HR: 14:40h
AN: B23D-05 INVITED     [Abstracts]
TI: Experimental Studies of Abiotic Organic Synthesis Under Hydrothermal Conditions
AU: * McCollom, T M
EM: mccollom@lasp.colorado.edu
AF: CU Center for Astrobiology and Laboratory for Atmospheric and Space Physics, Campus Box 392, University of Colorado, Boulder, CO 80309-0392 United States
AB: It has become widely accepted among geochemists that methane and other organic compounds are readily synthesized in hydrothermal systems, and that the products of this synthesis make a significant contribution to the hydrocarbons observed in vapors and fluids from hydrothermal environments. The mechanism most frequently invoked for this synthesis is the Fischer-Tropsch (FT) process, which involves the surface-catalyzed reduction of CO (or CO₂) to methylene followed by polymerization of methylene into more complex hydrocarbons. Owing to the significance of the Fischer-Tropsch process to industrial applications, it has been extensively studied and there are numerous publications on the process in the chemical engineering literature. However, the vast majority of these studies are performed under conditions with little relevance to natural systems, and the behavior of FT synthesis under geologic conditions remains largely unclear. While there is substantial geochemcial evidence for the occurrence of abiotic organic compounds in hydrothermal systems and elsewhere, experimental studies have thus far generally failed to produce significant organic compounds under analogous conditions. For instance, because of the strongly reducing environments created during serpentinization, serpentinites are frequently cited as a particularly favorable environment for abiotic organic synthesis. Yet, experimental studies of this process have thus far failed to yield any significant organic compounds other than methane. On the other hand, experiments performed with native iron present generate abundant hydrocarbons, but it remains uncertain whether the reaction conditions are suitable analogs to geologic environments. The reasons for this gap between the experimental results and environmental observations remain uncertain. One issue remaining unresolved is whether or not significant amounts of organic compounds can be synthesized under purely aqueous conditions. This issue is particularly relevant to submarine hydrothermal systems, since pressures there restrict occurrences of a separate vapor phase. To date, experimental attempts to synthesize organic compounds under purely aqueous conditions have not produced significant amounts of organic compounds except for methane, suggesting that a vapor phase may be required for synthesis of complex organic compounds in hydrothermal environments. Another unresolved issue is the isotopic fractionation resulting from abiotic synthesis. Recent analyses of natural samples by Sherwood
Lollar et al. ( Nature 416, 522, 2002) and other researchers have suggested that light hydrocarbon gases produced by abiotic synthesis may have a unique isotopic signature that can be used to distinguish them from hydrocarbons generated by thermal degradation of biogenic organic matter. Although experimental studies might confirm these trends, there have been very few isotopic studies of Fischer-Tropsch products. Initial results, however, have not confirmed the same isotopic trends among FT products, suggesting that alternative catalysts or other processes such as methane polymerization may be involved. Interestingly, the carbon isotopic signature of higher molecular weight hydrocarbons produced by FT synthesis appear to fall within the range of typical biological processes, suggesting it may be difficult to discriminate between biologic and non-biologic sources on the basis of carbon isotopes. Additional experimental study is required to resolve all of these issues in order to clarify the contribution of abiotic synthesis to the organic matter found in hydrothermal environments.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1055 Organic and biogenic geochemistry
SC: Biogeosciences [B]
MN: Fall Meeting 2005