This is true. Gases will blanket (layer) "with time" *IF* it is left undisturbed. Heavier gasses will sink and separate beneath lighter gasses. Liquids have this same effect, but they must be left undisturbed for this effect to happen.
People ask why CO2 in the atmosphere doesn't sink to the ground, but that's easy to explain. Wind currents prevent it. The gasses have to be left undisturbed for gravity to cause this layering effect to occur.
That said, if oxygen is in the carboy, it will have access to the wine early on and when you disturb the carboy.
No, it is not just wind currents. It is the kinetic energy of the motion of the gas molecules. The kinetic energy completely dominates the potential energy of gravity. Molecules bounce around at about 1150 mph. If you took a molecule going that speed, and you directed it upwards in a vacuum (i.e., it didn't hit another molecule), it would go 19 miles upwards before gravity would slow it to zero. These are not objects that are going to "settle out" due to gravity.
It is true that there is a slight enhancement at lower atmospheres; very slight. To cite an example, Ar is about 0.5% of the atmosphere at sea level. At 1000 m (3300 ft), it would be about 0.48%. Clearly, the difference between the top and bottom of your carboy will be negligible.
So, in particular, the gas will NOT blanket at room temperature even if left undisturbed. The temperature IS the disturbance that keeps them mixed.
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