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Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers

Kemeny, Preston C. and Torres, Mark A. and Lamb, Michael P. and Webb, Samuel M. and Dalleska, Nathan and Cole, Trevor and Hou, Yi and Marske, Jared and Adkins, Jess F. and Fischer, Woodward W. (2021) Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers. Earth and Planetary Science Letters, 562 . Art. No. 116838. ISSN 0012-821X.

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Pyrite oxidation plays a critical role in the relationship between weathering and climate, and its impact on the global carbon cycle has previously been constrained through inversion models utilizing observations of river sulfate (SO₄²⁻) and its ³⁴S/³²S isotope ratio (δ³⁴S_(SO₄)). However, measurements from some rivers have suggested that δ³⁴S_(SO₄) can be substantially impacted by processes such as microbial sulfate reduction and/or sulfur assimilation and cycling, rather than simply reflecting a weighted mixture of lithologic sulfur sources. To study the prevalence and controls on SO₄²⁻ transformations, we measured dissolved major element concentrations and δ³⁴S_(SO₄) in river water samples from throughout western Iceland. Our analyses focused on samples from a small catchment hosting the Efri Haukadalsá river, a system with relatively uniform and isotopically constrained basaltic bedrock. We also measured sediment δ³⁴S and sulfur speciation using sulfur K-edge X-ray absorption spectroscopy on sediment and vegetation samples from this catchment. Values of dissolved δ³⁴S_(SO₄) in the Efri Haukadalsá ranged from 2.5‰ to 23.7‰ and had a linear relationship with Cl⁻/SO₄²⁻ ratios, indicating that SO₄²⁻ predominantly derived from basalt weathering and meteoric precipitation. The lower δ³⁴S_(SO₄) values were found in fluvial valleys with V-shaped cross sections, while higher values of δ³⁴S_(SO₄) occurred in U-shaped, glacially eroded valleys with thick alluvial fills blanketing the valley floor. Spectroscopic observations identified organic sulfur phases in suspended river sediment, floodplain deposits, and vegetation. Mass balance calculations quantified the organic sulfur flux as less than 10% of SO₄²⁻ export, and sediment δ³⁴S values were comparable to river δ³⁴S_(SO₄). We interpreted these isotopic and chemical patterns as reflecting differences in the availability of unweathered bedrock across the Efri Haukadalsá catchment, with V-shaped valleys having greater access to fresh sulfide-bearing minerals than alluviated U-shaped valleys; this interpretation is in contrast to one in which the elevated δ³⁴S_(SO₄) values reflect fractionation during sulfur transformations along alluvial reaches. These results validated the application of river inversion models for constraining weathering fluxes and affirmed that pyrite oxidation globally, even in the presence of river sulfur cycling, modulates the abundance of atmospheric carbon dioxide.

Item Type:Article
Related URLs:
URLURL TypeDescription
Kemeny, Preston C.0000-0003-1693-4142
Torres, Mark A.0000-0002-9599-2748
Lamb, Michael P.0000-0002-5701-0504
Webb, Samuel M.0000-0003-1188-0464
Dalleska, Nathan0000-0002-2059-1587
Cole, Trevor0000-0001-5931-5400
Adkins, Jess F.0000-0002-3174-5190
Fischer, Woodward W.0000-0002-8836-3054
Additional Information:© 2021 Elsevier B.V. Received 28 August 2020, Revised 2 February 2021, Accepted 16 February 2021, Available online 4 March 2021. 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 (P.C.K.). M.A.T. gratefully acknowledges support from the Alfred P. Sloan Foundation. This research was supported by the Discovery Fund (to W.W.F. and M.P.L.) at the California Institute of Technology, and this work was partially support by the US National Science Foundation through grants 1349858 and 1834492. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We acknowledge DEMs provided by the Polar Geospatial Center under NSF-OPP awards 1043681, 1559691, and 1542736. We gratefully acknowledge A.J. West for loaning sampling equipment, A. Sessions, F. Wu, and A. Phillips for assistance with EA-IRMS measurements, G. Lopez, T. Present, and D. Johnson for assistance with MC-ICP-MS measurements, N. Edwards, S. Bone, and C. Roach for assistance with synchrotron measurements at SLAC, and A. Phillips and P. Mateo for advice on figure design. We thank Louis Derry and two anonymous referees for their constructive comments. CRediT authorship contribution statement: Preston C. Kemeny: Conceptualization, Formal analysis, Investigation, Project administration, Software, Visualization, Writing – original draft. Mark A. Torres: Conceptualization, Funding acquisition, Investigation, Project administration, Supervision, Writing – review & editing. Michael P. Lamb: Funding acquisition, Supervision, Writing – review & editing. Samuel M. Webb: Resources, Writing – review & editing. Nathan Dalleska: Resources, Writing – review & editing. Trevor Cole: Investigation, Writing – review & editing. Yi Hou: Investigation. Jared Marske: Writing – review & editing. Jess F. Adkins: Funding acquisition, Resources, Supervision, Writing – review & editing. Woodward W. Fischer: Conceptualization, Funding acquisition, Resources, Supervision, Writing – review & editing. 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.
Funding AgencyGrant Number
Fannie and John Hertz FoundationUNSPECIFIED
National Defense Science and Engineering Graduate (NDSEG) Fellowship32 CFR 168a
Alfred P. Sloan FoundationUNSPECIFIED
Caltech Discovery FundUNSPECIFIED
Department of Energy (DOE)DE-AC02-76SF00515
Subject Keywords:chemical weathering; pyrite oxidation; sulfur isotopes; XANES; Iceland
Record Number:CaltechAUTHORS:20210309-070118649
Persistent URL:
Official Citation:Preston C. Kemeny, Mark A. Torres, Michael P. Lamb, Samuel M. Webb, Nathan Dalleska, Trevor Cole, Yi Hou, Jared Marske, Jess F. Adkins, Woodward W. Fischer, Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers, Earth and Planetary Science Letters, Volume 562, 2021, 116838, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108352
Deposited By: Tony Diaz
Deposited On:10 Mar 2021 20:11
Last Modified:10 Mar 2021 20:11

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