Dissolved black carbon (DBC) represents the largest molecularly identifiable slow-cycling organic carbon pool in the ocean. However, its behavior remains debated due to large differences between its radiocarbon ages and residence times based on mass balance estimations, suggesting considerable removal in the source regions. Here, we show that DBC is predominantly derived from riverine sources and behaves conservatively throughout the entire water masses of the northwestern Pacific marginal seas—including the East China Sea, the Yellow Sea, and the East Sea (Japan Sea)—which are characterized by extreme biogeochemical alterations and long water residence times (~100 years). This conservative behavior is evidenced by a strong negative correlation between DBC and salinity, consistent mass balance estimates, and a uniform B6CA/B5CA marker ratio. Thus, we suggest that the discrepancy between DBC ages and residence times in the ocean is more likely due to substantially enhanced contemporary DBC production rather than removal pathways.