Presence of 236U and 237Np in a marine ecosystem: The northern Benguela Upwelling System, a case study

[Display omitted] •236U and 237Np were studied in Benguela Upwelling System of the South Hemisphere.•Seawater samples collected and analyzed using AMS.•237Np/236U global fallout ratio in seawater was 1.77 ± 0.20.•236U and 237Np concentrations at the 106 atoms kg−1 level.•Different 237Np behavior compared to 236U is observed at inshore stations. The Benguela Upwelling System (BUS), off the south-western African coast, is one of the four major eastern boundary upwelling ecosystems in the oceans. However, this area has been overlooked in the field of environmental radioactivity. In this work, 236U and 237Np were collected off the coast of Namibia within the northern BUS. Surface seawater exhibited similar 236U and 237Np concentrations, ranging from 3.9·106 to 5.6·106 atoms kg−1 and from 4.6·106 to 8.5·106 atoms kg−1, respectively. The observed inventories in a water column from the continental margin, of (2.10 ± 0.11)·1012 atoms m−2 for 236U and (3.48 ± 0.13)·1012 atoms m−2 for 237Np, were in agreement with the global fallout (GF) source term in the Southern Hemisphere that was the main source of actinides to the region. A pattern was observed in the surface samples, with 237Np concentrations that decreased by 25–30% when moving from inshore to offshore stations, but such an effect could not be clearly discerned in the case of 236U within the data uncertainties. An explanation based on the larger particle reactivity of GF 237Np compared to GF 236U was proposed. Such an effect would have been important at the studied site due to the enhance presence of particles in the continental shelf triggered by the upwelling phenomenon. A value of 1.77 ± 0.20 was obtained for the 237Np/236U atom ratio for the GF source term in the marine environment.