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

• Published in: Nature geoscience Vol. 9; no. 6; pp. 448 - 451
• Main Authors: Som, Sanjoy M., Buick, Roger, Hagadorn, James W., Blake, Tim S., Perreault, John M., Harnmeijer, Jelte P., Catling, David C.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-06-2016; Nature Publishing Group
• Subjects: 704/106/413; 704/2151/209; Astrobiology; Atmosphere; Atmospheric pressure; Brackish; Bubbles; Carbon dioxide; Earth science; Earth Sciences; Earth System Sciences; Gases; Geochemistry; Geology; Geophysics/Geodesy; Greenhouse effect; Greenhouse gases; Isotope studies; Lava; Lava flows; letter; Marine; Sea level; Australia; United States > US; Western Australia Australia; Colorado; Australia, Western Australia, Pilbara Craton
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo2713

The composition of the Earth’s early atmosphere is uncertain. The morphology of vesicles in basalts suggests that the air pressure 2.7 billion years ago was 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 negligible
1
O
2
and only moderate CO
2
levels
2
in the Archaean atmosphere, methane has been invoked as an auxiliary greenhouse gas
3
. Alternatively, pressure broadening in a thicker atmosphere with a N
2
partial pressure around 1.6–2.4 bar could have enhanced the greenhouse effect
4
. 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 enhancement
5
. This result is supported by nitrogen and argon isotope studies of fluid inclusions in 3.0–3.5 Gyr rocks
6
. 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
P
atm
= 0.23 ± 0.23 (2
σ
) bar, and combined with previous studies suggests ∼0.5 bar as an upper limit to late Archaean
P
atm
. The result implies that the thin atmosphere was rich in auxiliary greenhouse gases and that
P
atm
fluctuated over geologic time to a previously unrecognized extent.