Paleoclimatic evolution of the Uvs Nuur basin and adjacent areas (Western Mongolia)

Quaternary International 65/66 (2000) 171}192

Paleoclimatic evolution of the Uvs Nuur basin and adjacent areas (Western Mongolia) JoK rg Grunert , Frank Lehmkuhl
*, Michael Walther Geographisches Institut der Universita( t Mainz, Becherweg 21, D-55128 Mainz, Germany
Geographisches Institut der Universita( t Go( ttingen, Goldschmidtstr. 5, D-37077 Go( ttingen, Germany Freie Universita( t Berlin, Institut fu( r Geographische Wissenschaften, Malteserstr. 74-100, D-12249 Berlin, Germany Dedicated to Prof. Dr. F. Scholz, FU Berlin, on the occasion of his 60th birthday

Abstract The investigations presented in this paper focus on the shifts in Pleistocene glaciations and the geomorphic changes in landforms, as well as lake level changes and aeolian deposits of the last glacial}interglacial cycle, including the Holocene. Geomorphic evidence and high lake levels show that the climate was more humid before the last glacial maximum (LGM); however, at least one arid phase also occurred. During the second half of the LGM the climate was dry and cold, turning to wet and cold during the Late Glacial of the last Ice Age. Fluctuations in humidity and temperature occurred during the Holocene. Since about 2000 yr BP the impact of human activity has increased.  2000 Elsevier Science Ltd and INQUA. All rights reserved.

1. Introduction The Uvs Nuur Basin in the northwestern part of Mongolia stretches from 92330N to 96350N, and from 49347E to 50330E. At its maximum, it measures 312 km from west to east and 135 km from north to south. The northern part of the basin is surrounded by the Tannu Ulu Mountains with the Baruun Tagnyn Nuruu in the northwest, the 4000-m-high foothills of Kharkhiraa in the west, and the 2300-m-high Khan KhoK hiyn Nuruu range in the south. The latter comprises several individual massifs; of these, the Togtochyn Schil is considered to be a separate mountain. Eastwards, the basin interior rises gradually to elevations of about 1500 m and is bounded in the east by a lower mountain range, about 1900 m high, in the vicinity of Tes Somon. In the climatic classi"cation according to KoK ppen (1923) the medium and higher mountainous parts of the study area belong to the Dw climate (cold snowy forest with dry winters) and the lower basin zones to the Bs (steppe) region. According to Troll (1964), the study area is situated near the boundaries between the strongly

* Corresponding author. Fax: 551-398-006. E-mail address: #[email protected] (F. Lehmkuhl).

continental boreal climates with permafrost and the steppe climates with cold and dry winters and moist summers. The recent continental climatic conditions are characterised by a wide annual range of temperature, with winter values of below !20 to !303C and summer values up to and exceeding 203C (for example, the mean temperature at Ulaangom is !32.93C in January and 19.23C in July with an annual average of !3.73C). Annual rainfall averages about 200 mm at the altitude of 2000 m and is estimated to exceed 300 mm in the high mountain areas, whereas precipitation in the basins is less than 100 mm per year (Academy of Sciences of Mongolia and Academy of Sciences of USSR, 1990). The research work was part of the Uvs Nuur joint research project supported by the Deutsche Forschungsgemeinschaft (DFG, German Science Foundation). First results of the Mongolian}German joint research work including these topics were presented in a volume of Die Erde in the German language, with contributions by Grunert, Lehmkuhl, Naumann and Walther. In addition papers have been published by Lehmkuhl (1998) on the Pleistocene glaciations of Mongolia, and by Walther (1998), Walther and Naumann (1997) and Naumann and Walther (2000, in press) on the lake level changes and other geomorphic features of the Uvs Nuur Basin.

1040-6182/00/$20.00  2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 9 ) 0 0 0 4 3 - 9

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1.1. Study areas The individual study areas are located in the higher and medium elevations of the mountains, in the lower transitional zone to the basin interior, and in the basin interior itself. The entire inner basin of Uvs Nuur consists of two large units. The western part comprises the lake of Uvs Nuur; measuring 75;80 km at its maximum extent. Either alluvial fans (fanglomerates and pediments) or sizeable river deltas bound the lake. Towards the east, the relief rises gently across the deltas of the Tes Gol, Narrin Gol and Khara Ussu Gol and the dune"elds of BoK oK roK g Deliyn Els, grading into the second landscape unit. The broad river channel of Tes Gol bounds this unit in the north, while the middle and southern portions of the deeper parts of the basin consist of dune"elds and aeolian sands, with occasional lakes such as Bayan Nuur. The Turgen-Kharkhiraa Mountain system lies in the northernmost part of the Mongol Altai and is situated southwest of the Uvs Nuur Basin (Fig. 1). These mountain systems cover an area of approximately 5700 km. Planation surfaces (probably of Cretaceous and Tertiary age) were formed by Canozoic tectonism. There is still an active tectonism in this area. The Turgen-Kharkhiraa Mountains comprise mainly Palaeozoic rocks, whereas at the margins of the mountains pelites, sandstones, conglomerates and schists are dominant and the central part

of the mountains is built up by Palaeozoic granites and gneiss. At the margins of the mountains and in the basins, younger Quaternary sediments * mainly fanglomerates, sands, gravels and lacustrine sediments * overlie the basement rocks. 1.2. Current state of research Only a few results have been published in the western literature on the Quaternary geology of Mongolia and very little detail has been given about Pleistocene, Lateglacial and Holocene climatic #uctuations. According to Mongolian and Russian scientists the Pleistocene glaciations were divided into di!erent stages, and two to three main Pleistocene glaciations occurred in the mountains of Mongolia (Devjatkin, 1981; Florensov and Korzhnev, 1982). The Pleistocene stratigraphy of Mongolia has been made by Russian scientists, and the local names of the Siberian stratigraphy as published by, among others, Arkhipov et al. (1986) were used. According to this stratigraphy the Last Glaciation is divided into two glacial periods (Sartan and Early Zyrianka) interrupted by the Karginsky Interstadial. Devjatkin (1981) published further details for the middle and older Pleistocene glaciations. However, TL-ages of more than 100 ka BP for older glaciations as given by Devjatkin (1981, Table 25, p. 176) may be unreliable (see Wintle and Huntley, 1982). The present state of research on the

Fig. 1. Map of the study area.

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

Pleistocene glaciations of Western Mongolia is described by Lehmkuhl (1998). Little is known about the size of a huge palaeo-lake in the transition from Pliocene to Pleistocene, the so-called `Eopleistocenea (3}0.7 million years ago); according to Devjatkin (1981) this is due to the lack of outcrops and to the fact that lakes were smaller than during the Pliocene. However, in contradiction it was assumed that the climate was much wetter than it is today (Devjatkin, 1981). Early Pleistocene sediments, described in the Russian literature as `alluvial}proluviala deposits of lacustrine origin, were found at the foot of the Togtochyn Schil and the southern margin of the Uvs Nuur Basin, where they are 25 m thick, and in the Khirgis Basin immediately to the south, which is also part of the `Valley of the Lakesa. Here, these deposits correlate with a strandline at 1190 m above sea level, leading Devjatkin and Murzaev (1989) to conclude that an early Pleistocene transgression occurred up to this level. According to Devjatkin (1981) a drier climate led to lower lake levels at the start of the midPleistocene (about 360,000 years ago). The majority of Russian and Mongolian authors assume that enhanced precipitation and falling temperatures prevailed during the second half of the interglacial periods and the "rst half of the glacials. In this context, Murzaev (1954) and Devjatkin (1981) speak of a pluvial period during the remainder of the Middle Pleistocene in the intermontane depressions, while the westernmost range of the Mongolian and the Gobi Altai, the Prichubsugul, the Khangayn Nuruu and the Khenteyn Nuruu were apparently glaciated. Enhanced precipitation, combined with decreased evaporation and meltwater in#ows, resulted in a pronounced transgression of the lakes, causing Murzaev (1954) to speak of a lacustral stage during the mid-Pleistocene. It is thus assumed that a closed water body of some 92,000 km occupied the `Valley of the Lakesa, having an absolute lake level of 1500 m above sea level and hence a water depth of about 740 m. On the basis of TL dates of laminated silts outcropping in the lake terrace between Khar Nuur and Khar Us Nuur in the central part of the `Valley of the Lakesa this transgression has been attributed to the time span between 28,000 and 177,000 yr BP (Devjatkin and Murzaev, 1989). Evidence cited in support of a water body of this size consists largely of morphographic descriptions. Lake plains have been reported from both Uvs Nuur and from the Khirgis Nuur Basin immediately to the south. These plains incline gently towards the lake and are covered by proluvial debris and aeolian sands (Gluchowskaja, 1989). Strong tectonic activity is described at the terminal Middle Pleistocene, leading to considerable changes in the #uvial system. Uplift of the Khan KhoK hiyn Mountains separated the Uvs Nuur Basin from the Khirgis Nuur Basin in the south, the `Palaeo-Lakea already being in regression. This lake basin reached a stable state in the Uvs Nuur Basin.

173

2. Results 2.1. Glacial and periglacial environment of the mountains The Turgen-Kharkhiraa Mountain system, the northernmost part of the Mongolian Altai, can be roughly divided into seven main morphotectonic regions, comprising three mountain areas separated by tectonic basins of di!erent altitude (Lehmkuhl, 1998). Five modern geomorphic altitudinal belts can be distinguished according to typical morphologic forms and processes (Lehmkuhl, 1999). A glacier and nivation zone is found at elevations above 3000 m in the highest mountain ranges. The present ELA occurs at about 3500 m a.s.l. and the periglacial zone with soli#uction can be mapped down to 2600 m. Discontinuous permafrost and rock glaciers are found down to 2200 m on northern slopes. The morphological processes below 2600 m are weak and largely restricted to frost shattering of the bedrock. Below 2400 m at the southern #ank and 2200 m at the northern #ank of the Turgen-Kharikhiraa Mountains, small Holocene gullies cut into the bedrock and slope debris. In addition, in the lowermost, semiarid and arid areas below 1200 m, #uvial incision is stronger, especially in Pleistocene deposits (Lehmkuhl, 1999). During the cold stages of the Pleistocene, glacial and nivation processes in the higher mountain areas as well as periglacial processes down to the basins dominated in the ice-free areas. According to ice wedge casts in di!erent exposures down to 1000 m a.s.l. and up to more than 1 m in depth, the lowermost boundary of continuous permafrost was at least 1300 m lower than today and reached the basin of the Uvs Nuur. Changes of temperature and precipitation during the Pleistocene produced a signi"cant change in vegetation cover which re#ect large changes in the rate of sediment supply to the stream channels and alluvial fans. In the basins and at the foothills of the mountains large alluvial fans and fanglomerates or pediments (bajadas) were deposited. In the Russian literature these landforms are named bels (Murzaev, 1954). They extend several kilometres beyond the mountain front; in the Uvs Basin more than 20 km. This accumulation area has produced quite a lot of silt and sand during the Pleistocene (Lehmkuhl, 1997b). Since the beginning of the Holocene there has been a reduction in sediment yield due to the vegetation cover. In addition there is a fan trenching due to the reduction and concentration of precipitation in vast areas of Central Asia (HoK vermann, 1985, 1987; HoK vermann et al., 1993; Lehmkuhl, 1997a). Pleistocene valley and cirque glaciers covered an area of 1300 km in the Turgen-Kharkhiraa. Di!erent types of terminal moraines and ice margins can be mapped in the various valleys of the Turgen-Kharkhiraa Mountains. Six main ice margins terminated at elevations of between 1950 and 2250 m a.s.l. (Fig. 2). They provide evidence for

174

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

Fig. 2. Extent of LGM glaciers in the Turgen-Kharkhiraa Mountains.

the extent of Pleistocene valley glaciers during the Last Glaciation. The ELA of the last glaciation was at 2900 to 3000 m a.s.l., about 600 m lower than at present (see Lehmkuhl, 1998, for further details). The accumulation of ice marginal grounds and terminal moraines are controlled by the topography. Each of these ice margins is

connected to glacio#uvial outwash plains and terraces. The modern streams have incised into these deposits to depths of between 5 and 25 m (30 m) to create the `main terracea (¹ ) and younger terrace sequences. Beyond  (upstream) of these ice margins there is only one small terrace. The beginning of the main terraces represent the

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

LGM ice margins. Most remnants of older moraines were eroded and transported by meltwater during the deposition of glacio#uvial outwash. Only one small moraine ridge has been preserved in the middle reaches of the Kharkhiraa River 2 km in front of the LGM moraines on the outwash plain at an elevation of 1900 m and partly covered by these deposits (see Fig. 4). In the area outside these moraine deposits, glacial landforms or sediments are absent. However, well developed Late Glacial moraines are completely missing in all valleys of the Turgen-Kharkhiraa Mountains. This indicates a rapid glacier retreat in the deglaciation period of the Last Glaciation. On the other hand, outstanding modern end moraines can be recognised in front of all modern valley glaciers (Lehmkuhl, 1998). As they have only a little or no vegetation cover and soil formations they can be correlated with the global glacier advances of the `Little Ice Agea in the 16th}19th century (Grove, 1988). Moraines, with only sparse vegetation, have been observed in other mountain ranges of High Asia, e.g. on the Tibetan plateau or the Tian Shan (e.g. HoK vermann and Lehmkuhl, 1994; Kotlyakov et al., 1991; Meiners, 1997). In China and Tibet the Little Ice Age event coincides with cooling periods in the 14th}19th century (BraK uning and Lehmkuhl, 1996). For the adjacent Russian Altai, Serebryanny and Solomina (1996) reported on Holocene glacier advances beyond a warm interval (5000/6000 yr BP) with glacier advances at 6060$60 and 4700$600 yr BP. Younger moraines were dated in the Aktru valley (c. 250 km west of the study area) by remnants of wood in moraines with radiocarbon ages of 1560$50, 1640$50, 1635$35, and 1440$50 yr BP. The so-called Little Ice Age was dated by lichenometry to the 15th, 17th and the end of the 18th century and the limit of forest growth was about 60 m lower than nowadays during that period

175

(Serebryanny and Solomina, 1996). In the 20th century there was a general glacier retreat in all these mountain areas. However, in the investigation area there has been a remarkable glacier retreat in area and length since this maximum extent, which is presumably the maximum extent during the Holocene. Detailed "eld observations and aerial photo mapping calculated the glacier retreat since these Little Ice Age moraines for the Turgen and Kharkhiraa Mountains from 117.2 to 92.3 km (Lehmkuhl et al., 1998). The di!erent terraces can be distinguished by their relation to the main ice margins, weathering criteria, and the overlying aeolian sediments. Besides some smaller and younger Holocene and Late Glacial terraces with a thin aeolian mantle (¹ , see Table 1) the main terrace  has about 40}150 cm of aeolian silt on top. In some parts of the valley this terrace is divided into two levels (¹ ,  ¹ , see Table 1). In addition, remnants of a second and 
higher terrace (¹ ) with a weathered, up to 3 m thick  aeolian silt on top, and highest terraces (¹ , ¹ , in most   cases in bedrock), could be found in the valleys. The "rst two terrace sequences (¹ , ¹ ) are assigned to   the last and penultimate Glaciations. This preliminary stratigraphy is supported by the relation to the main ice margins and the weathering characteristics of the overlying stratum: the second and higher terrace previously classi"ed during the "rst expedition in 1995 as being of penultimate Glaciation in age, has strongly weathered aeolian sediments including minor calcrete and brownish clay skins often associated with stony horizons. The "rst terrace has a Holocene soil development with minor laminated white calcrete on the underside of stones. The fanglomerates in the Uvs Nuur Basin can also be separated by the calcretes (younger calcrete and residue of older calcrete weathered more strongly). OSL-samples at

Table 1 Quaternary Terraces of the KhoK ndlen and Kharkhiraa Rivers. Elevations of the di!erent terraces in metres above the modern river #oodplain. Letters A}F: see Fig. 3

Terrace sequence KhoK ndlen (A) Old Turgen Somon, 1760 m (B) Intramontane basin, 1640 m (pro"les 25}28) (C) Frontal range/Uvs Nuur Basin, 1300 m (D) Uvs Nuur Basin, 1110 m Terrace sequences Kharkhiraa (A) Basin of Pleistocene glacial terminus, 1860 m (pro"le 20) (B) Base camp, 1840 m (C) Middle reaches, graves, 1800 m (D) Middle reaches, 1700 m (pro"le 17}18) (E) Middle reaches, 1600 m (F) Frontal range/Uvs Nuur Basin, 1430 m

¹ 

¹ 

¹ 


1}2 & 4}5 1}2 1}2 Section: gravels

9 4 3}6 of ¹ : 0.5}1.5 m 

1}2

18

27

1}3 1}2 & 5}7 1}2 1}2 1}2

14}15 14}15 15 * 3}4

25}26

¹ 

¹ 

¹ 

* '30 14 32 120 (12}15) 20}25 50}60 '100 gravel on top and gravels of ¹ : '1 m gravel at the basis  45 78

25}27 25

40}45 40 25}30

80 70

200?

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Fig. 3. Cross-sections of KhoK ndlen and Kharkhiraa River from the highest peaks in the catchment area (north slope of Turgen Mountains) and the Uvs Basin (820 m). The numbers on top indicate di!erent morphotectonic units: I"Turgen massif, II"Huh Nuur intramontane basin, III"frontal range Huh Taschuu, IV"Uvs Nuur Basin. A}F indicate pro"les and terrace sequences (see Table 2).

the base of the aeolian cover sediments were taken at several places to provide a framework for the Pleistocene in this mountain system. Palaeosols and buried humic horizons date to the mid and younger Holocene. There are two main rivers from the Turgen-Kharkhiraa which reach the Uvs Nuur Basin, the KhoK ndlen River at the northwestern part, and the Kharkhiraa

River at the northeastern part of the mountains. The Kharkhiraa River has the larger catchment and is also related to the Kharkhiraa Mountains and the southern slope of the Turgen (Fig. 2). However, the main source of water are the widespread peat bogs in elevations between 2200 and 2900 m and the permafrost area, represented, for example, by rock glaciers down to 2400 m. Two cross-

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177

Table 2 Main characteristics of the Glaciations and mean river gradients of KhoK ndlen and Kharkhiraa Rivers

Highest peak Terminus elevation and length of modern glacier Terminus elevation and length of LIA glacier Terminus elevation and length of LGM glacier River gradients Upper reaches Middle reaches Steeper part in the frontal range Lower reaches

KhoK ndlen

Kharkhiraa

3965 m

3944 m 2850 m, 5.5 km

3080 m, 3.0 km 2790 m, 6.7 km 2980 m, 3.6 km 2000 m, 24.5 km 1800 m, 20 km

1}2.5% 0.7}2.5% 4}10% 0.3}2%

1}10% 2}7% 2}6% 0.7}2%

Table 3 Pleistocene stratigraphy for the terraces in the Turgen-Kharkhiraa. Russian scientists established the Pleistocene stratigraphy of Mongolia and local names of the Siberian stratigraphy were used Terrace

Period

Siberian Pleistocene stratigraphy

¹  ¹  ¹ 
¹  ¹ 

Holocene and Late Glacial Last Glacial (Stage 2?) Last Glacial (Stage 4?) Penultimate glaciation Middle or old Pleistocene; older than penultimate glaciation Early Pleistocene

Holocene and Late Glacial Sartan glaciation Early Zyrianka Taz glaciation Late Shaitan glaciation?

¹ 

sections from the northern slope towards the Uvs Nuur Basin were given in Fig. 3 and the main characteristics of these rivers are given in Table 2. As the two streams have di!erent gradients the vertical sequence of these terraces vary (see Table 1 and Fig. 3). In the upper reaches, the "rst 20 km of the rivers, both valley pro"les show an irregular river gradient above 2000 m due to Pleistocene glaciations. Owing to di!erential tectonic uplift the middle reaches of the KhoK ndlen are #atter than those of the Kharkhiraa. This di!erent base level induced di!erent vertical elevation of the terraces above the modern #ood plain, especially in the middle reaches (see Table 2). A "rst stratigraphy of the terraces and a correlation with the Siberian stratigraphy (Arkhipov et al., 1986) is given in Table 3. At the foothills of the mountains and in the intramontane basins and depressions widespread alluvial fans and fanglomerates were accumulated predominantly during the Pleistocene. The landscape is determined by the accumulation of gravel and debris. First descriptions and explanations of these alluvial fans or bels were summarised by Berkey and Morris (1927), Murzaev (1954)

Early Shaitan glaciation?

and Richter et al. (1963). Preliminary descriptions of the fanglomerates in the Uvs Nuur Basin are given by Walther and Naumann (1997). They divided two main upper pediments (P2 and P3) and one lower pediment in this area. A mid-Pleistocene age for the upper pediments and a young Pleistocene age for the lower pediment were supposed. Further on, two terraces were described at the Kharkhiraa east slope. For the southern Altai, Cunningham et al. (1996) and Owen et al. (1997) presented new evidence on alluvial fans and Cenozoic tectonism. However, the alluvial fans and fanglomerates (pediments) in the foothills of the Turgen-Kharkhiraa in the Uvs Nuur Basin can be correlated with the di!erent terrace sequences in the mountains. Four main systems can be distinguished (see Fig. 4): Al : modern #oodplain and Holocene alluvial fan; in  the middle and lower parts of the fans remnants of Late Glacial and Pleistocene gravels occur. Equivalent to: ¹ (¹ ).   Al : main alluvial fan and fanglomerate level, partly  dissected by small gullies. At the mountain front 5}20 m above the modern rivers; in the lowermost sections of

178

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Fig. 4. Map of Quaternary alluvial fans and fanglomerates in the Turgen-Kharkhiraa mountains and the piedmont towards the Uvs Nuur Basin.

the fan partly covered by younger alluvial sediments. Equivalent to: ¹ "Younger Pleistocene: last Gla 
ciation period. Al : older alluvial fans and fanglomerates or pedi ments, dissected by (mainly asymmetric) gullies. Equivalent to: ¹ "Middle Pleistocene (penultimate  Glaciation?). Al : oldest pediments and fanglomerates; 30}200 m  above modern rivers, dissected by (asymmetric) gullies and small creeks, divided by several tectonic faults and partly di!erentially uplifted. Equivalent to: ¹ , ¹ "   Middle and Old Pleistocene. The active accumulation periods of these gravels occurred mainly during glacial periods. All main alluvial fans in the mountains and basins were related to Al /¹   and were incised by modern streams, which create a smaller alluvial fan. The Holocene accumulation (Al )  covers less than a third to a "fth of the Pleistocene fans. The "rst level Al can be dated to the Last Glacial by the  connection of the main alluvial fan and fanglomerate accumulations to the main terrace sequence (¹ ) of the 

di!erent river systems in the Mongolian Altai. In addition the di!erent ice margins, terraces and alluvial fans can be distinguished on the basis of relative weathering criteria of the overlying aeolian deposits and palaeosols (see above). In the catchment of the KhoK ndlen River Al /¹ and Al /¹ can be distinguished according to     the di!erent ratio of granite boulders in the gravels. The older fan or pediment levels Al and Al south of the   Turgen Somon (see Fig. 2) and associated with the higher terraces in the mountains show a di!erentiated uplift due to major fault and rupture lines. Same major directions of these tectonic lineaments are NNW}SSE, NW}SE, and W}E and create steps up to 30 m in the pediment surfaces. One main fault line runs parallel to the mountain range Huh Taschuu (NNW}SSE). In the Russian literature some of these major ruptures and faults were estimated as highest lake shorelines at an altitude of 1180 m (e.g. Florensov and Korzhnev, 1982). The widespread aeolian mantles in this area were accumulated mainly in the Late Glacial or Early Holocene. In some pro"les Interstadial silt (loess-like) can be expected

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

179

Table 4 Stratigraphy and palaeoclimatic interpretation of an exposure on the alluvial fans in the Uvs-Nuur-Basin in front of the Turgen-Kharkhiraa-Massif (see Fig. 5) Morphology and sedimentology

Period

I. II.

Penultimate glaciation (Al )  Last Interglacial

III. IV. V. VI.

Accumulation of gravel Accumulation of aeolian sediments (deductive) Soil development and calcareous deposition in the gravel layer Erosion of the aeolian sediments and wind erosion (forming of wind-faceted pebbles) Forming of ice wedges (possible in the same time as IV) Accumulation of sand

VII.

Thawing of ice wedges and cryoturbation VIII. Accumulation of angular debris IX. X.

Fig. 5. Morphostratigraphy and landscape development deduced from a section at 1160 m on the north-eastern slope of the Turgen-Kharkhiraa Mountains (50310E, 91338N).

as silt is accumulated between gravels of ¹ /¹ , ¹ /¹  
  and Al /AL . Preliminary results from luminescence dat  ing con"rm these results based on the morphostratigraphy. Fig. 5 and Table 5 provide an example from the northeastern slope of the Turgen-Kharkhiraa Mountains. Sixteen di!erent phases can be deduced from a section in the gravels of the alluvial fan and will not be discussed here in detail. With regard to Holocene climatic #uctuations, fossil soils in the mountains show alternating #uvial activity and soil development during the last 4000 yr (Pro"le 1}8 in Table 4). In addition, two buried soils were found under soli#uction lobes, indicating a higher activity c. 1500 yr ago (Pro"le 9a, b). For the continental regions of Asia, Serebryanny and Gravis (1993) discussed a correlation between intensive soli#uction processes, seasonal thawing}freezing cycles and increased humidity since 2000 yr BP. Similar observations (more humid and/or cooler climatic conditions) were made in the same time frame for the Taklamakan (Tarim Basin; Yang, 1991) and Eastern Tibet (Lehmkuhl, 1995) and in the

Early Last Glaciation

First cycle of Last Glaciation Interstadial of Last Glaciation Last cycle of Last Glaciation (Al )  (equivalent OIS II) Late Glacial

Accumulation of sand De#ation period (forming of ventifacts) XI. Forming of ice wedges (possible in the same period as X) XII. Accumulation of aeolian Late Glacial sediments XIII. Thawing of ice wedges and BoK lling/AlleroK d "lling with aeolian sediments XIV. Cryoturbation Younger Dryas XV. Soil development and Holocene calcareous deposition in the gravel layer (about 30 cm) XVI. De#ation and partly erosion of the aeolian sediments, expansion of desert pavements with ventifacts

Khangay. Pekala and Repelewska-Pekalowa (1993) saw in the fossilisation of periglacial features and the degradation and transformation of soil pro"les in Mongolia a tendency to drier and cooler climatic conditions. In addition, they indicated human activity by burning down forests and devastating the steppe vegetation due to severe overgrazing as a signi"cant factor. However, "rst results of pollen analysis from peat bogs of the Turgen-Kharkhiraa by Dr. F. SchluK tz show wetter climatic conditions since c. 2500 yr BP (Lehmkuhl et al., 1998). These peat bogs are widespread above 2400} 2500 m a.s.l. and showed peat growth since the early Holocene (see Table 5, p1}11). In lower elevations they are limited to special sites close to the rivers (e.g. at the KhoK ndlen above permafrost: P2, 1640 m a.s.l.). 2.2. Lake level yuctuations As yet, the highest reliably dated lake level was found in the Uvs Nuur basin at Bayan Nuur at a present elevation of 1010 m a.s.l. (about 80 m above the present

Lab. No.

Hv 21,118

Hv 21,112

Hv 22,337

Hv 21,115

Hv 22,073

Beta 91,950

Hv 21,114

Hv 22,091

Hv 21, 116

Hv 22,069

Hv 22,340

Hv 21,117

Hv 22,093

Hv 22,071

Beta 99,135

Hv 22,341

Hv 22,072

Hv 21,119

Beta 124,957

Beta 113,750

Hv 22,338

LU 2662


No.

5

1

3a

2a

p7

100

2b

3b

2c

p2

9b

4

9a

p5

101

7

p6

P3

102

103

3c

104

2930$90

2460$70

2150$40

2070$40

1995$265

1830$120

1740$50

1740$60

1640$80

1560$80

1560$60

1555$80

1390$170

1235$120

1135$75

990$75

730$80

635$75

370$55

305$75

295$55

240$55

Uncal. yr BP

Table 5 Radiocarbon dates of the Uvs Nuur basin

1390}900 BC

770}405 BC

360}270 BC

180 BC}25 AD

350 BC}395 AD

70}375 AD

245}390 AD

130}430 AD

340}540 AD

420}605 AD

425}595 AD

420}610 AD

540}790 AD

665}965 AD

890}995 AD

995}160 AD

1060}1090 AD

1290}1405 AD

1450}1635 AD

1480}1665 AD

1515}1660 AD

1645}1954 AD

Cal. yr.

123

80}85

161}162

45

60?

61}75

44}52

50

70}70

44}46

52}54

30}40

82}92

93}94

70}71

40}45

65

61}67

27}29

45}48

51}55

51}55

Depth [cm]

937

2030

937

937

2600

2600

2275

980

2600

2540

1940

2540

1640

2150

2030

2150

759

2400

2150

2030

2150

1960

elev [in a.s.l.]

Peat

Humin acid of peat

Humin acid of peat

Organic sediment

Peat

Peat

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Peat

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Peat

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Peat

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Humin acid of a fossil Ah

Peat

Dated material

Site Bayan Nuur 2 GPS 183/95

Huh Nuur basin

Boring Bayan Nuur 1 GPS 343/96 (45) Boring Baga Nuur II; 161}162

Has

Has

Lateral moraine of Shiver river

Schavart Nuur

Has, 2600

Burried soils under soli#uction lobes

Huh lake

Buried soils under soli#uction lobes

Northwest-slope Turgen mountains

Huh Nuur basin

Huh Nuur basin

Huh Nuur basin

Site Eastbay of Uvs Nuur (33/95)

Boring Has (timber line)

Huh Nuur basin

Huh Nuur basin

Huh Nuur basin

Orlogyn river

Site

N49328 E91338 N49357 E95337 N49357 E95337 N49355 E91338 N49355 E91338 N5031255.7 E9331650.2 N49355 E91338 N4957 E95337 N49355 E91338 N49358 E91335 N49335 E91334 N49351 E91346 N49335 E91334 N49355 E91338 N4935511.6 E9334245.5 N49327 E91312 N49355 E91338 N49355 E91338 N5030030.5 E9335824.2 N5032251.1 E9231225.4 N49357 E95337 N5030144.0 E9430121.9

Coordinates

180 J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

Beta 113,749

Hv 22,339

Beta 113, 748

Hv 22,070

Hv 22,092

Beta 113,751

Beta 124,958

Hv 22,075

Beta 99,140

Beta 124,959

TA 2,286


Beta 99,133

Beta 91,953

Beta 106,783

Beta 99,134

Beta 91,954

TA 2,284


Hv 22,074

Hv 22,076

Hv22,086

UtC-5,730

Beta 106,784

Beta 99,141

Beta 124,960

Beta 101,484

105

6

106

p4

8

107

108

p9

109

110

128

111

112

113

114

115

127

p8

p10

p11

126

116

117

118

119

12,210$80

11,380$80

11,230$60

10,790$50

9690$60

9300$450

8905$230

8840$225

7440$90

7310$90

7170$80

7170$50

6770$60

6740$80

6510$170

6040$50

5880$90

5020$115

4350$50

4030$50

3940$195

3475$150

3250$70

3140$60

3010$50

1350}12600 BC

11420}11230 BC

11850}11750 BC

1116}11090 BC

9260}9090 BC

9020}7935 BC

8090}7620 BC

8065}7575 BC

6450}6150 BC

6390}6000 BC

6220}5880 BC

6170}6130 BC

5780}5600 BC

5780}5480 BC

5750}5050 BC

5055}4815 BC

4950}4490 BC

3960}3695 BC

3045}2895 BC

2620}2470 BC

2855}2140 BC

1970}1545 BC

1690}1390 BC

1445}1320 BC

1410}110 BC

565

804

*

576}577

*

120}135

61}68

91}103

260

256

345

455}456

235

321

260

390

41

81}87

220

198}199

70}75

60}65

50

60}63

226}227

937

937

980

937

980

1640

2570

2540

937

937

938

937

937

937

937

937

759

2570

937

937

1985

2600

940

2160

937

Wood

Organic sediment

Molluscs

Limnohumite

Molluscs

Peat

Peat

Peat

Peat

Peat

Wood

Organic sediment

Peat

Humin acid of peat

Peat

Organic sediment

Limnocalcite

Peat

Organic sediment

Humin acid of peat

Humin acid of a fossil Ah

Peat

Humin acid of peat

Humin acid of a fossil Ah

Humin acid of peat

Southern slope Kharkhiraa Northwestern slope Turgen mountains 48 m level at Bayan Nuur Boring Bayan Nuur I GPS 343/96 (576}577) 48 m-level at Bayan Nuur (SE-side) Boring Bayan Nuur I (GPS 343/96) 804 Boring Bayan Nuur IV

Boring Bayan Nuur 1 GPS 343/96 (390) Site Bayan Nuur 2 GPS 183/95 Site Bayan Nuur 12 GPS 343/96 : layer 6 Site Bayan Nuur 1 GPS 181 (176/95) Boring Bayan Nuur 1 GPS 343/96 (455}456) Site Bayan Nuur 12 GPS 349/96) layer 9 Site Bayan Nuur 2 GPS 183/95 (256/95) Site Bayan Nuur 2 GPS 183/95 Olan Nuur

Eastern bay of Uva Nuur

Boring Baga Nuur II; 198}199 Boring Baga Nuur I; GPS 343/96 (226) Southern slope Kharkhiraa

Ulaan Davaa

Site Bayan Nuur 1 (GPS 181/95) : peat 1 Has, 2600

Boring Baga Nuur 1 GPS 343/96 (226}227) Kharkiraa river

N5032251.1 E9231225.4 N49344 E91338 N5030206.4 E9430127.0 N49355 E91338 N50308 E91323 N5032251.1 E9231225.4 N5030030.5 E9335824.2 N49326 E91315 N5031255.7 E9331650.2 N5030030.5 E9335824.2 N5030144.0 E9430121.9 N5030146.2 E9430057.7 N5030206.4 E9430127.0 N5030030.5 E9335824.2 N5030146.2 E9430057.7 N5030144.0 E9430121.9 N5030144.0 E9430121.9 N49343 E91305 N49326 E91315 N49358 E91335 N4935745.7 E9430230.0 N5030030.5 E9335824.2 N4935745.7 E9430230.0 N5030030.5 E9335824.2 N4935944.4

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192 181

Chusutuin Gol (GPS 421/97) (Limnopellite) Molluscs

Narijn Gol (100/95) Molluscs

Molluscs

Molluscs

Numbers point at the text.
Published by Sevastianov et al. (1993).

Beta 109,824 125

46,700$1400

323 Beta 91,951 124

'45,760

Beta 91,952 123

'44,620

345

768 256 Beta 99,139 122

39,710$610

Beta 109,823 121

28,050$250 26700}25600 BC

960

765

Molluscs

E9335804.9 N5030030.5 E9335824.2 N5031813.0 E9331642.6 N5031813.0 E9331642.6 N5031459.5 E9431159.7 N5031459.5 E9431159.7 N495623.5 E9430518.7 (GPS 350/96) (565) Boring Bayan Nuur I (GPS 343/96) (1070) Lower cli!-site at the eastern shore of Uvs Nuur Upper cli!-site at the eastern shore of Uvs Nuur Narijn Gol (101/95) Wood 937 1070 14500}12900 BC Beta 101,483 120

13,210$90

Lab. No. No.

Table 5. Continued

Uncal. yr BP

Cal. yr.

Depth [cm]

elev [in a.s.l.]

Dated material

Coordinates

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

Site

182

lake level of Bayan Nuur; Figs. 6 and 7). This high lake stand can presently be attributed only to the MidPleistocene or the "rst phase of the Late-Pleistocene. This is supported by bones of wild horse, wild camel, lion, hyaena and mammoth in the lake silts of Chusutuin Gol and by C dates of lake carbonates (Beta-109,824: 46,700$1,400 yr BP) that date to well before the LGM and may be linked to this higher lake level. Presuming the existence of a hydrological linkage between Bayan Nuur and Uvs Nuur, the lake level of Uvs Nuur would have been 250 m above its present height. Only a few sedimentological features and radiocarbon dates give an indication of this higher lake level. There is no trace anywhere of lacustrine deposits or strandlines marking such a highstand, which suggests that lake-level indicators have been overlain by the extensive pediments that border the lake in many places (Walther and Naumann, 1997). We attribute this high lake stand to warmer and wetter interglacial or interstadial climatic conditions. Because of the uncertain dating it is not possible to obtain an unambiguous chronostratigraphical classi"cation. These dates show an age of 28,050$250 yr BP (Beta109,823) and 39,710$610 yr BP (Beta-99,139) for molluscs, which were deposited in sands and silts of a deep water environment. A similar situation was found at Narryin Gol, where molluscs have been dated to '44,620 yr BP (Beta-91,952) and '45,760 yr BP (Beta-91,951), which means that this high lake level could be dated to the last interglacial period or even older, or to an interstadial phase before the Last Glacial Maximum (Fig. 6). During the Last Glacial Maximum either the levels of the lakes in the Uvs Nuur Basin were much lower, or else the lakes had disappeared altogether, in which case the dry and unvegetated areas would have been a rich source of sand for the extensive areas of aeolian sand and dunes. The lake carbonates contained in the sand would also have been transported. By the Lateglacial/Holocene transition the lake basins had "lled again, reaching a further high stand in the upper Lateglacial and early Holocene of 48 m above the present lake level at Bayan Nuur, i.e. about 979 m a.s.l. Because of dune blocking there was no hydrological link to Uvs Nuur (Naumann, 1999). Molluscs from lake sediments have been dated yielding an age between 11,230$60 yr BP (Beta-99,141) and 9690 yr BP (Grunert et al., 1999, see Table 5) for this highstand. In the eastern bay of Uvs Nuur the lake transgressed up to some 40 m above the present lake level into a dune relief, resulting in the deposition of limnopsammites (carbonate sands and silts). However, neither molluscs nor other datable matter was discovered. Furthermore, carbonatic limnopsammites have been found at a few places on the top of the lower pediments at the south bank of the Uvs Nuur (Fig. 6). During a generally regressive phase, lake levels in excess of 10 m above the present Uvs Nuur level and

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

183

Fig. 6. Preliminary young Quaternary lake level #uctuations of Bayan Nuur and Uvs Nuur.

about 4.4 m above the present lake level in the Bayan Nuur area represent either a lengthy standstill or a slight short-term rise in generally falling water levels. At Bayan Nuur contemporaneous peats from palaeomeanders of the Choid Gol have been found to contain mammal remains (wild horse, argali sheep). Numerous C dates yield an age of between 7310$90 yr BP (Beta-91,954) and 3250$70 yr BP (Beta-113,748) (Table 5) for this Atlantic to Sub-Boreal transgressional phase in the Bayan Nuur area. In the eastern bay of Uvs Nuur the carbonate in this lake sediment was dated to 5880$90 yr BP (Beta-99,140). Further evidence of this mid-Holocene lake level of Uvs Nuur is provided by strandlines between 10 and 4 m above the present lake level. These strandlines have been measured by tachymeter at the south and northwest shores of Uvs Nuur. They surround lagoons (e.g. Baga Nuur) that are either shallow or already completely dry. Studies at

Baga Nuur (NW shore of Uvs Nuur) show the onset of lacustrine sedimentation to have occurred between 3010$50 yr BP (Beta-113,749) and 4030$50 yr BP (Beta-113,751) (Walther (1999). Hence, the lower strandlines must be younger (Figs. 6 and 7). At about 1 m lake-level marks are found at Uvs Nuur and Bayan Nuur that represent the most recent evidence of a higher lake level. They are probably late Holocene in age. At the southern shore of Uvs Nuur Phragmites peats were inundated during this youngest phase. In places young bars lie up to 1 m below the present water level. A comparison between maps, air photos and "eld observations shows that Uvs Nuur has risen by as much as 1 m during the past 30 years. Pollen analysis was carried out on one of the four cores taken at Bayan Nuur giving a complete picture of the vegetation record during the period beginning 15,000 yr BP. The 10.50 m long core BAN-1 was preliminarily

184

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

Fig. 7. Lake levels in the Uvs Nuur Basin.

divided into at least eight di!erent local pollen-assemblage zones. The Younger Dryas is noted for an abrupt drop in arboreal pollen, which however, increases again very quickly, marking the transition to the Pre-Boreal. Sedimentological evidence indicates considerable water depths since the Younger Dryas (10,790$50 yr BP; Beta-106,784), since fragile laminations have been preserved. The increase in arboreal pollen in relation to steppe elements (Artemisia, Chenopodiaceae, Ephedra distacha type) is a strong indication that the forests became denser and larger, which can also be interpreted as evidence of a lowered tree- and forest-line. During the Holocene additional drops are recorded on the AP curve, but at a much higher level than during the Lateglacial. Up to about 15,000 yr BP the Lateglacial shows three APricher sections, which in turn are separated by short but distinct phases with a greater proportion of non-arboreal pollen. The latest studies by Dorofeyuk and Tarasov (1998) on the vegetation and diatom record of Northern Mongolia have shown great variations * and even an anticyclical #uctuation * in the levels of the three lakes studied (Dood Nuur, Hovsgol Nuur and Gun Nuur). For example, these lakes displayed low lake levels and other transgressive tendencies during the mid-Holocene. The authors describe a lake level development that should be

di!erentiated according to the geographical location of the lakes (see Fig. 8). 2.3. Dunes The dune area of the Uvs Nuur Basin averages some 150 km in length, 30 km in width and about 40 m in depth. It is situated at the same latitude as central Europe (503N) and represents the northernmost dune"eld in Central Asia, indeed of the entire Asian arid belt. Beginning on the eastern shoreline of the huge Uvs Nuur Lake at 760 m a.s.l., the dune"eld rises continuously eastwards up to 1450 m a.s.l. In the same direction the mean annual rainfall rises from about 100 to 250 mm. Owing to higher humidity, the steppe vegetation of the basin lowland also changes from semi-desert steppe with a predominance of Ephedra sp. to long-grass steppe dominated by Stipa sp. (Hilbig, 1990). Here, at about 1400 m a.s.l., a very sparse forest of Larix sibirica indicates the position of the present-day lower timberline. The dune"eld mainly consists of WNW}ESE-oriented longitudinal dunes, indicating the prevailing winds during the period of sand accumulation. This wind direction has not changed to the present day. The dunes are fossilised by a dark-brown castanozem (brown steppe soil, see Dordschgotov, 1992; Haase, 1983; Opp, 1991) which is up to 1 m deep in the parts of the

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

185

Fig. 8. Comparison between north Mongolian lakes and the development of forest and steppe systems.

dune"eld that are higher than 1100 m a.s.l. In its drier parts, below 900 m, the soil is 0.5 m deep at the most. Furthermore, the castanozem is covered by a dense steppe vegetation. Owing to many small roots, the soil is solid and not susceptible to erosion by strong winds during the dry spring season, even in the case of overgrazing. This is astonishing because of its "ne sand

texture and low degree of compactness. In spite of the dark brown colour it contains little organic matter (about 0.5%). Particles of aragonite as well as the carbonate content of the entire pro"le, reaches 3% or more indicate that the dune sand derives from the Uvs Nuur Basin. The carbonate is concentrated at a depth of 0.5}1.0 m. Fossil roots of Caragana bushes have been

186

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

completely encrusted during a more humid period (probably mid-Holocene). Today, Caragana sp. grows only in the higher parts of the dune"eld with at least 200 mm of precipitation per year. In places where strong winds have cut erosion cli!s in steep dune #anks, hard pieces of encrusted roots have fallen down covering the dune sand on the bottom of the de#ation depression. Owing to their white colour they can be misinterpreted from a distance as fossil lake sediments. The other parameters of the soil are as follows: a high pH-value of 7}7.5, relatively high contents of P O (1}3 mg/100 g) and K O (3}5 mg/100 g) and very    low contents of N (about 0.1%). We conclude that  soil fertility is remarkably high and that, moreover, the physical parameters such as the pore volume and the homogeneous structure of the thick dune}sand layer are favourable for storing humidity for several months. (Dordschgotov, 1992; Opp, 1991). Along the margins of the dune"eld, where severe overgrazing has been practised for the last 2000 years or so, many dune cli!s and de#ation depressions have been formed by wind erosion. Correspondingly, new dunes sometimes more than 20 m high have been built up. In contrast to the old dunes, they are semi-active (parabolic) dunes or fully active barchans. Their surface is completely unprotected (no soil or vegetation cover). By contrast, up to 50% of the surface of the very common parabolic dunes is covered by steppe vegetation mostly composed of Elymus sp. and Hedysarm sp. (Hilbig, 1990). The "ne, calcareous dune sand has been transformed down to a depth of 0.5 m into an initial soil slowly compacted by roots as well as by disperse organic matter (0.1}0.3%). All other parameters, including the carbonate content, are similar to those of the old dunes. But, owing to the di!erent time of formation, there is a big di!erence between the initial soil and the castanozem. Even if the dune forms are not clearly distinguishable locally, they can be easily di!erentiated by their soil cover. Hence, two dune generations were mapped. The active barchans represent a third generation (JaK kel, 1996; VoK lkel and Grunert, 1990). As to the age of the old dunes, absolute OSL dates are unfortunately not yet available. However, it is possible to "x their minimum age by C-dated lake sediments which locally cover the dune #anks. Such a key area lies about 10 km east of Bayan Nuur in the valley of the small river Chusutuin Gol (960 m a.s.l.). Here, at a height of 20 m above the river and 48 m above the level of Bayan Nuur, well-bedded lake carbonates containing shells of snails and relicts of swamp vegetation are in contact with pure, well-bedded dune sand (5 and 4 in pro"le D, A7). Two C-dates were obtained. One based on vegetation relicts gave an age of 9619$60 yr BP (UtC-5730), and the other based on snail shells gave an age of 11,230$60 yr BP (Beta-99, 141). This means the dunes must be older than 12,000 yr BP.

However, to determine the length of the dune formation period it is necessary to study the base of the big dune site D, A7 (Fig. 9). It consists of well-bedded #uvial sand and granite gravel (2) which cover very old, "ne dune sands (1). They are slightly compacted and have therefore been eroded by the river Chusutuin Gol. Their ages as well as the age of the #uvial sediment can be determined as follows: The granite gravel with a diameter of several centimetres is well-rounded and shows a great petrographic variety. The components are similar to those of the lower terrace of Baruunturuun Gol which originates at the high mountains (2900 m) at the southern rim of the Uvs Nuur Basin. The granite gravel of the Chusutuin Gol indicates that, probably during the Last Glacial Maximum (LGM), the Baruunturuun Gol was a tributary of Bayan Nuur and was completely SE}NW oriented. Today the Baruunturuun Gol is blocked by the large dune"eld. At the contact point with the dunes it changes direction abruptly to E}W and follows the southern rim westward of the dune"eld. About 4 km south of the dunes, the big alluvial fan (lower terrace) of the Baruunturuun Gol is submerging under masses of sand #uvially deposited during the Holocene. It can therefore be assumed that the granite sediments of the lower terrace underlie the dune"eld over a distance of about 15 km. The 20}25 m high dune front, mostly composed of semi-active parabolic dunes, is interrupted by a 100 m long relict of the old valley. At this point, the modern river is very close to the dunes, and during the rainy period in summer, a considerable part of the water seeps into the dune sand. The ions of the water have been analysed and found to be identical with those of several springs in the upper part of Chusutuin Gol. A water #ux from SE to NW can be reconstructed. This proves again the assumption of an old pre-dune valley of Baruunturuun Gol. In conclusion, we can say that the dune"eld did not exist during the LGM or, at least, it was completely dissected by the broad valley of the Baruunturuun Gol. Therefore, the climate must have been relatively humid: this means similar precipitation and probably lower temperatures than today. The lower terrace of the Baruunturuun Gol is connected with moraines of the LGM in the uppermost part of the valley. Therefore, the assumption of lower temperatures at that time seems to be well founded. According to this evidence and the results that have been obtained from other parts of central Asia (Fang, 1991; Frenzel, 1994; Hofmann, 1993; Lehmkuhl and Haselein, 2000), it can be said that dune formation took place between 20,000 and 13,000 yr BP. It may have been facilitated by stronger winds than today and, especially, by sparse vegetation cover. This vegetation cover, on the other hand, must have been able to collect large quantities of "ne sand whose accumulation was determined by the laws of aerodynamics. As a result, large longitudinal

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

187

Fig. 9. East side of Bayan Nuur (east of Uvs Nuur).

dunes were formed. They represent the "rst dune generation. The very old dune sands at the base of the large site of Chusutuin Gol (1 in D, A7; Fig. 9) are older than the granite gravel (2) and therefore older than the LGM. They indicate a period of dry climate conditions at or

before 46,000 yr BP. If they were older than 40,000 yr BP, they could have formed the base of another lake sediment in the upper part of the valley of Chusutuin Gol reaching 81 m above the lake level of Bayan Nuur. The sediment has been dated by Naumann and Walther (2000) at 46,700$1400 yr BP (Beta-109, 824). The topographic

188

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

and climatic implications have also been discussed by the authors. In contrast to the very old dune sands, the sandy sediment of the 4-m terrace was formed by Chusutuin Gol, which has its source in the dune"eld. The terrace is mid- to young Holocene in age indicating slightly pluvial conditions at that time. Swampy areas on both sides of the river were typical. They can be reconstructed by one thick and several small peat layers within the sandy accumulation whose strong cryoturbation can be interpreted as the result of very cold winters. That means that the climate was only slightly di!erent from the present one. The very dry High Glacial to Late Glacial period was followed by an early to mid-Holocene pluvial period when mean annual precipitation must have been higher than today. This assumption is based on the high position (48 m"L.L 3, see Fig. 10) of lake sediments east of Bayan Nuur which indicates a strong lake level rise at that time. This evidence as well as the implications of the special relief forms of the Bayan Nuur Basin for the mechanism of lake level rising have been discussed by Walther and Naumann (1997) and Naumann and Walther (2000). By contrast, the analysis of the Castanozem described above gives no proof of real pluvial conditions. The pedogenic processes typical of intense weathering on sand, such as podzolisation and clay formation cannot be

recognised. Pollen diagrams taken from di!erent soil horizons also indicate moderate humidity. No timber forest, only a dense steppe vegetation like today's can be reconstructed. But the large quantity of Larix sibirica pollen suggests that the ancient timberline may have been lower, down to the 1000 m isohyet. This means a depression of about 400 m compared with the timberline of today. Another argument has to be considered: We know that increased humidity prevailed from about 11,500 to 5000 yr BP. This period, lasting as long as the preceding arid period, had little in#uence on pedogenesis. The amount of average precipitation per year can be concluded to have been between 25 and 50% of that today. The formation of parabolic dunes representing the second dune generation (see Fig. 10) and much more recent in age is presumed to have started during the transition phase between the mid-Holocene pluvial period and the drier young Holocene. Such dunes are very frequent in the higher eastern part of the dune"eld, indicating strong western winds. These must have been e!ective in overcoming the protection a!orded to the old dunes by a relatively dense vegetation cover. Supposing that the former precipitation rate increases eastward like today, there is a certain contradiction in this hypothesis. The second point not really clari"ed is the extent of

Fig. 10. Cross-section beginning at the southern shore of Bayan Nuur.

J. Grunert et al. / Quaternary International 65/66 (2000) 171}192

human impact, e.g. the in#uence of grazing sheep, cattle, and horses for the last 2000 years. As mentioned above, this must have been remarkable along the margins, especially the southern margin of the dune"eld. Starting at that time, barchans also could have been formed in places where overgrazing was severe. This means they are primarily man-induced and not climateinduced forms. In this respect, they can be seen as the third dune generation.

3. Paleoclimatic implications and conclusions 3.1. Pre-LGM Fig. 11 gives a summary of the di!erent morphologenic processes in the basins and mountains of western

189

Mongolia during the last 100,000 years. This framework for climatic #uctuations is derived from references on palaeoclimatic events in Central Asia and "rst dating results (see Lehmkuhl and Haselein, 2000). The glacier #uctuations are based on results from Tibetan glaciers (Lehmkuhl, 1995). In Tibet and the surrounding mountains maximum glacier advances took place in both stages of the Last Glaciation (70}50 ka and 32}15/14 ka: Benn and Owen, 1998; Lehmkuhl, 1995). However, in the southern parts of the Mongolian Altai two moraines of the last glacial cycle can also be found. It is possible to reconstruct lake level #uctuations from higher and lower lake levels in the Bayan Nuur depression and in the adjacent areas of Uvs Nuur. The higher lake levels are related to higher levels of palaeoprecipitation compared with present conditions. This means that vegetation was denser and more widespread than it is

Fig. 11. Landscape development including lake level changes, glacier #uctuations in the mountains and basins of Mongolia during the last 100,000 years.

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today. However, geomorphological features may have modi"ed these factors. For example, dammed lakes may be due to dune formations. On the basis of lake sediments and geomorphological mapping of the areas surrounding the lakes, it is possible to calculate a high lake level of Bayan Nuur at around 80 m above its present level before the Last Glacial Maximum. The oldest dune generation representing an arid period also dates to the time before 46,000 yr BP, i.e. pre-LGM. In the pro"le of Fig. 9 the dune sand is covered by a #uvial sediment which corresponds to the lower terrace of the Baruunturuun Gol. This is connected with moraines and is therefore High Glacial in age. The main dune"eld of the Borig Del Els occupies the largest area. The dunes were formed by strong winds from WNW. During the phase of wet and cool to cold glacier formation before the Last Glacial Maximum lake levels were probably high (Walther, 1999, among others). However, it is presently unclear whether this can be correlated with the 80 m level. The chronostratigraphic classi"cation of this high stand is shown as uncertain in Fig. 11, but it must belong to the phases of wet/cool glacier build-up or wet/cool interstadial or interglacial climate. Several workers, among them HoK vermann and SuK ssenberger (1986) assume a wet/cool anaglacial phase between 32 and 25 ka in Central Asia. WuK nnemann et al. (1998) present evidence of high lake levels in the Tengger Shamo and Badain Jaran Shamo between c. 40,000 and c. 21,000 yr BP. Frenzel and Gliemeroth (1998) point to a distinct improvement in the water budget in northern and central Tibet between c. 80,000 and c. 20,000 yr BP with lower temperatures than those today. The higher level of Uvs Nuur probably occurred in the "rst half of this time span. On the basis of these results, we assume that the oldest dune generation originated at the end of the "rst glacial stage of the last glaciation (Fig. 11).

3.2. Last Glacial Maximum The low-reaching glacier levels during the LGM apparently correlate with low lake levels. Minimally, it is likely that the lakes in the Uvs Nuur Basin even dried up completely during the dry/cold younger part of the High Glacial (dry/cold Kataglacial phase sensu HoK vermann and SuK ssenberger 1986: 24}15 (13) ka BP) parallel to the maximum extent of the glaciers. Hence, the Glacial Maximum should be set at the transition between the Anaglacial and the Kataglacial. Sands and silts were wind transported out of these dry lake basins and form the second dune generation. Even large rivers were blocked and diverted by dunes (longitudinal dunes of the "rst generation) at that time. Dune formation and reactivation probably played a major role up to the start of the Late Glacial.

3.3. Late Glacial period Melting of the ice masses in the mountains, the end of pediment formation on the lower mountain slopes and the slow regeneration of plant cover owing to rising temperatures and increased precipitation all lead to a relative quick rise in lake level, since the basins "lled up with both rainwater and meltwater from the rapidly melting glaciers. Lake levels were high in the terminal Late Glacial during the "nal phase of glacier melting. There was no hydrographic link between Bayan Nuur (lake level: 48 m) and Uvs Nuur (lake level: 40 m) owing to dune blockage of the present-day Bayan Nuur drainage eastward (Naumann, 1999).

3.4. Holocene period At Dood Nuur, Hovsgol Nuur and Gun Nuur (Dorofeyuk and Tarasov, 1998) and in the entire Uvs Nuur and Bayan Nuur there is evidence of relatively higher lake levels during the mid-Holocene (Fig. 8). During this period of higher water tables, enhanced precipitation and higher temperatures, there was a notable increase in the vegetation cover. It is likely that the forests became denser, even closed at locations with a high water table. Falling lake levels during the second half of the Holocene point to lower temperatures and less precipitation from c. 5 ka BP onward. During this `arid phasea dune sands were remobilised and soil formations were covered over. The next rise in lake level may have started at 3 ka BP at the earliest, but more probably about 2 ka BP. This was also a time of glacial readvances (Neoglacial). In palaeoclimatic terms, higher lake levels and glacier growth point to enhanced rainfall and lower temperatures. In the transitional phase to greater aridity younger than 5000 yr BP parabolic dunes (second generation) and, locally, barchans (third generation) were built up. These processes, resulting in the progressive destruction of the old longitudinal dunes, are di$cult to interpret as climatically induced because of intensive grazing by large nomadic herds. Widespread deserti"cation can be observed leading to accelerated aeolian processes. The dates of these soils vary in age. Increasing soli#uction, depression of the lower boundary of soli#uction, the sediment cover of humus horizons, and a lower forest line all show that much cooler conditions prevailed in the mountains too. A recent rise in lake level has been observed. This is con"rmed by the #ooding of sand bars and the lake's encroachment on the dune"eld on the eastern side of Uvs Nuur. Here, the interdunal depressions are "rst #ooded by the higher water table and then incorporated into Uvs Nuur itself as its shore line advances.

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Acknowledgements We thank the Deutsche Forschungsgemeinschaft and the Gesellschaft fuK r Technische Zusammenarbeit for their "nancial support. Thanks are also due to the Gesellschaft fuK r Erdkunde zu Berlin, its former chairman Professor Dr. F. Scholz, and its present chairman Dr. D. Biewald for bringing about a co-operation agreement with the Mongolian Academy of Sciences. We are grateful to the Mongolian Academy of Sciences, especially the Institute of Geoecology and its head, Prof. Dr. Dordschgotow, as well as the Institute's many scientists (in particular Dr. Batischik, Dr. Dasch, Dr. Batnasan) for their extensive support during "eldwork. We also wish to thank Professor Dr. von den Driesch of Munich University, who accompanied us on one of the "eld trips and kindly identi"ed the mammal remains, and Professor Dr. M.A. Geyh (Hannover) for the radiocarbon dating and valuable comments.

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