CO2 blanket is real, but it doesn't really protect all that much from oxygen unless enough new CO2 is being produced to replenish the blanket faster than the O2 diffuses in. As in, an actual "air" current flowing away from the surface of the liquid. (I might be fooling myself about CO2 replenishing faster than O2 diffuses in; I don't know the rates.) A layer of bubbles or foam should make a good barrier, until it collapses.
Gases mix all on their own due to something called "partial pressure". Otherwise we wouldn't need air locks.
Gases mix all on their own due to something called "partial pressure". Otherwise we wouldn't need air locks.