Modeling organic sulfur geochemistry and biogeochemistry of hydrothermal systems

M. Schulte¹* & K.L. Rogers¹

Corresponding author: schultemd@missouri.edu
¹University of Missouri, Department of Geological Sciences, Columbia, MO, 65211

Abstract:
Along with similar data for the alkyl thiols (Schulte and Rogers, 2004), thermodynamic properties for aqueous alkyl sulfides have been compiled and/or estimated (Schulte, 2009). These properties are used to investigate reactions among organic sulfur compounds in a variety of geological environments, ranging from sea floor hydrothermal systems to organic-rich sludge. Using thermodynamic data and the revised Helgeson-Kirkham-Flowers equations of state, along with geochemical constraints imposed by the environment, we can estimate the abiotic production of these classes of organic sulfur compounds. In hydrothermal systems where H2 and H2S concentrations are buffered by the pyrite-pyrrhotite-magnetite (PPM) mineral assemblage, calculated equilibrium activities of methanethiol (MSH) and dimethylsulfide (DMS) are as high as 10-² and 10-³, respectively, through formation reactions where the environment contains millimolal concentrations of CO2. Higher activities are obtained when MSH and DMS form from CO and more reducing mineral assemblages are considered.

Contributions to Integration and Synthesis:
My primary research interest in hydrothermal systems is understanding the distribution of carbon and potential for abiotic generation of organic matter during water/rock reactions and fluid mixing. I am also interested in the availability of inorganic and organic compounds to serve as substrates for microoganisms that inhabit hydrothermal environments. Additionally, I use geochemical models to determine the energetic potential for particular organic compounds to act as substrates for heterotrophic microorganisms (currently focusing on the organic sulfur compounds). A number of microorganisms have been identified or isolated from environments after they were postulated to inhabit particular fluids based on the chemistry and the availability of particular species to act as substrates. Thus, we can make predictions of bioenergy availability through geochemical modeling, helping microbiologists with media design. Currently I am focusing on understanding the interactions between carbon and sulfur compounds.

The presence of organic matter, and the identification of organic sulfur compounds found in hydrothermal fluids was predicted based on geochemical models of hydrothermal environments (Shock and Schulte, 1998; Schulte and Rogers, 2004; Schulte, 2009). Therefore, it is clear that theory provides hypotheses that can be tested through experiment and observation, while the latter also help make the models as realistic as possible. Geochemical models require complete and accurate compositions of vent fluids in order for us to make meaningful interpretations of chemical processes occurring in hydrothermal systems. They also require thermodynamic data for as many system components system as possible. Thus, we are working to develop the data needed for other classes of aqueous organic compounds. We are currently including organic sulfur compounds in our geochemical modeling database for mixing calculations to understand the potential for abiotic synthesis of organic matter in hydrothermal systems.

References:
Schulte M. (2009) Organic sulfides in hydrothermal solution: Standard partial molal properties and role in the organic geochemistry of hydrothermal environments. Aquatic Geochem., submitted.
Schulte M. D and Rogers K. L. (2004) Thiols in hydrothermal solution: Standard partial molal properties and their role in the organic geochemistry of hydrothermal environments. GCA 68, 1087-1097.
Shock E. L. and Schulte M. D. (1998) Organic synthesis during fluid mixing in hydrothermal systems. J. Geophys. Res. 103, 28,513-28527.