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 elastic 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] approximate to 6 mu mol/l and Sr-87/Sr-86 approximate to 0.717, with a large contribution from moderately radiogenic carbonate. Rivers draining HHC gneisses are very dilute with [Sr] approximate to 0.2 mu mol/l and Sr-87/Sr-86 approximate to 0.74. Lesser Himalayan streams also have low [Sr] approximate to 0.4 mu mol/l and are highly radiogenic (Sr-87/Sr-86 greater than or equal to 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 Sr-87/Sr-86 approximate to 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] approximate to 4 mu mol/l and Sr-87/Sr-86 approximate to 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 Aux is limited by uncertainties in the hydrologic budget. The G-B in Bangladesh show strong seasonal variability with low [Sr] and high Sr-87/Sr-86 during the monsoon. Sr in the Brahmaputra ranges from 0.9 mu mol/l and 0.722 in March to 0.3 mu mol/l and 0.741 in August. We estimate the seasonally weighted flux from the G-B to be 6.5 x 10(8) mol/yr with Sr-87/Sr-86 = 0.7295. Copyright (C) 1999 Elsevier Science Ltd.