Totally outside of anything that can happen on the Earth, but as a test of the buffering abilities of saltwater, an experiment that I may perform.
In all honesty, I have no idea what the pH results would be. You got me rather curious.
Totally outside of anything that can happen on the Earth,
Not really; depends on how much dry ice you add. I did the experiment in aquatic chemistry as an undergrad where you create the simple seawater carbonate buffer system (leaving out some of the minor stuff, like boric ion), and then just breathe into the flask. The increase in CO2 simply from your exhalations is sufficient to alter the pH of the system. But for that you have to wait 3-4 days. Dry ice will do it faster, because it will put a lot more carbonic acid into the system.
You'll need a decent pH meter.