carbonate. Thickness of the cemented layer varies a lot. The first layer is the 9435 +345 thickest one, it ranges normally from 30 to 40 cm, even 60 cm occasionally like in Huhejilin (see fig. 2). The second and third cementation surfaces are 20 31750+485 cm and 10 - 20 cm respectively. The fourth Fig. 3. Schematic profile of a megadune in the cementation consists of a group thin (2-8 mm) southeastern Badain Jaran Desert (for location see fig. 1) 1, Shifting dunes; 2, palaeo-dune surface and its age (aBP); plates, among which sand appears. The 3, basement rocks; 4, lake surface. sedimentological structure indicates that the oldest palaeo-dunesurface was a relatively smooth sandy desert plain. On the surface of some locations, there is a thin stratum of gravels with a diameter of 3-5 mm covered by desert varnish, and Artemisia and several other plants often grow hier. Dunes of the second oldest generation cover the desert plain and the color of sand becomes lighter, there exist relatively dense plants as well. The dunes of the third phase with weak cementation are most vastly distributed. The palaeo-dunes of the fourth epoch are semi-active and they appear on the surface of all other palaeo-dunes. The younger the dunes are, the lighter their color is. Modem active dunes are widely spread above the palaeo-dunes. The four cementation surfaces represent the landscapes of palaeo-dunes and should be formed during four different periods. Plenty of cemented plant roots exists on the old dunes. In the first cementation layer, this kind of roots may be 4-5 cm in diameter, densely distributed at the locations of outcrops. The tubes of plant roots are 2-3 cm in diameter in the second and the third cementation layer, and they are narrowest in the fourth cementation layer, frequently less than 1 cm. The radiocarbon ages of such tubes from the palaeo-dunes in the southeastern part of the desert (see fig. 3) are shown in table 1. Table 1 The radiocarbon ages of plant roots from different cementation layers Materials .. .- Location Lab. No. AgeIaBP Calcareous roots first palaeo-dune Hv* 15943 31 750+485 Calcareous roots second palaeo-dune Hv 15944 Calcareous roots third palaeo-dune HV 15938 Calcareous roots fourth palaeo-dune Hv 15937 * Hv-C- 14 und H-3 Labor Hannover. 3 Sedimentological evidence of dune evolutions The dunes were sedimentologically examined in the north, south and southeast of the Desert and in the marginal areas. Because the sand materials are not the same at different locations of a dune, the samples of every dune were taken from bottom, midst and ridge. The three kinds of samples were mixed together and were treated as representatives of the dune. In order to minimize the occasional influences, samples of two or three dunes were analyzed in every place and their average values of analyses were taken for further interpretation. The data show that the sand materials vary a lot in the Badain Jaran Desert, the grain size does not differentiate regularly in the N-S direction (fig. 4), and the composition of heavy minerals changes considerably from place to place (fig. 5). These data suggest 1 044 Chinese Science Bulletin Vol. 45 No. 11 June 2000 that the wind from different directions has possibly played an active role in the formation of these dunes. Besides, the sand sources in the desert might be different. It is to assume that the existence of many lakes may function as obstacle for sand transportation. In the whole desert, megadunes are mostly compound or have pyramid forms. This confirms the joint contribution of surface winds from multidirections. The observation of weather stations in the margin of the desert shows that the direction of dominant wind changes considerably from place to place. For instance, in Wentugaole (see fig. 1) the dominant sand-blown wind (36 m/s) comes from the east. But in the Wuliji (see fig.l), the western wind predominates. The dominant wind direction varies during a year too, it changes from the northwestern wind during spring to the northeastern one in ~u~ust''~~'~~.

+Relat. stable -A- Stablc Jt Extrem. stable ------x------x- /21314151617 18 112131415161718 Location ol'samples Location of samples ~i~,4, variation of mean grain size [ = ( @16+ @50+0 Fig. 5. Composition variation of heavy minerals along the N-S direction in the Badain Jaran Desert. For location 84),3, *=- log2 <, = diameter in mml of dune sand along the N-S direction in the Badain Jaran Desert. For see fig' location of samples see fig. I. The microstructures on the quartz grains reveal that the wind has played an important role in the constructing landscapes. Here 114 grains from 13 samples of this Desert were examined in detail under an electron probe machine. The results show that two types of microstructures occur most frequently on the quartz grains of the Badain Jaran Desert: i ) Mat structures formed by wind transportation; ii) solution and sedimentation of silica owing to the high amplitude of temperature (fig. 6). Mat surface, crescent and dish-like concavities were formed through the processes of percussion during aeolian transport. In this desert, the daily amplitude of temperature is extremely high. Water condenses among the sand grains when the temperature on the sand surface becomes lower than that of ground air, and dissolves salt. After having dissolved salt, the condensed water would reach a higher pH-value and can dissolve the SiOz of the grain surface. As the 35 - - - temperature goes up during the daytime, the - dissolved Si02 could be deposited on the grain 5- 30 surface again. Owing to the mechanical process 2 25 - and chemical weathering, many up-turned plates - - were formed on the edge of cleavage. While the 3 - SiOz is deposited on the plates, the lattices become # l5 unclear. Even so, the basic outline of the plates is E 10 - - still observable and looks like up-turned. The 6 5- frequent occurrence of up-turned plates on the quartz grains indicates that the strong wind and O 1'23'~'S1?' large daily amplitude of temperature are Types of microstructure characteristic in the region. Fig. 6. The occurrence frequency of various micr- Although- the old and new dunes look ostructures on quartz grains in the southeastern Badain Jaran different in color, they are almost the same in Desert. 1, Mat surface; 2, ceresent-like concavities; 3, up- turned plates on cleavages; 4, cleavage surface; 5, dish-like composition of heavy minerals. In both kinds of concavities; 6, trough; 7, solution features.

Chinese Science Bulletin Vol. 45 No. 11 June 2000 1045 NOTES dunes, the relatively stable minerals, stable minerals 100 and unstable minerals are dominant, and the - percentages of extremely stable minerals are very 5 90 B85 low. These data indicate that the formation 80 environments of old and young dunes are similar. In .= 70 E the areas of Badanjilinmiao, the old lacustrine 60 El4 deposits, old and new aeolian sands are extremely 2 50 identical in composition of heavy minerals (fig. 7). It % w3 means that the lake sediments and aeolian materials .sc 40 are closely related, and the lacustrine deposits 'z 30 & €l2 among the dunes are one of the main sand sources. g 20 4 Discussions and conclusions on the precipitat- 10 a1 tion changes during the last 30 000 years Lake Shifting Palaeo- The dry climate promotes the development of sediments dunes dunes shifting dunes, which becorrie stabilized under Fig. 7. The composition of heavy minerals of different moister conditions. B~~~~~~ ground water does not sediments in the Badanjilinmiao. 1, Hornblende; 2, epidote; flow out on the slopes of megadunes in the Badain 3, zoisite; 4, non-transparent minerals; 5, garnet. Jaran Desert and the upper parts of the dunes are far from rivers and lakes, the cementation of dune surface in a large area should be therefore treated as an indicator of increased precipitation. Taking the calcareous cementation on the dune surface, cemented tubes of plant roots (see fig. 1 and table 1) and lake terraces among the dunes into consideration, it can be claimed that the climate was much more humid at ca. 30 000 aBP and ca. 20 000 aBP than at present in the Badain Jaran Desert. This indicates that the environment was sometimes moister during the generally dry and cold period of the last glaciation than at present in the Desert. Additionally, it was more humid at the beginning of Holocene and at ca. 2 000 aBP. Field observations reveal that the lakes in the Badain Jaran Desert are mainly recharged by the ground water. There are springs on the foot of megadunes and they belong to descending types, which possess a circle-like distribution pattern. This shows that the spring water originates from the dunes, Isotope analysis denies a relationship between the ground water in the Badain Jaran Desert and the Ruoshui ~iver'"'.Therefore, it is reasonable to take the high levels of the lakes in the desert as an additional significant evidence for relatively more humid climate in the research area. The epochs with high lake levels on the northwestern margin of the desert coincide with the times of dune stabilization in the interior. This reconfirms the existence of periods with higher precipitation in a regional scale. The ''c dating shows that the highest lake level in the Sugunuoer occurred ca. 33 000 aBP [IJ.A core from Gashunnuoer, being located west to Sugunuoer, again indicates that the highest lake levels were during 39 000 aBP and 21 000 ~BP"~~"'.The records of higher lake levels happened during the last 13 000 years too. Detailed interpretation of satellite imageries and air photos reveal a large number of ancient river courses. It is worth mentioning that lakes of the lower reaches of the Ruoshui depend on the amount of the melting water from the Tibetan Plateau as well. Therefore, lake level changes here might not reflect the local climate history completely. Provided the I4c ages and sedimentation rates were correctly measured, the pollen composition and the content of CaC03 changed considerably in the past in a profile between the Badain Jaran Desert and ~abulai-~han"].The former water erosion could still be clearly seen on the basement rocks in the Yabulai-Shan. Besides, the continuous distribution of alluvial fans on its eastern side confirms stronger rainfall in the mountains. The formation of the palaeo-dunes in the Badain Jaran Desert shows that the main controlling factor of the dunes in the deserts of western China has been the aridity index. It was cold and dry during glaciations in the eastern Asia owing to the enhancement of the winter monsoon['81.And even at present the north boundary of the summer monsoon can only reach the Maowusu Desert and the Tengeli ~esert"~'.Therefore, the more humid conditions in the Badain Jaran Desert during the last glacial times have to be explained in two ways. Firstly, the enhancement of westerlies. This means the Badain Jaran Desert experienced relatively more humid periods under the influence of westerlies during the last glaciation. The moisture of the westerlies formed rain or snow in the deserts of arid China while it met 1046 Chinese Science Bulletin Vol. 45 No. 11 June 2000 the cold air from the Mongolian high pressure and then raised up. Secondly, despite the dominance of the winter monsoon, the summer monsoon might be strong and even stronger than at present for a short time at ca. 30 000 aBP and ca. 20 000 aBP. However, the moister periods during the Holocene should be attributive of summer monsoon because of the clear increase of intensities of summer monsoon during interglacials. In one word, the evolution of climate in the Badain Jaran Desert during the last 30 000 years is not simple process of aridification. There were several fluctuations with a relatively clear periodicity at an approximately 10 000 years scale, which changed the regional landscape too. Acknowledgements Prof. Juergen Hoevermann, Prof. Horst Mensching, Prof. LIU Tungsheng, Prof. ZHU Zhenda, Prof. Dieter Jaekel and Prof. Frank Lehmkuhl as well as others have provided me with important support and advice during the work. The I4c dating was carried out by Prof. Mebus A. Geyh. This work was jointly supported by the National Natural Science Foundation of China (Grant No. 49902015), the Chinese Academy of Sciences (Grant No. KZ952-51-438) and the Deutsche Forschungs- gemeinschaft as well as the Max-Planck-Gesellschaft. References I. Norin, E., Sven Hedin central Asia atlas, Memoir on maps, Vol. 111, Stockholm: Statens Etnografiska Museum, 1980, 94- 110. 2. Yu, S. Z., Li, B., Cai, W. Q. et al., Expedition into Gobi of western and the Badanjilin Desert (in Chinese), Research on Sand Stabilization, 1962,3: 96. 3. Lou, T. M., The desert between Minqing of and Badanjilinmiao: origin and utilization, Research on Sand Stabilization (in Chinese), 1962, 3: 90. 4. Tan, J. 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