Hannah Hickey reports on Joel Blum and Jana Meixnerová, concerning Earth's oxygenation 2.5 Bya. Apparently that's a round number.
The big carbon-crash and oxygen-dump, which threw us into a deep freeze, came 2.4 Bya. A hundred million years prior - Ariel Anbar et al. told us back in 2007 - came a false-dawn. Actually several. Blum and Meixnerová, and Roger Buick, and others, are here to explain these. It turns out there was... mercury, in those traces. Obviously not a signature for life.
A signature, rather, of volcanism. Blum's team says that animals without chlorophyll munched on the nitrates and phosphates that washed down from these new mountains, giving off oxygen as waste. Not a lot of it, and not sustainably - it takes energy to release oxygen from a molecule. But enough profusion of microbes, perhaps, came together that one microbe stumbled upon our inner solar system's greatest source of energy. Or several did.
I'm thinking the new green algae started along freshwater shallows, and that it took longer to colonise the briny deeps. I'm told that the first cyanobacteria tended to drown in their own oxygen, hardly a problem in the ocean, but certainly possible in a shallow puddle.
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