Sulfur Biogeochemical Cycling and Redox Dynamics in a Shale‐Dominated Mountainous Watershed

• Published in: Journal of geophysical research. Biogeosciences 2022-06, Vol.127 (6), p.n/a
• Main Authors: Fox, Patricia M., Carrero, Sergio, Anderson, Cam, Dewey, Christian, Keiluweit, Marco, Conrad, Mark, Naughton, Hannah R., Fendorf, Scott, Carroll, Rosemary, Dafflon, Baptiste, Malenda‐Lawrence, Helen, Dwivedi, Dipankar, Gilbert, Benjamin, Christensen, John N., Boye, Kristin, Beutler, Curtis, Brown, Wendy, Newman, Alexander, Versteeg, Roelof, Williams, Kenneth H., Nico, Peter S.
• Format: Article
• Language: English
• Subjects: Analytical methods; Bedrock; Biogeochemical cycles; Biogeochemistry; Chemical precipitation; Climate change; Earth crust; Exports; Floodplains; Fracture surfaces; Groundwater; Hydrologic cycle; Hydrological cycle; Hydrology; Hyporheic zone; Hyporheic zones; Identification; Mountains; Organosulfur compounds; Oxidation; Oxidoreductions; Pore water; Pyrite; Redox properties; Rivers; Rocks; Sediment; Sedimentary rocks; Sediments; Shale; Shales; Soil; Soils; Speciation; Spectroscopy; Sulfate reduction; Sulfates; Sulfur; Sulfur compounds; Sulfur oxidation; Sulphate reduction; Sulphur; Surface water; Trace metals; Unsaturated soils; Water quality; Watersheds; Weathering
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2021JG006769

Sulfur (S) is an essential macronutrient and important component of the earth’s crust, and its cycling has critical impacts on trace metal mobility, water quality, and human health. Pyrite weathering is the primary pathway by which sulfur enters surface waters. However, biogeochemical cycling of sulfur in soils and the river corridor mediates sulfate exports. In this study, we identified the major forms of sulfur across multiple compartments and scales in a pristine mountainous watershed, including shale bedrock weathering profiles, hillslope soils, and alluvial floodplain sediments, in order to provide insight into biogeochemical sulfur cycling in a hydrologically variable alpine system. X‐ray absorption near‐edge spectroscopy (XANES) analysis of shale weathering profiles showed clear evidence of pyrite oxidation to sulfate, with large accumulations of intermediate S(0) (20%–53%). Micro‐scale XANES showed evidence of reprecipitation of pyrite at fracture surfaces within the permanently saturated zone. Organic sulfur dominated S speciation in shallow hillslope soil and floodplain sediment, with little evidence of reduced inorganic S. However, mackinawite formation, representing active sulfate reduction, was observed in saturated oxbow sediments and saturated weathered shale underlying floodplain sediments. Further evidence of sulfate reduction from aqueous sulfur isotopic analysis was observed in shallow groundwater transects across an Fe‐reducing meander, whereas increases in pore water sulfate concentrations implied sulfur oxidation at other locations. The data present an integrated picture of sulfur cycling in a shale‐dominated watershed, where riverine sulfate exports are mediated by biological cycling, particularly in redox‐stratified and temporally dynamic hyporheic zone sediments. Plain Language Summary Sulfur is an essential macronutrient and biologically important component of the earth’s crust, and its cycling has critical impacts on water quality and human health. Weathering of the mineral pyrite from rock is the primary pathway by which sulfur enters surface waters, and alterations to the hydrologic cycle due to climate change may affect pyrite weathering rates. However, biological cycling of sulfur in soils and the river corridor mediates the release of sulfur to rivers and the ocean. In this study, we identified the major forms of sulfur across a pristine mountainous watershed, including shale bedrock weathering profiles, hillslope soils, and