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Earth's air pressure 2.7 billion years ago constrained to less than half of modern levels


How the Earth stayed warm several billion years ago when the Sun was considerably fainter is the long-standing problem of the ‘faint young Sun paradox’. Because of negligible1 O2 and only moderate CO2 levels2 in the Archaean atmosphere, methane has been invoked as an auxiliary greenhouse gas3. Alternatively, pressure broadening in a thicker atmosphere with a N2 partial pressure around 1.6–2.4 bar could have enhanced the greenhouse effect4. But fossilized raindrop imprints indicate that air pressure 2.7 billion years ago (Gyr) was below twice modern levels and probably below 1.1 bar, precluding such pressure enhancement5. This result is supported by nitrogen and argon isotope studies of fluid inclusions in 3.0–3.5 Gyr rocks6. Here, we calculate absolute Archaean barometric pressure using the size distribution of gas bubbles in basaltic lava flows that solidified at sea level 2.7 Gyr in the Pilbara Craton, Australia. Our data indicate a surprisingly low surface atmospheric pressure of Patm = 0.23 ± 0.23 (2σ) bar, and combined with previous studies suggests 0.5 bar as an upper limit to late Archaean Patm. The result implies that the thin atmosphere was rich in auxiliary greenhouse gases and that Patm fluctuated over geologic time to a previously unrecognized extent.

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Figure 1: Geology of the Beasley River field area, southwestern Pilbara, and sampling localities.
Figure 2: Beasley River locality (‘br’ in Fig. 1) with the locations of five conformable subaerial lava flows, along with the location of collected samples.
Figure 3: Beasley River geologic context and flow detail.

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This work was supported by NASA Exobiology/Astrobiology grant NNX08AP56G to R.B. Additional support came from NASA Astrobiology Institute grant NNA13AA93A. The Washington State University Geoanalytical Laboratory performed the major- and trace-element analyses. S.M.S. thanks E. Stüeken for insightful conversations on K+ replacement in clays. We thank S. Mikhail, D. Sahagian and B. Marty for helpful reviews.

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Authors and Affiliations



R.B. conceived the project and led the field work in Western Australia, T.S.B. discovered the locality, J.P.H. assisted in mapping the locality, D.C.C. supervised the data analysis and contributed to the geologic N cycle interpretation, J.W.H. supervised the X-ray work, J.M.P. assisted in extracting amygdale dimensions, S.M.S. assisted in the Beasley River field work, prepared samples for analysis, X-rayed the cores, developed the algorithm to analyse the X-ray images, led the amygdale dimension extraction task, analysed the data, and contributed to the geologic N cycle interpretation. S.M.S., R.B. and D.C.C. wrote the manuscript.

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Correspondence to Sanjoy M. Som.

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Som, S., Buick, R., Hagadorn, J. et al. Earth's air pressure 2.7 billion years ago constrained to less than half of modern levels. Nature Geosci 9, 448–451 (2016).

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