From Jeff Hecht at New Scientist:
Yet the scorched remains of 60 micrometeorites have survived 2.7 billion years in the limestone Tumbiana Formation of Western Australia. They are the oldest space rocks ever discovered on Earth.
What’s more, the fact that the meteorites contain oxides of iron show that the upper part of the atmosphere back then must have contained oxygen.
The survival of iron oxides is particularly unusual – and it may only have happened because of unusually fortuitous circumstances. The lake into which the micrometeorites fell was highly alkaline, with its deepest layers totally anoxic. This is probably what prevented the minerals from dissolving.
“Such conditions are rarely encountered in the geological record, which means that the Pilbara micrometeorites might be a one-off discovery,” Zahnle writes. “But one can wish upon a shooting star.”
It seems, from the stories, that oxygen isn’t necessary for life to get started on Earth but finding it is a big discovery anyway. This story does not even speculate on that subject. Okay. Here’s the file:
Researchers: Small amount of oxygen 3.8 billion years ago
Did a low oxygen level delay complex life on Earth? (October 31, 2014)
Early Earth was indeed “extremely oxygen-poor compared to today” (January 16, 2015)
Small pre-Cambrian oxygen jump kickstarted complex life
(July 24, 2015)
Oxygen Does Not Equal Life – Implications for Abiogenesis? (September 15, 2015)
Researchers: Cyanobacteria responsible for Earth’s early oxygen
(November 28, 2015)
Animals didn’t “arise” from oxygenation, they created it, researchers say
Theory on how animals evolved challenged: Some need almost no oxygen
New study: Oxygenic photosynthesis goes back three billion years
We really should organize a panel discussion—oops, a conference—with all these people
Here’s the abstract for the paywalled paper:
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 (2s) 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. – Sanjoy M. Som, Roger Buick, James W. Hagadorn, Tim S. Blake, John M. Perreault, Jelte P. Harnmeijer & David C. Catling
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