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Project Progress

Radiolysis experiments. Completed experimental results showed δ²H of H₂ can only be used as an indicator of in-situ water radiolysis for groundwater with relatively young ages and low temperatures. However, radiolytic H₂ yields derived from theoretical estimates for several natural settings seem sufficient to support lithoautotrophic microbial communities in continental crusts or enriched ore bodies, which have more abundant radioactive elements than those of oceanic crusts.

Hydrocarbon Yielding Water/Mineral Reactions. Substantial progress was made in three different sets of experiments under this Objective. Experiments generating abiogenic hydrocarbons under hydrothermal conditions (400°C, 500 bars) were conducted using gaseous sources of H₂ and CO₂, as well as the traditional method of using formic acid (in order to compare and contrast the influence of simple organic acids in the experiments). In addition, experiments were conducted to produce abiogenic hydrocarbon via Fischer-Tropsch synthesis using iron powder catalysts instead of magnetite, since mineral surfaces appear to introduce an additional source of carbon into the experiments due to residual carbon sorbed to the mineral surfaces even after careful cleaning. The initial results of the synthesis of abiogenic hydrocarbon via spark discharge experiments were also very positive. Initial results show that ethane produced from methane by polymerization in the spark discharge experiments do indeed show the pattern of inverse carbon isotope depletion and hydrogen isotope enrichment hypothesized by Sherwood Lollar et al. (2002) and first identified for the Kidd Creek mine gases in that paper in Nature.

The spark discharge results suggest that indeed production of abiogenic hydrocarbons via polymerization may have a characteristic isotopic signature that could be an important criteria for distinguishing abiogenic from biogenic sources of hydrocarbons. Recently there has been substantial interest in this approach as a more definitive means of resolving the abiogenic versus biogenic origin debate over methane in the Martian atmosphere. This issue is addressed in a manuscript in submitted to the Astrobiology Journal by T.C. Onstott, B. Sherwood Lollar and others. In addition, B. Sherwood Lollar was asked to discuss this research and its implications for Mars exploration at the NASA Astrobiology Institute sponsored workshop on “Methane on Mars” at NASA Goddard Space Flight Center in May 2005. The results of these experiments make an important step forward in trying to define a criteria for unambiguous identification of abiogenically produced hydrocarbons versus biogenic hydrocarbons and hence for establishing isotopic signatures of life.

Evolution of the C Cycle From Abiotic to Biotic Processes. As planned, comparison of results from Canadian Shield sites to results from the South Africa mines was used to develop a model to distinguish between microbial gases and abiogenic end-members in Precambrian Shield field settings. Most importantly, the geochemical, isotopic and microbiological data suggest that the methanogens might in fact be utilizing the abiogenic gases, specifically H₂, as an energy source in the deep subsurface. The association of high concentrations of H₂ with ¹³C-enriched CH₄ end-members, and H₂ depletion in the ¹³C-depleted methanogenic end-members suggests the possibility that abiogenic gases may support H₂ autotrophy linked to methanogenesis in the deep subsurface.

With continuing effort to increase the sensitivity of our analytical techniques, we have been able to observe details of the isotopic composition of the Shield gases not possible in previous research. In particular we are now building a data base documenting isotopic signatures for higher hydrocarbons (C5 and C6) and both the straight chain and branched chain forms of C4-C6. These observations have been used to develop a mass balance model, which adds substantially the evidence for an abiogenic origin via polymerization for the Shield gases.

Evolution of the N cycle from Abiotic to Biotic Processes. Nitrogen isotope values for South African samples ranged from -2‰ to 7‰ AIR, similar to that observed for the Canadian Shield and consistent with N₂ derived during devolatilization reactions associated with metamorphism. The Canadian sites had somewhat more enriched values δ¹⁵N (as high as 16‰AIR) and we are presently investigating whether these differences are a function of mixing with air, or whether they reflect differences in the thermal metamorphic histories.

Field Sites for Biosustainable Subsurface Environments.

Two field sampling trips were conducted by the Deep-Subsurface Life Team to Lupin Mine in May 2004 and March 2005 (Figure 1). The gases were predominantly methane with C1/C2+ ratios typically between 50-200 indicating a substantial component of C2+ (ethane, propane and butane) in these gases. Although the carbon and hydrogen isotope values for the Lupin methane samples appear to fall close to the field expected for microbially-produced gases, the high concentrations of C2+ in these boreholes argues against the gases being entirely microbially-derived. Gases produced by methanogenesis are typically dominated by methane, with C1/C2+ ratios > 10³-10⁵. In addition, abiogenic Shield gases found at other sites in Canada and South Africa have been shown to fall in a similar range as the Lupin samples — reflecting a predominantly abiogenic origin due to water-rock interaction, but with carbon isotope depletion and hydrogen isotope enrichment due to mixing with a component of microbially produced methane.

The study of deep subsurface, ecosystems in ancient groundwater is directly relevant to the exploration for extant life in the subsurface of Mars. In addition we are interested in the study of geological terranes where thermal overprinting is sufficient to eradicate microbial life and where tectonic processes keep the terrane isolated from Earth’s surface. We are planning to determine the types of prebiotic compounds that are formed or available in these thermally overprinted terranes, allowing assessment of the potential for life to be spawned and evolved beneath Earth’s surface.

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Highlights

• Ecosystems based on H₂ autotrophy are proposed as a possible basis for microbial life on Mars in a manuscript submitted to Astrobiology Journal.
• Abiogenic gases supporting H₂ autotrophy are linked to deep-subsurface methanogenesis in a paper by Sherwood Lollar et al. that has been accepted for publication in Chemical Geology.
• Results of stable hydrogen isotope experiments suggest a new criteria for unambiguous identification of abiogenically produced hydrocarbons versus biogenic hydrocarbons and hence for establishing isotopic signatures of life. Sherwood Lollar was invited to discuss this research and its implications for Mars exploration at the NAI sponsored workshop on “Methane on Mars” at NASA Goddard Space Flight Center in May 2005.