ConchL,

OK, carbon dioxide dissolved in water makes a weak acid, which will make it hard on the molluscs.  This has been noted here a few times.  But during the Cretaceous, the seas were also warm and huge blooms of calcareous algae (coccolithophores) made the famous chalk cliffs.  Why was there more than sufficient available CaCo3 then?  Was there a buffering affect then that we can't expect now?

Allen Aigen
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New Scientist offers these nifty free podcasts. "When the Sea Goes Sour" offers some sobering speculation about what the increased levels of carbon will do to those sea creatures who rely on calcium for their shells (and metabolism and everything else!)

They don't even mention how the extinction of some of these critters (coccolithophores) will worsen the atmospheric issues. Still, it's enough to send a chill through any sheller!

http://www.newscientist.com/podcast.ns