HR: 15:10h
AN: B23D-07 INVITED     [Abstracts]
TI: Thermodynamics of Organic Compound Alteration in Hydrothermal Systems
AU: * Shock, E L
EM: eshock@asu.edu
AF: Arizona State University, GEOPIG, Depts. of Geological Sciences and Chemistry & Biochemistry, Tempe, AZ 85287 United States
AB: Organic compounds enter hydrothermal systems through infiltrating surface waters, zones of microbial productivity in the subsurface, extracts of organic matter in surrounding host rocks, and abiotic synthesis. Owing to variations in pH, oxidation state, composition, temperature, and pressure throughout the changing pathways of fluid migration over the duration of the system, organic compounds from all of these sources are introduced to conditions where their relative stabilities and reactivities can be dramatically transformed. If those transformations were predictable, then the extent to which organic alteration reactions have occurred could be used to reveal flowpaths and histories of hydrothermal systems. Speciation and mass transfer calculations permit some insight into the underlying thermodynamic driving forces that result in organic compound alteration. As an example, the speciation of many geochemist's canonical organic matter: CH₂O depends strongly on oxidation state, temperature, and total concentration of dissolved organic matter. Calculations show that at oxidation states buffered by iron-bearing mineral assemblages, organic acids dominate the speciation of CH₂O throughout hydrothermal systems, with acetic acid (itself equivalent to 2 CH₂O by bulk composition) and propanoic acid generally the most abundant compounds. However, at more reduced conditions, which may prevail in organic-rich iron-poor sediments, the drive is to form ketones and especially alcohols at the expense of organic acids. The distribution of organic carbon among the various members of these compound classes is strongly dependent on the total concentration of dissolved organic matter. As an example, at a bulk concentration equivalent to average dissolved organic matter in seawater (45μm), the dominant alcohols at 100°C are small compounds like ethanol and 1-propanol. In contrast, at a higher bulk concentration of 500μm, there is a drive to shift large percentages of dissolved organic carbon into 1-pentanol and 1-hexanol. As the fugacity of H₂ increases so does the complexity of the mixture of organic compounds that would result in the lowest energy state. However, the number of dominant compounds in the mixture decreases with increasing temperature for similar extents of reduction referenced to mineral buffered conditions.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1055 Organic and biogenic geochemistry
SC: Biogeosciences [B]
MN: Fall Meeting 2005