A historical record of trace metal deposition in northeastern Qinghai-Tibetan Plateau for the last two centuries

Owing to rapid socio-economic development in China, trace metal emissions have increased and lakes even in remote areas have experienced marked changes in the last century. However, there are limited studies revealing long-term trends, anthropogenic fluxes, and spatial characteristics of trace metals in lakes. In this study, we present a geochemical record from Lake Qinghai in the northeastern Qinghai-Tibetan Plateau and reconstruct trace metal pollution history during the last two centuries. The lacustrine sediment core was dated by 137Cs and 210Pb, and sediments deposited prior to the 1850s were selected as the pre-industrial background. Factor analysis and enrichment factor indicated Cr, Cu, and Ni generally originated from natural sources, while Cd, Pb, and Zn have been influenced by human contamination since the mid-1980s. The anthropogenic Cd mainly derived from non-ferrous metal smelting in Gansu Province, and fluxes to Lake Qinghai sharply increased after the mid-1980s. The metal reconstruction is similar to other lake sediment records from China and corresponds well with rapid economic development in China. The spatial pattern of anthropogenic Cd fluxes to lakes is primarily attributed to regional industrial emission, phosphate fertilizers, and manure applied in agriculture.


Introduction
Aquatic ecosystems are habitats of great human importance as they provide essential water resource, help maintain biodiversity and regional ecosystem balance, and provide food (Brönmark & Hansson 2002;Ricciardi & Rasmussen 2001). In spite of their fundamental importance to humans, aquatic ecosystems have experienced unprecedented changes with rapid urbanization and industrialization, such as eutrophication and trace metal pollution, which have led to serious negative effects on the structure and function of these ecosystems (Dudgeon et al. 2006;Vanmaercke et al. 2015;Liu et al. 2013). Trace metals are among the most widespread of the various pollutants originating from anthropogenic activities, particularly from metal mining and smelting, burning of fossil fuels, and agriculture (Evenset et al. 2007;Hosono et al. 2016;Kuwae et al. 2013;Mikac et al. 2011). Lacustrine sediments are generally recognized as a primary sink for the potentially harmful elements. However, trace metals such as cadmium (Cd), lead (Pb), and zinc (Zn) may be released back into the water column under favorable conditions and bioaccumulated in food webs, which is hazardous to human health (Liu et al. 2013). Therefore, it is necessary to consider the abundance, transport, and persistence of trace metals as well as the effects of anthropogenic activities in the sediments.
In China, most of the previous studies have concentrated on large lakes from relatively populated areas, such as in the reach of the Yangtze River (Liu et al. 2012;Liu & Shen 2014). Certainly, studies on these lakes are of great significance, as they are often heavily affected by human activities, while lakes in relatively remote areas were previously considered pristine ecosystems with limited anthropogenic impacts. However, investigations of relatively remote mountainous lakes in west China indicate that most of them have  (Bing et al. 2016;Jin et al. 2010;Lin et al. 2018;Liu et al. 2013;Zeng et al. 2014). These studies suggested that the trace metals are transported with aerosols and dusts over long distances. Pb is probably the most extensively investigated trace metal. However, knowledge of temporal trends, anthropogenic deposition fluxes, and spatial characteristics are all still limited, which makes the understanding of the influence of human activities on regional to continental scale environmental quality unclear.
In this study, we present the temporal changes of trace metals in Lake Qinghai, which is the largest inland lake in China. The objectives of the study are first to determine the trace metal background values in Lake Qinghai sediments, then to reconstruct historical trends and distinguish anthropogenic contributions of trace metals, and further evaluate the spatial fluxes of anthropogenic sourced metals.

Study region and field sampling
Lake Qinghai (36°32′-37°15′N, 99°36′-100°47′E) lies on the northeastern margin of the Qinghai-Tibetan Plateau at an altitude of about 3194 m above present sea level. It is a hydrologically closed system with a surface area of about 4260 km 2 , and its catchment area is more than 29,660 km 2 . Catchment bedrock mainly consists of metamorphic and acidic igneous rocks while limestone outcrops are scattered throughout the western part of the catchment. The lake is fed mainly by direct rainfall and five large rivers and their tributaries, namely Buha, Shaliu, Hargai, Quanji, and Heima. The Lake Qinghai Basin region is mainly affected by a cold and semi-arid continental climate. In the winter, in the upper atmosphere, westerlies prevail and in the lower atmosphere, the polar cold air mass prevails. While in the summer, warm and humid air masses are transported by Asian summer monsoon from the tropical oceans. The mean annual air temperature is approximately 1.2 °C, mean annual precipitation is about 360 mm, and the evaporation is about 3-4 times higher than precipitation.
Lake Qinghai is located about 100 km west of Xining City, and total population in the watershed is about 0.2 million. Land use and land cover analysis showed that grassland (63%) and water body (18%) dominated in the watershed; in contrast, cropland and built-up areas accounted for about 1.1% and 0.1% of the total area ). Therefore, local human activities around Lake Qinghai are very limited.
A gravity core with length of 32 cm named QH-9 was collected in the center area of the lake in November 2017 ( Fig. 1). Inspection of the cores at the time of sample collection indicated intact recovery of the sediment-water interface. The sediments were sliced at intervals of 1 cm in the field, sealed into polyethylene bags, and stored in the cooler until they were returned to the laboratory. The samples were freeze-dried and weighed before and after drying to calculate the dry bulk densities (DBD, g cm −3 ) and mass depth (m z , g cm −2 ) in the laboratory:

Establishing the chronology
For dating, the weighed dry samples were sealed in plastic test tubes with caps. Radionuclides ( 210 Pb, 226 Ra, and 137 Cs) activities in samples were measured using an ORTEC HPGe GWL series well-type coaxial low background intrinsic germanium detector. The samples were sealed in a measuring pot and stored for more than 3 weeks to achieve equilibrium between 226 Ra and its daughter 214 Pb. 137 Cs was measured at 661.6 keV, while total 210 Pb was detected via gamma emission at 46.5 keV, and 226 Ra at 295 and 352 keVγrays emitted by its daughter isotope 214 Pb activities. Individual samples were subjected to > 40,000 s counting times, in order to provide a relative measurement precision of ≤ ± 10%. The unsupported 210 Pb ( 210 Pb ex ) in each sample was obtained by subtracting the activity of 226 Ra from the total activity of 210 Pb (Appleby 2001).
The Constant Rate of Supply (CRS) model is the most widely used 210 Pb ex dating model within lacustrine deposits (Appleby 2001). The model assumes that the supply of 210 Pb ex (P) to the lacustrine sediment was constant, but the sedimentation accumulation rate varied with time. This method relies on the ratio of the cumulative residual 210 Pb ex beneath a given depth to the total 210 Pb ex activity in the sediment column. The inventory and age t are calculated as follows: where C(m) is the mass specific 210 Pb ex concentration at mass depth m, and A(m Z ) is the 210 Pb ex inventory beneath a given mass depth m Z , A(0) is the total 210 Pb ex inventory of the sediment core, and λ is the 210Pb disintegration constant.
When there is significant discrepancy between the results provided by the CRS model and 137 Cs chronostratigraphic marker, the peak of 137 Cs corresponding to the year 1963 CE can be incorporated into the CRS model as a composite CRS model to improve the final result (Appleby 2001). The age t of each sediment layer between the surface and marked layer and below the layer is calculated as Eqs. (4) and (5), respectively: where T 0 is the sample year, P is the mean 210 Pb ex flux during the period between sample year and 1963 CE, and A(M) is the total 210 Pb ex inventory in the sediment core below the marked layer corresponding to the year 1963 CE.

Geochemistry analysis and data processing
For the geochemistry analysis, the freeze-dried samples were homogenized in an agate mortar, and then completely digested by HCl-HNO 3 -HF-HClO 4 in Teflon beakers. The concentrations of aluminum (Al) and Zn were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES; Leeman Labs, Profile DV), while the concentrations of arsenic (As), Cd, chrome (Cr), copper (Cu), nickel (Ni), and Pb were measured by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7700 ×). The detection limits of the analytical procedure are 20 (Al), 0.2 (Zn), 0.1 (As), 0.01 (Cd), 0.1 (Cr), 0.02 (Cu), 0.05 (Ni), and 0.01 (Pb) mg kg −1 , respectively. Data quality was ensured by measuring blanks and standard reference materials (GBW07358; Chinese geological reference materials) after every tenth sample. The accuracy, expressed as a recovery of the reference material, was 92.8-106.3% for the elements. Concentrations of elements in procedural blanks were very low or undetected. Factor analysis was performed using IBM SPSS19 statistical software for Windows to determine the relationships among the major and trace metals.

Sedimentary chronology
The activity profiles of 137 Cs, 210 Pb, and 226 Ra are shown in Fig. 2a and b. The first recognition of 137 Cs activity is at the depth of 10.5 cm in the Core QH-9, and the peak of the 137 Cs profile at the depth of 5.5 cm. Considering diffusion and post-depositional mobility of the radionuclide in sediments, the accuracy of the first 137 Cs occurring position is difficult to identify due to the weak activity in the early stage. Nevertheless, the influence of diffusion and diagenetic processes did not significantly change the position of the maximum peak of 137 Cs in sediments. Thus, the 137 Cs peak at the depth of 5.5 cm corresponds to about 1963 CE, which is similar with the widely recognized peak caused by the global fallout peak (Xu et al. 2010).
The 210 Pb and 226 Ra activities generally reached equilibrium at mass depth of 17.5 cm, and the 210 Pb ex profile did not follow the exponential variations with the mass depth (Fig. 2b). As shown in Fig. 2c, the CRS model underestimated the age of the sediments: the CRS model provided a date of ~ 1991 CE at the depth of 5.5 cm in Core QH-9, which deviated by ~ 28 years from the 137 Cs date. The deviations suggest variation in the supply rate of 210 Pb ex in the northern basin of Lake Qinghai. Thus, the composite model was chosen to develop the age-depth model (Fig. 2c).
The age for depth of 16.5 cm is ~ 1851 CE based on the composite CRS model. The sediment accumulation rate ranges generally increased from 0.02 to 0.11 g/cm 2 /year during the period from the middle nineteenth century to the middle twentieth century, and then the sediment accumulation rate decreased significantly to about 0.04 g/cm 2 / year (Fig. 2d). Such obvious high sediment accumulation rates during the middle twentieth century may be ascribed to the large-scale human migration for land reclamation in northwest China (Ta et al. 2006). Thus, the underlying part of the Core QH-9 was dated successively based on the average annual sediment accumulation rate of the second half of the nineteenth century. Finally, it is obtained that the Core QH-9 covered a period between ∼1560 s and 2017 CE.

Elemental concentrations in the sediments
The vertical distribution of Al, As, Cd, Cr, Cu, Ni, Pb, and Zn is shown in Fig. 3, and the basic statistics of the trace metal concentrations are shown in Table 1. Factor analysis shows two factors that explain 83.0% of the data variance (Table 2). Factor 1 is represented by Al, Cr, Ni, and Cu accounting for 45.8% of the data variance, while factor 2 is dominated by Cd, Pb, and Zn and it accounts for 37.2% of the total variance. In the Core QH-9, Al, Cr, Cu, and Ni concentrations display similar vertical trends (Fig. 3). The values are generally steady prior to 1986 CE and then followed by a gradual decrease in the upper layers of the core. In contrast, Cd, Pb, and Zn concentrations generally show increasing trends since 1986 CE. However, As concentration does not show any similar trend with other elements, the concentrations show an increasing trend prior to 1760s CE, then decrease from 1760 to 1980s CE, and subsequently increase to present.

Selection of background reference
An estimation of reference background is required to accurately determine a regional contamination level, identify anthropogenic contribution, and calculate inventory (Birch 2017). Empirical methods such as use of global mean concentrations of upper crust and marine shale, catchment geology and soils, and sedimentary core data have now been Pb tot (purple) activities, and (c) chronology (red square represents the composite CRS age model, and green square represents the interpolated age) of Core QH-9 used extensively to determine natural metal concentrations (Birch 2017;Gloaguen & Passe 2017;Song et al. 2014). Compared to the mean values of upper crust, lacustrine sediments of Lake Qinghai and background value in the soils of Qinghai Province are generally higher in As, Cd, Cr, and Ni concentrations (Chen et al. 2015;Taylor & McLennan 1995). Thus, the use of catchment geology and soils, and sedimentary core might be more appropriate than using global concentration due to the detritus mainly derived from a local source (Bindler et al. 2011). In addition, a previous study found that the concentrations of Cu, Pb, and Zn in soils were higher in the northern and western of the Lake Qinghai catchment, suggesting human activities had influenced the virgin topsoil .
The use of sedimentary cores for determining pre-industrial samples as non-contaminated and background is now the most straightforward method (Birch 2017;Li et al. 2017;Lin et al. 2018;Song et al. 2014;Wan et al. 2016). Northwest China is considered to be a copper smelting center during the Bronze Age, and the geochemical analysis of anthropogenic and adjacent lacustrine sediments showed influence of metallurgical activity on the natural environment (Dodson et al. 2009;Li et al. 2011a;Zhang et al. 2017). For remote areas, increases in Pb, Cu, and Zn in Hongyuan peatland from the northeast Qinghai-Tibetan Plateau were also suggested to be correlated with the intensification of metallurgical activity at between 5.4-4.0, 3.0-2.5, and 2.0-1.0 cal ka BP (Yu et al. 2010). However, the Pb isotopic composition and Pb/Sc ratios never varied beyond the range of soil weathering values and do not indicate a mining and smelting origin of the deposited Pb (Ferrat et al. 2012). The difference might be due to the trace metal concentrations within anthropogenic sediments decreased significantly from smelting sites (Bi et al. 2006;Li et al. 2015Li et al. , 2011b. Therefore, the established age model indicated that bottom sediments (17-32 cm) were deposited prior to the 1850s, which could represent the pre-industrial background in Lake Qinghai.  Table 1 Geochemical composition of lacustrine sediments in Lake Qinghai compare with the UCC (Taylor and Mclennan 1995), and background value in the soils of Qinghai Province (Chen et al. 2015) Al (

History of trace metal enrichment and anthropogenic contribution
To express the trace metal pollution, the enrichment factor (EF) is calculated as: where M is the concentration of the considered element; aluminum (Al) is used as the conservative element because anthropogenic sources are negligible, and minimally influenced by grain size effect and post-depositional diagenesis; and (M/Al) sample and (M/X) background are the ratios of the considered element and the reference element in the target and background samples, respectively (Bing et al. 2016;Kuwae et al. 2013;Lin et al. 2018). In this study, (M/Al) background is the ratio in the sediment dated prior to 1850s (Bing et al. 2016;Kuwae et al. 2013;Lin et al. 2018).
The EFs of As, Cd, Cr, Cu, Ni, Pb, Sb, and Zn are presented in Fig. 4. The EFs of Cr, Cu, and Ni in the cores are close to 1, indicating that the sediments are not enriched by these trace elements. The results of PCA analysis show that Al, Cr, Cu, and Ni clustered in the same group, also indicating these metals generally originated from the natural sources because Al is a lithogenic element enriched in silicates and clay minerals. Relatively stable Cd, Pb, and Zn EFs of about 1 prior to mid-1980s suggest that sedimentary Cd, Pb, and Zn also mainly originated from natural detritus. The group including Cd, Pb, and Zn showed distinctly different features from the group of Al, Cr, Cu, and Ni, combining with subsequent increasing of EFs suggest that Cd, Pb, and Zn had been influenced by human contamination. Especially, the degree of Cd pollution had reached moderate level. In contrast, the EF of As referenced to the preindustrial background is obviously lower than 1 during the twentieth century, and the absence of significant correlations between As and other elements in the sediments of Lake Qinghai, indicating a second-order diagenetic overprinting of the trace metal profiles over this period (Cooke & Abbott 2008).
Assuming that EF > 1 indicates the presence of pollution, the anthropogenic contribution of metals can be calculated by the following equation: where M anthropogenic is the anthropogenic concentration of a considered metal (Shotyk et al. 2003). Accumulation flux is much more effective in reflecting the anthropogenic contribution than their concentrations due to large variation of the sediment accumulation rates (Bing et al. 2016;Li et al. 2017;Liu et al. 2013). The anthropogenic flux of the   The temporal changes of total and anthropogenic fluxes of Cd, Pb, and Zn since 1850s CE are shown in Fig. 5. The variations of F Cd and F Pb are consistent with the EFs (Fig. 5). F Cd and F Pb increased from nearly 0 before the 1980s to a relatively high level (0.03-0.16 mg/m 2 /year and 0.35-5.77 mg/m 2 /year, respectively) since the mid-1980s, corresponding to 33.2-76.1% and 6.3-49.3% of the total sedimentary metals, respectively. In contrast, F Zn show a small peak at about 1950s but only accounting for 7.2% of sedimentary Zn, indicating that Zn pollution during the period was diluted by the higher fluxes of natural debris.

Spatio-temporal trends of Cd pollution in China
Anthropogenic fluxes of Cd in our study increased from the mid-1980s. Similar enrichments in anthropogenic Cd from around 1980s have been recorded elsewhere in lacustrine sediments from China (Bing et al. 2016;Li et al. 2018;Lin et al. 2018;Wan et al. 2016;Zeng et al. 2014). However, the flux of anthropogenic Cd varies in different archives. In Lake Sayram from northwest China, the F Cd increased from 0.04 to 0.07 mg/m 2 /year during the period 1990-2010 CE (Zeng et al. 2014). The average anthropogenic Cd accumulation was about 0.21 mg/m 2 /year in Lake Gonghai from central China between 1980 and 2014 CE, and the fluxes showed a dramatic increase during the early 1990s (Wan et al. 2016). In southwest China, Bing et al. (2016) investigated an alpine lake in the margin of Qinghai-Tibetan Plateau and showed that the F Cd accumulated at about 0.68 mg/m 2 /year since the mid-1980s, while Lin et al. (2018) reported that approximately 0.15 mg/m 2 /year anthropogenic Cd deposited in Lake Lugu during this period. In western Lake Taihu from East China, a relatively high average F Cd at about 0.94 mg/m 2 / year was reported during the period 1980-2016 CE (Li et al. 2018). Compared with previous studies in China, the deposition of anthropogenic Cd in lakes is lower in northwest China than other region (Fig. 6).
It is well known that anthropogenic Cd deposition rates to sediment are strongly correlated with industrial emissions (Pacyna et al. 2009). The estimated industrial Cd emission from 1949 to 2012 CE in China showed Cd emission from Fig. 5 (a-c) Sedimentary fluxes of Cd, Pb, and Zn (filled by light blue) and anthropogenic sources referenced to the pre-industrical background (filled by orange) in the sediment core from Lake Qinghai Fig. 6 Historical trends in the mass accumulation fluxes of anthropogenic Cd in Chinese lakes ((a) Lake Qinghai; (b) Lake Sayram (Zeng et al. 2014); (c) Lake Gonghai (Wan et al. 2016); (d) Lake Caohaizi (Bing et al. 2016); (e) Lake Lugu (Lin et al. 2018); (f) Lake Taihu (Li et al. 2018)), industrial Cd emissions ((g) Tian et al. 2015), and fertilizer used in agriculture in China during the last 60 years primary anthropogenic sources increased from 15.5 to 526.9 t/year, also with the most rapid increase occurring since the mid-1980s (Tian et al. 2015). Industrial Cd emissions in China are mainly derived from non-ferrous metal smelting and coal consumption by industrial boilers (Tian et al. 2015). Non-ferrous industries are flourishing in Jiangxi, Yunnan, Anhui, and Gansu Province, and poorly controlled smelting emissions have caused many areas in these provinces to have relatively higher emissions (Tian et al. 2012(Tian et al. , 2015. Although there are few modern industries and the population density is low in the catchment of Lake Qinghai and even in wider Qinghai Province, anthropogenic Cd can be transported by the long-range atmospheric dust derived from the adjacent Gansu Province. For example, Jinchang is Chinese largest nickel producer and well known for its non-ferrous metal production, and Baiyin is the base for non-ferrous metal mining in China. A number of large industrial enterprises were formed for Pb-Zn, Cu, and polymetallic extraction since the 1970s. As estimated by Tian et al. (2015), 27 tons of Cd was discharged in 2010 CE with over 90% sourced from non-ferrous metal smelting in Gansu Province.
Cd is also added to agricultural land through rock phosphate fertilizers and manure (Luo et al. 2009;Zhang & Shan 2008). The Cd contents in these fertilizers and manure in China range from 0.5 to 4.8 mg/kg, which are significantly higher than the background of Chinese lacustrine sediments Luo et al. 2009). These fertilizers have been increasingly applied in China since the late twentieth century, and the persistent application has led to Cd accumulation in the agricultural soils. In Chinese agricultural soils, livestock manures and fertilizers account for 63% of the total Cd inputs (Luo et al. 2009). With the steady increase in population and relative scarcity of agricultural land in China, agricultural cultivation in east China has been intensified with fertilizers and agrochemicals (Zhang & Shan 2008). The catchment of Lake Taihu is not only one of Chinese most industrial developed areas, but has also intensified use for agriculture, with the concomitant increase in use of phosphate fertilizers and manure. These should be one of the main sources of Cd accumulation in the watershed and anthropogenic Cd in the lake. Similar findings have also been found in lacustrine records from Huaihe region, where is mainly influenced by agriculture (Zhang & Shan 2008). Therefore, the spatial variation of F Cd in the Chinese lakes may be related to different levels of economic development, population density, and specific industries.

Conclusions
In this study, changes in the trace metal concentrations and sources in sediments of Lake Qinghai are examined over the past two centuries. Compared with the pre-industrial background, Cr, Cu, and Ni generally originated from natural sources, while Cd, Pb, and Zn have been influenced by human contamination since the mid-1980s. Based on enrichment factors, Cd reached moderate enrichment while Pb and Zn pollution were minimal. The anthropogenic Cd fluxes to Lake Qinghai sharply increased from 0.03 to 0.17 mg/m 2 /year after the mid-1980s, mainly derived from non-ferrous metal smelting in Gansu Province. The timing is similar to other lacustrine sediments from China corresponding with the initial Reform and Opening-up. We found that the fluxes of anthropogenic Cd to lakes were lower in northwest China than other region, primarily attributed to regional industrial emission, and phosphate fertilizers and manure applied in agriculture.