Systematic radiometric data on surface sediments and sediments at depth are relatively scarce for the Baltic Sea. This is mainly because focus has been on the water column and the biota and for some reason, little work has been devoted to the sediments. What is really needed are there-fore more and systematic data on sediments to properly decribe and elucidate the pollution situ-ation at present and in the last ca. 150 years. Recently published work on the radiological expo-sure in the Baltic Sea (Nielsen et al., 1995) using a box model suggested that most of the man-made radioactivities are contained in the water and not in the
sediments. However, the model calculations had a tendency to underestimate for instance 137Cs contents in sediments by a factor of about 3.
There is an increasing consensus that material transported to the shallow Sea is not a continuous process but has more the character of being caused by periodic events. Such trans-portation episodes due to changes in bottom current regimes probably triggered by climatic change allow the movement of remote material, e.g., from the North Sea, into the Skagerrak, further into the Kattegat and to the deeper parts of the Baltic Sea (Bornholm Basin and Gotland Basin). The Kattegat area probably acts as a natural barrier preventing fine particle transport further into the Baltic Sea
We present here some data from several areas of the semi-enclosed large estuary, the Baltic Sea, and its westernmost entrance (Christiansen and Kunzendorf, 1995; Kunzendorf, 1995). They include values of 210Pb (for dating purposes) and of anthropogenic radioisoptes, i.e. systematic data on sediment cores surveyed with depth. The main emphasis of the contribution lies in the outlining of the importance to conduct such studies before calculation of chemical fluxes in the sediments.
Fourteen cores from the Norwegian Channel, the Skagerrak, the Kattegat, the Bornholm and Gotland basins were dated by the 210Pb method using non-destructive low-energy gamma-spectrometry. Average sedimentation rates were found to vary greatly, between 0.5 and 9 mm per year. Using a constant rate of supply model for 210Pb, depth-age curves were calculated for the cores. Data for anthropogenic 137Cs, measured at the same time in the sediment cores, were then plotted against depth, i.e. time.
Maximum 137Cs activities in the sediment column may be ascribed to the time span 1970-1985 in all the cores from the westernmost part of the investigated area. This is exactly the time span where maximum Sellafield releases were observed. It should be mentioned, that the releases of material from the La Hague and the Dounreay facilities are regarded negligible compared to those from Sellafield. In general, the Cs vs. sediment depth plots suggest that the Sellafield component in the Skagerrak and Kattegat sediments is a significant factor. In most of the cores, the nuclear bomb testing component with a maximum in 1963, is also expressed, although only through weakly elevated 137Cs activities. The dominating activity in the cores of the Kattegat, Bornholm and Gotland Basin, however, is due to the Chernobyl event in 1986. The results of the study also suggest that a possible transport component from the central North Sea to the Norwegian Channel leading eventually to elevated activities is practically absent.
The present investigation suggests that the depth distribution of man-made 137Cs in the sediments is an important help when elucidating sedimentation history calculations. The previ-ously favoured monitoring of this isotope in a thin sediment surface layer is of little value because of the greatly varying sedimentation rates in the marine environment.
Christiansen, C. & Kunzendorf, H., GOBEX-Newsletter, 12/94-01/95, 6-10 (1995).
Kunzendorf, H., 1.Nordic Marine Sciences Meeting, Göteborg 23-26/3, abstr. (1995).
Nielsen, S.P., Øhlenschlæger, M. & Karlberg, O., Risø-R-819(EN), 54 pp. (1995).