5 
1 Introduction 
Bottom sediments play an important role in 
radioecological studies of the marine environment 
because a large proportion of radioactive sub 
stances entering the sea is adsorbed overtime 
onto suspended particulate matter and deposited 
in sediments. Generally, this is the final sink for 
most of the organic material produced in the water 
phase, as well as for other particles transported 
by water currents from other sea areas and from 
adjacent terrestrial areas. During their slow set 
tling, the particles tend to bind radionuclides from 
the water phase and carry them to the bottom. 
Under favourable conditions, the deposited par 
ticles form undisturbed laminae in a stratigraphic 
sequence on the seabed, and the bottom sedi 
ments create an archive from which the history 
of the area can be read. Various particle-bound 
substances can be identified as markers of spe 
cific historical events and, with the aid of marker 
horizons, the laminae can be dated. Modern 
dating methods provide reliable time scales in 
which time-dependent changes in the concentra 
tions of radionuclides can be recorded. As an 
example, the radioactive fallout from atmospheric 
nuclear weapons tests in the 1950s and 1960s 
and the accident at the Chernobyl Nuclear Power 
Plant in April 1986 have created useful markers 
in the sediments of many sea areas, especially in 
the Baltic Sea. 
The Baltic Sea offers exceptionally good oppor 
tunities to conduct sedimentological studies 
because the average rate of sedimentation there 
is much higher than in the oceans and in most 
other coastal seas. The anoxic conditions in the 
near-bottom water of the Baltic Proper, and hence 
the lack of benthic animals over large bottom 
areas, result in the formation of an undisturbed 
sedimentary medium, which enables the sampling 
of sediment laminae in an undisturbed strati 
graphic sequence. However, sediment sampling 
is extremely sensitive to errors, which can cause 
substantial differences in results. This fact should 
always be taken into account when considering 
sediment results. 
This report presents the results of a Sediment 
Baseline Study carried out by the HELCOM 
Project Group for Monitoring of Radioactive 
Substances in the Baltic Sea (HELCOM MORS- 
PRO) in 2000-2005. The goal of the study was 
to complement the knowledge and inventories of 
long-lived radionuclides in the seabed of the Baltic 
Sea by providing additional data from so-called 
“white areas” lacking data, and from radionuclides 
that were not measured in earlier surveys. 
Long-lived radionuclides in the seabed of the Baltic Sea