Marginal seas as potential sinks for refractory carbon

Abstract The Yellow Sea (YS) and East China Sea (ECS) are marginal seas in the Northwestern Pacific that receive large amounts of aged, terrestrial organic matter. In this study, we measured dissolved organic carbon (DOC) concentrations and radiocarbon contents (Δ 14 C) in these seas during summer and autumn, extending a previous winter study to provide a more comprehensive understanding of the DOC cycle, including its sources and removal. The significant negative correlation between DOC concentrations or Δ 14 C values and salinity shows that vertical and horizontal water mass mixing between coastal waters and the water intruding to the site from the Northwestern Pacific is the primary control on the distribution of DOC. The Δ 14 C values and the inverse of DOC concentrations show significant negative correlation, suggesting that marine primary production is the dominant DOC source in this region. However, deviations from this correlation imply inputs of aged DOC. Although freshwater input is highest in summer, the effects of aged DOC are greater in autumn and winter. Terrestrial organic matter delivered by rivers is rapidly degraded, and this process likely stimulates marine primary production. In addition, large seasonal differences in Δ 14 C values in Kuroshio‐derived waters indicate significant removal of marine refractory DOC on the continental shelf. The results show that continental shelves have a key role in the removal of terrestrial and marine refractory DOC.