Australia: The Land Where Time Began

A biography of the Australian continent 

Atmospheric Pressure of Earth 2.7 Ga Constrained to Less than Half of Modern Levels

A long-standing problem of the ‘faint young Sun paradox’ has been how the Earth stayed warm several billion years ago at a time when the Sun was considerably fainter than at present. In the atmosphere during the Archaean methane has been invoked as an auxiliary greenhouse gas (Kasting & Siefert, 2002) as the concentrations of O₂ were negligible (Holland, 2006) and the concentrations of CO₂ were moderate (Sheldon, 2006). Som et al. suggest that alternatively pressure broadening in a denser atmosphere in which N₂ partial pressure was around 1.6-2.4 bar could have enhanced the greenhouse effect (Goldblatt et al., 2009). It is indicated by fossilised raindrop imprints that atmospheric pressure 2.7 Ga was less than twice modern levels, and probably less than 1.1 bars, which precludes such enhancement of pressure (Som et al., 2012). Nitrogen and argon isotope studies of fluid inclusions in rocks dating from 3.5-3.0 Ga support this result (Marty et al., 2013). In this study Som et al. calculated absolute atmospheric pressure in the Archaean by using the size distribution of gas bubbles in basaltic lava flows in the Pilbara Craton, Western Australia, at sea level ~2.7 Ga. A surprisingly low atmospheric pressure of P_atm = 0.23 ± 0.23 (2σ) bar, is indicated by their data, and when combined with previous studies it is suggested that ~0.5 bar is an upper limit to the P_atm during the Late Archaean. It is implied by this result that the thin atmosphere was rich in auxiliary greenhouse gases and that over geological time the P_atm fluctuated to an extent that was previously unrecognised.

Sources & Further reading

Som, S. M., R. Buick, J. W. Hagadorn, T. S. Blake, J. M. Perreault, J. P. Harnmeijer and D. C. Catling (2016). "Earth's air pressure 2.7 billion years ago constrained to less than half of modern levels." Nature Geosci 9(6): 448-451.