Physical dynamics modulate deep‐sea carbon flux in the western Pacific marginal sea

Abstract Deep‐sea particulate organic carbon (POC) flux sustains deep‐sea ecosystems and contributes significantly to long‐term carbon sequestration. However, our understanding of its magnitude and governing mechanisms remains poorly constrained due to the limitations of traditional observational approaches. Using a biogeochemical profiling float equipped with a transmissometer, we present nearly 3 yr of high‐frequency observations of POC flux at 1000 m depth in the western Pacific marginal sea and examine its responses to upper‐ocean physical dynamics. The annual mean POC flux is 0.2 ± 0.1 mol C m −2 yr −1 , representing approximately 6% of carbon exported from the euphotic zone. A pronounced seasonal increase in deep POC flux occurs from July to September each year, driven primarily by upwelling‐enhanced biological productivity within seasonal cyclonic eddies. Variability in deep flux is largely governed by large particles, which exhibit shorter lags relative to surface carbon export (15 ± 4 d) compared with small particles (54 ± 6 d). The float also captures a coastal jet event that markedly enhances deep POC flux, highlighting the importance of cross‐shelf transport from continental margins in regulating deep carbon supply. Together, these results reveal the highly dynamic, process‐driven nature of deep carbon flux in low‐latitude marginal seas, and further demonstrate the potential of biogeochemical floats to resolve upper–deep ocean coupling and improve constraints on regional carbon budgets.