Research ArticleATMOSPHERIC SCIENCE

Atmospheric CO2 levels from 2.7 billion years ago inferred from micrometeorite oxidation

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Science Advances  22 Jan 2020:
Vol. 6, no. 4, eaay4644
DOI: 10.1126/sciadv.aay4644
  • Fig. 1 Single model run for a particle with an entry angle of 45° from zenith through a 39% CO2, 61% N2 atmosphere (50 wt % CO2, 50 wt % N2).

    The plots above show (A) micrometeorite temperature in kelvin, (B) micrometeorite velocity in kilometers per second, (C) micrometeorite radius in micrometers, (D) mass fraction of metallic Fe to oxidized FeO, and (E) micrometeorite’s altitude above Earth’s surface in kilometers. The orange part of each curve indicates the micrometeorite is molten. For plot (C), the micrometeorite increases in radius because the oxide layer is growing faster than it is evaporating. The oxidation of high-density Fe to the lower-density FeO results in a less dense particle and, thus, a larger radius. In this simulation, the micrometeorite is molten for 2.6 s between 11.9 and 14.5 s. The initial radius is 50 μm, the final radius is 54.8 μm, the final Fe fractional mass is 41% (37% by cross-sectional area), and the maximum temperature reached is 2275 K at an altitude of 81.2 km. See the Supplementary Materials for an animation of this figure.

  • Fig. 2 Comparison of unoxidized Fe area in CO2-N2 and CO2-N2-O2 atmospheres with increasing CO2.

    The black curve shows the mean model cross-sectional area of unoxidized Fe compared with the total cross-sectional area of the micrometeorite for a CO2-N2 atmosphere. It is the same curve as in Fig. 3. The orange contour shows the same simulated micrometeorites but entering a CO2-N2-O2 atmosphere where the O2 represents 1% by volume. The addition of 1% O2 to the atmosphere has little impact on the average.

  • Fig. 3 Simulated fractional area of unoxidized Fe with increasing atmospheric CO2.

    The atmosphere was assumed to be composed of pure N2 and CO2. The black curve shows the mean model prediction for the cross-sectional area of unoxidized Fe compared with the total cross-sectional area of the micrometeorite. The simulated micrometeorites were assumed to have spherical, central metal beads so the cross-sectional area of the unoxidized Fe bead is a maximum. The gray-shaded area shows the 2σ confidence interval of our model. The orange dot and solid error bar show the mean unoxidized Fe fractional area and 2σ confidence interval from the two Fe-FeO micrometeorites reported by Tomkins et al. (10). The corresponding uncertainty in atmospheric CO2 from the Tomkins et al. data is shown by the dashed orange error bars. The Tomkins et al. data point indicates the CO2 level was at 6458+36% (2σ). The dashed blue line shows the fraction of modeled micrometeorites that were fully oxidized in the atmosphere with no remaining metallic Fe. Such particles could lead to magnetite-rich micrometeorites and appear in our model once atmospheric CO2 reaches ~70%.

  • Fig. 4 Fractional area of unoxidized Fe in simulated sectioned micrometeorites compared with observed modern micrometeorites.

    The horizontal axis in this plot shows the fractional area of unoxidized Fe in cross-sectioned micrometeorites. The blue histogram shows the simulated unoxidized Fe fractional area from 500 randomly generated micrometeorites entering the modern atmosphere, which were oxidized by O2, and the orange histogram shows the data inferred from figure 4 of Genge et al. (15). The model mean and 2σ confidence interval are shown by the blue dot and error bar, while the mean and 2σ of the data from Genge et al. are shown by the orange dot and error bar.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/4/eaay4644/DC1

    Data file S1. A zipped file containing our model as a Python script and the data files necessary to reproduce our results and figures.

    Movie S1. An animated version of Fig. 1. The movie also shows a simulated micrometeorite (gray sphere) and the corresponding micrometeorite cross section.

  • Supplementary Materials

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    Other Supplementary Material for this manuscript includes the following:

    • Data file S1 (.zip format). A zipped file containing our model as a Python script and the data files necessary to reproduce our results and figures.
    • Movie S1 (.mp4 format). An animated version of Fig. 1. The movie also shows a simulated micrometeorite (gray sphere) and the corresponding micrometeorite cross section.

    Files in this Data Supplement:

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