Long-range transport of terrestrial particulate organic carbon to the open ocean by sediment resuspension revealed by δ13C and 234Th tracers

The transport of particulate organic carbon (POC) from land to deep-sea sediments is a key component of the global carbon cycle. However, the magnitude and mechanisms of terrestrial POC transport on continental shelves remain poorly understood due to the complexity of these systems. In this study, we investigated the vertical fluxes and fates of terrestrial versus marine POC using stable carbon isotope ratios (δ¹³C) and ²³⁴Th tracers in the southern coastal region of Korea. The total suspended matter concentrations were highest in the bottom layer, while the POC concentrations were higher in both the surface and bottom layers. Based on δ¹³C values, terrestrial POC accounted for 29 ± 24% of the total POC, with higher contributions at the innermost stations and in the bottom layer, while the contributions of marine POC were only higher in the surface layer. Based on ²³⁴Th-²³⁸U disequilibria, residence times of particulate ²³⁴Th (10 ± 6 days) were calculated to be significantly longer than those of dissolved ²³⁴Th (3.8 ± 2.3 days). Much higher vertical fluxes of terrestrial POC in the deeper layers than in the upper layers suggest that terrestrial POC undergoes multiple cycles of turnover through resuspension before burial, while marine POC undergoes preferential degradation during sinking. Our findings highlight that resuspension processes in coastal margins and the refractory nature of terrestrial POC likely facilitate its long-range transport (> 200 km) to the deep Ulleung Basin of the East/Japan Sea.