Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers
- California Institute of Technology (CalTech), Pasadena, CA (United States)
- Rice Univ., Houston, TX (United States)
- SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Synchrotron Radiation Lightsource (SSRL)
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 ($$SO^{2–}_{4}$$) and its 34S/32S isotope ratio (δ34SSO4). However, measurements from some rivers have suggested that SSO4 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 $$SO^{2–}_{4}$$ transformations, in this study we measured dissolved major element concentrations and δ34SSO4 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 δ34SSO4 in the Efri Haukadalsá ranged from 2.5‰ to 23.7‰ and had a linear relationship with Cl–/$$SO^{2–}_{4}$$ ratios, indicating that $$SO^{2–}_{4}$$ predominantly derived from basalt weathering and meteoric precipitation. The lower δ34SSO4 values were found in fluvial valleys with V-shaped cross sections, while higher values of δ34SSO4 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 $$SO^{2–}_{4}$$ export, and sediment δ34S values were comparable to river δ34SSO4. 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 δ34SSO4 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.
- Research Organization:
- SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
- Grant/Contract Number:
- AC02-76SF00515
- OSTI ID:
- 1786349
- Alternate ID(s):
- OSTI ID: 1781871
- Journal Information:
- Earth and Planetary Science Letters, Journal Name: Earth and Planetary Science Letters Vol. 562; ISSN 0012-821X
- Publisher:
- ElsevierCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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