Molecular evidence for abiotic sulfurization of dissolved organic matter in marine shallow hydrothermal systems

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• 作者：Gonzalo V. Gomez-Saez^a ; ^{gonzalo.gomez@uni-oldenburg.de" class="auth_mail" title="E-mail the corresponding author} ; Jutta Niggemann^b ; Thorsten Dittmar^b ; Anika M. Pohlabeln^b ; Susan Q. Lang^c ; ^d ; Ann Noowong^e ; Thomas Pichler^f ; Lars Wö ; rmer^g ; Solveig I. B&uuml ; hring^a
• 关键词：Marine shallow hydrothermal systems ; Dissolved organic matter (DOM) ; Dissolved organic sulfur (DOS) ; FT-ICR-MS ; Milos (Eastern Mediterranean) ; Dominica (Caribbean Sea) ; Iceland (North Atlantic)
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 October 2016
• 年：2016
• 卷：190
• 期：Complete
• 页码：35-52
• 全文大小：2635 K

文摘

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 (δ²H, δ¹⁸O 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.