Yes, I've read this before. It's not very convincing, except for the loons over at WUWT.
Acidification doesn't mean something has to already be an acid. It describes the water chemistry just fine, it is becoming more acidic. After all, the pH scale is measuring the H+ ions in an aqueous solution, and as the pH drops, you have more H+ ions. Of course the pH scale is logarithmic, so the difference between 8.14 and 8.25 is about 28.8% (10^8.25/10^8.14=1.28). Second, there will be huge problems before the ocean ever becomes acidic. It's an equilibrium problem. Check out this graph, and look at the CO3 ion, the carbonate ion that is so important to marine ecosystems.
At a pH of 8.14, the ocean is already low in carbonate, though the surface waters are still saturated. You add more H2CO3, that's carbonic acid, and the finely tuned balance that exists in nature between carbonate and bicarbonate (HCO3) very soon will be unbalanced, the equilibrium shifts more towards bicarbonate, and you no longer have enough carbonate, even when the surface waters are saturated with it. There will be not enough carbonate for shellfish, corals, diatoms, etc. to build/maintain their exo-skeletons.
And in regards to measuring pH, how do you know what the temperature was say 500 million years ago? They use proxies. If you know how much carbon dioxide is in the air, you can use equilibrium and air-ocean exchanges to estimate.
Except the Ordovician era didn't build up it's concentration in the atmosphere in less than two centuries. The deep ocean has time to mix when you have a building concentration over millions of years. When the ocean is well mixed, you don't have this problem. Our oceans today are not well mixed, and certainly would take millenia to come to an equilibrium if we stopped perturbing it today.
Ahh, the classic idiotic reply. It's idiotic because it requires no thought, which is probably why the folks at WUWT liked it so much.
The shallow ocean corals that we have today evolved from deep sea corals. In the deep sea, where the waters essentially never mix with surface waters, the higher atmospheric carbon dioxide would not be as large a problem. It's only a problem when they move into shallow waters 40 million years ago, which happens when atmospheric carbon dioxide is less than 600 ppm.
If you really want to learn something, and aren't interested in just posting some crap you found that you think sounds smart, try reading these. I know you won't, but I'll post it here anyways. Maybe some others who stumble by this will want to learn what science is telling us.
Dynamic patterns and ecological impacts of declining
ocean pH in a high-resolution multi-year dataset
Southern Ocean acidification: A tipping point
at 450-ppm atmospheric CO2
Imminent ocean acidification in the Arctic projected with the NCAR
global coupled carbon cycle-climate model
Anthropogenic ocean acidification over
the twenty-first century and its impact on
calcifying organisms