The Elements and Color of Lacustrine Record Revealed Lake Environmental Changes in Semiarid Northern China: A Case Study From Chagan Nur Lake of Southern Mongolian Plateau

The lakes at the southern Mongolian Plateau are sensitive to the variation of EASM changes during the Holocene, and hence the lacustrine records of these lakes provide prospects for understanding how lake environmental changes respond to EASM variation at different timescales. The interpretation of proxy indexes of lacustrine sediments is complex due to the various climatic or/and environmental factors influencing the processes during deposition and after deposition. In this study, the elements and colors of an 11.7-m lacustrine record from Chagan Nur at the southern Mongolian Plateau of northern China were analyzed to constrain the lake environmental changes during the Holocene. The results show that stable elements of Al, Si, K, Ti, Fe, and Rb are more related to surface erosion or/and vegetation coverage, and the variation of Sr is related to the lake-level change in Chagan Nur. The element and color of the lacustrine records show that a small and shallow lake environment occurred at Chagan Nur during 11.7–10.5 ka, the lake level had increased during 10.5–7 ka, and then the lake shrank with strength surface erosion during 7–6.2 ka and from 2 ka to the present. The lake level and surface erosion changes of Chagan Nur have shown a direct response to EASM precipitation changes as reconstructed from the pollen record from the Chagan Nur Lake and other records from northern China.


INTRODUCTION
East Asian summer monsoon (EASM) is the crucial atmospheric circulation system for East Asia, for it provided amounts of fresh water to all of the lives Chen et al., 2015;Chen et al., 2020). The research of variations and mechanism of EASM is of significance for understanding how human beings adapt to the climatic changes in EASM dominance in the future. The southern Mongolian Plateau located at the margin of EASM is an ideal place to study the mechanism and variation of EASM. The lake sediments in these regions are the ideal carriers to reconstruct the variations of EASM intensity in the past period (Jin et al., 2006;An et al., 2012;Zhang et al., 2016;Goldsmith et al., 2017;Wen et al., 2017;Li et al., 2020;Chen et al., 2021).
Chagan Nur Lake locates at the southern Mongolian Plateau, which is exactly located at the fringe of the modern EASM. The lacustrine sediments in this lake provide reliable records for detecting the variation of EASM during the Holocene. For example, Jiang et al. (2016) and Han and Liu (2017) have reported that the paleoclimatic variations were affected by the changes of summer insolation using the proxies of grain size and pollen from west Chagan Nur over the past 2 ka and 7 ka, respectively. Liu and Ha (2015) has reconstructed the lake level since the past 6.8 ka by using paleolake shoreline records, and it showed that the paleolake level was at least 7 m higher than the present and gradually shrank till now.  and Chun et al. (2018) has reported the variations of precipitation, temperature, and population in recent 50 years and further pointed out their effects on the changes of the lake level and vegetation coverage, and the results suggested that the variation of precipitation was the most fundamental factor for the recovery of vegetation and the rise of the lake level.
Element is one of the ideal proxies to detect the paleoclimate changes and can be superiorly and rapidly obtained by X-ray fluorescence (XRF) core scanning (Rothwell and Rack, 2006;Thomson et al., 2006;Liang et al., 2012). It has been widely applied in deep-sea sediments (Haug et al., 2001;Ziegler et al., 2008), lake sediments (Zhao et al., 2010;Li et al., 2016;Chen et al., 2021;Wu et al., 2012), and aeolian depositions (Liang et al., 2012;Liu et al., 2020). It has been proved to be well correlated with conventional XRF by contrast in lots of elements (Liang et al., 2012;Niu et al., 2019;. Different elements respond diversely to weathering, humidity, and temperature changes due to their own chemical and physical properties. The element ratio like Sr/Rb, Sr/Ca, Mg/Ca, Si/Fe, and Fe/Mn can be used to reflect the climate variation (Liang et al., 2012;Chen et al., 2021), and the single stable elements like Fe, Mn, Ti, Rb, and Zr also can be used to indicate the climate changes (Haug et al., 2001;Li et al., 2016). However, the paleoclimatic indications of elements under different climatic conditions are complicated and need to be further clarified or combined with other proxy indexes.
In this study, we presented the elements, color results of an 11.7-m lacustrine sediment core CG18B from Chagan Nur in the southern Mongolian Plateau. The changes of the lake level and surface erosion in the catchment during the Holocene were reconstructed. Furthermore, we discussed the relationship between lake environmental changes and the variation of EASM precipitation based on the pollen record from the same lake. Finally, it was figured out how the lake environment responded to EASM precipitation variation during the Holocene in northern China.

GEOGRAPHICAL SETTING
Chagan Nur (43°26′ 11.68″ N, 115°01′ 36.98″ E, 1,013 m a.s.l. Figure 1 and Figure 2), an inland-closed lake situates on the southeastern Mongolian plateau, northern China. It covers a total area of ∼ 108 km 2 consisting of two lakes, east and west, in which the west lake dried up in 2001 AD, only leaving the east lake with an area of 22 km 2 . The elevation of the Chagan Nur basin is lower than 1,500 m, with the Otindag Sandy Land lying to the south and the Abaga Lava Platform to the north, basically appearing as the pattern of high in the south and low in the north. The Gogusty and Engeer rivers originating from the Otindag Sandy Land from the southern direction provide most of the fresh water to Chagan Nur Lake in the rainy season (June to September). The Chagan Nur basin is dominated by the typical semiarid temperate continental climate. The mean annual temperature is ∼ 3.5°C. The annual precipitation in the basin is ∼ 270 mm Li et al., 2020), and more than 70% of the rainfalls occur in the rainy season (June to September) which is transported by the EASM from the Pacific and the Indian Ocean. The modern vegetation landscape in the region is typical dry steppes consisting primarily of grasslands, sandy deserts, and few forests.

MATERIALS AND METHODS
The X-ray Fluorescence Core Scanner The 11.7 m sediment core (CG18B) was retrieved in September 2018 from the center of east Chagan Nur Lake at the water depth of about 2.5 m by the UWITEC platform produced by Austria and then split it down from the middle in the lab. Before the scanning XRF measurement, the fresh sample surface was flattened and smoothed to eliminate scanning errors and then was quickly covered with preservative film to isolate the sample from the pollution of atmospheric dust. After the preparatory work, the elemental counts of the lake's core were measured by the XRF Core Scanner at the Key Laboratory of Western China's Environmental Systems (Ministry of Education) of Lanzhou University. The CG18B core was scanned in the conditions of 10 kV-10 mA, 5 mm, no filter, and 10 s scanning time to get the elements of Al, Si, S, K, Ca, Ti, and Fe and 30 kV-20 mA, 5 mm, Pd-Thick filter, and 20 s scanning time to get the elements of Cu, Zn, Ga, Br, Rb, Sr, and Pb (Richter et al., 2006;Weltje and Tjallingii, 2008). The validity of parameter setting was carried out by the inspection of instrument parameters. All elements were recorded by counts per second (cps).
A total of 27 bulk sediment samples were collected from CG18B core for accelerator mass spectrometry (AMS) radiocarbon dating. We compounded the graphite in the 14 C dating laboratory in Lanzhou University by the normative approach and dated by accelerator mass spectrometry (AMS) in Peking University. All 14 C ages were recalibrated to calendar years using the IntCal20 calibration curve (Reimer et al., 2020). The ages are expressed as years before present (BP) where "present" is defined as AD 1950. The top two samples (2 and 14 cm) were 184 and 136 cal yr BP. Therefore, we employed the 136 yrs as the reservoir age to correct all the ages. 10 luminescence dating samples are collected from the core CG18B for chronology determination in Lanzhou University. The measured luminescence dating ages from 728 cm, 926 cm, and 1,150 cm of the core are consistent to 14 C age, while other luminescence dating ages have shown a systematic underestimation comparing to their 14 C ages primarily due to the exact water content is underestimated. As a result, the chronology of core CG18 is  Frontiers in Earth Science | www.frontiersin.org September 2021 | Volume 9 | Article 713972 established by using all 14 C ages with Bacon age-depth modeling (Blaauw and Christen, 2011) and the results are modified in Figure 3 according to Li et al., 2020.

The Proxy Indexes of Color and Grain Size Measurement
The color and grain size from core CG18B are measured and analyzed at the Key Laboratory of Western China's Environmental Systems at Lanzhou University. The chroma of the samples was measured by using the Avaatech AV60626 XRF Core Scanner when the fresh samples were split immediately to avoid the influence of oxidation. A total of 469 samples from the core were used for grain size measurement. We treated the samples in the lab of Lanzhou University and followed the ways of Peng et al. (2005). For each sample, the 0.2-0.25 g dry sample was treated with 10% H 2 O 2 in the heating plate at 220°C to remove the organic material first, and then we added the 10 ml of 10% HCl to remove the carbonate content. The samples were soaked in distilled water to remove the acidic ions and stand for 10 h. After siphoning off the upper liquids by a rubber hose, we added 10 ml of 0.1 mol/L (NaPO 3 ) 6 with an ultrasonicator for 10 min to disperse the samples and then tested the dispersed samples using Malvern Master Sizer 2000.

Lithology and Chronology
All of these sediments from the core CG18B are consisted of lacustrine clay and silty clay. The details of the sedimentary structure are as follows: the uppermost 2.4 m of the core is black clay interbedded with sandy clay layers at depths of 0.9-1.1 m and 1.7-2 m; the light brown to blue-gray clay is present at the depth of 2.4-6.4 m; at 6.4-11.7 m, there is black mud with no obvious structure but the color changes to light black in the upmost 7.4-6.4 m of the core; poorly sorted and poorly rounded gray sandy gravels occur at 11.7-11.8 m (Li et al., 2020).
Both the 14 C dating ages and luminescence dating ages show the lake sediments were deposited since the early Holocene (Figure 3, after 11.7 ka). An age jump occurred between the 14 C age of 2.086 ± 0.067 cal kyr BP at 264 cm and 6.274 ± 0.050 cal kyr BP at 305 cm in the core, indicating that the deposition hiatus occurred between the depths of 2.64-3.05 m. As a result, the age-depth FIGURE 4 | Lithology, 14 C ages, elements, color, and carbonate indexes data of core CG18B. model was established separately for the depths of 0-3 m and 3-11.7 m. The resulting age-depth model for core CG18B is illustrated in Figure 3.

The Results of Chroma and Elements
The measured elements of samples from the CG18B core are illustrated in Figure 4. The contents of Al, Si, K, and Ti vary with the depth. The relatively stable and high contents of Al, Si, K, and Ti occur at the depth of 11.7-5 m and then increase to the highest content in 5-3 m, whereas the contents of Al, Si, K, and Ti drop obviously at the depth of 3-0 m. Fe and Rb contents show the identical trend of the gradual decrease in 11.7-6.5 m and then appear as a slight peak at the depth of 6.5-5.5 m, and the contents keep stable and low in 5.5-3 m, followed by a distinct fluctuation at the depth of 3-0 m. The high values of Sr and Zr occur at the depth of 11.7-9.5 m and the contents slightly reduce at the depth of 9.5-5 m. The following variations of Sr and Zr at the depth of 5-0 m exhibit discrepancies, in which the highest Zr content occurs at the depth of 4-2.5 m, while the Sr content reaches the maximum at the depth of 3-1.5 m. Correlation analysis of elements data indexes of core CG18B are further classified by tree cluster analysis ( Figure 5). Al and Si (0.97), K and Ti (0.88), and Fe and Rb (0.86) are highly correlated (R 2 ＞0.8); Zr and Sr (0.78) and S and Br (0.57) also have moderate correlations. Therefore, all the elements are divided into five branches by the tree cluster analysis initially, and then Al, Si, K, Ti, Fe, and Rb are classified as the same class according to their variation tendency and correlation.
For chroma analysis (Figure 4), the indices of a* are used to represent the degree of redness from green (negative value) to red (positive value), respectively. Chromas in Chagan Nur lake sediments showed different patterns. Two peaks of a* can be recognized in 11.7-9 m and 6.5-3 m and two low ebbs are in 9-6.5 m and 3-0 m.

The Variation of Stable Element Ti Recorded Surface Erosion at Chagan Nur Basin
Al, K, Ti, Fe, and Rb have the stable chemical property which is almost unaffected by surface chemical weathering and almost shows the same change (Figure 4), they are regarded as rockforming elements and are enriched in coarse particle and only transported to the lake by surface erosion in semiarid and arid lake basin (Shen et al., 2013b;Li et al., 2016;Zhang et al., 2017;Tao. 2020;Chen et al., 2021). The result of mean grain size proxy also shows the similar change with these stable elements (see Figure 6, Ti and mean grain size). In 10.5-7.5 ka, Ti shows a relatively low content (about 12 × 10 3 count) correspondingly with the decrease in mean grain size which indicated the weak surface erosion during this period. In 11.7-10.5 ka and 7-6.2 ka, the high content of Ti (15 × 10 3 count) correspondingly with the increase in mean grain size suggested that the surface erosion became strong. Over the past 2,500 years, the value of Ti becoming much more unstable indicated that the surface erosion process became more complex in this period.
The results of pollen concentration from core CG18B also supported the surface erosion result indicated by Ti. It has been reported that pollen concentration is one of the reliable proxies to reflect the vegetation coverage or humidity changes in the past period (Liu et al., 2002;Jiang et al., 2006;Zhao and Yu, 2012;Shen, 2013a). The vegetation recovery improvement in the Loess Plateau that had been reported could efficiently stop the water loss and soil erosion . At the same time, loess profiles also have revealed the degraded climate can result in vegetation reduction and desertification (mean grain size increase) and even cause the lack of loess sediments by the strong wind erosion (Stevens et al., 2018;Hao et al., 2013), so that the vegetation coverage variation is the appropriate proxies to reflect the change of surface erosion.
As illustrated in Figure 6, the Ti, mean grain size, and the pollen concentration show a similar trend. The low content of pollen concentration but the high value of mean grain size during 11.7-10.5 ka and 7-6.2 ka indicated the poor vegetation coverage, while the high pollen concentration and the low level of mean grain size during 10.5-7 ka indicated the lush vegetation. Correspondingly, the high content of Ti in 11.7-10.5 ka and 7-6.2 ka indicated strong surface erosion but low in 10.5-7 ka suggested surface erosion became low. The same situation also has been reported in Huahai Lake (Li et al., 2016) and Hurleg Lake (Zhao et al., 2010), which revealed that stable elements were abundant when the grain size turned coarse.
In the past 2,500 years, the instabilities and complexities of the pollen concentration and grain size might be caused by human activities. Zhang et al. (2012) has reported that a number of human activities have been recorded in the Chagan Nur basin such as the agricultural activities causing the vegetation degradation during the past 2000 years. In addition, human activities could cause the unconformity between the sand The Sr Element, Color (a*) in CG18B Core Revealed the Lake-Level Change of Chagan Nur Lake During the Holocene Sr is a kind of active element which can be leaching from land surface during the wet and warm period by strong chemical weathering in typical monsoon area and then enrich in lake sediments Liang et al., 2012;Jin et al., 2006;Wu et al., 2012). However, Sr in arid Chagan Nur Lake shows the opposite result since 11.7 ka (Figure 7) that high lake level correlated with low content of Sr or shallow lake level correlated with high content of Sr and it might be affected by lacking precipitation (Li et al., 2020) or weak chemical weathering. Zhang et al. (1994) have presented that the variations of Sr element and salinity that generally exists in the linear positive correlation in the arid Qinghai Lake, which indicates that the saltier the lake water, the higher the content of Sr, correspondingly, and Sr/Ca extracted from Eucypris inflate also used to rebuild the lake level change of Qinghai Lake. Therefore, Sr content in arid Chagan Nur Lake may be influenced by lake-level variation since 11.7 ka. a* (redness) also has some correspondence with Sr, for the value of a* varies synchronously with the content of ferric iron mineral (Ji et al., 2005) which could be influenced by oxidation environment caused by lake level in Chagan Nur Lake. As illustrated in Figure 7, the content of Sr exhibited high value during 11.7-10 ka in keeping with the small and shallow lake and less precipitation, leading to the lake water dissolving more oxygen and keeping the lake in strong oxidized condition, so that the low lake level not only provided enough oxygen content to the lake water but also promoted the enrichment of Sr in the lake water. In 10-6.2 ka, the Sr content decreased or attenuated in line with the rising of precipitation and the lake level, correspondingly, a* kept the lowest value in 10.5-8 ka, which meant the high lake level could reduce the oxygen in lake water and attenuate the concentration of Sr in lake water. The high a* value appeared in 8-6.2 ka might have been caused by the hiatus that happened during 2.5-6.2 ka, which caused the lake sediments formed in 8-6.2 ka to be exposed to the air and kept a highly oxidized state in the dried-up lake environment.
The high values of Sr during 2-0 ka were corresponding with the shrunken lake level, high a* value, and poor precipitation during the late Holocene which was the same situation with 11.7-10 ka that the small-shallow lake level and precipitation enhanced the oxidized condition and enriched the content of Sr in lake water (Figure 7). FIGURE 8 | Regional comparison by Sr content, (A) Ti content, and (B) lake level (C, reorganize according Li et al., 2020) with precipitation from Gonghai (D, Chen et al., 2015) and Bayan Chagan Nur (E, Jiang et al., 2006).
Frontiers in Earth Science | www.frontiersin.org September 2021 | Volume 9 | Article 713972 6 The Element Indicated Surface Erosion and Lake-Level Changes and Compared With Precipitation Recorded by Pollen and Lake Level Changes Rebuilt by Paleolake Shorelines The lake environmental changes since 11.7 ka in Chagan Nur were reconstructed by proxy indexes of elements and color. The results suggested that the shallow lake with relatively strong surface erosion occurred during 11.7-10.5 ka. The weakening surface erosion, high lake level, and fresh lake water environment occurred during 10.5-7 ka. The strong surface erosion, relatively low lake level, and salinity lake water occurred in 7-6.2 ka and 2-0 ka of Chagan Nur Lake. The influence of human activities on the lake environmental changes has increased significantly during the past 2 ka.
As illustrated in Figure 7, the precipitation changes have been reconstructed by the pollen record from the Chagan Nur Lake. The results suggested that the variation of EASM dominated the paleoenvironmental changes in Chagan Nur basin at the margin of EASM dominance, which not only influenced the lake level but also influenced the vegetation coverage and surface erosion. The variations of precipitation reconstructed by pollen records from Chagan Nur (Li et al., 2020), Bayanchagan (Jiang et al., 2006), and Gonghai (Chen et al., 2015) are basically consistent (Figure 8) and show a spatial trend of decreasing from the south to north, which indicated that the strength of EASM is gradually reduced from southern China to northern China. During 11.7-10 ka, reconstructed precipitation by pollen showed a relatively low value, and the values of Sr and Ti kept at high level, indicating the EASM was weak and coarse grains were transported to the lake in this period. Pollen records from the Chagan Nur lake showed the herbaceous plant predominated in this region (Li et al., 2020), and the vegetation landscape was typical dry steppe. The less precipitation reduces runoff into the lake, and the poor vegetation coverage enhances surface erosion, both of which cause an increase in Ti and Sr contents in the lake. In 10-7 ka, the precipitation increased steadily and hit the peak in 8.5-7 ka, and the low values of Sr and Ti indicated that the EASM became strong, as a result of the rising lake level, the improved vegetation coverage, the fine mean grain size, and the higher ratio of tree pollen in the neighborhood area (Jiang et al., 2006;Wen et al., 2017;Han et al., 2020;Li et al., 2020). During 7-6.2 ka and 2.5-0 ka, the precipitation decreased rapidly and the content of Sr and Ti shot up which indicated the weakening of EASM, the degradation of vegetation, and the intensifying of surface erosion. Furthermore, the drought event happened in Chagan Nur during 6.2-2.5 ka which caused the depositional hiatus. When the lake reappeared in 2.5 ka, it might have been small and shallow as a result of the low lake water as indicated by the Sr value, shoreline record Li et al., 2020), and strong surface erosion indicated by Ti value, which suggested the weakened EASM causing the diminishing precipitation during this period. Actually, the EASM intensity of the monsoonal region in northern China was weakest during the late Holocene because the northern Hemisphere summer insolation was minimum during the Holocene (Laskar et al., 2004).

CONCLUSION
Based on the chronology obtained by the radiocarbon and luminescence dating, in combination with the results of elements, color from an 11.7 m Chagan Nur lacustrine core, we have reconstructed variations of the surface erosion and lake level on the fringe of modern EASM dominance in northern China during the Holocene. The results show that the lake environment during the Holocene can be identified into four stages. The strong surface erosion and low lake water environment occurred at 11.7-10.5 ka. During 10.5-7 ka, the weakened surface erosion, rising lake level, and a better land coverage environment occurred at Chagan Nur lake basin. After that, the surface erosion gradually strengthened during 7-6 ka and from 2.5 ka to present, and lake level declined obviously during the Holocene period.
The variation in lake basin surface erosion, lake level, and land coverage of Chagan Nur have correlated with the EASM precipitation changes during the Holocene, indicating that the variation of EASM during the Holocene directly dominated the changes of surface erosion and lake level in arid northern China. During the past 2 ka, human activity has played a key role in the environmental changes at Chagan Nur Lake Basin.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, and further inquiries can be directed to the corresponding author.