Uranium in the Arabian Sea: role of denitrification in controlling its distribution

A total of 61 samples, from six vertical profiles between surface and 1800 m from the Arabian Sea, have been analysed for uranium concentration to assess its behaviour in oxic and sub-oxic waters. These samples cover a wide range of dissolved oxygen concentration, from 0.14 to 230 μM. Uranium concentration in most of the samples fall in the range of 3.2 ± 0.2 μg l^–1. In general, uranium exhibits a conservative behaviour in the Arabian Sea water column and denitrification processes do not seem to influence its abundance. The mean uranium/salinity ratio in waters with dissolved oxygen <2 μM is (9.17 ± 0.15) × 10^–8 g g^–1, nearly identical to the ratio (9.22 ± 0.27) × 10^–8 g g^–1 in waters with dissolved oxygen >100 μM, and in waters from other oceanic regions (9.34 ± 0.56) × 10^–8 g g^–1. In one of the profiles (18°N, 64°E) collected during summer monsoon (SK115/6; August 1996), two samples from 300 and 600 m have uranium concentration ∼10–15% lower than that expected based on salinity. Further studies are needed to confirm these results and to characterize the process(es) contributing to depletion of uranium at these depths.