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Environmental Processes

Spatio-temporal Trends of Elemental Carbon and Char/Soot Ratios in Five Sediment Cores from Eastern China Marginal Seas: Indicators of Anthropogenic Activities and Transport Patterns Yin Fang, Yingjun Chen, Tian Lin, Limin Hu, Chongguo Tian, Yongming Luo, Xin Yang, Jun Li, and Gan Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00033 • Publication Date (Web): 05 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Environmental Science & Technology

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Spatio-temporal Trends of Elemental Carbon and Char/Soot Ratios

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in Five Sediment Cores from Eastern China Marginal Seas:

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Indicators of Anthropogenic Activities and Transport Patterns

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Yin Fang,†,§,# Yingjun Chen,*,†,‡ Tian Lin,*,ǁ Limin Hu,⊥,¶ Chongguo Tian,§ Yongming

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Luo,§ Xin Yang, ‡,ψ Jun Li,# and Gan Zhang#

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†

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College of Environmental Science and Engineering, Tongji University, Shanghai,

Key Laboratory of Cities’ Mitigation and Adaptation to Climate Change in Shanghai,

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200092, China.

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‡

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China.

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§

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Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai,

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264003, China.

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ǁ

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Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China.

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⊥

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Institute of Oceanography, State Oceanic Administration, Qingdao, 266061, China.

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¶

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and Technology, Qingdao, 266061, China.

Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092,

Key Laboratory of Coastal Environmental Processes and Ecological Remediation,

State Key Laboratory of Environmental Geochemistry, Guiyang Institute of

Key Laboratory of Marine Sedimentology and Environmental Geology, First

Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science

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ACS Paragon Plus Environment


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#

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Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.

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ψ

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Department of Environmental Science and Engineering, Fudan University, Shanghai,

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200433, China

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention,

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*Corresponding authors:

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College of Environmental Science and Engineering, Tongji University, Shanghai,

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200092, China.

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Phone/fax: +86-21-65983869; E-mail: [email protected] (Yingjun Chen).

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State Key Laboratory of Environmental Geochemistry, Guiyang Institute of

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Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China.

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Phone/fax: +86-851-85895239; E-mail: [email protected] (Tian Lin).

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ABSTRACT: Elemental carbon (EC), the highly recalcitrant carbonaceous material

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released exclusively from fossil fuels combustion and biomass burning, is a preferred

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geochemical agent for evaluating anthropogenic activities. We investigated the

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spatio-temporal trends of EC and char/soot ratios (char and soot, the two subtypes of

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EC differing in formation mechanisms and physicochemical characters) in five

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sediment cores from eastern China marginal seas, spatially spanning from inshore

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coastal mud areas to offshore remote mud areas. The temporal profiles of EC

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depositional fluxes closely tracked socioeconomic development in China over the past

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~150 years, with the most pronounced increasing trend beginning in early 1980s,

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commensurate with the implementation of national Policy of “Reform and Open” in

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1978. The temporal EC profiles in China differed significantly from those in

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European/American countries, reflecting their different socioeconomic development

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stages. The spatio-temporal trends of char/soot ratios were also highly informative.

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Temporally, they decreased from bottom to subsurface layers, indicating the switch of

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China from an agricultural economy to an industrial economy during the twentieth

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century. Spatially, they decreased from inshore to offshore areas, suggesting the

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differential transport patterns of EC among these sampling regimes.

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1. INTRODUCTION

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Elemental carbon (EC), sometimes termed black carbon (BC) and pyrogenic

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carbon (PyC),1-3 is of great attention owing to its potential roles in global/regional

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carbon cycle,4 climate change,5 air quality,6 as well as public health.7 EC is

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characterized as highly aromatic and recalcitrant carbon species released exclusively

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from incomplete burning of organic matter, including fossil fuels (e.g., coal and

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petroleum) and biomass.8 Despite its identified potential importance, EC is not a

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chemically singular entity with well-characterized properties, but rather exists as a

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complex combustion continuum.2,9 EC is an operational term, which means that its

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precise definition depends on the applied quantification methodology.2,9 In general,

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EC can be formed by two fundamentally different processes, which correspond to two

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subtypes, i.e., char and soot.10-13 Char is the partially combusted solid residue formed

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at low temperatures (300–600 °C), retaining some chemistry and morphology of its

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parent precursors, with typical particle size distributions of 1–100 µm.10,13 By contrast,

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soot is comprised of submicron-sized particles of structurally grape-like clusters

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produced in flames from condensation of hydrocarbon radicals at higher temperatures

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(˃600 °C), and it does not maintain any morphology of its combusted parent

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precursors.10,13 Due to their distinct formation conditions (e.g., temperatures) and

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physicochemical properties (e.g., particle sizes and resultant transportability), the

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effective discrimination between char and soot in environments may provide further

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information revealing EC sources and transport pathways.10,11,14 For instance, Han et

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al.11 recently reported similar depositional fluxes of soot in northern and southern

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Lake Qinghai subbasins in Tibetan Plateau, but with distinct depositional fluxes for

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char, suggesting regional atmospheric soot deposition but local riverine char inputs.

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The purely pyrogenic origin and post-depositional resistance against degradation

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makes EC an increasingly preferred geochemical agent for reconstructing historical

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impacts of anthropogenic activities (e.g., changing energy strategies and mitigation

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measures) regionally and/or globally.10,11,15-19 Such temporal EC trends can be reliably

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retrieved from undisturbed environmental archives with chronologically constrained

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sequences, such as marine/lake sediments, polar/alpine ice cores, and peatlands.17

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China, owning 1.36 billion people in East Asia, has been regarded as world’s largest

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EC contributor, accounting for approximately one-fifth that of global emissions.8,20

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Consequently, over the latest decade there have been increasing investigations on EC

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historical records and their responses to anthropogenic activities in China. However,

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the available studies focused mainly on background areas on Tibetan Plateau (e.g.,

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Lake Nam Co and Lake Qinghai),11,21 and inland peatlands/wetlands/lakes in eastern

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China,10,16,22-27, with the former EC mainly from long-range transport and the latter

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EC primarily from local emission. Thus, for an all-sided understanding of historical

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influences of anthropogenic activities in China, EC needs to be studied at a national

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scale, especially in areas dominated by strong EC emissions.

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There are several marginal seas located to the east off mainland China.28 Driven

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by strong northerly/northwesterly winter and spring prevailing East Asian Monsoon,

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and large amounts of riverine outflow, in particular from two large rivers of Yellow

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River and Yangtze River, which drain highly industrialized Chinese mega-cities in

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their middle and lower reaches, the eastern China marginal seas are undoubtedly

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important sinks of EC from mainland China.8,29-31 Additionally, under strong influence

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of regional ocean current systems, several mud areas developed in these marginal

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seas,32,33 in which EC can be preferentially deposited and subsequently sequestrated in

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these regions.8 Therefore, the extensively distributed mud areas from eastern China

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marginal seas (Figure 1) are ideal places to extract sediment cores for reconstructing

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the nationwide historical anthropogenic activities primarily from China.

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In this study, we present the spatio-temporal trends of EC and char/soot ratios in

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five sediment cores from scattered mud areas in eastern China marginal seas, spatially

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spanning from inshore coastal mud areas (~50 km) in East China Sea to offshore

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remote central mud areas (~300 km) in Yellow Sea. The major objectives were: (1) to

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elucidate coupled relationship between the temporal trends of EC depositional fluxes

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and nationwide historical anthropogenic activities in China over the past ~150 years,

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and (2) based on the differences in formation mechanisms (e.g., temperatures) and

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physicochemical properties (e.g., particle sizes and therefore transportability) between

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char and soot, to explore the powerful indicators of the spatio-temporal trends of

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char/soot ratios. This study, as far as we know, is the first to evaluate both input

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history and transport patterns of pyrogenic EC on such a large spatial-scale eastern

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China marginal seas (covering four sea regions), which are hot topics focused by

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some global community programs.34,35 Furthermore, our results will also provide

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critical database for refining the significance of continental shelves in global EC

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cycling, with existing estimations demonstrating that >90% of global EC buries on

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continental shelves despite comprising only 10% of global ocean area.8,36

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2. MATERIALS AND METHODS

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2.1. Sediment Core Collection

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Five sediment cores, including one from Bohai Sea (BHS), one from northern

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Yellow Sea (NYS), one from southern Yellow Sea (SYS), and two from East China

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Sea (ECS1 and ECS2), were collected by gravity corer deployed from research

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vessels of Dong Fang Hong 2 and Ke Yan 59 from 2003 to 2011 (Figure 1; Table S1).

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All cores were retrieved from mud areas of each sea region. The core samples were

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cut into sections of 1- or 2-cm down the core length with a stainless steel cutter in situ

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on board the research vessels or in the laboratory. The sediment slices were packed in

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aluminum foil (combusted at 450 °C for 4 h before use). They were then stored at

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−20 °C until further analysis.

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2.2. Analyses of Sediment Grain Size and Sediment Dating

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The detailed analytical procedures for sediment grain size and sediment dating

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are described in Texts S1 and S2, respectively. Notably, only four cores, BHS, NYS,

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ECS1, and ECS2, were radionuclide 210Pb dated (Table S2). The sedimentation rate

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for core SYS was deduced from prior studies,37,38 and an approximate value of 0.20

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cm yr-1 was utilized as a reference.

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2.3. Quantification of Sedimentary EC, Char and Soot

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Several EC quantification methods have been developed,2 with determined EC

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concentrations of identified samples varying by up to a factor of 500 between the

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methods.39 This study adopted the wet-chemical treatment integrated with thermal

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optical reflectance (TOR) method to quantify EC. The TOR approach has been used

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for reconstructing EC pollution history, and it has powerful potential to effectively

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differentiate between char and soot.10,11,14 Prior to chemical treatment, the sediment

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samples were thawed, freeze-dried, and homogenized ( 63 µm) accounted for less than 5%. The vertical variation of sediment

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composition was small in all cores, with the exceptions of cores BHS and SYS,

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suggesting relatively stable dynamic depositional settings in these sampling regimes.

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Following the stable sedimentary environments, sediments are well-preserved at these

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sampling sites, and thus the collected sediment cores are well suitable for EC studies.

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For cores BHS and SYS, however, there were some small fluctuations in sediment

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composition (Figure S1).

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Cores ECS1 and ECS2 in Min-Zhe coastal mud area (MZCMA, Figure 1) had

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the highest sedimentation rates (0.98 and 1.05 cm yr-1, respectively) (Figure S2; Table

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S1). The sedimentation rate for core BHS (0.47 cm yr-1) was only half of those of

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cores ECS1 and ECS2, but it was still two times higher than those of cores NYS and

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SYS (both were 0.20 cm yr-1), which were collected in central mud areas of northern

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and southern Yellow Seas. The relatively higher sedimentation rates for cores ECS1,

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ECS2, and BHS were largely attributable to large amounts of sediment inputs from

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Yangtze River and Yellow River, respectively (Figure 1). In terms of sediment loads,

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Yangtze River and Yellow River are the fourth- and second-largest rivers in the world,

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respectively, and they each have been shown to impact significantly on sedimentary

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circumstances of MZCMA of East China Sea and central mud area of Bohai Sea.44,45

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3.2. EC Concentrations and Depositional Fluxes

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Cores BHS, NYS, SYS, ECS1, and ECS2 had EC concentration ranges of

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0.59−1.36, 0.56−1.32, 0.51−1.18, 1.03−2.37, and 0.59−1.48 mg g-1, with averages of

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0.93 ± 0.15, 0.78 ± 0.19, 0.77 ± 0.17, 1.46 ± 0.23, and 0.91 ± 0.22 mg g-1, respectively

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(Table S4). Considering that EC in sediments may be concentrated or diluted because

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of inputs of mineral and organic matter,16,36 the EC depositional fluxes were more

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favorable than EC concentrations for reflecting the real EC inputs. The ranges of EC

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depositional flux for cores BHS, NYS, SYS, ECS1, and ECS2 were 3.79–11.16,

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0.97–2.58, 0.97–2.25, 8.14–25.10, and 4.00–27.37 g m-2 yr-1, respectively, and their

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corresponding averages were 5.51 ± 1.55, 1.40 ± 0.43, 1.47 ± 0.32, 12.58 ± 3.78, and

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9.95 ± 5.65 g m-2 yr-1 (Table S4). The relative magnitude of average EC depositional

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fluxes for these cores were consistent with their average sedimentation rates, with

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higher values for cores ECS1, ECS2, and BHS, and relatively lower values for cores

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NYS and SYS, suggesting strong impact of large riverine discharges on

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anthropogenic EC from land to coastal ocean ecosystems.

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Compared with recent investigations in other regions using a similar thermal

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optical method (Table 1), our average EC depositional fluxes from eastern China

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marginal seas were of same magnitude as those observed in lakes and salt marshes

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from moderately to highly industrialized areas of eastern China (3.16–3.29 g m-2 yr-1)

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and North China Plain (1.78 g m-2 yr-1).10,16 However, they were up to nearly 50 times

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higher than values detected in background areas, such as Lake Nam Co (0.26 g m-2

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yr-1), and Lake Qinghai (0.27–0.39 g m-2 yr-1) on Tibetan Plateau.11,21 The

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significantly lower levels of EC depositional fluxes on Tibetan Plateau were mainly

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attributed to its relatively pristine environments characterized by extremely sparse

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populations and limited local industries, and long-range atmospheric transport and

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subsequent deposition (including dry and wet) dominated EC inputs.11,21

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3.3. Temporal Trends of EC Depositional Fluxes: Tracking Historical

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Anthropogenic Activities in China

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The temporal trends of EC depositional fluxes for five sediment cores from

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eastern China marginal seas are illustrated in Figure 2, and they closely tracked

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historical anthropogenic activities in China over the past ~150 years. The timeline

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could be broadly divided into the following four time periods.

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Early 1860s to Early 1900s. The EC depositional fluxes in all cores showed

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increasing trends beginning in 1860s, and reached peak values between 1890s and

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early 1900s, with peak fluxes ~1.5−2 times higher than that in 1860s (Table S4). The

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period of EC peak fluxes coincided well with the timing of “Westernization

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Movement” (1861−1894). The Westernization Movement after the two Opium Wars

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(first: 1840−1842; second: 1856−1860) pressured China to open for trade and start

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industrializing,28,46 which may have been responsible for the first EC increases and

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the subsequent peak EC depositional fluxes in these cores from these marginal seas.

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Late 1900s to Late 1970s. This long time period encompassed more diverse

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historical trends in EC depositional fluxes among the sediment cores. For core BHS,

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the EC depositional fluxes increased slightly from ~5.0 g m-2 yr-1 in late 1900s to ~7.0

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g m-2 yr-1 in early 1970s, but thereafter they abruptly decreased until reaching 4.0 g

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m-2 yr-1 in late 1970s, approaching the lowest EC depositional flux (3.8 g m-2 yr-1)

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throughout this whole century-long sedimentary record. The abrupt decline in EC

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depositional fluxes largely resulted from the diversion of the mouth of Yellow River.47

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Previous study48 using multiple geochemical indexes, such as CaO/TiO2 and chemical

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index of alteration (CIA), have shown that sediments in sampling region of core BHS

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were mainly derived from the mixture of clay/silt Yellow River sediments and

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fine/middle-fine sand Luan River sediments (Figure 1). The diversion of the terminal

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course of Yellow River in 1976 from Diaokou (DK, gray dotted line) in outer Bohai

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Bay to Qingshuigou (QSG, gray solid line) in Laizhou Bay (Figure 1) transported

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most of Yellow River sediments into the sea from central Bohai Sea to Laizhou Bay in

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southern Bohai Sea. This shift resulted in a decreased influence from Yellow River

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but conversely increased influence from Luan River on sediment compositions of core

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BHS, as clearly evidenced by elevated proportions of sand component (Figure S1).

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Compared with coarser sand component, the (sub)micrometer-sized EC particles were

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more prone to adsorption by fine clay/silt components.8 The abruptly elevated sand

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fraction following the shift of Yellow River mouth was not conducive to enrichment

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of fine EC particles, and thus EC depositional fluxes decreased in response.

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The EC depositional fluxes for core NYS from late 1900s to late 1970s were

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within a narrow range of 1.1−1.4 g m-2 yr-1 (average, 1.3 ± 0.2 g m-2 yr-1), while core

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SYS demonstrated somewhat larger fluctuations, which corresponded well with the

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changes in sediment composition (Figure S1). For cores ECS1 and ECS2, the EC

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depositional fluxes also showed small variations as those of core NYS, ranging from

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~9.0 to ~11.0 g m-2 yr-1 and ~6.0 to ~8.0 g m-2 yr-1, respectively. Of particular note,

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however, was that several anomalously high EC depositional flux values were

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concurrently observed in these two near-shore coastal mud sediment cores, i.e.,

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13.8−14.6 g m-2 yr-1 for core ECS1 in ~1956−1958, 10.5 g m-2 yr-1 for core ECS2 in

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~1958, and 11.2 g m-2 yr-1 for core ECS2 in ~1970. These time periods corresponded

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to the timing of several medium-to-large flood events in Yangtze River drainage basin

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(e.g., 1954 and 1969).44 The extremely stripping and accompanying leaching induced

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by flood transported more anthropogenic EC, previously stored in soil and water

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systems of Yangtze River drainage basin, into Yangtze River estuary. Subsequently, it

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was delivered to regions of cores ECS1 and ECS2 by southward Zhejiang-Fujian

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Coastal Current (ZFCC) (Figure 1).

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Early 1980s to Late 1990s. With the implementation of “Reform and Open”

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Policy by Chinese government in 1978, China entered rapid industrialization and

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urbanization period, and the requirement for energy from fossil fuels (coal and

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petroleum/oil) increased drastically (Figure S3). Consequently, early 1980s witnessed

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increasing trends of EC depositional fluxes in all sediment cores. It was noticeable,

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however, that such increasing trends were more pronounced in early 1990s, which

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was associated with widespread occurrence of small-scale coke production,49 and

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therefore increased EC emission from industrial sector. According to the latest

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compiled EC emission inventory,50 the annual EC emission from industrial sector in

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China was estimated to increase from 324 Gg in 1990 to 758 Gg in 1996 (Figure S4),

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for an average annual increase of 15.2%.

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After mid-1990s, the EC depositional fluxes in cores NYS and SYS both began

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to decrease. This was mainly attributed to the gradual phase-out of beehive coke

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ovens following the implementation of “Coal Law” in 1996 and the substitution of

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domestic coal stoves with natural gas/liquid petroleum gas stoves,50 which decreased

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industrial and residential EC emissions, respectively. The annual EC emissions in

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China from industrial and residential sectors decreased from 758 and 1,200 Gg in

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1996 to 521 and 911 Gg in 2000, respectively (Figure S4).50 In contrast to those of

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cores NYS and SYS collected in remote central mud areas and less affected by large

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local riverine inputs, EC depositional fluxes in cores ECS1 and ECS2 located in

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inshore coastal mud areas did not decrease in mid-1990s, but continued to increase

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significantly until they reached their peak values in late 1990s, i.e., 25.1 g m-2 yr-1 for

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ECS1 in ~1999 and 19.1−19.4 g m-2 yr-1 for ECS2 in ~1999−2000 (Table S4). The

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continuous increase of EC depositional fluxes for cores ECS1 and ECS2 during this

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period again reflected strong influence from the severe flood event that occurred in

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middle and lower reaches of Yangtze River drainage basin in 1998.44 Based on same

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sediment core ECS1, Guo et al.51 and Hao et al.52 also found a strong association of

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peaks of pyrogenic polycyclic aromatic hydrocarbons (PAHs) and heavy metal lead

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(Pb), respectively, in late 1990s with the 1998 catastrophic flood event.

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Early 2000s to Sampling Date. Due to the low time resolution and limited

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number of sediment slices from early 2000s until sampling date for cores BHS, NYS,

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SYS, and ECS1, only core ECS2 is discussed for this period.

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After the peak value for core ECS2 in late 1990s caused by the 1998 flood event,

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the EC depositional fluxes increased once more in the early 2000s, jumping from 15.0

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g m-2 yr-1 in ~2001 to 27.4 g m-2 yr-1 in ~2006, with mean annual increase of up to

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12.8%. Thereafter, the EC depositional fluxes maintained relatively constant values

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(24.2−27.4 g m-2 yr-1). During first several years of 21st century, another phase of

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industrialization and urbanization proceeded, and small- to medium-sized cities and

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towns with total populations

Soot Ratios in

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