Carbon-cycle habitability

Master of Orion allowed besides "difficulty" a bounded playfield small / medium / large, which could take on multiple NPC alien-races. The sequel adds the age of the playfield. So, we guess, if it formed in the centre of our Milky Way or if it is a holdout from a dead patch of space like some of those low-gas galaxies. Relevant to all this, from 10 April: carbon-cycle. (Presented by Bristol, but not well.)

Lewis J. Alcott, Craig Walton, Noah J. Planavsky, Oliver Shorttle, and Benjamin J. W. Mills propose, on the assumption of (somehow) photosynthesis being introduced, how the atmosphere became oxygen-rich. They say: not just the spread of alga. During the Boring Billion before land plants, photosynthesis was both two-dimenstional, and limited to shallow pools and "brownwater" coast.

So we had oxygen in our early atmosphere - but not much. Plus photosynthesis requires CO₂. The paper, as I read it, proposes rock carbonates as a proxy for atmospheric carbon generally. If free carbon monoxide, say from a volcano, reacts with a silicate, some gets buried and we can dig it out and measure its isotopes. Or the dioxide can get taken by those plants, which eventually die and leave some carbon-nitrogen sludge.

Over time, like a boring billion years, carbon slowly becomes carbonate and also dead plants. More plants grow. The winner as carbon is cycled in and out is - oxygen.

What the paper doesn't constrain is Great Oxidation Event, measured elsewhere 2.4-2.2 Gya. They'd define that G.O.E. as the point at which "oxygen overwhelms [carbon] sinks". But again: not yet breathable to modern mammals, maybe not even to dino/birds. There's just enough O₂ to exist without it all burning.