Evidence from molecular marker and FT-ICR-MS analyses for the source and transport of dissolved black carbon under variable water discharge of a subtropical Estuary

Dissolved black carbon (DBC) is the largest known refractory organic carbon pool in the ocean. However, processes in estuaries connecting rivers (a significant DBC source) and oceans are underexplored. We carried out two sampling campaigns in the Jiulong River estuary (JRE) and conducted a dissolved organic matter (DOM) mixing experiment. DBC was quantified by the benzenepolycarboxylic acid (noted as DBC-BPCA) method and characterized with Fourier transform ion cyclotron resonance mass spectrometry (noted as DBC-FT). The molecular formulae of DBC-FT in the JRE were also compared with that in soil and atmospheric deposition. Overall, DBC-BPCA was removed during seaward dispersion. Soil- and deposition-derived molecules were identified in the DBC-FT of the JRE, confirming deposition inputs to estuarine DBC for the first time. The salinity-dependent decrease in the intensity normalized modified aromatic index (AI_mod) of DBC-FT and in DBC-BPCA revealed that DBC composition and concentration were primarily controlled by mixing. However, photodegradation and particle adsorption/flocculation may play a role in removing DBC depending on the concentration of total suspended matter (TSM). The two campaigns showed that high discharge increased both riverine input and estuarine output of DBC. However, the removal patterns of DBC-BPCA were minimally influenced by discharge. We further estimated that approximately 20% DBC-BPCA was lost in this estuary annually. Future studies should investigate the transformation of DBC in coastal bays and the open ocean where the residence time is longer and the transparency of bulk water is higher.