ALBERT

Description: Shallow submarine hydrothermal systems are extreme environments with unique biogeochemical conditions, originating from the interaction of hot, reduced fluids and cold, oxygenated seawater. Hydrothermal fluids represent one of the marine sources of essential trace elements like iron, the flux of which is controlled by dissolved organic matter (DOM) in and between terrestrial and marine ecosystems. However, the selectivity of the DOM with iron in marine environments is not well understood. Here we characterized at a molecular level the marine hydrothermal iron?DOM interaction using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Our study site was a shallow-water hydrothermal system off Dominica Island (Lesser Antilles, Caribbean Sea), where reduced fluids were strongly enriched in dissolved Fe and Si as compared to surface seawater. The dissolved organic carbon (DOC) concentration was lower in the hydrothermal fluids, most likely due to low-DOC meteoric water influence, but also suggesting biotic or abiotic processes consuming or depleting DOC when seawater is re-circulated through the sediment. The formation of hydrous ferric oxides upon aeration of the hydrothermal fluids for 10 h leads to an 8% decrease in DOC, indicating co-precipitation of iron and DOM. Re-solubilization of iron precipitates revealed increased relative abundance of aromatic compounds in co-precipitated DOM, which is in accordance with iron-coagulation observed in terrestrial environments. In conclusion, we provide evidence of co-precipitation of DOM with iron at hydrothermal systems as a selective process, which characteristically alters the molecular composition of DOM released with hydrothermal fluids.

Description: Shallow submarine 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 when compared to surrounding seawater and can contain high concentrations of hydrogen sulfide (H2S). It is well known that sulfur in its various oxidation states 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 so far not well understood. We investigated DOS dynamics and its relation to the physicochemical environment by studying the molecular composition of dissolved organic matter (DOM) in three contrasting shallow hydrothermal systems off Milos (Eastern Mediterranean), Dominica (Caribbean Sea) and Iceland (North Atlantic). We used ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to characterize the DOM on a molecular level. The molecular information was complemented with general geochemical data, quantitative dissolved organic carbon (DOC) and DOS analyses as well as isotopic measurements (d2H, d18O and F14C). In contrast to the predominantly meteoric fluids from Dominica and Iceland, hydrothermal fluids from Milos were mainly fed by recirculating seawater. The hydrothermal fluids from Milos were enriched in H2S and DOS, as indicated by high DOS/DOC ratios and by the fact that 〉90% of all assigned DOM formulas that were exclusively present in the fluids contained sulfur. In all three systems, DOS from hydrothermal fluids had on average lower O/C ratios (0.26?0.34) than surrounding surface seawater DOS (0.45?0.52), suggesting shallow hydrothermal systems as a source of reduced DOS, which will likely get oxidized upon contact with oxygenated seawater. Evaluation of hypothetical sulfurization reactions suggests DOM reduction and sulfurization during seawater recirculation in Milos seafloor. The four most effective potential sulfurization reactions were those exchanging an O atom by one S atom in the formula or the equivalent + H2S reaction, correspondingly exchanging H2O, H2 and/or O2 by a H2S molecule. Our study reveals novel insights into DOS dynamics in marine hydrothermal environments and provides a conceptual framework for molecular-scale mechanisms in organic sulfur geochemistry.