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.2012 Jun 19;109(25):9750-4.
doi: 10.1073/pnas.1205223109. Epub 2012 Jun 7.

Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces

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Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces

Christopher Oze et al. Proc Natl Acad Sci U S A..

Abstract

Molecular hydrogen (H(2)) is derived from the hydrothermal alteration of olivine-rich planetary crust. Abiotic and biotic processes consume H(2) to produce methane (CH(4)); however, the extent of either process is unknown. Here, we assess the temporal dependence and limit of abiotic CH(4) related to the presence and formation of mineral catalysts during olivine hydrolysis (i.e., serpentinization) at 200 °C and 0.03 gigapascal. Results indicate that the rate of CH(4) production increases to a maximum value related to magnetite catalyzation. By identifying the dynamics of CH(4) production, we kinetically model how the H(2) to CH(4) ratio may be used to assess the origin of CH(4) in deep subsurface serpentinization systems on Earth and Mars. Based on our model and available field data, low H(2)/CH(4) ratios (less than approximately 40) indicate that life is likely present and active.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
CH4 (μmol kg-1), H2 (μmol kg-1), and H2/CH4 collected over time (h) for Exps. 1 and 2 (Table 1) at 200 °C and 0.03 GPa. Each data point represents one analysis per withdrawn sample; calibration and controls constrain uncertainties with errors smaller than the symbol.
Fig. 2.
Fig. 2.
SEM image of fine-grained magnetite and chrysotile with scale bar included.
Fig. 3.
Fig. 3.
A framework is presented for interpreting H2/CH4 ratios with the introduction and accumulation of magnetite.
Fig. 4.
Fig. 4.
Our model is shown for serpentinization-fueled H2 production and CH4 formation utilizing H2/CH4 ratios, data points, and rates from Fig. 1 (seeMaterials and Methods) and the framework presented in Fig. 3. Data from other studies demonstrate that abiotic serpentinization systems (i.e., laboratory experiments) have H2/CH4 ratios equal to or greater than approximately 40 and serpentinization systems where life is present have H2/CH4 ratios less than approximately 40.
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Comment in

References

    1. Kelley DS, et al. An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30 °N. Nature. 2001;412:145–149. - PubMed
    1. Sleep NH, et al. H2-rich fluids from serpentinization: Geochemical and biotic implications. Proc Natl Acad Sci USA. 2004;101:12818–12823. - PMC - PubMed
    1. Charlou JL, et al. High production and fluxes of H2 and CH4 and evidence of abiotic hydrocarbon synthesis by serpentinization in ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge. In: Rona PA, Devey CW, Dyment J, Murton BJ, editors. Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. Vol. 188. 2010. pp. 265–296. (Geophysical Monograph).
    1. Oze C, Sharma M. Serpentinization and the inorganic synthesis of H2 in planetary surfaces. Icarus. 2007;186:557–561.
    1. Lilley MD, et al. Anomalous CH4 and NH4+ concentrations at an unsedimented mid-ocean-ridge hydrothermal system. Nature. 1993;364:45–47.

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