DOI: 10.5061/dryad.r2280gbf6
title: Significance of the terrestrial sink in the biogeochemical sulfur cycle

description: 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 (&lt;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.
description: <strong>Sample
 collection and preparation</strong> 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 (&gt;2 mm), sand (2 mm -
 62.5 µm), and mud (&lt;62.5 µm) fractions. The fine-grained
 (&lt;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
 (&lt;62.5 µm) sediment samples were freeze dried and powdered for
 further analyses. <strong>Liquid sample
 chemistry</strong> 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. <strong>Sulfur isotope
 analysis</strong> 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 <sup>34</sup>S/<sup>32</sup>S ratios using a
 Neptune Plus multi-collector inductively coupled plasma mass spectrometer
 at Caltech. The measured
 <sup>34</sup>S/<sup>32</sup>S 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 δ<sup>34</sup>S value of -1.55‰ ± 0.16. Analytic
 reproducibility for δ<sup>34</sup>S has been evaluated as ±
 0.2‰. Total sulfur
 in bedrock samples was reduced to H<sub>2</sub>S by a boiling
 mixture of HI-HCl-H<sub>3</sub>PO<sub>2</sub>
 (Thode et al., 1961) and precipitated as Ag<sub>2</sub>S.
 Freeze-dried and powdered soil samples were fused with Eschka’s mixture
 (MgO and CaCO<sub>3</sub>) at 800 °C, converting available
 sulfur to BaSO<sub>4</sub> (Selvig & Fieldner, 1927).
 Sulfur isotope ratios of recovered Ag<sub>2</sub>S and
 BaSO<sub>4</sub> 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 δ<sup>34</sup>S values of 0.5‰ and 20.3‰ for
 IAEA SO-5 and NBS-127 (BaSO<sub>4</sub>), and -0.3‰, 22.7‰,
 and -32.3‰ for IAEA S-1, S-2, and S-3 (Ag<sub>2</sub>S).
 Analytical precision is within 0.2‰. <strong>Solid
 chemistry</strong> The “average bedrock” and
 fine-grained sediment samples were analyzed for major elements by
 inductively coupled plasma atomic emission spectroscopy.