Temporal (3H) and spatial variations of 90Sr, 239,240Pu and 241Am in the Arabian Sea: GEOSECS Stations revisited

In order to study the spatial and temporal variations of radionuclides in the world ocean, five Geochemical Ocean Sections (GEOSECS) stations, located in the different areas of the Arabian Sea, were revisited in 1998, 20 years after the GEOSECS expedition. Water samples were collected to study the vertical and horizontal distributions of 3H, 90Sr, 239,240Pu and 241Am. 239,240Pu and 241Am profiles generally showed subsurface maxima at 400–800 m water depth, while 3H and 90Sr profiles decreased smoothly with depth. 3H profiles measured in 1998 showed higher 3H levels at surface and intermediate depths as expected from the decay-corrected GEOSECS results. 239,240Pu and 241Am inventories increased towards the south, reaching a maximum at GEOSECS Station 419, situated in the southern Arabian Sea. 90Sr inventories showed the same tendency as transuranics, while 3H inventories were lowest at Station 419 and highest at Stations 413 (close to the Gulf of Aden). It appears that there are several processes viz. water transport from the Red Sea, different monsoon regimes (summer and winter), transport of tritium from the atmosphere to the ocean via isotope exchange with atmospheric moisture, rain and river inputs, and finally, circulation processes specific to the Arabian Sea and the Indian Ocean.