Dissolved redox sensitive elements, Re, U and Mo in intense denitrification zone of the Arabian Sea

Abstract

The concentrations of redox sensitive elements rhenium, uranium and molybdenum have been measured in the suboxic water column of the Arabian Sea to determine their response (behavior) to intense oxygen minima and denitrifying conditions in the water column. North of 12°N, within the core of the intense oxygen minimum zone (OMZ) of the Arabian Sea, the dissolved oxygen levels drop to values as low as < 5 μM accompanied by the presence of secondary nitrite maxima (SNM) indicating the occurrence of denitrification in the water column. The distributions of Re, U and Mo in the Arabian Sea show that their dissolved concentrations in the suboxic layer are indistinguishable from those in the overlying and underlying oxic waters suggesting that there is no discernible removal of these elements from the suboxic denitrifying layers. In contrast, the lateral and vertical distribution of dissolved Re, U and Mo concentrations vary as a function of salinity suggesting their conservative behavior. The salinity normalized (35) Re, U and Mo concentrations in the Arabian Sea are 40 pmol/kg, 13.8 nmol/kg and 114 nmol/kg respectively. These concentrations are nearly identical to their abundances in the Bay of Bengal, a basin adjacent to the Arabian Sea characterized by high freshwater influx and fluvial sediments. The similarity in concentration suggests that the large variability in the biogenic and detrital particulate fluxes and the suboxic/denitrifying conditions between these two oceanic regions do not affect the concentrations of Re, U and Mo and that the prevailing biogeochemical conditions in these regions are inadequate for their removal from the water column.