Biological carbon pump (BCP) has long been recognized as an essential process of ocean carbon sequestration and storage, and plays a crucial role in regulating the global carbon cycle, and the climatic system at large. The downward export flux of particulate organic carbon (POC) transported to the interior of the ocean is an important index for  evaluating the efficiency of BCP and one of the core parameters of the marine carbon cycle. However, constraining the POC export fluxes in the global ocean remains a daunting challenge. This study examines spatial patterns of particle size distributions (PSDs) and carbon flux within the upper 1000 m of the oligotrophic North Pacific Ocean based on in situ Underwater Vision Profiler 5 (UVP5) measurements and simultaneous ²³⁴Th samplings. We established an empirical relationship between particle size structure and POC flux applicable to the Northwest Pacific Ocean and obtain carbon flux results with high vertical resolution. The POC export fluxes averaged at 3.20±2.54 mmol C m⁻² d⁻¹ in Kuroshio Extension (KE) are significantly higher than the fluxes in North Pacific Subtropical Gyre (NPSG, averaged at 1.22±0.58 mmol C m⁻² d⁻¹). Furthermore, our results revealed a positive correlation between POC export fluxes at the base of the euphotic zone and the attenuation rate of POC fluxes in the twilight zone, with higher Marin curve b values (averaged at 0.44±0.10) in the KE than those in lower latitudes. By analyzing the variation of the attenuation rate in the twilight zone, we concluded that the vertical structure of the b value was highly corresponding to the PSD slope, with high b values generally characterized by flatter PSD slope, that is, a high relative abundance of large versus small. This suggests that the particle size distribution profoundly affects the POC export and remineralization process.

north pacific ocean,Biological carbon pump,particulate organic carbon export