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

Abstract

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.