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A baseline study on anthropogenic radioactivity in the Namibian marine ecosystem, which is part of the northern Benguela upwelling system, known as one of the most productive ocean areas in the world, has been performed. A scientific cruise carried out in 2014 covering inshore and offshore areas, exhibiting different oceanographic features, has provided a basis for better understanding the distributions, ratios and inventories of six anthropogenic radionuclides (Sr, Cs, Pu, Pu, Pu and Am) in seawater. Although H was also measured, due to extremely low levels, its behaviour was not studied. The main source of Sr, Cs, Pu, Pu and Am in the samples analysed was proven to be global fallout, a finding further confirmed by Pu/Pu and Sr/Cs ratios. Furthermore, the Pu SNAP-9A satellite accident signal was confirmed once again through the determination of the Pu/Pu activity ratio. Inshore and offshore samples showed different patterns due to the unique oceanographic features of this upwelling system. The levels of anthropogenic radionuclides, comprehensively assessed for the first time in this region, are comparable with the few existing data and filled a critical gap for the Southern Atlantic Ocean.

In the last years, 236U (T1/2=2.34·107 y), which is essentially an anthropogenic radionuclide, has been established as a new oceanographic tracer thanks to its conservative behaviour in seawater. At the Centro Nacional de Aceleradores (CNA), in Seville (Spain), it has been recently demonstrated that 236U can be determined at environmental levels on the 1 MV Accelerator Mass Spectrometry (AMS) system. In the framework of the existing collaboration between the National Marine Information and Research Centre (NatMIRC), in Namibia, and the IAEA Environment Laboratories in Monaco, 236U has been analysed by AMS at CNA in a set of seawater samples collected in the northern Benguela upwelling system, on the Namibian coast. In surface samples, 236U/238U atom ratios at the level of 10-10 have been obtained, in agreement with the expected values for the so-called global fallout. The 236U inventories reported up to now in seawater are discussed.

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 121 (2010): 206-214, doi:10.1016/j.marchem.2010.04.009., Precise measurements of the short lived radium isotopes 223Ra and 224Ra by means of the delayed coincidence counting system (RaDeCC) rely on an efficiency calibration of this system using Mn-fiber standards for which radium activities are exactly known. We prepared seventeen different standards by placing Mn-fibers in seawater spiked with various amounts of 227Ac (with 223Ra in radioactive equilibrium), 228Th (in radioactive equilibrium with 232Th and 224Ra) and 226Ra. We tested for quantitative adsorption of 227Ac and 228Th on the Mn-fibers by: (1) measuring 227Ac and 232Th in the residual solutions after preparing the Mn-fiber standards and (2) monitoring their 223Ra and 224Ra activities over a period of ~100 days. In the residual solutions, the activities of 227Ac and 232Th were < 1.0 % and < 5.3 %, respectively, of the activities initially added to the Mn-fibers. Our results indicate that Milli-Q water washing of the Mn-fibers is the major source of our observed losses of thorium. Measurements of 227Ac standards over 1½ years indicate a significant decrease of measurable 223Ra with time prohibiting the long-term use of 227Ac Mn-fiber standards. We found the 224Ra efficiency to be independent of the range of 227Ac, 228Th and 226Ra activities on the Mn-fibers standards used. The efficiency determination for 223Ra, however, may be biased in the case of relatively high 224Ra activities due to insufficient correction of chance of coincidence. Thus we suggest using a single 227Ac Mn-fiber standard for the efficiency determination for 223Ra., M. Charette and H. Dulaiova were supported by a grant from the National Science Foundation (OCE- 0751461).