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Australia: The Land Where Time Began |
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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 Patm
= 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 Patm 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
Patm fluctuated to
an extent that was previously unrecognised.
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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |