4.1. 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 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 (Wang et al. 2015).
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. 2015, Li et al. 2011b). Therefore, the established age model indicated that bottom sediments (17–26 cm) were deposited prior to the 1850s, which could represent the pre-industrial background in Lake Qinghai.
4.2. History of trace metal pollution and anthropogenic contribution
To express the trace metal pollution, the enrichment factor (EF) is calculated as:
EF= (M/Al)sample/ (M/Al)back (5)
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 post depositional diagenesis, and (M/Al)sample and (M/X)back 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) back is the ratio in the sediment dated prior to 1850s (Bing et al. 2016, Kuwae et al. 2013, Lin et al. 2018). The degrees of pollution may be classified in 5 scales (1 < EF ≤ 2, minimal pollution; 2 < EF ≤ 5, moderate pollution; 5 < EF ≤ 20, significant pollution; 20 < EF ≤ 40, very high pollution; EF > 40, extremely high pollution) (Sutherland 2000).
The EFs of As, Cd, Cr, Cu, Ni, Pb, Sb and Zn are presented in Fig. 5. 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 are obviously lower than 1 from 1867 to 1986 CE, 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:
Manthro=Msample− Alsample× (M/ Al)back (6)
where Manthro 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 considered metal was calculated by multiplying the mass accumulation rate and the anthropogenic concentration in sediments at each depth:
FM= MAR× Manthro (7)
The temporal changes of total and anthropogenic fluxes of Cd, Pb and Zn since 1841 CE are shown in Fig. 6. The variations of FCd and FPb are consistent with the EFs (Fig. 5). FCd and FPb increased from nearly 0 before the 1980s to a relatively high level (0.03–0.17 mg/ m2/ yr and 0.48–5.93 mg/ m2/ yr, respectively) since the mid-1980s, corresponding to 33.2–76.1% and 6.3–49.3% of the total sedimentary metals, respectively. In contrast, FZn show an obvious 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.
4.3. 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 FCd increased from 0.04 to 0.07 mg/ m2/ yr during the period 1990–2010 CE (Zeng et al. 2014). The average anthropogenic Cd accumulation was about 0.21 mg/ m2/ yr 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 FCd accumulated at about 0.68 mg/ m2/ yr since the mid-1980s, while Lin et al. (2018) reported that approximately 0.15 mg/ m2/ yr anthropogenic Cd deposited in Lake Lugu during this period. In western Lake Taihu from East China, a relatively high average FCd at about 0.94 mg/ m2/ yr 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. 7).
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 primary anthropogenic sources increased from 15.5 to 526.9 t/yr, 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). Shandong Province is characterized by rapid economy growth, there is a large volume of coal consumption and industrial output mean it is ranked as the province with the highest Cd emissions (Tian et al. 2015). In addition, nonferrous industries are flourishing in Jiangxi, Yunan, 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 were 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 0.5–4.8 mg/kg, which are significantly higher than the background of Chinese lacustrine sediments (Cheng et al. 2015, Luo et al. 2009). These fertilizers have been increasingly applied in China since the late 20th 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 Chinse 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 FCd in the Chinese lakes may be related to different levels of economic development, population density and specific industries.