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Cited by 177Space Science and Extraterrestrial LifePlanetary Science and ExplorationPaleontology and Stratigraphy of Fossils

Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life

Open access
Krissansen-Totton, Joshua, Olson, Stephanie, Catling, David C.·2018·Science Advances
DOI 10.1126/sciadv.aao5747PMC PMC5787383
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Abstract

Coexisting methane and carbon dioxide in atmospheres of habitable planets represent a disequilibrium biosignature.

AI snapshot

Written from the paper’s full text.

TL;DR

The paper quantifies thermodynamic disequilibrium in Earth’s atmosphere-ocean system through the Archean, Proterozoic, and Phanerozoic, finding a secular increase tied to biogenic oxygen and life. It argues that a CH4-N2-CO2-H2O disequilibrium is a plausible remotely detectable biosignature for anoxic exoplanets, especially because large abiotic CH4 fluxes are improbable.

Key findings

  • Earth’s atmosphere-ocean available Gibbs energy increased over time: Archean maximum 234 J/mol and minimum 5.1 J/mol; Proterozoic maximum 884 J/mol and minimum 9.5 J/mol; modern fluid-envelope disequilibrium cited as 2326 J/mol (modern gas-phase disequilibrium 1.5 J/mol).
  • In the maximum Proterozoic case, N2-O2-H2O reaction to nitric acid plus carbon speciation contributes ~640 J/mol (72%), with methane oxidation adding 75 J/mol; the minimum case remains nitrate-dominated but only 9.5 J/mol due to lower Po2.
  • In the maximum Archean case, CO2-N2-CH4-H2O reaction to NH4+ and HCO3− contributes ~170 J/mol (74%) and consumes 99.8% of initial CH4; even the minimum Archean case depletes CH4 by 99.99% at equilibrium.
  • Abiotic serpentinization/Fischer-Tropsch-type CH4 fluxes are much smaller than modern biological and plausible Archean biological fluxes; CH4 mixing ratios >10−3 imply source fluxes >7 Tmol/year and >10−2 imply >50 Tmol/year, likely biological when combined with CO2 and absence of CO.

Methods

  • Available Gibbs energy Φ was defined as the Gibbs free energy difference between initial atmosphere-ocean composition and equilibrium state, computed by Gibbs energy minimization conserving atoms and charge; MATLAB code was validated against Aspen Plus 8.6, generally agreeing within 10% or better.
  • Initial Archean and Proterozoic atmosphere-ocean compositions came from literature ranges, with maximum/minimum disequilibrium endmembers (Table 1); calculations used 1-bar surface pressure, Henry’s law for dissolved gases, pH/carbonate from alkalinity and Pco2, and Pitzer equations for aqueous activity coefficients.
  • Maximum abiotic CH4 flux from serpentinization plus Fischer-Tropsch-type synthesis was estimated by uniformly sampling ranges for crustal production (5.7×10^13 to 5.7×10^14 kg/year), FeO wt%, fractional FeO to H2, and H2 to CH4, calculating the flux 1 million times.

Limitations

  • Stated: solids and most nonvolatile aqueous species were neglected because the focus is remotely observable disequilibria; adding solids would not diminish calculated disequilibria but could make total available energy larger.
  • Stated: large ranges reflect uncertainties in atmospheric composition, mainly Archean Pch4 and Proterozoic Po2; ocean composition and volume are difficult or impossible to observe, and the paper did not attempt to capture changes from the advent of anoxygenic photosynthesis due to insufficient constraints.
  • Evident: exoplanet biosignature interpretation depends on future observational capabilities and on photochemical calculations that the paper says ought to be applied in the future; solid-state crustal disequilibria are excluded, so total Earth disequilibrium is underestimated.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1126/sciadv.aao5747

Cites

101 of 101 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 86 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    There was an important biogenic disequilibrium in the anoxic Archean atmosphere.

    “However, in this paper, we show that there was an important biogenic disequilibrium in the anoxic Archean atmosphere.”
    • Also stated in: § The Archean disequilibrium and species that contribute to it · plain
      “The available Gibbs energy for the maximum Archean case is 234 J/mol, which is dominated by the coexistence of CO2, N2, CH4, and liquid water.”
    Quote in the paperSubstantial claim
  • Findingown data

    Unless both NH4+ and HCO3− are extremely high, methane should be depleted in equilibrium.

    “Unless both NH4+ and HCO3− are extremely high, methane should be depleted in equilibrium.”
    Quote in the paperSubstantial claim
  • Findingown data

    The magnitude of the Archean thermodynamic disequilibrium typically varies by less than a factor of 2 across a wide range of assumptions about ocean pH, alkalinity, salinity, volume, temperatures, and pressure.

    “The magnitude of the Archean thermodynamic disequilibrium typically varies by less than a factor of 2 across a wide range of assumptions about ocean pH, alkalinity, salinity, volume, temperatures, and pressure.”
    Quote in the paperSubstantial claim
  • Findingown data

    The abiotic photochemical disequilibrium of Mars is comparable in magnitude to the biological disequilibrium of the early Earth.

    “The thermodynamic disequilibria of modern Mars and Titan are overplotted in Fig. 2, and it can be seen that the abiotic photochemical disequilibrium of Mars is comparable in magnitude to the biological disequilibrium of the early Earth.”
    Quote in the paperSubstantial claim
  • Findingreasoning

    Nitric acid formation and subsequent adjustment of carbon speciation contributes ~640 J/mol, the majority (72%) of Proterozoic atmosphere-ocean disequilibrium.

    “Nitric acid formation and subsequent adjustment of carbon speciation contributes ~640 J/mol, the majority of Proterozoic atmosphere-ocean disequilibrium (72%).”
    Quote in the paperSubstantial claim
  • May combine two claims. 47
  • Classified as probably not a substantial claim. 21

  • May combine two claims.

    Findingown data

    Earth's atmosphere-ocean disequilibrium was the smallest in the Archean, increased with the initial rise of oxygen during the Paleoproterozoic Great Oxidation Event, and then increased again after a second major increase in oxygen during the Neoproterozoic.

    “We find that Earth’s atmosphere-ocean disequilibrium was the smallest in the Archean, increased with the initial rise of oxygen during the Paleoproterozoic Great Oxidation Event, and then increased again after a second major increase in oxygen during the Neoproterozoic (Fig. 2).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    There was a secular increase in Earth's atmosphere-ocean disequilibrium over Earth history, correlated with the history of atmospheric oxygen.

    “We calculate a secular increase in Earth’s atmosphere-ocean disequilibrium over Earth history, correlated with the history of atmospheric oxygen.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    CO2, N2, CH4, and liquid water should not coexist but rather react to form ammonium and bicarbonate, depleting almost all atmospheric methane (99.8% of initial CH4 consumed).

    “These four species should not coexist but rather should react to form ammonium and bicarbonate, depleting almost all atmospheric methane (99.8% of the initial CH4 is consumed by reaction to equilibrium)”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Earth's atmosphere-only disequilibrium decreases with time and thus does not reflect the rise of oxygen or the growth of primary productivity since 3.5 Ga.

    “The atmosphere-only disequilibrium decreases with time and thus does not reflect the rise of oxygen or the growth of primary productivity since 3.5 Ga.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The available Gibbs energy for the maximum Archean case is 234 J/mol, dominated by the coexistence of CO2, N2, CH4, and liquid water.

    “The available Gibbs energy for the maximum Archean case is 234 J/mol, which is dominated by the coexistence of CO2, N2, CH4, and liquid water.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The available energy for the minimum Archean case is 5.1 J/mol, but even in this case, CH4 is 99.99% depleted with respect to its initial abundance at equilibrium.

    “The available energy for the minimum Archean case is 5.1 J/mol, but even in this case, CH4 is 99.99% depleted with respect to its initial abundance at equilibrium”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The largest source of disequilibrium in the Proterozoic is the coexistence of N2, O2, and liquid water, which should react to form nitric acid.

    “The largest source of disequilibrium in the Proterozoic is the same as for the modern Earth: The levels of N2, O2, and liquid water should not coexist but rather react to form nitric acid”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    For the minimum Proterozoic case, the disequilibrium is still dominated by nitrate formation, but the available energy is only 9.5 J/mol due to lower initial Po2.

    “For the minimum Proterozoic case, the disequilibrium is still dominated by nitrate formation, but the available energy is only 9.5 J/mol due to lower initial Po2 (partial pressure of oxygen).”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingreasoning

    The Earth's basaltic oceanic crust is ~10 wt % FeO, which implies a modern FeO production rate of 79 Tmol FeO/year.

    “The Earth’s basaltic oceanic crust is ~10 wt % FeO, which would imply a modern FeO production rate of 0.1 × 5.7 × 1013/(0.056 + 0.016) = 79 Tmol FeO/year.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    The H2 production rate from serpentinization is 0.25% of the total FeO production, two orders of magnitude less than a one-third theoretical maximum from the stoichiometry of Eq. 7.

    “which is 0.25% of the total FeO production, two orders of magnitude less than a one-third theoretical maximum from the stoichiometry of Eq. 7.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    Both the modern biological methane flux and plausible biological Archean fluxes are much larger than the distribution of maximum abiotic fluxes.

    “Both the modern biological methane flux (58) and plausible biological Archean fluxes (59) are much larger than the distribution of maximum abiotic fluxes.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [58] K. L. Denman, G. Brasseur, A. Chidthaisong, P. Ciais, P. M. Cox, R. E. Dickinson, Couplings between changes in the climate system and biogeochemistry in Climate Change 2007: The Physical Science Basis, S. Solomon, Ed. (Cambridge Univ. Press, 2007).

      Not checked in the free run.

    • cites [59] Kharecha P., Kasting J., Siefert J., A coupled atmosphere–ecosystem model of the early Archean Earth. Geobiology 3, 53– 76 (2005). source

      Not checked in the free run.

    • The authors state this as an assumption: All H2 produced by serpentinization is converted to CH4 via Fischer-Tropsch type synthesis.
    • The authors state this as an assumption: The entirety of an Exo-Earth’s FeO oxidation is due to H2-producing serpentinization through Eq. 7.

  • May combine two claims.

    Findingreasoning

    CH4 abundances in excess of 10−2 imply methane source fluxes in excess of 50 Tmol/year, which is very likely biological.

    “whereas CH4 abundances in excess of 10−2 imply methane source fluxes in excess of 50 Tmol/year, which is very likely biological (compare Fig. 6), when seen in combination with CO2 and the absence of CO.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    The available energy from oxidizing all surface biomass dwarfs the ~1 J/mol from atmospheric methane oxidation but is only half the size of the N2-O2-H2O disequilibrium on the modern Earth.

    “This dwarfs the ~1 J/mol from atmospheric methane oxidation but is only half the size of the N2-O2-H2O disequilibrium on the modern Earth.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingreasoning

    A smaller thermodynamic contribution to disequilibrium came from the CH4-O2 couple, which remains a compelling biosignature due to the short kinetic lifetime of CH4 in O2-rich atmospheres.

    “A smaller thermodynamic contribution to disequilibrium came from the CH4-O2 couple, which remains a compelling biosignature due to the short kinetic lifetime of CH4 in O2-rich atmospheres.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingnone given

    The magnitude of this disequilibrium has increased through time, correlated with increases in atmospheric oxygen and probable growth in biomass.

    “The magnitude of this disequilibrium has increased through time, correlated with increases in atmospheric oxygen and probable growth in biomass.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    CH4 abundances in excess of 10−3 imply methane source fluxes in excess of 7 Tmol/year, which is likely biological.

    “Calculations (section S6) suggest that CH4 abundances in excess of 10−3 imply methane source fluxes in excess of 7 Tmol/year, which is likely biological, whereas CH4 abundances in excess of 10−2 imply methane source fluxes in excess of 50 Tmol/year, which is very likely biological (compare Fig. 6), when seen in combination with CO2 and the absence of CO.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Chemical disequilibrium need not map to the amount of biological production to be a good biosignature; it merely has to reveal the existence of life.

    “However, chemical disequilibrium need not map to the amount of biological production to be a good biosignature—it merely has to reveal the existence of life.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Oxygenic photosynthesis is enzymatically complex and only evolved once on Earth.

    “Oxygenic photosynthesis is enzymatically complex and only evolved once on Earth (15).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [15] Knoll A. H., Cyanobacteria and earth history. Cyanobacteria Mol. Biol. Genom. Evol. 484 (2008).

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    The modern O2-CH4 redox couple is a compelling disequilibrium biosignature because of the short kinetic lifetime (~10 years) of methane in Earth's atmosphere.

    “The modern O2-CH4 redox couple is widely believed to be a compelling disequilibrium biosignature because of the short kinetic lifetime (~10 years) of methane in Earth’s atmosphere (21, 22)”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [21] Prinn R. G., Huang J., Weiss R. F., Cunnold D. M., Fraser P. J., Simmonds P. G., McCulloch A., Harth C., Salameh P., O’Doherty S., Wang R. H. J., Porter L., Miller B. R., Evidence for substantial variations of atmospheric hydroxyl radicals in the past two decades. Science 292, 1882– 1888 (2001). 11337586 source

      Not checked in the free run.

    • cites [22] Simoncini E., Virgo N., Kleidon A., Quantifying drivers of chemical disequilibrium: Theory and application to methane in the Earth’s atmosphere. Earth Syst. Dyn. 4, 317– 331 (2013). source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Earth's purely gas-phase disequilibrium, as measured by available Gibbs free energy, is 1.5 J/mol of atmosphere and largely attributable to the O2-CH4 thermodynamic disequilibrium.

    “Earth’s purely gas-phase disequilibrium, as measured by available Gibbs free energy, is 1.5 J/mol of atmosphere and largely attributable to the O2-CH4 thermodynamic disequilibrium.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [25] Krissansen-Totton J., Bergsman D. S., Catling D. C., On detecting biospheres from chemical thermodynamic disequilibrium in planetary atmospheres. Astrobiology 16, 39– 67 (2016). 26789355 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Oxygen levels in the Proterozoic eon (2.5 to 0.541 Ga) are disputed, but some proxy estimates imply remotely undetectable levels.

    “Oxygen levels in the Proterozoic eon (2.5 to 0.541 Ga) are disputed, but some proxy estimates imply remotely undetectable levels (14).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [14] Planavsky N. J., Reinhard C. T., Wang X., Thomson D., McGoldrick P., Rainbird R. H., Johnson T., Fischer W. W., Lyons T. W., Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635– 638 (2014). 25359975 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Oxygenic photosynthesis possibly originated by 3.0 Ga.

    “despite the possible origin of oxygenic photosynthesis by 3.0 Ga (13)”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [13] Planavsky N. J., Asael D., Hofmann A., Reinhard C. T., Lalonde S. V., Knudsen A., Wang X., Ossa F. O., Pecoits E., Smith A. J. B., Beukes N. J., Bekker A., Johnson T. M., Konhauser K. O., Lyons T. W., Rouxel O. J., Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event. Nat. Geosci. 7, 283– 286 (2014). source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductown data

    MATLAB calculations agree with Aspen Plus outputs to within 10% or better.

    “In general, MATLAB and Aspen outputs agree to within 10% or better.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    If the true atmosphere and ocean abundances are bounded by the values in Table 1, the minimum and maximum calculated disequilibria encompass the true disequilibrium of Earth's atmosphere-ocean system through time.

    “If the true atmosphere and ocean abundances are bounded by the values in Table 1, then the minimum and maximum disequilibria we calculate will encompass the true disequilibrium of the Earth’s atmosphere-ocean system through time.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The Phanerozoic Earth range was determined by varying initial oxygen mixing ratios from 0.1 to 0.3, the range inferred for the Phanerozoic.

    “the Phanerozoic Earth range was determined by using the abundances from (25) but varying initial oxygen mixing ratios from 0.1 to 0.3, which is the range inferred for the Phanerozoic (30).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [25] Krissansen-Totton J., Bergsman D. S., Catling D. C., On detecting biospheres from chemical thermodynamic disequilibrium in planetary atmospheres. Astrobiology 16, 39– 67 (2016). 26789355 source

      Not checked in the free run.

    • cites [30] Berner R. A., Beerling D. J., Dudley R., Robinson J. M., Wildman R. A. Jr, Phanerozoic atmospheric oxygen. Annu. Rev. Earth Planet. Sci. 31, 105– 134 (2003). source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductreasoning

    The initial water vapor abundance from Henry's law (1.6%) is consistent with the empirical range (0 to 4%).

    “using Henry’s law yields an initial abundance (1.6%) consistent with this empirical range.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Nitrogen absorbs at 4.15 μm due to N4, which could be used to infer N2 partial pressure.

    “Nitrogen absorbs at 4.15 μm due to N4, which could be used to infer N2 partial pressure (37).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [37] Schwieterman E. W., Robinson T. D., Meadows V. S., Misra A., Domagal-Goldman S., Detecting and constraining N2 abundances in planetary atmospheres using collisional pairs. Astrophys. J. 810, 57 (2015). source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    A nonlinear relationship exists between degree of serpentinization and H2 production, and for <50% serpentinization, negligible H2 is generated.

    “A nonlinear relationship also exists between degree of serpentinization and H2 production, and for <50% serpentinization, negligible H2 is generated (49).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [49] Cannat M., Fontaine F., Escartin J., Serpentinization and associated hydrogen and methane fluxes at slow spreading ridges. Geophys. Monogr. Ser. 241– 264 (2010).

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Hydrothermal alteration of crustal mafic rocks (serpentinization) produces H2, followed by Fischer-Tropsch type (FTT) synthesis.

    “A much discussed scenario for abiotic methane generation is through hydrothermal alteration of crustal mafic rocks (serpentinization), which produces H2, followed by Fischer-Tropsch type (FTT) synthesis.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [44] Fiebig J., Woodland A. B., Spangenberg J., Oschmann W., Natural evidence for rapid abiogenic hydrothermal generation of CH4. Geochim. Cosmochim. Acta 71, 3028– 3039 (2007). source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Most of the H2 produced from serpentinization is derived from ultramafic, slow-spreading crust, which only constitutes 24% of the total crustal production.

    “Most of the H2 produced from serpentinization is derived from ultramafic, slow-spreading crust, which only constitutes 24% of the total crustal production (48).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [48] Keir R. S. A note on the fluxes of abiogenic methane and hydrogen from mid-ocean ridges. Geophys. Res. Lett. 37 (2010). source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisenone given

    During an impact, Fe or Ni catalysts in an asteroid could produce CH4 from CO and H2.

    “During an impact, Fe or Ni catalysts in an asteroid could produce CH4 from CO and H2.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Empirically, H2/CH4 ratios in hydrothermal systems are highly variable, so the kinetic barriers to CH4 formation and CO2 limitations are generally present.

    “Empirically, H2/CH4 ratios in hydrothermal systems are highly variable (48), so it is clear that the kinetic barriers to CH4 formation and CO2 limitations are generally present.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [48] Keir R. S. A note on the fluxes of abiogenic methane and hydrogen from mid-ocean ridges. Geophys. Res. Lett. 37 (2010). source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Kress and McKay (60) modeled the kinetics of CH4 production during the cooling of an impactor fireball and concluded that this could produce an abiotic methane flux during the late heavy bombardment greater than the modern Earth's biological flux.

    “Kress and McKay (60) modeled the kinetics of CH4 production during the cooling of an impactor fireball and concluded that this could produce an abiotic methane flux during the late heavy bombardment greater than the modern Earth’s biological flux.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [60] Kress M. E., McKay C. P., Formation of methane in comet impacts: Implications for Earth, Mars, and Titan. Icarus 168, 475– 483 (2004). source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Premisenone given

    Hydrothermal alteration is ultimately limited by the availability of FeO.

    “Hydrothermal alteration is ultimately limited by the availability of FeO.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Underestimation of frH2 is implausible because for fast rates of crustal production, fractional conversion of FeO to H2 will, if anything, be lower than on Earth because water will permeate a smaller fraction of the total crust given faster spreading rates and thicker crust.

    “However, underestimation of frH2 is implausible because for fast rates of crustal production, fractional conversion of FeO to H2 will, if anything, be lower than on Earth because water will permeate a smaller fraction of the total crust given faster spreading rates and thicker crust.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    CO has few abiotic sinks and detectable spectral features.

    “because CO has few abiotic sinks and detectable spectral features (42), mantle-derived methane could readily be identified by coexisting CO.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [42] Schwieterman E. W., Meadows V. S., Domagal-Goldman S. D., Deming D., Arney G. N., Luger R., Harman C. E., Misra A., Barnes R., Identifying planetary biosignature impostors: Spectral features of CO and O4 resulting from abiotic O2/O3 production. Astrophys. J. Lett. 819, L13 (2016). PMC6108182 30147857 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Only 1 to 2% of H2 production is due to serpentinization; the remaining 10 to 11% is from oxidation by sulfate, which does not generate H2.

    “Furthermore, only 1 to 2% is due to serpentinization; the remaining 10 to 11% is from oxidation by sulfate, which does not generate H2 [(26), chap. 10; (45)].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [26] D. C. Catling, J. Kasting, Atmospheric Evolution on Inhabited and Lifeless Worlds (Cambridge Univ. Press, 2017). source

      Not checked in the free run.

    • cites [45] Lécuyer C., Ricard Y., Long-term fluxes and budget of ferric iron: Implication for the redox states of the Earth’s mantle and atmosphere. Earth Planet. Sci. Lett. 165, 197– 211 (1999). source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    It is unclear whether natural systems can overcome kinetic barriers to CO2-to-CH4 conversion on a global scale without biological catalysts.

    “but it is unclear whether natural systems can overcome kinetic barriers on a global scale without biological catalysts.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The conditions required to generate large fluxes of abiotic methane are specific and implausible: all unknown variables need to be at the high end of their ranges, and the upper estimate for frH2 must be an underestimate.

    “On the basis of current understanding, the conditions required to generate large fluxes of abiotic methane are specific and implausible: All unknown variables need to be at the high end of their ranges, and our upper estimate for frH2 must be an underestimate.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: All H2 produced by serpentinization is converted to CH4 via Fischer-Tropsch type synthesis.
    • The authors state this as an assumption: The entirety of an Exo-Earth’s FeO oxidation is due to H2-producing serpentinization through Eq. 7.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    CH4 production is further restricted by CO2 availability in the crust, limiting H2/CH4 ratios to ~13.

    “Guzmán-Marmolejo et al. (56) argued that CH4 production is further restricted by CO2 availability in the crust, limiting H2/CH4 ratios to ~13.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [56] Guzmán-Marmolejo A., Segura A., Escobar-Briones E., Abiotic production of methane in terrestrial planets. Astrobiology 13, 550– 559 (2013). 23742231 PMC3689174 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    High experimental methane production has been reported, but similar experiments with 13C-labeled carbon have shown that the methane produced is derived from background organic carbon contamination.

    “High experimental methane production has been reported (51, 52), but similar experiments with 13C-labeled carbon have shown that the methane produced is derived from background organic carbon contamination (53, 54).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [51] Oze C., Jones L. C., Goldsmith J. I., Rosenbauer R. J., Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces. Proc. Natl. Acad. Sci. U.S.A. 109, 9750– 9754 (2012). 22679287 PMC3382529 source

      Not checked in the free run.

    • cites [52] Neubeck A., Duc N. T., Bastviken D., Crill P., Holm N. G., Formation of H2 and CH4 by weathering of olivine at temperatures between 30 and 70°C. Geochem. Trans. 12, 6 (2011). 21707970 PMC3157414 source

      Not checked in the free run.

    • cites [53] McCollom T. M., Abiotic methane formation during experimental serpentinization of olivine. Proc. Natl. Acad. Sci. U.S.A. 113, 13965– 13970 (2016). 27821742 PMC5150391 source

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    If the inferred source flux is greater than any known plausible abiotic mechanism, or if plausible abiotic fluxes can be ruled out by contextual information, then life would be left as a reasonable hypothesis.

    “If the inferred source flux is greater than any known plausible abiotic mechanism (Fig. 6), or if plausible abiotic fluxes can be ruled out by contextual information, then life would be left as a reasonable hypothesis.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Large abiotic sources of CH4 are either improbable or identifiable with other observations.

    “We conclude that large abiotic sources of CH4 are either improbable or identifiable with other observations.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: All H2 produced by serpentinization is converted to CH4 via Fischer-Tropsch type synthesis.
    • The authors state this as an assumption: The entirety of an Exo-Earth’s FeO oxidation is due to H2-producing serpentinization through Eq. 7.

  • May combine two claims.

    Interpretationnone given

    If the modern Earth's atmosphere-ocean system were allowed to relax to equilibrium, much of the atmospheric O2 would react with the crust via oxidative weathering, and some dissolved carbon may form carbonate-bearing rocks.

    “In reality, if the modern Earth’s atmosphere-ocean system were allowed to relax to equilibrium, then much of the atmospheric O2 would react with the crust via oxidative weathering, and some dissolved carbon may form carbonate-bearing rocks.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Atmospheric N2 persists on Venus because it is in equilibrium with the CO2-dominated atmosphere and there is no mechanism to draw down N2 into the crust because of the lack of water.

    “Atmospheric N2 persists on Venus because it is in equilibrium with the CO2-dominated atmosphere (25) and there is no mechanism to draw down N2 into the crust because of the lack of water.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [25] Krissansen-Totton J., Bergsman D. S., Catling D. C., On detecting biospheres from chemical thermodynamic disequilibrium in planetary atmospheres. Astrobiology 16, 39– 67 (2016). 26789355 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    For terrestrial exoplanets, it may be possible to rule out large impactor fluxes from the system age, dust levels, or the absence of transiting planetesimals.

    “In any case, for terrestrial exoplanets, it may be possible to rule out large impactor fluxes from the system age, dust levels, or the absence of transiting planetesimals.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    The CO2-CH4 disequilibrium pair is more compelling than CH4 alone, which could be primordial from a migrated icy world or outgassed from an extremely reducing mantle.

    “This CO2-CH4 disequilibrium pair is more compelling than CH4 alone, which could be primordial from a migrated icy world or outgassed from an extremely reducing mantle.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    An atmosphere rich in CO2 and CH4 has been proposed to explain a warm early Mars, but this atmosphere would be very transient (<1 million years) because CH4 released from clathrates would be rapidly photodissociated.

    “An atmosphere rich in CO2 and CH4 has been proposed to explain a warm early Mars (62), but this atmosphere would be very transient (<1 million years) because CH4 released from clathrates would be rapidly photodissociated.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [62] E. S. Kite, M. A. Mischna, P. Gao, Y. L. Yung, Climate optimum on Mars initiated by atmospheric collapse. http://arxiv.org/abs/1709.08302 (2017).

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationreasoning

    The rich absorption spectra of CO2, CH4, and water vapor mean that they are the most readily detectable gases on an exoplanet with an anoxic atmosphere in the near future.

    “Furthermore, the rich absorption spectra of CO2, CH4, and water vapor mean that they are the most readily detectable gases on an exoplanet with an anoxic atmosphere in the near future.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    Both N2 and O2 are replenished by biology.

    “Both N2 and O2 are replenished by biology, and this disequilibrium would not persist in the absence of life because almost all the O2 would be converted to nitric acid in the ocean.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    The reduction of nitrate will yield ammonium, which is readily sequestered into silicates.

    “However, the reduction of nitrate will yield ammonium, which is readily sequestered into silicates (65, 66).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [65] Sleep N. H., Bird D. K., Rosing M. T., Biological effects on the source of geoneutrinos. Int. J. Mod. Phys. A 28, 1330047 (2013). source

      Not checked in the free run.

    • cites [66] Šucha V., Uhlík P., Madejová J., Petit S., Kraus I., Particle properties of hydrothermal ammonium-bearing illite-smectite. Clays Clay Miner. 55, 36– 44 (2007). source

      Not checked in the free run.

  • May combine two claims.

    Premisenone given

    Biology allows for the coexisting large redox separation of CH4 and CO2 and also readily consumes CO.

    “Biology allows for the coexisting large redox separation of CH4 and CO2 and also readily consumes CO.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    In principle, surface biomass is detectable through the vegetative red edge.

    “In principle, surface biomass is detectable through the vegetative red edge (16).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [16] E. W. Schwieterman, N. Y. Kiang, M. N. Parenteau, C. E. Harman, S. DasSarma, T. M. Fisher, G. N. Arney, H. E. Hartnett, C. T. Reinhard, S. L. Olson, V. S. Meadows, C. S. Cockell, S. I. Walker, J. L. Grenfell, S. Hegde, S. Rugheimer, R. Hu, T. W. Lyons, Exoplanet biosignatures: A review of remotely detectable signs of life. http://arxiv.org/abs/1705.05791 (2017). PMC6016574 29727196

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Adding solid phases would not diminish any of the disequilibria described in the analysis but would potentially make the total available energy larger.

    “Adding solid phases would not diminish any of the disequilibria described in our analysis but would potentially make the total available energy larger.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Large abiogenic fluxes of CH4 (>10 Tmol/year) needed to support high abiogenic CH4 abundances are very unlikely to occur on Earth-like exoplanets, and where they do occur, they can probably be identified through context.

    “Large abiogenic fluxes of CH4 (>10 Tmol/year) needed to support high abiogenic CH4 abundances are very unlikely to occur on Earth-like exoplanets, and where they do occur, they can probably be identified through context.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: All H2 produced by serpentinization is converted to CH4 via Fischer-Tropsch type synthesis.
    • The authors state this as an assumption: The entirety of an Exo-Earth’s FeO oxidation is due to H2-producing serpentinization through Eq. 7.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    The impact-generated abiotic methane flux could potentially be increased to 1 Tmol/year if all impact ejecta are serpentinized.

    “This could potentially be increased to 1 Tmol/year if all impact ejecta are serpentinized.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    For very high impactor fluxes, observable atmospheric consequences such as dust or CO may be detectable.

    “For very high impactor fluxes, observable atmospheric consequences such as dust or CO may be detectable.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Study design and conductown data

    Neglecting cation-cation and anion-anion interactions, neutral solute parameters, and triple particle parameters in the simplified Pitzer equations is a reasonable approximation, as confirmed by excellent agreement between MATLAB and Aspen calculations.

    “The excellent agreement between MATLAB and Aspen calculations confirms that neglecting these terms is a reasonable approximation.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Aqueous systems are electroneutral.
    • The authors state this as an assumption: Atoms and charge are conserved in the equilibrium calculation.

  • May combine two claims.

    Interpretationreasoning

    It is reasonable to expect an N2-O2-H2O disequilibrium to disappear without the continuing influence of life, although the precise time scale for the depletion of oxygen and nitrogen will depend on geological cycling that is difficult to quantify.

    “It is therefore reasonable to expect an N2-O2-H2O disequilibrium to disappear without the continuing influence of life, although the precise time scale for the depletion of oxygen and nitrogen will depend on geological cycling that is difficult to quantify.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Nitrogen-bearing crust may then be subducted, where some nitrogen may return to the atmosphere via arc volcanism, but the rest will continue to the mantle because of the stability of ammonium-bearing silicates at high temperatures.

    “Nitrogen-bearing crust may then be subducted, where some nitrogen may return to the atmosphere via arc volcanism, but the rest will continue to the mantle because of the stability of ammonium-bearing silicates at high temperatures (67).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [67] Watenphul A., Wunder B., Wirth R., Heinrich W. Ammonium-bearing clinopyroxene: A potential nitrogen reservoir in the Earth’s mantle. Chem. Geol. 270, 240– 248 (2010). source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    Tinetti et al. (70) showed that the vegetative red edge is potentially detectable with next-generation direct imaging, though challenging with realistic clouds.

    “Tinetti et al. (70) showed that the vegetative red edge is potentially detectable with next-generation direct imaging, though challenging with realistic clouds.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [70] Tinetti G., Meadows V. S., Crisp D., Kiang N. Y., Kahn B. H., Bosc E., Fishbein E., Velusamy T., Turnbull M., Detectability of planetary characteristics in disk-averaged spectra II: Synthetic spectra and light-curves of earth. Astrobiology 6, 881– 900 (2006). 17155887 source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    There are 3.7 × 1019 mol O2 in the atmosphere and oceans, 5.1 × 1020 mol O2 equivalent Fe3+ and sulfate in sedimentary rocks, and ~2 × 1021 mol O2 equivalent excess Fe3+ in igneous and metamorphic rocks.

    “Although there are 3.7 × 1019 mol O2 in the atmosphere and oceans, there are 5.1 × 1020 mol O2 equivalent Fe3+ and sulfate in sedimentary rocks, and ~2 × 1021 mol O2 equivalent excess Fe3+ in igneous and metamorphic rocks (68).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [68] Catling D. C., Zahnle K. J., McKay C. P., Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth. Science 293, 839– 843 (2001). 11486082 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    Confirmation of a liquid water ocean and atmospheric N2 that contribute to the full disequilibrium of an Archean Earth twin would require follow-up observations with next-generation space telescopes.

    “Confirmation of a liquid water ocean and atmospheric N2 that contribute to the full disequilibrium of an Archean Earth twin would require follow-up observations with next-generation space telescopes.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    The simultaneous detection of atmospheric oxygen and a vegetative red edge would be a more compelling biosignature than oxygen alone.

    “The simultaneous detection of atmospheric oxygen and a vegetative red edge would obviously be a more compelling biosignature than oxygen alone.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    One potentially detectable solid-state disequilibrium not considered here is the disequilibrium between atmospheric oxygen and reduced carbon on the surface in the form of biomass.

    “One potentially detectable solid-state disequilibrium not considered here is the disequilibrium between atmospheric oxygen and reduced carbon on the surface in the form of biomass [(23), p. 336].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [23] A. Kleidon, Thermodynamic Foundations of the Earth System (Cambridge Univ. Press, 2016). source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing). May combine two claims.

    Premisecitation

    Nitrate may be reduced to ammonia in mid-ocean ridge hydrothermal systems, which can then return to the atmosphere and be photochemically oxidized back to N2.

    “In contrast, it has been argued that nitrate may be reduced to ammonia in mid-ocean ridge hydrothermal systems, which can then return to the atmosphere and be photochemically oxidized back to N2 (64).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [64] Kasting J. F., Whitmire D. P., Reynolds R. T., Habitable zones around main sequence stars. Icarus 101, 108– 128 (1993). 11536936 source

      Not checked in the free run.

66 of 66

  • The paper relies on this without support.

    How Earth's atmosphere-ocean disequilibrium changed through Earth history had not been quantified before this study.

    “Although Earth’s chemical disequilibrium is large today, how the disequilibrium changed through Earth history has not been quantified—whether it was large in the Precambrian when there was less atmospheric oxygen, and whether biogenic species would have contributed in an anoxic atmosphere.”

  • The paper relies on this without support.

    Atmospheric chemical disequilibrium had not been considered for the early Earth before this study.

    “A more general biosignature, which has not been considered for the early Earth, is atmospheric chemical disequilibrium”

  • The authors state this as an assumption.

    All H2 produced by serpentinization is converted to CH4 via Fischer-Tropsch type synthesis.

    “For maximum conversion, we assume that all H2 makes CH4 via FTT synthesis”

  • The authors state this as an assumption.

    The entirety of an Exo-Earth’s FeO oxidation is due to H2-producing serpentinization through Eq. 7.

    “For the upper bound, we generously assume that the entirety of an Exo-Earth’s FeO oxidation is due to H2-producing serpentinization through Eq. 7, as might occur with more ultramafic crust, giving frFeO = 0.12/3 = 0.04.”

  • The authors state this as an assumption.

    Aqueous systems are electroneutral.

    “where the latter means that aqueous systems are electroneutral (25).”

  • The authors state this as an assumption.

    Atoms and charge are conserved in the equilibrium calculation.

    “we minimize Eq. 10 subject to the constraint that atoms and charge are conserved”
6 of 6

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Determining what the Archean atmosphere was made of and the influence of life could help distinguish biogenic gases in exoplanet atmospheres and find life elsewhere.

    “So, determining what the Archean atmosphere was made of, and the influence of life, could help us distinguish biogenic gases in exoplanet atmospheres and so find life elsewhere (2, 250).”

    From The Archean atmosphere (2020) · cites this paper as [2]

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Surface biomass is given in incompatible units

    Minor

    The biomass amount is stated as 4.4 × 1017 Tmol C, but the calculation immediately below uses 4.4 × 1017 mol. Those quantities differ by a factor of 1012.

  • FeO-content parameter is also used for H2-conversion bounds

    Minor

    The paper defines frFeO as crustal FeO weight fraction and assigns it a range of 10 to 25 wt %. It then assigns frFeO a value of 0.04 for the different parameter describing H2-producing conversion.

2 of 2

Cites

Flagged first, then by citations

Research works printed but not linked: 4

  1. A. H. Knoll, Cyanobacteria and earth history. Cyanobacteria Mol. Biol. Genom. Evol. 484 (2008).
  2. G. N. Lewis M. Randall Thermodynamics and the Free Energy of Chemical Substances (McGraw-Hill 1923).
  3. K. L. Denman G. Brasseur A. Chidthaisong P. Ciais P. M. Cox R. E. Dickinson Couplings between changes in the climate system and biogeochemistry in Climate Change 2007: The Physical Science Basis S. Solomon Ed. (Cambridge Univ. Press 2007).
  4. E. S. Kite M. A. Mischna P. Gao Y. L. Yung Climate optimum on Mars initiated by atmospheric collapse. http://arxiv.org/abs/1709.08302 (2017). · arXiv

Cited by

Sorted by citations

Authors
Krissansen-Totton, J., Olson, S., & Catling, D. C.
Affiliations
  • Department of Earth and Space Sciences/Astrobiology Program, University of Washington, Seattle, WA 98195, USA.
  • University of California, Riverside (US)
  • University of Washington (US)
ORCID
Venue
Science Advances
Publisher
American Association for the Advancement of Science (AAAS)
Published
2018-01-05
Volume
4
Issue
1
Pages
eaao5747-eaao5747
ISSN
2375-2548 (ISSN-L)
DOI
10.1126/sciadv.aao5747
PMC
PMC5787383
OpenAlex
W2784756873
Semantic Scholar
c5d96c7a2262eb836e55a5e0f52d5791d00ca9bc
Cited by
177
References
101
Citations by year
2026: 15 · 2025: 21 · 2024: 26 · 2023: 26 · 2022: 20 · 2021: 18 · 2020: 23 · 2019: 14 · 2018: 14
FWCI
9.04
Type
journal article
Language
en
Open access
Locations
  • Science Advances (publishedVersion)
  • PubMed Central (publishedVersion, cc-by-nc)
  • PubMed (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • arXiv (Cornell University)
Funders
National Aeronautics and Space Administration (award343610, NNX15AR63H, award343611, NNX15AL23G, award343612, NNA13AA93A)
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Space Science and Extraterrestrial Life
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Planetary Science and Exploration
  • Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
Keywords
methane, carbon monoxide, anoxic atmosphere, biogenicity, Earth history, atmospheric chemistry
MeSH terms
Atmosphere, Bicarbonates, Extraterrestrial Environment, Kinetics, Methane, Oceans and Seas, Probability, Thermodynamics, Earth, Planet, Exobiology, Ammonium Compounds

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-10.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-10.

Statistics and arithmetic are recomputed, not judged.

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