5

1  Introduction

Bottom  sediments  play  an  important  role  in
radioecological  studies  of  the  marine  environment
because  a  large  proportion  of  radioactive  substances ­
  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  settling, ­
  the  particles  tend  to  bind  radionuclides  from
the  water  phase  and  carry  them  to  the  bottom.
Under  favourable  conditions,  the  deposited  particles ­
  form  undisturbed  laminae  in  a  stratigraphic
sequence  on  the  seabed,  and  the  bottom  sediments ­
  create  an  archive  from  which  the  history
of  the  area  can  be  read.  Various  particle-bound
substances  can  be  identified  as  markers  of  specific ­
  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  concentrations ­
  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  opportunities ­
  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  stratigraphic ­
  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