The division of 137Cs vertical profile types on coastal area and its implications

As an anthropogenic radionuclide, the 137Cs tracer dating method has been widely used in coastal areas, and has served as an important means for reconstructing modern geological processes. Based on an analysis of the shape of the 137Cs vertical profiles and a comparison with ideal 137Cs vertical profiles, the authors collected more than 200 137Cs vertical profiles, which included 121 stations in China's coastal areas, and more than 100 stations in other areas of the world. The results show that there exist six main types, i.e., (1) discontinuous 137Cs curves, (2) continuous 137Cs curve without characteristic peak, (3) characteristic peak on the lower part of the continuous 137Cs curve, (4) characteristic peak on the upper part of the continuous 137Cs curve, (5) increasing activity of 137Cs with depth, and (6) decreasing activity of 137Cs with depth. The main 137Cs vertical profiles reflect a variety of sedimentary environments and imply that the main factors controlling the profiles of 137Cs are erosion, mixing and sediments supply; The key points of the application of 137Cs are undisturbed sampling and the consideration of the influence of sampling resolution on the interpretation of the data. In the coastal area without atmosphere fallout monitoring, the profile in a relative stable environment could serve as a background reference for tidal flat and shallow sea area.