Behavior of dissolved redox sensitive elements (U, Mo and Re) in the water column of the Bay of Bengal

Abstract

The dissolved concentrations of redox sensitive elements, U, Mo and Re have been measured in the water column of the Bay of Bengal (BoB) to track their distributions and behavior in its suboxic layers at the intermediate depths. The dissolved oxygen content in the intermediate waters (200–800 m) north of ∼11°N decreases to values as low as ∼3 μmol/ℓ. The distribution of U, Mo and Re in the surface waters along the 87°E transect shows a north–south gradient with significant correlation with salinity, underscoring the dominant influence of the Ganga–Brahmaputra (G–B) Rivers in controlling the geochemical budgets of these three elements. The concentrations of these three elements in surface and deep waters of the BoB are governed by salinity suggestive of their conservative behavior. On a macroscale, there is no discernable removal of U, Mo, or Re from the suboxic layer in the BoB water column, consistent with earlier studies in the suboxic zones of other oceanic regions. Further, their conservative behavior in the bay, suggest that the annual delivery of ∼10⁹ tons of suspended matter rich in organic matter by G–B and other rivers do not have any measurable impact in determining their distributions. This study suggests that the suboxic conditions and high particulate flux prevailing in the water column of the Bay of Bengal do not affect the conservative behavior of U, Mo and Re. The dissolved Re concentrations in the upper layers in the central BoB seem to be marginally higher compared to that expected based on their salinities. This seems to be contributed from the Godavari River, which drains into the BoB and has dissolved Re three times higher than seawater. The higher Re concentrations in the central Bay of Bengal suggest that mixing process by local gyre and/or eddies in the BoB transports riverine dissolved material to ∼400 km offshore. More detailed transects of Re from the Godavari coast to central bay are needed to confirm this.