STRONTIUM (SR) AND NEODYMIUM (ND) ISOTOPIC COMPOSITIONS OF RIVERS DRAINING CONTRASTING LITHOLOGIES FROM NEPAL HIMALAYA

Himalayan rivers have high radiogenic Sr and actively contribute to the oceanic Sr budget. Previous studies from Nepal either analyzed cores retrieved from the Bay of Bengal or sampled large rivers during the dry season. Hence, sampling during monsoons draining major lithologies in the mountainous terrain is required to assess these monsoon-dominated rivers. This study aims to systematically reassess the contribution of Sr and Nd isotopes from watersheds in various geological terrains within Nepal. We did seasonal sampling in rivers originating from the Tethyan Sedimentary Series (TSS) – Upper Kaligandaki (UKG), High Himalayan Crystalline Series (HHC) – Myagdikhola (MK), Lesser Himalaya (LH) – Aadhikhola (AK) and Siwaliks Foreland Basin Sequence - Tinahukhola (TK).

Our data showed marked variability in ⁸⁷Sr/⁸⁶Sr and ε_Nd in fluvial sediments due to different crustal rock types. The ⁸⁷Sr/⁸⁶Sr values in suspended sediments followed the order: AK (0.923 – 0.934) > TK (0.742 – 0.765) > MK (0.747) > UKG (0.712 – 0.716). The variability of ε_Nd recorded was as follows: AK (-23.57 to -21.06), TK (-17.07 to -14.39), UKG (-15.70 to -14.96), and MK (-20.12). There was also a considerable variability of ⁸⁷Sr/⁸⁶Sr in bedload samples along the river course and between different seasons. The ε_Nd data showed similar variability in bedload samples but comparatively small seasonal variability. During monsoons, AK was the major contributor of radiogenic Sr in river water (⁸⁷Sr/⁸⁶Sr ≈0.79), whereas the UKG recorded the lowest value (⁸⁷Sr/⁸⁶Sr ≈ 0.71). At these sites, dissolved Sr concentrations were 6 ppb and 500 ppb, respectively. TK and MK indicated similar values (⁸⁷Sr/⁸⁶Sr ≈ 0.73). The concentration of Sr for TK and MK were 80 and 30 ppb, respectively. While AK and TK rivers transported higher ⁸⁷Sr/⁸⁶Sr during the high flow season, Sr concentrations were comparable during the low flow season. These data support the inference that weathering of TSS is a major source of dissolved Sr in rivers. Moreover, the provenance of radiogenic Sr in river water and sediments is derived from silicate weathering in LH rivers.