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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.

No MeSH data available.


Schulz-Flory distribution of hydrocarbons produced in this study.The two solid lines show the least-squares fit of the data at 400 °C and 3.0 kbar (Table 1) using Origin 8.6 with correlation coefficients (R2).
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f3: Schulz-Flory distribution of hydrocarbons produced in this study.The two solid lines show the least-squares fit of the data at 400 °C and 3.0 kbar (Table 1) using Origin 8.6 with correlation coefficients (R2).

Mentions: The hydrocarbons produced in this study are probably abiotic, supported by the following evidence. First, blank experiments were performed at 311–500 °C and 3.0 kbar using peridotite loaded without any fluid. The quantities of H2 and hydrocarbons were below the detection limit of gas chromatograph after 27 days of reaction time. It suggests that hydrocarbons were not released from the decomposition of organic matter and long-chain hydrocarbons in peridotite4344. Otherwise, it would result in highly elevated hydrocarbons. Moreover, the log of the n-alkane concentrations is linearly correlated with the carbon numbers (Fig. 3), which is consistent with the Schulz-Flory distribution predicted for FTT synthesis31. All these indicate that hydrocarbons were formed through reactions between H2 and dissolved carbon dioxide from the atmosphere in the starting fluid.


The H 2 /CH 4 ratio during serpentinization cannot reliably identify biological signatures
Schulz-Flory distribution of hydrocarbons produced in this study.The two solid lines show the least-squares fit of the data at 400 °C and 3.0 kbar (Table 1) using Origin 8.6 with correlation coefficients (R2).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Schulz-Flory distribution of hydrocarbons produced in this study.The two solid lines show the least-squares fit of the data at 400 °C and 3.0 kbar (Table 1) using Origin 8.6 with correlation coefficients (R2).
Mentions: The hydrocarbons produced in this study are probably abiotic, supported by the following evidence. First, blank experiments were performed at 311–500 °C and 3.0 kbar using peridotite loaded without any fluid. The quantities of H2 and hydrocarbons were below the detection limit of gas chromatograph after 27 days of reaction time. It suggests that hydrocarbons were not released from the decomposition of organic matter and long-chain hydrocarbons in peridotite4344. Otherwise, it would result in highly elevated hydrocarbons. Moreover, the log of the n-alkane concentrations is linearly correlated with the carbon numbers (Fig. 3), which is consistent with the Schulz-Flory distribution predicted for FTT synthesis31. All these indicate that hydrocarbons were formed through reactions between H2 and dissolved carbon dioxide from the atmosphere in the starting fluid.

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.