SEDIMENT GEOCHRONOLOGY IN CHANGING COASTAL ENVIRONMENTS - POTENTIALS AND LIMITATIONS OF THE CS-137 AND PB-210 METHODS

This paper describes the combined use of Cs-137 and Pb-210 radiotracer s for obtaining information on sedimentation which in a changing coast al environment is subject to erosion, losses of sediments, and deposit ion of material that had been remobilized before from near-shore sedim ents in the course of storm surges. Geochronologies were established f or sediment cores taken from salt marsh cliffs on the Isle of Sylt (Ge rmany). Concentrations of Cs-137 and of excess Pb-210 were measured ga mma-spectrometrically. Micromorphological analyses of thin-sections sh owed that sediment mixing in all cores is negligible. An advection mod el was fitted to the vertical distributions of Cs-137. The results ind icate that cores taken from sparsely vegetated areas had been denudate d prior to sampling. This was confirmed independently by the lack of a ny Chernobyl-derived cesium in these cores. Taking denudation into acc ount, Pb-210 geochronologies are consistent with sediment dating based on the Cs-137 data. Grain size analyses showed that only some sedimen t layers include a coarse sand fraction. The dating of these sandy lay ers coincides with periods of elevated storm surge activities, giving an independent validation of our method. We can conclude that sediment ation rates at the sites studied remained almost constant during the l ast 50 years. It is shown that capabilities and limitations of the Cs- 137 and Pb-210 methods are complementary and that the combined use of both radiotracers is necessary for establishing reliable geochronologi es in coastal environments.