Normalization of excess Pb-210 with grain size in the sediment cores from the Yangtze River Estuary and adjacent areas: Implications for sedimentary processes

The fallout radionuclide Pb-210 has been widely used as an environment tracer for estimating deposition rates and revealing sedimentary processes. However, a comprehensive evaluation of different grain size fractions effects on the adsorption of Pb-210 is a prerequisite for effective utilization of the tracer. In this paper, 11 cores were collected from different sub-sedimentary environments in the Yangtze Estuary and its adjacent area, x-radiographs of the sediment cores were taken, and the grain size and radioactive Pb-210 levels were analyzed systematically; the relationship between grain size and Pb-210 was discussed in detail. The results indicated that Ra-226 was relatively stable, with no systematic variation from the Yangtze Estuary to inner shelf region or with depth and was unaffected by grain size. In contrast to Ra-226, significant positive correlations between the fine fraction (<32 mu m) and Pb-210(ex) were found, particularly between the clay component (<4 mu m) and Pb-210(ex), in all the sediment cores. The Pb-210(ex) profiles were improved significantly after using the linear relationships for quantitatively normalizing the activities to the <4 mu m components, and the calibrating Pb-210(ex) profiles of different zones matched the sub-sedimentary environments of the recent reduction in sediment discharge after Three Gorges Dam (TGD) impoundment very well. Also, through the normalization procedure, it was found that the Yangtze-derived clay component may play the most important role in the distribution of Pb-210(ex) in sediments, especially in upper layer 10-15 cm of the cores. When applying Pb-210 data to establish the geochronology or study sedimentary processes in the East China Sea or similar sedimentary environments, the grain size effect should be fully considered.