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

Abstract

The fallout radionuclide ²¹⁰Pb 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 ²¹⁰Pb 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 ²¹⁰Pb levels were analyzed systematically; the relationship between grain size and ²¹⁰Pb was discussed in detail. The results indicated that ²²⁶Ra 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 ²²⁶Ra, significant positive correlations between the fine fraction (<32 µm) and ²¹⁰Pb_ex were found, particularly between the clay component (<4 µm) and ²¹⁰Pb_ex, in all the sediment cores. The ²¹⁰Pb_ex profiles were improved significantly after using the linear relationships for quantitatively normalizing the activities to the <4 µm components, and the calibrating ²¹⁰Pb_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 ²¹⁰Pb_ex in sediments, especially in upper layer 10–15 cm of the cores. When applying ²¹⁰Pb 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.