Abstract:
A narrow sandy-muddy transitional zone exists offshore the Yangtze River estuary, separating the muddy subaqueous delta from the sandy East China Sea shelf, which is highly sensitive to the changes in fluvial sediment supply and marine hydrodynamic dynamics. We investigated decadal-scale changes in surface sediment grain size offshore the estuary and examined two sediment cores in YE16 site collected in two consecutive years from the transitional zone, focusing on grain size, elemental ratios, organic compositions, and radionuclides. Results reveal a recent landward migration of the outer boundary of the transitional zone, obvious surface sediment coarsening, and rapid thickening of sandy layers overlying muddy deposits in YE16 site. Grain-size end-member (EM) analysis via the grain size-standard deviation method and component partitioning identified a pronounced increase in EM3 (medium-fine sand end-member) in YE16 cores. Sediments dominated by EM3 display lower C/N ratios, elevated Sr/Ba and Ca/Ti ratios, and depleted excess
210Pb and
137Cs activities, indicating older provenance with marine biogenic dominance. Integrated analysis attributes the EM3 increase to resuspension and onshore transport of relict mid-shelf sandy sediments driven by high-energy events (e.g., storm waves). This mechanism of exogenic inputcoupled with drastic reductions in Yangtze-derived sediment, forms a dual driver for coarsening: exogenic sand compensates coarse fractions directly, while diminished fine sediment supply weakens the dilution effects. These combined processes explain recent landward migration, surface coarsening, and sandy layer thickening. Against intensifying extreme climatic events and watershed human activities, ongoing monitoring of the transitional zone’s evolution and environmental impacts is imperative.