Vertical distribution of 137Cs, 210Pb, 226Ra and 239,240Pu in bottom sediments from the Southern Baltic Sea in the years 1998-2000

This paper presents vertical distribution of 137Cs, 226Ra and 239,240Pu in bottom sediments collected from the Southern Baltic Sea in the years 1998-2000. In the southern part of Baltic Sea the highest concentrations of 137Cs and 239,240Pu were observed at the Gulf of Gdansk. Deposition of 137Cs ranged from 1990š402 Bq m-2 in the Bornholm Basin to 3260š820 Bq m-2 in the Gulf of Gdansk. Depositions of 239,240Pu in the Bornholm Basin were 28.1-30.4 Bq m-2 and in Gulf of Gdansk 162-174 Bq m-2. The concentration peaks of long-lived radionuclides, owing to the sedimentation processes, show the maximum fallout period in different sediment layers. The observed differences in distribution of radiocaesium and plutonium along the profile confirm two main sources of contamination - Chernobyl fallout for 137Cs and global fallout in case of 239,240Pu. In chosen core samples from the Gdansk Basin vertical distribution of 210Pb concentrations were determined. Evaluated sedimentation rates based on decrease of unsupported 210Pb ranged in the Gdansk Basin from 1.9 to 2.3 mm year-1. Calculations based on 239,240Pu peaks show sedimentation rate, in the range from 1.6-2.2 mm year-1 for P110 region

Sedimentation rates and dating of bottom sediments in the Southern Baltic Sea region

Sedimentation rates and dating of bottom sediments were estimated in two sampling stations of the Gulf of Gdańsk and in four stations in the open sea area. Estimations were based on vertical distributions of 210Pb, 137Cs and 239,240Pu activity concentrations in sediment core samples taken in 1998–2007. Two dating models based on changes of activity concentrations of 210Pbunsup were used: 1) CF:CS (Constant Flux Constant Sedimentation rate-model) and 2) CRS (Constant Rate of Supply-model). 137Cs and 239,240Pu were applied as time markers. 137Cs originates mostly from the Chernobyl accident in 1986, whereas 239,240Pu comes from the global fallout in 1963. The validation of the 210Pb methods was performed by activity peak of 137Cs and 239,240Pu. Sediment accumulation rate (g·cm–2·y–1) was constant along sediment core. Annually accumulated layer, (mm·y–1) decreased with sediment depth in all the locations. In the Gulf of Gdańsk sedimentation rate in the upper layer was about 3.6 mm·y–1, and it decreased in the deeper layers to about 1.1 mm·y–1. Sedimentation rates in the open sea area were lower than in the gulf region and the lowest was observed in the Bornholm Deep, being about 0.95 mm·y–1 in the upper layer and 0.35 mm·y–1 in the deeper layer. The growth of a 5 cm thick layer took 27–37 years in the Gulf of Gdańsk, and 61–105 years in the open sea area. It is suggested that the mean values obtained from the models would give a most reliable estimation of the sedimentation rates