Long term, all the digging we're doing is pulling down atmospheric O2, by exposing ferrous iron and reduced sulfides, manganese, etc. to oxidation. The amount

Long term, all the digging we're doing is pulling down atmospheric O2, by exposing ferrous iron and reduced sulfides, manganese, etc. to oxidation. The amount of crust that would have to be fully oxidized to deplete all atmospheric O2 is surprisingly small, I think just a few hundred meters. Atmospheric O2 is on quite a small scale compared to the reducing power of Earth as a whole.