Spatial distribution of Mo and δ98Mo in waters of the northern Indian Ocean: Role of suboxia and particle-water interactions on lighter Mo in the Bay of Bengal

Abstract

The dissolved Mo concentrations and Mo isotopic composition (δ98Mo) have been determined in the water column of the northern Indian Ocean to assess the impact of water column anoxia/suboxia, seasonal hypoxia, and enormous riverine particulate fluxes from the Ganga-Brahmaputra river system on Mo isotope composition of seawater. All the samples collected from the Arabian Sea and the central and southern Bay of Bengal display conservative and uniform Mo concentrations and δ98Mo, 112 ± 2 nmol/kg, and 2.25 ± 0.13‰, respectively, similar to the other global oceanic basins. Overall, this study suggests no modification in both Mo concentrations and Mo isotopic composition in the water columns of the Arabian Sea and the open ocean locations of the Bay of Bengal under the influence of suboxic/denitrifying conditions and high primary productivity. However, we report, for the first time, lighter Mo isotope composition in the water column of the northern coastal Bay of Bengal. These waters from the northern Bay of Bengal continental margin show significantly depleted/lighter δ98Mo (average = 1.64 ± 0.42‰; 1σ) than the global open ocean average value, with conservative Mo concentrations. In addition, δ98Mo of bulk sediments and leachable Fe-Mn oxyhydroxide phases from this region show a lower δ98Mo than the overlying water column, with the bulk sediments containing ∼ 5–11% of leachable Mo fraction adsorbed on Fe-Mn oxyhydroxide fraction. Intermediate water anoxia/suboxia in this region extends up to the sediment–water interface. The depletion in δ98Mo in the northern coastal Bay of Bengal waters could result from the in situ release of relatively lighter Mo from the reductive dissolution of Fe-Mn oxyhydroxide phases adsorbed on suspended particulate matter. Repeated cycles of deposition and reductive dissolution of sedimentary Fe-Mn oxyhydroxide phases in the shelf/slope regions could supply lighter Mo to the porewaters. Furthermore, the subsequent diffusion/advection of these porewaters to the water column and/or dissolution of Fe-Mn oxyhydroxide phases in the suboxic water column could supply lighter Mo to the waters of the northern coastal Bay of Bengal.