Research ArticleATMOSPHERIC SCIENCE

Archean kerogen as a new tracer of atmospheric evolution: Implications for dating the widespread nature of early life

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Science Advances  28 Feb 2018:
Vol. 4, no. 2, eaar2091
DOI: 10.1126/sciadv.aar2091
  • Fig. 1 Raman spectra determined on the kerogen isolated from the black chert sample, on the mineral matrix, and on the secondary hydrothermal veins.

    (A) Transmitted light photography showing that organic matter particles are present in the main mineral matrix but virtually absent in secondary hydrothermal veins. (B) Raman spectra determined on the mineral matrix and on the kerogen suggest that the kerogen mostly comprises organic matter from the main mineral matrix. No organic feature is detected in secondary hydrothermal veins. a.u., arbitrary unit.

  • Fig. 2 Isotopic composition of Xe and Kr in the kerogen of the MGTKS3 black chert.

    The sample is shown relative to the isotopic composition of the modern atmosphere (25) and expressed using the δ notation δiXeair = ((iXe/130Xe)ker/(iXe/130Xe)air − 1)*1000. (A) The isotope spectrum of Xe in Barberton quartz samples (3.3 Gy) from Avice et al. (2), the mass fractionation line of U-Xe (1), and the isotopic composition of Xe in a comparatively recent organic material (here the anthracite sample; table S1) are given for comparison. The error envelopes of the MGTKS3 kerogen and Barberton quartz mass fractionation lines are given at 2σ. No mass-dependent isotopic fractionation of Xe isotopes is observed for a recent organic material (table S1). (B) The MDF line derived from Xe data is reported for comparison. The abundance-weighted mean composition of Kr in the kerogen of the MGTKS3 black chert sample is essentially atmospheric. Errors are 2σ.

  • Fig. 3 Degree of mass fractionation (‰ amu−1) of Xe and Kr isotopes isolated from the MGTKS3 black chert kerogen.

    Degrees of MDF are given for each temperature step relative to the isotopic composition of the modern atmosphere (25). The fractions of 130Xe (A) and 84Kr (B) released for each extraction step relative to the total amount of 130Xe and 84Kr extracted from each aliquot, respectively, are given in percentage. Both series of Xe analysis are shown, with the first and second series represented by open and closed symbols, respectively. Percentage of total gas released during each heating step is displayed beside the symbols. The Xe isotope spectrum with the highest deviations from modern atmosphere (9.8 ± 2.1‰ amu−1, 500°C) is taken as being the most representative signature of the trapped component. This trapped component dominates the budget of Xe isotopes in the sample because more than 75% of the total amount of 130Xe is extracted at this temperature. Errors are 2σ.

  • Fig. 4 Mixing diagram of the 40Ar/36Ar ratios as a function of the 130Xe/36Ar ratios.

    The two series of experiments are represented in open and solid symbols, respectively. The linear trend depicted over stepwise heating of the kerogen isolated from the MGTKS3 black chert sample indicates that the initial 40Ar/36Ar of the fluid [625 ± 285 (2σ)] was higher than the Archean atmospheric 40Ar/36Ar ratio of 143.6 ± 48 (2σ) (36), thus confirming that the initial fluid had an excess of radiogenic 40Ar*, possibly related to interaction with crustal and/or hydrothermal reservoirs.

  • Fig. 5 Isotopic composition of Kr in the kerogen of the MGTKS3 black chert.

    The sample is shown relative to the isotopic composition of the modern atmosphere (25) and expressed using the δ notation δiKrair = ((iKr/84Kr)ker/(iKr/84Kr)air − 1)*1000. The chondritic krypton [Q-Kr; (37)] and solar Kr (38) compositions are given for comparison. Whereas Q-Kr is depleted in the light isotopes relative to modern air, solar Kr is mass dependently fractionated in favor of the light isotopes, relative to modern air, and thus appears to be a better candidate for the precursor atmospheric Kr, although additional and repeated analyses of Archean Kr signatures are required to confirm this interpretation.

  • Fig. 6 MDF of atmospheric Xe with time relative to modern atmosphere.

    The red curve was established from a power law (y = 0.238*x3.41) fitting the literature data of Xe isotope mass fractionation in ancient samples [see the study of Avice et al. (2) and references therein] as well as the initial [U-Xe, the precursor of Earth’s atmosphere (1)]) and final [modern atmosphere (25)] compositions of the atmosphere. Isotopic composition is expressed in per mil per atomic mass unit relative to the modern atmosphere (purple dot) (25). The degree of MDF obtained in this study for the kerogen isolated from the MGTKS3 black chert sample (9.8 ± 2.1‰ amu−1, 2σ) can be used to estimate the age of the sample from the evolution curve of the isotopic fractionation of atmospheric Xe. This method would provide a model age of 3.0 ± 0.2 Gy (2σ) for the kerogen embedded in silica, in excellent agreement with the geological age of the chert [2.95 Gy (15)].

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaar2091/DC1

    table S1. Xe isotopic composition of the kerogen isolated from the MGTKS3 black chert sample and from two Paleozoic—so comparatively younger—kerogens (an anthracite and a type III kerogen called Champclauson) by stepwise heating.

    table S2. Kr isotopic composition of the kerogen isolated from the MGTKS3 black chert sample by stepwise heating.

    table S3. Ar isotopic composition of the kerogen isolated from the MGTKS3 black chert sample by stepwise heating.

    fig. S1. Fission spectrum of MGTKS3 kerogen Xe corrected for mass fractionation relative to U-Xe.

    fig. S2. Comparison of power law/exponential law fit of the literature data [see the study of Avice et al. (2) and references therein] on the evolution of atmosphere Xe isotope fractionation over geological periods of time.

  • Supplementary Materials

    This PDF file includes:

    • table S1. Xe isotopic composition of the kerogen isolated from the MGTKS3 black chert sample and from two Paleozoic—so comparatively younger—kerogens (an anthracite and a type III kerogen called Champclauson) by stepwise heating.
    • table S2. Kr isotopic composition of the kerogen isolated from the MGTKS3 black chert sample by stepwise heating.
    • table S3. Ar isotopic composition of the kerogen isolated from the MGTKS3 black chert sample by stepwise heating.
    • fig. S1. Fission spectrum of MGTKS3 kerogen Xe corrected for mass fractionation relative to U-Xe.
    • fig. S2. Comparison of power law/exponential law fit of the literature data see the study of Avice et al. (2) and references therein on the evolution of atmosphere Xe isotope fractionation over geological periods of time.

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