| Black carbon(BC) has been of great interest due to its linkage with climate change, carbon cycle, air quality and public health. BC is the highly condensed carbonaceous residue produced exclusively from the incomplete combustion of biomass(including wood, grass, crop straw, and etc.) and fossil fuels(including coal and petroleum/oil). The provinces and municipalities adjacent to the Bohai Sea(BS) and Yellow Seas(YS) have the highest BC emission intensity, which contribute ~40% of the China’s total annual BC emission amount even if the area there is only ~9% of the Chinese territory. A significant amount of BC is predictably injected into the BS and YS through atmospheric deposition and riverine inputs, making the BS and YS continental shelf be an important BC sink. In this study, multi-medium samples, including 191 surface sediments, 3 sediment cores, 36 riverine water in Bohai Rim and 2 seawater sites with different seasons(i.e., spring, summer, autumn, and winter) and layers(surface, middle and bottom) were collected for analyzing BC using the method of wet-chemical treatment combined with thermal/optical reflectance(TOR) detection. Together with the extensive investigations of atmospheric BC conducted in the Bohai Rim, the objective of this study was to have a comprehensive understanding of the regional BC geochemical behaviors in the BS and YS, which might provide some scientific basis for the deep study of coastal BC biogeochemical processes and regional carbon cycle. The main conclusions are as follows.The BC concentrations in the BS and YS ranged from 0.02 to 3.55 mg/g, with an arithmetic mean value of 0.85 ± 0.57 mg/g. High and low BC values took place in sea areas characterized by weak and strong hydrodynamic conditions, respectively, indicating that the regional hydrodynamic condition might be an important factor affecting the spatial distribution of the BC concentrations in the BS and YS. The BC burial flux in the BS and YS ranged from 4 to 1100 μg/cm2/yr, and averaged 160 ± 200 μg/cm2/yr. The BC burial flux in the BS and YS was comparable to those detected in other continental shelf regimes, but was 2–4 orders of magnitude higher than those in the pelagic regimes. For the sub-regions, the BS had the highest burial flux, reaching up to 260 ± 252 μg/cm2/yr, followed by that in the NYS(100 ± 84 μg/cm2/yr) and the SYS(67 ± 51 μg/cm2/yr). In contrast to the BC concentration, the BC burial flux decreased with the increasing distance from the coastline in the BS and YS. The BC sink flux in the entire BS and YS was estimated to be ~325 Gg/yr. For the sub-regions, the BC sink fluxes were sorted in the order of BS(~157 Gg/yr) > SYS(~127 Gg/yr) > NYS(~41 Gg/yr), and it can be therefore concluded that the source intensity of BC input into the BS was significantly higher than that of the YS.Positive Matrix Factorization(PMF)-based source apportionment showed that the main sources of BC to the BS and YS continental shelf surface sediments consisted of coal combustion, biomass burning and oil combustion, with each accounting for ~75%, ~17% and ~8%. Therefore, the fossil fuels combustion was the main source(~83%) of BC in the BS and YS surface sediments, which was nearly five-fold to that from biomass burning. Spatially, the percentage of the fossil fuels combustion-derived BC increased with the increasing distance from the nearshore areas, and the resultant was the declined percentage of the biomass burning-derived BC. These trends were more pronounced in the Yellow River estuary. Within 5–30 km away from the Yellow River estuary, with the increasing distance from the Yellow River mouth, the percentage of the fossil fuels combustion-derived BC increased exponentially from ~10% to ~80%, and that of the biomass burning-derived BC declined exponentially from ~90% to ~20%. This spatial trend was to a large extent attributed to the differences in physical properties between these two BC sources and the regional hydrodynamic conditions.The BC inputs into the BS include atmospheric deposition(including dry deposition and wet deposition), riverine discharge, and import from the NYS through the Bohai Strait, with each contributing 93 ± 30 Gg/yr, 85 ± 14 Gg/yr, and 4 ± 4 Gg/yr. The outputs consist of sequestration to bottom sediments and export to the NYS through the Bohai Strait, and the fluxes were 157 ± 41 Gg/yr and 21 ± 6 Gg/yr, respectively. The difference between the inputs and the outputs, ~4 Gg/yr, was thus the BC internal sink, i.e., net increase in the BS seawater. It can be therefore concluded that atmospheric deposition and riverine discharge were the main BC input patterns to the BS, with each accounting for 51% and 47%. Sequestration to bottom sediments was the major BC output pattern, accounting for ~88% of the input BC. Water exchange in the Bohai Strait led to net BC transport from the BS to the NYS. The net export flux was 17 ± 7 Gg/yr, and the BS could be considered as a source of BC to the NYS.Sediments cores in the BS, NYS and SYS had BC concentrations ranges of 0.59?1.36 mg/g, 0.56-1.32 mg/g, and 0.51-1.18 mg/g, with mean values of 0.93 ± 0.15 mg/g, 0.78 ± 0.19 mg/g, and 0.77 ± 0.17 mg/g, respectively. The temporal trends of BC concentrations in these three cores closely followed the regional historical anthropogenic activities in China. The increasing trends occurring at the beginning of the 1860 s and 1980 s each corresponded well to the rapid economic development induced from the “Westernization Movement” and the “Reform and Open” Policy. The lowest BC concentration detected in the core of BS in the late 1970 s might stem from the shift of the Yellow River mouth, which made the grain size of the sediments coarser. The decreasing trends since ~1996 in the NYS and SYS cores was associated with the enforcement of the Coal Law and the substitution of high polluted domestic coal stoves with clean liquid petroleum gas/natural gas stoves during this period. The TOR method used in the present work has been demonstrated to effectively discriminate between char and soot, the two subtypes of BC with different formation mechanisms and physicochemical properties. The temporal trends of char/soot indicated that the energy structure in China has changed from low-temperature to high-temperature combustion processes, implying that China has shifted from an agricultural economy to an industrial economy. The spatial trends of char/soot indicated that BC in central mud areas of the NYS and SYS was from long range transport, and BC in central mud area of the BS was from short range transport. By comparing results from the present work and other sedimentary records retrieved around the world, we found that there were differences of sedimentary records in different countries or in different regions within a country. Of particular notable was that, however, energy consumption(including energy amount and structure), levels of economic and social development, pollution emission control measurements, energy strategies implemented by the countries and governments, and etc. could be well mirrored from the temporal trends of BC concentrations and fluxes. This study confirmed that BC was a good geochemical indicator, which could be used to reveal the influential history of anthropogenic activities on ecological environment. |
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