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My guess as to the answer to my own question:

Over a long period of time, no matter what the original composition of the air pocket was, it will exchange gases with the seawater just below it, which will in turn mix with the effectively unlimited supply of seawater beyond. As a result, the proportional gas composition of the air pocket will eventually match that of the seawater, which tends to be lower in dissolved N2, and higher in O2 and CO2 than the regular atmosphere. As a result, it would not be breathable by people because of the excess of CO2.

Is my reasoning sound, or have I missed something?

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  • $\begingroup$ This seems quite similar to this question asked three days earlier. $\endgroup$
    – gerrit
    Feb 23 at 8:04
  • $\begingroup$ It does seem quite similar; the devil is in the details: the cave in the other question is a real cave whose pocket of breathable air is above sea level, and is presumed to communicate with the regular atmosphere. My example is purely hypothetical (though I'd be intrigued to learn of real-world examples), and its pocket of air is many meters below sea level, so mixing with the regular atmosphere would be (by natural means) impossible. $\endgroup$
    – Gonzalo
    Feb 23 at 8:38
  • $\begingroup$ I think that the rate at which the air dissolves into the water probably outstrips the gas exchange - so by the time the composition of the air was substantially altered the bubble would have been replaced by water $\endgroup$
    – Andy M
    Feb 23 at 15:03
  • $\begingroup$ Interesting, Andy M. So to make sure I understood your suggestion: you believe all the air in the submerged chamber would eventually be forced into solution by the pressure? Are there observable examples of this, such as open-air, water-mediated pressure gauges (with a "pocket" of air in a transparent tube) collapsing over time? $\endgroup$
    – Gonzalo
    Feb 23 at 18:33
  • $\begingroup$ There is some discussion of this, more mathematically, at physics.stackexchange.com/questions/67970/…. This suggests a 1 m diameter bubble at a depth of 30 metres would 'last' 100 days $\endgroup$
    – Andy M
    Feb 27 at 9:34

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