Long‐range transport of terrestrial particulate organic carbon to the open ocean by sediment resuspension

Abstract

The transport of particulate organic carbon (POC) from land to deep‐sea sediments is a critical component of the global carbon cycle. However, the transport processes of terrestrial POC across continental shelves remain poorly understood due to the complexity of these systems. In this study, we investigated the vertical fluxes and fates of terrestrial vs. marine POC using stable carbon isotope ratios (δ13C) and 234Th tracers along two transects in the southern coastal region of Korea. The POC concentrations were highest in the surface layer and decreased with depth, with a slight increase near the seafloor. The δ13C values revealed that terrestrial POC contributed 29% ± 24% of the total POC, with higher contributions at the innermost stations and in the bottom layer. Based on 234Th–238U disequilibria, the residence times of particulate 234Th (8.1 ± 3.6 d) were significantly longer than those of dissolved 234Th (3.7 ± 2.2 d). The much higher vertical fluxes of terrestrial POC in the deeper layer relative to the upper layer suggest that terrestrial POC undergoes multiple turnover cycles through sediment resuspension before burial, while marine POC degrades preferentially in the course of settling. These findings highlight that effective sediment resuspension and the refractory nature of terrestrial POC allow for its long‐range transport (> 200 km) to the deep Ulleung Basin in the East Sea (Japan Sea). This study sheds new light on the mechanisms driving the transport of terrestrial POC from coastal regions to the deep ocean.