We would like to express our gratitude to D. Gerelsaikhan, G. Tserendorj and G. Narmandakh, research assistants of the Laboratory of Geochemistry and Geomorphology (LGG), for helping our fieldwork. This research was funded by National University of Mongolia (research grants: P2017-2388, P2017-2520). We also thank anonymous reviewers and Dr. Armelle Decaulne for stimulated suggestions that improved the current work.

1. Introduction

1Lake Yakhi sediments archive information on surface processes in the lake basin during geological time scales recording paleoclimate change in eastern Mongolia. Thick bottom sediments deposited in lakes are significant imprints of the Quaternary paleoenvironmental changes (Khosbayar, 2005). In the arid environment where a continuous terrestrial archive is scarce, lacustrine sequences often employ a paleoenvironmental repository (Yu et al., 2017).

2Because of eastern Mongolia's geographical position at the junction of the westerlies and East Asian Winter Monsoon, sedimentations in lakes of eastern Mongolia provide a key for reconstructing regional paleoclimate changes in Central and Northeast Asia. However, sedimentary features in lakes of eastern Mongolia have never been studied in comparison with those in southern Mongolia (Hülle et al., 2010; Lee et al., 2013), western Mongolia (Grunert et al., 2000; Klinge and Lehmkuhl, 2013), northern Mongolia (Prokopenko et al., 2007; Orkhonselenge et al., 2013) and central Mongolia (Schwanhart et al., 2009; Davaagatan et al., 2015). Eastern Mongolia had been a large drainage basin at the beginning of the Pliocene (Tsegmid, 1969; Jigj, 1975). In the region, Kang et al. (2015) found a relationship between lake area changes and precipitation since the late 1980s. Tserenpil et al. (2010) showed that Lakes Toson, Zegstei, and Utaat Minjuur were rich in dialkyl phthalates (DAPs) of dissoluble and insoluble organic matters. Moreover, paleofossils of plants were studied in Lake Yakhi (Khosbayar, 2005).

3Third largest lake in eastern Mongolia, Lake Yakhi plays an essential role in the surface water resources of the Pacific Ocean drainage basin. Lake Yakhi basin has been intensely impacting by global and regional warming today. The drought in eastern Mongolia noted the regional trend in warming over the past decades since the late 1900s (Davi et al., 2013), and a significant trend in large-scale drying during 1951–2005 across eastern Mongolia (Li et al., 2009). Lake Yakhi is rapidly losing its water resource due to the continuous rise in air temperature that has forced to decrease in the lake surface area over the past decades.

4Many attempts have been made to refine provenance models using the framework composition (Dickinson et al., 1983) and geochemical features (Roser and Korsch, 1986, 1988; Armstrong-Altrin et al., 2013). The geochemical analysis of sedimentary rocks is used to define the provenance characteristics, tectonic setting, and source rock composition (Roser and Korsch, 1986; Madhavaraju et al., 2016). Major elements are one of the reliable indicators in evaluations of tectonic setting, weathering processes, and provenance. The major elemental ratios are used to find the source rock composition because these elements are not redistributed by lithogenesis and metamorphism (Pamdey and Parcha, 2017). The source rocks' chemical composition preserves the history of sedimentary rocks’ evolutionary processes (Ferdous and Farazi, 2016) in the basin. Mineralogical composition, degree of weathering, and landscape evolution of the source area offer clues to the basin's tectonic setting.

5The sedimentary basin's tectonic setting may play a predominant role because different tectonic settings can provide other types of source materials with variable chemical signatures (Armstrong-Altrin and Verma, 2005). For instance, active margin or arc sediments are typically enriched in mafic rather than felsic signatures (Bhatia and Crook, 1986). Bhatia (1983) created a discriminant diagram of a simplified tectonic classification of continental margins and oceanic basins based on the geochemical composition of associated wacke for differentiating tectonic settings: oceanic island arc (OIA), continental island arc (CIA), active continental margin (ACM) and passive margin (PM), however, Roser and Korsch (1986) incorporated them into three groups of tectonic settings: passive margin (PM), active continental margin (ACM) and oceanic island arc (ARC).

6This work is the first attempt to understand the Lake Yakhi basin's landscape evolution in eastern Mongolia based on geomorphological imprints of shoreline features, changes in the lake area, and geochemical processes with indicators of provenance signature, tectonic setting, source area weathering, and source rock compositions in the lake sediments.

2. Site descriptions

2.1. Physiographic condition

7Lake Yakhi is located at 670 m a.s.l. in the Dornod plain of eastern Mongolia (fig. 1). There was a small marsh in the Yakhi Govi (or Gobi - “Govi” has been often spelled as “Gobi” in the international literature. The Govi is the correct English transliteration from Mongolian “Говь”) in the 1950s. By 1964, the marsh developed to become a lake covering 97.2 km² by 1964 due to the water level rise since 1960s (tab.1). The lake expands 20.4 km in length and 11.9 km in width (Tserensodnom, 2000).

8Lake Yakhi is formed in a valley-shaped dry depression extended over 100 km in length and 7–15 km in width (Tsegmid 1969), and it is surrounded by low mountains in the northwest, north, and east, and by a vast plain in the south (fig. 2). For instance, it is encompassed by Mt. Bayan Dun (934 m a.s.l.) in the northeast, Mt. Khundlun Tolgoi (826 m a.s.l.) in the east, Mt. Ikh Kholboo (785 m a.s.l.) in the southeast, an expansive marsh in the south (fig. 2), Mt. Shiveet (772 m a.s.l.) in the southwest, Mt. Bayan (831 m a.s.l.) in the west, and Mt. Tsagaan Tolgoi (856 m a.s.l.) and Mt. Zuun Tsagaan Undur (802 m a.s.l.) in the north (Orshikh et al., 2014) and an expansive marsh in the south (fig. 2).

9Lake Yakhi, a mineral lake, is fed by Gal River in the southwest (fig. 2) and streams Sumiin Bulag and Tugaliin Bulag, draining from the eastern Mt. Narst (1356 m a.s.l.) in the Ereen Mountain Range. The Gal River is about 200 km in length (Tsegmid, 1969) and it does not reach the lake depending on precipitation (Badarch et al., 1965). The Lake Yakhi drainage basin was more extensive in the past than today, and its drainage network was much denser (Tsegmid, 1969).

10Lake Yakhi shifted from a freshwater to a saline lake during the Neogene period (Tsegmid, 1969). The lake’s mineralization value is 5.0–5.2 g/l during the low water level and 5.4 mg/l during the high water level (Tserensodnom, 2000). The water depth of the modern-day lake is 1.2 m on average, 2.5 m at the maximum, and the total volume of the lake is 0.116 km³ (Davaa, 2015).

2.2. Geological setting

11The Dornod plain of eastern Mongolia is underlain by Mesozoic and Cenozoic sedimentary and volcanic rocks. Tectonically, Lake Yakhi borders the Cenozoic depositional complex in the southeast, the early Mesozoic volcanic depositional complex in the east, south and southwest, and the early Paleozoic orogeny complex in the north and northwest (Academy of Sciences of Mongolia and Academy of Sciences of USSR, 1990). Regarding lithology around the Lake Yakhi, it borders with the Permian dacites and rhyolites in the north, northeast and west, the early Jurassic and late Cretaceous volcanic deposits in the east, south, and southwest, the Paleogene and Neogene deposits in the southeast, and the early Permian intrusive alkaline granites, granosyenites, and quartz-syenites in the northwest (Academy of Sciences of Mongolia and Academy of Sciences of USSR, 1990).

12In the Lake Yakhi basin, there are the early Paleozoic to the early Mesozoic basalts, andesite-basalts and andesites in the east, the terrigenous gray sands-gravels and sands-pebbles in the southeast and south, the early Paleozoic to the late Mesozoic andesite-dacites, andesite-dacite-rhyolites, dacite-rhyolites and rhyolites in the southwest, west, north and northeast, and the early Paleozoic to the late Mesozoic granite-leucogranites, monzonite-syenite-granosyenites and granite-granosyenites in the northwest. For Quaternary deposits, Lake Yakhi is surrounded by modern-day lacustrine sediments from the north to southeast and by the Quaternary eluvial, deflection, desertion and diluvial deposits from the south to the northwest (Academy of Sciences of Mongolia and Academy of Sciences of USSR, 1990).

2.3. Climate condition

13Lake Yakhi basin is located under a semiarid steppe climate characterized by low precipitation and high evaporation in dry and cool summer and harsh winter (Academy of Sciences of Mongolia and Academy of Sciences of USSR, 1990) with an annual average wind velocity of over 3.0 m.s^-1 (Jambaajamts, 1989). In eastern Mongolia, the annual average air temperature ranges from -0.9 to 1.5°С (Jambaajamts, 1989), and the annual average precipitation is 188–391 mm (Altantsetseg and Munkhjargal, 2006). The multi-year time scale changes of average air temperature and precipitation show an increase of 0.6–1.9°С. yr^-1 and a decrease of 4–42 mm.yr^-1 (Altantsetseg, 2006)

14The local climate data recorded at the Sergelen meteorological station, located at 21 km southwest of Lake Yakhi, show the annual average air temperature of 1.0°С and precipitation of 222.6 mm during 1985–2019. The seasonal climate trend shows gradual rises in summer and spring air temperatures (fig. 3A) and winter precipitation (fig. 3B) during the last 35 years. The summer average air temperature is 18^oC (20.7^oC in July), whereas the winter average air temperature is -17^oC (-21.5^oC in January). The summer precipitation is 167 mm (42 mm in June, 68.5 mm in July, and 56.5 mm in August). Lake Yakhi basin became warmer since 1987, and the warmer years were recorded in 1987, 2004, 2007, 2008, and 2016 with the summer average air temperature of 21^oC (fig. 3A). Arid years were recorded in 1992–1994, 1997, 1999–2001, 2003–2004 and 2006–2007 with the summer precipitation of 20.3–87.0 mm (fig. 3B).

3. Methods

3.1. Fieldwork

15During fieldwork in April 2018, core samples (tab. 2) were collected from three sites in Lake Yakhi (fig. 2). Lake water is unstable, and it depends on annual and seasonal precipitations (fig. 3B, 4C, D). Lake Yakhi disappeared during the fieldwork in April 2018 (fig. 4C) when the monthly precipitation was 3.5 mm (fig. 3B). It still appeared on Landsat 8 image in June 2018 (fig. 2) when the monthly precipitation was 17.2 mm, and then it reappeared in August 2019 (fig. 4D) when the monthly precipitation was 243.0 mm (fig. 3B).

16We could not continue the coring after the Y18-3 (fig. 2) due to deep lake water conditions near the modern shoreline 3.3, downstream of Gal River on the central Lake Yakhi (fig. 4E-F, 5A). Spar (feldspar, calcite, barite) such as thenardite, soda and brine are widespread on the modern-day lake’s surface (fig. 4C, E-F).

3.2. Analytic work

17A topographic map (1:100,000) in 1970, Landsat 5 TM (path 127, row 26) in 1986, and Landsat 8 (path 126, row 26) in 2018 were used for compiling the changes in the area of Lake Yakhi at the years (fig. 2).

18For a lithological description of the Lake Yakhi sediments, the core Y18-1 consists of light grey (2.5Y 7/2) and light brownish grey (2.5Y 6/2) gravels, sands and silts. The core Y18-2 consists of light olive grey (5Y 6/2) and greenish-grey (10Y 6/1) sands, silts and muds. The core Y18-3 consists of pale olive (5Y 6/3), light olive grey (5Y 6/2), olive-grey (5Y 5/2, 5Y 4/2), and olive (5Y 5/3) sands, silts and muds.

3.3. Geochemical analysis

19Major elements in the Lake Yakhi sediments were analyzed at the Division of Radionuclide Analysis, the Central Geological Laboratory in Mongolia, using the Axios Max X-ray fluorescence spectrophotometer.

20In arid regions, where available climatic proxies are limited, a geochemical weathering index can provide useful information about climate changes (Lee et al., 2013). The weathering intensity indicates climate-induced erosional and depositional processes in the lake basin (Orkhonselenge et al., 2018a). The weathering intensity and climatic conditions of source area can be inferred from the chemical index of alteration (CIA).

4. Results

4.1. Shoreline changes

21The areal extents of the paleo- and modern-day Lake Yakhi and the paleo- and modern shoreline features in the lake basin are described. The shoreline landform features show that the paleo-Lake Yakhi may have covered an extensive area in the past.

22Paleostrandlines (at least four) are identified in the southeast and south of Lake Yakhi (fig. 5). These strandlines placed on the image would be determined with the further detailed field survey on their levels, extension size and direction, preservation, and even sediments constituting them. The paleostrandlines are interpreted as wave-cut terraces. The spit (fig. 5A) may have presumably formed by the deposition of materials by currents in the paleo-Lake Yakhi.

23Present-day shorelines (at least three) are identified (fig. 5A). The shorelines 1-2 and 3.1-3.3 (fig. 5A), coincide with the outer limits of the temporal extended areas of the lake (fig. 2). The surface inside of shoreline 3.2 appears as flat (fig. 4E). The modern-day shorelines may have resulted from the regressions during the last half-century (fig. 2, tab. 1). The lake’s exposed floor (fig. 4C, E-F) is often reworked by the aeolian process at the present warming and drying climatic conditions. This process may have effectively contributed to accumulating the light-grey and white spars on the lake’s exposed floor (fig. 4C, E-F). The modern-day shorelines 3.1 to 3.3 (fig. 5A) show the present deepest parts of the lake.

4.2. Changes in lake area

24Lake Yakhi lost an area of 43.44 km² during 1964–1986 except for 1971 and 18.73 km² during 1986–2018 (fig. 2, tab. 1). The continuous decrease of Lake Yakhi area by ~62.2 km² during the past five decades and the increase by 1.6 km² in the last five years may have been related to the gradually rising air temperature (fig. 3A) and the dropping in precipitation below 50 mm/yr (Fig. 11.3B) during the last 35 years.

25There are two periods of losing area during 1964–2010 and gaining area during 2010–2018 (tab. 1). The long-term decrease during 1964–2010, particularly during 1986–2010, is consistent with the continuous drop of precipitation between 1985 and 2007 with the exceptions of 1998 and 2002 and high evaporation caused by the high summer average air temperature since 1987. The short-term abrupt increase during 2010–2013 and the gradual increase during 2013–2018 coincides with the rise of rainfall during 2008–2019. Overall, the lake area changes during the past 50 years indicate that Lake Yakhi is gradually shifting into a playa lake.

4.3. Provenance signature and tectonic setting

26The discrimination plot shows mafic igneous, felsic igneous and quartzose sedimentary provenance signatures for the Lake Yakhi sediments (fig. 6). The function analysis using major elements (Roser and Korsch, 1988) indicates two groups of mafic igneous and quartzose sedimentary provenances (fig. 6A). The core Y18-1 sediments in the marginal part of Lake Yakhi (fig. 2) show a derivation from sedimentary rocks. In contrast, the core Y18-2 and Y18-3 sediments in the lake’s central part show derivations from mixed mafic igneous and sedimentary rocks (fig. 6A). The bivariate plot using the ratio of major element oxides to Al₂O₃ (Roser and Korsch, 1988) shows that Lake Yakhi sediments are dominantly in a quartzose sedimentary provenance field with felsic igneous provenance for some of the core Y18-1 sediments (fig. 6B).

27The tectonic setting discrimination diagram based on log[K₂O/Na₂O] vs. SiO₂ (fig. 7) shows the Lake Yakhi sediments on ACM and ARC settings. The tectonic discrimination diagram (Roser and Korsch, 1986) suggests that the lake sediments fall in the ACM field for the core Y18-1 sediments in the lake’s margin and the ARC field for the cores Y18-2 and Y18-3 sediments in the lake’s center (fig. 7A). The bivariate plot using oxides of Si, Ti, Al, Fe, Mn, Mg, Ca, Na and K (Bhatia, 1983) in tectonic settings shows the OIA field for the cores Y18-2 and Y18-3 sediments, the CIA field for the core Y18-2 sediments and the ACM field for the core Y18-1 sediments (fig. 7B).

4.4. Source area weathering and source rock composition

28Lake Yakhi sediments are rich in SiO2 (47.17% to 75.66%) and Al2O3 (11.28 to 15.37%) with a wide range of contents of the major elements (tab. 3). According to the geochemical classification (Herron, 1988), the Lake Yakhi sediments are mostly composed of arkose for the core Y18-1 sediments and shale for the cores Y18-2 and Y18-3 sediments (fig. 8A). The binary plot of Al2O3/TiO2 (Amajor, 1989) shows discriminant fields of source rocks for the core Y18-1 sediments on granite (felsic) and the cores Y18-2 and Y18-3 sediments on granite basalt (felsic to mafic) (fig. 8B).

29Source area weathering and source rock composition in the Al₂O₃-CaO+Na₂O-K₂O (A-CN-K) ternary diagram (Nesbitt and Young, 1984) show a low degree of chemical weathering in the Lake Yakhi basin (fig. 9A). Moreover, the CIA indicates more intensive weathering for the core Y18-1 in the marginal part than those for the cores Y18-2 and Y18-3 in the central part of Lake Yakhi. In the A-CN-K diagram, the sediments clustered along the left side of the K-feldspar-plagioclase join show that the Lake Yakhi sediments were derived from granite to granodiorite and tonalite source terrains (fig. 9A). The chemical maturity of Lake Yakhi sediments, based on a correlation between SiO₂ and total Al₂O₃+K₂O+Na₂O (Suttner and Dutta, 1986), shows shifting climatic conditions from semiarid to arid in the lake basin (fig. 9B).

5. Discussion

5.1. Shoreline changes

30Paleo- and modern-day shoreline features are identified in the Lake Yakhi basin (fig. 5). The paleoshorelines of Lake Yakhi (fig. 5B) observed in the southeast and south may have been wave-cut terraces. Simultaneously, the spit found in the southeast of the lake (fig. 5A) implies that Lake Yakhi was connected with a present-day small playa lake in the past. The physiographic condition of Lake Yakhi basin with the paleoshorelines coincides with a wide range of shoreline geomorphology commonly observed with well-studied paleolakes such as Lake Orog in southern Mongolia (Komatsu et al., 2001) and paleo-Lake Darkhad in northern Mongolia (Krivonogov et al., 2005). The formation of multiple wave-cut terraces requires geochronological results to conclude the extended periods of the paleolake Yakhi and a prolonged history of transgression and regression.

31The modern-day shorelines (fig. 5A) allow us to infer that the present-day Lake Yakhi has separated into the three parts shorelines 1-3 with the deepest ones 3.1-3.3 due to the rapid regressions strengthened by the rising temperature and evaporation at the arid climate today. The shoreline 3.1 in the southeast of the lake (fig. 5A) has already been separated. In contrast, the shoreline 3.2 in the east of the lake has recently been isolated that identified on Landsat image (fig. 2). The separated parts of the lake limited by the shorelines 3.1 and 3.2 (fig. 5A) may still be fed by numerous discontinuous channels. The deepest part bound by the shoreline 3.3 is intensely discharged by Gal River today.

32The landform features in the Lake Yakhi basin have formed with fluvial and aeolian erosion, transportation and deposition processes associated with regression and transgression at different climatic conditions in the past and present. Further geochronological data from the stratigraphic sequence of the wave-cut terraces would lead to infer the temporal distribution and extents of geomorphologic features and the paleolake.

5.2. Changes in lake area

33The water resource of Lake Yakhi is extremely sensitive to climate change, particularly the rising temperature and fluctuating precipitation (fig. 3). The lake area decrease concurs with the recent rapid regressions of Lake Yakhi (fig. 2, 5A) at the arid climate. The lake area reduction of Lake Yakhi by ~62.2 km² during the past 50 years (fig. 2, tab. 1) agrees with that occurred in other lakes of the Govi region in southern Mongolia (Orkhonselenge et al., 2018b). For instance, the impacts of annual precipitation fluctuation and an increase of air temperature cause a decrease in lake area in southern Mongolia (Szumińska, 2016). The decreasing trend of the lake area also coincides with the result given by Davi et al. (2013) who noted that the mean streamflow of Kherlen River in eastern Mongolia had decreased for more than a half from 2000 to 2008 when compared with prior decades. Moreover, it agrees with the result by Kang et al. (2015) who noted a general decreasing pattern of lake areas in eastern Mongolia between 1980 and 2008, and the higher sensitivity of lake area changes to precipitation in southern and southeastern Mongolia.

5.3. Geochemical view on geomorphology

34Lake Yakhi sediments are enriched in SiO₂ and Al₂O₃ (tab. 3). Source of silica is mainly quartz, chert, quartzite, feldspars and clay minerals. Al₂O₃ in the lake sediments supports the significant presence of K-feldspars (orthoclase and microcline), clay minerals (e.g., illite) and mica (fig. 9A). The spatial distribution of the major elements shows the shift from alkali metals' dominance in the lake’s margin toward alkaline earth metals in the lake’s center (tab. 3). This trend is related to the discharge of Gal River flowing through the andesite-dacites, andesite-dacite-rhyolites, dacite-rhyolites and rhyolites in the southwest (fig. 4A-B), identified in the provenance signatures (fig. 6), tectonic settings (fig. 7) and source rock compositions (fig. 8-9A). For instance, in the Lake Yakhi basin, the tectonic setting (fig. 7A) shows good evidence for the source area tectonic system consistent with the tectonostratigraphic terrain of the cratonic clastic sedimentary rocks (Badarch et al., 2002). Representative groundwater feeding lakes in eastern Mongolia is alkaline sourced from the local Cretaceous alkaline rhyolites and precipitated during evaporation of lakes (Linhoff et al., 2011). The major elements in the Lake Yakhi sediments are compatible with the passive margin sandstones (Bhatia, 1983), arkosic sandstones (Pettijohn, 1963) and sandstones (Zaid and Gahtani, 2015) (tab. 3).

5.4. Paleoweathering and paleoclimatic conditions

35The CIA values in Lake Yakhi sediments show that erosion is more intensive in the lake’s margin than those in the lake’s center (fig. 9A). The spatial pattern of the erosion inferred from the degree of chemical weathering in Lake Yakhi basin coincides with that from Lake Ulaan basin by Orkhonselenge et al. (2018a). In Lake Ulaan, CIA values had decreased since 3.0 kyr BP when the climatic conditions became drier (Lee et al. 2013), i.e., the CIA values may indicate moisture availability in the source areas of Lake Ulaan sediments (Orkhonselenge et al., 2018a). The degree of chemical weathering in the Lake Yakhi basin (fig. 9A) shows paleoweathering occurred in the source area at the arid climate (fig. 9B).

36The CIA values that decrease upwards with a decrease in kaolinite and an increase in feldspar (Zaid and Gahtani, 2015) are found in the Lake Yakhi sediments (fig. 9A). For shales (fig. 8A), Nesbitt and Young (1982) attributed CIA values of about 70–75. In the Lake Yakhi sediments the CIA values ranged at 50.06–58.04 on average from the cores Y18-3 to Y18-1 sediments (fig. 9A). According to Abayomi et al. (2016), the low CIA values suggest that some of the sediments could have been sourced from fresh basalt, granite (fig. 8B), granodiorite and feldspars (fig. 9A).

37Tonalites, granodiorites and granites, the source rocks of the Lake Yakhi sediments, are parallel to the A-CN line (fig. 9A), defining a steady-state weathering trend towards the plagioclase, which is probably due to the presence of Na-rich feldspar rather than diagenetic calcite. The source rock composition in the lake sediments is related to the composition of the cratonic clastic sedimentary rocks (fig. 6) that comprised gneisses of granodioritic to tonalitic composition. The low weathering conditions in the source area (fig. 9A) suggest the arid climate in the lake basin (fig. 9B) in the past.

6. Conclusion

38Landscape evolution in the Lake Yakhi basin of eastern Mongolia was considered with landform features and lake conditions. The paleoshorelines show the geomorphological history of paleo-Lake Yakhi, whereas the modern-day shorelines indicate the northward regression of the present lake. The rapid decrease of Lake Yakhi area by ~62.2 km² and the shift toward a playa lake during the last five decades may have directly related to the local climate trends in rising air temperature since 1987 and fluctuating precipitation since 1992. The major elements in the Lake Yakhi sediments show a mixture of derivation from sedimentary and igneous provenance signatures on the tectonic settings of the oceanic island arc and the active continental margin in the lake basin. The geochemical classification refers to arkose and shale derived from granitic (granites or felsic to granite basalt or felsic to mafic) and pre-existing sedimentary terrains. The A-CN-K plot shows the source rocks of granites, granodiorites and tonalities dominated in the lake sediments. The CIA values and chemical maturity reflect a low degree of chemical weathering in the source area and climate conditions shifting from semiarid to arid in the lake basin. The lacustrine geomorphology and geochemistry in the Lake Yakhi basin extended with the geochronology are essential for understanding paleohydrological dynamics of the lake and reconstructing regional paleoclimate change in eastern Mongolia.

*Corresponding author: Tel: +976-77307730-2453
rkhnslng@num.edu.mn (Orkhonselenge A.)