The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh

Himalayan rivers have very unusual Sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic. We present Sr, O, and C isotopic data from river and rain water, bedload, and bedrock samples for the western and central Nepal Himalaya and Bangladesh, including the monsoon season. Central Himalayan rivers receive Sr from several sources: carbonate and clastic Tethyan sediments, High Himalayan Crystalline (HHC) gneisses and granitoids with minor marbles, carbonates and metasediments of the Lesser Himalaya (LH), and Miocene-Recent foreland basin sediment from the Siwaliks group and the modern flood plain. In the Tethyan Himalaya rivers have dissolved Sr] ≈ 6 μmol/l and ⁸⁷Sr/⁸⁶Sr ≈ 0.717, with a large contribution from moderately radiogenic carbonate. Rivers draining HHC gneisses are very dilute with Sr] ≈ 0.2 μmol/l and ⁸⁷Sr/⁸⁶Sr ≈ 0.74. Lesser Himalayan streams also have low Sr] ≈ 0.4 μmol/l and are highly radiogenic (⁸⁷Sr/⁸⁶Sr ≥ 0.78). Highly radiogenic carbonates of the LH do not contribute significantly to the Sr budget because they are sparse and have very low Sr]. In large rivers exiting the Himalaya, Sr systematics can be modeled as a mixture between Tethyan rivers, where slightly radiogenic carbonates (mean ⁸⁷Sr/⁸⁶Sr ≈ 0.715) are the main source of Sr, and Lesser Himalaya waters, where extremely radiogenic silicates (>0.8) are the main source of Sr. HHC waters are less important because of their low Sr]. Rivers draining the Siwaliks foreland basin sediments have Sr] ≈ 4 μmol/l and ⁸⁷Sr/⁸⁶Sr ≈ 0.725. Weathering of silicates in the Siwaliks and the flood plain results in a probably significant radiogenic (0.72–0.74) input to the Ganges and Brahmaputra (G-B), but quantification of this flux is limited by uncertainties in the hydrologic budget. The G-B in Bangladesh show strong seasonal variability with low Sr] and high ⁸⁷Sr/⁸⁶Sr during the monsoon. Sr in the Brahmaputra ranges from 0.9 μmol/l and 0.722 in March to 0.3 μmol/l and 0.741 in August. We estimate the seasonally weighted flux from the G-B to be 6.5 × 10⁸ mol/yr with ⁸⁷Sr/⁸⁶Sr = 0.7295.