Datierung von Ostseesedimenten mit²¹⁰Pb

Storm Disturbance of Sediment Contaminants at a Hot-Spot in the Baltic Sea Assessed by 234Th Radionuclide Tracer Profiles

“…Instead of using a "constant rate supply" approach for a first approximation of the source function, "constant initial activity" ain of an instantaneous tracer input pulse (e.g., by the excess- 234 Th-laden SPM admixture) was assumed for the layers above the effective mixing depth Mm. At this depth, the excess 234 Th activity am is constant due to sediment mixture at time scales much faster than decay (with 234 Th decay constant λ), and related to the input 234 Th activity ain by Below that depth Mm, the excess tracer activity aex exponentially decreases with depth Mi according to (13,22) The fit curves of both equations depicted in Figure 3 matched well the measured tracer profiles. A quite extreme sedimentation rate S derived from such fitting is meaningless because of the relatively high decay rate λ of 234 Th (S 2 , 4D*λ).…”

“…This flux is less than the dry bulk density of a 1 cm wet sediment slice. It therefore corresponds to only a few millimeters of sediment depth, on the order of what has been found for the yearly net linear sediment accumulation rate at these sites (13), and also on the order of what has been found recently for transport bottoms in a freshwater environment using the particlereactive tracer 7 Be instead of 234 Th (25). The activity of the former tracer does not depend on salinity because of atmospheric processes as the source term, and its use may therefore become more appropriate at freshwater sites, or those of strong salinity gradients such as in estuaries (26).…”

“…For this natural attenuation effect by sediment capping, an average sedimentation rate of 2 mm yr -1 can be estimated, which yields a sediment dry mass accumulation rate of about 60 mg cm -2 yr -1 (2). This is on the order of the long-term linear net sedimentation rate determined from 210 Pb radionuclide dating (13).…”

“…Up to 7 m of mud is accumulated below the 20 ( 2 m isobath, which includes over half of the bay (1400 out of 2500 km 2 , Figure 1), the remainder being fineto coarse-grained sand. Net sediment accumulation rates of 30-65 mg cm -2 yr -1 (dry weight) were reported for mud sampling sites in Mecklenburg Bay on the basis of 210 Pb radionuclide dating of (albeit severely disturbed) sediment cores (13).…”