An imbalance in pyrite weathering and burial is regarded as one of the primary mechanisms responsible for the oxygenation of the atmosphere and oceans, but key processes governing the terrestrial sulfur cycle remain nebulous. Here, we investigate components of the terrestrial sulfur cycle in a highly productive, glacier-fed catchment, and use a global mass balance model to put constraints on the riverine sulfur fluxes. Chemistry of stream water and plant debris in the Jostedal watershed, Norway suggests sulfur isotope discrimination is occurring in the porewater. Global models also corroborate additional, previously overlooked pyrite burial with a modest isotope fractionation (<20‰), similar to values reported from freshwater ecosystems. Collectively, our results support the notion that a significant amount of sulfate produced by weathering remains trapped in terrestrial environments. This terrestrial sulfur sink might have waxed and waned over geologic time in response to major biogeochemical events such as terrestrial afforestation.

Sample collection and preparation

Samples of stream and lake water, snow and glacier meltwater, fluvial sediment, bedrock and soil were collected in July 2015 along the forefield of Jostedal Glacier, Norway, and water temperature and pH were measured during sampling. Sediment samples were wet-sieved to separate gravel (>2 mm), sand (2 mm - 62.5 µm), and mud (<62.5 µm) fractions. The fine-grained (<62.5 µm) sediments were reacted with buffered acetic acid (pH = 4.8) and hydrogen peroxide to remove carbonate and organic matter, respectively, and freeze-dried. Considering the wide variety of lithology within the igneous and metamorphic bodies in the area, the sands from the most proximal part of the stream were further sieved to isolate the medium sand (250-500 µm) fraction, which we used as a representative of the “average bedrock” in the drainage of the Storelvi River for the measurement of major element concentrations. Basement rock samples were used for concentration and isotope ratio of sulfur, since the exposure to weathering and depositional processes can result in loss of sulfur via oxidation. The bedrock samples, soil samples, and fine-grained (<62.5 µm) sediment samples were freeze dried and powdered for further analyses.

Liquid sample chemistry

Cation and anion concentrations in stream water were quantified by Dionex DX 500 ion chromatography equipped with both cation-exchange column CS-12 and anion-exchange column AS-11.

Sulfur isotope analysis

Sulfate in stream water, snow, and glacier meltwater was purified and concentrated by anion chromatography on Bio-Rad AGX8 anion exchange resin and analyzed for ³⁴S/³²S ratios using a Neptune Plus multi-collector inductively coupled plasma mass spectrometer at Caltech. The measured ³⁴S/³²S ratio was calibrated using a linear interpolation between the two bracketing standard values, and sulfur isotope compositions were reported using conventional delta notation with respect to VCDT (Vienna Cañon Diablo Troilite). The bracketing standard was calibrated against the IAEA S-1 reference material and has a δ³⁴S value of -1.55‰ ± 0.16. Analytic reproducibility for δ³⁴S has been evaluated as ± 0.2‰.

Total sulfur in bedrock samples was reduced to H₂S by a boiling mixture of HI-HCl-H₃PO₂ (Thode et al., 1961) and precipitated as Ag₂S. Freeze-dried and powdered soil samples were fused with Eschka’s mixture (MgO and CaCO₃) at 800 °C, converting available sulfur to BaSO₄ (Selvig & Fieldner, 1927). Sulfur isotope ratios of recovered Ag₂S and BaSO₄ were determined using a Delta V Plus continuous-flow isotope ratio mass spectrometer operated by the Department of Earth and Planetary Sciences at Northwestern University. All measured sulfur isotope compositions were reported in the delta (δ) notation as isotope ratio variations relative to VCDT, and linear scale correction was applied to yield δ³⁴S values of 0.5‰ and 20.3‰ for IAEA SO-5 and NBS-127 (BaSO₄), and -0.3‰, 22.7‰, and -32.3‰ for IAEA S-1, S-2, and S-3 (Ag₂S). Analytical precision is within 0.2‰.

Solid chemistry

The “average bedrock” and fine-grained sediment samples were analyzed for major elements by inductively coupled plasma atomic emission spectroscopy.