High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils


Solar luminosity on the early Earth was significantly lower than today. Therefore, solar luminosity models suggest that, in the atmosphere of the early Earth, the concentration of greenhouse gases such as carbon dioxide and methane must have been much higher1,2. However, empirical estimates of Proterozoic levels of atmospheric carbon dioxide concentrations have not hitherto been available. Here we present ion microprobe analyses of the carbon isotopes in individual organic-walled microfossils extracted from a Proterozoic ( 1.4-gigayear-old) shale in North China. Calculated magnitudes of the carbon isotope fractionation in these large, morphologically complex microfossils suggest elevated levels of carbon dioxide in the ancient atmosphere—between 10 and 200 times the present atmospheric level. Our results indicate that carbon dioxide was an important greenhouse gas during periods of lower solar luminosity, probably dominating over methane after the atmosphere and hydrosphere became pervasively oxygenated between 2 and 2.2 gigayears ago.

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Figure 1: Photographs of D. delicata specimens used in this study.
Figure 2: Relationship between carbon isotopic fractionation of the Calvin cycle (ɛp) and CO2 concentration in sea water (Ce) at different growth rates (µ, in units of d-1).
Figure 3: Atmospheric p CO 2 levels in the Archaean and Proterozoic.


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We thank the following for technical assistance; E. Hauri, J. Wang, J. Orloff, R. Dotson, K. Livi, D. Veblen and P. Piccoli. We also thank A. Knoll and L. Yin for providing samples for bulk rock analyses, and J. Hayes for comments on an earlier version of this manuscript. This research was supported by NASA Exobiology, NSF Geology and Paleontology, and China MOST 973 programs.

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Correspondence to Alan J. Kaufman or Shuhai Xiao.

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Kaufman, A., Xiao, S. High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils. Nature 425, 279–282 (2003). https://doi.org/10.1038/nature01902

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