Source and distribution of lead in the surface sediments from the South China Sea as derived from Pb isotopes

Marine Pollution Bulletin 60 (2010) 2144–2153

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Source and distribution of lead in the surface sediments from the South China Sea as derived from Pb isotopes Laimin Zhu a,*, Laodong Guo b, Ziyou Gao c, Guan Yin c, Ben Lee a, Fei Wang a, Jiang Xu d a

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, PR China Department of Marine Science, University of Southern Mississippi, 1020 Balch Blvd., Stennis Space Center, MS 39529, USA c College of Geology Science, Chengdu University of Technology, Chengdu, 610059, PR China d Third Institute of Oceanography, State Oceanography Administration, Xiamen 361005, PR China b

a r t i c l e

i n f o

Keywords: Lead Pb isotope ratio Marine sediments South China Sea

a b s t r a c t Rapid economic development in East Asian countries has inevitably resulted in environmental degradation in the surrounding seas, and concern for the environment and its protection against pollutants is increasing. Identification of sources of contaminants and evaluation of current environmental status are essential to environmental pollution management, but relatively little has been done in the South China Sea (SCS). In order to investigate the abundance, distribution, and sources of Pb within the SCS, stable Pb isotopes and their ratios were employed to assess the contamination status and to differentiate between natural and anthropogenic origins of Pb in the surface sediments. The total Pb concentrations in sediments varied from 4.18 to 58.7 mg kg1, with an average concentration of 23.6 ± 8.9 mg kg1. The observed Pb isotope ratios varied from 18.039 to 19.211 for 206Pb/204Pb, 15.228 to 16.080 for 207 Pb/204Pb, 37.786 to 39.951 for 208Pb/204Pb, 1.176 to 1.235 for 206Pb/207Pb, and 2.468 to 2.521 for 208 Pb/207Pb. The majority of these ratios are similar to those reported for natural detrital materials. Combined with Pb enrichment factor values, our results show that Pb found within most of the SCS sediments was mainly derived from natural sources, and that there was not significant Pb pollution from anthropogenic sources before 1998. Further studies are needed to reconstruct deposition history and for trend analysis. Ó 2010 Elsevier Ltd. All rights reserved.

The South China Sea (SCS) is the largest marginal sea, with an area of 3.3 million km2 (Morton and Graham, 2001), largely surrounded by land (Fig. 1). There are many countries surrounding the rim of the SCS. Among those, China in the north, the Philippines in the east, Malaysia in the south and Vietnam in the west are most important in terms of their coastal margins. Other surrounding countries, territories, and cities including Brunei, Cambodia, Indonesia, Singapore, Thailand, Taiwan, Hong Kong, and Macau also have some influence on the SCS, as these areas face many of the common environmental issues such as deforestation and soil erosion, pollution from rapid industrialization, and discharge of untreated urban, agricultural and industrial wastes into the coastal waters of the SCS (Morton and Graham, 2001). Moreover, the countries around its rim are some of the most densely populated, fastest growing and, until 1997, the most vibrant economies on earth. About 270 million people live in the coastal cities or sub-regions of the South China Sea (Morton and Graham, 2001), such as Guangzhou, Hong Kong, Hanoi, and Ho Chi Minh City. Several major world rivers also drain into the SCS, * Corresponding author. Tel.: +86 29 88302202. E-mail address: [email protected] (L. Zhu). 0025-326X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2010.07.026

including the Pearl River from southern China, the Red in North Vietnam, the Mekong in South Vietnam. These rivers carry vast amounts of suspended sediments and dissolved substances such as nutrients, trace elements, and carbon species, derived from continental runoff (domestic, agricultural, and industrial) and river bank erosion. For example the northern source is mainly the Asian continent and Taiwan while the southern source consists of islands or volcanic arcs that bound the SCS to the east and south (Li et al., 2003). Weathering products from the Asian continent are transported to the SCS mainly by rivers, including the Yangtze, Mekong, and Red rivers, which discharge 768  106 tons of sediments annually (Liu et al., 2003). About 30% of the Yangtze River sediment discharge is transported southward by nearshore currents along the China coast (Milliman et al., 1985). Over the last two decades the South China Sea has received increasing attention in research fields such as paleoceanography, Cenozoic tectonism, and paleoenvironmental reconstruction in relation to East Asian monsoon evolution and plate reorganization during the Cenozoic (Wang et al., 1999, 2003; Lüdmann et al., 2001; Wehausen and Brumsack, 2002; Tamburini et al., 2003; Liu et al., 2003; Li et al., 2003; Yang et al., 2008). In recent years, a strong link between sediment supply from the Pearl River and East

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Fig. 1. Sampling locations and Pb distribution of surface sediments in the South China Sea (SCS).

Asian monsoon activity during the Quaternary has been established on the basis of paleoceanographic, mineralogical, and geochemical studies on the SCS sediments (Wang et al., 1999; Clift et al., 2002; Wehausen and Brumsack, 2002; Liu et al., 2003; Yang et al., 2008). Heavy metal contamination is still an environmental problem today in the margin sea (Marchand et al., 2006; Miguel et al., 2008; Xu et al., 2009). As the economy in Asian countries continues to grow, inputs of heavy metals and other contaminants to the coastal regions are likely to increase. Many of the pollutants and particle-reactive elements will eventually be scavenged and removed to bottom sediments (Shumilin et al., 2002; Zhang et al., 2007). Therefore, marine sediments may provide a proxy for contamination status and sources of Pb in the coastal environments (Soto-Jiménez and Péez-Osuna, 2001; Miguel et al., 2008; Xu et al., 2009). However, human impacts on the SCS and geochemical consequences remain poorly understood (Zhao and Yan, 1994; Morton and Graham, 2001; Liu et al., 2002b; Guo et al., 2004; Zhang and Du, 2005; Zhu et al., 2007). There are four naturally occurring stable Pb isotopes: 204Pb, 206 Pb, 207Pb and 208Pb. While 204Pb is a non-radiogenic 206Pb, 207 Pb and 208Pb are the decay products of the 238U, 235U and 232 Th decay series (Gunter, 1986). Sources of Pb in marine environments include natural sources from rock weathering and riverbank and coastal erosion, and anthropogenic sources from urban and industrial emissions. Recent applications have demonstrated that measurements of Pb concentrations and isotopes could be an ideal tool for identifying sources and transport pathways of Pb in marine environments (e.g., Bindler et al., 2001; Millot et al., 2004; Choi et al., 2007; Komárek et al., 2008; Kelly et al., 2009). Indeed, Pb

isotope ratios have been widely used to investigate sources of Pb and other pollutants in marine sediments (e.g., Gobeil et al., 2001; Bindler et al., 2001; Hiniches et al., 2002; Choi et al., 2007; Zhang et al., 2008; Komárek et al., 2008; Soto-Jiménez and Flegal, 2009). It has been useful to differentiate pollutant sources for multi-source environments using isotopic tracers in the Yellow Sea (Choi et al., 2007) and the Liaodong Bay in the northwest of the Bohai Sea (Xu et al., 2009). However, Pb isotopic tracing has not yet been done systematically for sediments in the SCS. Here, we report the total Pb concentration and isotopic composition (204Pb, 206Pb, 207Pb and 208Pb) in 52 sediments from the SCS. Based on the Pb concentrations and isotope ratios, we have identified the main source of Pb deposited in sediments and evaluated the relative contribution from anthropogenic influences for Pb in surface sediments in the SCS before 1998. Our measurements here could also service as baseline of heavy metals in the sediments of the SCS. Sediment samples were collected onboard the R/V ‘‘Xiang Yanghong 4” from the State Oceanic Administration of China during 1998, using the Chinese Dayang grab sampler designed to sample the top seabed sediment. Our surface sediment samples here conform to the top 5 cm of the sediments, although this depth may vary slightly depending on the compaction of the sediment (Guo et al., 2004; Zhang and Du, 2005; Zhu et al., 2007). Sites of sampling are shown in Fig. 1 for the 52 locations in the South China Sea, covering an area of 5°300 –23°000 N and 108°000 –119°300 E (Fig. 1). Samples were packaged and transported to the laboratory according to the Oceanographic Investigation Criteria of China (GB/ T13909-92) for processing and analysis. Subsamples for analysis were removed from the center of the grab with a plastic spoon

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to avoid contamination. Immediately after collection, the sediments were placed in a polyethylene bag for transport to the laboratory. Sediment samples were divided into two portions, one for measuring grain size and the other for chemical analysis, respectively. Grain size analysis was performed using a laser separator-size analyzer (Model MAM 5005, UK). Subsamples for the analysis of Pb and Sc were dried at 80 °C and then ground to a powder with a mortar and pestle, sieved through an 80-mesh sieve and kept in a pre-cleaned container. Lead and Sc content were measured using a Finnigan Element-2 ICP-MS at the Institute of Geochemistry, Chinese Academy of Sciences, following the procedures described in Qi et al. (2000). Standard samples (e.g., GBPG-1, AMH-1 and OU-6) were used to monitor the performance of the instrument and the data quality. Coefficient of variation was generally lower than 10%. For lead isotopes, the analytical method employed was similar to that described by Birkeland (1990). Briefly, sediment samples

were first digested with concentrated hydrochloric and hydrofluoric acids in sequence. Pb was separated and purified using an ion exchange chromatographic column with AGV-X8 resin (200– 400 mesh, Bio-Rad, USA). The purified Pb samples were then placed on a Re filament together with a solution of silica gel and phosphoric acid. The samples were analyzed on a MAT 261 mass spectrometer. The total procedural blank was less than 10 ng; the 2r variations were 0.1%, 0.09%, and 0.30% for the 206Pb/204Pb, 207 Pb/204Pb, and 208Pb/204Pb ratios, respectively. During the sample analysis, the international standard NBS981 was also measured as a sample, which yielded an error of ±0.002% for the isotopic ratios. The results of grain size, total Pb and Sc concentrations, and Pb isotopic ratios for the sediment samples from the SCS are listed in Table 1. The total Pb concentrations varied from 4.18 to 58.7 mg kg1, with an average concentration of 23.6 ± 8.9 mg kg1. Scandium (Sc) concentrations in the sediments varied from 0.33 to 20.6 mg kg1 with an average concentration of 10.6 ± 4.2 mg kg1

Table 1 Granularity, Pb and Sc contents, as well as Pb isotopic composition of surface sediments.

a

Station no.

Latitude (N)

Longitude (E)

Mz (U)

Pb (mg kg1)

Sc (mg kg1)

208

S001 S003 S005 S007 S009 S011 S013 S016 S018 S021 S023 S024 S026 S028 S029 S031 S033 S035 S037 S039 S041 S043 S045 S047 S049 S050 S052 S053 S056 S057 S059 S060 S062 S064 S066 S068 S071 S073 S075 S081 S083 S086 S088 S090 S092 S094 S096 S097 S098 S100 S102 S103

21°00 20°00 19°00 17°590 18°00 17°00 16°00 14°590 15°00 14°00 12°00 11°590 12°00 12°00 11°00 10°300 9°590 9°300 9°00 8°300 8°30 7°300 7°00 6°300 6°00 5°440 7°150 8°00 10°00 11°00 12°00 12°00 12°00 11°590 12°590 13°590 14°590 15°00 15°00 18°00 17°590 17°590 19°00 19°590 21°00 22°00 22°400 23°00 19°290 20°310 21°300 21°590

111°00 112°00 113°00 114°00 110°00 109°590 110°00 110°300 111°29 113°00 113°00 112°00 111°00 110°00 110°00 110°00 112°00 113°00 108°00 109°00 109°520 111°00 111°590 113°00 114°00 114°300 115°290 115°590 115°00 115°00 115°590 116°590 118°00 118°560 119°00 119°00 118°300 117°300 116°00 117°00 118°00 119°290 119°300 119°300 119°300 119°300 118°400 118°00 117°00 116°00 115°00 114°300

6.30 5.26 4.33 6.94 5.65 7.10 7.00 7.44 7.03 6.51 7.58 7.42 6.84 6.76 7.06 6.62 6.35 1.97 4.00 6.80 6.97 7.16 7.09 6.64 6.63 6.10 4.68 6.70 3.18 5.85 5.44 6.25 7.12 5.97 6.85 6.33 6.66 6.86 7.05 7.14 7.17 7.44 7.31 7.39 7.31 5.27 6.09 6.26 6.30 5.26 4.33 6.94

36.1 18.0 10.2 21.9 27.4 21.3 18.4 21.2 20.6 22.0 28.8 27.7 29.1 26.0 26.1 29.8 28.6 19.3 17.0 17.3 23.2 25.8 27.1 25.0 29.0 18.7 22.6 13.0 20.4 10.6 26.2 4.18 27.2 12.7 21.7 25.4 58.7 28.6 33.8 31.2 33.0 21.9 32.1 30.9 26.5 27.7 5.94 8.17 28.4 12.5 14.8 31.5

10.7 6.18 5.04 12.6 6.03 11.8 10.1 10.5 10.1 10.6 14.8 13.4 13.3 12.6 10.3 12.3 10.7 7.94 4.91 4.93 10.7 11.7 12.6 11.6 12.9 8.78 10.2 6.28 7.77 3.97 12.9 1.20 8.19 5.38 11.8 15.8 13.4 15.0 14.2 15.4 15.6 20.6 16.8 17.6 14.5 14.7 0.63 0.33 15.5 6.34 6.98 11.3

39.450 ± 0.008 38.715 ± 0.006 39.015 ± 0.004 39.391 ± 0.004 38.432 ± 0.006 39.344 ± 0.010 37.970 ± 0.007 39.319 ± 0.008 39.057 ± 0.004 38.876 ± 0.004 38.843 ± 0.003 39.184 ± 0.003 39.163 ± 0.002 39.361 ± 0.003 39.415 ± 0.002 39.332 ± 0.006 38.961 ± 0.006 39.570 ± 0.003 39.651 ± 0.003 39.540 ± 0.001 39.178 ± 0.005 39.379 ± 0.006 38.778 ± 0.001 39.305 ± 0.009 39.190 ± 0.001 38.729 ± 0.004 39.006 ± 0.007 39.694 ± 0.002 39.517 ± 0.004 39.344 ± 0.005 39.951 ± 0.004 38.367 ± 0.001 39.112 ± 0.005 39.557 ± 0.004 38.870 ± 0.005 38.435 ± 0.006 38.455 ± 0.002 38.781 ± 0.002 38.594 ± 0.003 39.527 ± 0.009 39.360 ± 0.003 38.235 ± 0.003 38.021 ± 0.005 38.024 ± 0.003 37.786 ± 0.004 37.997 ± 0.004 38.461 ± 0.005 38.345 ± 0.003 38.890 ± 0.005 38.953 ± 0.005 38.984 ± 0.006 38.846 ± 0.005

From calculation of the

206

Pb/204Pb and

207

Pb/204Pb ratios.

Pb/204Pb

207

Pb/204Pb

15.917 ± 0.003 15.685 ± 0.002 15.750 ± 0.005 15.866 ± 0.002 15.560 ± 0.002 15.872 ± 0.004 15.275 ± 0.003 15.850 ± 0.003 15.734 ± 0.002 15.668 ± 0.002 15.648 ± 0.001 15.566 ± 0.001 15.761 ± 0.001 15.640 ± 0.001 15.679 ± 0.001 15.673 ± 0.003 15.711 ± 0.003 15.712 ± 0.001 15.731 ± 0.001 15.707 ± 0.001 15.566 ± 0.002 15.646 ± 0.003 15.609 ± 0.001 15.844 ± 0.001 15.764 ± 0.001 15.625 ± 0.002 15.712 ± 0.003 16.012 ± 0.001 15.954 ± 0.001 15.891 ± 0.002 16.080 ± 0.002 15.542 ± 0.001 15.775 ± 0.002 16.006 ± 0.003 15.671 ± 0.002 15.496 ± 0.002 15.519 ± 0.001 15.578 ± 0.002 15.554 ± 0.001 15.928 ± 0.004 15.858 ± 0.001 15.452 ± 0.001 15.338 ± 0.002 15.343 ± 0.001 15.228 ± 0.002 15.323 ± 0.001 15.567 ± 0.003 15.489 ± 0.001 15.663 ± 0.002 15.698 ± 0.0011 15.703 ± 0.003 15.665 ± 0.002

206

Pb/204Pb

18.998 ± 0.004 18.485 ± 0.002 18.673 ± 0.004 18.889 ± 0.002 18.493 ± 0.003 18.858 ± 0.005 18.171 ± 0.004 18.860 ± 0.004 18.738 ± 0.002 18.659 ± 0.001 18.692 ± 0.002 19.221 ± 0.002 18.801 ± 0.001 19.111 ± 0.001 19.120 ± 0.001 19.041 ± 0.003 18.667 ± 0.003 19.178 ± 0.002 19.220 ± 0.002 19.216 ± 0.001 18.989 ± 0.003 19.120 ± 0.003 18.653 ± 0.001 18.872 ± 0.001 18.837 ± 0.001 18.652 ± 0.002 18.767 ± 0.003 19.069 ± 0.001 19.022 ± 0.002 18.873 ± 0.002 19.219 ± 0.002 18.280 ± 0.001 18.784 ± 0.002 18.995 ± 0.003 18.635 ± 0.002 18.376 ± 0.003 18.409 ± 0.001 18.629 ± 0.002 18.494 ± 0.001 18.901 ± 0.004 18.787 ± 0.002 18.290 ± 0.002 18.157 ± 0.004 18.172 ± 0.002 18.039 ± 0.002 18.091 ± 0.003 18.346 ± 0.002 18.279 ± 0.001 18.584 ± 0.003 18.583 ± 0.002 18.606 ± 0.002 18.574 ± 0.003

206

Pb/207Pb

208

Pb/207Pba

1.194 ± 0.00001 1.179 ± 0.00001 1.186 ± 0.00007 1.191 ± 0.00001 1.188 ± 0.00005 1.188 ± 0.00003 1.190 ± 0.00002 1.190 ± 0.00002 1.191 ± 0.00001 1.191 ± 0.00001 1.195 ± 0.00004 1.235 ± 0.00001 1.193 ± 0.00001 1.222 ± 0.00004 1.219 ± 0.00001 1.215 ± 0.00004 1.188 ± 0.00003 1.221 ± 0.00006 1.222 ± 0.00003 1.223 ± 0.00001 1.220 ± 0.00003 1.222 ± 0.00009 1.195 ± 0.00001 1.191 ± 0.00001 1.195 ± 0.00003 1.194 ± 0.00001 1.194 ± 0.00002 1.191 ± 0.00001 1.192 ± 0.00001 1.188 ± 0.00001 1.195 ± 0.00001 1.176 ± 0.00001 1.191 ± 0.00001 1.187 ± 0.00005 1.189 ± 0.00004 1.186 ± 0.00003 1.186 ± 0.00001 1.196 ± 0.00006 1.189 ± 0.00003 1.187 ± 0.00003 1.185 ± 0.00001 1.184 ± 0.00004 1.184 ± 0.00006 1.184 ± 0.00002 1.185 ± 0.00002 1.181 ± 0.00006 1.179 ± 0.00001 1.180 ± 0.00001 1.186 ± 0.00006 1.184 ± 0.00004 1.185 ± 0.00006 1.186 ± 0.00001

2.478 2.468 2.477 2.483 2.470 2.479 2.486 2.481 2.482 2.481 2.482 2.517 2.485 2.517 2.514 2.510 2.480 2.518 2.521 2.517 2.517 2.517 2.484 2.481 2.486 2.479 2.483 2.479 2.477 2.476 2.485 2.469 2.479 2.471 2.480 2.480 2.478 2.489 2.481 2.482 2.482 2.474 2.479 2.478 2.481 2.480 2.471 2.476 2.483 2.481 2.483 2.480

EF (Pb) 1.86 1.60 1.11 0.96 2.50 0.99 1.00 1.11 1.11 1.14 1.07 1.13 1.20 1.13 1.39 1.33 1.47 1.33 1.90 1.93 1.19 1.21 1.18 1.18 1.24 1.17 1.22 1.14 1.44 1.47 1.12 1.92 1.83 1.29 1.01 0.88 2.41 1.05 1.31 1.11 1.17 0.58 1.05 0.96 1.00 1.04 5.19 13.62 1.01 1.09 1.16 1.53

L. Zhu et al. / Marine Pollution Bulletin 60 (2010) 2144–2153

(Table 1). Grain size ranged from 1.97 to 7.58 U (U = log2D, where D is the diameter of a particle in mm). As shown in Table 1, the ratio of 206Pb/204Pb varied from 18.039 to 19.211 with an average of 18.715 ± 0.324. The ratio of 207 Pb/204Pb ranged from 15.228 to 16.080 with an average of 15.675 ± 0.188. The ratio of 208Pb/204Pb changed from 37.786 to 39.951, with an average of 38.567 ± 0.914. The averaged 206 Pb/207Pb ratio was 1.194 ± 0.014, ranging from 1.176 to 1.235, and the average 208Pb/207Pb ratio was 2.486 ± 0.015, ranging from 2.468 to 2.521. The average Pb concentration (23.6 ± 8.9 mg kg1) in the SCS surface sediments is comparable to that of the lower reach of Yangtze River sediments (average of 35.0 ± 8.5 mg kg1, Choi et al., 2007) (Table 2), the upper continent crust (20 mg kg1, Taylor and Mclennan, 1995), and the Yangtze River intertidal zone sediments (27.3 ± 5.6 mg kg1, Zhang et al., 2008). However, the Pb concentrations in the SCS are significantly lower than those determined for sediments in the Pearl River Estuary, Shenzhen Bay, Jiaozhou Bay, Western Xiamen Bay (all on the China coast) and other large industrialized/urban ports in the world such as New York/New Jersey Harbor (USA) (Table 2). Based on the marine sediment quality standard (e.g., GB 186682002) that was issued by the China State Bureau of Quality and Technical Supervision (CSBTS), the GB 18668-2002 has three standard criteria for marine sediments: the primary sediment standard criterion, which is the strictest, the secondary standard criterion that is applied to regulating general industrial use and coastal tourism, and the third standard criterion which is for defining harbors and special use for ocean exploration (e.g., Zhang et al., 2009). Compared to the GB 18668-2002, the overall average Pb concentration in sediments from the SCS well meets the primary standard criterion (Table 2). Among the 52 stations sampled, we found no cases of Pb concentrations exceeding the primary standard criteria (Tables 1 and 2). According to the guidelines suggested by Long et al. (1995), Pb concentrations of 46.7 mg kg1, which are considered toxic to biota, are referred to as effects range-low (ERL). We use the sediment quality guidelines (Long et al., 1995) to determine Pb values at which adverse biological effects may occur in the SCS (Tables 1 and 2). After examining the Pb concentrations, we found that there was only one station in this study (i.e., S071) where Pb concentra-

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tions were found above ERL in the SCS. Therefore, integrated with comparable results mentioned above and the guidelines suggested by Long et al. (1995), we argue that most of the sediments in the SCS do not seem to be significantly contaminated by anthropogenic Pb based on samples collected in 1998. The abundance and distribution of Pb are depicted in Fig. 1. In general, higher Pb concentrations were measured mostly at nearshore stations, with lower concentrations occurring at the offshore stations. However, elevated Pb concentrations were observed not only in the shallow continental shelf areas of main river mouths, but also in central area in the South China Sea (Fig. 1). This is likely due to the hydrological control and granularity effect of surface sediments. Granulometry could be an important factor governing the concentration of trace metals in estuarine and marine environments. Due to its high specific surface areas, fine-sized particles tend to contain a relatively higher level of metal concentration (McCave, 1984; Horowitz and Elrick, 1987; Zhao and Yan, 1994; Lin et al., 2002; Zhang et al., 2007, 2009). As shown in Fig. 2, the Pb and Sc concentrations linearly increased with increasing U values (the median grain size) (R2 = 0.122, n = 52, p = 0.0112 for Pb and R2 = 0.218, n = 52, p < 0.01 for Sc) in the sediments from the SCS, suggesting that the difference in Pb and Sc concentrations could mainly be caused by particle grain size differences which cause specific surface area differences on the particles and the amount of Pb scavenged by the particles. Previous studies have shown that Sc can be used as a reference element for tracing terrestrial sources (Shumilin et al., 2002; Shevchenko et al., 2003; Lee et al., 2009). This element, similar to Al and Li, is a reliable indicator of the contribution of terrestrial, crust-derived materials. Hence, the evaluation of the correlation between Sc concentration and other heavy metal concentrations using regression analysis methods can be used to distinguish whether the metals originate from anthropogenic sources or from natural weathering sources (Soto-Jiménez and Péez-Osuna, 2001; Zhang et al., 2009). Here, we use the Sc and Pb data to distinguish anthropogenic Pb sources

Table 2 A comparison of Pb concentration in surface sediments between the SCS and other regions. Region

Pb (mg kg1)

Reference

South China Sea (Mean ± S.D.) Yangtze River intertidal zone sediments (Mean ± S.D.) Pearl River Estuary (China) Shenzhen Bay (China) Jiaozhou Bay (China) Quanzhou Bay (China) Western Xiamen Bay (China) New York Harbor (USA) Bremen Harbor (Germany)

23.6 ± 8.9 27.3 ± 5.6

This study Zhang et al. (2009)

59.4 46.0 30.9 34.3 ± 16.9 50.0 109–136 122

Izmir Harbor (Turkey)

97

Boston Harbor (USA)

86

Marine sediment quality Primary standard criteria Marine sediment quality Secondary standard criteria Sediment guideline for effects range-low (ERL)

60

Liu et al. (2002a) Huang et al. (2003) Xu et al. (2005) Yu et al. (2008) Zhang et al. (2007) USEPA et al. (1999) Hamer and Karius (2002) Filibeli and Yilmaz (1995) Bothner et al. (1998) CSBTS (2002)

130

CSBTS (2002)

46.7

Long et al. (1995)

Fig. 2. Correlation of Pb and Sc concentrations with median particle diameter. The Pb and Sc concentration shows a linear increase with increasing Phi (U) value of particles, suggesting that higher Pb and Sc concentrations are associated with smaller particles (U = log2D, where D is diameter of particle in mm).

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Fig. 3. A significant positive correlation between Pb and Sc concentrations in the sediments from the SCS, implying a strong effect from natural weathering processes on Pb concentration in the sediment.

from natural weathering sources by adopting regression analysis methods (Soto-Jiménez and Péez-Osuna, 2001; Zhang et al., 2009). Based on the correlation between Sc and Pb concentrations, we found the Pb concentrations increased linearly with Sc (Fig. 3). Most of the points fell within or below the upper limit 95% prediction band, but nine points lay above the 95% confidence line (Fig. 3), indicating a few sediment samples were potentially influenced by anthropogenic Pb sources. The regression analysis showed most samples in the SCS have remarkable correlation between Pb and Sc concentrations (R2 = 0.5137, n = 52, p < 0.0001) (Fig. 3), suggesting that these most ‘‘trimmed clean’’ Pb data could represent ‘‘natural’’ sediment background values varying with grain size at varying degrees. The results also indicated that localized individual Pb contaminant sources could have significant influence on reducing the natural Pb correlation with Sc (10 out of 52 stations, Fig. 3). Hence, it should be mentioned here that although the majority of SCS sediments demonstrate Pb concentrations within the natural level, the localized individual Pb contaminant sources only existed in few sediments of the SCS before 1998. As mentioned above, there exists a significant positive correlation between Pb and Sc concentrations, implying a natural terrestrial source of Pb in the sediments mostly derived from continental runoff to the SCS. The variability in Pb concentrations was controlled by particle size effects and weathering processes. In order to better understand the current environmental status and the extent of Pb contamination with respect to the natural environment, other approaches have to be applied in addition to using the

Fig. 4. The distribution of Pb enrichment factor values of the surface sediments in the SCS. Red dots indicate Pb contamination stations based on Zhang and Liu (2002) and Han et al. (2006). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

L. Zhu et al. / Marine Pollution Bulletin 60 (2010) 2144–2153

sediment quality criteria. Enrichment factor (EF) has been proven a useful tool in determining the degree of anthropogenic heavy metal pollution (e.g., Windom et al., 1989; Shumilin et al., 2002; Feng et al., 2004; Zhang et al., 2007, 2009). To identify possible anomalous metal concentrations, geochemical normalization of the heavy metal concentration to a conservative element was employed. It has been suggested that Sc can be used as a reference element for tracing terrestrial sources (Shumilin et al., 2002; Shevchenko et al., 2003; Lee et al., 2009), which is a reliable indicator of the contribution of terrestrial, crust-derived materials. Hence, the EF is defined here using Sc as a reference element as:

EF ¼ ðX=ScÞsample =ðX=ScÞcrust ;

ð1Þ

where (X/Sc)sample is the metal to Sc ratio in a sample, and (X/Sc)crust is the average ratio in the continental crust. Background values of Sc and Pb from the SCS were adopted from Taylor and McLennan (1995), who used a Sc concentration of 11 mg kg1 and a Pb concentration of 20 mg kg1 for the upper continental crust. Interestingly, this Pb background value is close to that measured for the SCS sediment (23.6 ± 8.9 mg kg1) (Table 2). When the EF of metals reflects the status of environmental contamination, the assessment criteria are generally based on the EF values (Windom et al., 1989; Klerks and Levinton, 1989; Zhang and Liu, 2002; Han et al., 2006). Zhang and Liu (2002) recommend the use of EF = 1.5 as an assessment criterion, i.e., EF values between 0.5 and 1.5 (0.5 6 EF 6 1.5) suggest that the trace metals may be entirely from crustal materials or natural weathering processes, while EF values greater than 1.5 (EF > 1.5) suggest that a significant portion of trace metal is delivered from non-crustal materials. Han et al. (2006) divided the contamination into different categories based on EF values, i.e., an EF 6 2 suggests deficiency to minimal enrichment, an EF of 2–5

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indicates moderate enrichment, an EF of 5–20 signals significant enrichment, an EF of 20–40 constitutes very high enrichment, and an EF > 40 is an indication of extremely high enrichment. As shown in Table 1 and Fig. 4, the sediment samples from the SCS had a Pb enrichment factor ranging from 0.58 to 13.62, with an average EF value of 1.60 (Table 1). Except for ten samples, which were collected near Hainan island (S009), the Pearl River plume (S001 and S003), southwestern Taiwan Island (S094 and S097), Mekong River Estuary (S037 and S039), and near the Philippines (S060, S062, and S071), the SCS sediments had relatively low EF values for Pb (generally less than 1.5) (Table 1 and Fig. 4), indicating no significant Pb contamination in most of the SCS sediments, although low to high grade Pb contamination may exist in some locations based on rules of Zhang and Liu (2002) and Han et al. (2006). Considering the fact that the total Pb content was relatively low in most sediments and varied with sediment grain size and Sc content (Figs. 2 and 3), we therefore suggest that the Pb in most surface sediments is mainly derived from natural origins, and that most of the surface sediments of the SCS did not contain significant anthropogenic Pb. However, there are localized areas of Pb contamination in the SCS, which can be attributed to local point sources resulting from rapid urbanization and economic development in adjacent areas. The Pb isotope ratios of the SCS sediments reported here varied over a broad range, but the distribution fell along a regression line (R2 = 0.9109, n = 52, p < 0.001) near the Pb growth curve presented by Cumming and Richards (1975) (Fig. 5). Also as shown in Fig. 5, the Pb isotope data of Yangtze River sediments, as well as single Kfeldspars from the Mekong and Red rivers all partitioned along the line of the SCS sediments, but slightly shifted upward from the Pb growth curve (208Pb/204Pb vs. 206Pb/204Pb relation). This tendency is likely due to the influence of the Th-rich continental crust in

Fig. 5. Relationship between Pb isotope ratios (208Pb/204Pb and 206Pb/204Pb) in the SCS. The long solid line is the best-fitting Pb growth curve from Cumming and Richards (1975). The broken line is the best-fitting Pb isotopes line of SCS sediments (R2 = 0.9109, n = 52, p < 0.0001). Data of the Yangtze River are from Zhang et al. (2008) and Millot et al. (2004). Data of the Mekong and Red rivers are after Bodet and Scharer (2001).

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China and southeast Asia (Bodet and Scharer, 2001; Mukai et al., 2001; Millot et al., 2004; Zhang et al., 2008), suggesting that SCS sediments are mainly of catchment origin, and the lithogenic material from rock weathering and river inputs. To further assess whether the Pb is carried with lithogenic particles from rock weathering or from local anthropogenic sources, we examined the relationship between the Pb enrichment factor values and Pb isotopes (Fig. 6). As shown in Fig. 6, although 206 Pb/207Pb and 208Pb/206Pb ratios do not change significantly with the Pb enrichment factor values, the samples with a EF value greater than 1.5 (EF > 1.5 suggests anthropogenic Pb sources) generally have lower 206Pb/207Pb and 208Pb/206Pb ratios (206Pb/207Pb < 1.19

Fig. 6. Relationship between Pb enrichment factor (EF) values and 208 Pb/206Pb isotope ratios.

206

Pb/207Pb and

and 208Pb/206Pb < 2.48). Contrarily, the samples with an EF value lower than 1.5 (EF < 1.5 suggests natural Pb sources from rock weathering) often have higher Pb isotopic ratios (206Pb/207Pb > 1.19 and 208Pb/206Pb > 2.48). This coincides with the conclusion of the recent study by Xu et al. (2009) that the surface sediments affected by anthropogenic activities show the characteristics of higher heavy metal contents and lower 206Pb/207Pb ratio. To identify the sources of Pb deposited in SCS sediments, the measured isotope ratios were further compared to those of source-specific materials. Table 3 shows the Pb isotopic compositions of some possible natural and anthropogenic sources to the SCS. For natural sources, the terrigenous materials from the neighboring continents should be considered. We also compared Pb isotope ratios in the SCS sediments with those reported for other areas including upstream and downstream along major river watersheds (Zhu, 1995; Bodet and Scharer, 2001; Millot et al., 2004; Zhang et al., 2008). The Pb isotope ratios for 208Pb/206Pb and 207Pb/206Pb of possible natural sources are listed in Table 3, including granites in eastern Cathaysia and the Pearl River Delta (PRD) of China, volcanic rocks in Foshan, uncontaminated soils in the PRD, sediments from the Mekong and Yangtze rivers, single K-feldspars from sediments of the Mekong and Red rivers, and country park soils in Hong Kong. As shown in Table 3, the SCS sediments have similar average Pb isotope ratios to those natural source materials, both with ratios of 206Pb/207Pb > 1.17 and 208 Pb/207Pb > 2.46, suggesting that Pb in most of the sediments in the SCS originated mainly from natural weathering processes of rocks, consistent with the conclusion derived from the Pb enrichment factor observed for the same samples. With respect to anthropogenic sources, generally, atmospheric particulate Pb has been regarded as the main source of Pb in the marine environment (Chester, 2000). Although Pb isotope ratios for aerosols over the SCS were not measured in this study, data are available for aerosols from major cities and nearby countries surrounding the SCS, including Hong Kong (Lee et al., 2007),

Table 3 A comparison of Pb isotope ratios from different sources and media to the SCS. Samples

206

Pb/207Pb

208

Pb/207Pb

Surface sediments of SCS (n = 52)

1.1940

2.4859

This study

Natural sources Granite in eastern Cathaysia (n = 102) Granite in the PRD (n = 6) Volcanic rocks in Foshan (n = 8) Uncontaminated soils in the PRD (n = 2) Sediment samples from Mekong river (n = 1) Sediment samples from Yangtze river (n = 57) Single K-feldspars from sediments of Mekong river (n = 63) Single K-feldspars from sediments of Red river (n = 50) Country park soils in Hong Kong (n = 11)

1.1834 1.1842 1.1993 1.1952 1.1958 1.1853 1.1910 1.1780 1.1996

2.4680 2.4824 2.4965 2.4815 2.4888 2.4810 2.4720 2.4670 2.4953

Zhu (1995) Zhu et al. (2001) Zhu et al. (2001) Zhu et al. (2001) Millot et al. (2004) Millot et al. (2004) Bodet and Scharer (2001) Bodet and Scharer (2001) Lee et al. (2007)

Anthropogenic sources Fankou Pb–Zn deposit (n = 26) Automobile exhaust in the PRD (n = 3) Aerosols in Xiamen (n = 40) Vehicle exhaust in Shanghai (leaded) (n = 5) Vehicle exhaust in Shanghai (unleaded) (n = 5) Coal in Shanghai (n = 23) Coal combustion dust in Shanghai (n = 3) Coal fly ash in Shanghai (n = 3) Aerosols in Taipei (n = 77) Aerosols in Hong Kong (n = 51) Aerosols in Shanghai Aerosols in Guangzhou (n = 45) Aerosols in Ho Chi Minh, Vietnam Aerosols in Bangkok, Thailand Aerosols in Kuala Lumpur, Malaysia Aerosols in Jakarta, Indonesia

1.1716 1.1604 1.1660 1.1099 1.1468 1.1628 1.1669 1.1655 1.1450 1.1609 1.1617 1.1684 1.155 1.127 1.141 1.131

2.4725 2.4228 2.4590 2.4349 2.4358 2.4548

Zhu et al. (2001) Zhu et al. (2001) Our unpublished data Zheng et al. (2004) and Chen et al. (2005) Chen et al. (2005) Chen et al. (2005) Chen et al. (2005) Chen et al. (2005) Hsu et al. (2006) Lee et al. (2007) Chen et al. (2005) Lee et al. (2007) Bollhöfer and Rosman (2001) Bollhöfer and Rosman (2001) Bollhöfer and Rosman (2001) Bollhöfer and Rosman (2001)

2.4050 2.4499 2.4454 2.4571 2.430 2.404 2.410 2.395

References

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Taipei (Hsu et al., 2006), Xiamen (our unpublished data), Guangzhou (Lee et al., 2007), Ho Chi Minh City (Bollhöfer and Rosman, 2001), Bangkok, (Bollhöfer and Rosman, 2001), Kuala Lumpur, Malaysia (Bollhöfer and Rosman, 2001) and Jakarta, Indonesia (Bollhöfer and Rosman, 2001) (see also Table 3). Since anthropogenic Pb is also emitted to the atmosphere through burning of leaded gasoline and coal and other industrial activities (Nriagu and Pacyna, 1988; Mukai et al., 1993; Zheng et al., 2004; Chen et al., 2005), Table 3 also lists the isotope ratios of anthropogenic Pb of other possible source-related materials, including local ores from South China (Zhu et al., 2001), automobile exhaust in the PRD (Zhu et al., 2001), vehicle exhaust in Shanghai (Zheng et al., 2004; Chen et al., 2005), coal combustion dust and fly ash in Shanghai (Zheng et al., 2004; Chen et al., 2005), and coals used in China (Mukai et al., 1993, 2001; Zheng et al., 2004; Chen et al., 2005). Comparing the 206Pb/207Pb and 208Pb/206Pb isotope ratios in the SCS sediments with those commonly found in various other sources listed in Table 3, it is obvious that 206Pb/207Pb and 208 Pb/206Pb isotope ratios in the SCS sediments are significantly

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different from those of vehicle emissions, coal combustion, and aerosols from other sources (Table 3, Fig. 7). Although anthropogenic Pb isotope ratios had very wide ranges and scattered patterns in ratio–ratio plots, their isotope ratios such as 206Pb/207Pb and 208Pb/207Pb were often lower than those of natural sources (Table 3 and Fig. 7), usually with a ratio of <1.17 for 206Pb/207Pb and <2.46 for 208Pb/207Pb (Table 3, Fig. 7). Overall, Pb isotope ratios of most SCS sediments resembled those in natural sources and were significantly different from those found in anthropogenic materials (Fig. 7). Based on Pb concentration and isotope ratios, our data indicate that the contributions of Pb from local anthropogenic sources such as aerosols, vehicle emissions, and urban industrial sources (coal burning, ore-refining) are minimal in most areas of the SCS. However, it is likely that the discharge of terrestrial materials and sediments from the Yangtze, Mekong, and Red rivers is so large (ca. 768  106 tons of sediments annually, Liu et al., 2003) that this may considerably dilute and thus mask the local anthropogenic Pb input signals. Further studies are needed to compare Pb deposition in the SCS during recent years

Fig. 7. Comparison of the Pb isotopic ratios of surface sediments in the South China Sea and other environmental samples (data sources and references are the same as those listed in Table 3).

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and to reconstruct the relative contributions between natural and anthropogenic Pb inputs over the last few hundred years of sedimentation history in the SCS. From our investigation of SCS sediments, Pb concentrations and isotopic ratios provide useful information about the contamination status and sources of Pb in the study area. The average Pb concentration in the SCS sediments was 23.6 ± 8.9 mg kg1 and Pb isotopic compositions (mean ± S.D.) were 18.715 ± 0.324, 15.228 ± 0.021, 38.967 ± 0.514, 1.194 ± 0.014, and 2.486 ± 0.015 for the 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 206Pb/207Pb, and 208 Pb/207Pb ratios, respectively. The Pb isotope ratios show that Pb in the SCS is mainly derived from surrounding watersheds as a consequence of sediment transport from river basins, lateral and along shore transport, and sediment accumulation. Local anthropogenic Pb influences from anthropogenic sources such as vehicle emission, ore-refining, and coal combustion seemed to be insignificant contributors of Pb to most of the SCS sediments before 1998. The relatively low values of the Pb enrichment factor (average EF = 1.60 ± 1.82) indicate that the degree of Pb contamination in most of the SCS sediments was not significant. The trend and extent of Pb accumulation over the last few hundred and recent years of sedimentation in the SCS are needed to be studied. Acknowledgements We gratefully thank the crew members and science party of the R/V Xiang Yang Hong 4 for their assistance during sample collection, Dr. Qi Liang for technical assistance during sample analysis in the laboratory, Diana Flosenzier and Victor Johanson for critical reading, and Dr. Bruce Richardson and anonymous reviewers for constructive comments on the manuscript. This research is jointly supported by the National Natural Science Foundation of China (Grants Nos. 40872071, 40060005), the National Basic Research Program of China (Grant No. 2006CB403502), MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University (Grant No. BJ091349), and the Program of State Key Laboratory of Environmental Geochemistry (Grant No. SKLEG5001). References Bindler, R., Renderg, I., Anderson, N.J., Appleby, P.G., Emteryd, O., Boyle, J., 2001. Pb isotope ratios of lake sediments in West Greenland: inferences on pollution sources. Atmospheric Environment 35, 4675–4685. Birkeland, A., 1990. Pb-isotope Analysis of Sulfides and K Feldspars; A Short Introduction to Analytical Techniques and Evolution of Results, vol. 15. Mineralogist Museum, University of Oslo, Internal Skriftserie, pp. 1–33. Bodet, F., Scharer, U., 2001. Pb isotope systematics and time-integrated Th/U of SEAsian continental crust recorded by single K-feldspar grains in large rivers. Chemical Geology 177, 265–285. Bollhöfer, A., Rosman, K.J.R., 2001. Isotopic source signatures for atmospheric lead: the northern hemisphere. Geochimca et Cosmochimca Acta 65, 1727–1740. Bothner, M.H., Buchholtz ten Brink, M., Manheim, F.T., 1998. Metal concentrations in surface sediments of Boston Harbor-changes with time. Marine Environmental Research 45, 127–155. Chen, J.M., Tan, M.G., Li, Y.L., Zhang, Y.M., Lu, W.W., Tong, Y.P., Zhang, G.L., Li, Y., 2005. A lead isotope record of Shanghai atmospheric lead emissions in total suspended particles during the period of phasing out of leaded gasoline. Atmospheric Environment 39, 1245–1253. Chester, R., 2000. Marine Geochemistry, second ed. Blackwell Science Ltd. Choi, M.S., Shou, H.Y., Yang, S.Y., Lee, C.B., Cha, H.J., 2007. Identification of Pb sources in Yellow Sea sediments using stable Pb isotope ratios. Marine Chemistry 107, 255–274. Clift, P., Lee, J.I., Clark, M.K., Blusztajn, J., 2002. Erosional response of South China to arc rifting and monsoonal strengthening: a record from the South China Sea. Marine Geology 184, 207–226. CSBTS (China State Bureau of Quality and Technical Supervision), 2002. Marine Sediment Quality. Standards Press of China, Beijing, China, p. 10 (in Chinese). Cumming, G.L., Richards, J.R., 1975. Ore lead isotope ratio in a continuously changing earth. Earth and Planetary Science Letters 28, 155–171. Feng, H., Han, X., Zhang, W., Yu, L., 2004. A preliminary study of heavy metal contamination in Yangtze River intertidal zone due to urbanization. Marine Pollution Bulletin 49, 910–915. Filibeli, A., Yilmaz, R., 1995. Dredged material of Izmir Harbor: its behavior and pollution potential. Water Science Technology 32, 105–113.

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