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 (δ¹³С_BC) 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 (BC_TOR, including char and soot), and the conventional method of chemothermal oxidation (BC_CTO, related to the soot fraction) was also used to determine BC_CTO concentrations and δ¹³С_BC compositions. In the JRE and adjacent WTS, the concentrations of BC_TOR (0.77 to 3.79 mg g⁻¹) were higher than those of BC_CTO (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 δ¹³С_TOC and δ¹³С_BC 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 δ¹³С_BC 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.