Reconstruction of the historical deposition environment from 210Pb and 137Cs records at two tidal flats in China

To examine the historical depositional environment with the evolution of vegetation at tidal flats, sediment samples were collected and grain-size distribution, 137Cs and 210Pb characteristics, heavy metals content, stratigraphy and the vegetation information were measured at Linhong and Chongming Dongtan tidal flat in China. Sediment grain size parameters exhibited transitional lines of the unvegetated flat to the vegetated flat. The vertical profiles of the 137Cs and 210Pbex reflected different depositional environment at the tidal flats. The linear/slumps profiles of 210Pbex indicated complex hydrodynamics or rapid changes in sediment supply. The maximum peaks of 137Cs activity were observed in some cores. The sedimentation rates from the 137Cs maximum deposition peaks in 1963–1964 were comparable with the results from the 210Pbex-CRS (Constant Rate of Supply) method. Positive relationships between the average activities of 137Cs, 210Pbex, inventory of 210Pbex and the mean clay composition, suggested that grain size control effect was an important factor to influence the characteristics of 137Cs and 210Pb distribution at the tidal flats. For heavy metals, only Hg, Cd and Cr were moderately polluted at the Linhong tidal flat and the Chongming Dongtan, respectively. Integrating the results above and the stratigraphy, the vegetation introduction historical information, the clearer sedimentation evolution processes of the tidal flats were reconstructed, which is necessary for the coastal zone management and the pollution evaluation.