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

There are very few data available of 236 U in marine sediment cores. In this study we present the results from the first oceanic depth profile of 236 U in a sediment core sampled in the North Atlantic Ocean, at the PAP site (4500 m depth, Porcupine Abyssal Plain (PAP) site, 49°0' N, 16°30' W). Additionally, the sediment core was radiologically characterized through the measurement of anthropogenic 137 Cs, 239 Pu, 240 Pu, 129 I and 14 C and natural 210 Pb, 40 K and 226 Ra. The measured 236 U 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 ). 236 U/238 U 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 236 U, 80 Bq m-2 for 137 Cs, 45 Bq m-2 for 239+240 Pu and 2.6·1012  at m-2 for 129 I. Atomic ratios for 236 U/239 Pu, 137 Cs/236 U and 129 I/236 U, 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 236 U, 137 Cs, 239+240 Pu and 129 I 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.