Sulfur Cycling in Lake Superior: Characterizing the Role of Organic Sulfur in Oligotrophic Systems

Abstract

Microbially mediated sulfate reduction is important in lake sediment due to its impact on numerous other biogeochemical cycles, including phosphorus, iron, and mercury, and its contribution to carbon remineralization in lake settings. Recently, modelling studies in oligotrophic lake settings have demonstrated that organic sulfur is a significant portion of the total sulfur pool, capable of sustaining cryptic sulfur cycling within the sediment. Organic sulfur consists of freshly deposited biogenic sulfur-containing molecules, their degradation products, and abiotic “sulfurized” non-labile organic matter, which may play a significant role in organic matter preservation. Few, if any, studies have quantified these organic forms of sulfur to the total sulfur cycle in oligotrophic lake settings, indicating that characterization of the total sulfur budget in a low-nutrient lacustrine system is warranted. Here, organic and inorganic species of sulfur are characterized in Lake Superior sediment from four offshore locations. Inorganic contributions to the sulfur pool consist of sulfate in porewater; and sulfides, elemental sulfur, and pyrite in the sediments. Organic contributions are determined by difference from total sulfur by mass. Contributions from organic and inorganic sources of sulfur are evaluated from the surface of the sediment at 1-cm depths to 12 cm, then again at 15, 20, and 25 cm. Results show that in low-deposition settings, organic sulfur comprises between 70-90% of total sulfur in Lake Superior sediment, indicating the necessity of further exploration of the contribution of organic sulfur to carbon sequestration in oligotrophic lacustrine settings.