Environmental evolution records reflected by radionuclides in the sediment of coastal wetlands: A case study in the Yellow River Estuary wetland

Vertical profiles of environmental radionuclides (²¹⁰Pb, ¹³⁷Cs, ²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K) in a sediment core (Y1) of the Yellow River Estuary wetland were investigated to assess whether environmental evolutions in the coastal wetland could be recorded by the distributions of radionuclides. Based on ²¹⁰Pb and ¹³⁷Cs dating, the average sedimentation rate of core Y1 was estimated to be 1.0 cm y^-1. Vertical distributions of natural radionuclides (²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K) changed dramatically, reflecting great changes in sediment input. Concentrations of ²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K all had significant positive relationships with organic matter and clay content, but their distributions were determined by different factors. Factor analysis showed that ²³⁸U was determined by the river sediment input while ²²⁶Ra was mainly affected by the seawater erosion. Environmental changes such as river channel migrations and sediment discharge variations could always cause changes in the concentrations of radionuclides. High concentrations of ²³⁸U and ²²⁶Ra were consistent with high accretion rate. Frequent seawater intrusion decreased the concentration of ²²⁶Ra significantly. The value of ²³⁸U/²²⁶Ra tended to be higher when the sedimentation rate was low and tide intrusion was frequent. In summary, environmental evolutions in the estuary coastal wetland could be recorded by the vertical profiles of natural radionuclides.