Variation in sedimentary 210Pb over the last 60 years in the Yangtze River Estuary: New insight to the sedimentary processes

Abstract

Anthropogenic activities have decreased the sediment transport from catchment to the river-dominated continental margin mainly through river damming that controlled the estuarine and associated shelf modern sedimentation. In this study, radioisotopes 210Pb (210Pbex and 226Ra) and 137Cs, grain size, and X-radiograph were combined to evaluate the modern sedimentary processes in the Yangtze River Estuary (YRE). We found that seabed cores collected on the estuary and associated shelf show a direct temporal correlation between decreases in initial activity and burial flux of 210Pbex and suspended sediment load in relation to river damming and soil conservation from 1950 to 2016. Most of sediment cores are characterized by low, uniform and reverse 210Pb patterns at surface and negligible sediment accumulation rates (SARs), indicating that due to the limited sediment supply following the construction of the Three Gorges Dam in 2003, erosion and remobilization of fine-grained sediments by energetic tidal currents and biological mixing tends to have become determinant forces in controlling the fate of modern sediment burial in the YRE. Moreover, these results indicated that decline in sediment load and additional input of eroded materials will largely violate the basic assumption of constant 210Pb flux, raising a substantial challenge for 210Pb chronology in the river-dominated continental margins. However, we suggest that radiotracer 210Pb could provide critical information for understanding the complex the sedimentary processes (e.g., terrestrial sediment supply and redistribution by tidal currents) and assessing past environmental status on the river- and/or tidal-dominated estuaries subject to human modification.