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Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints


According to the ‘Faint Young Sun’ paradox, during the late Archaean eon a Sun approximately 20% dimmer warmed the early Earth such that it had liquid water and a clement climate1. Explanations for this phenomenon have invoked a denser atmosphere that provided warmth by nitrogen pressure broadening1 or enhanced greenhouse gas concentrations2. Such solutions are allowed by geochemical studies and numerical investigations that place approximate concentration limits on Archaean atmospheric gases, including methane, carbon dioxide and oxygen2,3,4,5,6,7. But no field data constraining ground-level air density and barometric pressure have been reported, leaving the plausibility of these various hypotheses in doubt. Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m−3, compared to today’s 1.2 kg m−3, but because such drops rarely occur, air density was more probably below 1.3 kg m−3. The upper estimate for air density renders the pressure broadening explanation1 possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO2 levels required for hot Archaean climates8.

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Figure 1: The 2.7-billion-year-old Ventersdorp Supergroup raindrop imprints lithified in tuff at Omdraaivlei, South Africa.
Figure 2: Experimental relationship between raindrop area and dimensionless momentum.
Figure 3: Theoretical predictions of the variation of air density with terminal velocity and dimensionless momentum at the surface.
Figure 4: Atmospheric density given the maximum raindrop diameter that created the largest imprints at Omdraaivlei, South Africa.

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This work was supported by NASA Exobiology/Astrobiology grant NNX08AP56G. We thank W. Van der Westhuizen of the University of the Free State in South Africa, and E. and D. Jackson of Omdraaivlei for their assistance when sampling in the field. We also thank E. Stüeken, A. Chen and K. Huntington at the University of Washington for laboratory assistance, and the staff at Metron Corporation for data acquisition. XRF measurements were performed by the Washington State University Geoanalytical Laboratory. Funding and field logistics for the Iceland fieldwork was supported by the Coordination Action for Research Activities on life in Extreme Environments (CAREX), a project supported by the European Commission Seventh Framework Programme. Funding and field logistics for the Hawaiian fieldwork was supported by the University of Washington Department of Earth and Space Sciences, and its Geoclub. D.C.C. was also supported by NASA Exobiology/Astrobiology grant NNX10AQ90G.

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D.C.C. conceived the research project and established maximum-raindrop-size terminal velocity dependence on air density, R.B. led the field work in South Africa, collected the latex peels and analysed the grain sizes of the Ventersdorp tuff, J.P.H. found additional geographic and stratigraphic occurrences of raindrop imprints while performing field work and helping collect latex peels, P.M.P. collected experimental data and measured the Eyjafjallajökull and Pahala ash grain sizes, and S.M.S. developed the method of dimensionless momentum, collected the volcanic ash from Hawaii and Iceland, analysed the data, and led the experimental work. S.M.S., R.B. and D.C.C. discussed results and prepared the manuscript.

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Correspondence to Sanjoy M. Som.

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Som, S., Catling, D., Harnmeijer, J. et al. Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints. Nature 484, 359–362 (2012).

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