Deciphering sources and processing of dissolved black carbon in coastal seas

Dissolved black carbon (DBC) is the largest known slow‐cycling organic carbon pool in the ocean; yet, its sources and processing in the coastal seas remain poorly understood. To address this knowledge gap, we conducted a study in the East China Sea and South Yellow Sea to quantify and molecularly characterize DBC using benzene polycarboxylic acids (DBCBPCA) and ultra‐high resolution mass spectrometry (DBCFT), respectively. The concentration of DBCBPCA was 0.70–1.9 μmol C L−1 and exhibited a significant negative correlation with salinity. Significant correlations were also observed between salinity and molecular characters (oxygen‐to‐carbon ratio; double bond equivalent; molecular weight) of DBCFT. These findings collectively suggest that mixing processes primarily control the concentration and composition of DBC. A two‐end‐member mixing model revealed that riverine input and offshore water contributed ~ 22% and 63%, respectively, to the DBCBPCA in the study region. Additionally, atmospheric deposition and potentially other unidentified sources contributed at least ~ 15% DBCBPCA, assuming no removal. Furthermore, ~ 70% of DBCFT in the coastal seawater could be found in the river water, and aerosols, which was in line with the information from DBCBPCA. Our study provides the first comprehensive assessment of both the sources and influential processes of DBC in coastal seas, highlighting the importance of additional sources, such as atmospheric deposition, and emphasizing the need to identify previously unknown sources of DBC.