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2008 Annual Science Report

Indiana University, Bloomington Reporting  |  JUL 2007 – JUN 2008

Isotopic Signatures of Methane and Higher Hydrocarbon Gases From Precambrian Shield Sites: A Model for Abiogenic Polymerization of Hydrocarbons

Project Summary

Methane and higher hydrocarbon gases in ancient rocks on Earth originate from both biogenic and abiogenic processes. The measured carbon isotopic compositions of these natural gases are consistent with formation of polymerization of increasing long hydrocarbon chains starting with methane. Integration of carbon isotopic compositions with concentration data is needed to delineate the origin of hydrocarbon gases.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

Isotopic Signatures of Methane and Higher Hydrocarbon Gases from Precambrian Shield Sites: A Model for Abiogenic Polymerization of Hydrocarbons

Previous studies of methane (CH4) and higher hydrocarbon gases in Precambrian Shield rocks in Canada and the Witwatersrand Basin of South Africa identified both microbial and abiogenic origins for gas types. Relatively fresh paleometeoric waters were dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogenic microbes utilizing a CO2 reduction pathway. In contrast, the deepest and most saline fracture waters contained gases that did not resemble the products of microbial methanogenesis and were characterizedg by high concentrations of both H2 and CH4 gas and by isotopic compositions of CH4 and higher hydrocarbon gases aligned with abiogenic processes in water-rock reactions at high rock/water ratios.. Based on new data obtained for the higher hydrocarbon gases in particular, a model is proposed to account for carbon isotope variation between CH4 and the higher hydrocarbon gases (specifically ethane, propane, butane, and pentane) consistent with abiogenic polymerization. Values of δ13C for CH4 and the higher hydrocarbon gases predicted by the model are shown to match proposed abiogenic hydrocarbon gas end-members identified at 5 field sites (2 in Canada and 3 in South Africa) suggesting that the carbon isotope patterns between the hydrocarbon homologues reflect the reaction mechanism. Recent experimental and field studies of proposed abiogenic hydrocarbons such as those found at mid-ocean spreading centers and off-axis hydrothermal fields such as Lost City have begun to focus not only on the origin of CH4 but on the compositional and isotopic information contained in the higher hydrocarbon gases. Although the extent of fractionation in the first step in the hydrocarbon synthesis reaction chain may vary as a function of different reaction parameters, δ13C values for the higher hydrocarbon gases may be predicted by a simple mass balance model from the δ13C values of the lower molecular weight precursors, consistent with abiogenic polymerization. Integration of isotopic data for the higher hydrocarbon gases in addition to CH4 may be critical for delineation of the origin of the hydrocarbons and investigation of formation mechanisms.

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    Barbara Sherwood Lollar Barbara Sherwood Lollar
    Ken Voglesonger
    Tullis Onstott

    Lisa Pratt

    G Lacrampe-Couloume
    Research Staff

    Stefanie Tille
    Doctoral Student

    Objective 1.1
    Models of formation and evolution of habitable planets

    Objective 2.1
    Mars exploration

    Objective 3.1
    Sources of prebiotic materials and catalysts

    Objective 4.1
    Earth's early biosphere

    Objective 4.2
    Foundations of complex life

    Objective 6.1
    Environmental changes and the cycling of elements by the biota, communities, and ecosystems

    Objective 7.1
    Biosignatures to be sought in Solar System materials

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems