Anthropogenic radionuclides in Indian Ocean surface waters—the Indian Ocean transect 1998

Results of the analyses of 3H, 90Sr, 137Cs, 239,240Pu and 241Am in surface-water samples collected during the 1998 Indian Ocean Transect cruise from New Zealand to Italy are reported and discussed. Latitudinal variations in the concentrations of these radionuclides still can be seen over 40 yr after the main input, but the observed distribution no longer corresponds to the atmospheric deposition in the study area. Low levels of conservative radionuclides characterise the Tasman Sea and the South Australian Basin. At about 30°S the concentrations increase and remain very similar until about 10°N. A slight decrease is observed in the Gulf of Aden and in the Red Sea. The distribution of transuranic elements shows marked differences with respect to that of conservative radionuclides, particularly in the Tasman Sea and in the south-eastern Indian Ocean. The observed latitudinal trend is mainly dependent on the surface oceanic circulation for conservative radionuclides, while for the non-conservative transuranic elements it is also related to primary productivity and enhanced scavenging. l investigated radionuclides, the Mediterranean is the area with the highest concentrations. The observed radionuclide concentrations in addition to 137Cs/3H, 90Sr/3H, 137Cs/90Sr, and 241Am/239,240Pu activity ratios confirm the global origin of the radionuclides studied in surface waters, except for the area below 30°S, where the average 238Pu/239,240Pu ratios (0.20±0.04) suggest the possible influence of the SNAP Satellite that burned-up over the Indian Ocean in 1964.