Sources and dynamics of dissolved black carbon in the Pearl River Estuary and Shelf, Northern South China Sea

Estuarine processes regulate the transport of dissolved black carbon (DBC) and associated contaminants to the ocean. However, there is limited understanding of the geochemical behavior of DBC in estuaries. In this study, DBC in the Pearl River Estuary (PRE) and the northern shelf of the South China Sea were examined using the benzene polycarboxylic acid (BPCA) method. DBC, bulk dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM) exhibited distinct behaviors during their transport from the PRE to the sea. DOC and CDOM decreased during the initial mixing of river water and seawater but increased at the lower PRE. In contrast, high aromaticity DBC inputs were observed throughout the PRE, likely originating from local terrestrial sources, such as runoff from nearby islands, as indicated by the high R-H/L values (i.e., the ratio of BPCA containing 5 and 6 carboxyl groups to that containing 3 and 4 carboxyl groups; 2.03-2.30). In the Pearl River-plume zone (salinity < 33.0), DOC, CDOM, and DBC showed quasi-conservative behaviors against salinity, indicating that their geochemical behaviors were primarily governed by physical mixing between plume water and seawater. Using a flux model, it was estimated that the discharge of riverine DBC from the Pearl River Delta ranged from 11.2 to 16.3 Gg year(-1), representing an important source of bio-resistant DOC to the northern South China Sea.