H2/CH4 ratios cannot reliably distinguish abiotic vs. biotic methane in natural hydrothermal systems

September 25, 2012
109 (47) E3210
Research Article
Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces
Christopher Oze, L. Camille Jones [...] Robert J. Rosenbauer
In their paper, Oze et al. (1) used H2/CH4 ratios to assess the origin of methane in serpentinization systems on Earth and Mars, with ratios <33 indicating that “life is likely present and active.” The production of CH4 and H2 in oceanic and continental hydrothermal systems has been intensively studied for several decades by using numerous techniques (e.g., 13C, 14C, thermodynamic modeling) in conjunction with comprehensive fluid analyses. Based on this body of work, we argue that H2/CH4 ratios are inappropriate for determining the origin of methane on this planet and likely on others. Artificially high H2/CH4 ratios in benchtop hydrothermal serpentinization experiments do not place realistic constraints on the complex multiple processes affecting CH4 and H2 in natural hydrothermal systems, despite their claims.
Of the nine “biotic serpentinization systems” cited (1), eight have temperatures (>150 °C) that argue against biological methanogenesis [Ashadze I and II, Broken Spur, Endeavour, Logatchev, Lucky Strike, Menez Gwen, and Rainbow (2)] and four are hosted in magmatically robust basaltic crust not undergoing serpentinization (Broken Spur, Endeavour, Lucky Strike, and Menez Gwen). Furthermore, thermogenic decomposition of buried sedimentary organic matter is considered the primary source of CH4 at Endeavour (3), which has the lowest cited H2/CH4 ratio (∼0.1), and is therefore not directly “biotic.” Oze et al. (1) cited Ashadze II as having the highest ratio (∼33), serving “as an upper bound for natural systems with dominantly abiogenic CH4 and minimal biological activity,” but fail to include higher H2/CH4 ratios (∼46) from serpentinite-influenced systems (4), or similarly high values from other basaltic systems in which serpentinization does not influence either volatile. Also, dissolved hydrocarbons at Ashadze II show 13C enrichments with carbon chain length (2) consistent with a thermogenic, not abiotic, origin (5). For the remaining systems, multiple lines of evidence still argue for a dominantly abiotic origin for methane without significant biotic inputs. These include methane δ13C and ∆14C signatures, relative concentrations and δ13C signatures of C1–C4 hydrocarbons, a lack of sedimentary inputs, and a lack of clear evidence for significant microbial methanogenesis (2, 5). Minor contributions of magmatically degassed methane are also neglected, yet this is likely the dominant source at Broken Spur (1), where H2 and CH4 are low, despite “biotic” ratios according to Oze et al. (1).
Similar to CH4, H2 can have multiple sources, as well as sinks, and is more reactive. High-temperature reactions with mineral assemblages (aside from serpentinization), microbial oxidation, and sulfate reduction can influence H2 production or consumption, and therefore H2/CH4 ratios, without impacting CH4. Additionally, H2 production by serpentinization of olivine-rich igneous rock is highly complex, and can vary over a wide range of conditions in nature that are often highly simplified in hydrothermal experiments. It is therefore unreasonable to apply purely experimental H2/CH4 ratios to diverse natural hydrothermal systems. The distinction made by Oze et al. (1) between “biotic serpentinization systems” and “abiotic serpentinization experiments” is instead better explained as a difference between the complexity of natural systems and the inability to replicate them with simple benchtop experiments.

References

1
C Oze, LC Jones, JI Goldsmith, RJ Rosenbauer, Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces. Proc Natl Acad Sci USA 109, 9750–9754 (2012).
2
JL Charlou, 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. Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, eds PA Rona, CW Devey, J Dyment, BJ Murton (American Geophysical Union, Washington, DC), pp. 265–296 (2010).
3
MD Lilley, et al., Anomalous CH4 and NH4+ concentrations at an unsedimented mid-ocean-ridge hydrothermal system. Nature 364, 45–47 (1993).
4
K Nakamura, et al., Serpentinized troctolites exposed near the Kairei Hydrothermal Field, Central Indian Ridge: Insights into the origin of the Kairei hydrothermal fluid supporting a unique microbial ecosystem. Earth Planet Sci Lett 280, 128–136 (2009).
5
G Proskurowski, et al., Abiogenic hydrocarbon production at lost city hydrothermal field. Science 319, 604–607 (2008).

Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 109 | No. 47
November 20, 2012
PubMed: 23012469

Classifications

Submission history

Published online: September 25, 2012
Published in issue: November 20, 2012

Authors

Affiliations

Susan Q. Lang1 [email protected]
Department of Earth Sciences, Eidgenössische Technische Hochschule Zürich, 8092 Zürich, Switzerland;
Gretchen L. Früh-Green
Department of Earth Sciences, Eidgenössische Technische Hochschule Zürich, 8092 Zürich, Switzerland;
Deborah S. Kelley
School of Oceanography, University of Washington, Seattle, WA 98195; and
Marvin D. Lilley
School of Oceanography, University of Washington, Seattle, WA 98195; and
Giora Proskurowski
School of Oceanography, University of Washington, Seattle, WA 98195; and
Eoghan P. Reeves
MARUM Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany

Notes

1
To whom correspondence should be addressed. E-mail: [email protected].
Author contributions: S.Q.L., G.F.G., D.S.K., M.D.L., G.P., and E.P.R. wrote the paper.

Competing Interests

The authors declare no conflict of interest.

Metrics & Citations

Metrics

Note: The article usage is presented with a three- to four-day delay and will update daily once available. Due to ths delay, usage data will not appear immediately following publication. Citation information is sourced from Crossref Cited-by service.


Citation statements




Altmetrics

Citations

If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download.

Cited by

    Loading...

    View Options

    View options

    PDF format

    Download this article as a PDF file

    DOWNLOAD PDF

    Get Access

    Login options

    Check if you have access through your login credentials or your institution to get full access on this article.

    Personal login Institutional Login

    Recommend to a librarian

    Recommend PNAS to a Librarian

    Purchase options

    Purchase this article to get full access to it.

    Single Article Purchase

    H2/CH4 ratios cannot reliably distinguish abiotic vs. biotic methane in natural hydrothermal systems
    Proceedings of the National Academy of Sciences
    • Vol. 109
    • No. 47
    • pp. 19035-19509

    Media

    Figures

    Tables

    Other

    Share

    Share

    Share article link

    Share on social media