Organic carbon cycling and preservation in aquatic environments: Origin, forms and mechanisms using bulk, molecular and isotopic methods

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I propose to continue research initiated in the past 5 years on the forms, sources, reactions, and fates of quantitatively important forms of organic matter (OM) in aquatic environments. The central themes of our program are to develop new analytical methods (bulk, molecular and isotopic) that can be applied to organic mixtures, and to use these methods to test hypotheses about processes affecting OM cycling in aquatic systems. During the previous DG cycle, our work targeted the interactions between reactive iron oxides and sedimentary OM and the role played by these interactions on carbon preservation in sediments. We also developed methods and optimized instrumentation for the measurement of the 13C signature of dissolved organic carbon in seawater, and used the stable isotope signature of methane to decipher its sources in ground water, and its cycle in lacustrine sediments. The research proposed in the next 5 years will answer some of the questions that were raised by our previous work, and describes new and original projects that will be undertaken during the next DG cycle.******The following major themes will be addressed in the next 5 years: (1) we will now target the interactions between iron sulfides and OM in sediments, and their role in OM cycling and preservation; (2) we will probe the effect and mechanism of dissolved sulfide exposure on free and OM-bound iron oxides; (3) we will assess the reactivity of natural pore water extracted from anoxic sediments for iron sulfide particles and examine the speciation of sulfides in the corresponding solid phase; (4) we will use molecular modeling and single molecule atomic force microscopy to better understand the interactions between organic compounds and the surface of iron sulfides nanoparticles; (5) in a new initiative, we will use compound-specific stable isotope analysis (CSIA; 13C and D) on petroleum derived hydrocarbons to understand the effect of weathering (e.g., dispersion, evaporation, photooxidation and biodegradation) on the relative abundances and stable isotope signatures of these hydrocarbons, and we will build a dilution model that will allow spatially and temporally tracking these contaminants in the St. Lawrence River, Estuary and Gulf; and (6) we will keep developing our CSIA methods toolbox in a series of small projects for which the application of CSIA could be rewarding. ******Mechanistically understanding the dynamics of shielded OM at the redox interface represents one of the greatest analytical frontiers remaining in biogeochemistry. Developing new analytical strategies is thus a fundamental prerequisite for obtaining a mechanistic understanding of OM preservation in lakes, estuaries and the ocean. Our work on OM interactions with Fe sulfides, in particular, will prove very promising.