Deciphering sources and processing of dissolved black carbon in coastal seas

Abstract

Abstract 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 (DBC BPCA ) and ultra‐high resolution mass spectrometry (DBC FT ), respectively. The concentration of DBC BPCA was 0.70–1.9 μ mol 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 DBC FT . 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 DBC BPCA in the study region. Additionally, atmospheric deposition and potentially other unidentified sources contributed at least ~ 15% DBC BPCA , assuming no removal. Furthermore, ~ 70% of DBC FT in the coastal seawater could be found in the river water, and aerosols, which was in line with the information from DBC BPCA . 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.