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

There are very few data available of U in marine sediment cores. In this study we present the results from the first oceanic depth profile of 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 Cs, Pu, Pu, I and C and natural Pb, K and Ra. The measured U concentrations decrease from about 90·10 at g at the seafloor down to 0.5·10 at g 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 2700·10 to 7500·10 at g). U/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 1·10 at m for U, 80 Bq m for Cs, 45 Bq m for Pu and 2.6·10 at m for I. Atomic ratios for U/Pu, Cs/U and I/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 U, Cs, Pu and 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.