Sulfate sulfur isotopes and major ion chemistry reveal that pyrite oxidation counteracts CO2 drawdown from silicate weathering in the Langtang-Trisuli-Narayani River system, Nepal Himalaya
Abstract
Drawdown of atmospheric carbon dioxide (CO2) due to silicate weathering in the Himalaya has previously been implicated in Cenozoic cooling. However, over timescales shorter than that of the removal of marine sulfate (SO42-), the oxidation of pyrite (FeS2) in weathering systems can counteract the alkalinity flux of silicate weathering and modulate pCO2. Here we present evidence from sulfur isotope ratios in dissolved SO42- (δ34SSO4), together with dissolved major ion concentrations, that reveals FeS2 oxidation throughout the Langtang-Trisuli-Narayani River system of the Nepal Himalaya. River water samples were collected monthly to bi-monthly throughout 2011 from 16 sites ranging from the Lirung Glacier catchment through the Narayani River floodplain. This sampling transect begins in the High Himalayan Crystalline (HHC) formation and passes through the Lesser Himalayan (LH) formation with upstream influences from the Tethyn Sedimentary Series (TSS). Average δ34SSO4 in the Lirung Glacier outlet is 3.6‰, increases downstream to 6.3‰ near the confluence with the Bhote Kosi, and finally declines to -2.6‰ in the lower sites. Using new measurements of major ion concentrations, inversion shows 62-101% of river SO42- is derived from the oxidation of sulfide minerals and/or organic sulfur, with the former process likely dominant. The fraction of H2SO4-driven weathering is seasonally variable and lower during the monsoon season, attributable to seasonal changes in the relative influence of shallow and deep flow paths with distinct residence times. Inversion results indicate that the primary control on δ34SSO4 is lithologically variable isotope composition, with the expressed δ34S value for the LH and TSS formations (median values -7.0‰ to 0.0‰ in 80% of samples) lower than that in the HHC (median values 1.7‰ to 6.2‰ in 80% of samples). Overall, our inversion indicates that FeS2 oxidation counteracts much of the alkalinity flux from silicate weathering throughout the Narayani River system such that weathering along the sampled transect exerts minimal impact on pCO2 over timescales >5-10 Kyr and
Citation
Kemeny , P C , Lopez , G I , Dalleska , N F , Torres , M , Burke , A , Bhatt , M P , West , A J , Hartmann , J & Adkins , J F 2021 , ' Sulfate sulfur isotopes and major ion chemistry reveal that pyrite oxidation counteracts CO 2 drawdown from silicate weathering in the Langtang-Trisuli-Narayani River system, Nepal Himalaya ' , Geochimica et Cosmochimica Acta , vol. 294 , pp. 43-69 . https://doi.org/10.1016/j.gca.2020.11.009
Publication
Geochimica et Cosmochimica Acta
Status
Peer reviewed
ISSN
0016-7037Type
Journal article
Rights
Copyright © 2020 Elsevier Ltd. All rights reserved. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1016/j.gca.2020.11.009
Description
P.C.K. is supported by the Fannie and John Hertz Foundation Cohan-Jacobs and Stein Families Fellowship. This research was conducted with government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. This research was supported by the US National Science Foundation (grants 1349858 and 1834492). N.F.D. is grateful to the Linde Center for support. The Caltech Environmental Analysis Center is supported by the Linde Center and the Beckman Institute at Caltech. This research was also supported by the German Research Foundation DFG through the Cluster of Excellence ‘CliSAP’ (EXC177), Universität Hamburg. The authors acknowledge the Department of Hydrology and Meteorology (DHM), Government of Nepal, for discharge measurements. Initial computing costs were covered by startup research funds provided by Caltech to F. Tissot. T. Jappinen and P. Bartsch helped with logistics and analysis.Collections
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