Content and distribution of trace metals in surface sediments from the northern Bering Sea, Chukchi Sea and adjacent Arctic areas

Content and distribution of trace metals in surface sediments from the northern Bering Sea, Chukchi Sea and adjacent Arctic areas

Marine Pollution Bulletin 63 (2011) 523–527 Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/l...

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Marine Pollution Bulletin 63 (2011) 523–527

Contents lists available at ScienceDirect

Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Content and distribution of trace metals in surface sediments from the northern Bering Sea, Chukchi Sea and adjacent Arctic areas M.H. Cai a,b, J. Lin c, Q.Q. Hong c, Y. Wang c, M.G. Cai c,d,⇑ a

State Oceanic Administration Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai 200136, China UNEP-Tongji Institute of Environment for Sustainable Development, Shanghai 200092, China c College of Oceanography and Environmental Science, Xiamen University, Xiamen 361005, China d State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China b

a r t i c l e

i n f o

a b s t r a c t

Keywords: Trace metal Sediment Northern Bering Sea Chukchi Sea

Concentrations of trace metals (Zn, Cr, Cu, V, Cd and Pb), total organic carbon (TOC), black carbon (BC) and their granulometry were examined in 25 surface sediment samples from the northern Bering Sea, Chukchi Sea and adjacent areas. Trace metal concentrations in the sediments varied from 21.06– 168.21 mg kg1 for Zn, 8.91–46.94 mg kg1 for Cr, 2.69–49.39 mg kg1 for Cu, 32.46–185.54 mg kg1 for V, 0.09–0.92 mg kg1 for Cd, and 0.95–15.25 mg kg1 for Pb. The geoaccumulation index (Igeo) indicated that trace metal contamination (Zn and Cd) existed in some stations of the study area. The distribution of grain size plays an important role in influencing the distribution of trace metals (Zn, Cr, Cu, V, and Pb) in sediments from the Chukchi Sea and adjacent areas. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction

quality (in particular the current trace metal contamination) using the geoaccumulation index (Igeo); and to investigate the relationship between trace metal concentrations and some physical– chemical parameters of the sediments. This data could also form the foundation for the protection of sediment quality in the study area.

Naturally occurring sources of trace metals in marine sediments include the weathering and erosion of the lithosphere, while anthropogenic sources comprise industrial processing, urban sewage, and agricultural run-off (Kaushik et al., 2009). When released into the marine environment, most metals will soon be removed from the water column by interacting with organic matter, clay, sulfides, and Fe/Mn oxides etc., and then incorporated them into the marine sediments (Wang and Chen, 2000). Therefore, sediment metal concentrations are orders of magnitude greater than the overlying water column, and this result in sediments being considered as the primary sinks for most trace metals (Yang et al., 2009). High biological productivity is found in the Bering and Chukchi Seas (Chen and Gao, 2007), and thus the environmental quality of the Bering and Chukchi Seas is worth assessing. Several studies on trace metals have been conducted in the Arctic (Evenset et al., 2007; Outridge et al., 2005; Trefry et al., 2003). However, there are few studies concerning metal contamination in the sediments of the Bering and Chukchi Seas (Naidu et al., 1997). The objectives of our study were to investigate the content and distribution of several trace metals in the sediments in and around the northern Bering Sea and Chukchi Sea to accurately assess the environmental

2. Materials and methods 2.1. Sampling The Bering Sea is one of the largest marginal seas in the world, with an area of 2.29  106 km2 and a volume of 3.75  106 km3. The wide shallow shelf of the Chukchi Sea, with a depth of 30– 50 m, extends up to 300 km from shore (Wang et al., 2005). During the 3rd Chinese National Arctic Research Expedition (CHINARE) 25 Surface sediment samples were collected from the study area in July to September, 2008 (Fig. 1). In this study, the sampling region was separated into the northern Bering Sea, the Chukchi Sea and adjacent areas, the northernmost station B84A in the Canada Basin. Surface sediments were collected using a grab sampler and sediments were placed carefully into acid-rinsed glass vials using a plastic spatula and then stored at 20 °C before analysis. 2.2. Sample analysis

⇑ Corresponding author at: College of Oceanography and Environmental Science, Xiamen University, Xiamen 361005, China. Tel./fax: +86 592 218 0188. E-mail address: [email protected] (M.G. Cai). 0025-326X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2011.02.007

The samples were freeze-dried at 80 °C pulverized to fine powder using an agate mortar and pestle, and then sieved through

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Fig. 1. Study area and sampling stations.

a 160 mesh sieve to remove large particles. Sediment sample (0.1 ± 0.001 g, dry weight) was dissolved with 10 mL of HNO3 in an acid-washed PTFE vessel and heated at 180 °C until almost dry. HNO3 (1.0 mL) and 2.0 mL of HClO4 were subsequently added and heating continued until a wet paste was formed. Finally, ultra pure water was added and the solution maintained at 180 °C until almost dry (this step was repeated three times). After cooling, samples were diluted to 50 mL with HNO3 (1% v/v) (National standard of PRC, GB 17378.5-1998). The sample was diluted to 50 mL with Milli-Q water. Solutions were analyzed with flame atomic absorption spectrometry for Zn, and for Cu, Pb, Cr, Cd and V by graphite furnace atomic absorption spectrometry (SOLLAAR M6, Thermo Electron). Granulometry was analyzed using a laser particle size analyzer (Mastersizer 2000, Malvern). The measurement methods for total organic carbon (TOC) and black carbon (BC) followed the

procedures of Luo et al. (2010). A brief description is as follows: For TOC, about 1.0 g freeze-dried sediment sample was treated with dilute HCl solution (1:3 v/v) to remove carbonates and dried overnight at 60 °C. BC content was measured using a modified method of Gustafsson et al. (1997). Subsamples were then oxidized thermally in muffle furnace at 375 °C for 24 h in the presence of excess oxygen (air) after acidification. The samples were weighed again for mass balance calculation after cooling in the desiccator. Pretreated samples (20 and 30 mg) were weighed and packed in the clean tinfoil for TOC and BC determination, respectively. TOC and BC content were performed with Vario EL III elemental analyzer (Elementar, Germany). All reagents were pure superior grade. During sample collection and analysis, strict QA/QC measures were taken including method blanks, analysis of standard reference and analysis of duplicate

Table 1 Physical–chemical parameters and trace metal concentrations in surface sediments from the study area (mg kg1). Stations

Clay (%)

TOC (%)

BC (%)

Zn

Cr

Cu

V

Pb

Cd

NB22 NB24 BR09 BR14 BS01 BS05 BS07 BS09 C13 C15 C19 C23 C35 R07 R09 R11 B11 M07 P23 P31 R15 R17 S14 S23 B84A

8.28 1.16 1.75 10.07 1.41 2.25 2.32 2.96 8.37 15.69 5.18 8.37 10.62 13.77 15.51 7.71 36.73 27.50 36.09 34.98 30.91 23.46 29.66 18.55 17.66

1.51 0.78 1.59 0.94 1.30 0.17 1.02 1.13 1.80 0.40 1.91 1.17 1.39 0.34 0.86 0.64 0.21 0.20 1.19 0.52 1.63 0.25 1.17 0.23 0.21

0.13 0.05 0.07 0.24 0.03 0.03 0.04 0.05 0.15 0.20 0.07 0.16 0.17 0.17 0.14 0.40 0.19 0.18 0.18 0.05 0.19 0.16 0.15 0.15 0.02

76.05 45.52 67.90 102.52 36.39 24.36 25.59 21.06 84.84 111.04 62.35 80.59 106.6 96.52 83.12 125.16 150.04 168.21 145.64 128.96 164.31 132.69 146.73 142.36 100.96

38.80 17.88 25.65 32.07 17.44 11.95 12.73 8.91 29.17 40.08 16.53 31.18 26.38 29.43 28.71 35.74 46.94 37.85 35.74 28.22 36.56 36.40 32.21 36.00 22.41

13.41 4.64 15.72 17.76 6.05 2.69 2.97 3.85 14.18 18.34 9.76 12.52 19.13 15.28 11.36 18.60 37.66 36.78 12.24 43.39 26.00 32.61 38.44 19.79 49.39

82.90 59.00 77.20 83.30 58.80 33.90 32.50 35.10 83.10 111.00 67.00 82.60 85.60 84.20 77.30 104.00 176.00 149.00 186.54 121.00 155.00 125.00 161.00 137.00 129.00

2.29 2.64 1.99 3.61 3.61 1.23 1.64 0.95 4.01 6.42 2.74 3.84 5.48 5.91 4.82 5.82 15.25 8.30 9.55 3.74 9.37 8.53 10.39 9.16 8.15

0.20 0.14 0.31 0.37 0.37 0.10 0.09 0.11 0.16 0.22 0.17 0.28 0.39 0.27 0.33 0.24 0.12 0.34 0.21 0.21 0.21 0.50 0.92 0.61 0.16

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samples. Detection limits were 1.80 lg L1 for Zn, 0.03 lg L1 for Cr, 0.21 lg L1 for Pb, 0.009 lg L1 for Cd, 0.06 lg L1 for Cu, and 1.01 lg L1 for V. All samples were analyzed in duplicate. The analytical precision (coefficient of variation) was <6% for all selected metals. The quality of metals analysis methods was checked with a Standard Reference Material (NBS 1646, estuarine sediment from China), which yielded satisfactory results.

3. Results and discussion 3.1. Contents of trace metals in the surface sediments As shown in Table 1, the concentrations of the six trace metals in surface sediments from the study area varied from 21.06 to 85 °N

168.21 mg kg1 for Zn, 8.91–46.94 mg kg1 for Cr, 2.69–49.39 mg kg1 for Cu, 32.50–185.54 mg kg1 for V, 0.09–0.92 mg kg1 for Cd, and 0.95–15.25 mg kg1 for Pb. The concentrations in B84A, the northernmost station in the CHINARE, were 97.18, 28.60, 19.30, 99.86, 5.58, and 0.28 mg kg1 for Zn, Cr, Cu, V, Pb, and Cd, respectively. The spatial distribution of Zn, Cr, Cu, V, and Pb were similar to each other with high concentration in the northern Chukchi Sea and adjacent areas (Fig. 2). However, Cd showed a different spatial distribution from the other metals with high concentrations at stations S14, S23, and R17 (0.92 mg kg1, 0.61 mg kg1, and 0.5 mg kg1, respectively). The lowest concentrations of metals were found in the Bering Strait, while the highest were in the sediments from the Chukchi Plateau and areas adjacent to the Beaufort Sea and Mendeleev Ridge, which had higher contents of clay in the surface sediments. It is reported that the

85 °N B84A

B84A

Cd conc in mg kg-1

Cr conc in mg kg-1

80 °N

80 °N

Canada Basin P31

Canada Basin

P31 P23

P23 R17

M07

75 °N

B11

R15 R11 R09

Chukchi SeaR07

70 °N 65 °N

BS01

M07

75 °N

S14

R15 S14

S23 C13

C15

R11

C19

R09

C23

Chukchi SeaR07

70 °N

C35

65 °N

BS07

BS05 BS09 NB22 NB24

C19

C23 C35

BR14

BR09

BR09

60 °N

Bering Sea

55 °N 50 °N 170 °E

S23 C15

C13

BS01 BS07 NB22 BS05 BS09 NB24

BR14

60 °N

B11

R17

Bering Sea

55 °N

170 °W

180 °

150 °W

160 °W

140 °W

85 °N

50 °N 170 °E

170 °W

180 °

150 °W

160 °W

85 °N B84A

B84A

Pb conc in mg kg-1

Cu conc in mg kg-1

80 °N

80 °N

Canada Basin

P31

Canada Basin P31 P23

P23 M07

75 °N

B11

R17

S14

R09

Chukchi SeaR07

70 °N 65 °N

BS01

M07

75 °N

R15 R11

C15

R11

C19

R09

Chukchi SeaR07

70 °N

C35

65 °N

BS07

BS05 BS09 NB22 NB24

BS01

S23 C15

C13

C19

C23 C35

BS07

BS05 BS09 NB22 NB24

BR14

60 °N

B11

R17 R15

S14

S23 C13 C23

BR14

BR09

BR09

60 °N

Bering Sea

Bering Sea

55 °N

55 °N 50 °N 170 °E

170 °W

180 °

150 °W

160 °W

140 °W

85 °N

50 °N 170 °E

170 °W

180 °

150 °W

160 °W

140 °W

85 °N B84A

B84A

V conc in mg kg-1

Zn conc in mg kg-1

80 °N

80 °N

Canada Basin

P31

Canada Basin

P31

P23 M07

75 °N

P23

B11

R17 R15

R09 R07

Chukchi Sea

70 °N 65 °N

BS01

M07

75 °N

S14 R11

C15

R11

C19

R09 R07

C23

Chukchi Sea

70 °N

C35

65 °N

BS07

BS05 BS09 NB22 NB24

BS01

S23 C13

C15

C19

C23 C35

BS07

NB22 BS05 BS09 NB24 BR14

BR09

60 °N

B11

R17 R15

S14

S23 C13

BR14

BR09

60 °N

Bering Sea

55 °N 50 °N 170 °E

140 °W

Bering Sea

55 °N

180 °

170 °W

160 °W

150 °W

140 °W

50 °N 170 °E

180 °

170 °W

Fig. 2. Distribution of six trace metals in surface sediments from the study area (mg kg

160 °W 1

).

150 °W

140 °W

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Table 2 Pearson’s correlation coefficients for the metal concentrations and sediment properties. Clay Northern Bering Sea Clay 1 TOC BC Zn Cr Cu V Pb Cd

TOC 0.159 1

BC 0.931** 0.168 1

Chukchi Sea and adjacent areas (excluding station B84A) Clay 1 0.325 0.179 TOC 1 0.209 BC 1 Zn Cr Cu V Pb Cd * **

Zn

Cr

Cu

V

Pb

0.788* 0.372 0.905** 1

0.765* 0.500 0.755* 0.909** 1

0.691 0.534 0.811* 0.951** 0.871** 1

0.610 0.562 0.714* 0.930** 0.931** 0.919** 1

0.799** 0.423 0.250 1

0.552* 0.617* 0.480 0.713** 1

0.710** 0.455 0.129 0.693** 0.428 1

0.887** 0.296 0.123 0.900** 0.707** 0.574* 1

0.357 0.244 0.507 0.612 0.520 0.493 0.660 1

0.708** 0.399 0.226 0.771** 0.794** 0.503* 0.860** 1

Cd 0.341 0.477 0.514 0.670 0.549 0.724* 0.739* 0.821* 1 0.110 0.145 0.073 0.276 0.003 0.309 0.214 0.246 1

Significance at the 0.05 level. Significance at the 0.01 level.

distribution of metal in sediments is significantly influenced by the sediment characteristics. 3.1.1. Northern Bering Sea In the northern Bering Sea, trace metal concentrations in surface sediments varied from 21.06 to 102.52 mg kg1 for Zn, 8.91–38.8 mg kg1 for Cr, 2.69–17.76 mg kg1 for Cu, 32.50–83.3 mg kg1 for V, 0.09–0.37 mg kg1 for Cd, and 0.95–3.61 mg kg1 for Pb. Sediment concentrations of Zn, Cr, Cu, and V at stations NB22, BR09, and BR14 were higher than those at other stations. Pb and Cd concentrations in sediments of stations BR14 and BS01 were higher than those of other stations. The values showed that all selected metals concentrations were lower than the background shale values (Forstner and WittMann, 1981) in sediments of the northern Bering Sea except Zn and Cd in some stations. It indicated that sediments in the northern Bering Sea had not been contaminated by metals. 3.1.2. Chukchi Sea and adjacent areas Compared to the northern Bering Sea, levels of trace metals in sediments are higher in the Chukchi Sea and adjacent areas, which varying from 62.35 to 168.21 mg kg1 for Zn, 16.53–46.94 mg kg1 for Cr, 7.96–43.39 mg kg1 for Cu, 67.00–186.54 mg kg1 for V, 0.12–0.92 mg kg1 for Cd, and 2.74–15.25 mg kg1 for Pb. High concentrations of metals were observed at stations B11, M07, P23, P31, R15, R17, S14, and S23, the depth of which are deeper than other stations. Everaarts and Nieuwenhuize (1995) found that the concentrations of metals in surface sediments increased with the depth of station. The concentrations of Zn, V, and Cd in some stations were slightly higher than the background shale values, indicating that sediments from the Chukchi Sea and adjacent areas had been contaminated by these metals. It may be due to the transportation of sea ice and atmospheric conditions in the Arctic Ocean (Pfirman et al., 1995). The results obtained from the current study were compared with those from other regions of the Arctic Ocean. The mean concentrations of all metals except Cd in the northern Bering Sea were lower than those of the northeastern Chukchi Sea (Naidu et al., 1997), Beaufort Sea Inner Shelf (Sweeney and Naidu, 1989), Laptev Sea (Holemann et al., 1999), and Kara Sea (Sericano et al., 2001).

Sweeney and Naidu (1989) note that the Beaufort Sea is a relatively pristine environment presumably due to a lack of large-scale industrialization in nearby areas. Thus we suggested that the lower trace metal concentration was not likely to indicate any increased anthropogenic input of metal contaminants into the northern Bering Sea. With the exception of Zn, Cu and Cd, the concentrations of all other metals measured in the Chukchi Sea and adjacent areas were within range of or lower than the levels determined in sediments from other regions in the Arctic.

3.2. Assessment of trace metal pollution Geoaccumulation index (Igeo) was calculated to assess the extent of sediment contamination in studied area. It was originally defined by Müller’s (1979) and can be calculated by the following expression:

 Igeo ¼ log2

Cn 1:5Bn



where Cn is the measured content of the examined metal ‘‘n’’ in the sediment, Bn is the geochemical background content of the metal ‘‘n’’, and factor 1.5 is the background matrix correction factor because of lithogenic effects. Because we did not have the background values of metals in the sediment from the studied area, geoaccumulation index had been calculated by using the earth crust values (Forstner and WittMann, 1981). Müller has distinguished seven classes of geoaccumulation index from class 0 (Igeo 6 0) to class 6 (Igeo P 5) (Müller, 1981). The results indicated that Cr, Cu, V, and Pb showed an unpolluted situation at all stations (Igeo < 0). Zn showed a light pollution at stations B11, M07, P23, R15, and S14 (Igeo, 0.03–0.24), and other stations showed an unpolluted situation (Igeo < 0). For Cd, medium pollution was observed at station S14 (Igeo = 1.03) while light pollution was observed at stations R17 and S23 (Igeo = 0.14–0.45). All other stations showed an unpolluted situation of Cd. The results suggested that stations B11, M07, P23, R15, R17, S14, and S23 were polluted to some extent by the trace metals Zn or Cd or both whereas none of the other stations showed any signs of metal pollution.

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3.3. Relationships between trace metals and some physical–chemical properties of the sediments Grain size, TOC, and BC are the most important factors controlling the distribution of trace metals in sediments (Zhang et al., 2009; He and Zhang, 2009). Clay contents were low in all our sediment samples, ranging from 1.16% to 36.73%. Significant spatial variation was observed, with increased clay content in sediments of the Chukchi Sea and adjacent areas. TOC concentrations ranged from 0.17% to 1.91%, with a mean of 0.90%. Station C19 in the Chukchi Sea had the highest TOC content; while station BS05 in the Bering Strait had the lowest TOC content. BC content in sediments varied from 0.02% to 0.40%, with a mean of 0.13%. The lowest BC was found at station B84A (0.02%). To explore the relationship among trace metals, clay, TOC, and BC contents in the surface sediments, Pearson’s correlation coefficients were calculated (Table 2). Because of the difference of sediment component and hydrology condition, we divided into two areas to discuss, northern Bering Sea and Arctic (here including Chukchi Sea and adjacent areas), respectively. In the northern Bering Sea, significantly positive correlations were found between metals (Zn and Cr) with clay contents indicating that these trace metals may have considerable association with the clay. The correlations between metals (Zn, Cr, Cu, and V) with BC contents were significant. Similar result is also observed in other study (He and Zhang, 2009). Significant correlations among most metals (Zn, Cr, Cu, and V) were also found, which suggested these metals were redistributed in the sediments by the same physico-chemical processes or had a similar source (Baptista Neto et al., 2006; Bai et al., 2011). No significant correlations were observed between metals (Cd and Pb) with the tested factors, indicating that the distribution of Cd and Pb in the sediments from the northern Bering Sea is possibly affected by other factors. In the Chukchi Sea and adjacent areas, the results showed low correlations between trace metals (Zn, Cr, Cu, V, and Pb) with TOC and BC, while considerable positive correlation with clay content. It suggested that the distribution of grain size was of greater importance than TOC or BC in influencing the metal distribution in the sediments from the Chukchi Sea and adjacent areas. Significant correlations among Zn, Cr, Cu, V, and Pb were also found. However, low correlations found between Cd with other parameter in the Chukchi Sea and adjacent areas indicated that the distribution of Cd in sediments is influenced by many other factors. 4. Conclusions Analyses of trace metals (Zn, Cr, Cu, V, Pb, and Cd) in surface sediments from the study area suggested that metal concentrations in the sediments of the Chukchi Sea and adjacent areas were higher than those from the northern Being Sea. This disparity could be due to several factors. The geoaccumulation index (Igeo) of selected metals indicated that some metals (Cr, Cu, V, and Pb) were not major pollutants, while other metals (Zn and Cd) showed some extent of contamination in the study area. Acknowledgements Financial support for this study mainly came from the National Natural Science Foundation of China (NSFC) through grants Nos. 40776040, 40306012 and 40776003. It was also funded partly by the National High Technology Research and Development Program

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