Anthropogenic radionuclides (3 H, 14 C, and 129I) stemmed from nuclear weapons tests were found in 1999 to be very abundant in the surface of the southern Indian Ocean, comparable to those in the subtropical Northwest Pacific Ocean. The observed radionuclide variations with latitude/longitude in the southern Indian Ocean are not due to deposition patterns of global fallout, but due to transport of water masses from the western Pacific through the Indonesian seas, and different water fronts present in the Crozet Basin of the Indian Ocean. High radionuclide concentrations observed in the latitudinal belt of 20-40°S are associated with the Indian Ocean Subtropical Gyre which acts as a reservoir of radionuclides, maintaining their high concentrations on a time scale of several decades. 14 C data documents that the southern Indian Ocean is an important for sink of anthropogenic carbon. The isotopic tracers reveal the evidence of the most intense surface gradients and presence of several water masses in the southern Indian Ocean, which makes the region one of the most dynamic places of the World Ocean.