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Holocene Landform Evolution of Lake Khuvsgul basin, Mongolia

Evolution holocène du bassin du lac Khuvsgul, Mongolie
Alexander Orkhonselenge, Sergey K. Krivonogov, Kenta Mino, Kenji Kashiwaya, Masayoshi Yamamoto et Toshio Nakamura
p. 343-354

Résumés

Cette étude présente l'évolution de l'environnement du lac Khuvsgul (également connu sous les noms de Hovsgol et Khubsugul) dans le nord de la Mongolie durant l'Holocène : barres de plage et flèches littorales, terrasses lacustres et processus fluviatiles (composantes sédimentaires et organiques) de la rivière Borsog qui se jette dans le lac. Le lac Borsog, ancienne baie orientale du lac Khuvsgul, en fut séparée lors de sa trangression post-glaciaire. Il offre un enregistrement à haute résolution de l'évolution holocène du paysage. Une séquence sédimentaire de 13 m de profondeur issue du sondage BB03 dans le lac Borsog montre trois phases de forte sédimentation et de courte durée à 7,4-7,1 ka cal. BP, 4,8-4,5 cal. BP et 1,0-0,9 cal. BP, qu'il est possible de relier à des abaissements du niveau du lac, des réductions de sa surface, à la progradation du delta de la rivière Borsog et à l'érosion des rivages exposés du lac. Les abaissements de niveau du lac font écho à ceux déjà enregistrés sur le lac Khuvsgul à 7,2-7,0, 4,5-4,1 et 2,1-0,2 ka cal. BP., suggérant une interrelation et un contrôle climatique continu de l'évolution des deux lacs après leur séparation.

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Notes de la rédaction

Article soumis le 22 décembre 2013, accepté le 8 août 2014.

Texte intégral

Introduction

1Interpreting sequences of lake sediments is important for understanding landform evolution and climate and lake level changes which is focus of this publication. Few reliable proxy series reflecting Holocene climate changes and landform evolution have been obtained for Mongolia in lakes Hoton, Uvs and Telmen in western Mongolia (Grunert et al., 2000; Tarasov et al., 2000; Peck et al., 2002; Fowell et al., 2003; Rudaya et al., 2009), Ugii in central Mongolia (Schwanghart et al., 2009; Wang et al., 2011), and Dood and Gun in northern Mongolia (Dorofeyuk and Tarasov, 1998; Feng et al., 2005; fig. 1A). These data were partly summarized in An et al. (2008), who suggested warm and humid early Holocene, highly humid early-middle Holocene, arid middle Holocene and humid late Holocene with cold intervals.

2Lake Khuvsgul (fig. 1B) is an important paleoclimatic archive in Northern Mongolia. It is located in the center of the Eurasian continent, and hosts the Siberian-Mongolian (Central Asian) winter anticyclone, which interacts with westerly and monsoon atmospheric circulations. Lake Khuvsgul has been studied within national and international drilling projects to reveal long-term events of the last 1 Ma (Fedotov et al., 2004a, 2007; Prokopenko et al., 2007; Hovsgol., 2009). Additionally, the investigation of short-term events is of special importance to reconstruct the Holocene and Late Pleistocene history.

3The Holocene paleoclimatic proxies obtained in Lake Khuvsgul indicate higher precipitation/runoff from 11 to 7 ka cal. BP, and a warmer period at 6-3.5 ka cal. BP (Prokopenko et al., 2005, 2007). Drop in level of Lake Khuvsgul to 100 m during Late Pleistocene glaciation is indicated by seismic data (Fedotov et al., 2002, 2007), bottom and coastal morphology (Zolotarev et al., 1982; Krivonogov et al., 2003), and sedimentology (Fedotov et al., 2004b; Prokopenko et al., 2005). The lake level fluctuated with smaller magnitudes of several meters during the Holocene (Dorofeyuk and Tarasov, 1998; Krivonogov et al., 2003; Krivonogov, 2006).

4High topography of the Lake Khuvsgul basin is responsible for long winters and permafrost development (Sodnom and Yanshin, 1990), which has existed since initial glaciation and reached its peak during the LGM (Owen et al., 1998; Bertran et al., 2003). The modern processes of permafrost aggradation/degradation strongly affect basin landscapes and sediment transportation patterns. During the Holocene, the permafrost expanded and retreated in response to climatic changes (Goulden et al., 2006) which may be reflected in local sediment records, e.g., Lake Borsog situated along the eastern shore of Lake Khuvsgul (fig. 1). This paper presents in detail results of sedimentological study of Lake Borsog, which were briefly discussed in Alexander Orkhonselenge et al. (2011). These data contribute to better understanding of climate-hydrological regimes and landform evolution around these lakes.

Fig. 1 – Study area.
Fig 1 – La zone d'étude.

Fig. 1 – Study area. Fig 1 – La zone d'étude.

A: Locations of Lake Borsog and other lakes mentioned in the text. B: Topography of Lake Khuvsgul and its catchment: 1: Lake Khuvsgul in the last glacial time (100 m level drop); 2: modern outline; 3: the Ulhen Sair alluvial fan, which controls the level of Lake Khuvsgul; 4: directions of river-flow; 5: boreholes; 6: Lake Borsog and its surroundings as mapped in Fig. 2; 7: villages. C: Schematic section along Line a–b–c indicating structural and tectonic features.
A : Localisation du lac Borsog et des autres lacs mentionnés dans le texte. B : Topographie du lac Khuvsgul et son bassin versant. 1 : Le lac Khuvsgul durant la dernière glaciation (niveau abaissé de 100 m) ; 2 : limite moderne ; 3 : cône de déjection de l'Ulhen Sair, qui contrôle le niveau du lac Khuvsgul ; 4 : Sens de l'écoulement fluvial ; 5 : Sondages ; 6 : Le lac Borsog et ses environs comme cartographié dans la fig. 2 ; 7 : Villages. C : Profil schématique le long de la ligne a-b-c indiquant des traits structuraux et tectoniques.

Study area

5The Lake Khuvsgul area is a south-western part of the Baikal Rift Zone developing since the late Miocene. The Khuvsgul rift, formed in the Pliocene (Zorin et al., 1989), is a typical asymmetric half-graben. Its western side is bounded by the Bayan-Zurkhiin and Khoridol-Saridagiin Ranges, which have dome-blocked structures. Its eastern side is a relatively low hilly area, which is tilted to the west and covered by basaltic flows (Zolotarev et al., 1982) (fig. 1C). The basaltic flows extend beneath the Lake Khuvsgul shoreline, suggesting recent subsidence of the eastern coast (Zolotarev et al., 1982). These structural features were responsible for the formation of the modern relief including the drainage network of the eastern coast of the lake.

6The structure of the lower reaches of the eastern-coastal valleys reflects the latest Late Pleistocene to Holocene development of Lake Khuvsgul and its adjacent areas. Lake Khuvsgul dramatically dropped during glaciation, and its shoreline retreated by several kilometers, disturbing drainage and sediment transportation. As soon as the lake returned to its high level during the LGM termination and late glacial (Prokopenko et al., 2005), the valleys were flooded to form estuaries (Rogozin, 1993). The lower reaches of the Lake Khuvsgul-feeding rivers are characterized by beach ridges, spits, and traces of old bays suggesting formerly 2-3 m higher lake levels (Krivonogov et al., 2003). In places, the spits intersect bays and form lagoon lakes, such as Lake Borsog (fig. 2).

Fig. 2 – Lake Borsog area.
Fig. 2 – Le secteur du lac Borsog.

Fig. 2 – Lake Borsog area. Fig. 2 – Le secteur du lac Borsog.

A: Geomorphological map of Lake Borsog area. 1: hills; 2: valley bottom and slopes; 3: former part of Borsog Bay filled by sediments; 4: floodplain; 5: Lakes Khuvsgul and Borsog; 6: beach bars and spits; 7: outcrop; 8: borehole. B: Altitudinal profile along Line a–b–c–d (SRTM DEM data). C: Schematic section along Line b–c showing the structure of Lake Borsog and its sedimentary fill. The distribution of frozen/thawed ground is shown approximately.
A : Carte géomorphologique du secteur du lac Borsog. 1 : collines ; 2 : fond de vallée et pentes ; 3 : ancienne partie de la baie de Borsog comblée par des sédiments ; 4 : plaine alluviale ; 5 : lacs Khuvsgul et Borsog ; 6 : barres de plage et flèches littorales ; 7 : butte ; 8 : sondage. B : Profil en travers le long de la ligne a-b-c-d (SRTM données MNT). C : Coupe schématique le long de la ligne b-c montrant la structure du lac Borsog et son remblaiement sédimentaire. La limite entre le sol gelé et dégelé est approximative.

7Lake Borsog, a lagoon-type, is situated along the middle part of the eastern shore of Lake Khuvsgul. This lake was separated from the Lake Khuvsgul by three spits (fig. 2A). The two older spits do not currently dam the lake, although their heights suggest levels of Lake Khuvsgul were 2-3 m higher at the time of their formation. The modern spit, which is periodically restored by breaking waves and ice hummocking, dams Lake Borsog and keeps its level ca. 0.5 m higher than that of Lake Khuvsgul. Water freely discharges to Lake Khuvsgul through a small channel in the spit. The level of Lake Borsog can slightly fluctuate due to the erosion/restoration of the spit. Currently, the lake is approximately 800x500 m in dimension. However, its area was twice as large during the last highest stand of Lake Khuvsgul, which is marked by the former bay bounded by the old beach ridge (fig. 2A). The depth of the lake currently exceeds 10 m. However, the lake was obviously much deeper before separation from Khuvsgul by the oldest spit, as it is filled by >13 m of sediments in addition to the 3 m increase in level of Lake Khuvsgul. The southwestern part of the lake is the deepest. The depth of the shallower northeastern part gradually decreases from 5 to 3 m, and abruptly falls to 1.5-2 m near the shore, possibly due to the recent increase of the lake level and/or the effect of permafrost.

8The shores of the lake and a part of its bottom are frozen. Permafrost is present on the lake bottom several tens of meters from the northern shore. The depth to the frozen layer is ca. 3 m, and it is covered by a thin layer of soft sediments. In the northern part of the lake, the bottom sediments were displaced upwards by permafrost and form a ca. 4 m high frozen island (fig. 2A and fig. 2C). The island is separated from the shore by a trench formed due to thermal erosion. The sedimentary sequence exposed at the lakeside wall of the island is silty peat. A sample taken at a depth of 4 m from the surface yielded a 14C age of 4510 ± 40 BP (Krivonogov et al., 2003).

9Lake Borsog is fed by the Borsog River. The bottom of the river valley is filled by slope sediments in the lower reaches (fig. 2B and fig. 2C) and by boggy floodplain deposits in the upper reaches. With negative average annual temperatures (Bogoyavlensky, 1989), the climate is cold enough to retain permafrost, which is typical of the whole valley. The total annual precipitation is 300-600 mm with 60-70% occurring between June and August. The water discharge through the Borsog River increases after heavy rains, which may cause flooding and transportation of large masses of peat and silt into the lake. The melting of permafrost contributes to this process by supplying more water and making sediments unstable and susceptible to erosion.

Materials and methods

10A 13 m long core (BB03) was drilled using a Livingston-type corer at a point with coordinates N 50°59’09.5, E 100°42’40.7” and water depth of 9.1 m (fig. 2A and fig. 2C). The core BB03 consists of seven lots, totally. The sediments of each lot were packed into plastic liners 200 cm long and 3.6 cm in diameter. The BB03 core was subsampled at 1 cm intervals and examined for the physical-chemical properties at Kanazawa University, Japan.

11The core possesses variable lamination, making its visual division into layers problematic. The clay to silt sediments of the core show laminated structures. Laminae of different colors are irregularly separated by 2-3 mm thick bands of fine-grained sand. The lithological units used in this paper have been recognized by the following methods.

12The physical-chemical properties of the BB03 core sediments include the measurements of water content, grain density, grain size, organic matter (OM), biogenic silica (BiSi) and mineral fractions. The content of water was measured directly by drying a given amount of the sediment (Lambe and Whitman, 1969; Dingman, 2002). The grain density was analyzed using a Micrometrics analyzer AccuPyc1330 using helium as a carrier gas (Thompson and Isaacs, 1967; Gustafson and Hall, 1972). The grain size was measured with a laser diffraction particle size analyzer Shimadzu SALD 2200J in two modes: in the total sediment sample and in the mineral fraction remaining after the successive removal of biogenic and authigenic fractions. The biogenic silica measurement routine followed the spectrophotometric method (Mortloch and Froelich, 1989): the insoluble mineral residue was used in grain size analysis.

13The age of the sediments is constrained by 14C dates obtained by Accelerator Mass Spectrometry at the Center for Chronological Research of Nagoya University (Japan). The materials used for dating were bulk organic matter, plant remnants, and wood fragments.

Results and discussions

Sediment structure and sedimentation rates

14Features of the water content and grain density curves, total sediment and mineral fraction grain size curves, organic matter, biogenic silica and mineral fraction curves (fig. 3), and dating of the BB03 core sediments (fig. 4) from Lake Borsog demonstrate the lake level fluctuations of Lake Khuvsgul and landform evolutions of its drainage basin during the Holocene. Drops in lake level, reductions of lake area, progradation of the delta of Borsog River and erosion of the exposed lake shores are crucial contributors for Holocene rapid sedimentations in Lake Borsog, while periodic formations of alluvial fan in Ulkhen Sair and incision of Egiin River are main impact factors for the lake level fluctuations of Lake Khuvsgul during Holocene and Late Pleistocene.

15The records were divided into seven intervals, the boundaries of which are intersections of curves with their linear trends (fig. 3).

Fig. 3 – Sediment properties of core BB03 from Lake Borsog.
Fig. 3 – Caractéristiques des sédiments extraits du carottage BBO3 dans le lac Borsog.

Fig. 3 – Sediment properties of core BB03 from Lake Borsog. Fig. 3 – Caractéristiques des sédiments extraits du carottage BBO3 dans le lac Borsog.

Water content and grain density; grain size median of mineral fractions and grain size median of total sediments; sediment compositions of biogenic silica, organic matter and mineral fractions.
Teneur en eau et densité du grain ; médiane des fractions granulométriques et médiane de l'échantillon ; part de la silice biogénique, de la matière organique et des fractions minérales.

16The intervals at 0-72, 72-210, 210-580, 580-830, 830-1010, 1010-1110 and 1110-1300 cm reflect the main lithological units of the sedimentary sequence. The curves of these physical-chemical properties in figure 3 clearly show the considerable fluctuations on sediment depositions during the Holocene and the dependent of sediment properties on the content of the mineral fraction. The values for water content show a clear feature in lacustrine sedimentations induced by water level fluctuations around Borsog Bay, with several signals implying changes in lake level due to the certain climatic changes during the Holocene. It is shown that the variations in water content (fig. 3) are consistent with other components of grain size, organic matter and biogenic silica, i.e., finer grains, higher organic matters and biogenic silica suggest high-stand of the lake levels at the intervals 0-72, 210-580, 830-1010, and 1110-1300, respectively.

17High values in the grain density at 72-210 cm and 580-830 cm intervals, except a peak of dropping at a depth of 637 cm (Orkhonselenge et al., 2011)., are plausibly consistent with grain sizes and water content (fig. 3) mentioned above, i.e., they suggest the abrupt discharges, induced by rapid climatic changes resulting in the dense and coarse sedimentations. The intervals with higher percentages of the mineral fraction are characterized by higher grain density, lower water content and coarser grain size at the intervals 72-210 cm, 580-830 cm, and 1010-1110 cm, respectively. In addition, mineral content shows a familiar pattern to distributions of the grain sizes (fig. 3), i.e., it indicates sediments greatly deposit once the lake level drops or area of erodible shorelines extents. The curves of biogenic components (BiSi and OM) are relatively similar, suggesting a hydrological regime of high-stand lake level in the lacustrine catchment (fig. 3). Table 3 shows sedimentation rates and sediment fluxes for the main lithological units.

18The intervals of rapid sedimentation at 72-210 cm, 580-830 cm and 1010-1110 cm accumulated 3-12 times faster than the slow sedimentation intervals. The rapid sedimentation intervals are characterized by 1.5 times higher content of silty and clayey minerals, and by a lower influx of plant organics. These intervals can be interpreted as short periods (ca.130, 330 and 260 years, respectively) of increased water turbidity. Shallowing could result in stronger erosion of the lake shores. The effect of Borsog River on the lake resulted from the expansion of the river delta. These two processes increased the sediment flux into the lake. Moreover, the aeolian and slope processes became stronger due to the climatically-driven decrease in precipitation/runoff, degradation of vegetation, and expansion of permafrost.

Radiocarbon dating and age constraints

19Radiocarbon dating for core BB03 indicates the Holocene age of the sediments (tab. 1).

Tab. 1 – Results of 14C dating of core BB03.
Tab. 1 – Datations radiocarbone issues du carottage BB103.

Sample no.

Depth, cm

*Material

Lab No (NUTA2)

Age,yr14C BP

1

2.4

BO

13426

2560 ± 30

2

5.9

BO

14357

3030 ± 35

3

60.4

BO

14358

3770 ± 35

4

103.0

BO

13427

3010 ± 35

5

150.3

BO

13428

2760 ± 30

6

208.0

BO

13431

2750 ± 30

7

330.3

BO

12783

3830 ± 35

8

432.8

BO

12784

4710 ± 35

9

546.4

BO

12785

5960 ± 35

10

624.6

BO

12432

5940 ± 35

11

641.2

PR

12859

5210 ± 35

12

677.2

W

12860

4080 ± 30

13

737.4

BO

12876

5870 ± 40

14

842.5

BO

12787

6120 ± 35

15

940.8

BO

12788

7040 ± 40

16

1020.6

BO

14359

8210 ± 40

17

1040.2

BO

12789

7880 ± 40

18

1103.4

BO

14360

7970 ± 45

19

1147.7

BO

12791

8330 ± 40

20

1260.7

BO

12792

8980 ± 45

* BO – bulk organic; PR – plant remnants; W - wood

20There are inconsistencies in the dataset, including inversions and mismatched bulk organic and wood dates. The bulk organic (BO) dates obtained for the 600-800 cm interval (samples 10, 11, 13, 27, 28) are 1500±400 years 14C older than the age of wood (tab. 1, sample 12) at a depth of 677 cm. Moreover, the uppermost sample in core has 14C age of ca. 2600 BP.

21The age-to-depth plot (fig. 4) shows that the distribution of the ages is not linear.

Fig. 4 – The age-to-depth model for Lake Borsog sedimentation based on the 14C ages of core BB03.
Fig. 4 – Le modèle âge-profondeur de la sédimentation du lac Borsog, basé sur les âges radiocarbone du carottage BB103.

Fig. 4 – The age-to-depth model for Lake Borsog sedimentation based on the 14C ages of core BB03. Fig. 4 – Le modèle âge-profondeur de la sédimentation du lac Borsog, basé sur les âges radiocarbone du carottage BB103.

Line A is a piecewise linear approximation of the ages of bulk organic samples. Line A1 is a linear trend of Line A. Line B is a model built through two age points: zero and wood sample (no. 12 in Table 1). Line C is the most probable model based on the ages of the bulk organic samples corrected to the age of the wood sample. Sedimentation: S- slow, F- fast.
La ligne A est une approximation linéaire des âges des échantillons organiques. La ligne A1 est une tendance linéaire de la ligne A. La ligne B est un modèle construit à partir de deux âges : zéro et un échantillon de bois (n° 12 dans le tab. 1). La ligne C est le modèle le plus probable basé sur les âges des échantillons organiques corrigés par l'âge des échantillons de bois. Sédimentation : S- lente, F-rapide.

22There are two types of intervals: i) dates in a normal stratigraphic sequence (normal-type); and ii) disordered dates (disordered-type). These intervals coincide with the main layers of the core. The boundaries separating the intervals are at depths of 210, 580, 830, 1010, and 1110 cm. In addition, the 72-cm boundary is based on the records of sediment properties (fig. 3). The bends of Line A at 2730, 5660, 5870, 7820, and 8090 14C BP match the six boundaries between the main layers of the core (fig. 4). The bottom sediments at a depth of 13 m have a conventional age of 9230 14C BP. According to the corrected model (Line C), the age of these boundaries may be ca. 1500 years younger. The different behavior of the approximated lines in the normal type and disordered-type intervals suggests variable rates of sedimentation. Therefore, three episodes of dramatic acceleration of sedimentation in Lake Borsog during the Holocene are recognized.

23A shift in age determinations to older values is a common problem for Lake Khuvsgul and Baikal sediments. In Lake Khuvsgul, 14C ages of bulk organic are 0.4-4 ka older than those of wood fragments (Watanabe et al., 2009) and the ages of its surface sediments average at ca. 0.5 ka (Prokopenko et al., 2005). The age of Baikal surface sediments ranges from 0.47 to 1.22 ka, and the age of trapped suspended algae is 0.61 ka (Colman et al., 1996). The older ages of Baikal and Khuvsgul surface sediments were attributed to the input of terrestrial organics and to the re-suspension of previously deposited algal carbon (Colman et al., 1996), and were considered as a lake reservoir effect (Prokopenko et al., 2005). If so, the dates obtained from deeper layers of sediments in both lakes also require reservoir effect correction.

24The Borsog sediments contain 10-35% organics (fig. 3) suggesting delivery of re-deposited terrestrial organic material by streams (see Section 2). The dated BO samples contain a mixture of coeval and older humic fractions, which could have shifted the ages to older values. For corrections, the dates obtained from plant macrofossils are considered more reliable than those obtained from bulk organics (e.g., McGeehin et al., 2001). As it is difficult to evaluate the degree of BO contamination and therefore the correctness of each date, the trends indicated by the whole BO dataset were used. For correction, the only available wood date (tab. 1, sample 12) was used, which is 1500 ± 400 14C years younger than the BO dates at the same level. Line B in figure 4 connects the zero point and the age point of sample 12. Line B is parallel to Line A, and therefore the correction appears valid and is referenced to zero in the corrected age model, i.e., Line C.

25Table 2 shows the corrected ages of the main lithological boundaries of Lake Borsog sediments, which are used for discussing environmental and climatic changes. This age model cannot define the age of the 72 cm boundary, because the whole 0-210 cm interval is of the disordered-type.

Tab. 2 – Corrected ages of main lithological boundaries in Lake Borsog sediments.
Tab. 2 – Âges corrigés des principales limites lithologiques des sédiments du lac Borsog.

Boundary, cm

Correctedage, yr BP

14C-based

Calibrateda

72 b

1000

890

210

1120

1020

580

4020

4500

830

4240

4830

1010

6200

7090

1110

6430

7350

1300

7600

8400

aCALIB v.6.0 Radiocarbon calibration program (Reimer et al., 2009). b aless reliable estimate.

Tab. 3 – Sedimentation rate, sediment flux and plant organic index for main lithological units of Lake Borsog.
Tab. 3 – Taux de sédimentation, flux de sédiments et indice de matière organique pour les principales unités lithologiques du lac Borsog.

Interval, cm

Sedimentation

Sedimentation rate, cm/yr

Sediment flux, g/(cm2*yr) a

Plant organic index b

Time, ka

0-72

slow

0.08

0.18

0.5

0-0.9

72-210

fast

1.1

2.6

0.3

0.9-1.0

210-580

slow

0.11

0.24

0.7

1.0-4.5

580-830

fast

0.76

1.86

0.3

4.5-4.8

830-1010

slow

0.08

0.18

0.7

4.8-7.1

1010-1110

fast

0.38

0.9

0.4

7.1-7.4

1110-1300

slow

0.18

0.42

0.5

7.4-8.4

aratio of average sediment density to sedimentation rate.
bratio of plant organic (%) to mineral component (%).

The level of Lake Khuvsgul

26The level of Lake Khuvsgul considerably increased in response to glacial termination. The highest level during the Holocene was ~ 6 m compared to the present one. The value of 6 m is the height of the pre-Holocene Ulkhen Sair fan at the southern end of the lake (fig. 1), above which the Egiin River flows out of the lake. Lake Khuvsgul probably reached its highest level in early post-glacial time, and since then the level has been decreasing and irregularly fluctuating within a few meters (Krivonogov et al., 2003). The lake level depends on two mutually-related factors: the continuing incision of the Egiin River outlet and the periodic restoration of the Ulkhen Sair fan. Both factors are climatically controlled: increased precipitation led to stronger erosion of the outlet on one hand, and contributed to the damming by the sediment accumulated in the fan on the other hand.

27The increases of the lake level due to sedimentary pulses on the Ulkhen Sair fan are episodic and therefore correlate with the phases of high precipitation (Rogozin, 1993). The torrents, which carry stones and mud into Lake Khuvsgul, affect the Egiin River outlet and occur approximately every 7 years along the western shore and every 10-11 years along the eastern shore (Batsukh et al., 1976). Sedimentary pulses significantly changed the level of Lake Khuvsgul in 1886 (Cherkasov et al., 1972) and in 1971 (Rogozin and Yakimov, 1977). The hydrometric data show that significant, up to 60 cm, rises of the level occurred in 1973-1977 and 1982-1987 (Kumagai et al., 2006). The data cited by Kumagai et al. (2006) are the annually averaged values, whereas the annual fluctuation of Lake Khuvsgul levels is within 80 cm (Batsukh et al., 1976). Thus, the progressive reduction of the Egiin River headwaters results in the lowering of Lake Khuvsgul levels.

28The pebble spit separating Lake Borsog keeps its level 0.5 m higher than that of Lake Khuvsgul. The outflow water of Lake Borsog can easily erode the spit, and both lakes would be expected to have the same level. The level in Lake Borsog cannot significantly change without a respective change in Lake Khuvsgul. Therefore the recorded periods of shallow water in Lake Borsog at ca. 7.4-7.1, 4.8-4.5 and 1.0-0.9 ka cal. BP (tab. 2 and tab. 3) suggest respective drops in the level of Lake Khuvsgul.

29The sedimentary record suggests that Lake Borsog separated from Lake Khuvsgul before 8.4 ka cal. BP. The old spits show that the separation occurred during the ~3 m high stand of Lake Khuvsgul, possibly, in the beginning of the Holocene.

Climatic implications

30The patterns of climate-related level changes of other lakes situated at latitudes close to that of Lake Khuvsgul are variable. For example, in Late Glacial time, Lake Uvs, a closed lake, was 40 m higher than now and dramatically decreased during the Holocene, with several drops ca. 10.7-9.3, 6.3, 5.2 and 3.2 ka cal. BP. The minimal level of Lake Uvs (-7 to -5 m) occurred ca. 3.2-1.0 ka cal. BP, and later the lake rose to its present level (Grunert et al., 2000). The pollen data obtained from Lake Telmen indicate a relatively dry period of 7.5-4.5, a wet period of 4.5-1.6, and a fluctuating dry to wet period from 1.6 ka cal. BP to the present. At ca. 1.6-1.2 ka cal. BP there was a short dry period, after which the humidity increased in response to the Medieval Warm Epoch (1.2-0.4 ka BP) and the following Little Ice Age (0.4 ka BP to present) (Fowell et al., 2003). Another example is Lake Gun, which was low at 10.7-7.7, high at 7.7-2.2, and low again at 2.2-1.5 ka cal. BP (Feng et al., 2005). Proxies of Lake Ugii indicate a slightly higher moisture supply at ca. 10-8 ka cal. BP compared to the present time, and Lake Ugii was considerably larger in the middle Holocene (ca. 8-4 ka cal. BP) but shallower at ca. 4-2.8 ka cal. BP due to increased aridity. During the last 2 ka the level of Lake Ugii rose as a result of increased moisture supply (Schwanghart et al., 2009). In addition, a period of dry climate occurred in Lake Ugii at ca. 6.6-3.3 ka cal. BP (Wang et al., 2011). Thus, the diverse data obtained on several localities suggest either dry or humid climates in the Holocene. However, they appear valid if Holocene climate was variable in different areas of Central Asia, which could be dominated by the Westerly or Pacific patterns of atmospheric circulation (e.g., Chen et al., 2008; Rudaya et al., 2009).

31Several recent overviews of climatic records obtained from Asian lakes including Lake Khuvsgul showed the following trend of moisture supply into arid regions of Central Asia (Chen et al., 2008): low before ca. 8, increased at 8-4, and gradually decreased after 4 ka cal. BP (fig. 5, Line a).

Fig. 5 – The main trend of the Holocene moisture evolution for arid Central Asia (Line a; Chen et al., 2008), Mongolia (Line b; modified from An et al., 2008), and the summarized curve of Lake Khuvsgul level changes (Line c; data from this paper and Krivonogov et al., 2003).
Fig. 5 – Principale tendance de l'évolution holocène de l'humidité pour l'Asie centrale (Ligne a ; Chen et al., 2008), la Mongolie (ligne b, modifié de An et al., 2008) et courbe synthétique des changements du niveau du lac Khuvsgul (ligne c, données de cet article et Krivonogov et al., 2003).

Fig. 5 – The main trend of the Holocene moisture evolution for arid Central Asia (Line a; Chen et al., 2008), Mongolia (Line b; modified from An et al., 2008), and the summarized curve of Lake Khuvsgul level changes (Line c; data from this paper and Krivonogov et al., 2003). Fig. 5 – Principale tendance de l'évolution holocène de l'humidité pour l'Asie centrale (Ligne a ; Chen et al., 2008), la Mongolie (ligne b, modifié de An et al., 2008) et courbe synthétique des changements du niveau du lac Khuvsgul (ligne c, données de cet article et Krivonogov et al., 2003).

The gray bars indicate the intervals of fast sedimentation in Lake Borsog.
Les barres grises indiquent les périodes de sédimentation rapide dans le lac Borsog.

32If applied to Mongolia, this trend suggests a humid and warm early Holocene, an arid and warm middle Holocene, and a humid and colder late Holocene (An et al., 2008) (fig. 5, Line b). For Lake Khuvsgul, this trend indicates warmer and drier conditions at ca. 6-3.5 ka cal. BP (Prokopenko et al., 2007). However, this trend is a mirror reflection of the reconstruction for level changes of Lake Khuvsgul (fig. 5, Line c). The level of Lake Khuvsgul was high during the dry phases and low during the wet periods. This phenomenon reflects the balance between the incision of the Egiin River (lowering level) and the renovation of the Lake Khuvsgul outflow dam (rising level). Such a model suggests a positive water balance in the lake and continuous discharge during the Holocene.

Conclusions

33There have been three short-term events of abruptly increased sedimentation in Lake Borsog. These short events (100-300 years) of high rate sedimentation coincide with the low stands of the lake, which resulted in the progradation of the delta of the Borsog River and in erosion of exposed lake shores. The low stands of Lake Borsog generally match the drops of Lake Khuvsgul at ca. 7.2-7.0, 4.5-4.1 and 2.1-0.5 ka cal. BP. The Borsog sediment data confirm the correctness of the previous conclusions about the changes of Lake Khuvsgul levels (Tarasov et al., 1996; Dorofeyuk and Tarasov, 1998; Krivonogov et al., 2003, 2006).

34The level of Lake Khuvsgul is controlled by a balance of two opposite processes: the incision of the Egiin River outlet and the episodic restoration of the Ulhen Sair fan. The highest level of Lake Khuvsgul was during a period from 14 to 12 ka cal. BP, i.e., the time of glacial recession, where in addition to precipitation, glacier and permafrost melting increased inflow to the lake. The geomorphology of the water discharge area restricts the former level of Lake Khuvsgul to ~6 m as compared with the present level. The incision of the outlet was very fast during the early Holocene, and consequently the lake level dropped. The Holocene minimum level of the lake occurred ca. 8 ka cal. BP. After this time the lake level started to rise, although with fluctuations.

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Annexe

Version française abrégée

L'interprétation des séquences sédimentaires lacustres est importante pour comprendre l'évolution des paysages, du climat et les variations de niveau des lacs, qui font l'objet de cette publication. Peu de proxies fiables reflétant les changements climatiques et l'évolution des paysages à l'Holocène ont été obtenus dans les lacs Hoton et Uvs situés dans l'ouest de la Mongolie (Grunert et al., 2000), dans le lac Ugii en Mongolie centrale (Wang et al., 2011), et dans les lacs Dood and Gun dans le nord de la Mongolie (Dorofeyuk and Tarasov, 1998). Le lac Khuvsgul constitue une archive paléoclimatique importante en Mongolie. Il est localisé dans le centre du continent eurasiatique, sous l'influence de l'anticyclone sibérien l'hiver, en interaction avec les circulations des vents d'ouest et de la mousson. Le lac Khuvsgul a déjà fait l'objet d'études dans le cadre de programmes nationaux et internationaux de forages pour identifier les événements à long-terme du dernier million d'années (Prokopenko et al., 2007). Mais les investigations portant sur le court terme sont d'une grande importance pour reconstruire l'histoire du Pléistocène supérieur et de l'Holocène. Cet article participe à une meilleure compréhension de l'évolution des paysages dans la proximité de ces lacs mongols, thème qui avait déjà fait l'objet d'une brève présentation par A. Orkhonselenge et al. (2011).

Un carottage de 13 m de profondeur (BB03) a été réalisé dans le lac Borsog avec une foreuse de type Livingston, sous 9,1 m d'eau, positionné N 50059’09.5”, E 100042’40.7”. La carotte sédimentaire montre des laminations variées permettant une division visuelle en plusieurs couches. Les laminations de différentes couleurs sont séparées de manière irrégulière par des niveaux de 2-3 mm d'épaisseur de sables fins. Les unités lithologiques ont été identifiées par différentes méthodes. Des analyses physico-chimiques ont été réalisées, teneur en eau, densité et taille des particules, matière organique (OM), silice biogénique (BiSi) et fractions minérales. L'âge des sédiments est défini par des datations radiocarbone obtenues par spectrométrie de masse par accélérateur au « Center for Chronological Research » de l'université de Nagoya (Japon), sur de la matière organique, des restes de plantes et des fragments de bois.

Les analyses ont permis de reconstituer les fluctuations de niveau du lac Khuvsgul et l'évolution du réseau hydrographique de son bassin versant durant l'Holocène. Le niveau du lac Khuvsgul s'est considérablement relevé en réponse à la déglaciation. Les plus hauts niveaux sont enregistrés à environ 6 m au-dessus du niveau actuel. La flèche littorale de galets qui isole le lac Borsog porte son niveau à 0,5 m au-dessus de celui du lac Khuvsgul. Le niveau du lac Borsog ne peut pas changer de manière significative sans un changement également significatif du niveau du lac Khuvsgul. Ainsi, la période de bas niveau lacustre enregistrée dans le lac Borsog vers 7,4-7,1, 4,8-4,5 et 1,0-0,9 ka cal. BP permet d'envisager une évolution comparable dans le lac Khuvsgul. Par ailleurs, les enregistrements sédimentaires laissent penser que le lac Borsog s'est individualisé du lac Khuvsgul avant 8,4 ka cal. BP.

Plusieurs synthèses récentes portant sur les enregistrements climatiques des lacs asiatiques, incluant le lac Khuvsgul, montrent une tendance à l'apport d'humidité dans les régions arides de l'Asie centrale (Chen et al., 2008): faible apport avant 8 ka cal. BP, en augmentation vers 8-4 ka cal. BP et en décroissance régulière après 4 ka cal. BP. Appliquée à la Mongolie, cette tendance suggère un Holocène ancien humide et chaud, un Holocène moyen aride et chaud et un Holocène récent humide et plus froid (An et al., 2008).

Dans le cas du lac Khuvsgul, cette tendance climatique interfère avec les variations du niveau du lac qui sont importantes pendant les phases sèches et faibles pendant les périodes humides. Ce phénomène reflète le bilan de l'incision de l'émissaire, l'Egiin (niveau du lac abaissé) et de la reconstitution du barrage de l'exutoire (niveau du lac relevé; Krivonogov et al., 2003).

Un tel modèle suggère un bilan hydrologique positif dans le lac et un débit continu durant l'Holocène. Les bas niveaux du lac Borsog sont généralement corrélés aux abaissements du lac Khuvsgul vers 7,2-7,0, 4,5-4,1 et 2,1-0,5 ka cal. BP. Les sédiments du lac Borsog confirment l'exactitude des conclusions antérieures portant sur les changements de niveau du lac Khuvsgul. Le niveau de ce lac est contrôlé par l'occurrence de deux processus opposés : l'incision de l'exutoire de la rivière Egiin et la restauration épisodique des cônes de déjection de l'Ulhen Sair. Le niveau le plus haut du lac Khuvsgul est observé entre 14-12 ka cal. BP, c'est-à-dire durant la période de récession glaciaire, où, aux précipitations, s'ajoutent les eaux de fonte du glacier et du permafrost, accroissant ainsi les apports dans le lac. La géomorphologie du secteur d'évacuation des eaux réduit l'ancien niveau du lac Khuvsgul d'environ 6 m, comparé au niveau actuel. Le niveau minimum du lac à l'Holocène est atteint vers 8 ka cal. BP. Après cette période, le niveau du lac commence à monter, avec des fluctuations.

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Table des illustrations

Titre Fig. 1 – Study area. Fig 1 – La zone d'étude.
Légende A: Locations of Lake Borsog and other lakes mentioned in the text. B: Topography of Lake Khuvsgul and its catchment: 1: Lake Khuvsgul in the last glacial time (100 m level drop); 2: modern outline; 3: the Ulhen Sair alluvial fan, which controls the level of Lake Khuvsgul; 4: directions of river-flow; 5: boreholes; 6: Lake Borsog and its surroundings as mapped in Fig. 2; 7: villages. C: Schematic section along Line a–b–c indicating structural and tectonic features. A : Localisation du lac Borsog et des autres lacs mentionnés dans le texte. B : Topographie du lac Khuvsgul et son bassin versant. 1 : Le lac Khuvsgul durant la dernière glaciation (niveau abaissé de 100 m) ; 2 : limite moderne ; 3 : cône de déjection de l'Ulhen Sair, qui contrôle le niveau du lac Khuvsgul ; 4 : Sens de l'écoulement fluvial ; 5 : Sondages ; 6 : Le lac Borsog et ses environs comme cartographié dans la fig. 2 ; 7 : Villages. C : Profil schématique le long de la ligne a-b-c indiquant des traits structuraux et tectoniques.
URL http://geomorphologie.revues.org/docannexe/image/10781/img-1.png
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Titre Fig. 2 – Lake Borsog area. Fig. 2 – Le secteur du lac Borsog.
Légende A: Geomorphological map of Lake Borsog area. 1: hills; 2: valley bottom and slopes; 3: former part of Borsog Bay filled by sediments; 4: floodplain; 5: Lakes Khuvsgul and Borsog; 6: beach bars and spits; 7: outcrop; 8: borehole. B: Altitudinal profile along Line a–b–c–d (SRTM DEM data). C: Schematic section along Line b–c showing the structure of Lake Borsog and its sedimentary fill. The distribution of frozen/thawed ground is shown approximately. A : Carte géomorphologique du secteur du lac Borsog. 1 : collines ; 2 : fond de vallée et pentes ; 3 : ancienne partie de la baie de Borsog comblée par des sédiments ; 4 : plaine alluviale ; 5 : lacs Khuvsgul et Borsog ; 6 : barres de plage et flèches littorales ; 7 : butte ; 8 : sondage. B : Profil en travers le long de la ligne a-b-c-d (SRTM données MNT). C : Coupe schématique le long de la ligne b-c montrant la structure du lac Borsog et son remblaiement sédimentaire. La limite entre le sol gelé et dégelé est approximative.
URL http://geomorphologie.revues.org/docannexe/image/10781/img-2.png
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Titre Fig. 3 – Sediment properties of core BB03 from Lake Borsog. Fig. 3 – Caractéristiques des sédiments extraits du carottage BBO3 dans le lac Borsog.
Légende Water content and grain density; grain size median of mineral fractions and grain size median of total sediments; sediment compositions of biogenic silica, organic matter and mineral fractions. Teneur en eau et densité du grain ; médiane des fractions granulométriques et médiane de l'échantillon ; part de la silice biogénique, de la matière organique et des fractions minérales.
URL http://geomorphologie.revues.org/docannexe/image/10781/img-3.png
Fichier image/png, 3,2M
Titre Fig. 4 – The age-to-depth model for Lake Borsog sedimentation based on the 14C ages of core BB03. Fig. 4 – Le modèle âge-profondeur de la sédimentation du lac Borsog, basé sur les âges radiocarbone du carottage BB103.
Légende Line A is a piecewise linear approximation of the ages of bulk organic samples. Line A1 is a linear trend of Line A. Line B is a model built through two age points: zero and wood sample (no. 12 in Table 1). Line C is the most probable model based on the ages of the bulk organic samples corrected to the age of the wood sample. Sedimentation: S- slow, F- fast. La ligne A est une approximation linéaire des âges des échantillons organiques. La ligne A1 est une tendance linéaire de la ligne A. La ligne B est un modèle construit à partir de deux âges : zéro et un échantillon de bois (n° 12 dans le tab. 1). La ligne C est le modèle le plus probable basé sur les âges des échantillons organiques corrigés par l'âge des échantillons de bois. Sédimentation : S- lente, F-rapide.
URL http://geomorphologie.revues.org/docannexe/image/10781/img-4.png
Fichier image/png, 331k
Titre Fig. 5 – The main trend of the Holocene moisture evolution for arid Central Asia (Line a; Chen et al., 2008), Mongolia (Line b; modified from An et al., 2008), and the summarized curve of Lake Khuvsgul level changes (Line c; data from this paper and Krivonogov et al., 2003). Fig. 5 – Principale tendance de l'évolution holocène de l'humidité pour l'Asie centrale (Ligne a ; Chen et al., 2008), la Mongolie (ligne b, modifié de An et al., 2008) et courbe synthétique des changements du niveau du lac Khuvsgul (ligne c, données de cet article et Krivonogov et al., 2003).
Légende The gray bars indicate the intervals of fast sedimentation in Lake Borsog. Les barres grises indiquent les périodes de sédimentation rapide dans le lac Borsog.
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Alexander Orkhonselenge, Sergey K. Krivonogov, Kenta Mino, Kenji Kashiwaya, Masayoshi Yamamoto et Toshio Nakamura, « Holocene Landform Evolution of Lake Khuvsgul basin, Mongolia », Géomorphologie : relief, processus, environnement, vol. 20 - n° 4 | 2014, 343-354.

Référence électronique

Alexander Orkhonselenge, Sergey K. Krivonogov, Kenta Mino, Kenji Kashiwaya, Masayoshi Yamamoto et Toshio Nakamura, « Holocene Landform Evolution of Lake Khuvsgul basin, Mongolia », Géomorphologie : relief, processus, environnement [En ligne], vol. 20 - n° 4 | 2014, mis en ligne le 01 janvier 2016, consulté le 23 novembre 2017. URL : http://geomorphologie.revues.org/10781 ; DOI : 10.4000/geomorphologie.10781

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Auteurs

Alexander Orkhonselenge

School of Arts & Sciences National University of Mongolia Ulaanbaatar 15160 Mongolia (rkhnslng@gmail.com).

Articles du même auteur

Sergey K. Krivonogov

Institute of Geology & Mineralogy Siberian Branch of Russian Academy of Sciences Novosibirsk 630090 Russia.
Novosibirsk State University Novosibirsk 630090 Russia.

Kenta Mino

Institute of Nature and Environmental Technology Kanazawa University Kanazawa 920-1192 Japan.

Kenji Kashiwaya

Institute of Nature and Environmental Technology Kanazawa University Kanazawa 920-1192 Japan.

Masayoshi Yamamoto

Institute of Nature and Environmental Technology Kanazawa University Kanazawa 920-1192 Japan.

Toshio Nakamura

Center for Chronological Research Nagoya University Nagoya 464-8602 Japan.

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