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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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Plot of H2/CH4 ratios as a function of time (in days), showing strong temperature dependence.(a) 311 °C and 3.0 kbar. (b) An enlargement of the rectangle in (a). The critical number 40 is shown as a horizontal curve. (c) 400–500 °C and 3.0 kbar.
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f1: Plot of H2/CH4 ratios as a function of time (in days), showing strong temperature dependence.(a) 311 °C and 3.0 kbar. (b) An enlargement of the rectangle in (a). The critical number 40 is shown as a horizontal curve. (c) 400–500 °C and 3.0 kbar.

Mentions: Molecular hydrogen, methane, ethane, and propane were formed. At 311 °C and 3.0 kbar, the H2/CH4 ratios ranged from 58 to 2,120, much higher than the critical value of 40 (Fig. 1a,b). The ratios increased as a function of time, implying that rates of H2 production are faster than the rates of CH4 formation. In experiments using peridotite with initial grain sizes <30 μm, the H2/CH4 ratios varied from 58 to 91. By contrast, for those using larger grain sizes (100–177 μm), the H2/CH4 ratios were much higher, from 360 to 2,120. At 400–500 °C and 3.0 kbar, the H2/CH4 ratios decreased greatly, 0.1–8.2 (Fig. 1c). In experiments at 500 °C and 3.0 kbar using peridotite with initial grain sizes of <30 μm, the H2/CH4 ratios increased during the first 20 days to a maximum value and then decreased slightly during the subsequent 16 days. This decrease suggests an increase of CH4 production (Table 1). A similar trend was also observed at 400 °C and 3.0 kbar with grain sizes of 42–59 μm, whereas it was not detected in experiments with larger grain sizes.


The H 2 /CH 4 ratio during serpentinization cannot reliably identify biological signatures
Plot of H2/CH4 ratios as a function of time (in days), showing strong temperature dependence.(a) 311 °C and 3.0 kbar. (b) An enlargement of the rectangle in (a). The critical number 40 is shown as a horizontal curve. (c) 400–500 °C and 3.0 kbar.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Plot of H2/CH4 ratios as a function of time (in days), showing strong temperature dependence.(a) 311 °C and 3.0 kbar. (b) An enlargement of the rectangle in (a). The critical number 40 is shown as a horizontal curve. (c) 400–500 °C and 3.0 kbar.
Mentions: Molecular hydrogen, methane, ethane, and propane were formed. At 311 °C and 3.0 kbar, the H2/CH4 ratios ranged from 58 to 2,120, much higher than the critical value of 40 (Fig. 1a,b). The ratios increased as a function of time, implying that rates of H2 production are faster than the rates of CH4 formation. In experiments using peridotite with initial grain sizes <30 μm, the H2/CH4 ratios varied from 58 to 91. By contrast, for those using larger grain sizes (100–177 μm), the H2/CH4 ratios were much higher, from 360 to 2,120. At 400–500 °C and 3.0 kbar, the H2/CH4 ratios decreased greatly, 0.1–8.2 (Fig. 1c). In experiments at 500 °C and 3.0 kbar using peridotite with initial grain sizes of <30 μm, the H2/CH4 ratios increased during the first 20 days to a maximum value and then decreased slightly during the subsequent 16 days. This decrease suggests an increase of CH4 production (Table 1). A similar trend was also observed at 400 °C and 3.0 kbar with grain sizes of 42–59 μm, whereas it was not detected in experiments with larger grain sizes.

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 &lt;40 for biotic methane. The definition of the criterion was based on two serpentinization experiments at 200&thinsp;&deg;C and 0.3&thinsp;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&ndash;500&thinsp;&deg;C and 3.0&thinsp;kbar using natural ground peridotite. Our results demonstrate that the H2/CH4 ratios strongly depend on temperature. At 311&thinsp;&deg;C and 3.0&thinsp;kbar, the H2/CH4 ratios ranged from 58 to 2,120, much greater than the critical value of 40. By contrast, at 400&ndash;500&thinsp;&deg;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