Black Carbon In Sediments Research Articles

Distribution, sources, and burial flux of black carbon in sediment of Daye lake, central China

Black carbon (BC), primarily originating from fossil fuel and biomass combustion, holds significance for global carbon cycling, climate change, and human health. Despite a lake's role as a carbon sink, detailed information about BC sedimentary burial flux and sink in its sediment remains insufficiently explored. The current study investigates the distribution, sources, and burial flux of BC and its subtypes (char and soot) in the surface sediment of Daye Lake, the largest lake in Huangshi City, central China. BC concentrations in the sediment ranged from 0.10 to 3.60 mg/g, corresponding to 0.40%–17.02% of organic carbon (OC). Higher values of BC and BC/OC observed in the western region suggest direct terrestrial input via river discharge and surface runoff, influenced by anthropogenic activities. In contrast, variations in char/soot ratios reflect diverse combustion sources and hydrological dynamics in different regions. The indications from BC/OC and char/soot ratios imply that fossil fuel combustion is the predominant sources. The weak correlations between BC and OC suggest that they may come from different sources or undergo different processes that affect their distribution in the lake sediment. However, a stronger correlation was observed between BC and soot, as well as between char and soot, indicating potential similarities in their input pathways. The BC burial flux displays notable variations across the lake, ranging from 0.69 to 24.07 g/m2/yr, with elevated values observed in the western region. The BC sink in the sediments of Daye Lake was estimated to be 0.635 Gg/yr. Though locally small, it significantly contributes to the broader picture of BC burial in Chinese lakes and the global distribution of BC in lake ecosystems.

Quantification and isotopic characterization of benzene polycarboxylic acids (BPCA)-derived black carbon in deep oceanic sediments: Towards assessing pyrogenic inputs from marine sources

Methodologies based on benzene polycarboxylic acids (BPCA) selectively target the polymeric aromatic fraction of black carbon (BC) and are considered adequate to quantify pyrogenic inputs in environmental samples such as soils, lakes, and marine dissolved organic carbon. However, the usefulness of these methodologies to quantify BPCA-derived BC in deep-sea sediments has not been fully evaluated. In this manuscript we describe and validate a procedure to quantify BPCAs in deep oceanic sediments with very low organic carbon content. The resulting analytical procedure has produced reproducible quantitative data for BPCAs over a period of 10 months (coefficient of variation, CV = 6.4 − 6.6%). The stable carbon isotopes (δ13C) of BC_BPCA have been characterized using an LC Isolink™-irMS system with an accuracy better than 0.5‰. The quantitative and isotopic composition of several marine sediments has been characterized to investigate the relative contributions of marine/diagenetic and continental/pyrogenic sources to the BC accumulated in oceanic sediments from different contexts, ranging from upwelling systems to remote oceanic locations. Overall, a significant fraction of the sedimentary BC is of marine origin and should be considered in inventories of pyrogenic materials accumulated in the world oceans. However, the continental/pyrogenic sources can be largely dominant in marine settings with large inputs of pyrogenic materials.

Characterizing sedimentary black carbon in the Pearl River Estuary, Southern China

Sedimentary black carbon in coastal regions plays an important role in global carbon cycles due to its refractory nature in sediments. In this study, we conducted the first comprehensive investigation on black carbon (BC) in surface sediments of the Pearl River Estuary (PRE), a large tropical estuary system in China. We used two different analytical methods: IMPROVE-A thermal optical reflectance (TOR) and chemothermal oxidation (CTO-375) to constrain the distribution, and TOR can further constrain the composition (i.e., char and soot). The BC concentration determined by the TOR method (BCTOR) ranged from 0.07 to 7.13 mg/g, with char and soot contents ranging from 0.04 to 6.98 mg/g and 0.03–0.43 mg/g, respectively. The BC concentrations measured by the CTO-375 method (BCCTO) ranged from 0.27 to 1.88 mg/g. Char was the predominant constituent (87%) of sedimentary BCTOR in the PRE. Both char and soot had higher concentrations locating near the outlets and on the Shenzhen coast, indicating that they were sourced through direct fluvial transport and surface runoff. Soot particles were transported farther from fluvial sources than char particles. We estimated that the burial flux of BCTOR in the PRE sediments was 2.97 ± 1.61 mg/cm2/yr, acting as an important sink of BC. This study implies that large river estuaries are hotspots for BC burial, sinking a large amount of organic carbon from terrestrial reservoirs.

Char and soot records of the Holocene fire history and its implications for climate-vegetation change and human activities within the Guanzhong Basin, southern Loess Plateau, China

Black carbon in sediments has been widely used as a proxy for biomass burning/fire activity to reconstruct fire history and its evolution. Wildfire studies have revealed that different types of black carbon (char and soot) are formed due to changes in combustion efficiency. In this study, we obtained black carbon and its two subtypes, char and soot, from a typical Holocene aeolian loess-paleosol section in the Chilanqiao Ruins within the Guanzhong Basin, southern Loess Plateau, China. Combined with environmental proxies such as magnetic susceptibility, loss on ignition, and geochemical elements, along with AMS14C and optically stimulated luminescence (OSL) dates, we reconstructed the Holocene fire history and its evolution on the southern Loess Plateau at local and regional scales. The findings indicate that the limited vegetation during the relatively dry and cold early Holocene may have inhibited the spread of fires. In the warmer and wetter middle Holocene, there was higher local smoldering fire activity, likely influenced by both wet climatic conditions and an increase in the proportion of woody plants. Additionally, the fire history in relation to human activities at Shang Dynasty (1600–1046 BCE) including land reclamation, house construction, and bronze casting has also been identified. There has been a significant increase in regional flaming fire activity in the late Holocene as a result of drier climate and increased human activity. Notably, the significant increase in regional flaming fire activity since ~1.00 ka can be primarily linked to human-set fires with the usage of gunpowder in frequent wars. This research holds great importance in enhancing our understanding of the long-term interactions among fire activities, climate change and human activities.

Black Carbon in Deep-Sea Seamount Sediment Cores: Vertical Variation and Non-negligible Char Black Carbon.

Deep-sea sediments (>1000 m) are often considered to be the ultimate sink for black carbon (BC), and the long-term buried BC in these sediments is believed to potentially provide a negative feedback effect on climate warming. The burial flux of BC in marine sediments is predominantly estimated based on soot BC (SBC) in most studies, frequently ignoring the contribution of char BC (CBC). While this methodology may result in an underestimation of the BC burial flux, the precise extent of this underestimation is yet to be determined. This study used the benzene poly(carboxylic acid) (BPCA) method and chemothermal oxidation (CTO) method to analyze CBC and SBC in four deep-sea sediment cores from the Zhongnan seamount in the South China Sea, respectively. The CBC content increased from 0.026 ± 0.010% at the seamount upper part (1432 m) to 0.039 ± 0.012% at the seamount foot (4278 m), constituting approximately 25 to 42% of the SBC content. The content disparity between CBC and SBC diminishes as depth increases. In deep-sea sediments, biogeochemical factors influence the variation of CBC molecules with depth. In the seamount middle-upper part (1432 and 2465 m), highly condensed CBC gradually accumulated along the core downward profile. In the sediment core profile of the seamount middle-lower part (3497 m), benzenetricarboxylic acid and benzenetetracarboxylic acid content decreased while the BC condensation degree rose, i.e., less condensed CBC was preferentially consumed. Afterward, CBC molecules reached a relatively stable state at the seamount foot. This study reveals that CBC possesses the capacity for long-term carbon sequestration in deep-sea sediments, and its content is not negligible.

Distribution of black carbon in sediments from mangrove wetlands in China

Black carbon (BC) is a carbonaceous residue generated by the incomplete combustion of fossil fuels and biomass. It forms an important component of the mangrove carbon pool. Although coastal and marine sediments have long been recognized as important sinks for BC, there are few reports on its distribution in mangrove sediments. This study investigated BC distribution and the associated environmental implications in sediments of various size fractions. Mangrove sediments were collected from 44 sites in the Guangdong, Guangxi, Hainan, and Fujian Provinces of China. Sediments from the Hainan Province were found to have much higher organic carbon (OC) and BC content than those from other provinces. Sediment OC and the BC content showed a significant positive relationship. Sediments from the Guangdong, Guangxi, and Hainan provinces showed significant BC accumulation with an enrichment factor > 1, especially in the size fractions 0.053–0.25 mm and 0.25–2 mm. Sediments from different sampling sites did not show significantly different BC distribution between the various size fractions. A higher mass loading of BC was observed among fine particles in samples from the Fujian Province, and among the larger particles in samples from the Guangdong, Guangxi, and Hainan provinces. Among all the samples, the BC/OC ratio ranged from 0.21 to 0.29, indicating that BC originates from a combination of biomass and mineral combustion. The average δ13C values in the sediments varied between the sampling plots. The highest δ13C values were recorded in the Fujian province, which indicates the proportion of BC from biomass combustion in mangroves in Fujian Province is higher than that in other Provinces.

Differential mobilization and sequestration of sedimentary black carbon in the East China Sea

Black carbon (BC) derived from incomplete combustion of biomass and fossil fuels on land can be mobilized and transported to the ocean. Burial of BC in the ocean sequesters atmospheric CO2 into a long-term carbon sink, likely exerting a positive influence on mitigating global warming. However, the abundances, sources, and burial of sedimentary BC in marine sediments remain poorly constrained, hindering us from accurately understanding the mobilization and sequestration of BC and its roles in the ocean carbon cycle. Here, we investigate concentrations and isotopes (13C and 14C) of BC among grain size-fractionated surface sediments along a across-shelf transect from the Yangtze River prodelta to the Okinawa Trough to decipher the fate of BC in the East China Sea (ECS). Our results show that the bulk BC concentrations decrease firstly from the Yangtze River prodelta to the outer shelf and then increase to the Okinawa Trough. Grain size-fractionated BC concentrations vary along the transect, which we mainly attribute to the differential mobilization of BC driven by hydrodynamic processes. We argue that BC is aged during the mobilization, which results in an older 14C ages of BC found seaward. After considering biomass- and fossil-derived BC apportionments based on 14C balance calculation, we think that BC aging may be verified by more fossil-derived BC burial in the Okinawa Trough. We estimate that BC may account for ∼15% of sedimentary organic carbon (SOC), and up to ∼30% of terrestrial SOC buried in the ECS. BC burial fluxes decrease along the transect, and are heterogeneous in different size fractions, indicating differential sequestration of BC in the shelf and trough. We further estimate that 685 Gg/yr of BC is sequestered in the ECS, and 491 Gg/yr in the prodelta area, with ∼30% being continental biomass-derived BC. We suggest that increasing biomass-derived BC production on land and burying it in the ocean may serve as a powerful means for sequestrating atmospheric CO2, potentially contributing to carbon neutrality.

Sedimentary Black Carbon Isotope Record of Holocene Climate Changes on the Northeastern Tibetan Plateau

AbstractThe northeastern Tibetan Plateau (TP) is an unique area for studying the interactions between the Asian Monsoonal climate and the westerlies. However, significant contraries on Holocene paleoclimate evolution are still exist. To better understand the climate history on the northeastern TP particularly its variability, we here investigated the stable carbon isotope compositions of Black Carbon (BC) (δ13CBC) in Lake Gyaring sediment core from the source area of the Yellow River. The δ13CBC values were relatively positive before ∼6.2 cal kyr BP indicating that the climate on northeastern TP was warm and dry during the first half of the Holocene, while the decreasing trend thereafter reflects that the climate on the northeastern TP has become generally cooler and wetter since the middle Holocene. Superimposed on the long‐term paleoclimate evolution pattern, at least seven centennial‐scale cold‐wet events characterized by significant δ13CBC depletions occurred at about 8.4, 7.5, 6.2, 5.0, 4.3, 2.7, and 0.9 cal kyr BP. Those abrupt climate events well corresponded to the weak Asian Monsoon episodes and the ice‐rafted debris records in the North Atlantic. We speculate that the cold climate signals from the North Atlantic resulted from the sea‐ice expansion have been transported to the northeastern TP through the westerlies leading to a cold conditions there. Besides, the anomalous easterly water vapor transportation over the northern periphery of the TP during weak monsoon intervals has generated the wet conditions on the northeastern TP.

Holocene fire history in southwestern China linked to climate change and human activities

Global warming and human activities have significantly increased the wildfire risk in highly populated subtropical East Asia over recent decades; however, the factors driving fire occurrence on multiple timescales remain unclear. We present and analyze an observational dataset of fire for the Yunnan Plateau in southwestern China for the past 20 years, together with a Holocene fire history documented by four sedimentary black carbon records from four lakes on the Yunnan Plateau: lakes Dian, Xingyun, Qilu, and Yilong. Our aims were to explore the linkage between wildfires and natural and anthropogenic variables driving wildfire occurrence on multiple timescales. The results show that on a short timescale, more wildfires occurred in drier seasons, and on a longer timescale, more wildfires occurred during 6–2.8 cal kyr BP, when humidity decreased. In contrast, there was a moderate wildfire incidence during the relatively humid interval from 11.7 to 6 cal kyr BP. These observations indicate that fire activity is controlled by the humidity, against the background of the abundance of biofuel. The lowest degree of biomass burning occurred after ∼2.8 cal kyr BP and was linked to an anthropogenic reduction of biomass availability, despite an increase in drought. This suggests the importance of active fire management and prescribed clearance measures, especially given projections of continued drought and increased vegetation cover.

Sources, burial flux and mass inventory of black carbon in surface sediments of the Daya Bay, a typical mariculture bay of China

The contents of chemothermal oxidation (CTO)-derived black carbon (BC) and organic carbon (OC) and their stable isotopes (δ13CBC and δ13COC), including major elemental oxides, and grain sizes were measured to constrain the sources, burial flux, and mass inventory of BC in surface sediments of the Daya Bay. Surface sediments were mainly clayey silt (>90%) and contained 0.28–1.18% OC and 0.05–0.18% BC. Fossil fuel emission and physical erosion contributed to the sedimentary BC sources. High BC/OC ratio (6–30%), burial flux (154.88–922.67 μg cm−2 y−1), and mass inventory (22–34 Gg y−1) of BC in the upper 5 cm of surface sediments indicated that the Daya Bay is a significant sink of BC. The high accumulation of BC in sediments is attributed to a strong affinity to fine-grained sediments due to the enrichment of muddy biodeposits excrements from the cultured species in the bay.

Use of molecular markers and compound-specific isotopic signatures to trace sources of black carbon in surface sediments of Peninsular Malaysia: Impacts of anthropogenic activities

Black carbon (BC) acts as a reservoir of carbon in sediment due to its high persistency. Southeast Asia (SEA) is a main source region for BC emissions across the globe due to extensive biomass burning and escalating fossil fuel consumption. However, our understanding of the sources and sinks of BC in SEA is limited. Here, we have analysed BC structure using benzene polycarboxylic acid (BPCA) method in combination of stable carbon isotope (δ13C) signatures for B5CA and B6CA using HPLC-IRMS in sediment samples from riverine, coastal and shelf areas in Peninsular Malaysia. BPCA molecular markers indicated higher degree of aromatic condensation and lighter isotopic composition in relatively pristine environment of the East coast compared to developed environment of the West and South coast. n-Alkane biomarkers clearly demonstrated higher anthropogenic impacts on the sediments of the West and South coast compared to the East coast. Biomass burning with the predominance of C3 plants and the large share of natural gas in the energy mix in Malaysia are possibly the main reasons for the isotopically light composition of sedimentary BC (−45.4 to −26.4‰). Atmospheric soot and petrogenic BC are the possible main sources of BC in the East coast sediments, while char residues of low temperature biomass burning likely contribute more to sedimentary BC in the West and South coast. The n-alkane indices implied that the sediments of the Kelantan adjacent shelf area receive great proportion of terrestrial organic matter and the associated BC.

Transport of dissolved black carbon from marginal sea sediments to the western North Pacific

Pyrogenic carbon, also called black carbon (BC), is produced by biomass burning as well as by fossil fuel combustion and is an important slow-cycling component in the global carbon cycle. BC occurs in a fraction of marine dissolved organic carbon, and this is called dissolved BC (DBC). Marine DBC has been considered the ultimate repository for fluvial DBC; however, the environmental dynamics of marine DBC have not been well documented. In this study, vertical DBC profiles from the surface to the deep layers of the western subarctic Pacific and its marginal seas were analyzed using the benzenepolycarboxylic acid (BPCA) method. DBC concentrations tended to be highest in the surface layer and decreased with increasing depth, except for those in the Bussol’ Strait locatedin the Kuril Islands between the Sea of Okhotsk and the North Pacific. Vertical distributions of the condensation degree index of DBC (i.e., the ratio of B5CA and B6CA to all BPCAs) did not show a general trend with depth. Atmospheric deposition of BC is likely a major source of DBC in the surface layer. In the intermediate layer, the DBC concentration and the condensation degree index in the Bussol’ Strait were higher than those in the western subarctic Pacific. The occurrence of highly condensed DBC observed in the Bussol’ Strait was accompanied by low salinity and a high DO concentration, indicating that DBC is transported from the shelf sediments of the Sea of Okhotsk to the Bussol’ Strait by dense shelf water (DSW) and Okhotsk Sea Intermediate Water (OSIW). The DBC concentration and the condensation degree index in the intermediate layer of the Bering Sea and the Kamchatka Strait (locatedin the Aleutian Islands between the Bering Sea and the North Pacific) were higher than those in the western subarctic Pacific but lower than those in the Bussol’ Strait; this suggests that DBC from the slope sediments in the Bering Sea is transported to the Kamchatka Strait by the East Kamchatka Current and that the DBC flux from the slope sediments of the Bering Sea is lower than that involving the DSW in the Sea of Okhotsk, which accompanies the resuspension of shelf sediments. The results of this study imply that sedimentary BC is an important source of water column DBC in marginal seas and the adjacent open ocean.

Abundance and δ13C of sedimentary black carbon indicate rising wildfire and C4 plants in Northeast China during the early Holocene

Global warming and human activity are thought to be the primary drivers of the globally rising occurrences of forest fires. Due to the importance of local to regional environmental drivers in mediating fire frequencies, reconstructing fire histories of representative geographic regions is important to understand fire regimes and predict future changes. The present study aims to establish a relatively high-resolution fire record of boreal forests in northeastern (NE) China and identify the primary environmental drivers. We analyzed the concentration and stable carbon isotopic composition of black carbon (BC and δ13CBC) of a 34,000 cal yr BP sedimentary sequence from a crater lake (Tianchi). The results show that fire events and C4 plants were scarce during the late Last Glacial but increased since the deglaciation. Both fire and the abundance of combusted C4 plants reached their maxima during the early Holocene, which may be attributed to rising temperatures due to intensified solar radiation and summer monsoons. During mid-to-late Holocene, fire events remained relatively stable and were primary associated with C3 plants burning. Fire was particularly infrequent and C3 trees were abundant during the periods of 5500–3100 cal yr BP and 1500–150 cal yr BP, which can be attributed to a wet climate. Fires exhibited evident increases from 150 cal yr BP to the present, suggesting a substantial role of human activities. Both BC and δ13CBC during the Holocene show a significant correlation with pollen-based temperatures but not with precipitation, suggesting that temperature rises played a primary role in stimulating forest fires via mediating effective moisture. Collectively, these findings suggest that forest fires and C4 plants would likely increase in NE China in response to global warming and increasingly intensive anthropogenic disturbance, highlighting the importance of establishing a long-term understanding of fire regimes to create more sustainable, science-based fire management strategies.

Fire history and human activity revealed through poly cyclic aromatic hydrocarbon (PAH) records at archaeological sites in the middle reaches of the Yellow River drainage basin, China

Disentangling the role of natural and anthropogenic factors is a major challenge in paleofire studies. Here, we introduce the molecular biomarkers of polycyclic aromatic hydrocarbon (PAHs), combined with charcoal and black carbon in sediments of archaeological sites, to identify anthropogenic signals of environmental change during the middle Holocene. In the prehistoric period, an intensified-fire regime period, inferred on the basis of PAHs, char and macro-charcoal levels, has been attributed to intensified anthropogenic biomass burning from dry crop cultivation between 6500 and 5000 and 3900–3600 yr BP., corresponding to the earliest Chinese settlements during the Miaodigou & Banpo IV Culture and the development of the Erlitou Culture under drier climate conditions. Subsequently, the fire frequency decreased notably between 4500 and 4000 yr BP. as the climate became wetter and there were shifts in the frequency of high-magnitude floods, corresponding to the abandonment of late Neolithic settlements in the floodplain. Changes in the PAH fluxes in sediments are positively correlated with intensified-fire regime periods, consistent with the charcoal and black carbon fluxes during those periods. Principal component analysis of PAH fluxes suggests that these intensified-fire events have a pyrogenic signature, implying that the PAHs were derived from biomass burning of fuelwood and other woody plants. In this study, PAHs in loess sediments have been used as the indicators of anthropogenic biomass burning through time. Meanwhile, PAHs also record the shift of modes of production and the evolution of human settlements as well as the choices made in response to limited natural resources. To comprehensively analyze the paleofire records of PAHs, charcoal and black carbon combined with multiproxy evidence are needed to understand wildfire history as well as human land use and social and cultural development across the study regions in depth.

Combining and competing effects between precipitation and temperature on Holocene fire regime evolution inferred from a sedimentary black carbon record in southwestern China

AbstractWildfires are sensitive to climate change, but their response to changes in temperature and precipitation on long timescales is still disputed. In this study, we present a ~9.4 ka black carbon mass sedimentation rate (BCMSR) record from Lake Ximenglongtan (XMLT), southwestern China, to elucidate the Holocene fire regime and its linkages to climatic conditions. The results indicate that the regional fire activity was low during the early Holocene (before 7.6 cal ka BP), increased notably at 7.6 cal ka BP, and continued to increase gradually during the mid- to late Holocene until 2.2 ka. The episodes of higher fire occurrence reflected by higher BCMSR over the last 2.2 ka might be more likely related to the intensified human activities. The cool and humid climate during the early Holocene limited the spread of fire, while warming and drying at ~7.6 cal ka BP triggered higher fire occurrence. Instead of temperature, changes in precipitation dominated fire regime variation during the mid- to late Holocene. On millennial timescales, we suggest that Holocene fire variability has been predominantly controlled by the combined effects of Northern Hemisphere (NH) summer and winter insolation that influenced monsoonal precipitation and fire season temperature, respectively. Indian Ocean Dipole (IOD) events may also have affected fire incidence through influencing monsoon intensity.

Reduced bioavailability of polycyclic aromatic hydrocarbons (PAHs) in sediments impacted by carbon manufacturing plant effluent: Evaluation by ex situ passive sampling method

Potential risks of polycyclic aromatic hydrocarbons (PAHs) in sediments of a Japanese bay contaminated by carbon manufacturing plant effluent were evaluated by calculating toxicity units (TUs). TUs calculated from the measured whole-sediment concentrations (Cwhole) were often higher than or close to 1, signaling a possible toxicity concern to benthic organisms. In contrast, TUs based on the freely dissolved pore water concentrations (Cfree) measured by an ex-situ passive sampling method with polyethylene strips were 0.0007–0.005, much lower than 1, indicating no effect. We also found that the fractions of black carbon in sediments of the contaminated bay were significantly higher than those of reference sites. Overall, we conclude that carbon manufacturing plant effluent substantially increases Cwhole of PAHs in sediments but also increases the fraction of carbonaceous particles that strongly retain PAHs. As a combined result, bioavailable concentrations (as expressed by pore water Cfree) of PAHs do not increase as much as Cwhole. The results of this study indicate that ecotoxicological risks of PAH contamination by carbon manufacturing plants should be evaluated by directly measuring pore water Cfree instead of Cwhole.

Distribution and isotopic composition of sedimentary black carbon in a subtropical estuarine-coastal region of the western Taiwan Strait: Implications for tracing anthropogenic inputs

Estuarine and coastal margins are strongly influenced by anthropogenic inputs. To trace anthropogenic inputs to the subtropical Jiulong River Estuary (JRE) and the adjacent western Taiwan Strait (WTS), black carbon (BC) and its stable carbon isotope composition (δ13СBC) in surface sediments were investigated as an indicator of human activities. The concentrations of sedimentary BC were measured by an emerging method of thermal/optical reflectance with wet-chemical treatment (BCTOR, including char and soot), and the conventional method of chemothermal oxidation (BCCTO, related to the soot fraction) was also used to determine BCCTO concentrations and δ13СBC compositions. In the JRE and adjacent WTS, the concentrations of BCTOR (0.77 to 3.79 mg g−1) were higher than those of BCCTO (0.55 to 2.46 mg g−1), and both were similar to the moderate ranges obtained in other coastal sediments around the world. The small offsets between δ13СTOC and δ13СBC and the relatively low char/soot ratios revealed that fossil fuel combustion-derived contributions were likely more significant compared with inputs from biomass burning. The decreasing BC concentrations and increasing δ13СBC values with increasing distance from the JRE towards the adjacent WTS, indicates the decline of land-based anthropogenic inputs through fluvial transport. Furthermore, the differences in BC/TOC and char/soot values between the southern and northern WTS, indicated an effective preferential dispersal of the fluvial BC to the southern coast. The estimation for mass inventories of sedimentary BC in the coastal WTS showed that direct riverine discharge from the JRE was nearly equivalent to atmospheric deposition, and both of them contributed half of the sedimentary BC sink. To balance the sedimentary BC budget in the coastal WTS, long-range alongshore sediment transport driven by the Fujian-Zhejiang coastal current containing Yangtze River derived materials (indirect riverine discharge) could be another significant input pathway to contribute sedimentary BC.

Large-river dominated black carbon flux and budget: A case study of the estuarine-inner shelf of East China Sea, China

Mobilization of terrestrial-derived and recalcitrant black carbon (BC), including char and soot, from land to ocean exerts a significant influence on the global carbon cycle. This study elaborated the occurrence and spatial distributions of BC, char, and soot concentrations, as well as their burial fluxes, in the estuarine-inner shelf surface sediments of the East China Sea (ECS), an epicontinental sea adjacent to Chinese high-intensity BC emission source regions. Using a combination of BC measurements in the Yangtze River water and coastal ECS aerosol samples, a preliminary BC budget was concurrently constrained. The spatial distribution of char concentrations resembled largely that of BC, but differed significantly from that of soot, indicating that char and soot exhibited different geochemical behaviors. In contrast to concentrations, BC, char, and soot burial fluxes exhibited highly consistent spatial patterns, and all declined as the distance from the coastline increased. For the coastal ECS, riverine discharge dominated (~92%) the total BC input, with the Yangtze River alone accounting for as high as ~72%. The area-integrated sedimentary BC sink flux (630 ± 728 Gg/yr) in the coastal ECS was equivalent to the total BC influx (670 ± 153 Gg/yr), which coincided well with the regional sediment budget. This suggested that the terrestrial-derived and recalcitrant BC could be regarded as an alternative geochemical proxy for tracing the sediment source-to-sink processes in this region. Comparisons between BC and co-generated polycyclic aromatic hydrocarbons (PAHs) budgets in the coastal ECS revealed similarities in their input pathways, but dramatic differences in their ultimate fates. Despite these, the ECS estuarine-inner shelf could serve as a major sink of these terrestrial-based materials in the global ocean.

Comparison of black carbon chemical oxidation and macroscopic charcoal counts for quantification of fire by-products in sediments

Black carbon (BC) ranges in size from submicron to millimeter scales and represents incompletely combusted or pyrolysed organic fuels (e.g., coal, petroleum, biomass etc). The content of BC in sediments or in the atmosphere has been widely used to infer fossil fuel combustion from local to regional sources. However, the assessment of natural fires (biomass burning) in the past has been conventionally performed by counting charcoal particles in different size fractions (greater than ca. 10 and 100 µm for microscopic and macroscopic charcoal, respectively) using a microscope. Because similarly sized charcoal particles can have large differences in mass, counting using microscopes may induce significant error for quantifying fire-produced BC in sediments. A popular method oxidizes BC with nitric acid to produce benzene polycarboxyclic acids (BPCAs), which can be subsequently quantified using a gas chromatograph and mass spectrometer (GC-FID and GC–MS). In this paper, we provide the first parallel comparison of charcoal and BPCA analyses to assess their ability to quantitatively record BC content in experimental sediments. We find that while qualitatively comparable, the BPCA method more accurately quantifies changes in BC content than does charcoal microscopy. Additionally, we use metadata analyses of the Global Charcoal Database to contextualize our results. Further, we explore possible applications of these two methods in tandem which could provide novel insight into paleofire characteristics.

Sources and burial fluxes of soot black carbon in sediments on the Mackenzie, Chukchi, and Bering Shelves

Black carbon (BC) has been recognized as a climate forcing and a major component in the global carbon budget. However, studies on BC in the Arctic Ocean remain scarce. We report here variations in the abundance, sources and burial fluxes of sedimentary soot black carbon (soot-BC) in the western Arctic Ocean. The soot-BC contents averaged 1.6 ± 0.3, 0.46 ± 0.04 and 0.56 ± 0.10 mg-C g−1 on the Mackenzie, Chukchi and Bering Shelves, respectively, accounting for 16.6%, 10.2% and 10.4% of the total organic carbon in surface sediment. Temporally, contents of soot-BC remained fairly stable before 1910, but increased rapidly after the 1970s on the Mackenzie Shelf, indicating enhanced source input related to warming. Comparable δ13C signatures of soot-BC (− 24.95‰ to − 24.57‰) to C3 plants pointed to a major biomass source of soot-BC to the Beaufort Sea. Soot-BC showed similar temporal patterns with large fluctuations in the Chukchi/Bering shelf regions, implying the same source terms for soot-BC in these areas. Two events with elevated soot-BC corresponded to a simultaneous increase in biomass combustion and fossil fuel (coal and oil) consumption in Asia. The similar temporal variability in sedimentary soot-BC between the Arctic shelves and Asian lakes and the comparable δ13C values manifested that anthropogenic emission from East Asia was an important source of soot-BC in the western Arctic and subarctic regions. The burial fluxes of soot-BC, estimated from both 137Cs- and 210Pb-derived sedimentation rates, were 2.43 ± 0.42 g-C m−2 yr−1 on the Mackenzie Shelf, representing an efficient soot-BC sink. Soot-BC showed an increase in buried fluxes from 0.56 ± 0.02 g-C m−2 yr−1 during 1963–1986 to 0.88 ± 0.05 g-C m−2 yr−1 after 1986 on the Chukchi Shelf, and from 1.00 ± 0.18 g-C m−2 yr−1 to 2.58 ± 1.70 g-C m−2 yr−1 on the Bering Shelf, which were consistent with recent anthropogenically enhanced BC input observed especially in Asia. Overall, the three Arctic shelves could bury more than 3000 Gg soot-BC each year, attesting to an important role of the Arctic and subarctic shelves in global BC budget and carbon cycle.