Natural and artificial radionuclides in a marine core. First results of 236U in North Atlantic Ocean sediments

The measured 236U concentrations decrease from about 90·106 at g−1 at the seafloor down to 0.5·106 at g−1 at 6 cm depth. They are several orders of magnitude lower than the reported values for soils from the Northern Hemisphere solely influenced by global fallout (i.e. from 2700·106 to 7500·106 at g−1). 236U/238U atom ratios measured are at least three orders of magnitude above the estimated level for the naturally occurring dissolved uranium. The obtained inventories are 1·1012 at m−2 for 236U, 80 Bq m−2 for 137Cs, 45 Bq m−2 for 239+240Pu and 2.6·1012 at m−2 for 129I. Atomic ratios for 236U/239Pu, 137Cs/236U and 129I/236U, obtained from the inventories are 0.036, 0.11 and 2.5 respectively. Concentration profiles show mobilization probably due to bioturbation from the abundant detritivore holothurian species living at the PAP site sea-floor. The range of 236U, 137Cs, 239+240Pu and 129I values, inventories and ratios of these anthropogenic radionuclides are more similar to the values due to fall-out than values from a contribution from the Nuclear Fuel Reprocessing Plants dispersed to the south-west of the North Atlantic Ocean. However, signs of an additional source are detected and might be associated to the nuclear wastes dumped on the Eastern North Atlantic Ocean.