T220 GSA Environmental and Engineering Geology Division

Wednesday, 12 October 2022
8:00 AM - 12:00 PM
207

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

Paper number: T220-10
Program: Technical Sessions
Time: 10:05 AM – 10:20 AM

Authors

Abstract

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 εNd 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 εNd 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 εNd 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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380061

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.