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Distribution and sources of polycyclic aromatic hydrocarbons in surface sediments from the Bering Sea and western Arctic Ocean
Marine Pollution Bulletin 104 (2016) 379–385
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
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Distribution and sources of polycyclic aromatic hydrocarbons in surface sediments from the Bering Sea and western Arctic Ocean Mengwei Zhao a,b, Weiguo Wang b,⁎, Yanguang Liu c, Linsen Dong c, Liping Jiao b, Limin Hu c, Dejiang Fan a a b c
College of Marine Geosciences, Ocean University of China, Qingdao 266100, China Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
a r t i c l e
i n f o
Article history: Received 16 September 2015 Received in revised form 8 January 2016 Accepted 14 January 2016 Available online 21 January 2016 Keywords: Polycyclic aromatic hydrocarbon Surface sediment Western Arctic Ocean Bering Sea
a b s t r a c t To analyze the distribution and sources of polycyclic aromatic hydrocarbons (PAHs) and evaluate their potential ecological risks, the concentrations of 16 PAHs were measured in 43 surface sediment samples from the Bering Sea and western Arctic Ocean. Total PAH (tPAH) concentrations ranged from 36.95 to 150.21 ng/g (dry weight). In descending order, the surface sediment tPAH concentrations were as follows: Canada Basin N northern Chukchi Sea N Chukchi Basin N southern Chukchi Sea N Aleutian Basin N Makarov Basin N Bering Sea shelf. The Bering Sea and western Arctic Ocean mainly received PAHs of pyrogenic origin due to pollution caused by the incomplete combustion of fossil fuels. The concentrations of PAHs in the sediments of the study areas did not exceed effects range low (ERL) values. © 2016 Elsevier Ltd. All rights reserved.
There is a growing awareness of persistent organic pollutants and the harm they cause to human health and ecosystems (Gschwend and Hites, 1981; Brown et al., 1998; Jones and Devoogt, 1999), and polycyclic aromatic hydrocarbons (PAHs) are among the major persistent organic pollutants that are discharged from the land into the sea (MacDonald et al., 1998; Grebmeier et al., 2010). Due to their chemical stability, PAHs tend to remain in the environment for a long time, and they can migrate globally from areas of human activity through marine biogeochemical processes or settle into high-latitude alpine regions through distillation and condensation (Goldberg, 1975; Friedman and Selin, 2012). Once transported to the marine environment, PAHs can be easily adsorbed by suspended particles and carried into seabed sediments, from which they may be re-released into the water or even biological tissues, resulting in secondary pollution (Stout and Emsbo-Mattingly, 2008; Achten and Hofmann, 2009; Harris et al., 2011). Therefore, marine sediments comprise both a “sink” and a “source” of PAHs. Studies of persistent organic pollutants in marine sediments have great scientific significance, as such research can clarify pollutant levels, distributions, and source characteristics as well as the environmental risks posed by such pollutants (Baumard et al., 1998a, b; Magi et al., 2002; Nakata et al., 2005; Boonyatumanond et al., 2006; Chen et al., 2013; Rahmanpoor et al., 2014). The Arctic region, which experiences low temperatures throughout the year, is an area that collects materials transported by the atmosphere and ocean currents (Chernyak et al., 1996; Jantunen and ⁎ Corresponding author. E-mail address: [email protected] (W. Wang).
http://dx.doi.org/10.1016/j.marpolbul.2016.01.016 0025-326X/© 2016 Elsevier Ltd. All rights reserved.
Bidleman, 1998). The harsh Arctic climate and environment create difficulties for sampling, and research on PAHs has mainly been conducted in densely populated areas, estuaries, bays, and coastal zones of developed regions (Melnikov et al., 2003; Ding et al., 2007; Yunker et al., 2014). The natural environment and ecology of the Arctic region are substantially affected by human social interactions and human settlement and industrialization (Jaffrezo et al., 1994; Yunker et al., 1995, 2005; Stein and MacDonald, 2004; Stein, 2008). In the present study, we have quantitatively analyzed PAHs in sediment samples from the Bering Sea and western Arctic Ocean to examine their distribution and sources as well as pollution levels. Forty-three surface sediment samples (depth: 0–2 cm) were collected from the Bering Sea and western Arctic Ocean using the “Xuelong” icebreaker during the Fourth Chinese National Arctic Research Expedition (July–August, 2010). Sampling stations were selected from the Aleutian Basin to the north, through the Bering Strait, Chukchi Plateau, Canada Basin, Alpha Ridge, and Makarov Basin (53–89°N; Fig. 1). Sediment samples were sealed in aluminum boxes on site, stored at −20 °C, and, after transportation to the laboratory, lyophilized and milled. Each of the sediment samples (5.00 g each) was weighed and mixed with 1 g of diatomaceous earth, after which the mixture was placed into an extraction pool and the internal standard was added to extract the PAHs. The PAHs were extracted using an accelerated solvent extraction system (ASE200, Dionex, USA) under the following conditions: n-hexane and methylene chloride (1:1 volume ratio) as the extraction solvent, a temperature of 100 °C, a pressure of 1000–1500 psi (6.9–10.3 × 106 Pa), two repetitions of a 5-min static extraction, an elution volume of 60%, and a purge time of 8 s.
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Fig. 1. Location of the sampling stations in the Bering Sea and western Arctic Ocean.
Sixteen PAHs were analyzed using a Shimadzu QP-2010 plus/GC–MS, and the following chromatographic conditions: DB-5 MS capillary column (30 m × 0.25 mm × 0.25 μm), high-purity helium carrier gas, splitless injection (1 μL injection volume), and ion source and quadrupole temperatures of 250 °C and 150 °C, respectively. The PAH samples were combined with alternative internal standards (naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysened12, and perylene-d12), while blank samples, duplicate samples, and spiked blank samples were prepared to control for the recovery of the analytical method. Recoveries of the PAH monomer from the spiked blank samples ranged from 86.1%–119.06%. No target contaminants were detected in the blank samples. Prior to grain size analysis, sediment samples were soaked in 10% H2O2 to remove organic matter, and grain size was determined using the Malvern 2000 laser particle size analyzer (Malvern Instruments Ltd., Worcestershire, UK). Measurements ranged from 0.02–2000 μm. The resolution of the grain size measurements was 0.01 Φ, and the relative error of repeated analyses was b 3%. Total organic carbon (TOC) was determined using a Vario EL Cube CN organic elemental analyzer. Sediment samples were dried at a low temperature (50 °C) before being ground, and approximately 0.15 g of the ground samples was weighed and treated with hydrochloric acid to remove carbonate. Following completion of the reaction, each sample was repeatedly washed with deionized water, after which it was centrifuged, and the supernatant was decanted until neutral pH was reached. After drying, 10 mg of each sample was weighed, using a millionth balance, and wrapped with a tin cup.
To delineate the spatial distribution of the PAHs, the study area was divided into seven regions according to geographic location: the Aleutian Basin, Bering Sea shelf, southern Chukchi Sea, northern Chukchi Sea, Chukchi Basin, Canada Basin, and Makarov Basin. The compositions and concentrations of PAHs in the surface sediments of the study area are presented in Table 1, and the distribution of total PAH (tPAH) concentrations is shown in Fig. 2a. Of the 16 typical PAHs under environmental priority control, 15 (anthracene was the exception) were detected in the sediment samples (Table 1). The mean grain sizes of the surface sediments ranged from 3.7 Φ (SR01, southern Chukchi Sea) to 8.03 Φ (MS03, Canada Basin) in the Bering Sea and western Arctic Ocean study area. In general, the mean grain sizes of the surface sediments decreased gradually from the Bering Sea to the deep western Arctic Ocean. The TOC percentage in the sediments analyzed ranged from 0.017% (BN13, Makarov Basin) to 2.559% (BB06, Bering Sea shelf), with highest levels corresponding to the Bering Sea shelf (Fig. 2). PAHs were detected at every station in the study area, and tPAH concentrations (dry weight) ranged from 36.95–150.21 ng/g with a mean value of 70.61 ng/g. The distribution (decreasing) of PAHs in the surface sediments of the study area was as follows: Canada Basin N northern Chukchi Sea N Chukchi Basin N southern Chukchi Sea N Aleutian Basin N Makarov Basin N Bering Sea shelf. Of the simple components of the PAHs, the phenanthrene content was highest in the study area, and the second highest content was observed for benzo[g,h,i]perylene. A comparative analysis of the total contents of low-ring (two- and three-ring) and high-ring (four- to
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Fig. 2. Distributions of PAH /ng × g−1 (a), TOC /% (b), Mz/Φ (c) and Clay/% (d) in the surface sediments from the study area.
six-ring) PAHs revealed that high-ring PAHs constituted more than 50% of the contents at various stations. The three-ring PAHs constituted the highest content (28.23–41.0%, Fig. 3), followed by four-ring PAHs (17.6–36.72%) and five-ring PAHs (13.92–24.83%). The high-ring PAH contents pertaining to the different regions were as follows: in descending order, southern Chukchi Sea N Aleutian Basin N northern Chukchi Sea N Canada Basin N Chukchi Basin N Makarov Basin N Bering Sea shelf. In general, the PAH contents of marine sediments are believed to be affected primarily by TOC content and grain size (Kim et al., 1999; Yang, 2000; Yamamoto and Polyak, 2009). The TOC content within surface sediments depends on ocean currents and terrigenous inputs, and fine-grained sediments contain abundant TOC. The larger the sediment grain size, the lower the TOC content and the smaller the amount of adsorbed PAHs. In the present study, a positive correlation was found between tPAH concentration and TOC content in four regions (Fig. 4), as follows: Canada Basin N Chukchi Basin N southern Chukchi Sea N Makarov Basin. However, no significant correlations were found between TOC content and PAH accumulation in the sediments of the Aleutian Basin, Bering Sea shelf, or northern Chukchi Sea.
In the Aleutian Basin, the concentration of tPAHs in surface sediments gradually decreased with increasing latitude, and similar trends have been observed in the atmosphere of the same region (Ding et al., 2007); thus, atmospheric deposition appears to significantly affect the accumulation of PAHs. Due to atmospheric circulation within the Bering Sea slope, the deposition rate is relatively high in the Bering Sea shelf. This region is one of the world's most productive marine areas and is referred to as the “Green Belt” (Wang et al., 2005). Deposition rates vary in different areas of the sea, and surface sediment samples collected at a depth of 2 cm represent different ages. The highest TOC content was detected in surface sediments from the Bering Sea shelf. However, the high deposition rate in this area has a “dilution” effect on sediment tPAH concentrations, which may be the predominant reason behind the correlation between high TOC values and low tPAH values in the Bering Sea shelf. Relatively warm and nutrient-rich Pacific water masses profoundly impact the Chukchi Sea as well as the entire Arctic Ocean. Due to the seasonality of open water conditions and the substantial inputs of terrigenous material, the Chukchi Sea has high primary productivity with a high deposition rate (Backman et al., 2004). The TOC content of surface
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Fig. 3. Percent content of polycyclic aromatic hydrocarbons with varying numbers of rings at sampling stations in the study area. AB, Aleutian Basin; BSS, Bering Sea Shelf; SCS, Southern Chukchi Sea; NCS, Northern Chukchi Sea; CSB, Chukchi Basin; CB, Canada Basin; MB, Makarov Basin.
sediments of the northern Chukchi Sea was second only to the Bering Sea shelf. At stations in the northern Chukchi Sea, tPAH concentrations in surface sediments decreased as the distance from the coastline increased. The clockwise flow of the Beaufort Gyre is conducive to the transportation of river-borne materials westward from the Beaufort Sea and along the Northwind Ridge to the northern Chukchi Sea (MacDonald et al., 1998). High-value stations (tPAHs N 100 ng/g) were distributed through Barrow Point and the Prudhoe Bay offshore area, close to the mouth of the Mackenzie and Colleyville Rivers. As the distance from the polluted area increased, tPAH concentrations gradually decreased, providing a spatial pattern that is consistent with the conclusions of Baumard et al. (1998a,b) and Kim et al. (1999). Environmental sources of PAHs are complex. On the one hand, the various origins of PAHs indicate differences in their structures and compositions; on the other hand, these compounds remain relatively stable throughout their migration and deposition (Sicre et al., 1987; Basheer et al., 2003). According to Sicre et al. (1987), Fla/Pyr b 1 indicates that the corresponding PAHs originate from petroleum, while Fla/Pyr N 1 indicates fossil fuel combustion as their origin. We calculated the Flu/Pyr ratio to determine PAHs sources (Fig. 5) and found that Fla/Pyr values ranged from 0.937–2.15. Low Fla/Pyr values (b1) were only obtained for three
Fig. 5. The Fla/Pyr and Pyr/BaP ratios in the surface sediments from the study area (dry weight). Fla, fluoranthene; Pyr, pyrene; Ben, benzo[a]pyrene.
stations in close proximity to one another, namely, Mor2 (Canada Basin), SR11 (northern Chukchi Sea), and SR12 (northern Chukchi Sea); and high values (N1) were obtained for the remaining stations. Based on these ratios, there appear to have been petroleum inputs in the region surrounding stations Mor2, SR11, and SR12 (74–74.5°N, 159–169°W); however, the remaining regions received PAHs primarily from the incomplete combustion of minerals. The Pyr/BaP ratio is one of the important parameters used to distinguish between pollutants emanating from gasoline combustion (exhaust) versus coal combustion. Pyr/BaP b 1 is generally believed to correspond to coal combustion emissions, while a Pyr/BaP ratio of 1–6 indicates exhaust emissions (Zhu et al., 2001). A comparative analysis showed that Pyr/BaP values ranged from 0 to 2.16 in our study area (Fig. 5). In the present study, Pyr/BaP values remained below 1, excepting the high-PAH stations (B02, B04, BB05 and NB03) in the Bering Sea, indicating that the predominant source of PAHs in the area evaluated resulted from coal combustion emissions. In the western Arctic Ocean (excepting 74–75.65°N and 87–88.39°N), the much higher value of Pyr/BaP suggests that gasoline combustion (exhaust) might be more important. The pollutants in Siberia mainland and Alaska can move into the Arctic region easily by atmospheric and river transportation. Yunker et al. (2002) noted that Fla/(Fla + Pyr) b 0.4 indicates PAHs of petroleum origin, while 0.4 ≤ Fla/(Fla + Pyr) ≤ 0.5 indicates origination from the combustion of liquid fossil fuels (unprocessed crude oil
Fig. 4. Correlation of PAH and TOC in the surface sediments from the study area.
M. Zhao et al. / Marine Pollution Bulletin 104 (2016) 379–385
Fig. 6. The BaA/(BaA + Chry) and Fla/(Fla + Pyr) ratios in the surface sediments from the study area (dry weight). BaA, benzo[a]anthracene; Chry, chrysene; Fla, fluoranthene; Pyr, pyrene.
combustion and exhaust emissions), and Fla/(Fla + Pyr) N 0.5 indicates origination from the burning of herbaceous and woody plants and coal combustion. In the area evaluated, the present study determined
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Fla/(Fla + Pyr) values between 0.48 and 0.68. The Fla/(Fla + Pyr) values ranged from 0.48–0.5 only at the following three adjacent stations: Mor2 (Canada Basin), SR11 (northern Chukchi Sea), and SR12(northern Chukchi Sea). The PAHs at these three stations possibly originated from unprocessed crude oil combustion. With the exception of these three stations, Fla/(Fla + Pyr) values were all N0.5 in the study area evaluated herein, indicating that the PAHs in the sediment samples analyzed originated predominantly from the combustion of herbaceous and woody plants and coal (Fig. 6). To further determine the sources of PAHs in the surface sediments of the study area, we utilized the BaA/(BaA + Chry) ratio according to Yunker et al. (2002). BaA/(BaA + Chry) b 0.2 indicates a petroleum origin; 0.2 ≤ BaA/(BaA + Chry) ≤ 0.35 indicates a mixed origin; and BaA/(BaA + Chry) N 0.35 indicates a combustion origin. The BaA/ (BaA + Chry) values ranged from 0.21–0.48 in our study area (Fig. 6), indicating that almost no PAHs originated from petroleum. The BaA/ (BaA + Chry) values determined herein exhibited specific trends within the different study regions. For instance, BaA/(BaA + Chry) values were N0.35 at all the stations in the Bering Sea shelf, at three stations in the northern Chukchi Sea (SR10, SR11, and SR12), and at two stations at the entrance of the Bering Strait (SR01 and SR02), indicating that the PAHs in these sediments originated mainly from combustion. On the other hand, BaA/(BaA + Chry) values were generally b0.35 for the Aleutian Basin, the central south portion of the Chukchi Sea, the Canada Basin, and the Makarov Basin, indicating PAHs of mixed origin.
Table 1 PAH concentration, grain size, total organic (TOC) and total organic/total nitrogen (C/N mole ratios) of surface sediments from the study area. Station B02 B04 B06 B11 B14 NB01 BB01 NB03 NB04 BB05 BB06 SR01 SR02 CC4 SR04 R06 R08 C04 S21 C09 S23 R09 C07 SR10 SR11 SR12 BN03 S25 S26 MS01 MS02 MS03 Mor2 M07 M05 M03 M02 BN06 BN07 BN09 BN10 BN12 BN13
Lon. (°)
Lat. (°)
169.96 171.40 174.49 179.92 −177.69 −175.08 −177.48 −172.20 −170.63 −175.33 −174.38 −168.97 −168.98 −167.86 −169.00 −168.98 −168.98 −167.03 −154.72 −159.71 −153.76 −168.94 −165.33 −169.00 −168.99 −169.00 −158.90 −152.50 −153.55 −154.71 −156.37 −157.30 −158.99 −172.03 −172.13 −171.83 −171.99 −164.94 −166.47 −167.13 −178.64 −170.49 −176.63
53.33 54.59 57.01 59.99 60.92 61.23 61.29 61.51 61.58 62.54 63.01 67.00 67.50 68.13 68.50 69.50 71.00 71.01 71.62 71.81 71.93 71.96 72.54 73.00 73.99 74.50 76.45 72.34 72.70 73.17 73.68 74.07 74.55 74.99 75.65 76.50 77.00 81.46 82.48 84.19 85.50 87.07 88.39
Depth (m) 1937 3873 3780 2603 130 92 130 61 49 79 78 49 51 52 56 52 44 46 46 50 338 51 51 77 184 187 1443 2830 3521 3814 3743 3890 1224 393 1617 2298 2300 3566 3627 2500 2434 4000 3995
Note: Mz means grain size; “/” means no data.
Range
Aleutian Basin
Bering Sea Shelf
Southern Chukchi Sea
Northern Chukchi Sea
Canada Basin
Chukchi Basin
Makarov Basin
ΣPAHs (ng/g) 76.56 66.84 59.21 41.62 46.73 39.95 52.40 88.75 37.94 97.41 43.75 39.28 48.13 59.47 89.81 76.05 70.09 63.68 127.00 68.38 94.72 80.58 63.64 44.35 45.23 44.09 56.19 145.89 71.40 107.53 88.33 150.21 130.75 62.20 38.94 71.71 90.13 74.16 70.83 62.13 64.75 48.33 36.95
Sand (%)
Silt (%)
Clay (%)
Mz (Φ)
TOC (%)
46.34 6.31 11.39 16.20 22.36 11.45 21.20 15.67 / 27.98 9.83 66.13 47.76 33.42 7.21 13.69 27.08 21.07 23.59 25.35 7.26 10.49 0.76 8.67 1.04 28.71 19.43 6.60 5.44 5.97 14.03 0.06 22.03 2.83 6.49 8.87 2.90 20.02 19.61 20.58 16.27 7.33 11.51
41.18 78.82 72.94 76.05 71.78 81.23 70.49 52.94 / 63.49 80.37 31.74 49.10 58.21 83.04 73.85 67.62 72.21 63.73 60.21 71.79 71.56 81.47 75.44 71.28 63.01 53.18 78.67 58.07 68.76 48.12 52.32 50.81 82.62 55.78 67.80 62.63 43.21 37.93 36.38 44.15 42.36 48.89
12.48 14.87 15.67 7.76 5.87 7.33 8.31 31.39 / 8.53 9.80 2.13 3.14 8.38 9.75 12.46 5.31 6.73 12.68 14.44 20.94 17.95 17.78 15.89 27.68 8.29 27.39 14.74 36.49 25.27 37.86 47.62 27.17 14.55 37.74 23.33 34.47 36.77 42.46 43.04 39.58 50.31 39.61
4.76 6.36 6.14 5.55 5.24 5.69 5.47 6.65 / 5.43 5.89 3.77 4.35 4.79 5.92 5.96 5.01 5.31 5.65 5.62 6.56 6.33 6.79 6.37 7.22 5.12 6.31 6.40 7.45 7.11 7.02 8.03 6.28 6.56 7.48 6.84 7.49 6.59 6.86 6.48 6.91 7.84 7.17
0.52 1.37 0.77 0.36 1.45 1.80 2.16 0.83 / 1.61 2.56 0.27 1.03 0.28 1.49 1.66 1.17 0.03 1.38 / 1.70 / 1.35 1.99 1.80 1.29 1.42 1.67 / 1.21 0.87 0.95 1.08 1.02 0.70 0.90 1.20 1.38 1.11 0.76 0.44 0.43 0.02
C/N 3.77 6.63 6.03 4.60 16.33 7.91 9.11 5.46 / 8.49 35.99 4.27 6.20 3.14 6.57 11.18 7.72 0.74 10.58 / 5.99 / 8.22 8.11 7.44 6.50 13.35 8.62 / 7.13 6.03 6.01 9.80 5.38 4.96 6.82 8.93 6.99 6.83 7.76 4.33 4.22 0.47
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Table 2 Risk assessment for PAHs in the sediments from the study area. (ng/g dry weight). PAHs
Content range
ERL
ERM
Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benz[a]anthracene Chrysene Benz[b]acephenanthrylene Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Dibenz[a,h]anthracene Benzo[g,h,i]perylene ΣPAHs
3.36–14.22 N.D.–5.77 2.84–5.84 2.61–8.96 3.46–36.9 N.D. 2.03–11.66 1.79–9.37 1.82–6.54 1.95–23.34 2.89–14.51 1.42–4.21 N.D.–11.12 N.D.–9.64 N.D.–6.5 2.73–21.4 36.95–150.21
16 44 16 19 240 85.3 600 665 261 384 320 280 430 – 63.4 430 3912.4
2100 640 500 540 1500 1100 5100 2500 1600 2800 1880 1620 1600 – 260 1600 25,340
Long et al. (1995) proposed the effects range low (ERL, probability of adverse biological effects b 10%) and the effects range median (ERM, probability of adverse biological effects N 50%) to determine the potential ecological risks of organic pollutants in marine and estuarine sediments. The concentrations of the various PAHs detected at the stations evaluated herein were lower than the ERL (Table 2), indicating that these pollutants have a low potential of posing ecological risks to surrounding marine life and have few toxic effects on living organisms. This study provides information regarding the distribution and sources of PAHs in surface sediments from the Bering Sea and western Arctic Ocean. The spatial distribution of tPAH concentrations followed the following decreasing trend: Canada Basin N northern Chukchi Sea N Chukchi Basin N southern Chukchi Sea N Aleutian Basin N Makarov Basin N Bering Sea shelf. This distribution of PAHs was controlled by numerous factors but were predominantly affected by the TOC content and mean grain size in the southern Chukchi Sea, Chukchi Basin, and Makarov Basin; the distance from the source area, TOC content, and ocean current system in the Canada Basin; atmospheric deposition in the Aleutian Basin; and the distance from the source area, deposition rate, and ocean current system in the Chukchi Sea shelf and the northern Chukchi Sea. Combustion of herbaceous and woody plants as well as fossil fuels such as coal constituted the primary sources of PAHs in the study area evaluated herein. Acknowledgments This study was funded by the “Fourth Chinese National Arctic Research Expedition”, under contract number CHINARE-2010, and the Chinese Polar Environment Comprehensive Investigation & Assessment Programs, under contract numbers CHINARE2014-03-02 and CHINARE2014-04-03-03. The authors thank various colleagues for assisting with samples collection and processing, and the anonymous reviewers for their constructive comments and helpful suggestions, which have improved the manuscript. References Achten, C., Hofmann, T., 2009. Native polycyclic aromatic hydrocarbons (PAH) in coals—a hardly recognized source of environmental contamination. Sci. Total Environ. 407, 2461–2473. Backman, J., Jakobsson, M., Løvlie, R., Febo, L., 2004. Is the central Arctic Ocean a sediment starved basin? Quat. Sci. Rev. 23, 1435–1454. Basheer, C., Obbard, J.P., Keelee, H., 2003. Persistent organic pollutants in Singapore's coastal marine environment: part II, sediments. Water Air Soil Pollut. 149, 315–323. Baumard, P., Buzinski, H., Michon, Q., Garrigues, P., Burgeat, T., Bellocq, J., 1998a. Origin and bioavailability of PAHs in the Mediterranean Sea from mussel and sediment records. Estuarine 47, 77–90. Baumard, P., Budzinski, H., Garrigues, P., 1998b. Polycyclic aromatic hydrocarbons in sediments and mussels of the western Mediterranean Sea. Environ. Toxicol. Chem. 17, 765–776. http://dx.doi.org/10.1002/etc.5620170501.
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Baseline
Distribution and sources of polycyclic aromatic hydrocarbons in surface sediments from the Bering Sea and western Arctic Ocean Mengwei Zhao a,b, Weiguo Wang b,⁎, Yanguang Liu c, Linsen Dong c, Liping Jiao b, Limin Hu c, Dejiang Fan a a b c
College of Marine Geosciences, Ocean University of China, Qingdao 266100, China Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
a r t i c l e
i n f o
Article history: Received 16 September 2015 Received in revised form 8 January 2016 Accepted 14 January 2016 Available online 21 January 2016 Keywords: Polycyclic aromatic hydrocarbon Surface sediment Western Arctic Ocean Bering Sea
a b s t r a c t To analyze the distribution and sources of polycyclic aromatic hydrocarbons (PAHs) and evaluate their potential ecological risks, the concentrations of 16 PAHs were measured in 43 surface sediment samples from the Bering Sea and western Arctic Ocean. Total PAH (tPAH) concentrations ranged from 36.95 to 150.21 ng/g (dry weight). In descending order, the surface sediment tPAH concentrations were as follows: Canada Basin N northern Chukchi Sea N Chukchi Basin N southern Chukchi Sea N Aleutian Basin N Makarov Basin N Bering Sea shelf. The Bering Sea and western Arctic Ocean mainly received PAHs of pyrogenic origin due to pollution caused by the incomplete combustion of fossil fuels. The concentrations of PAHs in the sediments of the study areas did not exceed effects range low (ERL) values. © 2016 Elsevier Ltd. All rights reserved.
There is a growing awareness of persistent organic pollutants and the harm they cause to human health and ecosystems (Gschwend and Hites, 1981; Brown et al., 1998; Jones and Devoogt, 1999), and polycyclic aromatic hydrocarbons (PAHs) are among the major persistent organic pollutants that are discharged from the land into the sea (MacDonald et al., 1998; Grebmeier et al., 2010). Due to their chemical stability, PAHs tend to remain in the environment for a long time, and they can migrate globally from areas of human activity through marine biogeochemical processes or settle into high-latitude alpine regions through distillation and condensation (Goldberg, 1975; Friedman and Selin, 2012). Once transported to the marine environment, PAHs can be easily adsorbed by suspended particles and carried into seabed sediments, from which they may be re-released into the water or even biological tissues, resulting in secondary pollution (Stout and Emsbo-Mattingly, 2008; Achten and Hofmann, 2009; Harris et al., 2011). Therefore, marine sediments comprise both a “sink” and a “source” of PAHs. Studies of persistent organic pollutants in marine sediments have great scientific significance, as such research can clarify pollutant levels, distributions, and source characteristics as well as the environmental risks posed by such pollutants (Baumard et al., 1998a, b; Magi et al., 2002; Nakata et al., 2005; Boonyatumanond et al., 2006; Chen et al., 2013; Rahmanpoor et al., 2014). The Arctic region, which experiences low temperatures throughout the year, is an area that collects materials transported by the atmosphere and ocean currents (Chernyak et al., 1996; Jantunen and ⁎ Corresponding author. E-mail address: [email protected] (W. Wang).
http://dx.doi.org/10.1016/j.marpolbul.2016.01.016 0025-326X/© 2016 Elsevier Ltd. All rights reserved.
Bidleman, 1998). The harsh Arctic climate and environment create difficulties for sampling, and research on PAHs has mainly been conducted in densely populated areas, estuaries, bays, and coastal zones of developed regions (Melnikov et al., 2003; Ding et al., 2007; Yunker et al., 2014). The natural environment and ecology of the Arctic region are substantially affected by human social interactions and human settlement and industrialization (Jaffrezo et al., 1994; Yunker et al., 1995, 2005; Stein and MacDonald, 2004; Stein, 2008). In the present study, we have quantitatively analyzed PAHs in sediment samples from the Bering Sea and western Arctic Ocean to examine their distribution and sources as well as pollution levels. Forty-three surface sediment samples (depth: 0–2 cm) were collected from the Bering Sea and western Arctic Ocean using the “Xuelong” icebreaker during the Fourth Chinese National Arctic Research Expedition (July–August, 2010). Sampling stations were selected from the Aleutian Basin to the north, through the Bering Strait, Chukchi Plateau, Canada Basin, Alpha Ridge, and Makarov Basin (53–89°N; Fig. 1). Sediment samples were sealed in aluminum boxes on site, stored at −20 °C, and, after transportation to the laboratory, lyophilized and milled. Each of the sediment samples (5.00 g each) was weighed and mixed with 1 g of diatomaceous earth, after which the mixture was placed into an extraction pool and the internal standard was added to extract the PAHs. The PAHs were extracted using an accelerated solvent extraction system (ASE200, Dionex, USA) under the following conditions: n-hexane and methylene chloride (1:1 volume ratio) as the extraction solvent, a temperature of 100 °C, a pressure of 1000–1500 psi (6.9–10.3 × 106 Pa), two repetitions of a 5-min static extraction, an elution volume of 60%, and a purge time of 8 s.
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Fig. 1. Location of the sampling stations in the Bering Sea and western Arctic Ocean.
Sixteen PAHs were analyzed using a Shimadzu QP-2010 plus/GC–MS, and the following chromatographic conditions: DB-5 MS capillary column (30 m × 0.25 mm × 0.25 μm), high-purity helium carrier gas, splitless injection (1 μL injection volume), and ion source and quadrupole temperatures of 250 °C and 150 °C, respectively. The PAH samples were combined with alternative internal standards (naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysened12, and perylene-d12), while blank samples, duplicate samples, and spiked blank samples were prepared to control for the recovery of the analytical method. Recoveries of the PAH monomer from the spiked blank samples ranged from 86.1%–119.06%. No target contaminants were detected in the blank samples. Prior to grain size analysis, sediment samples were soaked in 10% H2O2 to remove organic matter, and grain size was determined using the Malvern 2000 laser particle size analyzer (Malvern Instruments Ltd., Worcestershire, UK). Measurements ranged from 0.02–2000 μm. The resolution of the grain size measurements was 0.01 Φ, and the relative error of repeated analyses was b 3%. Total organic carbon (TOC) was determined using a Vario EL Cube CN organic elemental analyzer. Sediment samples were dried at a low temperature (50 °C) before being ground, and approximately 0.15 g of the ground samples was weighed and treated with hydrochloric acid to remove carbonate. Following completion of the reaction, each sample was repeatedly washed with deionized water, after which it was centrifuged, and the supernatant was decanted until neutral pH was reached. After drying, 10 mg of each sample was weighed, using a millionth balance, and wrapped with a tin cup.
To delineate the spatial distribution of the PAHs, the study area was divided into seven regions according to geographic location: the Aleutian Basin, Bering Sea shelf, southern Chukchi Sea, northern Chukchi Sea, Chukchi Basin, Canada Basin, and Makarov Basin. The compositions and concentrations of PAHs in the surface sediments of the study area are presented in Table 1, and the distribution of total PAH (tPAH) concentrations is shown in Fig. 2a. Of the 16 typical PAHs under environmental priority control, 15 (anthracene was the exception) were detected in the sediment samples (Table 1). The mean grain sizes of the surface sediments ranged from 3.7 Φ (SR01, southern Chukchi Sea) to 8.03 Φ (MS03, Canada Basin) in the Bering Sea and western Arctic Ocean study area. In general, the mean grain sizes of the surface sediments decreased gradually from the Bering Sea to the deep western Arctic Ocean. The TOC percentage in the sediments analyzed ranged from 0.017% (BN13, Makarov Basin) to 2.559% (BB06, Bering Sea shelf), with highest levels corresponding to the Bering Sea shelf (Fig. 2). PAHs were detected at every station in the study area, and tPAH concentrations (dry weight) ranged from 36.95–150.21 ng/g with a mean value of 70.61 ng/g. The distribution (decreasing) of PAHs in the surface sediments of the study area was as follows: Canada Basin N northern Chukchi Sea N Chukchi Basin N southern Chukchi Sea N Aleutian Basin N Makarov Basin N Bering Sea shelf. Of the simple components of the PAHs, the phenanthrene content was highest in the study area, and the second highest content was observed for benzo[g,h,i]perylene. A comparative analysis of the total contents of low-ring (two- and three-ring) and high-ring (four- to
M. Zhao et al. / Marine Pollution Bulletin 104 (2016) 379–385
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Fig. 2. Distributions of PAH /ng × g−1 (a), TOC /% (b), Mz/Φ (c) and Clay/% (d) in the surface sediments from the study area.
six-ring) PAHs revealed that high-ring PAHs constituted more than 50% of the contents at various stations. The three-ring PAHs constituted the highest content (28.23–41.0%, Fig. 3), followed by four-ring PAHs (17.6–36.72%) and five-ring PAHs (13.92–24.83%). The high-ring PAH contents pertaining to the different regions were as follows: in descending order, southern Chukchi Sea N Aleutian Basin N northern Chukchi Sea N Canada Basin N Chukchi Basin N Makarov Basin N Bering Sea shelf. In general, the PAH contents of marine sediments are believed to be affected primarily by TOC content and grain size (Kim et al., 1999; Yang, 2000; Yamamoto and Polyak, 2009). The TOC content within surface sediments depends on ocean currents and terrigenous inputs, and fine-grained sediments contain abundant TOC. The larger the sediment grain size, the lower the TOC content and the smaller the amount of adsorbed PAHs. In the present study, a positive correlation was found between tPAH concentration and TOC content in four regions (Fig. 4), as follows: Canada Basin N Chukchi Basin N southern Chukchi Sea N Makarov Basin. However, no significant correlations were found between TOC content and PAH accumulation in the sediments of the Aleutian Basin, Bering Sea shelf, or northern Chukchi Sea.
In the Aleutian Basin, the concentration of tPAHs in surface sediments gradually decreased with increasing latitude, and similar trends have been observed in the atmosphere of the same region (Ding et al., 2007); thus, atmospheric deposition appears to significantly affect the accumulation of PAHs. Due to atmospheric circulation within the Bering Sea slope, the deposition rate is relatively high in the Bering Sea shelf. This region is one of the world's most productive marine areas and is referred to as the “Green Belt” (Wang et al., 2005). Deposition rates vary in different areas of the sea, and surface sediment samples collected at a depth of 2 cm represent different ages. The highest TOC content was detected in surface sediments from the Bering Sea shelf. However, the high deposition rate in this area has a “dilution” effect on sediment tPAH concentrations, which may be the predominant reason behind the correlation between high TOC values and low tPAH values in the Bering Sea shelf. Relatively warm and nutrient-rich Pacific water masses profoundly impact the Chukchi Sea as well as the entire Arctic Ocean. Due to the seasonality of open water conditions and the substantial inputs of terrigenous material, the Chukchi Sea has high primary productivity with a high deposition rate (Backman et al., 2004). The TOC content of surface
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Fig. 3. Percent content of polycyclic aromatic hydrocarbons with varying numbers of rings at sampling stations in the study area. AB, Aleutian Basin; BSS, Bering Sea Shelf; SCS, Southern Chukchi Sea; NCS, Northern Chukchi Sea; CSB, Chukchi Basin; CB, Canada Basin; MB, Makarov Basin.
sediments of the northern Chukchi Sea was second only to the Bering Sea shelf. At stations in the northern Chukchi Sea, tPAH concentrations in surface sediments decreased as the distance from the coastline increased. The clockwise flow of the Beaufort Gyre is conducive to the transportation of river-borne materials westward from the Beaufort Sea and along the Northwind Ridge to the northern Chukchi Sea (MacDonald et al., 1998). High-value stations (tPAHs N 100 ng/g) were distributed through Barrow Point and the Prudhoe Bay offshore area, close to the mouth of the Mackenzie and Colleyville Rivers. As the distance from the polluted area increased, tPAH concentrations gradually decreased, providing a spatial pattern that is consistent with the conclusions of Baumard et al. (1998a,b) and Kim et al. (1999). Environmental sources of PAHs are complex. On the one hand, the various origins of PAHs indicate differences in their structures and compositions; on the other hand, these compounds remain relatively stable throughout their migration and deposition (Sicre et al., 1987; Basheer et al., 2003). According to Sicre et al. (1987), Fla/Pyr b 1 indicates that the corresponding PAHs originate from petroleum, while Fla/Pyr N 1 indicates fossil fuel combustion as their origin. We calculated the Flu/Pyr ratio to determine PAHs sources (Fig. 5) and found that Fla/Pyr values ranged from 0.937–2.15. Low Fla/Pyr values (b1) were only obtained for three
Fig. 5. The Fla/Pyr and Pyr/BaP ratios in the surface sediments from the study area (dry weight). Fla, fluoranthene; Pyr, pyrene; Ben, benzo[a]pyrene.
stations in close proximity to one another, namely, Mor2 (Canada Basin), SR11 (northern Chukchi Sea), and SR12 (northern Chukchi Sea); and high values (N1) were obtained for the remaining stations. Based on these ratios, there appear to have been petroleum inputs in the region surrounding stations Mor2, SR11, and SR12 (74–74.5°N, 159–169°W); however, the remaining regions received PAHs primarily from the incomplete combustion of minerals. The Pyr/BaP ratio is one of the important parameters used to distinguish between pollutants emanating from gasoline combustion (exhaust) versus coal combustion. Pyr/BaP b 1 is generally believed to correspond to coal combustion emissions, while a Pyr/BaP ratio of 1–6 indicates exhaust emissions (Zhu et al., 2001). A comparative analysis showed that Pyr/BaP values ranged from 0 to 2.16 in our study area (Fig. 5). In the present study, Pyr/BaP values remained below 1, excepting the high-PAH stations (B02, B04, BB05 and NB03) in the Bering Sea, indicating that the predominant source of PAHs in the area evaluated resulted from coal combustion emissions. In the western Arctic Ocean (excepting 74–75.65°N and 87–88.39°N), the much higher value of Pyr/BaP suggests that gasoline combustion (exhaust) might be more important. The pollutants in Siberia mainland and Alaska can move into the Arctic region easily by atmospheric and river transportation. Yunker et al. (2002) noted that Fla/(Fla + Pyr) b 0.4 indicates PAHs of petroleum origin, while 0.4 ≤ Fla/(Fla + Pyr) ≤ 0.5 indicates origination from the combustion of liquid fossil fuels (unprocessed crude oil
Fig. 4. Correlation of PAH and TOC in the surface sediments from the study area.
M. Zhao et al. / Marine Pollution Bulletin 104 (2016) 379–385
Fig. 6. The BaA/(BaA + Chry) and Fla/(Fla + Pyr) ratios in the surface sediments from the study area (dry weight). BaA, benzo[a]anthracene; Chry, chrysene; Fla, fluoranthene; Pyr, pyrene.
combustion and exhaust emissions), and Fla/(Fla + Pyr) N 0.5 indicates origination from the burning of herbaceous and woody plants and coal combustion. In the area evaluated, the present study determined
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Fla/(Fla + Pyr) values between 0.48 and 0.68. The Fla/(Fla + Pyr) values ranged from 0.48–0.5 only at the following three adjacent stations: Mor2 (Canada Basin), SR11 (northern Chukchi Sea), and SR12(northern Chukchi Sea). The PAHs at these three stations possibly originated from unprocessed crude oil combustion. With the exception of these three stations, Fla/(Fla + Pyr) values were all N0.5 in the study area evaluated herein, indicating that the PAHs in the sediment samples analyzed originated predominantly from the combustion of herbaceous and woody plants and coal (Fig. 6). To further determine the sources of PAHs in the surface sediments of the study area, we utilized the BaA/(BaA + Chry) ratio according to Yunker et al. (2002). BaA/(BaA + Chry) b 0.2 indicates a petroleum origin; 0.2 ≤ BaA/(BaA + Chry) ≤ 0.35 indicates a mixed origin; and BaA/(BaA + Chry) N 0.35 indicates a combustion origin. The BaA/ (BaA + Chry) values ranged from 0.21–0.48 in our study area (Fig. 6), indicating that almost no PAHs originated from petroleum. The BaA/ (BaA + Chry) values determined herein exhibited specific trends within the different study regions. For instance, BaA/(BaA + Chry) values were N0.35 at all the stations in the Bering Sea shelf, at three stations in the northern Chukchi Sea (SR10, SR11, and SR12), and at two stations at the entrance of the Bering Strait (SR01 and SR02), indicating that the PAHs in these sediments originated mainly from combustion. On the other hand, BaA/(BaA + Chry) values were generally b0.35 for the Aleutian Basin, the central south portion of the Chukchi Sea, the Canada Basin, and the Makarov Basin, indicating PAHs of mixed origin.
Table 1 PAH concentration, grain size, total organic (TOC) and total organic/total nitrogen (C/N mole ratios) of surface sediments from the study area. Station B02 B04 B06 B11 B14 NB01 BB01 NB03 NB04 BB05 BB06 SR01 SR02 CC4 SR04 R06 R08 C04 S21 C09 S23 R09 C07 SR10 SR11 SR12 BN03 S25 S26 MS01 MS02 MS03 Mor2 M07 M05 M03 M02 BN06 BN07 BN09 BN10 BN12 BN13
Lon. (°)
Lat. (°)
169.96 171.40 174.49 179.92 −177.69 −175.08 −177.48 −172.20 −170.63 −175.33 −174.38 −168.97 −168.98 −167.86 −169.00 −168.98 −168.98 −167.03 −154.72 −159.71 −153.76 −168.94 −165.33 −169.00 −168.99 −169.00 −158.90 −152.50 −153.55 −154.71 −156.37 −157.30 −158.99 −172.03 −172.13 −171.83 −171.99 −164.94 −166.47 −167.13 −178.64 −170.49 −176.63
53.33 54.59 57.01 59.99 60.92 61.23 61.29 61.51 61.58 62.54 63.01 67.00 67.50 68.13 68.50 69.50 71.00 71.01 71.62 71.81 71.93 71.96 72.54 73.00 73.99 74.50 76.45 72.34 72.70 73.17 73.68 74.07 74.55 74.99 75.65 76.50 77.00 81.46 82.48 84.19 85.50 87.07 88.39
Depth (m) 1937 3873 3780 2603 130 92 130 61 49 79 78 49 51 52 56 52 44 46 46 50 338 51 51 77 184 187 1443 2830 3521 3814 3743 3890 1224 393 1617 2298 2300 3566 3627 2500 2434 4000 3995
Note: Mz means grain size; “/” means no data.
Range
Aleutian Basin
Bering Sea Shelf
Southern Chukchi Sea
Northern Chukchi Sea
Canada Basin
Chukchi Basin
Makarov Basin
ΣPAHs (ng/g) 76.56 66.84 59.21 41.62 46.73 39.95 52.40 88.75 37.94 97.41 43.75 39.28 48.13 59.47 89.81 76.05 70.09 63.68 127.00 68.38 94.72 80.58 63.64 44.35 45.23 44.09 56.19 145.89 71.40 107.53 88.33 150.21 130.75 62.20 38.94 71.71 90.13 74.16 70.83 62.13 64.75 48.33 36.95
Sand (%)
Silt (%)
Clay (%)
Mz (Φ)
TOC (%)
46.34 6.31 11.39 16.20 22.36 11.45 21.20 15.67 / 27.98 9.83 66.13 47.76 33.42 7.21 13.69 27.08 21.07 23.59 25.35 7.26 10.49 0.76 8.67 1.04 28.71 19.43 6.60 5.44 5.97 14.03 0.06 22.03 2.83 6.49 8.87 2.90 20.02 19.61 20.58 16.27 7.33 11.51
41.18 78.82 72.94 76.05 71.78 81.23 70.49 52.94 / 63.49 80.37 31.74 49.10 58.21 83.04 73.85 67.62 72.21 63.73 60.21 71.79 71.56 81.47 75.44 71.28 63.01 53.18 78.67 58.07 68.76 48.12 52.32 50.81 82.62 55.78 67.80 62.63 43.21 37.93 36.38 44.15 42.36 48.89
12.48 14.87 15.67 7.76 5.87 7.33 8.31 31.39 / 8.53 9.80 2.13 3.14 8.38 9.75 12.46 5.31 6.73 12.68 14.44 20.94 17.95 17.78 15.89 27.68 8.29 27.39 14.74 36.49 25.27 37.86 47.62 27.17 14.55 37.74 23.33 34.47 36.77 42.46 43.04 39.58 50.31 39.61
4.76 6.36 6.14 5.55 5.24 5.69 5.47 6.65 / 5.43 5.89 3.77 4.35 4.79 5.92 5.96 5.01 5.31 5.65 5.62 6.56 6.33 6.79 6.37 7.22 5.12 6.31 6.40 7.45 7.11 7.02 8.03 6.28 6.56 7.48 6.84 7.49 6.59 6.86 6.48 6.91 7.84 7.17
0.52 1.37 0.77 0.36 1.45 1.80 2.16 0.83 / 1.61 2.56 0.27 1.03 0.28 1.49 1.66 1.17 0.03 1.38 / 1.70 / 1.35 1.99 1.80 1.29 1.42 1.67 / 1.21 0.87 0.95 1.08 1.02 0.70 0.90 1.20 1.38 1.11 0.76 0.44 0.43 0.02
C/N 3.77 6.63 6.03 4.60 16.33 7.91 9.11 5.46 / 8.49 35.99 4.27 6.20 3.14 6.57 11.18 7.72 0.74 10.58 / 5.99 / 8.22 8.11 7.44 6.50 13.35 8.62 / 7.13 6.03 6.01 9.80 5.38 4.96 6.82 8.93 6.99 6.83 7.76 4.33 4.22 0.47
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Table 2 Risk assessment for PAHs in the sediments from the study area. (ng/g dry weight). PAHs
Content range
ERL
ERM
Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benz[a]anthracene Chrysene Benz[b]acephenanthrylene Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Dibenz[a,h]anthracene Benzo[g,h,i]perylene ΣPAHs
3.36–14.22 N.D.–5.77 2.84–5.84 2.61–8.96 3.46–36.9 N.D. 2.03–11.66 1.79–9.37 1.82–6.54 1.95–23.34 2.89–14.51 1.42–4.21 N.D.–11.12 N.D.–9.64 N.D.–6.5 2.73–21.4 36.95–150.21
16 44 16 19 240 85.3 600 665 261 384 320 280 430 – 63.4 430 3912.4
2100 640 500 540 1500 1100 5100 2500 1600 2800 1880 1620 1600 – 260 1600 25,340
Long et al. (1995) proposed the effects range low (ERL, probability of adverse biological effects b 10%) and the effects range median (ERM, probability of adverse biological effects N 50%) to determine the potential ecological risks of organic pollutants in marine and estuarine sediments. The concentrations of the various PAHs detected at the stations evaluated herein were lower than the ERL (Table 2), indicating that these pollutants have a low potential of posing ecological risks to surrounding marine life and have few toxic effects on living organisms. This study provides information regarding the distribution and sources of PAHs in surface sediments from the Bering Sea and western Arctic Ocean. The spatial distribution of tPAH concentrations followed the following decreasing trend: Canada Basin N northern Chukchi Sea N Chukchi Basin N southern Chukchi Sea N Aleutian Basin N Makarov Basin N Bering Sea shelf. This distribution of PAHs was controlled by numerous factors but were predominantly affected by the TOC content and mean grain size in the southern Chukchi Sea, Chukchi Basin, and Makarov Basin; the distance from the source area, TOC content, and ocean current system in the Canada Basin; atmospheric deposition in the Aleutian Basin; and the distance from the source area, deposition rate, and ocean current system in the Chukchi Sea shelf and the northern Chukchi Sea. Combustion of herbaceous and woody plants as well as fossil fuels such as coal constituted the primary sources of PAHs in the study area evaluated herein. Acknowledgments This study was funded by the “Fourth Chinese National Arctic Research Expedition”, under contract number CHINARE-2010, and the Chinese Polar Environment Comprehensive Investigation & Assessment Programs, under contract numbers CHINARE2014-03-02 and CHINARE2014-04-03-03. The authors thank various colleagues for assisting with samples collection and processing, and the anonymous reviewers for their constructive comments and helpful suggestions, which have improved the manuscript. References Achten, C., Hofmann, T., 2009. Native polycyclic aromatic hydrocarbons (PAH) in coals—a hardly recognized source of environmental contamination. Sci. Total Environ. 407, 2461–2473. Backman, J., Jakobsson, M., Løvlie, R., Febo, L., 2004. Is the central Arctic Ocean a sediment starved basin? Quat. Sci. Rev. 23, 1435–1454. Basheer, C., Obbard, J.P., Keelee, H., 2003. Persistent organic pollutants in Singapore's coastal marine environment: part II, sediments. Water Air Soil Pollut. 149, 315–323. Baumard, P., Buzinski, H., Michon, Q., Garrigues, P., Burgeat, T., Bellocq, J., 1998a. Origin and bioavailability of PAHs in the Mediterranean Sea from mussel and sediment records. Estuarine 47, 77–90. Baumard, P., Budzinski, H., Garrigues, P., 1998b. Polycyclic aromatic hydrocarbons in sediments and mussels of the western Mediterranean Sea. Environ. Toxicol. Chem. 17, 765–776. http://dx.doi.org/10.1002/etc.5620170501.
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