5.1 Sedimentation dynamics
The sedimentation rate in the eastern part of Lake Ulaan just near the downstream of Ongi River is remarkably higher than that of the western part (Fig. 8b) because the presently dried out Ongi River (Fig. 2g, h) may intermittently feed the lake with high meltwater discharge transporting sands (Fig. 9). The spits may block the sand transportation and water discharge, and it may slow down the fluvial sedimentation rate in the western part, where the filling by water is more significantly delayed than the eastern part (Figs. 3, 10). The eastern part tends to be covered with the water from the Ongi River and the groundwater for more extended periods and consequently have less aeolian deflation (Figs. 9, 10). The possibility of sand transport across the ice covering the lake in the Depression of Great Lakes in western Mongolia during the long winter (Stolz et al. 2012) when most of the sand storms caused by strong west and northwest winds (Hempelmann 2010) may support the aeolian sedimentations (i.e., deposition) in the Lake Ulaan area in winter and spring.
Lakes in the Depression of Great Lakes and the Valley of Lakes find high stands due to increased snowfall caused by a more humid climate resulting in a considerable glaciation at high elevations (Lehmkuhl and Lang 2001; Lehmkuhl et al. 2018b). This snow accumulation may contribute to the meltwater runoff in spring and early summer and the fluvial sedimentations in the lake’s eastern part (Fig. 9). However, the climate in the Lake Ulaan region still stays under the arid climate throughout the year (Figs. 11, 12) and permits deflation of the lake depression and strengthened aeolian sedimentation (Figs. 9, 10), probably increased by high wind velocities of up to 34–40 m/s (Amarjargal 2016) passing through the Valley of Lakes. The near-surface sediments of Lake Ulaan down to 3.9 m depth correspond to wind deposits (Lee et al. 2011) and are interpreted to be the lake terrain (Badarch et al. 2002). According to Lehmkuhl et al. (2018a), the silts deposited in Lake Ulaan during the humid middle Holocene may have been exposed to wind activity when the lake is dried out as a playa bed. The early to late Holocene sediments in the Lake Ulaan basin indicate the aeolian dominant sediment-transport mechanism (Lee et al. 2011). Moreover, the sediments in the Lake Ulaan basin were transported by local westerly winds blowing along the Valley of Lakes during the last 11.2 ka BP (Lee et al. 2013).
In general, the aeolian sedimentations predominated in the Lake Ulaan basin since the early Holocene. For instance, the fine aeolian sand at 4.2 m depth in a 6 m high terrace of Ongi River 55 km northeast of Lake Ulaan indicated that the deposition had already started during the late Glacial period (Lehmkuhl et al. 2018a). However, the fluvial sedimentation strengthened in the middle Holocene, and aeolian sedimentation increased during the late Holocene (Fig. 9). Some large alluvial fans or deltas may have formed around the Lake Ulaan basin in the past that are now buried by wind-derived surface sediments (Sternberg and Paillou 2015). The aeolian deflation in the Lake Ulaan basin during the Anthropocene (Figs. 9c, 10) had intensified since the 1950s because of the historical records of water coverage of Lake Ulaan (Table 4). Continuous deflations occurred in the Lake Ulaan basin in winter, spring, and autumn since that time, except for summer when the lake was filled due to rainfall in 1960, 1970, and 2013–2014 (Table 4, Fig. 2b, d). The lake has been an end-transfer base floor for aeolian sedimentations blown from the Depression of Great Lakes in the northwest through the Valley of Lakes, and also a continuous source for dust storms and suspension transport of silts towards the Pacific Ocean in the southeast.
5.2 Lake evolution history
Like other lakes in the Govi region, Lake Ulaan experienced significant changes in water level and coverage area during the late Quaternary. Historical analysis of changes in lake level and size (Table 4) shows that Lake Ulaan is highly sensitive to ongoing climate changes of precipitation, air temperature, and wind velocity. Lake Ulaan has experienced dramatic fluctuations of its extent over several thousand years. A giant paleolake of ~43 m depth might have existed at OSL-dated 162±13 ka, i.e., the paleolake formed after the Marine Isotope Stage (MIS) 6 (Lehmkuhl et al. 2018a; Table 4). The early time of a large lake covering an area of 19,000 km2 being associated with wetter climates corresponded to a water level at 1285 m a.s.l. (Sternberg and Paillou 2015; Table 4). Then the lake was defined as an intermediate-sized lake covering a surface of ~6,900 km2 at a water elevation of 1150 m a.s.l. (Sternberg and Paillou 2015; Table 4); however, the precise ages of such extended lakes are still unclear.
In the Holocene, the lake reduced to 1,700 km2 in the area, corresponding to a water level at 1070 m a.s.l. (Sternberg and Paillou 2015; Table 4) occurring as the stage of the lake before the present-day dry basin (Fig. 9). In the middle Holocene (Fig. 9a), Lake Ulaan occupied an area of approximately 500 km2 in size (Lehmkuhl et al. 2018a; Table 4). In the late Holocene (Fig. 9b), Lake Ulaan may have been exposed aerially for the first time. It coincides with the desiccated Lake Bayan Tukhum at 3.5 cal. ka BP (Felauer et al. 2012), ~105 km south from Lake Ulaan. If the remobilized and redeposited sediments are taken into account, the lake has been re-exposed to wind deflations at 0.6–1.3 cal. ka BP for the sites LU18-1, 2, 4, except for deep sediments at 2.4 cal. ka BP for the site LU18-3 (Table 2, Fig. 9c).
In the Anthropocene, Lake Ulaan may still have been a permanent lake (Table 4, Fig. 9c). The most recent high stand before the 1960s was when the lake level was maintained 2–3 m above the present dry lake bottom (Lehmkuhl et al. 2018a). The lake condition in addition to the former lakes determined by Murzaev and Bespalov before the 1950s (Table 4) is in accordance with the climate condition of the wettest epoch from the 1940s to the 1950s, reconstructed by Fang et al. (2010). Recently, Lake Ulaan has abruptly dried out and shrunk with sharply dropped areas (e.g., Davaa, 2015; Orkhonselenge et al. 2018a, b), and the former lake floor is partly covered by sedge plants (Fig. 2c). Once the largest lake in the Govi, Lake Ulaan was identified in 1991 but did not appear in the Landsat images since 2000 (Kang et al. 2015). According to Davaa (2015), Lake Ulaan disappeared in 2010 and recovered an area of 20.9 km2 in 2013 (Table 4). The frequent shrinkage of the lake since the 1950s has contributed to the aerial exposition of the lake floor to wind deflation and rapid redeposition (Figs. 9c, 10).
Although Lake Ulaan has experienced fluctuations of shrinking and/or filling depending on the annual precipitation and temperature (Orkhonselenge et al. 2018a) since the 1950s, the lake has finally shifted to become a playa (Fig. 9c). This was due to the strong westerly wind effects exceeding 20–25 m/s (Amarjargal 2016) and extreme over usage of groundwater by mining operation in the basin in addition to the rapidly rising air temperature (Orkhonselenge et al. 2018a). The playa condition (Fig. 9c) is reflected in intensified aeolian sedimentations (Lehmkuhl et al. 2018a), chemical weathering (Fig. 7b), and the climate change from semiarid to arid conditions (Fig. 11).
Although the eastern part of the lake is occasionally filled by the pulsating sedimentations by Ongi River during the heavy rainfalls, the lake basin has been thoroughly exposed to the westerly wind deflations (Figs. 9, 10). When the eastern part of the lake fills with water by Ongi River, it feeds the western part temporarily through the only channels C2 and C3 in the spit (Figs. 3, 10). Ongi River may fill only the eastern part, but due to the spit's presence, the lake water needs to reach the spit height before the two systems start connecting to each other fully. It implies that the filling of the western part should be delayed even though the groundwater feeds it (Fig. 10). The present playa condition of Lake Ulaan (Fig. 9c) is consistent with numerous other observations showing that most lakes in the eastern Govi have been exposed aerially and dried out entirely due to the recent rapid rise in air temperature and evaporation during the last two decades (Orkhonselenge et al. 2019).
5.3 Holocene climate changes
In Mongolia, there is a common trend with the warm and humid early Holocene, the humid early to middle Holocene, the arid middle Holocene, and the humid late Holocene reconstructed paleoclimate records (An et al. 2008). However, the Holocene climate change has differed in each region of Mongolia. For instance, the middle Holocene climate was recorded as humid in southern Mongolia and as arid in western Mongolia with well-developed ~9 m deep Lake Bayan Tukhum in southern Mongolia and a younger shallow Lake Ereen in western Mongolia (Grunert et al. 2009; Table 5). Moreover, the late Holocene climate in Mongolia was found as humid in northern Mongolia, and as an arid in southern Mongolia started since ~3.2 cal. ka BP and strengthened since 1.5 cal. ka BP (Orkhonselenge et al. 2018b; Table 5). The late Holocene climate in southern Mongolia at 1.5 cal. ka BP is more comparable with the results from northern China, while the paleoclimate pattern in northern Mongolia is much closer to the records from southern Siberia (Orkhonselenge et al. 2018b). In terms of the Holocene climate change in Mongolia, the climate in northern Mongolia has been remarkably in agreement with the paleoclimate changes in the East Asian winter monsoon (EAWM) and the mid-latitude westerlies dominated regions, whereas the climate in southern Mongolia has been coincident with the East Asian summer monsoon (EASM) and the westerlies dominated areas. For instance, the Holocene climate in the Govi region in southern Mongolia, including the Lake Ulaan area, coincides with the paleoclimate records from the EASM areas (Chen et al. 2008), showing a humid early Holocene and a drier late Holocene.
During the early Holocene, the paleoclimate in the Govi region is described relatively well (Table 5, Fig. 12). In the early Holocene, a humid and warm climate was recorded in lakes of southern Mongolia (e.g., Felauer et al. 2012; Lehmkuhl et al. 2018a). For example, Lake Ulaan contained an extensive network of paleohydrological complex (Holguin and Sternberg 2016) with a large area of 1,700 km2 in the early Holocene (Sternberg and Paillou 2015). This coincided with the most humid time in the Lake Ulaan area because the EASM occurred in the north of Lake Ulaan at that time (Lee et al. 2013; Fig. 12). Since the beginning of the Holocene, there had been a reduction in sediment yield due to vegetation cover in the lakes within the Valley of Lakes (Lehmkuhl and Lang 2001), where fluvial sands were in dominant production (Lehmkuhl et al. 2018a). Around ~8.5 ka BP lakes in the Valley of Lakes were extended (Lehmkuhl and Lang 2001) and held high water levels in the early Holocene (Komatsu et al. 2001).
In the middle Holocene, the paleoclimate in the Valley of Lakes in southern Mongolia is shown as a continuation of the humid early Holocene (Table 5, Fig. 12), i.e., it was humid between 11.0 and 4.0 cal. ka BP (Felauer et al. 2012). The humid climate at 6.0–2.7 cal. ka BP caused the high sedimentation rate of 4.6 cm/ka in the lake's margin for the western part of Lake Ulaan (Orkhonselenge et al. 2018b; Figs. 9a, 12). It matched with the wet climate predominated at 8.6–4.7 cal. ka BP in the Lake Ulaan area (Lee et al. 2013). Lake levels in the Valley of Lakes were high (Grunert et al. 2009; Lehmkuhl et al. 2018a) because the northern limit of the EASM around the north of Lake Ulaan was close to the southern Khangai Mountain Range, and it ended at 4.0 ka BP (Lee et al. 2013; Fig. 12). The presence of the EASM around the Lake Ulaan area is consistent with the stronger EASM system during the middle Holocene contributed to the high precipitation, high water tables, and the halophytic desert vegetation growing around saline ponds in the Ikh Nart area (Rosen et al. 2019), locating at the higher latitude than that of Lake Ulaan. The humid middle Holocene revealed at Lake Ulaan coincided with the evidence of relatively higher lake levels during the middle Holocene recorded at Lakes Dood, Khuvsgul and Gun (Dorofeyuk and Tarasov 1998) in northern Mongolia, and Lakes Uvs and Bayan (Grunert et al. 2000) in the northern margin of the Depression of Great Lakes in western Mongolia (Orkhonselenge et al. 2018b). The high lake levels throughout most of arid Central Asia, recorded at 8.5 and 6.0 cal. ka BP (Li and Morril 2010) may have been related to the high precipitation associated with the westerlies increased from the early to middle Holocene (Chen et al. 2008).
The late Holocene climate in the Govi region is recorded from lakes as arid (Table 5, Figs. 9b, 11, 12). The dry climate is shown by numerous studies in southern Mongolia (e.g., Felauer et al. 2012; Szumińska 2016). The dry climate since 3.2–2.7 cal. ka BP in the Lake Ulaan basin may have induced the slow sedimentation rate of 1.6–1.8 cm/ka in the western part of Lake Ulaan (Orkhonselenge et al. 2018b) and strengthened the recent shrinkage of the shallow lake (Fig. 9b, c), a phenomenon seems to be continuing up to the modern time. The dry climate in the Lake Ulaan basin (Figs. 9b, c, 11, 12) coincides with the weakened EASM after 4.3 cal. ka BP contributed to the vegetation changing into an increase of steppe grasses in the Ikh Nart area (Rosen et al. 2019) in the northeastern Govi. Moreover, the late Holocene dry climate in the Govi region (Table 5, Figs. 11, 12) has contributed to have the lakes to be dried out and exposed to wind deflations (Grunert et al. 2009), and the arid and warm climate after 4.0 ka BP has influenced aeolian activity and dune remobilization (Felauer et al. 2012). The reactivation in the Khongor sand dunefield of the Govi region during the late Holocene representing the ongoing aridity (Hülle et al. 2010). In the arid late Holocene (Table 5, Fig. 12) Lake Ulaan may have been exposed to wind erosion since 3.2–2.7 cal. ka BP (Fig. 9b), and aeolian deflation has strengthened particularly since 0.6–1.3 cal. ka BP (Fig. 9c). The phenomena in the Lake Ulaan basin is in agreement with the desiccation of Lake Bayan Tukhum at 3.5 cal. ka BP (Felauer et al. 2012) and dried-out Lake Juyan in northern China during the last 2.0 ka BP (Chen et al. 2008). The strengthened arid climate in the Govi region since 1.5 cal. ka BP (Orkhonselenge et al. 2018b) prevailed during the periods before and after the time of high-standing lakes in the Valley of Lakes at 1.4–1.5 ka BP (Lehmkuhl and Lang 2001), and the more arid climate conditions during the late Holocene might have enhanced dust emission (Lehmkuhl 2015).
In the Anthropocene, the Govi region’s climate is indicated as a dry (Table 5, Figs. 9c, 11). The trend in air temperature of the Govi region between 1961 and 2014 shows that the minimum air temperature in January was warmer than -21oC in 1961–1987 and -19oC in 1988–2014, whereas the maximum air temperature in July increased by 2–3oC (Dulamsuren 2016). A drought record from 1970 to 2006 across the Govi region using the Standard Precipitation Index (SPI) showed cyclical fluctuations with broadly wetter conditions in the 1970s and 1990s, a notably drier period in the 1980s and alternating wet-dry episodes in the 2000s, and an arid year in 2006 (Sternberg et al. 2011). The intense dryness or aridification causes the rapid shrinkage of lakes and a decrease in lake levels in the Govi regions (Figs. 2, 3, 9c, 11). The recent abrupt rising air temperature since 1995 and decreasing precipitation since 1987 in the Lake Ulaan area (Orkhonselenge et al. 2018a; Fig. 11) has caused the playa lake condition observed today (Fig. 9c). The trend is confirmed by a distinct tendency towards drier conditions since the 1980s reconstructed for the eastern central High Asia (Fang et al. 2010), and a statistically significant increase in the annual surface thawing index at a rate of 29°C-days/yr in Mongolia during the past 19 years, which is far greater than that in the high latitudinal regions of the Northern Hemisphere during recent decades (Wu et al. 2011).
By the end of this century, potential evaporation will rise by 200–300 mm/yr, with an increase in annual average surface temperature by 5–6oC, and the Govi region will extend 600 km toward the north (Dulamsuren 2016). This means that the present arid Govi region will shift into the dry desert, and the semiarid steppe regions will become arid Govi regions. This trend has been previously noted by Davi et al. (2015), showing that the current drought conditions in Mongolia associated with annual average temperature increase by ~2°C over the last 60 years have resulted in the expansion of desert areas from the warm and arid southern Mongolia towards central and northern regions of the country. The dry climate in the Govi region may rapidly contribute to the playa environments covering the semiarid steppe in a few years and result in a shortage of surface water resources, especially lakes and rivers. This trend has been confirmed by Lake Ulaan sediments showing the climate around the lake basin has been significantly shifted from the semiarid into the arid (Fig. 11).