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
Graphical abstract
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.
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