The fate of terrestrial particulate organic carbon in the northwestern Pacific marginal seas

Rivers annually transport ~200 Tg of particulate organic carbon (POC) to the global ocean, of which ~30% is known to be buried in continental-shelf sediments, including deltas. Given that ~35% of riverine POC is considered labile and may undergo remineralization in marine environments, the fate of the remaining “missing” terrestrial POC remains unknown. In this study, we investigated the distribution, fluxes, and fate of terrestrial POC in the northwestern Pacific marginal seas, including the East China Sea shelf and East/Japan Sea. These regions provide an ideal setting to address critical knowledge gaps regarding the fate of terrestrial POC, given substantial terrestrial inputs and extensive shelf areas connected to a semi-enclosed deep sea. Using stable carbon isotope ratios (δ¹³C) and ²³⁴Th tracers, we estimate that ~2.7 Tg C yr^-1 of terrestrial POC is transported to the deep sea of the East/Japan Sea, accounting for ~80% of terrestrial POC in these regions. Our results suggest that sediment resuspension on the continental shelf and the refractory nature of terrestrial POC facilitate its effective transport to the deep sea, which serves as its major sink. These findings offer valuable insights into the global carbon cycle across the land-to-ocean continuum.