Particulate and Dissolved Black Carbon in Bohai and Laizhou Bays, China: Distributions, Sources, and Contrasts Under Two Distinct Fluvial Hydrological Regimes

Abstract

Coastal seas, including coastal bays, are the geographically critical transitional zone that links terrestrial and open oceanic ecosystems. Organic carbon cycling in this area is a dynamic and disproportionally key component in the global carbon cycle and budget. As the thermally-transformed organic carbon produced exclusively from the incomplete combustion of biomass and fossil fuels, the recalcitrance and resultant longer environmental residence times result in important implications of black carbon (BC) in the global carbon budget. However, the environmental dynamics of BC in coastal seas have not well been constrained. In this study, we conducted one seawater sampling campaign in the high-intensity BC emission influenced Bohai Bay (BHB) and Laizhou Bay (LZB) in 2013, and quantified both particulate and dissolved BC (PBC and DBC). We elaborated the distributions, sources, and associated influencing factors of PBC and DBC in BHB and LZB in 2013, and simultaneously contrasted the PBC and DBC quantity and quality under two distinct fluvial hydrological regimes of 2013 and 2014 [discussed in Fang et al. (Environ. Sci. Technol., 2021, 55, 788–796)]. Except for the overwhelmingly high PBC in northern BHB caused by anthropogenic point-source emission, horizontally, both PBC and DBC showed a seaward decreasing trend, suggesting that riverine discharge was the major source for PBC and DBC. Vertically, in contrast to the uniform concentrations of DBC between surface and bottom waters, the PBC levels in bottom waters was significantly higher than that in surface waters, which was primarily resulted from the intense sediment re-suspension process during this sampling period. The nearly simultaneous investigations in 2013 and 2014 revealed consistent spatial patterns of PBC and DBC quantity and quality. But significantly lower PBC and DBC quantity and quality were found in 2014 than in 2013, which were largely due to the significantly different climatic conditions (including the watershed hydrology and sunlit radiation) between these 2 years.

Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments

“…Our findings, however, are in good agreement with recent studies, indicating that DBC has much higher Δ 14 C values (50 to −200‰) and modern to younger 14 C ages in the Yangtze, Yellow, and Heilongjiang Rivers in China , and in the Amazon River . Our study, together with other studies, demonstrates that a large amount of BC produced from biomass burning on the land can be transformed into the dissolved phase and then mobilized and transported by streams and rivers to coastal seas. ,,,, Similarly, after PBC deposition, the biomass-derived can be further transferred to porewater DBC during early diagenesis. Thus, the fate and transformation of DBC are important processes in BC cycling.…”

“…The Yangtze and Yellow Rivers are the two largest rivers in China, and their estuaries and adjacent shelf regions are highly dynamic due to large inputs of freshwater and terrestrial materials. The Yangtze and Yellow Rivers together deliver over 90% of the fresh water to the East China Sea and Bohai Sea. , Recent studies have shown that large amounts of BC are transported by the two rivers into coastal regions. ,, Our previous studies have demonstrated that the Yangtze River exports ∼192 Gg (1 Gg = 10 9 g) of particulate BC (PBC) to the East China Sea annually, and the Yellow River delivers ∼12 Gg of PBC to the Bohai Sea annually. , In addition, atmospheric deposition could contribute 55 ± 19 Gg aerosol PBC to the East China Sea and ∼88 Gg aerosol PBC to the Bohai Sea annually. , Large amounts of this river- and atmospherically transported PBC are deposited and buried in the sediments of the estuaries, making the two estuaries ideal regions for studying the transformation and cycling of BC in sediments.…”

“…34,35 Recent studies have shown that large amounts of BC are transported by the two rivers into coastal regions. 18,32,33 Our previous studies have demonstrated that the Yangtze River exports ∼192 Gg (1 Gg = 10 9 g) of particulate BC (PBC) to the East China Sea annually, and the Yellow River delivers ∼12 Gg of PBC to the Bohai Sea annually. 29,32 In addition, atmospheric deposition could contribute 55 ± 19 Gg aerosol PBC to the East China Sea and ∼88 Gg aerosol PBC to the Bohai Sea annually.…”

“…3 Our study, together with other studies, demonstrates that a large amount of BC produced from biomass burning on the land can be transformed into the dissolved phase and then mobilized and transported by streams and rivers to coastal seas. 16,19,29,33,67 Similarly, after PBC deposition, the biomass-derived can be further transferred to porewater DBC during early diagenesis. Thus, the fate and transformation of DBC are important processes in BC cycling.…”

“…The results from many studies indicate that the contents of BC buried in river-influenced coastal sediments are usually much higher than those in river-unaffected areas. − In recent studies, dissolved BC (DBC) transported by rivers has received substantial amounts of attention. ,,, It is estimated that 18 ± 4 Tg year –1 of BC is transported in the dissolved phase by rivers, accounting for 12 ± 5% of the global flux of riverine-dissolved organic carbon (DOC). , In our recent study, we found that large Chinese rivers transport significant amounts of DBC and that this DBC is unlikely to be refractory in nature and could be decomposed by bacteria, which suggest a possible mechanism for the phase transformation and cycling of BC in natural systems. Although many studies have investigated BC in large rivers and in the coastal sediments of China in recent years, ,,− the fate of BC transformation and cycling in marine sediments has not been studied. Therefore, the goal of this study is to investigate the sources and transformation of BC in the sediments of the Yangtze and Yellow River estuaries.…”