Origin and composition of dissolved and particulate matter in mountain rivers of the Nepal Himalayas: Insights from Sr and Nd isotopes and elemental ratios

The Ganga River exhibits elevated concentrations of dissolved strontium (Sr) and a higher radiogenic Sr ( 87 Sr/ 86 Sr) among global rivers, which impacts oceanic Sr composition. This has implications for understanding weathering and sediment flux into open oceans. The Nepal Himalayan section is a significant part of the upstream Ganga River catchment. We analyzed Sr and Nd isotopes and major element concentrations in water and sediments to trace compositional and seasonal variability in dissolved and particulate matter. The Sr and Nd isotopes, in particular, are crucial in this study as they provide unique signatures that can be used to trace the origin and composition of the dissolved and particulate matter. Representative rivers draining monolithological terrains were selected to investigate the isotopic and elemental provenance and weathering intensity. The elemental ratios (Ca/Na, Mg/Na) indicate the watersheds are carbonate-rich terrains. The Lesser Himalaya (LH) rivers transport radiogenic Sr with high values compared to rivers draining the Tethyan Sedimentary Series (TSS), Higher Himalayan Crystalline (HHC), Mahabharat, and Siwalik Hills. The dry season records a higher 87 Sr/ 86 Sr ratio than the monsoon. Due to the faster dissolution of carbonates compared to silicates, monsoon waters transport less radiogenic Sr. The significant correlation of 87 Sr/ 86 Sr in dissolved and particulate phases signifies that short-term interactions between sediment and water may alter the 87 Sr/ 86 Sr composition. Notably, sediments originating from continental rocks exhibit an inverse correlation between 87 Sr/ 86 Sr and ε Nd . In conclusion, the Sr and Nd isotopic records in this study categorize the fluvial catchments into geological clusters aligned with the TSS, LH, and Siwaliks, which advance our understanding regarding the provenance besides providing crucial insights into geological processes, weathering, landscape evolution, and sediment flux. • Provenance of Sr and Nd isotopes and elements in rivers hints carbonate dominates over silicates in the Nepal Himalayas. • Seasonal variability in 87 Sr/ 86 Sr is the outcome of dissolution kinetics control weathering of catchment rocks. • Correlation of 87 Sr/ 86 Sr in water, bedload, and suspended sediments implies short-term water-rock interactions. • Samples exhibit an inverse correlation between 87 Sr/ 86 Sr ratios and ε Nd values. • Sr and Nd signatures in samples from different physiographic regions in Nepal Himalayas cluster distinctly.