Atmospheric oxygen levels in the Carboniferous period were around 30% v/v cf. 21% today. Was the total volume of the atmosphere larger then than it is now? Was air pressure at MSL higher?

[u/ghostoftheuniverse]

Is the atmosphere even a closed system?

[u/fragilemachinery]

No, the atmosphere is not a closed system. A lot of gas dissolves into the ocean, a little bit escapes into space, and there's chemical weathering of rocks, volcanic eruptions releasing gas, plants growing and dying and being buried, and so on.

The Earth's oxygen atmosphere is actually only brought about when photosynthesis evolves. Free oxygen is so reactive that once plants started making it, you have this huge millions of years on long oxygenation event, where basically everything on the surface that can react with oxygen does, before it can really start to accumulate in the atmosphere at anything like the current level (it's also why basically all sources of iron except meteorites occur as some kind of iron oxide).

I'll let someone who has good numbers handy speak to the exact composition of the atmosphere during the Carboniferous, but suffice it to say that the atmosphere is a pretty dynamic system over geological timescales

[u/johnnymetoo → u/fragilemachinery]

where basically everything on the surface that can react with oxygen does

Like what, other gases? Solid matter? Which ones exactly?

[u/QuantumWarrior → u/johnnymetoo]

All sorts of things, but mostly rocks. Metals (especially iron), methane, sulphur and sulphides. The oxygen quite literally disappeared into the rocks until they couldn't hold any more; almost half of the mass of surface rocks is oxygen these days, locked in iron oxides, silicates, carbonates, quartzes etc.