MOLYBDENUM AND δ98MO IN TWO TROPICAL RIVERS OF WESTERN INDIA: ASSESSING THE IMPACT OF CHEMICAL WEATHERING, GROUNDWATER DISCHARGE, PARTICLE-WATER INTERACTIONS, AND REDOX TRANSFORMATIONS ON STABLE MO ISOTOPES

The stable molybdenum (Mo) isotopic composition (defined as δ⁹⁸Mo) of organic-rich marine sediments are often used as an important proxy to understand the changes in redox state of the oceans over the earth's geological history. However, these assessments are heavily dependent on the isotopic composition of terrestrial Mo supply to the ocean primarily via riverine dissolved loads. Some of the earlier works in this context considered a fairly constant δ⁹⁸Mo isotope value of the riverine input to the ocean, similar to the upper continental crustal value of earth (δ⁹⁸Mo ~ 0‰). However, recent studies have highlighted that δ⁹⁸Mo of the riverine component is significantly heavier and susceptible to the climatically and tectonically driven changes in different weathering regimes. Lack of accurate assessment of the δ⁹⁸Mo of the global riverine input may lead to significant inaccuracies in the estimations on the degree of oceanic redox states. To understand the influences of lithology, redox transformation and terrestrial processes (e.g., chemical weathering, secondary mineral formation, groundwater discharge, anthropogenic input) on the distribution of Mo and δ⁹⁸Mo isotopic composition on a spatial scale in tropical rivers, water samples from the Narmada and Tapi mainstream and their tributaries were collected. Mo concentrations in the Narmada and Tapi mainstream water vary significantly from 1.70 to 4.75 nM and 2.00 to 7.52 nM respectively. The δ⁹⁸Mo isotopic composition of the Narmada and Tapi mainstream water varies from 0.46 to 0.74‰, and 0.46 to 0.70‰ respectively; that are considerably heavier than the average crustal components. An increasing trend of δ⁹⁸Mo values and Mo concentrations was documented along the river courses from upstream to downstream. The contrasting variability in the distribution of Mo documented in the Narmada and Tapi mainstream water highlights the significant influences of congruent and incongruent weathering of the multiple bedrocks occurring at the river drainage basins. Additional influences of redox transformations, ground water discharge and localized signatures carried by the tributaries controlling the riverine δ⁹⁸Mo composition were documented. Further, a quantitative estimation of the riverine Mo flux during the dry pre-monsoon time period was done in this study.