Selenium and tellurium fractionation, enrichment, sources and chronological reconstruction in the East China Sea

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Estuarine, Coastal and Shelf Science xxx (2014) 1e10

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Selenium and tellurium fractionation, enrichment, sources and chronological reconstruction in the East China Sea Q1

Li-Qin Duan a, Jin-Ming Song a, *, Hua-Mao Yuan a, Xue-Gang Li a, Ning Li a, Jikun Ma b a b

Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PR China Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2013 Accepted 19 March 2014 Available online xxx

Surface and core sediments of the East China Sea (ECS) were collected to study spatio-temporal distributions and chemical fractions of Se and Te and to evaluate their enrichment and sources. Higher Se and Te concentrations in surface sediments appeared in the inner shelf and near the Cheju Island. It seemed to be controlled by sources and sediment characteristics. Enrichment factors (EFs) showed that Se and Te were at minor and moderate enrichment, respectively. Sequential extraction suggested that non-residual fractions of Se and Te accounted for high percentages (29.5
16.2% and 50.9
13.2%) in total, combined with risk assessment code (RCA), indicating that Se and Te were at medium and high risks, respectively. All temporal profiles of abundances, EFs and burial fluxes (BFs) of Se and Te displayed higher values before 1900, in 1989 and 2009, and in the period of 1960e1980 with a peak in 1970. These higher values were closely associated with biological and anthropogenic activities. Ó 2014 Published by Elsevier Ltd.

Keywords: selenium tellurium chemical speciation enrichment sediments the East China Sea

1. Introduction Coastal waters and estuaries are often surrounded by urban and industrial areas and are frequently loaded to a significant level with various pollutants. The ECS adjacent to the metropolis Shanghai and Ningbo Cities, receives massive agricultural, municipal, residential and industrial waste products, containing lots of nutrients, heavy metals and trace elements (e.g., Se and Te). A good deal of research on nutrients and heavy metals in the ECS has been studied (Lin et al., 2002; Wang et al., 2003; Yuan et al., 2004; Fang et al., 2009), whereas rather less attention has been focused on trace elements, such as Se and Te, which are particularly sensitive to the surrounding environment because of their very low abundances. Selenium is an essential element to organisms, but it can cause anomalies in organisms at low concentration and it is toxic at high concentration. Of the two oxidized forms, selenite (Se(IV)) is generally the more toxic form and more easily bioaccumulated (Maier and Knight, 1993). Tellurium in the same group as Se in the periodic table, is a relatively rare element without significant biological role. However, Te and its compounds are considered to be toxic and need to be handled with  care (Rezanka and Sigler, 2008). Organotellurium compounds can * Corresponding author. E-mail address: [email protected] (J.-M. Song).

damage cells, e.g., by oxidizing sulfhydryl groups and depleting endogenous reduced glutathione in a variety of tissues. Thus, due to their bioavailability, Se and Te can induce environmental problems. Natural biogeochemical cycles of Se and Te have been changed by anthropogenic activities, which transferred Se and Te from continents to seas via rivers and atmosphere. After entering into marine environments, Se and Te were adsorbed on suspended particulates or utilized by organisms and then deposited eventually on sediments, leading to Se and Te enrichment in sediments and making sediments being an important deposit of Se and Te in aquatic environment (Szefer et al., 1995; Duan et al., 2010). Sediments could be a secondary source of pollutants, once environmental condition was changed. When sedimentary environmental condition changed, Se and Te associated with sediments would be released to overlying water, threatening the aquatic biota. Therefore, it was necessary to assess enrichments and potential environmental risk levels of sedimentary Se and Te. Different methods, such as enrichment factor and geoaccumulation index have been widely applied to evaluate enrichment of trace elements and apportion their natural vs. anthropogenic contributions (Silva et al., 2009; Chandía and Salamanca, 2012; Hasan et al., 2013; Zahra et al., 2014). Moreover, depending on sediment conditions, Se and Te were mainly associated with carbonates, FeeMn oxides, sulfur and various organic compounds, either complexed or built into molecular and high-molecular. All these species could be

http://dx.doi.org/10.1016/j.ecss.2014.03.024 0272-7714/Ó 2014 Published by Elsevier Ltd.

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presented in sediments concomitantly, which made the geochemical behaviors of Se and Te in environmental compartments quite complex (Wright et al., 2003). Hence, estimating the potential bioavailability and risks associated with Se and Te in sediments required a detailed understanding of their biogeochemical behaviors and chemical speciation. Sequential extraction has been generally used to determine chemical speciation of Se and Te (Tessier et al., 1979; Ponce de Leon et al., 2003; Wright et al., 2003; Tolu et al., 2011; Savonina et al., 2012). Chemical speciation normally included exchangeable, carbonate, reducible, oxidable and residual fractions. In the absence of anthropogenic influence, trace metals in sediments were mainly associated with silicates and primary minerals, and therefore had limited mobility. Trace elements introduced by human activities were mainly associated with carbonates, FeeMn oxides and organic matters, showing high mobility (Passos et al., 2010). When environmental conditions (e.g., salinity, pH, temperature and redox state) changed, Se and Te existing as labile fractions would be migrated, released and used by organisms (Atkinson et al., 2007; Duan et al., 2010). In recent years, eutrophication become heavier and red tides occur frequently in the ECS due to a large amount of nutrients is discharged into the ECS (Liu et al., 2013), which might introduce more biological Se and Te to sediments. Thus, in order to reflect the traces left by anthropogenic and biological activities in marine environment, it is essential to reconstruct environmental evolution over the last several decades (Legesse et al., 2002). Due to their stability and non-biodegradation, Se and Te could accumulate and left fingerprints in sediments over time. Therefore, temporal variations of Se and Te in sediments could be used as effective indicators to essentially reflect their historical inputs by natural, anthropogenic and biological activities. Due to their species having diverse properties, Se and Te are of environmental and ecotoxicological concern. To understand and evaluate biogeochemical behaviors and environmental impacts of Se and Te, the distributions, fractionations, geochemical enrichments and historical evolutions of Se and Te in sediments of the ECS were studied in this paper. The objectives of this study were to (1) determine Se and Te distributions and their influencing factors in sediments of the ECS; (2) analyze chemical fractions of Se and Te in surface sediments of the ECS; (3) evaluate geochemical enrichments and risks of Se and Te in the ECS; and (4) trace the historical evolutions of Se and Te in the ECS.

2. Materials and methods 2.1. Study area and sampling The ECS is one of the larger marginal seas in the western Pacific Ocean of the northern hemisphere and surrounded by Chinese mainland to the west, the Kuroshio Current to the east, Taiwan and the Taiwan Strait to the south, and the Yellow Sea to the north with the width and depth of 4500 km and 130 m, respectively (Fang et al., 2009). The ECS receives a large amount of terrigenous sediment from the Changjiang River, which is the world’s fourth largest river based on suspended load. The average freshwater and sediment fluxes of the Changjiang River are approximately 9.25  1011 m3/yr and 4.61  108 t/yr, respectively (Hori et al., 2001; Zhang and Liu, 2002). According to the latitude of 29e32 N and longitude of 122e 126 E, 38 surface sediments and a sediment core from the ECS were sampled in May 2009 with the cruise of “Kexue 1” (Fig. 1). Surface sediments (0e2 cm) were collected using an Ekman-Birge box sampler. Immediately after collection, samples were placed in pre-cleaned polyethylene bags, sealed and refrigerated until lab analysis. Sediment core G1 (122 31.050 E, 29 30.250 N) was collected at the extension of the Changjiang River using a gravity corer (Fig. 1). Core G1 is located in the inner shelf along the northern Zhejiang and Fujian Provinces with water depth of 32 m. Immediately after collection, core G1 was sectioned at 2 cm intervals and stored in pre-cleaned polyethylene bags, sealed and refrigerated until lab analysis. Both of surface and core sediment samples were dried in an oven at 60  C for 72 h. Dried aliquots were ground using an agate mortar and pestle for homogenization, and prepared for analysis. Data reported in this study were calculated as dry weight. 2.2. Sediment analyses The analytical technique for Se and Te was performed by modifying a method of Mercone et al. (1999). Briefly, about 0.2 g dry sample was digested with a mixture of 10 ml aqua regia in Teflon digestion vessel heated in water bath for 1 h. After cooling, solution was pre-reduced by boiling at an acidity of 5 M HCl for 30 min. And then the reduced solution was determined by atomic fluorescence spectrometry coupled with a hydride generator (HGAFS) using 0.7% KBH4 with 0.05 M NaOH under Se and Te hollow cathode lamps for Se and Te determination, respectively.

Fig. 1. Location of the stations in the East China Sea. C indicates the surface sediments, B indicates core G1. The currents were pointed out by arrows. CDW: the Changjiang Dilute Water; KC: the Kuroshio Current; JCC: the Jiangsu Coastal Current; ZFCC: the Zhejiang-Fujian Coastal Current; TW: Taiwan Warm Current.

Please cite this article in press as: Duan, L.-Q., et al., Selenium and tellurium fractionation, enrichment, sources and chronological reconstruction in the East China Sea, Estuarine, Coastal and Shelf Science (2014), http://dx.doi.org/10.1016/j.ecss.2014.03.024

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Five chemical fractions of Se and Te in sediment included exchangeable (F1), carbonates (F2), FeeMn oxides (F3), organic matter (F4) and residual (F5) fractions. Among them, the first four fractions were extracted using sequential selective extraction analyses by the method of Tessier et al. (1979). After each extraction, the residue was centrifuged at 4000 rpm for 10 min, and supernatants were decanted into volumetric Nalgene flasks. Sediment residues were afterwards washed with Milli-Q water, centrifuged again, and the supernatants were combined. Each extractant was treated with the same way as total Se and Te and then were determined by HG-AFS. Se or Te in residual fraction was obtained as the difference between total concentrations and the sum of these four extracted fractions. Total organic carbon (TOC) was analyzed by the Walkey-Blake method (Gaudette et al., 1974), with the relative standard deviation (RSD) of <5%. Sediment grain size was determined using a Laser Particle Size Analyzer. The grain sizes were <4 mm for clay, 4e63 mm for silt and >63 mm for sand. The relative error of the duplicate samples was less than 3% (n ¼ 6). Chlorophyll a (Chl a) was collected from SeaBird 911 CTD equipped with a fluorescent probe. Inductively coupled Plasma-Mass Spectrometry (ICP-MS) was used to analyze for Cs in acid digestion with HNO3þHClO4þHF performed according to Duan et al. (2010). Sequential selective extraction analysis for Cs also was performed by the method of Tessier et al. (1979). 2.3. Data quality control Quality assurance and quality control were controlled by method blanks, field duplicate samples, spiked samples and standard reference materials (SRMs). Blank samples were conducted throughout experiments of total concentration and sequential extraction analyses. Field duplicate samples were collected once every 10 samples in the field. The relative percent difference for Se and Te in duplicate samples was <10%. Precision was assured by determining all samples in triplicate with RSD <10%. Accuracy of chemical fractions was assured by determining recovery of spiked samples in each extractant solution, with recoveries of 95e105%. Accuracy of total analysis concentrations was assured using SRMs (GSD-9 and GSS-15), with recoveries of 90e110%. 2.4. Environment factor (EF) To gain information about sources, enrichment and influencing factors of Se and Te, their EFs were calculated according to the following equation:

. EF ¼ ðX=YÞsample ðX=YÞbackground

where (X/Y)sample is ratio of Se or Te to reference element concentration in samples; (X/Y)background is ratio of Se or Te to reference element in background values. Five-category systems were generally recognized: EF < 2, deficiency to minimal enrichment; 2  EF < 5, moderate enrichment; 5  EF < 20, significant enrichment; 20  EF < 40, very high enrichment; and EF > 40, extremely high enrichment (Sutherland, 2000). 2.5. Burial flux (BF) BFs of Se and Te in core sediments could be obtained from following equation (Ingall and Jahnke, 1994):

BFi ¼ Ci  rd  Si where BFi (mg/(yr$cm2)) is burial fluxes of sedimentary Se and Te; Ci (mg/g) is Se and Te concentrations in sediments; Si (cm/yr) is

3

sedimentation rate; and rd (g/cm3) is sediment dry bulk density, which was calculated by equation of Snoeckx and Rea (1995):

rd ¼ ð1  Wc Þ=ðð1  Wc Þ=rs þ Wc =rw Þ where Wc (%) is the water concentration in sediment; rs (g/cm3) is the sediment grain density; and rw (g/cm3) is the seawater density. 2.6. Statistical analysis Data were analyzed using SPSS 13.0. Pearson correlation analysis has been used to assess the relationships of Se, Te and their chemical fractions to environmental factors (e.g., clay and TOC contents, pH, temperature, salinity) and the relationship between reference element Cs and grain sizes. In this work, a value of p < 0.05 was considered to indicate a significant difference in all statistical analysis. 3. Results and discussion 3.1. Sediment characteristics Physico-chemical parameters (i.e., clay, silt, sand, TOC, salinity, pH and temperature) in surface sediments and Chl a in seawaters of the ECS were determined (Table 1). Clay, silt and sand contents were 1.02e51.2%, 0e82.8% and 0e96.1%, with averages of 18.5
11.2%, 35.1
23.6% and 47.2
31.0%, respectively. Surface sediments of the ECS mainly consisted of sand, silty sand and clayey silt. Clay distribution in the ECS displayed a large spatial variation with higher clay contents in the inner shelf along the northern Zhejiang and Fujian Provinces and near the Cheju Island and with lower clay contents in the middle and outer shelves. TOC content ranged from 0.09% to 0.60%, with average of 0.32
0.13%. Similar to clay distribution, higher TOC contents mainly appeared in the inner shelf along the northern Zhejiang and Fujian Provinces and near the Cheju Island with lower contents in the middle and outer shelves. Salinity, pH and temperature were in the ranges of 30.3e34.5, 7.22e8.15 and 12.8e19.7  C, with averages of 32.5
1.4, 7.55
0.19 and 17.2
1.7  C, respectively. Salinity displayed an increasing trend whereas temperature presented a decreasing trend with distance away from shore due to the influence of the Changjiang and Qiantangjiang River inputs. Chl a concentration was 0.18e 8.74 mg/L, with average of 1.18
1.46 mg/L. There were two higher value areas for Chl a near the Hangzhou Bay mouth and in the northeastern ECS. 3.2. Total and chemical fractions 3.2.1. Total concentrations Total Se and Te concentrations in surface sediments of the ECS were 0.036e0.381 mg/g and 0.020e0.096 mg/g, with averages of 0.099
0.063 mg/g and 0.049
0.015 mg/g, respectively. Se and Te concentrations had similar spatial distribution (Fig. 2), with a significant relationship of 0.786 (p < 0.01, Fig. 3). Generally, both of them displayed the higher concentrations in the inner shelf (especially near the Changjiang Estuary and Hangzhou Bay mouth) and near the Cheju Island. Away from the inner shelf and Cheju island areas, Se and Te concentrations decreased in a southeast direction. The distribution pattern was related to sediment characteristics (e.g., grain sizes and TOC contents), sources and sedimentary environment characteristics (e.g., pH, temperature and salinity). Sediment characteristics (i.e., grain sizes and TOC content) controlled Se and Te distributions in a large extent. Se and Te concentrations displayed similar spatial distributions to clay and

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Table 1 Physical parameters (salinity, pH and temperature), sediment components (sand, silt, clay and TOC contents), and Se and Te concentrations in surface sediments of the East China Sea. Station S (&) T ( C) pH

Sand (%) Silt (%) Clay (%) TOC (%) Se (mg/g) Te (mg/g)

A1 A2 A3 A4 A5 B1 B2 B3 C1 C2 C3 C4 D1 D2 D3 D4 D5 E1 E2 E3 E4 F1 F2 F3 F4 F5 G1 G2 G3 G4 G5 H1 H2 H3 H4 H5 H6 H7

1.97 22.1 62.6 74.3 10.2 69.2 96.1 89.5 15.7 nda 0.0 52.8 nda 3.25 5.21 79.3 75.0 36.8 70.8 66.9 58.2 5.37 56.3 58.7 70.5 79.7 0.0 45.4 46.6 32.0 74.6 0.0 0.0 52.8 59.4 61.4 79.9 84.3

a

30.3 30.9 30.5 30.8 32.8 31.1 30.9 32.2 33.1 31.9 31.8 32.6 nda nda 33.6 33.1 32.8 34.1 33.8 33.5 33.5 30.6 34.3 34.2 33.8 33.8 33.5 32.3 34.0 33.9 34.0 30.4 34.3 34.5 34.3 34.1 34.1 34.3

18.1 18.0 17.3 14.3 nda 17.6 15.8 13.0 17.2 nda 17.7 12.8 nda nda 16.9 16.4 14.1 17.8 18.1 17.1 16.0 18.5 18.1 18.8 16.8 16.6 17.9 17.9 19.1 18.9 17.0 19.7 18.0 18.1 18.3 19.5 18.4 17.8

7.48 7.55 7.55 7.80 7.50 7.69 8.15 7.57 7.83 nda 7.75 7.74 nda nda 7.52 7.68 7.22 7.54 7.47 7.37 7.51 7.33 7.44 7.54 7.70 7.45 7.69 7.23 7.25 7.25 7.38 7.42 7.48 7.53 7.57 7.51 7.64 7.62

82.8 64.8 24.9 16.0 57.5 23.2 2.92 0.0 54.6 nda 48.8 33.8 nda 72.4 75.6 16.26 12.1 42.6 12.4 16.0 4.92 74.2 30.2 27.0 20.3 13.0 68.0 35.5 34.8 41.7 16.5 65.6 57.8 33.8 24.9 25.8 11.6 9.33

15.3 13.1 12.5 9.76 32.4 7.63 1.02 10.5 29.7 nda 51.2 13.4 nda 24.3 19.2 4.44 12.9 20.6 16.9 17.2 36.9 20.4 13.6 14.3 9.2 7.23 32.0 19.2 18.6 26.4 8.93 34.4 42.2 13.4 15.7 12.8 8.46 6.34

0.30 0.31 0.26 0.25 0.52 0.22 0.23 0.22 0.49 nda 0.53 0.23 nda 0.60 0.50 0.19 0.36 0.45 0.21 0.27 0.30 0.32 0.28 0.24 0.11 0.09 0.58 0.37 0.37 0.23 0.34 0.39 0.52 0.24 0.25 0.27 0.21 0.24

0.107 0.116 0.107 0.121 0.222 0.076 0.055 0.097 0.067 0.093 0.036 0.147 0.183 0.381 0.060 0.048 0.090 0.121 0.079 0.051 0.058 0.072 0.081 0.069 0.062 0.079 0.189 0.104 0.095 0.109 0.076 0.086 0.140 0.065 0.039 0.081 0.048 0.053

0.048 0.039 0.045 0.049 0.083 0.038 0.046 0.038 0.046 0.020 0.046 0.046 0.096 0.091 0.046 0.039 0.041 0.050 0.047 0.040 0.045 0.049 0.043 0.043 0.040 0.046 0.080 0.055 0.048 0.046 0.045 0.048 0.048 0.044 0.050 0.043 0.046 0.045

nd ¼ not determined.

TOC contents, which was supported by relationships of Se and Te to clay and silt content (r ¼ 0.682, p < 0.01; r ¼ 0.636, p < 0.01) and TOC content (r ¼ 0.729, p < 0.01; 0.760, p < 0.01; Fig. 3). It suggested that Se and Te were liable to be enriched in fine-grained sediments with high organic matters. Sources containing terrigenous and marine inputs played an important role in controlling Se and Te distributions. The main approaches possible for Se and Te to enter the ECS included direct discharge, riverine input and biological activities. To facilitate discuss the influence and contribution of sources on Se

and Te distribution, the study area was geographically divided into five zones (Fig. 1): zone I, the northern ECS bound to the Yellow Sea which was influenced by the Jiangsu Coastal Current (JCC); zone II, the estuary where Changjiang River sediment mainly deposited; zone III, inner shelf along the Zhejiang and Fujian Provinces; zone IV, near the Cheju Island and fringe of the offshore upwelling area; zone V, middle and outer shelves. Se and Te concentrations in five zones were presented in Table 2. Se and Te had the highest concentrations in the inner shelf (zones II and III), followed by offshore upwelling area (zone IV), with lower concentrations in the northern ECS (zone II) and the lowest concentrations in the middle and outer shelves (zone V). Higher Se and Te concentrations in zones II and III near the Changjiang Estuary and Hangzhou Bay mouth suggested the major influence of the Changjiang and Qiantangjiang Rivers. There was about half of sediments (w2.3  108 t/yr) from the Changjiang River deposited on the river-mouth area with the rest being delivered southward to the inner shelf by the coastal currents (Zhang and Liu, 2002; Liu et al., 2010). An elongated subaqueous mud wedge was formed extending from the river mouth southward off the Zhejiang and Fujian coast. In agreement with the dispersal of river sediment, mud content and total Se and Te mainly distributed along the coast and decreased toward sea. This suggested that the Changjiang River was a major source of Se and Te to the inner shelf. Besides, higher Se and Te concentrations also existed in the offshore upwelling area (zone IV, especially at station A5), located at cold-eddy area. There was little sediment from the Changjiang River transported northeastwards, however, some fine sediment was transported from the eroded old Yellow River delta by the JCC to the cold-eddy area, where the mud (clay þ silt) content was 89.8%, suggesting that some terrigenous Se and Te associated with fine sediments were carried there. Although the contribution of the old Yellow River input to the cold-eddy area was important, the sedimentary rates only were 0.2e0.5 cm/a there (Lim et al., 2007), suggesting that the supply of terrestrial sediment was limited in the offshore upwelling area. Thus, authigenic input likely was another important contribution to higher Se and Te concentrations at station A5. There were two possible approaches for authigenic Se and Te to enter sediment: direct deposition as a part of organisms; authigenic deposition associated with organic matters. The distribution of Chl a concentration showed that there were two productive areas in the ECS which were near the Hangzhou Bay mouth and in the northeastern ECS, respectively (Fig. 4). These two productive areas were in accordance with the higher value areas of Se and Te. Se and Te concentrations had significant correlations with Chl a concentration (r ¼ 0.857, p < 0.01; r ¼ 0.868, p < 0.01), suggesting that biological activities played an important role in Se and Te distribution, especially in higher

Fig. 2. Spatial distributions of total Se and Te concentrations in surface sediments of the East China Sea.

Please cite this article in press as: Duan, L.-Q., et al., Selenium and tellurium fractionation, enrichment, sources and chronological reconstruction in the East China Sea, Estuarine, Coastal and Shelf Science (2014), http://dx.doi.org/10.1016/j.ecss.2014.03.024

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Fig. 3. Relationships of Se, Te and their chemical fractions to sediment parameters (i.e., clay and silt content, TOC content, salinity and pH) and chlorophyll a (Chl a) concentrations.

Se and Te concentrations near the Cheju Island. Besides, there was a research suggesting that 66e79% of organic matters at the cold-eddy area were from marine authigenic input (Yu et al., 2012). Thus, the significant relationships of Se and Te to TOC content further suggested that higher Se and Te concentrations at

station A5 were closely relate to biological activities. In zone V, Se and Te had the lowest concentrations, which was consistent with much less terrestrial sediment input and the coarsest grain size. In addition, Se and Te distributions and transport also were affected by sedimentary environment characteristics (e.g., pH,

Please cite this article in press as: Duan, L.-Q., et al., Selenium and tellurium fractionation, enrichment, sources and chronological reconstruction in the East China Sea, Estuarine, Coastal and Shelf Science (2014), http://dx.doi.org/10.1016/j.ecss.2014.03.024

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Table 2 Total, organic matter fraction (F4) and residual fraction (F5) concentrations of Se and Te in each zone divided as Fig. 1. Zone Se (mg/g)

Te (mg/g)

I Total F4 F5 Total F4 F5

0.101 0.003 0.079 0.046 0.004 0.023

II







0.018 0.001 0.02 0.003 0.001 0.009

0.128 0.004 0.11 0.055 0.005 0.036

III







0.112 0.001 0.11 0.025 0.002 0.029

temperature and salinity), which mainly responded to chemical fractions of Se and Te.

3.2.2. Chemical fractions Chemical fractions of Se and Te were determined by sequential extraction (Fig. 5). Results suggested that Se and Te mainly occurred in residual fraction ranging from 26.4 to 97.2% and 23.5e87.2% with average percentages of 70.5
14.5% and 49.1
13.2%, respectively. High proportion of residual fraction implied that Se and Te in the ECS were mainly from natural lithogenic origin via the Changjiang River and JCC sediment load. The residual fraction concentrations of Se and Te in zones I, II and III were higher than that in zones IV and V (Table 2), suggesting that Se and Te in the inner shelf were more influenced by fluvial input. In non-residual fractions, Se mainly existed as exchangeable fraction (0.08e37.1% with an average of 12.1
8.22%) with equivalent percentages in other three nonresidual fractions. Te mainly existed as exchangeable and carbonates fractions (nd-29.5% and 0.53e26.4% with averages of 15.3
7.63% and 17.2
5.84%). Te also had high percentages in Fee Mn oxides and organic matter fractions ranging from 1.64 to 19.1% and 0.44e44.8% with averages of 7.94
4.33% and 10.4
6.49%, respectively. The existing of organic matter fraction for Se and Te was mainly related to organic matters and sulfides. The possible mechanism for Se and Te associated with organic materials was the microbial reduction of Se(VI), Se(IV), Te(VI) and Te(IV) to Se(0) and Te(0) or Se and Te incorporation in organic matter of sediments, which was the major process removing Se and Te from water and there transferring to sediments (Siddique et al., 2006; Ryu et al., 2011). Higher organic matter fraction concentrations of Se and Te appeared in zone IV than other zones (Table 2), indicating that Se and Te in zone IV were more affected by marine biological processes.

Fig. 4. Spatial distribution of chlorophyll a (Chl a) concentrations in surface sediments of the East China Sea.

0.101 0.004 0.081 0.051 0.005 0.026

IV







0.043 0.001 0.059 0.012 0.002 0.013

0.099 0.005 0.078 0.048 0.006 0.023

V







0.058 0.001 0.041 0.014 0.003 0.003

0.071 0.002 0.047 0.043 0.004 0.021









0.023 0.001 0.025 0.005 0.001 0.007

Mobility and availability of Se and Te were mainly controlled by chemical processes, e.g., adsorption-desorption process, dissolutioneprecipitation reaction and oxidationereduction reaction. These processes were influenced by environmental parameters (e.g., salinity, pH and temperature), which mainly worked on nonresidual fractions of Se and Te. Salinity, one of the factors affecting aggregation and deposition of sediment, may contribute to different concentrations of Se and Te in non-residual fraction among different sampling sites. Non-residual fractions of Se and Te had significant negative correlations with salinity (r ¼ 0.573, p < 0.01; 0.584, p < 0.01; Fig. 3), indicating that non-residual fraction concentrations of Se and Te decreased with an increasing salinity. This was closely related to the complexation of Se and Te with anions. Dissolved Se and Te could be mobilized as soluble chloride complexes. Upon the formation of these complexes, activity of free Se and Te in seawaters would decrease, followed by increase of sediment desorption. Besides the effect of complexation, an increase of salinity was associated with an increase in major cations (e.g., Naþ, Kþ, Ca2þ and Mg2þ) concentrations that competed with Se and Te for sorption sites. Thus, with salinity increase, complex anions and major cations increased, resulting in that the adsorption and precipitation of Se and Te on sediments decreased and desorption increased. Besides, since pH and temperature could affect solubility and behaviors of Se and Te in aquatic environment, so they can influence Se and Te distributions in sediments directly (Fianko et al., 2007). A drop in pH and increase in temperature could prevent Se and Te transfer from water phase to sediments and cause desorption from sediments (Gambrell et al., 1991; Calmano et al., 1993). At low pH, the negative surface charge of organic matter, clay particles and Fe and Al oxides was reduced (Du Laing et al., 2009), resulting in desorption of Se and Te from sediments. pH and temperature mainly worked on exchangeable and carbonates fractions (F1þF2). In this respect, there were positive correlations

Fig. 5. Se and Te concentrations in five chemical fractions in surface sediments of the East China Sea. F1: exchangeable fraction; F2: carbonates fraction; F3: FeeMn oxides fraction; F4: organic matte fraction; F5: residual fraction.

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between F1þF2 and pH, with r ¼ 0.573 (p < 0.01) for Se and r ¼ 0.760 (p < 0.01) for Te, respectively (Fig. 3). However, the correlation between temperature and F1þF2 of Se and Te was not found, suggesting that the effect of temperature on Se and Te distributions was not obvious in this study.

3.2.3. Enrichment assessment To determine the relative degree of Se and Te enrichment, EFs of Se and Te were calculated. Cs had been chosen as the reference element due to its higher percentage in residual fraction (89e 98% with an average of 93
1.7%) and its significant correlation with clay content (r ¼ 0.950, p < 0.01). This reference element already has been selected by Duan et al. (2010) for sediments of Bohai Bay, by Roussiez et al. (2005) for sediments in the Gulf of Lions and by N’guessan et al. (2009) for stream bed sediments. Se and Te values in Chinese marine sedimentary parent material were adopted as background values. Contour plots of EF distributions for Se and Te were shown in Fig. 6. EFs of Se and Te were 0.89e2.91 and 1.27e5.68 with averages of 1.78
0.43 and 2.60
0.93, respectively. EFs of Se in most stations were lower than 2 with some exception station near the Hangzhou Bay mouth and Cheju Island being greater than 2, corresponding to two productive areas. Se enrichment near the Hangzhou Bay mouth likely was the combined result of anthropogenic and biological burdens whereas Se enrichment near the Cheju Island seemed to be due to a marked biological burden. EFs of Te in most stations were greater than 2 with higher values at the north part of central area, which was closely related to Cs concentrations. Although Te concentration near the Hangzhou Bay mouth was 1e2 times higher than that at other area, Cs concentration near the Hangzhou Bay mouth was 3e4 times higher than that at other area, resulting in that Te did not present higher EFs near the Hangzhou Bay mouth. Generally, according to the classification proposed by Sutherland (2000), Se enrichment in sediments of the ECS was minor, whereas Te enrichment was moderate. These enrichments were contributed by the common-effect of anthropogenic and biological activities. EFs based on total concentrations of Se and Te suggested that their enrichment levels in the ECS sediments were not significant. However, the low enrichment of Se and Te did not mean they were the low potential bioavailability and risks, which needed to be assessed by chemical fractions of Se and Te. Risk assessment code (RAC) based on chemical fractions was always used to assess potential bioavailability and risks of metals (Singh et al., 2005; Sundaray et al., 2011). RAC assessed potential bioavailability based on the percentage of exchangeable and carbonates fractions in sediments. The classification was as follows (Singh et al., 2005): metals in exchangeable and carbonates

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fractions <1%, no risk; 1e10%, low risk; 11e30%, medium risk; 31e50, high risk; >75%, very high risk. The percentages of exchangeable and carbonates fractions for Se and Te were 0.2e 51.2% and 5.5e49.6% with averages of 18.8
11.4% and 32.5
10.0%, suggesting that Se and Te in the ECS generally were at medium and high risks, respectively. 3.3. Environmental evolution To trace historical sedimentary fingerprints of Se and Te left by natural, anthropogenic and biological activities, sediment core G1 was chosen at mud area located along the Zhejiang-Fujian Provinces coast. Sediment chronology of core G1 was determined by 210Pb radiometric technique. The determination of total and excess 210Pb activities in core G1 has been completed and published by our laboratory (Yu et al., 2012; Duan et al., 2013). Two sedimentation rates were estimated by the CIC model in core G1: a lower rate of 0.39 cm/yr below 43 cm and a higher rate of 0.91 cm/yr above 43 cm, corresponding to the periods before and after the 1960s, respectively. According to sedimentation rate and depth, core G1 could provide information as far back as about 1847. Combined with sediment chronology, temporal profiles of abundances, EFs and BFs of Se and Te in core G1 were studied. 3.3.1. Abundances Temporal profiles of Se and Te were examined in order to evaluate their historical changes in sediments. Vertical profiles of Se and Te concentrations in core G1 were presented in Fig. 7. Se and Te concentrations were 0.089e0.275 mg/g and 0.029e0.139 mg/g with averages of 0.15
0.04 mg/g and 0.06
0.02 mg/g, respectively. Se and Te showed a similar vertical variation, with correlative coefficient of 0.742 (p < 0.01). Although vertical variation of Se and Te was fluctuant, it generally could be divided into three sections including below 45 cm (before 1960), 19e45 cm (1960e1990) and above 19 cm (after 1990). Before 1960, anthropogenic input to the ECS was minor, thus, Se and Te variations were mainly related to natural input (i.e., terrigenous detrital and biological inputs). There was the research on other metals (Zn, Cu, Pb and Cd) in the ECS inner shelf sediments suggested that their concentrations before 1960 had little variation, indicating their natural terrigenous materials carried by the Changjiang River (Lin et al., 2002). Compared with heavy metals, Se and Te concentrations before 1960 had some variations and enrichments, suggesting that there also was other natural origin (i.e., biological input) besides the natural terrigenous materials carried by river. Se and Te were prone to be adsorbed by marine algae and also closely associated with organic matters and then delivered to the sediments. To understand the influence of

Fig. 6. Spatial distributions of enrichment factors (EFs) for Se and Te in surface sediments of the East China Sea.

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Cities) were mainly discharged into the Suzhou and Huangpujiang Rivers; however, in order to reduce pollution of the Suzhou and Huangpujiang Rivers, these raw industrial effluent and urban sewage were intensively discharged into the Changjiang River and ECS since 1960. At the same time, urbanization and industrialization developed rapidly, resulting in more industrial effluent and urban sewage with Se and Te into the ECS. The second spike in 1989 was related to the performance of environmental regulation and biological activities. Marine environmental management began to attract attention in the mid-1980s, and was enhanced in the late1980s, when the Environmental Protection Law of the People’s Republic of China (NPC, 1989) was drafted and carried out. However, the red tides frequently occurred in the late-1980s (Wang, 2006; Li et al., 2007; Zhou et al., 2008) with phytoplankton biomass having a peak value in 1989, when phytoplankton biomass was 8 times higher than that before and after 1989 (Wang et al., 2004). Thus, it seemed that biological activities made an important contribution to Se and Te enrichment in 1989. From 1990 to the early-2000s, fairly constant patterns of Se and Te concentrations were observed. There were two possible reasons for reduction in Se and Te accumulations during this period, in spite of the industrialization development and population increase: (1) effect of the Environmental Protection Law of the People’s Republic of China (NPC, 1989); and (2) the Three Gorges Project closure led to the decrease of sediment load and associated elements. In succession, a slight enrichment of Se and Te at surface sediments appeared, which may be due to Se and Te at seawaters were firstly absorbed into top sediment. In conclusion, except the spike during 1960e 1980 was attributed to anthropogenic input, Se and Te enrichment in other times was mainly associated with biological activities.

Fig. 7. Temporal profiles of total Se and Te concentrations, enrichment factors (EFs) and buried fluxes (BFs) and TOC content in core G1 sediments of the East China Sea.

biological processes on temporal variation of Se and Te, TOC content in core G1 were determined, with the range of 0.28e0.76% (average of 0.49
0.09%, Fig. 7). Se and Te concentrations had a similar variation to TOC content before 1960 with the correlation coefficients of 0.661(p < 0.01) and 0.600 (p < 0.01) for Se and Te, suggesting that sediment organic matters were the major factor of Se and Te deposition before 1960 at G1 site, which is located at upwelling area. In the second section, Se and Te concentrations increased with two spikes in around 1970 and 1989. During the period of 1960e1980, although terrigenous input of nutrients increased, primary productivity of the ECS did not increase significantly (Yu et al., 2012). Besides, Se and Te had negative relationships with TOC (r ¼ 0.700, p < 0.01; r ¼ 0.456, p < 0.05) in this period. These indicated that biological process was not main contribution to Se and Te enrichment, which likely was attributed to the aggravation of anthropogenic activities. The influence of anthropogenic activities on heavy metal (Zn, Cu, Pb and Cd) enrichment since 1960 also was found by Lin et al. (2002), whose study demonstrated that the rapid industrialization especially since the opening of China in 1972 was most likely the culprit in alternating the “natural” heavy metal deposition in the ECS continental shelf sediment. Before 1960, the industrial effluent and urban sewage of cities adjacent to the ECS (i.e., Shanghai and Ningbo

3.3.2. Enrichment factors To reduce Se and Te variability caused by grain sizes and mineralogy of sediments, and to identify non-natural contributions, EFs of Se and Te in core G1 were determined. Temporal distributions showed that EFs of Se and Te had significant variations along core G1 (Fig. 7). EFs of Se and Te were 0.92e3.05 and 0.80e3.22 with averages of 1.68
0.57 and 1.76
0.56, respectively. Although EFs of Se and Te were not exceptionally high, they still presented enrichments in some layers. Generally, temporal variations of EFs for Se and Te were similar to their concentration variations: higher EFs (>2) appeared before 1900 and in around 1970, 1989 and 2009. According to above discussion, higher enrichments of Se and Te before 1900, in 1989 and 2009 was mainly attributed to biological activities, whereas that in 1970 was caused by anthropogenic activities.

3.3.3. Burial fluxes Depositional Se and Te calculations for sedimentary sequence were carried out in order to understand how Se and Te accumulation in sediments varied with time. Since sedimentation rates varied significantly with time, burial flux might be a better parameter to assess temporal Se and Te accumulation changes. BFs of Se and Te were 0.03e0.25 and 0.01e0.12 mg/(yr$cm2), with averages of 0.10
0.05 and 0.04
0.02 mg/(yr cm2), respectively (Fig. 7). It could be seen from Fig. 7, BFs of Se and Te showed a sudden change since 1960 due to the increasing sedimentation rate. Increasing BFs might be related to the increasing influx of riverine suspended particles to the inner shelf. BF variations of Se and Te after 1960 were similar to their concentration and EF variations: higher BFs in the period of 1960e1980 were due to anthropogenic activities; other higher BFs in 1989 and 2009 were closely related to biological input.

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4. Conclusions Higher Se and Te concentrations appeared in the inner shelf and near the Cheju Island. Away from the inner shelf and Cheju Island, Se and Te concentrations decreased in a southeast direction. This finding was agreement with clay, TOC and Chl a distributions, indicating that sediment characteristics and sources were main factors influencing the Se and Te distributions. Sequential extraction showed that Se and Te mainly existed in residual fraction with exchangeable and carbonates fractions of 18.8
11.4% and 32.5
10.0%, indicating that Se and Te were at medium and high risks, respectively. Enrichment factors implied that Se and Te in the ECS sediments were minor and moderate enrichments. These enrichments were contributed by the common-effect of anthropogenic and biological activities. Temporal profiles of abundances, EFs and BFs for Se and Te in core G1 could be divided into three sections: below 45 cm (before 1960), 19e45 cm (1960e1990) and above 19 cm (after 1990), with higher values before 1900, in the period of 1960e1980, and in 1989 and 2009. Higher Se and Te values in the period of 1960e 1980 were mainly attributed to anthropogenic input, whereas those in other times were dominantly associated with biological activities. Acknowledgments This paper was supported by the National Natural Science Foundation of China (No. 41306070), Fund for Creative Research Groups by NSFC (No. 41121064), the National Key Project for Basic Research of China (No. 2011CB403602) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCXZ-YWQ07-02). References Atkinson, C.A., Jolley, D.F., Simpson, S.L., 2007. Effect of overlying water pH, dissolved oxygen, salinity and sediment disturbances on metal release and sequestration from metal contaminated marine sediments. Chemosphere 69, 1428e1437. Calmano, W., Hong, J., Förstner, U., 1993. Binding and mobilisation of heavy metals in contaminated sediments affected by pH and redox potential. Water Sci. Technol. 28, 223e235. Chandía, C., Salamanca, M., 2012. Long-term monitoring of heavy metals in Chilean coastal sediments in the eastern South Pacific Ocean. Mar. Pollut. Bull. 64, 2254e2260. Du Laing, G., Rinklebe, J., Vandecasteele, B., Meers, E., Tack, F.M.G., 2009. Trace metal behaviour in estuarine and riverine floodplain soils and sediments: a review. Sci. Total Environ. 407, 3972e3985. Duan, L.Q., Song, J.M., Xu, Y.Y., Li, X.G., Zhang, Y., 2010. The distribution, enrichment and source of potential harmful elements in surface sediments of Bohai Bay, North China. J. Hazard. Mater. 183, 155e164. Duan, L.Q., Song, J.M., Yuan, H.M., Li, X.G., Li, N., 2013. Spatio-temporal distribution and environmental risk of arsenic in sediments of the East China Sea. Chem. Geol. 340, 21e31. Fang, T.H., Li, J.Y., Feng, H.M., Chen, H.Y., 2009. Distribution and contamination of trace metals in surface sediments of the East China Sea. Mar. Environ. Res. 68, 178e187. Fianko, J.R., Osae, S., Adomako, D., Adotey, D.K., Serfor-Armah, Y., 2007. Assessment of heavy metal pollution of the Iture estuary in the central region of Ghana. Environ. Monit. Assess. 131, 467e473. Gambrell, R.P., Wiesepape, J.B., Patrick Jr., W.H., Duff, M.C., 1991. The effects of pH, redox, and salinity on metal release from a contaminated sediment. Water Air Soil Pollut. 57e58, 359e367. Gaudette, H.E., Flight, W.R., Toner, L., Folger, D.W., 1974. An inexpensive titration method for the determination of organic carbon in recent sediment. J. Sediment. Petrol. 44, 249e253. Hasan, A.B., Kabir, S., Reza, A.H.M.S., Zaman, M.N., Ahsan, A., Rashid, M., 2013. Enrichment factor and geo-accumulation index of trace metals in sediments of the ship breaking area of Sitakund Upazilla (Bhatiary-Kumira), Chittagong, Bangladesh. J. Geochem. Explor. 125, 130e137. Hori, K., Saito, Y., Zhao, Q., Cheng, X., Wang, P., Sato, Y., Li, C., 2001. Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression. Mar. Geol. 177, 331e351.

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