Are permeable coastal sediments hotspots for the formation of non-volatile dissolved organic sulfur (DOS) in the ocean?

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Organic sulfur compounds are ubiquitous in marine sediments. The main formation pathway of these compounds is the abiotic reaction of inorganic sulfur species with common biomolecules. Because of the importance of this process for global biogeochemical cycles, for the exploration of oil fields, and for the preservation of the molecular paleorecord, there is a wealth of studies on the topic. Very little attention, however, has been paid on water-soluble compounds that may be formed during the process of sulfurization and that may eventually enter the oceanic water column as dissolved organic sulfur (DOS). Based on the little available information, sulfur could be the third most abundant heteroelement in marine dissolved organic matter (DOM), after oxygen and nitrogen. Some sulfur compounds, in particular thiols, determine the mobility of hazardous elements and also essential trace elements. Important climate-relevant volatile sulfur compounds originate from DOS. Thus, the marine cycling of DOS has both atmospheric and oceanic consequences. Despite this important role, the sources of marine DOS, its cycling in the ocean and its function for marine life are unknown. Also the molecular composition of DOS is elusive. In this project we will perform pioneering research in a new field of marine biogeochemistry. We seek to answer fundamental questions with respect to the formation and distribution of non-volatile DOS in the ocean. Our main hypotheses: * Formation of DOS: (1) Sulfate-reducing sediments are prime sites of DOS formation. (2) Reduced forms of organic sulfur (mainly thiols) predominate in the formation sites. (3) DOS is formed primarily via abiotic sulfurization during early diagenesis. * Transport and fate of DOS in the ocean: (4) DOS is discharged from sulfate-reducing intertidal groundwater into the ocean. (5) In the oxic water column, DOS quickly oxidizes (e.g., into sulfonic acids).(6) DOS from intertidal sediments survives transport in an oxidized form over large distances across continental shelves. Aside the scientific objective of answering these hypotheses, this project will provide three PhD students, one in Germany and two in Brazil, with the extraordinary opportunity to perform their graduate research within the framework of an international project. Joining the strength of the two partners, we will apply a combination of field sampling, laboratory experiments and elemental, isotopic and molecular analysis. Sampling includes sharply contrasting regions in the German Wadden Sea and Brazilian mangroves (Rio de Janeiro and Amazonia), including the adjacent shelf regions on a large scale. Sulfurization experiments will complement the field studies. For the quantification and molecular characterization of DOS we will apply novel approaches that were recently established in the PIs working groups, involving ultrahigh-resolution mass spectrometry (FT-ICR-MS), and other mass spectrometry and chromatography techniques.