The Abiotic Synthesis and Distribution of Aqueous Alkyl Thiols and Organic Sulfides in Marine Hydrothermal Systems

A.F. Bennett¹*, M.D. Schulte¹, & K.L. Rogers¹

Corresponding author: afbbpc@mizzou.edu
¹Univeristy of Missouri, Dept. of Geological Sciences, Columbia, MO, 65201

Abstract:
The purpose of this study was to model geochemically the extent to which thiols and organic sulfides effect the total distribution of organic species as fluids mix in hydrothermal environments. Solution composition and speciation is determined through the minimization of Gibbs free energy. These calculations were made with the use of the thermodynamic reaction path modeling code, EQ3/6, as one kg of 2° seawater was mixed, incrementally, into one kg of vent fluid at 350°. Initial compositions were constrained to average bottom seawater elemental concentrations while a sample from EPR 21°N represented a typical hydrothermal fluid from a mid-ocean ridge. Initial vent fluid oxidation state was consistent with the fayalite-magnetite-quartz mineral buffer assemblage to simulate more reducing conditions. A customized database incorporated the estimated reaction properties of thiols and organic sulfides. Results of this study indicate that organic sulfur compounds, thiols in particular, have a great effect on the transformation of inorganic carbon with nearly complete abiotic conversion (>99%) to organic species. Consistent with past studies of these systems investigating organic synthesis potential, thiols and sulfides also demonstrate a shift towards longer-chained compounds with decreasing temperature. While current study results suggest sulfur-bearing organic compounds exhibit a critical influence on the resulting fluid composition, more studies are necessary to determine the specific geochemical and biogeochemical roles of these compounds. Interdisciplinary studies, including those that examine crustal rock composition and hydrodynamic flow, are essential for understanding the abiotic organic synthesis potential of submarine hydrothermal systems.

Contributions to Integration and Synthesis:
The model described above was directed towards understanding aqueous organic compound distribution and circumstances leading to abiotic synthesis, even though there are certainly mineral precipitation/dissolution reactions and generalized water/rock interactions occurring. Another notable contribution of this study, as well as similar investigations, is the wealth of information geochemical energetics is able to provide biogeochemists and geomicrobiologists. While the above study was applied to a specific site, models can be constructed to address geochemical interactions in other compositional settings, allowing. for a greater understanding of the organic geochemistry of the whole mid-ocean ridge system.