Sources and radiocarbon ages of aerosol organic carbon along the east coast of China and implications for atmospheric fossil carbon contributions to China marginal seas

https://doi.org/10.1016/j.scitotenv.2017.11.201Get rights and content

Highlights

  • 14C and 13C of aerosol samples from the east coast of China were analyzed.

  • Fossil carbon was an important component of coastal aerosols.

  • Strong seasonal variations of fossil carbon contribution were shown at Changdao.

  • Atmospheric deposition is important for fossil carbon burial in the CMS.

Abstract

Aerosol deposition is an important mechanism for the delivery of terrestrial organic carbon (OC) to marginal seas, but OC age characteristics of aerosols are not well constrained and their contributions to sediment OC burial have not been quantified. Total suspended particle samples were collected along the east coast of China at Changdao (CD), Qingdao (QD) and Huaniao Island (HNI), and were analyzed for total organic carbon (TOC) isotopes (13C and 14C) in order to bridge this information gap. TOC δ13C and Δ14C values ranged from − 23.6 to − 30.5‰, and − 153 to − 687‰, respectively, with the latter corresponding to 14C ages ranging from 1280 to 9260 yr. Estimated contributions of fossil carbon to TOC based on 14C mass balance approach ranged from 26 to 73%, with strong seasonal variations in fossil carbon observed at CD. Fossil carbon at CD showed the highest proportion (73%) in winter, reflecting anthropogenic emissions and the lowest proportion (26%) in summer, caused by biomass contribution (annual ave., 52% ± 17%). In contrast, the fossil carbon at both QD (57–64%) and HNI (57–67%) dominated throughout the year, reflecting local anthropogenic influences and long-range transport. Mass balance estimates indicate that atmospheric deposition and riverine export accounted for 31% and 69% of fossil carbon inputs to the China marginal seas (CMS) respectively, with fossil carbon burial efficiencies approaching 100% in the CMS. On a global scale, an atmospheric fossil carbon deposition flux of 17.2 Tg C yr 1 was estimated, equivalent to 40% of the estimated fluvial flux to the ocean, and potentially accounting for 24–41% of fossil OC burial in marine sediments. Therefore, the atmospheric deposition constitutes an important source of fossil carbon to marine sediments, and could play a key role in regional and global scale OC budgets and biogeochemical cycles.

Introduction

Atmospheric transport represents a significant pathway for the transfer of natural and anthropogenic materials from land to oceans, augmenting riverine input. Atmospheric deposition of nutrients, trace metals and pollutants has influenced coastal and open ocean biogeochemical cycles (Duce et al., 1991, Mahowald, 2011). Aerosol deposition also plays a key role in the global carbon cycle (Jurado et al., 2008, Willey et al., 2000). A total of 58 Tg C yr 1 of particulate organic carbon (POC) is delivered to the global ocean through dry and wet deposition (Jurado et al., 2008), equivalent to almost 30% of the annual river POC flux (~ 200 Tg C yr 1) (Galy et al., 2015) and also to ~ 40% of total OC (TOC) burial (~ 160 Tg C yr 1) in marine sediments (Burdige, 2005). Besides the riverine inputs, 14C data suggest that carbonaceous aerosols may also entrain a significant portion of pre-aged and fossil carbon (Heal, 2014, Matsumoto et al., 2001), and once buried in marine sediments, both carbon inputs may represent a long-term carbon sink. The transport and reburial of the non-modern carbon exerts minimal short-term influence on atmospheric CO2 concentrations (Galy et al., 2008), however its mineralization – both in the terrestrial and marine environments – would result in an increase atmospheric CO2. Therefore, it is important to constrain the different sources and fate of OC and their influence on atmospheric CO2 and climate forcing on different timescales. Fossil carbon may derive from natural weathering processes of continental rocks that is then transported oceanwards via riverine or atmospheric processes (Blair et al., 2003) or may be emitted as carbonaceous aerosols from fossil fuel combustion stemming from anthropogenic activities (Liu et al., 2013, Huang et al., 2014). Thus, increasing anthropogenic activity may enhance both the transport and burial of fossil carbon to the ocean.

Marginal seas are major loci of carbon sequestration, accounting for up to 90% of sediment OC burial in the global ocean (Hedges and Keil, 1995). The China marginal seas (CMS) in the western Pacific Ocean, including the Bohai Sea (BS), Yellow Sea (YS) and East China Sea (ECS), are important carbon sinks due to large-scale riverine and atmospheric inputs. With respect to the latter, the CMS are located in the downwind of the Asian continental outflow in spring and winter when the northerly wind prevails, during which atmospheric deposition of nutrients, heavy metals, toxic organic pollutants derived from anthropogenic activity could significantly influence marine ecosystems and biogeochemical processes (Shang et al., 2017, Wang et al., 2016a, Wang et al., 2017b). From a carbon cycle perspective, aerosol deposition has been also shown to be a significant source of carbon to the CMS, as indicated by studies of polycyclic aromatic hydrocarbons (PAHs, Lin et al., 2011, Wang et al., 2017a) and black carbon (BC, Fang et al., 2015, Huang et al., 2016). For example, a study of BC budget in the BS suggested that contributions from atmospheric deposition were as important as those from riverine transport (Fang et al., 2015); and that atmospheric deposition contributed nearly 72% of PAHs to the CMS (C. Wang et al., 2017).

Natural abundance variations in radiocarbon (14C) provide a powerful diagnostic for distinguishing fossil and modern (biomass) carbon sources. 14C-based source apportionment studies have also indicated that fossil carbon can comprise an important fraction of BC and PAHs in carbonaceous aerosols and sediments (Hanke et al., 2017, Huang et al., 2016, Uchida et al., 2010). The deposition of fossil carbon may thus also influence marine ecosystems, biogeochemical processes and carbon cycling. However, previous OC budgets for CMS sediments have primarily focused on riverine inputs. For example, Wu et al. (2013) estimated that about 2 Tg C yr 1 of fossil OC was buried in the ECS inner shelf, exceeding annual inputs from the Yangtze River. Tao et al. (2016) estimated an unidentified contribution of 0.72 Tg C yr 1 of pre-aged OC in the BS and YS. It could be inferred from both of the studies that atmospheric aerosols could serve as an important source of non-modern carbon to CMS sediments. Nevertheless, the importance of aerosol carbon contributions to the ocean carbon cycle remains poorly constrained. It is therefore necessary to characterize the fluxes and sources of aerosol OC in order to assess contributions to both regional and global ocean carbon budget.

In this study, aerosol total suspended particle (TSP) samples were collected along the east coast of China at Changdao (CD), Qingdao (QD) and Huaniao Island (HNI). There have been numerous prior studies on the composition, transport and deposition of major ions, trace elements and organic compounds in total suspended particulates and fine particulates at these sites (Feng et al., 2007, Feng et al., 2012, Guo et al., 2003, Wang et al., 2016a), however, there has been no study of seasonal variations in 13C and 14C isotopic characteristics and no assessments of fossil carbon contributions to the CMS via atmospheric deposition. Hence, the main objectives of this study are to identify sources, to quantify radiocarbon ages of aerosol OC at these sites, and to estimate the contributions of aerosol-derived fossil carbon to the CMS sediment carbon budget.

Section snippets

Study sites and sample collections

Aerosols were collected seasonally as total suspended particle (TSP) samples at CD, QD and HNI sites along the east coast of China (Fig. 1). Changdao (area, 56 km2) is near the demarcation line of the BS and YS and is located ~ 7 km north of the Shandong Peninsula, with limited local industrial activities. The sampling site at CD (37.90°N, 120.76°E, 90 m above sea level) was located on the rooftop of a radar station near the coast. Qingdao is a major coastal city situated in the southern tip of the

TOC and TSP concentrations

Table 1 lists properties of the TSP samples at the three sites. During the sampling period, TSP concentrations at the CD site ranged from 64.2 to 142.0 (avg. = 109.1 ± 29.8) μg/m3 with the highest value in 2012 autumn and the lowest value in 2014 summer. TOC concentrations ranged between 4.9 and 10.1 (avg. = 7.8 ± 2.0) μg/m3 with the highest value in 2014 winter. The TSP and TOC concentrations at the QD site ranged from 114.8 to 251.9 (avg. = 186.0 ± 56.3) μg/m3 and from 5.3 to 19.7 (avg. = 12.3 ± 5.9) μg/m3,

Spatiotemporal variations of aerosol OC characteristics and sources

Previous studies showed that most air parcels at CD (Feng et al., 2007) and HNI (F. Wang et al., 2015) derived from the open sea in summer, but were transported from the northwest, driven by the East Asian monsoon, in winter. In spring and autumn, air parcels mostly emanated from the land with wind directions varying between northwest to north, and southeast or southwest to south (Feng et al., 2007, Wang et al., 2015a). The backward trajectories in our sites were consistent with the wind

Conclusions

A wide range in OC δ13C (− 23.6 to − 30.5‰) and Δ14C (− 153 to − 687‰, 1280 to 9260 14C age) values of total suspended particles at three coastal sites (CD, QD and HNI) bordering the China marginal seas highlight spatial and temporal variations in carbon sources in terms of biomass and fossil OC contributions. Generally low Δ14C values indicate a significant contribution from fossil-fuel emissions to the carbonaceous aerosols, and higher δ13C values in winter samples than in summer is attributed to

Acknowledgements

We would like to thank Fengwen Wang and Shixin Guo for sampling help, and Julian Sachs for constructive suggestions and comments on the manuscript. This study was supported by the National Key Research and Development Program of China (Grant No. 2016YFA0601403), and by the National Natural Science Foundation of China (Grant Nos. 41520104009, 41521064), and the “111” Project (B13030). This is MCTL (Key Laboratory of Marine Chemistry Theory and Technology) contribution #152.

References (65)

  • A.L. Lamb et al.

    A review of coastal palaeoclimate and relative sea-level reconstructions using δ13C and C/N ratios in organic material

    Earth-Sci. Rev.

    (2006)
  • A.P. McNichol et al.

    Ten years after–the WOCE AMS radiocarbon program

    Nucl. Inst. Methods Phys. Res. B

    (2000)
  • D. Shang et al.

    Effects of continental anthropogenic sources on organic aerosols in the coastal atmosphere of East China

    Environ. Pollut.

    (2017)
  • X. Sun et al.

    14C-based source assessment of carbonaceous aerosols at a rural site

    Atmos. Environ.

    (2012)
  • S. Tao et al.

    Pre-aged soil organic carbon as a major component of the Yellow River suspended load: regional significance and global relevance

    Earth Planet. Sci. Lett.

    (2015)
  • S. Tao et al.

    Diverse origins and pre-depositional histories of organic matter in contemporary Chinese marginal sea sediments

    Geochim. Cosmochim. Acta

    (2016)
  • M. Uchida et al.

    Radiocarbon-based source apportionment of black carbon (BC) in PM 10 aerosols from residential area of suburban Tokyo

    Nucl. Inst. Methods Phys. Res. B

    (2010)
  • F. Wang et al.

    Characterization of carbonaceous aerosols over the East China Sea: the impact of the East Asian continental outflow

    Atmos. Environ.

    (2015)
  • Q. Wang et al.

    Probing the severe haze pollution in three typical regions of China: characteristics, sources and regional impacts

    Atmos. Environ.

    (2015)
  • F. Wang et al.

    The contribution of anthropogenic sources to the aerosols over East China Sea

    Atmos. Environ.

    (2016)
  • L. Xing et al.

    Multiple proxy estimates of source and spatial variation in organic matter in surface sediments from the southern Yellow Sea

    Org. Geochem.

    (2014)
  • L. Xing et al.

    Assessment of the sources of sedimentary organic matter in the Bohai Sea and the northern Yellow Sea using biomarker proxies

    Estuar. Coast. Shelf Sci.

    (2016)
  • S.H. Yoon et al.

    Source, composition and reactivity of sedimentary organic carbon in the river-dominated marginal seas: a study of the eastern Yellow Sea (the northwestern Pacific)

    Cont. Shelf Res.

    (2016)
  • Y.L. Zhang et al.

    Wet deposition of fossil and non-fossil derived particulate carbon: insights from radiocarbon measurement

    Atmos. Environ.

    (2015)
  • L. Zhou et al.

    Coastal erosion as a major sediment supplier to continental shelves: example from the abandoned Old Huanghe (Yellow River) delta

    Cont. Shelf Res.

    (2014)
  • R. Bao et al.

    Widespread dispersal and aging of organic carbon in shallow marginal seas

    Geology

    (2016)
  • N.E. Blair et al.

    The fate of terrestrial organic carbon in the marine environment

    Annu. Rev. Mar. Sci.

    (2012)
  • D.J. Burdige

    Burial of terrestrial organic matter in marine sediments: a re-assessment

    Glob. Biogeochem. Cycles

    (2005)
  • J.J. Cao et al.

    Spatial and seasonal distributions of carbonaceous aerosols over China

    J. Geophys. Res.

    (2007)
  • B. Chen et al.

    Source forensics of black carbon aerosols from China

    Environ. Sci. Technol.

    (2013)
  • B. Deng et al.

    Recent sediment accumulation and carbon burial in the East China Sea

    Glob. Biogeochem. Cycles

    (2006)
  • E.R. Druffel et al.

    Cycling of dissolved and particulate organic matter in the open ocean

    J. Geophys. Res. Oceans (1978–2012)

    (1992)
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