Seasonally varied cross-shelf transport of sedimentary particles in the river-dominated margin sea elucidated by radionuclides

The fate of riverine sedimentary organic carbon and nitrogen, whether buried in the coast or exported to the deep sea, is of considerable interest because the relocated pool of carbon and nitrogen has a bearing on the phytoplankton evolution and climate change. To investigate sediment export fluxes to the East China Sea (ECS), six cruises were conducted between 2011 and 2013. 162 surface sediments were collected and analyzed for 7Be, 137Cs, and excess 210Pb (210Pb_ex) to investigate the cross-shelf transport of sedimentary particles in a river-dominated margin, the ECS. Distribution of these radionuclides exhibited a clear seasonal variation corresponding to seasonal sediment transport. From the distributions of 7Be/210Pb_ex and 137Cs/210Pb_ex activity ratios, four pathways (I, II, III, and IV from north to south) can be identified, which existed in different seasons: section I for spring and summer, II for spring, III for all seasons, and IV for spring and summer. Using 210Pb mass balance, the estimated cross-shelf transport particle fluxes (×106 ton yr−1) are: 1.5, 5.0, 28, and 0.32 for sections I, II, III, and IV, respectively, which in total account for 18% of the annual sediment discharge of Changjiang to the ECS in 2011. The cross-shelf (inner to outer) transport of particulate nitrogen and particulate organic carbon is estimated to be 2.8 × 1011 g yr−1 and 2.5 × 1012 g yr−1, respectively, which is comparable to the Changjiang annual input POC. Quantification of the cross-shelf transport provides insights into particle dynamics in the coastal ocean, as well as insights into nutrients and pollutants as sinks in the marginal sea.