[labster]

I think other commenters have answered your question, but I thought I'd talk a little on carbonic acid.

Carbon dioxide partitions into water as carbonic acid. This is the key mechanism in ocean acidification. Of course the rate is affected by pH of the oceans, there's a buffering action going on between carbonate, bicarbonate, and carbonic acid depending on how many protons are available. It's also why pure water sitting open to the air will have a slightly acidic pH.

The oceans at the bottom have a decent layer of carbonate material accumulated from shells, which are also at equilibrium with the ocean, but the other direction (accepting protons). But since the oceans are so big, to increase the pH of the oceans you'd have to send the surface water to the bottom. Mixing the oceans is kind of a slow process -- around 500 years. But in theory we'll get some long term climate help from the ocean floor (well, if the methane clathrates don't get us first).