Differential Nonconservative Behaviors of Dissolved Black Carbon Along a Major River-Estuary-Shelf Continuum

Existing dual-carbon isotopic evidence suggests the nonconservative behaviors of dissolved black carbon (DBC) during its river-to-open ocean transport. Estuaries and shelves, characterized by highly dynamic and complex hydrological conditions, serve as pivotal transitional zones linking rivers and open ocean ecosystems, making them potential hotspots for DBC transfer and transformation. However, the nonconservative behaviors of DBC in the estuarine-shelf regimes remain poorly understood. In this study, we examined and elucidated the possible nonconservative behaviors of DBC and their underlying mechanisms along the globally representative major river Changjiang-Estuary-Shelf Continuum. Our findings reveal that the summertime DBC profiles were primarily shaped by the physical mixing between Changjiang Diluted Water and Taiwan Warm Current, which exhibited distinct DBC concentrations and compositions (as indicated by the molecular markers ratios of B6CA/B5CA). Despite this predominant control, differential nonconservative behaviors of DBC were found in subregions of the continuum. Specifically, DBC removal in the Changjiang River Channel was due to sorption onto resuspended sediments, while within the Changjiang River Estuary, it resulted from a combination of photoaging and Ca²⁺ bridging effects. In contrast, DBC addition in the East China Sea Shelf was driven by benthic diffusion, which represents an underappreciated yet significant DBC source to the coastal waters. Insights into the nonconservative behaviors of DBC within such a major river-estuary-shelf continuum enable a more robust assessment of the crucial roles of the relatively slow-cycling DBC component as a negative feedback mechanism in the global carbon cycle.