Molybdenum isotopes in two Indian estuaries: mixing characteristics and input to oceans

Abstract

The distributions of dissolved and particulate Mo and their isotope composition (δ⁹⁸Mo) have been measured in the Narmada and the Tapi estuaries draining into the Arabian Sea. During monsoon, the δ⁹⁸Mo of dissolved Mo in the Narmada estuary ranges from 0.49‰ to 2.19‰ in the salinity range 0–17.2 practical salinity unit (psu) quite similar to that in the Tapi estuary, 0.99–2.36‰, in the salinity range 0–20.3 psu. Mo concentration in suspended sediments of the Narmada estuary collected during monsoon average 512 ± 44 ng/g (range 459–602 ng/g) similar to that measured in one sample from the Tapi estuary 560 ng/g Mo. δ⁹⁸Mo of particulate Mo in the Narmada ranges from −0.21‰ to 0.48‰ with an average −0.03 ± 0.2‰. Dissolved Mo in the Narmada and the Tapi rivers display isotopically heavier Mο compared to that in basalts, the major lithology of their drainage. This could result from a variety of processes, preferential weathering of Mo rich sulphide minerals dispersed in the basalts, preferential removal of isotopically lighter Mo during transport or contribution from marine cyclic salts supplied via rain or chemical weathering of organic rich shales in the basins. The distribution of δ⁹⁸Mo in the Narmada and the Tapi estuaries with salinity does not follow the theoretical mixing line between river and seawater endmembers suggesting its non-conservative behavior. Particulate Mo and δ⁹⁸Mo show concomitant increase with salinity in the Narmada estuary indicating loss of dissolved Mo by adsorption onto Fe–Mn oxyhydroxide. Balancing the Mo budget along the course of these estuaries using inverse model suggests that in the Narmada estuary there could be loss up to 8% of the dissolved Mo and that in the Tapi supply from anthropogenic sources could be up to 27%. The results obtained in this study bring out the processes modifying riverine input of Mo and its δ98Mo in the estuaries, oxic sink in the Narmada and anthropogenic input in the Tapi. Repetitive adsorption and desorption of Mo in the Narmada estuary can modify the supply of dissolved Mo and its δ⁹⁸Mo relative to riverine supply by up to 40%, this can significantly impact the Mo isotope budget of the oceans. In contrast, in the Tapi estuary there is enhancement in the dissolved supply of Mo relative to that from river due to anthropogenic input of Mo. The investigations in these two estuaries underscore the importance of solute particle interactions and anthropogenic input in determining the Mo flux and its δ⁹⁸Mo to the open Arabian Sea.