Sources and Fate of Dissolved Organic Sulfur at the Redox Interface of Marine Shallow Hydrothermal Systems

Shallow hydrothermal systems are extreme environments with strong redox gradients at the interface of hot, reduced fluids and cold, oxygenated seawater. Hydrothermal fluids are often depleted in sulfate and can contain high concentrations of hydrogen sulfide (H₂S). It is well known that sulfur plays an important role in processing and transformation of organic matter. However, the formation and the reactivity of dissolved organic sulfur (DOS) in the water column at hydrothermal systems are not well understood. We investigated DOS dynamics by studying the molecular composition of DOM in three contrasting systems off Milos (Aegean Sea), Dominica (Lesser Antilles) and Iceland (North Atlantic). We used ultra-high resolution mass spectrometry (FTICR-MS) to characterize the DOM on a molecular level. The molecular information was complemented with geochemical data, quantitative DOC and DOS analyses and isotopic measurements (δ²H, δ¹⁸O, F¹⁴C). In contrast to the predominantly meteoric fluids from Dominica and Iceland, fluids from Milos were mainly fed by recirculating seawater. Milos hydrothermal fluids were enriched in H₂S and DOS, as indicated by high DOS/DOC ratios and by the fact that 93% of all assigned DOM molecular formulas exclusively present in the fluids contained sulfur. In all three systems, DOS from the fluids had on average lower O/C ratios (0.31-0.33) than surrounding seawater DOS (0.47-0.49), suggesting shallow hydrothermal systems as a marine source of reduced DOS, which will likely get oxidized upon contact with oxygenated seawater. Evaluation of hypothetical pathways suggested DOM reduction and sulfurization during seawater recirculation in Milos. The four most effective pathways were those exchanging an O atom by one S atom in the formula or the equivalent +H₂S reaction. In conclusion, our study reveals novel insights about DOS dynamics in marine hydrothermal environments and provides a conceptual framework for molecular-scale mechanisms in organic sulfur geochemistry.