Environmentally sensitive grain-size component records and its response to climatic and anthropogenic influences in Bosten Lake region, China

Using 137Cs and 210Pb dating and multi-proxy evidence from a 41-cm sediment core from Bosten Lake in China, the responses of sediment grain size to environmental changes were reconstructed over the past 150 years. After the end of the Little Ice Age, the climate of the Bosten Lake region became warmer and drier, and the lake water level decreased. The results indicated that the lowest water storage periods occurred at approximately 1920–1930 AD. Decreases in the Siberian High intensity and water vapour transport from the Indian Ocean during this period led to a reduction in the water vapour supply, which resulted in reduced lake levels in the period 1920–1930 AD. Then, the lake was at a high level until the 1960s. The water storage then declined in the 1960s. Since the 1960s, the contents of total organic carbon and total nitrogen have significantly decreased, which is closely related to the significant decline in water level and increased water salinity caused by enhanced water demands. Increased irrigation water demand as a result of expanding cultivated areas and climate change, coupled with a reduced input of water vapour, resulted in the worst water environment in approximately 1980–1990 AD. Since the late 1980s, the water level of the lake has risen, and the lake primary productivity of Bosten Lake has improved. Through the application of statistical methods to grain size data from Bosten Lake combined with the abovementioned data on climate change and human activities, two major potential factors influencing the grain size of terrigenous clastic material were revealed. The first factor, consistent with a grain size of 3.31 μm, is related to the recent increase in agricultural acreage in the Bosten Lake watershed and may reflect increases in atmospheric dust. The second factor, correlated with grain sizes of 11.48 μm and 69.18 μm, can be used to reflect changes in the lake hydrological state. It is suggested that the grain sizes of these lake sediments sensitively reflect changes in the hydrological characteristics of the basin and can be used to reconstruct the history of climate change and human activities.


Geographical Setting
The Bosten Lake basin lies between the Tian Shan Mountains and the Taklamakan Desert (Fig. 1, digital elevation data from the CGIAR-CSI SRTM 90 m Database 35 ) and has a typical arid climate 36 . The total annual precipitation is only 76.1 mm; however, evaporation amounts to 2000 mm year −1 18 . The Bosten Lake catchment comprises four counties, namely, Yanqi, Hejing, Heshuo, and Bohu, which have experienced rapid economic development over the past half century. Bosten Lake was once the largest inland freshwater lake in China, with a surface area of 1005 km 2 and a catchment area of 4.5 × 10 4 km 2 , 37 . The lake receives water from several rivers, including

Results
Variations in multiple environmental proxies from Bosten lake sediments. The specific activity of excess 210 Pb was determined by measuring the amount of 210 Pb and 226 Ra in the same layer. 210 Pb and 226 Ra were equilibrated at 39 cm, and the chronology of the sediment was established by the constant rate of supply (CRS) model with supported 210 Pb ex ( 210 Pb ex = 210 Pb-226 Ra) 40 . The specific activity of 137 Cs is shown in Fig. 2a. The global fallout of 137 Cs in 1954 AD 41 has been identified in the Lake Bosten sediment core at 29 cm. In general, the 137 Cs fallout peak should correspond to 1963 AD; however, the Lake Bosten watershed was a unique nuclear test zone in China. The core sediments recorded local bomb tests in 1976 AD 36 (the time horizon of 1976 AD was also recorded in BST04H 36 in Fig. 1) (Fig. 2c), and the results are in good agreement with the 210 Pb dating model 42 (Fig. 2b).
The variations in the organic geochemical proxies in the core sediments from Lake Bosten are shown in Fig. 3. The average value of magnetic susceptibility (MS) is 4.7 × 10 −8 m 3 ·kg −1 . Before 1955 AD, the MS is low, with an average of 3.5 × 10 −8 m 3 ·kg −1 . Since 1955 AD, the MS has increased obviously, with an average of 5.2 × 10 −8 m 3 ·kg −1 . The average content of total organic carbon (TOC) is 2.7%, with a maximum of 3.5% and a minimum of 2.1%. From the base to 1960 AD, there is an increasing trend in the TOC content. Then, the TOC content decreases until 1980 AD, with a minimum value of 2.1%. Since 1980 AD, the TOC content has gradually increased again. The average content of total nitrogen (TN) in the sediments is 0.53%, with a maximum of 0.75% and a minimum value of 0.40%. There are obvious similarities between the TOC and TN contents. The average C/N ratio in the sediments of Bosten Lake is 6.1. The maximum value is 8.4 in ca. 1960 AD. For Bosten Lake sediments, the Environmentally sensitive grain-size components extracted via the grain-size standard deviation method and factor analysis. Based on extraction via the grain size-standard deviation method ( Fig. 4), there are four peaks in the grain-size standard deviation curve, which are sensitively influenced by the sedimentary environments. The three boundaries for the sub-populations of Bosten Lake sediments were    Fig. 5.
Factor analysis is generally used to determine source apportionments and to identify environmental influencing factors in limnological studies [43][44][45] . As shown in Table 1, four factors explain 94.6% of the total variance. The factor scores of the first four factors are shown in Fig. 5. The four factors (F1, F2, F3 and F4) account for 43.6%, 23.2%, 18.7%, and 9.2% of the total variance, respectively. The factor score distributions for the abovementioned four factors are reported in Fig. 5.
The first factor (F1) (accounting for 43.604% of the total variance) has a good relationship with C1 (r = 0.896, p < 0.001) ( Table 2). F2 (accounting for 23.181% of the total variance) has a good relationship with C2 (r = 0.943, p < 0.001). The correlation between F3 and C3 is poor (r = 0.333); however, there is a good negative correlation between C2 and C3 (r = −0.687, p < 0.001). In addition, C4 has good correlations with F3 (r = 0.566, p < 0.001)  www.nature.com/scientificreports www.nature.com/scientificreports/ and F4 (r = 0.717, p < 0.001). The above analysis shows that F1 controlled the trend of the environmentally sensitive component C1. F2 controlled the trend of the sensitive component C2. There was a significant negative correlation between C2 and C3, which was affected by the same factor, F2. F3 and F4 together influenced the sensitive component C4. Additionally, because the content of the C4 component is too small, i.e., the content of sub-population S4 is less than 2.65%, F3 and F4 are not discussed separately.

Discussion
Changes in land cover and land use will change the grain size of terrigenous debris 46 , and a change in the lake hydrodynamic intensity will lead to a grain-size redistribution of terrigenous detritus in lake sediments 4 . The organic materials, including TOC and TN, in the lake sediments are mainly controlled by the initial productivity of the lake and by the input of terrigenous organic debris 47,48 . Different sources of organic matter have different C/N ratios. Aquatic phytoplankton are mainly rich in protein and organic nitrogen, so the C/N ratio is low (4 to 10), whereas terrestrial vascular plants are composed mainly of lignin and cellulose, with C/N values of 20-30 49 . The results suggest that the organic matter in the sediments of Bosten Lake was mainly from aquatic plants and therefore reflects the primary productivity in the lake. Starting the early 1960s, the TOC and TN contents significantly decreased in response to the significant decline in water level and increased water salinity caused by enhanced water demands. Since the late 1980s, the water level of the lake has risen, and the lake primary productivity of Bosten Lake has improved. The organic matter in the sediments of Bosten Lake does not reflect changes in land cover in the basin, and thus, it is impossible to explore the influence of changes in vegetation on the particle size of terrigenous debris.
The results of the Pearson correlation analysis show that the factor score F1 is correlated with the content C1 (the 3.31 µm size fraction) ( Table 2), indicating that cultivated land, through the alternation of the underlying surface, caused an increase in the atmospheric dust concentration [50][51][52][53] . This finding is supported by the fact that the results of the factor analysis show that the five-point running average of the factor score (F1) is consistent with that of the change in the agriculture acreage in the Bosten Lake watershed (Fig. 6). Thus, it can be concluded that increasing human activity caused an increase in aeolian dust deposition in Bosten Lake. Increases in MS values in lake sediments suggest higher land surface erosion in the lake watershed resulting from human activity 54 . From the MS variation (Fig. 3), the land surface erosion caused by human activities in the Bosten Lake watershed has increased significantly since the 1950s. Land surface erosion caused by human activities is superimposed on the natural state, which could also influence the grain size of terrestrial debris. However, the terrigenous clastic materials that entered the lake through surface runoff are affected by the hydrological conditions of Bosten Lake. This can be inferred from the variation in factor F2. The correlation analysis results suggest that the contents C2 (the 11.48 μm size fraction) and C3 (the 69.18 μm size fraction) are mostly sensitive to the influences of factor F2. The five-point running average curve of the factor score F2 is consistent with the trend of lake water storage and therefore reflects environmental aspects of the lake, including lake water level, surface area, and lake inflow runoff.
During the Little Ice Age (1500-1900 AD), the climate was cold and humid in this region 36 . Since 1900 AD, the Bosten Lake region has been warm and dry 36 , and the lake water level has decreased. From the five-point running average curve, we can detect two significant periods with the lowest water storage, that is, 1920-1930 and 1980-1990. The regional drought in the 1920s has been studied using other records from lake sediments and tree rings [55][56][57] , which show that the climate was dry during this period, indicating that the drought was a regional event. In previous research by Shi et al. (2007), a warm-moist transition was found in 1985 AD using instrumental data and data regarding changes in glaciers and lakes over the past 50 years in arid northwestern China 58 .
As highly vulnerable terrestrial ecosystems, the arid regions of Central Asia are heavily influenced by the westerly circulation, the Arctic Ocean airflow and the Indian Monsoon Current, making them sensitive to global changes 59,60 . On the one hand, winter temperatures in this region have a strong relationship with the Siberian High 61 , and an increase in winter temperatures is not conducive to the formation of precipitation. Additionally, weakening of the Siberian High can result the transport of less water vapour from the Arctic Ocean to Xinjiang. During 1920-1930, the Siberian High was weak 62 , and the precipitation in southern Xinjiang was minimal. In addition, it can also be seen that with weakening of the intensity of the Indian monsoon (Fig. 6), the amount of water vapour entering Xinjiang is bound to decrease. In the late 1980s, the North Atlantic Oscillation (NAO) was in the negative phase 63 , as shown in Fig. 6 Table 2. The Pearson correlation coefficient matrix among the four factor scores (F1, F2, F3, and F4) and the contents of the four peaks (C1, C2, C3, and C4) extracted from the curve of the standard deviation versus grain size. a Correlation is significant at the 0.01 level (2-tailed).
crosses the western mountains into Xinjiang, producing precipitation there. When the NAO is in a negative phase, the westerly wind is relatively weak 64,65 , enabling only a small amount of southwestern warm and humid air to enter Central Asia and resulting in less precipitation there. It can also be seen from Fig. 6 that during this period, the Siberian High was low 62 , and the Indian monsoon was weak 66 , resulting in a decrease in the Indian Ocean water vapour entering Xinjiang. However, since the late 1980s, decreases in water consumption due to drip irrigation, mulching and other agricultural water-saving technologies and enhanced watershed precipitation 67 have induced an increase in water levels.
The greatest influences on the lake and its catchment since the Little Ice Age have been anthropogenic 68,69 . The characteristics of the formation of and changes in the ecological environment since the Little Ice Age can be used to identify the impact of human activities and to predict future environmental changes. A comparison of the Bosten Lake watershed 15 and the Aral Sea basin 70 over the past 50 years reveals that these regions have experienced similar climatic trends. It is interesting that the water storage in these two lakes continued to decline in the 1960s, and the water environment of both lakes was the worst in approximately 1980-1990. A pumping station was constructed in the eastern part of Lake Bosten (Fig. 1) in 1983 AD to adjust the outflow to the Peacock River. Due to prominent contradictions in the allocation of water resources between Central Asian countries and a lack of centralized and effective water resource utilization policies, the inflow of water entering the lake has been consistently low 71 compared with the increased precipitation in the Aral Sea basin since the late 1980s 70 . It is inferred that human activity has had a profound impact on lake change in arid Central Asia over the past 50 years.

Conclusions
Using 137 Cs and 210 Pb dating of a short, 41-cm lacustrine sediment core from Bosten Lake, Central Asia, the sediment grain-size distribution in response to the changes in the lake environments was studied under climatic and anthropogenic influences, and lake water level changes over the past 150 years were reconstructed. The results are as follows: Figure 6. The factor scores of F1 and F2 with five-point running averages compared with regional environmental indicators, including the Siberian High intensity 62 , Indian monsoon index 66 , North Atlantic Oscillation (NAO) 63 , water level of Bosten Lake 15 , and agricultural acreage of the Bosten Lake watershed 67 .

Scientific RepoRtS |
(2020) 10:942 | https://doi.org/10.1038/s41598-020-57921-y www.nature.com/scientificreports www.nature.com/scientificreports/ (1) Environmentally sensitive grain-size components were extracted from the grain-size standard deviation method and factor analysis. The components with grain sizes of 11.48 μm and 69.18 μm were sensitive to variations in the lake water level. (2) The history of lake water level changes over nearly 150 years has been established, thus extending the 50-year instrumental record. There are two significant periods with the lowest water level: 1920-1930 AD and 1980-1990 AD. Weakening of the Siberian High and reduction of the water vapour transported from the Indian Ocean during this period led to a reduction in the local water vapour, which resulted in reduced lake levels in 1920-1930 AD.

Methods
Sampling and laboratory analysis. In June 2016, a lake sediment sampler (Uwitec, Austria) was used to extract a short sediment core with a length of 41 cm (41°55.665′N, 86°49.078′E) from a depth of 7.0 m in Bosten Lake (Fig. 1). The sediment core was sliced at 1 cm intervals in situ. The freeze-dried sub-samples were analysed for 210 Pb, 226 Ra and 137 Cs by an EG&G Ortec gamma spectrometer (HPGe GWL-120-15), while the activity of 226 Ra in the lake sediment was evaluated by averaging the activities of 214 Pb (295 keV and 352 keV) and 214 Bi (609 keV). The total 210 Pb was 46.5 keV, and the total 137 Cs was 662 keV, with standard counting errors of less than 10% 72 . After treating samples with 1 N HCl, the determination of total organic carbon (TOC) and total nitrogen (TN) were performed using a CE-440 elemental analyser (EAI Company) 73 . Magnetic susceptibility was measured using a Bartington MS2 meter (Bartington Instruments Ltd.), and the detailed procedure is described in reference 68 . Organic matter and carbonates were removed from the sediment samples with 10-20 ml of 30% H 2 O 2 and 10 ml of 10% HCl. Then, 10 ml of 0.05 M (NaPO 3 ) 6 was added to the residues, which were then subjected to ultrasonic treatment for 10 min. Grain-size analysis of the lake sediments was carried out using a Malvern Mastersizer 2000 with a Hydro 2000 MU dispersion unit (Malvern, Worcestershire, UK) with a relative error of <1%. The percentages of the related size fractions of a sample were calculated by the Mastersizer 2000 68 .
Data analysis. The factor analysis model 74,75 was used to explain the potential factors affecting the grain-size composition of the lacustrine sediments. The data matrix D can be formulated by the equation D[m,r] = C[m,n] × R[n,r], where D represents the grain size data matrix; C represents the factor-loading matrix representing the latent factor composition; and R represents the factor score matrix for factor contributions. In addition, m, n, and r are the number of grain-size fractions, factor, and sample number, respectively. The Pearson correlation method 76,77 was used to establish the quantitative relationship between the various variables. The curve of standard deviation values versus the grain size of the sediments 68,78,79 was calculated to reveal the environmentally sensitive grain-size population.