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

Abstract

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.