Significance of the terrestrial sink in the biogeochemical sulfur cycle

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.Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: EAR-1225162Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: EAR-1418716Funding provided by: Korea Polar Research InstituteCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100004230Award Number: PE20900Funding provided by: National Research Foundation of KoreaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100003725Award Number: 2019R1C1C1005550Sample 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 (