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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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The H2/CH4 ratio as a function of temperature.(a) Comparisons of H2/CH4 ratios in this study (in blue) and those in previous work (in black)2731454953. (b) An enlargement of the yellow rectangle in (a).
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f4: The H2/CH4 ratio as a function of temperature.(a) Comparisons of H2/CH4 ratios in this study (in blue) and those in previous work (in black)2731454953. (b) An enlargement of the yellow rectangle in (a).

Mentions: A plot of H2/CH4 ratios as a function of temperature is illustrated in Fig. 4, showing that the H2/CH4 ratios greatly depend on temperature. They reached their maximum values at ~300 °C, from 58 to 4,000 (Fig. 4a)3145. By contrast, the values were much lower at 400–500 °C, much less than 40 (Fig. 4), resulting from the dramatic decrease in H2 production and increase in CH4 formation. The decrease in H2 production may be induced by very slow rates of olivine serpentinization at temperatures higher than 350 °C464748, supported by infrared spectra of solid products with a sharp peak centered at 503 cm−1 for the Mg-O group of olivine and a weak band at 3677 cm−1 for the –OH group of talc (Fig. 2d). It suggests that H2 is mostly derived from orthopyroxene alteration. As indicated by experimental studies, the quantities of H2 produced during orthopyroxene alteration at >350 °C were one to two orders of magnitude less than those formed after olivine serpentinization at 300 °C3132. Consequently, H2 production at 400–500 °C decreases greatly. By contrast, CH4 concentrations increased at higher temperatures (Table 1), which possibly results from sufficient Fe-Ni alloys that highly enhance CH4 production29.


The H 2 /CH 4 ratio during serpentinization cannot reliably identify biological signatures
The H2/CH4 ratio as a function of temperature.(a) Comparisons of H2/CH4 ratios in this study (in blue) and those in previous work (in black)2731454953. (b) An enlargement of the yellow rectangle in (a).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: The H2/CH4 ratio as a function of temperature.(a) Comparisons of H2/CH4 ratios in this study (in blue) and those in previous work (in black)2731454953. (b) An enlargement of the yellow rectangle in (a).
Mentions: A plot of H2/CH4 ratios as a function of temperature is illustrated in Fig. 4, showing that the H2/CH4 ratios greatly depend on temperature. They reached their maximum values at ~300 °C, from 58 to 4,000 (Fig. 4a)3145. By contrast, the values were much lower at 400–500 °C, much less than 40 (Fig. 4), resulting from the dramatic decrease in H2 production and increase in CH4 formation. The decrease in H2 production may be induced by very slow rates of olivine serpentinization at temperatures higher than 350 °C464748, supported by infrared spectra of solid products with a sharp peak centered at 503 cm−1 for the Mg-O group of olivine and a weak band at 3677 cm−1 for the –OH group of talc (Fig. 2d). It suggests that H2 is mostly derived from orthopyroxene alteration. As indicated by experimental studies, the quantities of H2 produced during orthopyroxene alteration at >350 °C were one to two orders of magnitude less than those formed after olivine serpentinization at 300 °C3132. Consequently, H2 production at 400–500 °C decreases greatly. By contrast, CH4 concentrations increased at higher temperatures (Table 1), which possibly results from sufficient Fe-Ni alloys that highly enhance CH4 production29.

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