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Changes in the lake-grassland ecosystem revealed by multiple proxies in a sediment core from Ganggeng Nur Lake, northern China

Published online by Cambridge University Press:  28 July 2023

Liang Li Open the ORCID record for Liang Li [Opens in a new window] ,
Zhi Zhang ,
Zhenyu Ni ,
Weiwei Sun ,
Xianqiang Meng ,
Enlou Zhang ,
Guangwei Zhu ,
Yunlin Zhang  and
Boqiang Qin Open the ORCID record for Boqiang Qin [Opens in a new window]
Liang Li*
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China University of Chinese Academy of Sciences, Beijing 100049, China
Zhi Zhang
Affiliation:
School of Ecology and Environment, YuZhang Normal University, 1999 Meiling Avenue, Nanchang 330022, China
Zhenyu Ni
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
Weiwei Sun
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
Xianqiang Meng
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
Enlou Zhang
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
Guangwei Zhu
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
Yunlin Zhang
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
Boqiang Qin
Affiliation:
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China
*
Corresponding authors: Liang Li; Email: lee911214@163.com; Boqiang Qin; Email: qinbq@niglas.ac.cn
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Abstract

As the main global terrestrial ecosystem component, grasslands are extremely sensitive to global climate change. With increasing human activities over the last century, grassland ecosystems have been degraded to different degrees. However, the evolution of lake-grassland ecosystems in recent centuries remains unclear due to the dearth of high-resolution records. Here, we present high-resolution lacustrine sediment grain size, pollen (Artemisia, Myriophyllum), Pediastrum, and n-alkane records from Ganggeng Nur Lake to investigate vegetation, lake evolution, and human effects in semiarid northern China. Four stages were identified from the last ca. 150 years: (1) the natural evolution stage (AD 1870–1945), in which there was a wet climate around Ganggeng Nur and the lake level rose from increased runoff; (2) the human disturbance stage (AD 1945–1967), in which the regional climate got drier and human activities began having a detectable effect on the grassland ecosystem; (3) the human transformation stage (AD 1967–2005), in which a completely arid climate coupled with the implementation of a series of land reclamation policies resulted in a large reduction in grassland areas, extensive soil erosion, exacerbated climate change, and shrinking lake areas; and (4) the posttreatment stage (AD 2005–2018), in which soil erosion was alleviated by policy implementation and a favorable humid climate.

Keywords

Lacustrine sedimentSediment grain sizePollenN-alkanesLake-grassland ecosystemHuman activity
Type
Thematic Set: Asian Climate
Information
Quaternary Research , Volume 117 , January 2024 , pp. 30 - 42
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Copyright
Copyright © University of Washington. Published by Cambridge University Press, 2023

INTRODUCTION

As an important component of freshwater resources, lakes are crucial for the reproduction and biodiversity of various organisms (Tao et al., Reference Tao, Fang, Zhao, Zhao, Shen, Hu, Tang, Wang and Guo2015; Feng et al., Reference Feng, Han, Hu and Chen2016). This is especially true for lakes of Inner Mongolia—a typical arid and semiarid region (Gao et al., Reference Gao, Ryan, Li and Sun2017) that accounts for ~6.8% of the total area in China (Ma et al., Reference Ma, Yang, Duan, Jiang, Wang, Feng and Li2011). Some of these lakes, such as Hulun Lake, play an irreplaceable role in maintaining biodiversity by protecting endangered species and migratory waterfowls (Liu et al., Reference Liu, Yin, Piao, Zhao, Engels and Ciais2013; Tao et al., Reference Tao, Fang, Zhao, Zhao, Shen, Hu, Tang, Wang and Guo2015), in addition to providing essential support for ecosystem services related to human well-being (Chen et al., Reference Chen, Chuai, Yang and Zhao2012; Li et al., Reference Li, Gao, Li, Yan and Xu2017). The last century has been a period of human activities combined with natural changes, especially rapid climate changes due to precipitation and temperature changes in recent decades, during which time global warming has greatly influenced changes in lakes, such as substantial lake shrinkage (Tao et al., Reference Tao, Fang, Zhao, Zhao, Shen, Hu, Tang, Wang and Guo2015; Chen et al., Reference Chen, Chen, Huang, Michishita and Xu2018; Wang et al., Reference Wang, Peng, Liu, Wu, Chen and Zhang2018). Lake shrinkage poses significant threats, including dust release, water salinization, and waterfowl decline, to the regional environment and ecosystems (Ma et al., Reference Ma, Duan, Hu, Feng, Li, Ju, Jiang and Yang2010; Liu et al., Reference Liu, Yin, Piao, Zhao, Engels and Ciais2013; Zhang et al., Reference Zhang, Yao, Piao, Bolch, Xie, Chen and Gao2017). Importantly, the climate of Inner Mongolia is influenced by the low-latitude East Asian summer monsoon (EASM), mid-latitude westerlies, and winter monsoon systems controlled by the Siberian High. The lake systems in this region are therefore sensitive to natural forcing factors and serve as valuable meteorological archives for revealing variations in the climate system (Chen et al., Reference Chen, Tao, Zhao, Jin, Wang, Li, Ren and Li2021).

China's natural grasslands, which cover 400 million hectares and account for 40% of the country's territory, not only play an important role in the construction of the national ecological environment, but also form the main basis of national ecological security and animal husbandry development in China (Yang et al., Reference Yang, Yang and Meng2016). According to Chinese government reports, 10% of the total grassland area in China was degraded by the 1970s, with the degraded portion rising to 30% in the 1980s and to 50% in the mid-1990s. By the 2000s, ~90% of Chinese grasslands were degraded to varying degrees (Waldron et al., Reference Waldron, Brown and Longworth2010).

Grassland ecosystems and lakes play an important role in supporting regional socioeconomic development. There is an inextricable link between the growth condition of grassland vegetation and change in lake water volume. In areas with strong vegetation water-supply capacity, the recharge of lake water by atmospheric precipitation and ice melt is stable, which ensures that the water volume of the lake changes with a stable or slightly increasing trend. However, in areas with poor vegetation water-holding capacity, seasonal rainfall or glacial meltwater will increase the total water volume of the lake and its coastal wetlands in the short term, but due to the low vegetation cover and poor water-holding capacity around the lake, the soil will erode and cause vegetation degradation as soil sinks into the lake.

However, the evolution of lake-grassland ecosystems in recent centuries remains unclear due to the dearth of high-resolution records. The Dali Lake Basin mainly consists of Dali Lake, Ganggeng Nur Lake, and several rivers. The area of Dali Lake shrunk from 225 km2 in 1983 to 191 km2 in 2018 (Li et al., Reference Li, Wang, Zhang, Wu, Zeng and Tuo2021). Grassland ecosystems also show varying degrees of degradation. In recent years, many studies have provided valuable references for studying climate and environmental changes in Dali Lake (Liu et al., Reference Liu, Deng, Xiao, Li, Paterson, Chang, Yi, Qin and Zhu2016; Fan et al., Reference Fan, Xiao, Wen, Zhang, Huang, Yue and Wang2019; Zhen et al., Reference Zhen, Li, Xu, Zhang and Zhang2021; Zhang et al., Reference Zhang, Shen, Zhang, Chen, Chen, Li and Zhang2023). However, most of these studies have focused on millennial- to centennial-scale environmental changes during the Holocene or the last deglaciation in the region, and there is a paucity of information from the last century, when human activities were most intense.

In this study, we use a sediment core from the freshwater Ganggeng Nur Lake in the Dali Lake Basin, in Chifeng City, Inner Mongolia (Fig. 1). We constructed a chronological framework from 210Pb and 137Cs dating and used multiple proxies of pollen, sediment grain size, and n-alkanes for comparative analysis to construct the climatic and environmental ecological changes on the scale of the past century. These results will be of great significance to the future ecological environment evaluation and resource utilization in this region, and will provide a reliable palynological study in revealing centennial- and decadal-scale changes of vegetation, climate, and human activities.

Figure 1. Location of Ganggeng Nur Lake in northern China and DEM (Digital Elevation Model) of the surrounding area. The sediment core sampling site in Ganggeng Nur Lake is indicated by the red star.

STUDY AREA

Ganggeng Nur Lake is located in Heshigten Banner, Chifeng City (116°53′29″E–116°57′52″E, 43°15′52″N–43°18′42″N), Inner Mongolia Autonomous Region, which is part of the Mengxin Plateau region, with a temperate continental monsoon climate and a frost-free period of 60–150 days/year. The lake area is ~21.5 km2, with an average depth of 1.7 m, and an altitude of 1247 m. Ganggeng Nur Lake is on the northern margin of Otindag Sand Land and is mainly fed by the Shali River.

According to 1960–2020 meteorological data from the Chifeng City Meteorological Station, the average annual temperature of Ganggeng Nur Lake is 7.5°C, with an average temperature of −8°C in winter (December–February) and 22.3°C in summer (June–August), and the multiyear average precipitation is 361.2 mm, with an average summer precipitation of 248.8 mm, accounting for ~69% of the total annual precipitation (Fig. 2).

Figure 2. Climate diagram showing average monthly temperature and precipitation at the Chifeng City meteorological station from AD 1960–2020.

The modern vegetation type of the Ganggeng Nur Lake region is grassland vegetation. The main vegetation types of the alkaline soils around Ganggeng Nur Lake are Phragmites australis, Tephroseris palustris, Typha latifolia, Hippuris vulgaris, Myriophyllum verticillatum, and others. The grassland vegetation types are mainly Eurasian steppe plants, such as Stipa grandis, mostly belonging to Asteraceae, Poaceae, Fabaceae, Rosaceae, Ranunculaceae, and others (Mo et al., Reference Mo, Zheng, Jin, Bao and Jia2019).

MATERIALS AND METHODS

Sediment core sampling

A 34-cm core, GGNE-2 (43°17′34.38″N, 116°54′51.04″E), was collected by a gravity corer from Ganggeng Nur Lake at a water depth of 2.2 m in July 2018. The sediment core was sliced at 1 cm intervals, wrapped in sterile plastic bags, and stored in the laboratory refrigerator. The entire core lithology is black clay.

Dating and environmental parameter analysis

The GGNE-2 core 210Pb and 137Cs dating and analyses of sediment grain size, n-alkanes, and pollen were carried out at 1-cm resolution. Of these, 210Pb, 137Cs, grain-size analyses, and n-alkanes were carried out at the State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences; pollen analysis was carried out at the Palynology and Paleoecology Laboratory of Nanjing University, Nanjing, China.

210Pb and 137Cs dating

The age model based on 210Pb and 137Cs was computed with the serac R package (https://github.com/rosalieb/serac; Bruel and Sabatier, Reference Bruel and Sabatier2020). The serac package uses the CRS (constant rate of supply) piecewise model, which simulates data from the CRS model combined with 137Cs values to identify the depth of the peak. The age-depth curves of the CIC (constant initial concentration), CFCS (constant flux constant sedimentation rate), CRS, and CRS piecewise models can be calculated and plotted based on measured 210Pb and 137Cs data to construct the best age-depth framework (Bruel and Sabatier, Reference Bruel and Sabatier2020).

Sediment grain size analysis

Approximately 0.5 g was taken from each sample. Then, 10% H2O2 was used to remove organic matter, and 10% HCl was used to remove carbonate. Then, 10 ml of 10% Na(PO3)6 solution was added as a dispersing agent. Finally, the sediment grain size distributions were measured using a Malvern Mastersizer 3000.

Extraction and analysis of n-alkanes

A 5 g (± 0.0001 g) subsample was extracted from each lyophilized sample and sonicated three times (15 min each) at 110°C with a mixture of 60 ml dichloromethane and methanol (9:1, v:v) and then concentrated for drying under nitrogen. Neutral lipids containing n-alkanes were extracted by n-alkane silica gel column chromatography using hexane. The n-alkanes were measured using an Agilent 8890 gas chromatography (GC) DB-5 GC column (30 m × 0.25 mm, 0.25 μm film thickness) with an external C8–C30 standard for quantification.

We chose an index, the proportion of aquatic n-alkanes (Paq) (Ficken et al., Reference Ficken, Li, Swain and Eglinton2000), calculated from the measured n-alkane data, which is defined as:

$${\rm Paq} = ({\rm C_{23}} + {\rm C_{25}})/({\rm C_{23}} + {\rm C_{25}} + {\rm C_{29}} + {\rm C_{31}})$$

Pollen analysis

Thirty-four subsamples of ~1 g each were collected for pollen analysis. One tablet of Lycopodium spores (27560 grains/tablet) was added to each subsample to calculate pollen concentration and pollen accumulation rate. All subsamples were then treated with 10% HCl to dissolve carbonate, 10% NaOH to dissolve organic fractions, and 40% HF to dissolve silicate, and then were filtered in an ultrasonic bath with a 7 μm sieve. Finally, the pollen concentrates were stored in glycerol. Pollen taxa were identified under a ZEISS light microscope at × 400 magnification, and at least 500 grains of terrestrial plant taxa were counted for each subsample. Identification was aided by published literature (Xi and Ning, Reference Xi and Ning1994; Jankovská and Komárek, Reference Jankovská and Komárek2000; Tang et al., Reference Tang, Mao, Shu, Li, Shen and Zhou2016).

The percentage of terrestrial pollen was calculated from the total terrestrial pollen taxa, while pollen percentages of wetland and aquatic herbs and algae were calculated using the sum of all pollen and spore taxa (Xiao et al., Reference Xiao, Yao, Hillman, Shen and Haberle2020). The pollen diagram was plotted using Tilia 2.6.1 (Grimm, Reference Grimm2004). CONISS was used to identify pollen assemblage zones (Grimm, Reference Grimm1987).

RESULTS

Sediment chronology

We analyzed the GGNE-2 210Pb and 137Cs data using the serac package. The results (shown in Fig. 3) indicated that the overall fluctuation of 210Pb in Ganggeng Nur Lake was small, with a peak value of 476.2 Bq/kg. The sediment accumulation rate (SAR) in GGNE-2 was 7.07 mm/year. Depths of 18 cm and 24 cm (these correspond to 17.5 cm and 23.5 cm, respectively, in Fig. 3, due to the setting in the model) were selected as the depths corresponding to the 137Cs outbreaks in 1986 and 1963, respectively, based on the chronological data from the CRS model and the peak value of 137Cs. The piecewise model was used to finalize the GGNE-2 age-depth framework. Based on this, we provide a discussion of the climatic and ecological changes in Ganggeng Nur Lake over the past 150 years.

Figure 3. The age-depth framework of core GGNE-2 based on 210Pb and 137Cs dating. The 137Cs outbreaks in 1986 and 1963 are indicated at depths of 18 cm and 24 cm, respectively. 210Pbex = excess 210Pb; NWT = nuclear weapons text peak; C = Chernobyl peak. Abbreviations used by the serac package: CRS = constant rate of supply; CIC = constant initial concentration; CFCS = constant flux constant sedimentation rate.

Sediment grain size

Sediment grain size over the last 150 years was classified and plotted according to the designations of clay (<4 μm), silt (4–64 μm), sand (>64 μm), median size and mean size (Fig. 4).

Figure 4. Grain-size distribution of core GGNE-2 in Ganggeng Nur Lake. Note grain size change before and after AD 1945.

The clay fraction ranged from 6.2–23.3%, with a mean of 16.1%; the silt fraction ranged from 63.5–78.5%, with a mean of 73.7%; and the sand fraction ranged from 2.3–25.7%, with a mean of 10.2%. The median size varied from 10.2–33.4 μm, with a mean of 17.1 μm; the mean size ranged from 8.7–28.4 μm, with a mean of 13.7 μm (Fig. 4). The main component of GGNE-2 by grain size was silt. The trends of clay and sand variation were opposite, while the median and mean grain sizes were consistent with the sand content variation, indicating that the GGNE-2 sediment grain size was mainly influenced by sand variation.

The variation in each of the grain-size components is evident. The sand, median size, and mean size contents were higher, and the clay content was lower prior to AD 1945, while the sand, median size, and mean size contents decreased, and the clay content increased after AD 1945.

Pollen data

Forty-one terrestrial pollen taxa, three aquatic pollen taxa, and one algae taxon (Pediastrum) were identified in the GGNE-2 core (Fig. 5). The main terrestrial pollen taxa were Pinus, Corylus/Carpinus, Betula, Alnus, Poaceae, Artemisia, Aster, Brassicaceae, Apiaceae, Ranunculaceae, Humulus, Caryophyllaceae, Lamiaceae, Chenopodiaceae, and Cyperaceae, while the aquatic pollen taxa were mainly Typha and Myriophyllum. The terrestrial plants of the GGNE-2 core were characterized by Pinus, Chenopodiaceae, and Artemisia. Pollen records can be divided into four zones according to CONISS, the major pollen percentage diagram, and A/C (Fig. 5).

Figure 5. Diagram of pollen percentages and A/C (Artemisia/Chenopodiaceae) in core GGNE-2. Shadows show the exaggerations of the percentage data to make the key taxa more visible. Zone boundaries indicated by red, horizontal lines. Zone 1 = natural evolution stage (AD 1870–1945); Zone 2-a = human disturbance stage (AD 1945–1967); Zone 2-b = human transformation stage (AD 1967–2005); Zone 2-c = posttreatment stage (AD 2005–2018). Poaceae <37 = Poaceae <37 μm.

Zone 1: the natural evolution stage (AD 1870–1945, 34–28 cm)

In this zone, the proportion of upland herbs is the highest (~85.5%), dominated by Artemisia and Chenopodiaceae; the proportion of trees and shrubs is ~14.5%. The proportion of wetland herbs (Cyperaceae) is ~3.5% and the proportion of aquatic plants is ~1.8%, of which Myriophyllum is the largest (~1.7%); Pediastrum has a higher proportion of the total (~61.2%). Zone 1 is characterized by the highest percentage of Artemisia (~69.5%), the lowest percentage of Chenopodiaceae (~9.4%), and a high percentage of Pinus (11.8%).

Zone 2-a: the human disturbance stage (AD 1945–1967, 28–23 cm)

In this zone, the percentage of upland herbs (87.5%), trees and shrubs (12.5%) change little; the percentage of wetland herbs (Cyperaceae) increases to 4.5%; the percentages of Myriophyllum (0.9%) and Pediastrum (~33.4%) decrease.

Among the three dominant terrestrial taxa, Artemisia, Chenopodiaceae, and Pinus, the content of Pinus (8.8%) and Artemisia (64.6%) start to show a decreasing trend, while the content of Chenopodiaceae (14.5%) increases. However, the percentage of Artemisia is still ~50%, which is higher than in either Chenopodiaceae or Pinus.

Zone 2-b: the human transformation stage (AD 1967–2005, 23–9 cm)

The proportion of upland herbs is slightly lower in this zone (83.6%). The proportions of trees and shrubs (16.4%) and wetland herbs (Cyperaceae, 6.2%) increase, while Myriophyllum (0.5%), and Pediastrum (12.5%) decline.

Artemisia, Chenopodiaceae, and Pinus were still the dominant terrestrial taxa in this zone. The content of Artemisia shows a continuously decreasing trend and reaches the lowest value (51.5%), while the content of Chenopodiaceae increases and reaches an unprecedented high value (23.6%). Pinus also shows an increasing trend (11.6%).

Zone 2-c: the posttreatment stage (AD 2005–2018, 9–0 cm)

The proportions of trees and shrubs (16.6%), upland herbs (83.4%), wetland herbs (Cyperaceae, 5.9%), and aquatic plants (0.7%) change little, while Pediastrum increases in this zone (17.9%). Among the three dominant terrestrial taxa in this zone, Artemisia, Chenopodiaceae, and Pinus, the percentage of Chenopodiaceae (21.8%) and Pinus (10.5%) decline while Artemisia (52.5%) increases.

Distribution of n-alkanes

A range of n-alkanes (nC17–nC33) was detected in the GGNE-2 core. Many studies have classified n-alkanes into three categories according to chain length: long-chain n-alkanes, medium-chain n-alkanes, and short-chain n-alkanes. Long-chain alkanes are defined as n-alkanes with carbon numbers greater than 26, short-chain n-alkanes are concentrated in n-alkanes with carbon numbers less than 21, and n-alkanes with carbon numbers between long-chain and short-chain are defined as medium-chain n-alkanes (Ficken et al., Reference Ficken, Li, Swain and Eglinton2000; Sojinu et al., Reference Sojinu, Sonibare, Ekundayo and Zeng2012; Fang et al., Reference Fang, Wu, Xiong, Li, Du, An and Wang2014). Variation in the percentages of short-chain (nC17–nC20), medium-chain (nC21–nC26), and long-chain (nC27–nC33) n-alkanes with age are shown in Figure 6.

Figure 6. The proportions of n-alkanes in sediment core GGNE-2.

The proportion of short-chain n-alkanes ranged from 4.8–12.6% with a mean of 7.2%; medium-chain n-alkanes ranged from 35.4–58.4% with a mean of 43.4%; and long-chain n-alkanes ranged from 33–59.7% with a mean of 49.9%. GGNE-2 was dominated by medium-chain and long-chain n-alkanes with a slight variation in short-chain n-alkanes.

Two stages can be distinguished based on changes in the content of the individual n-alkane components: (1) high content of short-chain (8.5%) and medium-chain (52.5%) n-alkanes with low content of long-chain n-alkanes (39%) before AD 1945, and (2) increased content of long-chain n-alkanes (52.3%) while the content of short-chain (6.8%) and medium-chain n-alkanes (40.9%) decreased after AD 1945.

DISCUSSION

Paleoenvironmental significance of environmental proxies in Ganggeng Nur Lake

In lacustrine sediment studies, different grain sizes in different regions imply different environmental changes, including changes in regional precipitation (Fan et al., Reference Fan, Jiang, Shi, Guo, Zhang, Wei and Xu2021; Li et al., Reference Li, Wang, Zhang, Wu, Zeng and Tuo2021), lake level fluctuation (Xiao et al., Reference Xiao, Chang, Si, Qin, Itoh and Lomtatidze2009, Reference Xiao, Fan, Zhou, Zhai, Wen and Qin2013; Dong et al., Reference Dong, Li, Li, Wang, Lu and Ning2022), dust storms (Chen et al., Reference Chen, Chen, Zhang, Chen, Wang, Gowan and Qiang2020), and glacial advance and retreat (X. Zhang et al., Reference Zhang, Xu, Li, Xie and Gleixner2021). Furthermore, what lacustrine sediment grain size indicates about the environment is not the same at different timescales and resolutions (Chen et al., Reference Chen, Wan, Zhang, Zhang and Huang2004). It is generally accepted that in long-term, low-resolution sequences (centennial and millennial scales), coarse grain size indicates low lake water levels, reflecting an arid climate, while fine grain size indicates high lake water levels, reflecting a wet climate (Xiao et al., Reference Xiao, Chang, Si, Qin, Itoh and Lomtatidze2009; Fan et al., Reference Fan, Jiang, Shi, Guo, Zhang, Wei and Xu2021). In contrast, the available interpretations for short-term, high-resolution (interannual, decadal) series are that coarse grain size indicates more precipitation and a wetter climate, and that fine grain size indicates less precipitation and a dryer climate (Liu et al., Reference Liu, Wang and Shen2003; Zhang et al., Reference Zhang, Jin, Chen, Battarbee and Henderson2003; Chen et al., Reference Chen, Wan, Zhang, Zhang and Huang2004; Wang et al., Reference Wang, Yang, Zhao, Chen, Chen and Wang2011; Li et al., Reference Li, Ming, Zhang, Duan and Zhang2014). However, the climatic significance of coarse and fine grain sizes varies depending on the lake area, topography, watershed vegetation cover, and other influencing factors.

Considering the small area of Ganggeng Nur Lake, coupled with the confluence of the Shali River into the lake, an increase in precipitation on a short time scale would cause surface runoff to develop, and the stronger mechanical transport could bring coarse grains into the lake basin where they would settle, leading to coarser sediment grains. We interpret the grain size change in Ganggeng Nur Lake simply to indicate regional precipitation change as well as the lake water level.

N-alkanes are common components of plant waxes and can originate from algae, bacteria, phytoplankton, macrophytes, and terrestrial vascular plants (Ratnayake et al., Reference Ratnayake, Suzuki, Okada and Takagi2006; Fang et al., Reference Fang, Wu, Xiong, Li, Du, An and Wang2014; Wang et al., Reference Wang, Yang, Zhang, Xu and Wu2015; Seopela et al., Reference Seopela, McCrindle, Combrinck and Augustyn2020). The proportion of aquatic n-alkanes (Paq) is an index based on n-alkanes that assesses the relative contribution of submerged/floating aquatic plants to terrestrial plants in lacustrine sediments (Ficken et al., Reference Ficken, Li, Swain and Eglinton2000). Studies of Paq in lacustrine sediments have shown that Paq is sensitive to water depth and can be used to reflect fluctuations in lake water levels (Jiang et al., Reference Jiang, Meng, Liu, Wang, Kolpakova, Krivonogov, Song, Zhou, Liu and Liu2021). A high Paq suggests an association with lake expansion due to high precipitation, resulting in the input of large submerged/floating aquatic macrophytes into lacustrine sediments, while a low Paq indicates low precipitation and lake shrinkage, resulting in less aquatic plant input (Sun et al., Reference Sun, Xie, Shi, Zhang, Lin, Shang, Wang, Li, Liu and Chu2013; Chu et al., Reference Chu, Sun, Xie, Lin, Shang, Zhu and Shan2014).

Arid and semiarid regions are sparsely vegetated, and grassland vegetation generally grows mainly in riparian environments (Gong et al., Reference Gong, Ma, Gui, Yuan and Lu2007; Metrak et al., Reference Metrak, Sulwinski, Chachulski, Wilk, Suska-Malawska, Öztürk, Hakeem, Faridah-Hanum and Efe2015). Among the vegetation taxa, Artemisia and Chenopodiaceae are the dominant herbaceous taxa in arid and semiarid areas. Since Artemisia requires more moisture than Chenopodiaceae during the growing season, the Artemisia-Chenopodiaceae ratio (A/C) can be used in arid and semiarid areas to indicate dry-wet variation. A/C was proposed by El-Moslimany (Reference El-Moslimany1990), who pointed out that A/C needs to be used with caution and can only be applied within narrow geographical limits and within non-forested areas. Zhao (Reference Zhao, Liu, Li, Huang, Sun, Zhao, Herzschuh and Tang2012) found that A/C is applicable to study areas with annual precipitation <500 mm and where the summed percentage of Artemisia and Chenopodiaceae is greater than 45–50%. The annual precipitation in the area of Ganggeng Nur Lake is 361.2 mm, which is less than 500 mm, and the sum of Artemisia and Chenopodiaceae in the GGNE-2 core is 76%, which is much higher than 50%. The percentage of arboreal pollen in the GGNE-2 core is very low (<20%), which indicates that there is no forest near the study site. In conclusion, A/C can be used to give a qualitative indication of humidity variation in Ganggeng Nur Lake.

Pediastrum is a common genus of green algae that grows mainly in freshwater environments, especially in ponds, swamps, and lakes (Whitney and Mayle, Reference Whitney and Mayle2012; Wu et al., Reference Wu, Zhou, Chen, Yu, Zhang and Sun2015a). Related studies have shown that the abundance of Pediastrum increases with increasing lake water level (Zhao et al., Reference Zhao, Yu, Chen, Ito and Zhao2007; Turner et al., Reference Turner, Zhu, Lü, Peng, Ma, Wang, Hou, Lin, Yang and Frenzel2016). Myriophyllum is a genus of aquatic plants that is often found in standing or slow-moving water environments (Hussner et al., Reference Hussner, Meyer and Busch2009). In aquatic ecosystems, Myriophyllum pollen can be used to infer water-level fluctuations in shallow lakes (Shulmeister and Lees, Reference Shulmeister and Lees1995; Prebble et al., Reference Prebble, Sim, Finn and Fink2005). CCA (Canonical Correspondence Analysis) analysis of pollen from cores in Baiyangdian Lake showed that water level was more closely related to Myriophyllum, with increases in Myriophyllum pollen content indicating deeper lake water (Guo et al., Reference Guo, Yang and Chen2012). The general trends of Pediastrum and Myriophyllum in the GGNE-2 core were consistent with Artemisia and opposite of Chenopodiaceae, therefore we suggest that Pediastrum and Myriophyllum can be simple indicators of changes in lake water level here.

Since Artemisia pollination is anemophilous, short-term changes in Artemisia content are influenced by wind speed. Based on this, we selected wind speed, the difference between evaporation and precipitation (E−P) in meteorological data for Chifeng City (which indicates humidity), A/C, and the percentage of Artemisia in core GGNE-2 for comparison from AD 2005–2018 (Fig. 7). The percentage of Artemisia, A/C, and wind speed show a clear inverse relationship and a better positive relationship with E−P, which indicates that vegetation changes in this stage were influenced by the combined effect of wind speed and humidity.

Figure 7. Comparison of wind speed, E−P(difference of evaporation and precipitation) at the Chifeng City meteorological station, Artemisia pollen content, and A/C in core GGNE-2, from AD 2005–2018.

Vegetation and paleoenvironmental reconstruction of Ganggeng Nur Lake

The vegetation and paleoenvironmental evolution of Ganggeng Nur Lake over the past 150 years were reconstructed (Figs. 5 and 8) and are discussed below in the sections on the recognized stages in the area.

Figure 8. Comparison of various environmental proxies in core GGNE-2. (a–d) Percentages of Artemisia, Chenopodiaceae, Myriophyllum pollen, and Pediastrum; (e) A/C (Artemisia/Chenopodiaceae); (f, g) median size and mean size, respectively; (h) proportion of aquatic n-alkanes (Paq).

Stage 1: the natural evolution stage (AD 1870–1945, 34–28 cm)

The contents of Artemisia, Myriophyllum, and Pediastrum reached the highest values with the coarse grain size, high A/C, and Paq, indicating that the vegetation type of Ganggeng Nur Lake was grassland, coupled with a humid climate and high lake level. Artemisia was the main dominant species in this stage.

Stage 2 (AD 1945–2018, 28–0 cm)

The proportions of Artemisia, Myriophyllum, and Pediastrum showed a decreasing trend with the fine grain size, low A/C, and Paq compared to Stage 1. The proportion of Artemisia is still very high, indicating that the vegetation type of Ganggeng Nur Lake was grassland with Artemisia as the main dominant species in this stage, coupled with a dry climate and low lake level compared with Stage 1. Stage 2 can be further divided into three stages (Stage 2-a–2-c) based on Myriophyllum, Pediastrum, and A/C changes as follows (Fig. 5).

Stage 2-a: the human disturbance stage (AD 1945–1967, 28–23 cm) The content of Artemisia, Myriophyllum, Pediastrum, and A/C showed a slight decrease in Stage 2-a compared to their content in Stage 1. The proportion of Artemisia was still higher than Chenopodiaceae, indicating that Artemisia was still the main dominant genus for the grassland, and that the climate in this stage was in the wet-dry transition period.

Stage 2-b: the human transformation stage (AD 1967–2005, 23–9 cm) The lowest percentages of Artemisia, Myriophyllum, and Pediastrum with low A/C at the same time suggest that the climate in this area became dry and the lake level was low.

Stage 2-c: the posttreatment stage (AD 2005–2018, 9–0 cm) The ecosystem was still a grassland ecosystem where Artemisia and Chenopodiaceae were the dominant taxa. The proportion of Artemisia increased compared to in Stage 2-b, while Chenopodiaceae decreased, the aquatic plant Myriophyllum did not change much, and Pediastrum and A/C showed increasing trends compared to Stage 2-b, indicating that the climate became slightly wetter in Stage 2-c compared to Stage 2-b.

The chronological results obtained from the GGNE-2 core show that sediment was deposited during the modern period, which includes the modern instrumental observation period. Precipitation and evaporation data for the last 60 years in Chifeng City from the meteorological station were selected for analysis to verify the accuracy of the climatic environmental changes revealed by the proxies used in this study (Fig. 9). The difference between evaporation and precipitation (E−P) was calculated and used to represent the humidity change in this area.

Figure 9. Comparison of records in core GGNE-2 and other indicators. (a) The difference between evaporation and precipitation (E−P) and its five-year moving average for Chifeng City; (b) A/C in sediment core GGNE-2; (c) proportion of Pediastrum in sediment core GGNE-2; (d) median size in sediment core GGNE-2; (e) proportion of aquatic n-alkanes (Paq) in sediment core GGNE-2. Vertical shaded area on the left represents AD 1945 to AD 2005; vertical shaded area on the right = AD 2005 to AD 2018.

E−P and its five-year moving average for Chifeng City show that the climate in Chifeng City was arid from the beginning of the modern instrumental record to AD 2005. After AD 2005, climate shifted to a more humid environment, which is consistent with the stage of climate change revealed by pollen. This suggests that pollen is comparatively more accurate in revealing changes in the climatic environment over the past 150 years.

Regional comparison and the effect of human activity

Because the last century has been a period of human activities combined with natural changes, the effect of human activities in Ganggeng Nur Lake over the last 150 years is inevitable. We selected the population of China in the last 150 years, the population of Inner Mongolia, China, since AD 1949, the utilization rate of cropland and grassland after cropland retirement in Inner Mongolia since AD 2000 (Yin et al., Reference Yin, Pflugmacher, Li, Li and Hostert2018), the area of grassland and cropland in the Daihai Lake watershed in the last 40 years (Y. Zhang et al., Reference Zhang, Xu, Liao, Han, Ji, Liang and Liu2021), and the A/C, median size, and Paq indices of Ganggeng Nur Lake for comparison (Fig. 10).

Figure 10. Comparison of Ganggeng Nur Lake proxies and other indicators. (a) The population of China over the past 150 years (red line, Jiang et al. Reference Jiang, Han, Guo and Gong2022) and the population of Inner Mongolia since AD 1949 (blue line); (b) the utilization rate of cropland and grassland after cropland retirement in Inner Mongolia since AD 2000 (Yin et al., Reference Yin, Pflugmacher, Li, Li and Hostert2018); (c) the area of grassland and cropland in the Daihai Lake watershed over the past 40 years (Y. Zhang et al., Reference Zhang, Xu, Liao, Han, Ji, Liang and Liu2021); (d) A/C in sediment core GGNE-2; (e) median size in sediment core GGNE-2; (f) proportion of aquatic n-alkanes (Paq) in sediment core GGNE-2. Vertical gray bar = AD 1967 to AD 2005.

Since the founding of the People's Republic of China, as the country recovered from the wounds of war, its economy began to recover, its population grew, and human activities gradually increased. From the establishment of the People's Republic of China in AD 1949 to the end of the twentieth century, an increasing number of natural grasslands in Inner Mongolia were converted into arable land due to the migration of agricultural populations from neighboring provinces and due to land reclamation policies for economic development (Wu et al., Reference Wu, Zhang, Li and Liang2015b; Yin et al., Reference Yin, Pflugmacher, Li, Li and Hostert2018). Moreover, the Inner Mongolian government implemented the ‘household contract responsibility system’ from AD 1983 to the beginning of the twenty-first century, which made overgrazing the direct and main cause of large-scale grassland degradation (Wu et al., Reference Wu, Zhang, Li and Liang2015b). Because grasslands play a role in water and soil conservation, large-scale grassland degradation led to large-scale soil erosion in Inner Mongolia. Coupled with the arid natural environment, environmental degradation was aggravated, and large amounts of silt accumulated, blocking rivers, and causing large-scale lake shrinkage. Given these conditions, China proposed the Grain for Green Project program in AD 1999 to control soil erosion and improve land quality in the face of economic development (Fu et al., Reference Fu, Wang, Liu, Liu, Liang and Miao2017). The program, which has been used to improve soil erosion through terracing, afforestation, and restoration of grassland vegetation, already has achieved positive results in northern China.

The effect of human activities on Ganggeng Nur Lake is consistent with that of northern China. A series of measures led to the large-scale degradation of grasslands in Inner Mongolia and exacerbated the climate of Ganggeng Nur Lake, which began to shift to arid from AD 1945–1967; especially after AD 1967, Ganggeng Nur Lake was in a more arid state, and the lake water level was lower. Implementation of the Grain for Green Project program improved local soil erosion, which allowed the grassland vegetation to recover during this period, and the climate slowly became wetter, and the lake level became more stable.

CONCLUSIONS

A study of sediment in the GGNE-2 core using 137Cs and 210Pb dating, sediment grain size, pollen, n-alkanes, and instrumental data from Chifeng City, Inner Mongolia, reconstructed the lake-grassland ecosystem change in Ganggeng Nur Lake over the past 150 years. Overall, our study provides a reference case for short-scale, high-resolution changes in a localized lake-grassland ecosystem. The results showed that vegetation types of the grassland ecosystem of Ganggeng Nur Lake in the Dali Lake basin over the past 150 years were mainly dominated by Artemisia, Chenopodiaceae, and Pinus, and that the lake-grassland ecosystem can be divided into four stages.

Natural evolution stage (AD 1870–1945)—The lake-grassland ecosystem with Artemisia as the dominant vegetation species, in which the climate was humid, the water level of the lake was high, and the effect of human activities was low.

Human disturbance stage (AD 1945–1967)—This stage was early during the establishment of the People's Republic of China, when the population began to increase, human effect on the local environment increased, and the climate transitioned from humid to arid.

Human transformation stage (AD 1967–2005)—The content of Artemisia in the vegetation type at this stage reached its lowest value, the climate was arid, and the lake level was low. During this stage, China converted increasingly more grassland into arable land to develop the economy, and overgrazing-exacerbated soil loss. The effect of human activities exacerbated the arid climate and lowered the lake level, and the lake-grassland ecosystem of Ganggeng Nur Lake steadily deteriorated.

Posttreatment stage (AD 2005–2018)—During this stage, the content of Artemisia showed an increase in the vegetation types, and the overall climate became more humid. Implementation of the Grain for Green Project program improved the lake-grassland ecosystem of Ganggeng Nur Lake.

Acknowledgments

Thanks to Yiman Fang of Zhejiang University for help with the paper and to the reviewers for their professional advice. This research was funded by the National Natural Science Foundation of China (41790423).

References

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Figure 0

Figure 1. Location of Ganggeng Nur Lake in northern China and DEM (Digital Elevation Model) of the surrounding area. The sediment core sampling site in Ganggeng Nur Lake is indicated by the red star.

Figure 1

Figure 2. Climate diagram showing average monthly temperature and precipitation at the Chifeng City meteorological station from AD 1960–2020.

Figure 2

Figure 3. The age-depth framework of core GGNE-2 based on 210Pb and 137Cs dating. The 137Cs outbreaks in 1986 and 1963 are indicated at depths of 18 cm and 24 cm, respectively. 210Pbex = excess 210Pb; NWT = nuclear weapons text peak; C = Chernobyl peak. Abbreviations used by the serac package: CRS = constant rate of supply; CIC = constant initial concentration; CFCS = constant flux constant sedimentation rate.

Figure 3

Figure 4. Grain-size distribution of core GGNE-2 in Ganggeng Nur Lake. Note grain size change before and after AD 1945.

Figure 4

Figure 5. Diagram of pollen percentages and A/C (Artemisia/Chenopodiaceae) in core GGNE-2. Shadows show the exaggerations of the percentage data to make the key taxa more visible. Zone boundaries indicated by red, horizontal lines. Zone 1 = natural evolution stage (AD 1870–1945); Zone 2-a = human disturbance stage (AD 1945–1967); Zone 2-b = human transformation stage (AD 1967–2005); Zone 2-c = posttreatment stage (AD 2005–2018). Poaceae <37 = Poaceae <37 μm.

Figure 5

Figure 6. The proportions of n-alkanes in sediment core GGNE-2.

Figure 6

Figure 7. Comparison of wind speed, E−P(difference of evaporation and precipitation) at the Chifeng City meteorological station, Artemisia pollen content, and A/C in core GGNE-2, from AD 2005–2018.

Figure 7

Figure 8. Comparison of various environmental proxies in core GGNE-2. (a–d) Percentages of Artemisia, Chenopodiaceae, Myriophyllum pollen, and Pediastrum; (e) A/C (Artemisia/Chenopodiaceae); (f, g) median size and mean size, respectively; (h) proportion of aquatic n-alkanes (Paq).

Figure 8

Figure 9. Comparison of records in core GGNE-2 and other indicators. (a) The difference between evaporation and precipitation (E−P) and its five-year moving average for Chifeng City; (b) A/C in sediment core GGNE-2; (c) proportion of Pediastrum in sediment core GGNE-2; (d) median size in sediment core GGNE-2; (e) proportion of aquatic n-alkanes (Paq) in sediment core GGNE-2. Vertical shaded area on the left represents AD 1945 to AD 2005; vertical shaded area on the right = AD 2005 to AD 2018.

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Figure 10. Comparison of Ganggeng Nur Lake proxies and other indicators. (a) The population of China over the past 150 years (red line, Jiang et al. 2022) and the population of Inner Mongolia since AD 1949 (blue line); (b) the utilization rate of cropland and grassland after cropland retirement in Inner Mongolia since AD 2000 (Yin et al., 2018); (c) the area of grassland and cropland in the Daihai Lake watershed over the past 40 years (Y. Zhang et al., 2021); (d) A/C in sediment core GGNE-2; (e) median size in sediment core GGNE-2; (f) proportion of aquatic n-alkanes (Paq) in sediment core GGNE-2. Vertical gray bar = AD 1967 to AD 2005.