Heterogeneous Sources, Distribution, and removal processes of dissolved black carbon from East China Sea shelf to open ocean of Northwest Pacific

Absract: Dissolved black carbon (DBC) can be transported from land to the ocean, representing a significant slowcycling component in the global carbon cycle. However, the source, distribution, and degradation of DBC in the ocean are not well understood. In this study, water samples were collected from the East China Sea (ECS) shelf to the Northwest Pacific Ocean. The benzene polycarboxylic acid (BPCA) method and ultraviolet-visible spectroscopy were used to analyze DBC and dissolved organic matter, respectively. The concentration of DBC and the (B6CA + B5CA)/BPCAs ratio in surface water showed a decreasing trend from the ECS inner shelf towards the open ocean. This trend indicates the preferential removal of condensed aromatic compounds by photodegradation, while also implicating dilution by open ocean water as a significant factor contributing to the reduction of DBC concentrations. A significant negative correlation between DBC concentration and salinity was observed in surface water samples (r = - 0.88; p < 0.01). However, this relationship weakened and disappeared in offshore samples, suggesting a reduced influence of riverine inputs. Furthermore, vertical profiles of water samples (0 - 1000 m) revealed a decrease in DBC concentrations and an increase in the ratio of (B6CA + B5CA)/ BPCAs, indicating the preferential photodegradation of condensed aromatic DBC molecules in the euphotic zone and the selective accumulation of these molecules in the ocean interior. The DBC flux into the Northwest Pacific Ocean was estimated to be 1.46 Tg/year with 51.7 to 73.1 % removed through photodegradation, while the remaining DBC was either transported into the deep ocean or biodegraded. Given the important role of photodegradation in regulating the DBC flux in the Northwest Pacific Ocean, we recommend further research into its impact on the refractory carbon cycle in global oceans.