Sediment Records of Polycyclic Aromatic Hydrocarbons (PAHs) in the

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Sediment Records of Polycyclic Aromatic Hydrocarbons (PAHs) in the Continental Shelf of China: Implications for Evolving Anthropogenic Impacts Liang-Ying Liu,†,§ Ji-Zhong Wang,† Gao-Ling Wei,†,§ Yu-Feng Guan,† Charles S. Wong,‡ and Eddy Y. Zeng*,† †

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡ Department of Environmental Studies and Sciences and Department of Chemistry, Richardson College for the Environment, University of Winnipeg, Winnipeg, Manitoba, R3B 2E9 Canada § Graduate School, Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Sources, compositions, and historical records of polycyclic aromatic hydrocarbons (PAHs) in sediment cores collected from the Yellow Sea and the South China Sea were analyzed to investigate the influence of anthropogenic activities. The occurrence of PAHs was mainly derived from various combustion sources, especially the combustion of biomass and domestic coal. Uniform composition of sedimentary PAHs (52−62% of phenanthrene, benzo[b]fluoranthene, indeno[1,2,3-cd]pyrene, and benzo[g,h,i]perylene) suggested air-borne mixtures intractable to degradation. The concentrations of the sum of 15 PAHs (16 priority pollutants designed by the United States Environmental Protection Agency minus naphthalene; designed as Σ15PAH) in Yellow Sea sediment cores were generally higher than those in the South China Sea. The profiles of Σ15PAH concentrations recorded in the sediment cores closely followed historical socioeconomic development in China. In general, Σ15PAH concentrations started to increase from the background pollution level posed by agricultural economy at the turn of 20th century. In addition, a Σ15PAH concentration reduction was observed during the Chinese Civil War (1946−1949) and Great Cultural Revolution (1966−1976), suggesting them as setbacks for economic development in Chinese history. Increasing PAH emissions as a result of increasing coal combustion associated with the rapid urbanization and industrialization since the implementation of the Reform and Open Policy (since 1978) accounted for the fast growth of Σ15PAH concentrations in sediment cores. The decline of Σ15PAH concentrations from subsurface maximum until sampling time was inconsistent with current-day economic development in China, and may possibly suggest emission reductions due to decreasing proportional use of domestic coal and increasing consumption of cleaner energies (natural gas and liquefied petroleum gas).

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Reform and Open Policy in 1978.5 Energy consumption continued to rise (Supporting Information (SI) Figure S1a),5 which consequently caused increased PAH emissions, e.g., from ∼18 000 tons in 1980 to ∼116 000 tons in 2003.6 Combustionderived PAHs can widely distribute into various environmental media through two major sources, riverine runoff and atmospheric transport.7,8 In both cases, coastal marine sediments are an important reservoir for terrestrially derived PAHs.9 The eastern coast of China has several marine systems, which from north to south are the Bohai Sea, Yellow Sea, East China Sea (ECS), and South China Sea (SCS) (Figure 1). These

INTRODUCTION Due to their toxic, carcinogenic, and mutagenic characteristics, sixteen polycyclic aromatic hydrocarbons (PAHs) have been identified as priority pollutants by the United States Environmental Protection Agency.1 These compounds are mainly formed from incomplete combustion of fossil fuels and biomass.2 Therefore, PAHs are not only environmental pollutants, but also useful geochemical markers of anthropogenic impacts, since they are typically well correlated with anthropogenic activities. For example, previous studies demonstrated that PAH concentrations in sediments from eleven Michigan inland lakes were positively correlated with watershed population density,3 and that temporal trends of PAH concentrations and depositional fluxes correlated well with historical energy consumption.4 China has experienced rapid economic development in the past several decades, especially after the implementation of the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6497

February 7, 2012 May 4, 2012 May 29, 2012 May 29, 2012 dx.doi.org/10.1021/es300474z | Environ. Sci. Technol. 2012, 46, 6497−6504

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Bohai Sea and Yellow Sea,24 no work has been done to date on a large spatial scale to examine the similarities and differences in spatial and temporal variations between two or among more than two marine regions off China. Therefore, evaluating the spatial and temporal differences of sedimentary PAHs between the Yellow Sea and the SCS would be helpful in understanding the potential differences of terrestrial anthropogenic activities between northern and southern China. The present study, to our knowledge, is the first to simultaneously assess the input history of combustion-derived PAHs in the Yellow Sea and the SCS. The aims of this study were to determine the sources and spatial and temporal variations of PAHs in the coastal marine systems off China, to reveal the historical changes of anthropogenic activities, and to identify the factors relevant for the historical variations of PAH emissions.

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MATERIALS AND METHODS Sample Collection. Two sediment cores in the Yellow Sea and two in the SCS basin (Figure 1 and SI Table S1) were collected using a stainless steel box-corer in 2007 deployed from R/V KEXUE I and R/V SHIYAN III, respectively. Cores were preserved with ice after collection, transported to the laboratory, and frozen at −20 °C until analysis. Sample Preparation. The plastic tubes were split and sediment cores were placed on a precleaned table and manually sliced with a stainless steel saw along the core length. To reduce error associated with manual slicing, the core length was measured after each section was sliced. Core slices were immediately packed in aluminum foil prebaked at 450 °C, then freeze-dried. Detailed procedures for the extraction and fractionation of PAHs were described in a previous study.12 Briefly, freeze-dried and homogenized sediment samples were weighed and wrapped with clean filter paper. After addition of surrogate standards (naphthalene-d8, acenaphthene-d10, phenanthrened10, chrysene-d12, and perylene-d12) and copper sheets (for removal of elemental sulfur), samples were Soxhlet-extracted with 200 mL of hexane/acetone (1:1 in volume) for 48 h. Extracts were concentrated, solvent exchanged to hexane, and further concentrated under a gentle flow of nitrogen with a Zymark Turbo-Vap 500 (Hopkinton, MA) to ca. 1 mL before column fractionation with 1:2 (by volume) alumina/silica gel (3% distilled water deactivated). The first fraction, eluted with 25 mL of hexane, was collected for other purposes, and is not further discussed here. The second fraction, containing PAHs, was eluted with 80 mL of hexane/dichloromethane (7:3 in volume), further concentrated, solvent-exchanged to hexane, and reconcentrated to 0.5 mL volume. Instrumental analysis was conducted after internal standards (2-fluoro-1,1-biphenyl, p-terphenyl-d14, and dibenzo[a,h]anthracene-d14) were added. Instrumental Analysis. Quantification of perylene and the 16 priority PAHs (naphthalene (Na), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Flo), phenanthrene (Phe), anthracene (An), fluoranthene (Fl), pyrene (Py), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (IcdP), dibenzo[a,h]anthracene (DahA), and benzo[g,h,i]perylene (BghiP) was performed with a Shimadzu GCMS-QP2010 plus equipped with an AOC-20i auto injector (Kyoto, Japan). The gas chromatographic separation of individual PAH components was conducted with a 60 m × 0.25 mm-i.d. (0.25-μm film

Figure 1. Map showing the sampling sites of sediment cores in coastal marine systems off China: stars (present study), diamonds (Wu Y. et al.22), triangles (Zhang et al.13), cross (Guo et al.23). The fingerprint suggests deposition zone, and the red arrows depict the ocean currents in Yellow Sea.23,52 YSCC and YSWC refer to Yellow Sea Coastal Current and Yellow Sea Warm Current, respectively. In the SCS, the South China Sea Warm Current was season dependent, i.e., cyclonic and anticyclonic in winter and summer, respectively,53 but not depicted.

continental shelves are much closer to mainland China than any other nations. Due to the East Asian monsoon, a general eastward transport of atmospheric PAHs was suggested.10,11 As a result, sediments in the Yellow Sea presumably represent contamination mainly from northern/central China, and in the SCS may record inputs largely from southern China. Atmospheric input mainly accounted for sedimentary PAHs in the Yellow Sea and the SCS.12 Differences in emissions resulting from regionally different energy usage structures and environmental conditions between northern and southern China might potentially have caused differences in the levels, accumulation rates, and compositions of sedimentary PAHs between the Yellow Sea and the SCS. Reconstruction of the pollution history of combustionderived PAHs using sediment cores is helpful in revealing the socioeconomic development and historical energy consumptions of nearby land areas.3,4,13−15 Increasing trends of PAH concentrations and depositional fluxes, due to the widespread application of fossil fuels since the Industrial Revolution, and decreasing trends, due to the substitution of coal by petroleum and/or natural gas,16,17 were commonly observed. However, these studies mainly focused on inland lakes,3,4 rivers,15,18 and near-shore estuarine areas,19,20 while few studies have been carried out in the continental shelves.21 Previous studies conducted in coastal marine systems off China focused on a specific open sea area, e.g. the Yellow Sea,22 southern Yellow Sea,13 and central continental shelf mud area of the ECS.23 In addition, subtle changes in contamination levels from specific regional- or national-scale events, such as economic and political changes affecting combustion activity, are often difficult to detect in sediment cores, given limited temporal resolution in most cores.13,22 Except for a study which investigated the sources of PAHs in surface sediments of the 6498

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thickness) DB-5 column, as detailed elsewhere.12 Mass spectrometry was operated in the positive electron impact mode and full scan mode was employed for peak confirmation. Quality Assurance and Quality Control (QA/QC). For every 15 field samples, a procedural blank, spiked blank, and matrix spiked sample were processed. Surrogate standard recoveries (mean ± standard deviation) for QA/QC samples were 56 ± 11, 69 ± 9, 89 ± 13, 117 ± 15, and 115 ± 22% for naphthalene-d8, acenaphthene-d10, phenanthrene-d10, chrysened12, and perylene-d12, respectively. Recoveries of the same surrogate standards for field samples were 47 ± 12, 69 ± 12, 87 ± 16, 108 ± 25, and 113 ± 25%, respectively. The lowest calibration concentration (50 ng mL−1 for IcdP, DahA, and BghiP and 10 ng mL−1 for other compounds) divided by the average sample weight (22 g) was defined as the reporting limit (RL) of a target analyte with a procedural blank concentration less than the lowest calibration concentration (1.14 ng g−1 for IcdP, DahA, and BghiP and 0.23 ng g−1 for other compounds with exception of Flo and Phe). Otherwise, the average procedural blank level divided by average sample weight was defined as the RL (1.0 and 2.3 ng g−1 for Flo and Phe, respectively). Sedimentation Rate. Briefly, sedimentation rate was calculated by excess 210Pb activity, which was the subtraction of 210Pb and 226Ra. The activity of 210Pb was indirectly obtained by analyzing the α-radioactivity of its decay product 210Po assuming that they are in equilibrium. Po was used as yield monitor and tracer in quantification, with extraction and purification procedures detailed elsewhere.25 A multichannel αspectrometer interfaced with gold−silicon surface barrier detectors was used for the measurement of Po. The sedimentation rates were calculated using a constant initial 210 Pb concentration model and the results are given in SI Table S1 and Figure S2. The sedimentation rate in core YS2 (0.32 cm yr−1) was similar to those reported in previous studies.22,26 A sediment mixing depth of 5 cm in core S1 was suggested by a steep slope of 210Pb near the surface (SI Figure S2). Data Analysis. All concentrations were normalized to dry sample weight, and blank corrected but not surrogate standard recovery corrected. Naphthalene was found to be unusually high in several samples; therefore it was excluded from the target analyte list. The sum of the remaining 15 priority compounds was defined as Σ15PAH. For any PAH compound whose concentration was lower than its RL, zero and 1/2 RL were used for the calculation of concentration and compositional indices, respectively.7 No differences were found in concentration sums and compositional indices when other values (0−RL) were used to substitute for nondetect values (Acy, Ace, and DahA in several layers), suggesting that nondetect values had little effect on statistical analyses in the present study.27 The PAH accumulation rate was estimated as Ciρdi(1 − Ciw), where Ci is the Σ15PAH concentration in “i” section, ρ is the sediment density (assumed to be 2.45 g cm−3),28 di is the depth of i section, and Ciw is the water content of depth i section.

focusing factor. Plotting unsupported 210Pb activity against sediment mass accumulation rate produces the 210Pb inventory. Atmospheric deposition of 210Pb in soil cores around Taihu Lake (1.11 Bq cm−2 or 66.6 dpm cm−2) was applied.30 The resulting focusing factors for the SCS (5.4 for core S1 and 5.8 for core S2) were greater than that for Yellow Sea (1.6 for core YS2) (SI Table S1). Concentrations and Compositional Profiles of PAHs. The concentration ranges of Σ15PAH (45−203, 19−133, 32− 82.8, and 30−80 ng g−1 in YS1, YS2, S1, and S2, respectively) were comparable to those of Σ 16 PAH (Σ 15 PAH plus naphthalene) observed in the southern Yellow Sea (26−77 ng g−1)13 and central continental shelf mud area of the ECS (27−132 ng g−1),23 which were lower than those in the Yellow Sea (470−3800 ng g−1) obtained in another study.22 Historical Σ15PAH concentrations (SI Figure S3) were generally higher in sediment cores from the Yellow Sea than from the SCS. When corrected for sediment focusing (SI Table S1), this difference was even clearer (SI Figure S3). This observation agreed with the generally higher surficial PAH concentrations in the Yellow Sea, compared to the SCS.12 Greater consumption of coal and a more abundant coke industry in North China compared to South China are one of the causes identified for the generally higher PAH levels in the Yellow Sea compared to SCS sediments.12 Historically, the northern Chinese provinces consumed greater amounts of coal than the southern Chinese provinces (SI Figure S1b)5 partially due to the cold winter weather in North China. The relative proportions of individual PAH compounds, calculated as their concentrations relative to the Σ15PAH concentration for each sediment core averaged over the entire depth, were used to evaluate sedimentary PAH compositions and potential spatial and temporal divergences. Five- and sixring PAH compounds dominated in all four cores, while 2- and 3-ring PAHs only constituted a minor fraction (Figure 2). Sedimentary PAH distributions were opposite those in emission sources which were dominated by 2- and 3-ring PAHs (Figure 2).31 This apparent difference was not caused by

Figure 2. Compositional profiles of PAH compounds in sediment cores (average over the entire depth) off China and from typical combustion emissions (data were summarized and reanalyzed from a previous study).31 AP, OP, TP, IC, CI, DC, FI, and ST are the acronyms of aluminum production, other petroleum, transport oil, industrial coal, coking industry, domestic coal, firewood, and straw, respectively. Full names of abbreviated PAH compounds can be found in the main text. From left to right on the x-axis, stacked bar represents individual PAH compounds whose legends are depicted on the right, from top to bottom.

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RESULTS AND DISCUSSION Sediment Focusing. Focusing factor, defined as the ratio of unsupported 210Pb inventory to atmospheric deposition of 210 Pb activity, was used to measure the degree of sediment focusing.29 A three-parameter exponential decay model described in a previous study29 was used to calculate sediment 6499

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Figure S5), with slight differences. The onset of increasing high molecular weight (HMW) PAH (BbF, IcdP, and BghiP) concentrations occurred in the middle of the 1880s (Figure 3 and SI Figure S5), earlier than those of low (Phe) and moderate (Fl, Py, BaA, and Chr) molecular weight PAHs. The relative abundances of other PAHs remained generally constant with time, except for an obvious shift for that of BaP (not shown), which was suggested to be a potential marker for PAH pollution due to the widely observed excellent correlation between BaP and total PAH concentrations.33,34 Therefore, the historical changes of concentrations and accumulation rates will be evaluated with the sum of parent PAHs (Σ15PAH) in the subsection on temporal trends. Source Assessment. Diagnostic ratios calculated from LMW PAHs, e.g., An/(An + Phe) and BaA/(BaA + Chr), are more susceptible to changes during transport from sources to receptor sites than HMW ones, because of transformation and phase transfer processes.35 Therefore, only the Fl/(Fl + Py) and IcdP/(IcdP + BghiP) ratios are discussed in the present study (SI Figure S6). Fl/(Fl + Py) < 0.4, 0.4−0.5, > 0.5 and IcdP/(IcdP + BghiP) < 0.2, 0.2−0.5, > 0.5 both indicate PAHs from petrogenic origin, liquid fossil fuel combustion, and coal/ wood combustion, respectively.36 Values of both Fl/(Fl + Py) and IcdP/(IcdP + BghiP) in the present study were generally near or greater than 0.5 (SI Figure S6). These observations suggest that sedimentary PAHs mainly originated from combustion of coal and/or biomass. This assessment is consistent with a previous conclusion that biomass burning and domestic coal combustion are the top two emission sources of Σ16PAH in China, accounting for 60% and 20%, respectively, of total Σ16PAH emissions in 2003.31 Other previous studies also identified emissions from combustion of coal and biomass as the dominant source of sedimentary PAHs in the Yellow Sea24 and the SCS.37 Further comparison of sediment PAH compositional profiles with those of coal and biomass combustion emissions38−40 (SI Figure S7) suggested no further obvious correlations which might be a result of profile changes due to degradation and phase distribution during the geochemical transport processes.8,10 This observation is consistent with those discussed previously on PAH composition, in that only those compounds recalcitrant to degradation survived long-range transport to deposit in sediments.32,41 The emission amounts of individual PAH compounds were greater from the combustion of domestic coal and/or biomass rather than industrial coal (SI Figure S8).31 In summary, sedimentary PAHs in the present study were mixtures of various combustion sources, of which domestic coal and biomass combustion were dominant. Temporal Trends. Generally, the historical trends of Σ15PAH concentrations observed in the Yellow Sea and SCS sediment cores were characterized with an initial increase from the background level to a subsurface maximum, and a subsequent decrease until sampling date (SI Figure S3), as generally observed around the world, e.g., Europe,17 the United States,4 and Asia.14,42,43 The subsurface maximum of PAH concentrations and accumulation rates in developed countries (e.g., Japan, Europe, and the United States) was observed in the 1930s−1980s,4,17,42,43 at the same time as PAH concentrations in China started to increase rapidly.44 The historical profiles of Σ15PAH concentrations in the present study closely followed the historical socioeconomic development in China (Figure 4). Generally, China’s economy remained feudal before 1911. Therefore, the low Σ15PAH

the analytical procedures used because the compositions of sedimentary PAHs with and without surrogate recoverycorrection were quite similar. Specifically, either BbF or IcdP was the most abundant compound, followed by BghiP in all cores except S1. In core S1, IcdP was the most abundant compound, followed by Phe and BbF. Generally, PAHs in coastal marine sediments off China exhibited a uniform distribution (dominated by Phe, BbF, IcdP, and BghiP), without obvious historical changes in compositional profiles, which was in accordance with the results deduced from principal component analysis (not shown). The sum of these four compounds accounted for 52−62% of Σ15PAH. Similar uniform distributions of pyrolytic PAHs were also observed in sediments of remote mountain lakes17 and Lake Michigan,4 and were attributed to airborne combustion mixtures recalcitrant to degradation during long-range transport.32 Therefore, sedimentary PAHs in the present study were most probably a mixture of a variety of combustion-related emissions among which low molecular weight (LMW) compounds vulnerable to photodegradation and chemical decomposition failed to survive long-range atmospheric transport. This is consistent with the fact that atmospheric deposition mainly accounted for sedimentary PAHs in Yellow Sea and the SCS.12 Temporal variations in PAH compositions have been regarded as an indicator of the shift in PAH sources.3,23 An obvious up-core decrease (50% to 20%) in the fractions of 2and 3-ring PAHs (sum of Na, Acy, Ace, Flo, Phe, and An) and an increase (24% to 45%) in the relative abundances of 5- and 6-ring PAHs (sum of BbF, BkF, BaP, IcdP, DahA, and BghiP) in a sediment core from the central continental shelf mud area of the ECS reflected China’s transformation from an agricultural to an industrial economy.23 However, obvious changes in the relative abundances of 2- and 3-ring, 4-ring, and 5- and 6-ring PAHs were absent in the present study (SI Figure S4). Therefore, depth profiles of the concentrations and relative abundances of individual PAH compounds were investigated. Individual PAH compounds, except for perylene, had similar depth profiles, and therefore historical trends (Figure 3 and SI

Figure 3. Historical trends of individual PAH concentrations (focusing factor corrected; ng g−1 dry weight) in sediment cores from the Yellow Sea (YS2) and the South China Sea (S1). Upward arrows indicate up ticks and downward arrows suggest bottom ticks. 6500

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Figure 4. Historical records of total PAH (Σ15PAH) concentrations (yellow dots) and accumulations (red dots) in YS2 and S1.

Figure 5. Correlation analysis between historical rural populations (RP; 104 person)5 in China and Σ15PAH concentrations in core YS2, and between historical energy consumption (3 × 107 tons)5 in China and Σ15PAH (designated as 15PAH in the plot) concentrations in core YS2. RP (107 persons) and energy consumption (3 × 107 tons) follows the left y-axis. Solid lines indicate the lines of correlations between RP (green bar) and Σ15PAH concentrations (b1), and between energy consumption (blue bar) and Σ15PAH concentrations (b2).

concentrations (29 ± 4 and 37.8 ± 3.8 ng g−1 in YS2 and S1, respectively) in deeper layers (Figure 4) indicated a low background pollution level due to an agricultural economy.45 The Westernization Movement (1860s) after the Opium Wars (1840−1842 and 1856−1860) forced the initiation of industrialization, probably accounting for the first peak in Σ15PAH concentrations in the mid 1860s (Figure 4). The subsequent slight decrease of Σ15PAH concentrations observed in both YS2 (34 to 27 ng g−1) and S1 (52 to 43 ng g−1) from

the mid 1860s to the mid 1880s (Figure 4) may have been partially ascribed to the Taiping Rebellion (1851−1871) which devastated the Qing Dynasty’s economy and claimed over 20 million lives.45 Σ15PAH concentrations gradually increased (27 to 60 ng g−1 in YS2 and 43 to 70 ng g−1 in S1) (Figure 4) from the mid 1880s to late 1940s. The Great Depression (1930s) slowed development in North America and Europe but not in China. The growth of industries related to the military under the 6501

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decline of Σ15PAH concentrations in the present study (Figure 4), because combustion of domestic coal and biomass accounts for the majority of PAH emissions in China.31 Our recent study47 on the temporal variation of PAH levels in sediment of the Dongjiang River, encircling one of the most industrialized regions in South China, also suggested that sediment PAH levels had remained steady from 2002 to 2008. In summary, the reduction of PAH emissions from the combustion of domestic coal and biomass, due to the decline of rural populations, and the increasing use of liquefied petroleum gas in rural areas may have been one of the reasons for the decrease of PAH concentrations from subsurface maximum to the present date. Implications for the Occurrence of Perylene. The historical profile of perylene was markedly different from that of any other parent PAH compound (Figure 3 and SI Figure S5), suggesting a totally different source for perylene. The relative abundances of perylene to Σ16PAH (Σ15PAH plus perylene) increased from ∼10% and ∼15% in the surface layers to ∼40% and ∼50% in bottom layers in YS1 and YS2 (Figure 3 and SI Figure S5), respectively. This down-core increasing trend and the predominance of perylene in deeper sediment layers have been widely observed in both marine and freshwater systems, such as remote mountain lakes in Europe,17 Lake Michigan,4 Lake Ontario,48 and the Namibian coastal shelf,49 where perylene was believed to be formed from early diagenesis of either terrestrial or aquatic precursors.49−51 Perylene was also suggested to be an indicator of depositional conditions rather than the source of organic precursors if the hypothesis that perylene’s formation from nonspecific precursors was accurate.48 Perylene concentrations in S1 and S2 accounted for 17 ± 4% and 25 ± 6% of total concentrations, respectively, greater than the typical values in pyrolytic mixtures (1−5%).17 Perylene concentrations in sediment cores from the SCS (17 ± 4 and 16 ± 4 ng g−1 in S1 and S2, respectively) were generally lower than those from Yellow Sea (32 ± 14 and 28 ± 5 ng g−1 in YS1 and YS2, respectively), which may have been caused by generally higher levels of terrestrial hydrocarbons in the Yellow Sea than in the SCS (data not shown) to some extent. Another plausible reason for the lack of increases in perylene concentration with sediment depth in the SCS (Figure 3 and SI Figure S5) is the possibly different depositional conditions48 because the water depths in the SCS where the sediment cores were collected were deeper than those in the Yellow Sea.

Nanjing decade (1927−1937) was probably one of the reasons for the concentration increase.45 Σ15PAH concentrations exhibited two slight reductions in the subsequent 30 years (early 1940s to late 1970s) in both YS2 and S1 cores (Figure 4). The Chinese Civil War (1946−1949) and Great Cultural Revolution (1966−1976) were considered bad setbacks for the economic development, and are thus deemed as one of the reasons for these two declines. The apparent slight increase of Σ15PAH concentrations (81 to 105 ng g−1 for YS2, and 53 to 66 ng g−1 for S1 from 1956 to 1966) observed within this period (early 1940s to late 1970s) was probably ascribed to the reconstruction work and economic development following the establishment of the People’s Republic of China in 1949.45 After the Great Cultural Revolution, and especially after the implementation of the Reform and Open Policy in 1978, the resuscitation of China’s economy and consequent urbanization and industrialization resulted in rising energy consumption (SI Figure S1)5 and consequently increased PAH emissions,31 consistent with the rapid increase in Σ15PAH concentrations from late 1970s until the subsurface maximum (mid 1990s for YS2 and late 1980s for S1 (Figure 4). The time period (late 1980s) when the subsurface maximum of Σ15PAH concentration in S1 occurred was earlier than that in YS2 and the Pearl River Estuary,20 in which PAH concentrations peaked in the early 1990s.20 A possible reason is that the sedimentation rate in the SCS (0.2 cm yr−1) is much lower than that of the Pearl River Estuary (0.61 cm yr−1),20 so every single section (about 2 cm) of S1 represented 10 years. Thus, the peak date in the early 1990s could not be measured in S1. Another explanation is that widespread use of petroleum gas and natural gas started earlier in South China than in North China because economic development was accelerated earlier in South China than in North China. Clearly, the continuously rapid development of China’s economy (SI Figure S9) since the 1990s did not cause any further increase of Σ15PAH concentrations (Figure 4 and SI Figure S3). Instead, the sediment cores in this study exhibited a Σ15PAH concentration decline from the subsurface maximum to the sampling time (Figure 4 and SI Figure S3). The decrease from subsurface maximum until the present day in Europe and the United States was ascribed to the substitution of some coal combustion by natural gas.4,17 Despite increasing consumption amounts and percentages of petroleum (SI Figure S10a), coal is still the dominant energy source in China (SI Figure S10b).5 Therefore, the reasons for the PAH concentration decline in the present study are potentially different from those in Europe and the United States, and need further assessment. Excellent correlation (r2 = 0.98; Figure 5) exists between historical energy consumption5 and Σ15PAH concentrations in YS2 before the decrease from the subsurface maximum, confirming the linkage between anthropogenic activities and pollution histories. Historical rural population5 correlated well with the historical PAH concentrations and accumulation rates over the same time period (Figure 5), which was in accordance with the importance of PAHs emissions from the combustion of domestic coal and biomass (80% of total emissions).31 The household energy usage structure in China evolved over time due to the replacement of domestic coal with natural gas initiated since the early 1990s,31 with a decreasing proportion from coal combustion and increasing proportion from cleaner energy (natural gas and liquefied petroleum gas) (SI Figure S11).46 This is probably one of the reasons for the recent

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ASSOCIATED CONTENT

S Supporting Information *

An additional table and figures containing information about the sampling sites and selected results from data analyses not presented in the main text. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-20-85291421; fax: +86-20-85290706; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (41121063 and 40588001) and 6502

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Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS 135 project Y234081001). We thank the crews of the South China Sea Open Cruise and the Open Research Cruise Offshore China, administered by the South China Sea Institute of Oceanology and the Institute of Oceanology, Chinese Academy of Sciences, respectively, for sample collection. C.S.W. acknowledges the Canada Research Chairs Program and the Natural Sciences and Engineering Research Council of Canada for funding. This is contribution IS-1515 from GIGCAS.

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