Introduction

In estuarine and coastal zones, land runoff and oceans interact intensively and human activities have significant impacts1,2. Substantial amounts of substances generated by human activities and natural rock weathering enter estuaries and coastal zones through river runoff and atmospheric deposition and accumulate in the sediment of estuarine and coastal areas3. Sediments play a crucial role in complex coastal-estuarine systems and are the primary enrichment sites for pollutants, including trace metals, from different sources in near-shore waters4,5. Therefore, it is important to identify the sources of trace metals in sediments in estuarine and coastal areas, which will help assess the influences of human activities and natural sources on marine ecosystems. However, trace metals from anthropogenic activities and natural rock weathering are often mixed in coastal estuarine sediments through various physical, chemical, and biological processes during deposition and postdeposition, such as adsorption/desorption, precipitation/dissolution, consolidation, resuspension, bio-irrigation, bioturbation, and early diagenesis of organic matter, which obscures the original signals of the sources and makes it difficult to differentiate between natural and anthropogenic inputs6.

Scholars have applied various methods to identify the source of trace metals in marine sediment, including physicochemical/mineralogical signatures7, enrichment factor (EF), and positive matrix factorization8and statistical analysis, including correlation coefficient, cluster analysis, principal component analysis, and multiple linear regression9,10. Unfortunately, precise quantification of source apportionments (e.g., contributions from natural and anthropogenic sources) is not possible based only on traditional methods when a multi-source system is mixed, especially in complex estuarine and coastal sediments.

As a harmful metal to human health, lead (Pb) in aquatic environments can originate from natural processes such as weathering, erosion, and transport of bedrock, and from anthropogenic activities such as shipping, urban sewage discharge, aerosol deposition from vehicle emissions, and industrial activities11,12,13.

Radioisotopes are effective tracers for tracking the origin and transport of dissolved and detrital constituents in sedimentary, hydrologic and biogeochemical cycles and ecosystems14. Because the isotopic composition of Pb in the environment is not significantly affected by physiochemical fractionation processes15, the differences in Pb isotopic ratios, such as 207Pb/206Pb and 208Pb/206Pb in environmental reservoirs, can directly reflect their source areas16, which have been widely used to trace different sources of Pb17,18. For instance, the isotopic composition of Pb has been employed in the tracing contributions of different sources of Pb in soils19, lakes20, marine sediments21, seawater22, rain23, ice core24, aerosols25, and road dust26or other metals18,27. Similarly, strontium (Sr) has recently attracted considerable interest because of its harmful effects on human health in people with diets low in calcium and protein28. The 87Sr/86Sr ratio is often used in combination with Pb or other elemental isotopes to trace sources of heavy metals20,29.

The Bohai Sea, a semi-enclosed inland shallow sea in China, includes three bays: Liaodong Bay to the north, Bohai Bay to the west, and Laizhou Bay to the south8. Surrounded by the Beijing-Tianjin-Hebei region and the northeast industrial region with intensive human activities, Bohai Bay and Liaodong Bay are exposed to severe environmental pollution owing to the discharge of wastewater from hundreds of coastal drainage channels12,30,31,32and atmospheric deposition33,34. Many studies have focused on the status of marine pollution in the Bohai Sea12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36, the migration of pollutants37,38, and their negative ecological effects39; however, tracking the containment history in the coastal zone of the Bohai Sea is insufficient because of the difficulty in identifying complex sources of pollutants, which hinders the scientific assessment of the impacts of natural and human activities. This study aimed to identify the anthropogenic effects and natural input history in the several decades in the estuarine and coastal zones of the Bohai Sea by studying the vertical profiles of Pb and Sr isotopic compositions and their total contents in two tidal flat sediment cores.

Materials and methods

Sampling

Three sediment cores were collected from the tidal flat of the coastal zone of Bohai Bay (core BB) and Liaodong Bay (core LB and STZ) in the Bohai Sea (Fig. 1). Core BB was located in the Dashentang coastal zone, Hangu District, Tianjin City in 2017, and had a length of 56 cm. Core LB and STZ were collected at the Liao River estuary in Panjin City, Liaoning Province in 2012, and 2008, and had a length of 60 cm and 72 cm, respectively. The core LB was adjacent to the core STZ. After being sliced, a total of 27 samples were acquired from the sediment core BB and LB, respectively. The core STZ was sliced with an interval of 4 cm for dating only. All sediment samples were stored at −20 °C in a refrigerator until analysis.

Fig. 1
figure 1

Location maps of sampling points showing the marine flow-current patterns of the Bohai Sea (Black dashed lines represent currents). Modified after Hu et al.74.

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Analytical methods

Granulometric composition determination

Two grams of air-dried samples were dissolved in deionized water and H2O2 (30%) was added to remove organic matter and then HCl (10%) was added to remove inorganic carbon, and after settling, the particle size was measured using a laser grain-size analyzer (Malvern Mastersizer 2000).

Organic carbon and determination

The total organic carbon (TOC) content of the sediments was determined using a TOC analyzer (Vario TOC Cube, Elementar, Langenselbold, Germany). The pH of the sediments was determined using a pH meter (Orion Star A211).

Metal analysis

The collected samples were subjected to natural air drying in the laboratory followed by grinding to a particle size of 200 meshes. Subsequently, the concentrations of Pb and Sr were measured using an Inductively Coupled Plasma Mass Spectrometer (ELEMENT XR) following the GB/T 14506.30–2010 guidelines. The Al content was determined by X-ray fluorescence (AB-104 L, PW2404). The standard used for elemental analysis was GBW07309. All measurements were conducted at the Beijing Research Institute of Uranium Geology, Beijing, China, with qualification procedures including blank and parallel tests throughout the entire process. A duplicate experiment was done for every four samples and the pass rate of duplicate samples was 100%. The recovery of the standard reference material was 96–106%.

Pb and Sr isotopes determination

The Pb and Sr isotopes (208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb, 87Sr/86Sr) were measured for each powdered sample using thermal ionization mass spectrometry (ISOPROBE-T and Phoenix, respectively). A total of 14 and 27 sediment samples were used for Pb and Sr isotope analyses for cores BB and LB, respectively. The analysis of Pb isotopes followed the method of determination of Pb isotopic composition in rock and minerals (DZ/T 0184.12–1997), whereas the measurement procedure for Sr followed the determination of strontium isotopes in rock samples (GB/T 17672 − 1999). A duplicate experiment was determined for every sample as quality control. A supplementary file has been added to present the quality results. The testing accuracy was controlled within 2σ for both Pb and Sr isotope analysis.

Pollution indices

EF was used to assess the degree of chemical element enrichment quantitatively. EF is defined by Eq. (1)40

$$\:\text{EF}\text{=}\frac{{\left[{\text{C}}_{\text{n}}/{\text{C}}_{\text{Al}}\right]}_{\text{Sample}}}{{\left[{\text{B}}_{\text{n}}/{\text{B}}_{\text{Al}}\right]}_{\text{Background}}}$$
(1)

where Cn and CAl are the concentrations of metals n and Al, respectively, in the samples and Bn and BAlare the concentrations of n and Al in the background, respectively. The metal concentrations in the East Crust of China41were used as background values. Heavy metal sources can be assessed using EF values: EF > 0.5 and < 1.5 indicates a crustal source, while EF > 1.5 indicates a non-crustal source42. When EFs exceed 1.5, the effect of human activities on heavy metal enrichment should be suspected43.

Possible metal enrichment in marine sediments was evaluated using the geoaccumulation index (Igeo) according to Li et al.44, as given in Eq. (2).

$$\:\text{I}\text{geo}={\text{log}}_{\rm 2}\left(\frac{{\rm C}_{\rm n}}{1.5{\rm B}_{\rm n}}\right)$$
(2)

where Cn is the concentration of metal n in the sediments and Bn is the background concentration value for metal n. Sediments can be classified into six grades from non-polluted (Igeo < 0) to polluted (Igeo> 5)44.

Sediment dating

137Cesium radionuclide records were measured on the sediment core STZ by a γ-ray spectrometer gamma-spectrometer. The activities measurements of 137Cs were conducted at the Nanjing Institute of Geography and Limnology, Chinese Academy of Science (Nanjing, China). The sedimentation rate of core LB and BB could be referred in our previous studies45,46.

Statistical analyses

The MixSIAR package in R language (version 4.4.1) was utilized to quantify the potential source contributions towards Pb and Sr in the sediment core LB and BB. The model uses mean and standard deviations along with concentration of sources to produce model fits for user supplied data of individual samples (mixtures) through Markov Chain Monte Carlo (MCMC) simulations47. This model has been previously utilized for source apportionment of Pb in atmospheric particle, dusts, sediments and lichens with variable pollution sources48,49,50. The MCMC parameters were set as follows: chain length = 1,00,000 and number of chains = 3. The convergence of the models was evaluated using the Gelman–Rubin diagnostic method.

Principal component analysis (PCA) was adopted to identify the sources of Pb and Sr by SPSS 22.0 (IBM SPSS Inc.). The varimax method was used for rotation. Extraction of principal components based on eigenvalues (> 1) and cumulative variance (> 80%) in principal component analysis.

A geochemical model was applied to quantitatively distinguish the sources of the metals51: 

$$[\rm M]_{\rm N} = [\rm Al]_{S} \times (M/Al)_{\rm B}$$
(3)
$$[{\rm M}]_{A} = [{\rm M}]_{\rm T} - [{\rm M}]_{\rm N}$$
(4)
$$[{\rm M}]_{{\rm A}}\% = 100 \times [{\rm M}]_{\rm A}/[{\rm M}]_{\rm T}$$
(5)

where [M]T, [M]N, and [M]A are the total, natural, and anthropogenic concentrations of metals, respectively; [Al]S is the Al concentration in the sample; and (M/Al)B is the ratio of metal to Al in the background.

87Sr/86Sr vs 1/[Sr] plots can be used to identify the characteristics of Sr sources. The 1/[Sr] values for coal combustion and industrial dust are < 0.005, for atmospheric aerosols are between 0.005 and 0.08, and for vehicular exhaust are between 0.9 and 1.836.

The metal flux in sediments represents the input of dry matter mass and largely reflects the influence of natural and anthropogenic factors on the accumulation of elements in sediments52. The excess metal flux (MF) was calculated using Eq. (6)53,54:

$${\rm MF}_{\rm M} = {\rm S}_{\rm M} \times {\rm S} \times {\rho}$$
(6)

where SM is the excess Pb concentration at the Mth sediment depth (calculated as the total Pb concentration at the Mth sediment depth minus the background value of Pb), S is the average sedimentation rate in the study area (cm yr−1), and ρ is the dry sediment density (g cm−3). The dry sediment density was assumed to be 0.95 g cm-355.

Results

Sediment core chronology

Radionuclide 210Pb and 137Cs chronology techniques are widely used to estimate modern sedimentation rates and material mixing in lakes, estuaries, coasts, and oceans56,57,58. The first detectable 137Cs activity in undisturbed sediment profiles marked the appearance of significant bomb-derived 137Cs fallout in global environments in 195459. Major periods of global deposition of 137Cs fallout occurred in 1958/1959 and 1963/1964 in the Northern Hemisphere, corresponding to maximum and sub-maximum peaks in sediments, respectively60,61,62,63,64,65. The 137Cs fallout peak from the nuclear reactor accident in Chernobyl in 1986 is also used to identify the 1986 depth.

Vertical profile of 137Cs activity of the core STZ was shown in Fig. 2. The 137Cs profile displays three well defined peaks at 58, 45 and 22 cm, respectively, and the 137Cs activity was not detected below 70 cm. Assuming three peaks at 58 cm, 45 cm and 22 cm in the 137Cs profile corresponding to 1959, 1963 and 1986, respectively, there is a significant difference of sedimentation rate for the core LB (5.4 cm yr−1)45 and STZ (2.6 cm yr−1) between 1959 and 1963, although the two cores are adjacent to each other. The average sedimentation rate for the core STZ is 1.32 cm yr−1 from 1954 to 2008, which is approximately to be same as that of the core LB (1.3 cm yr−1) from 1954 to 200845. Therefore, an average sedimentation rate seems more reliable for the dating of both the core LB and STZ.

Fig. 2
figure 2

Depth profiles of 137Cs of core STZ.

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Coastal zone is open to tidal areas. Previous studies indicated that the constant initial concentration (CIC) model is more reliable than the constant of rate supply (CRS) model when using excess 210Pb to determine age of sediment in coastal areas, due to sediments in the areas have been transported and mixed by tidal currents66. Therefore, the CIC model of excess 210Pb activity has been applied to calculate the sedimentation rates of the core BB, on account of the fact that the core is located in open tidal flat46.

The average sedimentation rate was determined to be 1.87 cm yr−1in Bohai Bay46 and 1.3 cm yr−1 in Liaodong Bay, corresponding to a sedimentary record between 1987 and 2017 for core BB and 1974 and 2012 for core LB. Our results are also consistent with the previous studies on the sedimentation rate in the Liaohe River Estuary based on the 137Cs and 210Pb profiles67,68, and that in Bohai Bay57.

By comparison, the sedimentation rate results of the core LB and BB in the past 50 years of this study fell within the range of the sedimentation rate study results of 0.15∼3.27 cm yr−1in the Bohai Sea55,56,69,70,71,72, confirming the dating results of the two cores are reliable.

Physical and chemical parameters

The physical and chemical properties of the sediment samples are listed in Table 1 and vertical variations of Mz (grain size), TOC and pH in the sediment cores in Fig. 3. The TOC content ranged from 0.89 to 1.28% and 1.84–7.83%, with averages of 1.07% and 5.18% for cores BB and LB, respectively. The pH ranged from 7.81 to 8.1 and 7.2 to 8.69, with averages of 8.43 and 8.08, for cores BB and LB, respectively, indicating neutral sediments. The average grain size (Mz) ranged from 7.23 to 9.92 μm (average of 8.35 μm) and 20.83 to 67.26 μm (average of 34.86 μm) for cores BB and LB, respectively. The sediment samples in core LB consisted of silt (average of 62.5%), sand (average of 19.4%), and clay (average of 18.1%), while those in core BB mainly consisted of silt (average of 85.3%), followed by clay (average of 13.3%), and sand (average of 1.41%).

Table 1 Physical and chemical parameters of sediment samples.
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Fig. 3
figure 3

Vertical variations of Mz (grain size), TOC and pH in the sediment cores.

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Vertical variations of Pb and Sr isotopes

As shown in Fig. 4, the Pb isotope ratios in core BB ranged from 38.354 to 38.683 for 208Pb/204Pb; 15.575 to 15.643 for 207Pb/204Pb; 18.34 to 18.447 for 206Pb/204Pb; 2.091 to 2.099 for 208Pb/206Pb; 0.8471 to 0.8518 for 207Pb/206Pb; and 2.275 to 2.460 for 208Pb/207Pb. The vertical variation in 208Pb/204Pb was similar to that of 206Pb/204Pb, whereas the variation in 207Pb/204Pb was closely aligned with those of 208Pb/204Pb and 206Pb/204Pb, except for a peak in 2003 (the dotted lines), suggesting a high input of anthropogenic activities. Additionally, 207Pb/206Pb generally increased with depth from 1988 to 2003 and then decreased until 2013, similar to the vertical variation in 207Pb/204Pb. The Pb isotope ratios of our samples in the surface layers were comparable with the Pb isotope ratio results for the surface sediments of Bohai Bay (207Pb/206Pb: ranging from 0.844 to 0.863, 208Pb/207Pb: from 2.456 to 2.482) by Gao et al.73.

Fig. 4
figure 4

Vertical variations of Pb isotopes in the sediment cores. The dote line represents year 2003. in core BB, and 2006, 1997 in core LB.

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For core LB, the Pb isotope ratios ranged from 38.186 to 38.328 for 208Pb/204Pb; 15.533 to 15.566 for 207Pb/204Pb; 18.199 to 18.275 for 206Pb/204Pb; 2.0985 to 2.0912 for 208Pb/206Pb; 0.8535 to 0.8510 for 207Pb/206Pb; and 2.456 to 2.463 for 208Pb/207Pb. Our results were slightly lower than the isotope ratios of 208Pb/206Pb (2.1208) and 207Pb/206Pb (0.86557) reported by Hu et al.74. Vertical variations in 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios showed some similarities (Fig. 4), with a sharp decreasing trend from 1997 to 2006 and a slow increase from 1973 to 1997.

The vertical variations in the Sr isotope compositions of the two cores are presented in Fig. 5. As for core BB, the 87Sr/86Sr ratio showed a maximum value of 0.7155 in 2008 and minimum value of 0.7142 in 1996, with a generally increasing trend from the bottom to the 18 cm depth of the core. The vertical variation in 87Sr/86Sr value in core LB was generally similar to that in core BB, except for the time difference before reaching their maxima.

Fig. 5
figure 5

Vertical variation of Sr isotopes in the sediment cores.

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Vertical variation of total Pb and Sr concentration

The Pb content in core BB ranged from 28.8 to 36.8 µg g−1, with an average of 32.91 µg g−1 (Fig. 6). The Pb content in the core remained relatively stable with no significant variations, showing a variation trend similar to that of 207Pb/206Pb and 207Pb/204Pb, with a maximum in 2003. The Pb content in core LB ranged from 18 to 25.9 µg g−1, with an average of 21.76 µg g−1 (Fig. 6). The Pb content in core LB showed a vertical variation extremely similar to that of 208Pb/207Pb and 208Pb/204Pb, with a stable increase from 1980 to 1998, followed by a rapid decline from 1998 to 2010. An increase in the Pb concentration in core LB between 1980 and 2000 was also reported by Xu et al.75in a study on a sediment core from Liaodong Bay. This was consistent with the increase in the non-residual fractions of Pb in the sediment core from the Shuangtaizi Estuary (namely Liaohe Estuary after 2011), suggesting an increase in pollutant input into the sediments45.

Fig. 6
figure 6

Pb contents, EF, Igeo, PbA, MFPb, and contribution rates in the sediment cores.

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The Sr contents in cores BB and LB ranged from 170 to 186 µg g−1 and from 201 to 241 µg g−1, respectively. Vertically, the variation in Sr content was extremely similar to that of the 87Sr/86Sr ratio in core BB. However, the vertical variation in Sr content in core LB showed a converse trend to that of the 87Sr/86Sr ratio, especially between 35 cm depth and the surface layer of the core.

EF, I geo, anthropogenic pb (PbA), and MFPb

The EF of Pb in core BB ranged from 1.39 to 1.75, with an average of 1.55 (Fig. 6), suggesting that the enrichment of Pb in the core could be associated with anthropogenic inputs to some extent. The EFs of Pb in core LB were generally less than 1.5 (Fig. 6), suggesting that the sources of Pb in core LB were mainly natural inputs. The Igeo values of core BB were between 0.18 and 0.53, suggesting slight Pb pollution, while the Igeo values of Pb in core LB were less than 0, indicating an unpolluted level. However, the contribution of anthropogenic source ranged from 22.5 to 32.5% for the core BB, and from 24.2 to 30.4% for the core LB, with a slow increase from 1988 (23.4%) to 2003 (32.5%) in core BB and from 1973 (24.2%) to 2006 (30.4%) for the LB. The anthropogenic contribution rate of Pb showed a similar peak at 2003 with those of EF, Igeo, PbA, and MFPb for the core BB.

This suggests that although anthropogenic activities accounted for a relatively larger percentage in Liaodong Bay, the degree of enrichment of Pb was still lower than that in Bohai Bay. The MFPbin core BB ranged from 20.96 to 35.17 µg cm-2yr-1(average of 28.27 µg cm-2yr-1), while that in core LB ranged from 12.76 to 28.27 µg cm-2yr-1(average of 20.14 µg cm-2yr-1). There was a time difference in reaching the maximum EF, Igeo, PbA percentage, and MFPb for cores LB and BB, although the two cores showed a similar variation trend in the vertical profile to some extent. In addition, the vertical variations in MFPb and total Pb concentrations correlated well with the observed isotope variation in core BB (Figs. 4 and 6). However, there were significant discrepancies between the isotope and metal flux curves in the LB core. The MFPb and total Pb concentrations in the LB core peaked in 1998, whereas the isotope ratios reached peaked in 2006. This could be related to an extreme event in 1998, which can be interpreted as the dilution of deposited Pb with geogenic Pb during a well-known extreme high-water event in the year 1998.

All the EFs of Sr in core BB and core LB were below 1.5 (Fig. 7), suggesting that there was no Sr enrichment at the two sites. The Igeo values of Sr were less than zero, indicating that there was no Sr pollution in the two cores. However, obvious increases in Sr content, EF, and Igeo might suggest an increase in anthropogenic activity in Bohai Bay (after 1995) and LB Bay (after 1998).

Fig. 7
figure 7

Sr contents, EF, Igeo, and contribution rates in the sediment cores.

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Discussion

Potential sources identification of Pb

Previous studies have inferred that anthropogenic Pb is often characterized by a higher 207Pb/206Pb ratio (> 0.84), whereas geochemical background Pb usually has a lower 207Pb/206Pb ratio (~ 0.84)76,77,78,79. Between 1900 and 1930, when the Bohai coast and adjacent areas were considered unpolluted by human activities, the Pb isotopic composition of core samples from Liaodong Bay sediments ranged from 0.8410 to 0.8414 for 207Pb/206Pb and 2.0870 to 2.0885 for 208Pb/206Pb74,80. These values can be regarded as the baseline of Pb isotope ratios in the coastal zone of Bohai Bay and are comparable with those of unpolluted Yellow River sediments (207Pb/206Pb: 0.8401 ± 0.0025, 208Pb/206Pb: 2.0887 ± 0.0025) and loess81. Moreover, Northern China has been one of the most polluted areas in East Asia and worldwide, mainly because of emissions from widespread heavy industries and large amounts of coal combustion since the mid-to late-1900s. Since the reform and opening up of China in the late 1970s, the high-energy industry and nonferrous smelting have been developing rapidly82. Atmospheric Pb originated mainly from leaded gasoline combustion before 2000 and coal consumption after 200083. According to Duan et al.84, from 2000 to 2015, coal and oil consumption in China increased two to three times and pig iron and cement production doubled. The total atmospheric Pb emissions from coal combustion in China increased from 2,671 tons in 1980 to 12,561 tons in 200885. Particularly in northern China, large amounts of coal are consumed for home heating in winter, which causes heavy atmospheric pollution. Coal consumption and non-ferrous smelting emissions may be the main sources of atmospheric Pb contamination in North China, particularly after 200086,87,88. In addition, there are two major ports (Qinhuangdao Port - the largest conveyance port for coal in the world, and Tianjin Port) in the Northeast of Bohai Bay, where coal is frequently used89. Therefore, Pb from coal combustion related to shipping traffic is also an important contributor to the isotopic Pb composition of sediments in the western Bohai Sea90. Anthropogenic activities adjacent to Bohai and Liaodong bays include coal combustion, aerosols (including vehicle exhaust emissions), and non-ferrous smelting emissions, as reported by Liang et al.91.

Accordingly, the Pb isotope data for the different end-source members in the two cores aligned roughly linearly with the biplot of 208Pb/206Pb vs. 207Pb/206Pb (Fig. 9a). The positions of different natural and anthropogenic sources on the plot relative to the samples reflect the contribution of these sources to the samples. Natural sources, such as loess, Yellow River sediment, and Liao River sediment, exhibited lower 207Pb/206Pb and 208Pb/206Pb ratios. In contrast, anthropogenic sources, such as coal combustion, the Dongsheng Pb Mine in the Liaoning Province, and North China aerosols, are characterized by higher 207Pb/206Pb and 208Pb/206Pb ratios. As shown in Fig. 9a, the locations of the LB and BB cores are between the two groups of sources, indicating a mixture of sources with different contributions from natural (e.g., the Yellow River and loess) and anthropogenic sources (e.g., coal combustion, automobile exhaust, and Pb ore-related activities), while the positions of the Loess and Yellow River sources are relatively close to the positions of cores BB and LB, indicating that natural sources make a significant contribution to Pb isotopes.

Fig. 9
figure 8

Biplot for indicating potential sources. (a): 208Pb/206Pb vs. 207Pb/206Pb; (b): 87Sr/86Sr vs. 207Pb/206Pb). References: Yellow River sediment81; Loess81; Liao River sediment75; Coal combustion117,118; Automobile exhaust98,119; Liaoning Dongsheng Pb Mine120; North China aerosol95,111; Atmospheric dustfall111; Atmospheric aerosols25; BB and LB (this study).

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Sediment cores BB and LB generally showed increases in the amount of total Pb, EF, Igeo, PbA, and MF values over the last few decades, particularly in the 1980s and the 1990s (Fig. 6), which corresponded to the reform and opening up of China in the late 1970s and the large increase in automobiles using leaded gasoline during this period13. Similar variations in Pb have also been observed in Caohaizi Lake sediment in the Eastern Tibetan Plateau92. Gasoline is the major source of Pb93. After emission from vehicle exhaust, Pb enters the ocean via atmospheric pathways94. The phase-out of leaded gasoline in China began in the 1990s, with leaded gasoline being banned in several major cities, including Beijing, Shanghai, and Tianjin, from 1997 to 1998, followed by a nationwide ban in 2000. Despite the slightly decreasing trends, Pb concentrations remained relatively high in aerosol samples collected in Tianjin after the phasing out of leaded gasoline95. Therefore, vehicle exhaust might also be considered an important source of Pb in sediments, particularly before 2000, as confirmed by Tian et al.83 and Liang et al.91, although the Pb isotope ratios in automobile exhaust were far higher than those of the sediments. Therefore, it was assumed that the time difference reaching the maximum EF and MFPb for core LB and core BB occurring between 1998 and 2003 might, to some extent, be related to leaded gasoline being banned in different periods in the cities around the Bohai Sea. In addition, the observed trend highlighted the significance of the resuspension of particles deposited from earlier vehicle emissions with relatively high Pb isotopic ratios, similar to those in Shanghai and Changchun, as confirmed by Hu et al.90. Relatively high Pb concentrations between 1997 and 2002 were observed in Mount Everest ice cores96, eastern Tienshan ice cores24, lake sediment cores in the Eastern Tibetan Plateau92and aerosol particles in Xiamen, China97, reflecting the atmospheric deposition of Pb related to vehicle emissions and/or coal combustion in China.

Quantitative contribution of historical natural and anthropogenic activities using pb isotopic ratios

To determine the contribution rate of historical national and anthropogenic activities based on the MixSIAR model, the ratios of Pb isotopes of the loess and the mixture of coal combustion, Liaoning Dongsheng Pb mine, and North China aerosols, gasoline and vehicle exhaust were selected as anthropogenic source of Pb. Loess, Liao River and Yellow River sediment were chosen as natural source of Pb.

As shown in Figs. 6 and 8, the contribution of anthropogenic source of Pb showed an obvious increase from 1988 (23.4%) to 2003 (32.5%) in core BB and a stable increase from 1973 (24.2%) to 2006 (30.4%) for the LB. For core BB, the increase in anthropogenic activities also influenced the sediment Pb isotope composition between 1988 and 2003, possibly related to local industrial activities since the 1990s98. With the industrialization of Tianjin City since the 1990s, its surrounding waters have been subject to varying degrees of pollution99,100,101,102, which might have had a significant influence on the Pb content in core BB. As for core LB, between 1998 and 2006, the contribution rate of anthropogenic sources increased significantly, being consistent with the increase in 208Pb/206Pb and 207Pb/206Pb (Fig. 4) and the increase in EF and PbA (Fig. 6) in spite of the decrease of the total Pb concentration and MFPb, suggesting a significant increase in anthropogenic input, which might be due to rapid industrial development, pollutant emissions, and the discharge of wastewater from nearby factories during this period103,104,105,106,107. In particular, the largest zinc plant in Asia, the Huludao zinc plant, together with multiple nearby industries, caused serious pollution by heavy metals, including Pb, in seawater and sediment along the coast of Jinzhou Bay108,109. Seawater and resuspended sediment in Jinzhou Bay can be transported to the southwest of Liaodong Bay109,110, as well as the atmospheric deposition of Pb from the Huludao Zinc Plant109, which might potentially contribute to the anthropogenic input of Pb in Liaodong Bay. In addition, as discussed above, aerosol deposition, such as in Beijing, Tianjin, and Dalian, where coal combustion is primarily responsible for air Pb pollution95,111, is an important pathway for anthropogenic Pb input into the Bohai Sea. The impact of anthropogenic activities quickly decreased for cores BB (after 2003) and LB (after 2006), possibly because of increased environmental control measures and public environmental awareness.

Fig. 8
figure 9

MixSIAR results for mean contribution of Yellow River sediment, Gas and vehicle exhaust particles, Liao River, Loess, coal combustion, Liaoning Pb mine, and North China aerosol, Oyster shell towards core BB and core LB samples. (a) core BB-Pb; (b) core LB-Pb; (c) core BB-Sr and (d) core LB-Sr.

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It should be noted that PbA in core BB was extremely close to the contribution rate of anthropogenic sources of Pb isotopes. However, the PbA in core LB was slightly lower than the contribution rate of Pb isotopes, possibly related to relatively high atmospheric influxes of Pb in Liaodong Bay due to more coal combustion activities in Northeast China in winter and dilution of a great deal of geogenic materials transported by the Yellow River into Bohai Bay. The dilution of the Yellow River-derived sediments by anthropogenic input in Bohai Bay was also confirmed by Hu et al.90.

Our results showed that the contribution of natural sources was between 67.5% and 77.5% for core BB, and between 69.6% and 75.8% for core LB (Figs. 6 and 8). Zhu et al.8also identified a source of Pb in surface sediments from the Bohai Sea, indicating that natural river runoff was the main source of Pb, which is consistent with our results. Although many rivers flow into the Bohai Sea, the Yellow River accounts for the largest proportion of sediment transport (> 90%)112. In particular, water and sediment discharges from other main rivers flowing into the Bohai Sea, such as the Hai River, Luan River, Daliaohe, and Liao River (previously Shuangtaizi River), have dramatically decreased since the 1980s because of water diversion in the upper reaches84. However, since the 1970s, the Yellow River sediment flux has also been reduced by 12.2–50.3% due to climate change113and human activities (e.g. excessive water consumption in North China), which might have resulted in the sharp decrease of natural sources in the Bohai Bay between 1988 and 2002 and a stable decrease in the Liaodong Bay between 1973 and 2006. According to114, the riverine influxes of Pb from the Daliaohe and Liaohe rivers into Liaodong Bay (2,792 t yr-1) were generally comparable to those of the Haihe River into Bohai Bay (2,922 t yr-1). Although the Yellow River enters the Bohai Sea in the southwest, the influx of Pb by the Yellow River reached 4,264 t yr-1, which was approximately 1.5 times those of the Hai, Daliaohe, and Liaohe Rivers, respectively. Therefore, it was assumed that the input of Pb transported by the Yellow River into Bohai Bay accounted for a larger proportion than that into Liaodong Bay owing to the closer distance of Bohai Bay to the Yellow River estuary and the influence of clockwise circulation in the Bohai Sea in summer and coastal currents in winter (Fig. 1), which drives the sediment or re-suspended materials discharged from the Yellow River into the sea to move westward and northward112,115. Therefore, natural sources contribute more Pb to Bohai Bay than to Liaodong Bay via riverine inputs.

Potential sources and their contribution by using the related Sr isotopic ratios

Similar to the Pb isotopic indicators, two sources of Sr isotopes in the two sediment cores were identified based on the MixSIAR model as follows: anthropogenic activities (coal combustion and atmospheric dust fall for core LB; atmospheric aerosols for core BB) and natural sources (Yellow River sediment, Loess and Oyster shell). Oyster shell was chosen as another source of Sr which represents the marine source due to the high concentrations of Sr in seawater98. The variation of the contribution rates of anthropogenic sources with depth is shown in Fig. 6. The mean contribution rates of different sources are shown in Fig. 7.

For core BB, the contribution rates of Sr sources from anthropogenic activities ranged from 17.4 to 24.3% (Fig. 7). The contribution rates of anthropogenic sources generally remained vertically stable, except for a valley and peak between 1995 and 2000. This suggests that natural sources, including materials from land-rock weathering, soil erosion, and resuspended sediments, may be the primary sources of Sr in Bohai Bay. The contribution of atmospheric aerosols was high for Sr.

For core LB, the contribution rates from anthropogenic activities were approximately 70.7–78.5% (Fig. 7). The contribution of anthropogenic activities was far greater than that of natural sources. The contribution of anthropogenic sources in the core LB increased significantly from 1999 to 2011.

Coal combustion and industrial activities are significant sources of atmospheric pollution emissions and remain the primary sources of Sr contamination in current atmospheric pollution116, especially for core LB, as shown in Fig. 8, suggesting that coal combustion and industrial activities have significant influences on the estuarine and coastal environments in Liaodong Bay, reinforcing the assumptions discussed in Sect. 4.1 and 4.2.

Source indication by the combined Pb and Sr isotopic ratios

A better understanding of the potential sources and their contributions when the Pb isotopes (207Pb/206Pb) and Sr isotopes (87Sr/86Sr) were combined is shown in Fig. 9b. The Pb-Sr isotope compositions of the sediment cores from both locations differed significantly from those of vehicle exhaust but fell between coal combustion, atmospheric dust fall, atmospheric aerosols, loess, and Yellow River sediment. The data points in the scatter plot formed a line, indicating the influence of two primary sources: anthropogenic activities (coal combustion, atmospheric aerosols, and atmospheric dust fall) and natural sources (Yellow River sediment and loess). The sediment samples of core LB plotted nearer to atmospheric dust fall and coal combustion than those from core BB, suggesting that there were more anthropogenic activities in Liaodong Bay than in core BB. The Yellow River and Loess were relatively close to the BB core, suggesting that the Yellow River and Loess contribute significantly to the material source of the BB core, which was consistent with the quantitative results of MixSIAR. As discussed above, because the Yellow River may transport more natural Pb into Bohai Bay than into Liaodong Bay, core LB may have received more anthropogenic Pb through atmospheric input. Additionally, the isotopic composition of atmospheric dust fall was the closest to that of coal combustion (Fig. 9b), indicating that coal combustion remained the primary source of Sr and Pb pollution from anthropogenic activities25,36.

Natural sources, such as loess and Yellow River sediments, may have a greater influence on Bohai Bay, which is consistent with the results in Sect. 4.2, as confirmed by Zhu et al.8. This study indicated that Pb and Sr isotopes could be potentially applicable for tracing the various sources of trace metals in sediment and quantitatively differentiating between the contribution rates of natural and anthropogenic inputs in estuarine and coastal zones.

PCA analysis

Principal component analysis (PCA) was performed on Al, Pb, Sr, 208Pb/206Pb, 207Pb/206Pb, 208Pb/207Pb and 87Sr/86Sr. Data for the seven components can be reflected by two principal components (Fig. 10). The three principal components (PCs) accounted for 91.22% and 70.03% of the variability of all the studied variables in the core BB and LB, respectively. The result of PCA was represented in Fig. 10. In the core BB, factors with high loadings (> 0.7) in PC1 are Al, 208Pb/207Pb, 207Pb/206Pb and 87Sr/86Sr, and factors with high loadings (> 0.6) in PC2 is Sr. According to Sect. 4.3, Sr may be affected by natural activities, and Al is the main component of aluminosilicate, which mainly comes from rock weathering on land. Therefore, the combination of PC1and PC2 might represent natural sources, accounting for 70.3% of the total contribution rate of all variables. This is consistent with the results from natural sources analyzed by MixSIAR of the core BB (69.6% and 75.8%).

Fig. 10
figure 10

PCA results of the elements and isotope ratios in core BB and LB. Left: loading plots of the PC1 and PC2; right: scores plots marking the year. The yellow dots represent samples before 2000 and blue dots after 2000 on the right panels.

Full size image

As for the core LB, factors with high loadings (> 0.8) in PC1 are Al, Pb, Sr, 208Pb/206Pb, 208Pb/207Pb and 87Sr/86Sr, and factors with high loadings (> 0.6) in PC2 is 207Pb/206Pb. Therefore, in the core LB, PC1 may be interpreted as natural sources, PC2 may be interpreted as anthropogenic source. The contribution rate of PC1 was 68.84%, which was not much different from the results of natural sources analyzed by MixSIAR of the core LB (69.6% and 75.8%). It could be seen from the score plot of core LB (Fig. 10), pre-2000 points scored lower on PC1 while post-2000 points scored higher on PC2. Therefore, anthropogenic sources have significant influences on the core LB after 2000.

Conclusions

The Pb and Sr isotope ratios and total metal concentrations in sediment cores from Bohai Bay and Liaodong Bay in the Bohai Sea were determined to better understand their historical sources and distinguish between natural and anthropogenic contributions. The Pb isotope ratio compositions of cores BB and LB fell between natural sources and anthropogenic activities and were influenced by both compositions. The natural sources were mainly loess-related materials, while the anthropogenic sources were presumed to be a mix of coal combustion, the Dongsheng Pb mine in Liaoning, and North China aerosols. Loess has a significant influence on Pb deposition in the Bohai Sea and the influence of human activities in the Liaodong Bay area is greater than in the Bohai Bay area. The sources of Sr are complex and influenced by a variety of potential sources, including natural sources (e.g., Chinese loess and Yellow River sediment) and anthropogenic activities (mainly coal combustion, atmospheric dust fall, and atmospheric aerosols). Coal combustion and atmospheric dust fall are regarded as the major sources of Sr from anthropogenic activities in Liaodong Bay, whereas atmospheric aerosols are the main source of Sr in Bohai Bay. With rapid economic development in the late 1970s in China, anthropogenic activities have introduced more Pb into estuarine and coastal waters and sediments, as confirmed by the increases in the concentrations of Pb, EF, Igeo, PbA, and MFPb values in recent decades in cores BB and LB, posing a potential threat to coastal ecosystems in the Bohai Sea. Since the late 1990s, with the ban on leaded gasoline and the strengthening of environmental measures and awareness, the impact of human activities has declined, especially for core BB, with an obvious decrease in the contribution rate of anthropogenic Pb and Sr between 1997 and 2008. However, Liaodong Bay was still more influenced by anthropogenic activities than Bohai Bay, with a stable increase in the contribution rate of anthropogenic Pb and Sr from 2000 to 2010. This is likely related to the widely distributed heavy industry in Northeast China, mining activities, and coal combustion, resulting in more inputs from anthropogenic activities.