Paul Monfils raises some interesting questions about calcium availability.  
I don't know if anyone has traced out the life cycle of a calcium atom in 
the oceans.  Certainly I don't know the details.  I suspect the
availability of carbonate ions is at least as important.  Indeed, Vermeij's
hint that the equilibrium calcium carbonate concentration decreases as
temperature increases is probably an empirical fact implying that carbon
dioxide availability is a controlling factor.  That is, the solubility of 
gases in water decreases with temperature increases, unlike the behavior of
salts.  That the salt calcium carbonate has a gas-like solubility suggest 
to me that the solubility of carbon dioxide dominates the concentration of
calcium carbonate.  Where that leaves Paul's calcium ions I don't know.  
That raises another point.  The pH of fresh water is not 7.0 unless extreme
care is taken to prevent contact with carbon dioxide.  Water just sucks
that stuff up.  Our chemistry dept. still churns out water at about 6.0
and, if it is allowed to sit very long in the open air, that drops even
lower into the 5.5 range.  That is why so many soils have a pH in the area
of 5.5.  This has nothing to do with acid rain which has even lower pH
values.   
Incidently, this relates to the question of global warming.  The ability of
the oceans to absorb more carbon dioxide is unknown (and possibly
underestimated).  I think that means that no one has an answer to Paul's
question about where the calcium is and what it is doing.
Acidity is everywhere in freshwater.  That makes life tough for freshwater
shells.  The ocean is basic and far more congenial to shells although, as 
people have noted, pH and calcium carbonate concentrations fluctuate
sustantially, depending on temperature and other conditions.