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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

Kemeny, P. C. and Lopez, G. I. and Dalleska, N. F. and Torres, M. and Burke, A. and Bhatt, M. P. and West, A. J. and Hartmann, J. and Adkins, J. F. (2021) 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. Geochimica et Cosmochimica Acta, 294 . pp. 43-69. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20201202-083406031

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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 ³⁴S/³²S isotope ratios in dissolved SO₄²⁻ (δ³⁴S_(SO₄)), 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 bimonthly 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 δ³⁴S_(SO₄) 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 elevation 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 δ³⁴S SO₄ is lithologically variable isotope composition, with the expressed δ³⁴S value for the LH and TSS formations (median values −7.0–0.0‰ in 80% of samples) lower than that in the HHC (median values −1.7–6.2‰ in 80% of samples). Overall, our analysis 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 <10 Myr. Moreover, reanalysis of prior datasets suggests that our findings are applicable more widely across several of the frontal Himalayan drainages.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2020.11.009DOIArticle
ORCID:
AuthorORCID
Kemeny, P. C.0000-0003-1693-4142
Dalleska, N. F.0000-0002-2059-1587
Burke, A.0000-0002-3754-1498
West, A. J.0000-0001-6909-1471
Adkins, J. F.0000-0002-3174-5190
Alternate Title: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
Additional Information:© 2020 Elsevier Ltd. Received 14 April 2020, Revised 2 November 2020, Accepted 11 November 2020, Available online 19 November 2020. 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. The authors are grateful to W. Fischer for helpful conversations and to A. Philips and P. Mateo for advice on the design of figures. We thank the associate editor and three anonymous reviewers for providing insightful comments. Data availability: The measurements of δ³⁴S_(SO₄) and dissolved major ion concentrations described in this article are available as supplementary material. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funders:
Funding AgencyGrant Number
Fannie and John Hertz FoundationUNSPECIFIED
Air Force Office of Scientific Research (AFOSR)UNSPECIFIED
National Defense Science and Engineering Graduate (NDSEG) Fellowship32 CFR 168a
Ronald And Maxine Linde Center for Global Environmental ScienceUNSPECIFIED
Caltech Beckman InstituteUNSPECIFIED
Deutsche Forschungsgemeinschaft (DFG)EXC177
NSFEAR-1349858
NSFOCE-1834492
Subject Keywords:Silicate weathering; Pyrite oxidation; Sulfur isotopes; Carbon dioxide; Himalaya
Record Number:CaltechAUTHORS:20201202-083406031
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20201202-083406031
Official Citation:P.C. Kemeny, G.I. Lopez, N.F. Dalleska, M. Torres, A. Burke, M.P. Bhatt, A.J. West, J. Hartmann, J.F. Adkins, 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, Geochimica et Cosmochimica Acta, Volume 294, 2021, Pages 43-69, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2020.11.009.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106873
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:02 Dec 2020 17:09
Last Modified:16 Dec 2020 20:19

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