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The H 2 /CH 4 ratio during serpentinization cannot reliably identify biological signatures

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ABSTRACT

Serpentinization potentially contributes to the origin and evolution of life during early history of the Earth. Serpentinization produces molecular hydrogen (H2) that can be utilized by microorganisms to gain metabolic energy. Methane can be formed through reactions between molecular hydrogen and oxidized carbon (e.g., carbon dioxide) or through biotic processes. A simple criterion, the H2/CH4 ratio, has been proposed to differentiate abiotic from biotic methane, with values approximately larger than 40 for abiotic methane and values of <40 for biotic methane. The definition of the criterion was based on two serpentinization experiments at 200 °C and 0.3 kbar. However, it is not clear whether the criterion is applicable at a wider range of temperatures. In this study, we performed sixteen experiments at 311–500 °C and 3.0 kbar using natural ground peridotite. Our results demonstrate that the H2/CH4 ratios strongly depend on temperature. At 311 °C and 3.0 kbar, the H2/CH4 ratios ranged from 58 to 2,120, much greater than the critical value of 40. By contrast, at 400–500 °C, the H2/CH4 ratios were much lower, ranging from 0.1 to 8.2. The results of this study suggest that the H2/CH4 ratios cannot reliably discriminate abiotic from biotic methane.

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Identification of solid products by scanning electron microscope and Fourier transformed infrared spectroscopy analyses.(a) HR61, 311 °C and 3.1 kbar, with the formation of fibrous chrysotile (Ctl). (b) HR79, 400 °C and 3.0 kbar, with tabular shaped lizardite (Lz). (c) HR81, 500 °C and 3.3 kbar, with lizardite. (d) Infrared spectra of solid products. The spectra indicate that serpentine formed at 311–400 °C, whereas serpentine and talc were produced at 500 °C and 3.3 kbar. Talc was identified based on its stretching mode at 671 cm−1 for Si-O-Mg and a stretching vibration at 3677 cm−1 for the –OH group42.
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f2: Identification of solid products by scanning electron microscope and Fourier transformed infrared spectroscopy analyses.(a) HR61, 311 °C and 3.1 kbar, with the formation of fibrous chrysotile (Ctl). (b) HR79, 400 °C and 3.0 kbar, with tabular shaped lizardite (Lz). (c) HR81, 500 °C and 3.3 kbar, with lizardite. (d) Infrared spectra of solid products. The spectra indicate that serpentine formed at 311–400 °C, whereas serpentine and talc were produced at 500 °C and 3.3 kbar. Talc was identified based on its stretching mode at 671 cm−1 for Si-O-Mg and a stretching vibration at 3677 cm−1 for the –OH group42.

Mentions: At 311 °C and 3.0 kbar, the major secondary hydrous mineral was fibrous chrysotile (Fig. 2a), whereas tabular shaped lizardite formed at 400 °C and 3.0 kbar (Fig. 2b). Serpentine was identified based on infrared spectra with stretching modes at 954 and 1087 cm−1 for the Si-O group and a stretching vibration at 3686 cm−1 for the –OH group (Fig. 2d)373839. Chemical compositions of secondary minerals in HR61 were provided in an experimental study40, consistent with compositions of serpentine41. At 500 °C and 3.0 kbar, the secondary hydrous minerals produced were talc and lizardite. Talc is characterized by a stretching mode at 671 cm−1 for Si-O-Mg and a stretching vibration at 3677 cm−1 for the –OH group (Fig. 2d)42.


The H 2 /CH 4 ratio during serpentinization cannot reliably identify biological signatures
Identification of solid products by scanning electron microscope and Fourier transformed infrared spectroscopy analyses.(a) HR61, 311 °C and 3.1 kbar, with the formation of fibrous chrysotile (Ctl). (b) HR79, 400 °C and 3.0 kbar, with tabular shaped lizardite (Lz). (c) HR81, 500 °C and 3.3 kbar, with lizardite. (d) Infrared spectra of solid products. The spectra indicate that serpentine formed at 311–400 °C, whereas serpentine and talc were produced at 500 °C and 3.3 kbar. Talc was identified based on its stretching mode at 671 cm−1 for Si-O-Mg and a stretching vibration at 3677 cm−1 for the –OH group42.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036043&req=5

f2: Identification of solid products by scanning electron microscope and Fourier transformed infrared spectroscopy analyses.(a) HR61, 311 °C and 3.1 kbar, with the formation of fibrous chrysotile (Ctl). (b) HR79, 400 °C and 3.0 kbar, with tabular shaped lizardite (Lz). (c) HR81, 500 °C and 3.3 kbar, with lizardite. (d) Infrared spectra of solid products. The spectra indicate that serpentine formed at 311–400 °C, whereas serpentine and talc were produced at 500 °C and 3.3 kbar. Talc was identified based on its stretching mode at 671 cm−1 for Si-O-Mg and a stretching vibration at 3677 cm−1 for the –OH group42.
Mentions: At 311 °C and 3.0 kbar, the major secondary hydrous mineral was fibrous chrysotile (Fig. 2a), whereas tabular shaped lizardite formed at 400 °C and 3.0 kbar (Fig. 2b). Serpentine was identified based on infrared spectra with stretching modes at 954 and 1087 cm−1 for the Si-O group and a stretching vibration at 3686 cm−1 for the –OH group (Fig. 2d)373839. Chemical compositions of secondary minerals in HR61 were provided in an experimental study40, consistent with compositions of serpentine41. At 500 °C and 3.0 kbar, the secondary hydrous minerals produced were talc and lizardite. Talc is characterized by a stretching mode at 671 cm−1 for Si-O-Mg and a stretching vibration at 3677 cm−1 for the –OH group (Fig. 2d)42.

View Article: PubMed Central - PubMed

ABSTRACT

Serpentinization potentially contributes to the origin and evolution of life during early history of the Earth. Serpentinization produces molecular hydrogen (H2) that can be utilized by microorganisms to gain metabolic energy. Methane can be formed through reactions between molecular hydrogen and oxidized carbon (e.g., carbon dioxide) or through biotic processes. A simple criterion, the H2/CH4 ratio, has been proposed to differentiate abiotic from biotic methane, with values approximately larger than 40 for abiotic methane and values of <40 for biotic methane. The definition of the criterion was based on two serpentinization experiments at 200 °C and 0.3 kbar. However, it is not clear whether the criterion is applicable at a wider range of temperatures. In this study, we performed sixteen experiments at 311–500 °C and 3.0 kbar using natural ground peridotite. Our results demonstrate that the H2/CH4 ratios strongly depend on temperature. At 311 °C and 3.0 kbar, the H2/CH4 ratios ranged from 58 to 2,120, much greater than the critical value of 40. By contrast, at 400–500 °C, the H2/CH4 ratios were much lower, ranging from 0.1 to 8.2. The results of this study suggest that the H2/CH4 ratios cannot reliably discriminate abiotic from biotic methane.

No MeSH data available.


Related in: MedlinePlus