Occurrence, spatiotemporal distribution and environmental behavior of dissolved, particulate, and sedimentary black carbon in a subtropical coastal environment: Differences between semi-enclosed and open estuaries.

Black carbon (BC) plays a critical role in climate change and the carbon cycle as a long-term carbon sink. Estuaries, as key interfaces between land and ocean, are important not only for BC transport but also for its transformation and deposition. This study used the benzene polycarboxylic acid (BPCA) method to quantify BC concentrations in the surface waters and sediments of semi-enclosed and open estuaries in the North Beibu Gulf, South China Sea, across different seasons. Results showed that hydrological factors, such as urban riverine input, particle settling, and sediment resuspension, significantly influence BC dynamics. Dissolved BC (DBC) concentrations were highest in summer (125.5 ± 67.3 μg/L), driven by riverine input, while particulate BC (PBC) concentrations decreased rapidly along the river-estuary-coastal gradient (4.1-187.7 μg/L) due to particle settling. In winter, reduced riverine input increased the role of sediment resuspension, elevating PBC concentrations in the open estuary compared to the semi-enclosed estuary. Seasonal variations in BPCA composition showed differences in BC aromatic condensation, influenced by photodegradation, adsorption, and resuspension. Sedimentary BC (SBC) levels were stable (0.087 ± 0.048%dw), accounting for 14.0 ± 4.21 % of sedimentary organic carbon (SOC), which is a higher proportion compared to the contributions of DBC to dissolved organic carbon (DOC) (5.58 ± 2.74 %) and PBC to particulate organic carbon (POC) (6.81 ± 2.94 %). The aggregation of PBC and SBC was higher than that of DBC, emphasizing the sequestration potential of sedimentary environments. Semi-enclosed bays, less influenced by tides and waves, may provide better long-term BC storage, supporting coastal carbon sequestration.