Sediment Geochronology in Changing Coastal Environments: Potentials and Limitations of the 137CS and 210Pb Methods

G. Kirchner, H. Ehlers

Abstract


This paper describes the combined use of 137CS and 210Pb radiotracers for obtaining information on sedimentation which in a changing coastal environment is subject to erosion, losses of sediments, and deposition of material that had been remobilized before from near-shore sediments in the course of storm surges. Geochronologies were established for sediment cores taken from salt marsh cliffs on the Isle of Sylt (Germany). Concentrations of 137CS and of excess 210Pb were measured y-spectrometrically. Micromorphological analyses of thin-sections showed that sediment mixing in all cores is negligible. An advection model was fitted to the vertical distributions of 137CS. The results indicate that cores taken from sparsely vegetated areas had been denudated prior to sampling. This was confirmed independently by the lack of any Chernobyl-derived cesium in these cores. Taking denudation into account, 210Pb geochronologies are consistent with sediment dating based on the 137CS data. Grain size analyses showed that only some sediment layers include a coarse sand fraction. The dating of these sandy layers coincides with periods of elevated storm surge activities, giving an independent validation of our method. We can conclude that sedimentation rates at the sites studied remained almost constant during the last 50 years. It is shown that capabilities and limitations of the 137Cs and 210Pb methods are complementary and that the combined use of both radiotracers is necessary for establishing reliable geochronologies in coastal environments.


Keywords


Salt marsh geochronology; radiotracers; sedimentation rates; denudation

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