Contributors to this discussion have already noted that:  (1)  Marine
organisms constantly extract calcium from ocean water in the
construction of their shells; and  (2)  The concentration of calcium
in ocean water remains quite constant, at or near saturation level,
year in and year out.  The inescapable conclusion is that some
mechanism exists for replacement of dissolved calcium, at a rate
matching the rate of extraction.  Otherwise, the calcium
concentration would be gradually reduced over time until finally it
would be inadequate for shell production, and conchologists would
become extinct.  Since most of the non-dissolved calcium in the ocean
is tied up in animal life, it seems that animals (that is, their
postmortem remains) must figure prominently as a source of
replenishment.
Let's remember that mollusks are not the only marine harvesters of
calcium.  Various kinds of annelids (worms) and crustaceans build
tubes or shells of calcium salts.  On many rocky beaches here in the
northeastern United States, barnacles far outweigh mollusks in sheer
mass.  Vertebrates also - marine mammals, reptiles, and especially
fish have calcified skeletons which ultimately came from dissolved
calcium in the water.  But the champions among calcium-extractors are
those tiny, gelatinous, colonial polypoid organisms which we refer to
collectively as corals.  It would take even more shells than I have
in my basement to match the calcium content of the Great Barrier
Reef.
Aydin stated that a shell in sea water would not dissolve at all,
because sea water is already at saturation level.  I would modify
that statement - a shell will not dissolve unless the calcium
concentration of the water falls below saturation level.  In areas
where organisms abound, calcium is constantly extracted, and must
therefore constantly be replaced.  It is this ongoing replacement
that maintains the saturation level - it is not a static condition.
This can happen in one of two principle ways - either by direct
dissolution of nearby calcium salts (such as dead shells and coral),
or by dissolved minerals contained in runoff from the land.  One
could theorize that the first mechanism might be the primary one in
areas where the local geology is not rich in limestone or other
calcific minerals, or in areas distant from rivers and other sources
of inflow.  The physical breakdown of shells into smaller pieces, and
eventually into sand, by the various mechanisms already mentioned,
greatly increases the rapidity of dissolution by increasing the
surface area exposed to the water.
There is one characteristic of water which will greatly accelerate
the dissolution of carbonates, and that is pH.  For those who might
be unfamiliar, pH is a measurement of acidity and alkalinity, based
on a scale from 1 to 14 where values below 7 are acidic, values above
7 are basic (or alkaline), and 7.0 is neutral.  Carbonates dissolve
readily in acid solutions, and the more acidic the solution is (the
lower the pH), the faster they dissolve.  This is not usually an
issue in marine environments because the pH of sea water usually
falls around 8 - mildly but distinctly alkaline.  Even in the ocean
depths, where the pH is lower, it seldom if ever gets below 7.5.
However, in fresh water habitats it's a different story.  In bogs,
swamps, marshes, and small woodland ponds, the water pH may be below
7.0, due primarily to acids released in the decay of vegetation.  I
am not sure, but I suspect that such acidic conditions may be at
least partially responsible for the extreme erosion (or perhaps
corrosion is more appropriate) often seen in fresh water shells (does
anyone know whether this is true?).  Streams originating in such
areas, and then flowing through limestone or similar deposits may
dissolve a lot of calcium, and carry it along, ultimately depositing
it in the ocean.  Acid rain from industrial air pollution is another
source of dissolved minerals in runoff.   Of course, no matter how
much calcium is carried in, the final concentration in the sea water
cannot exceed the saturation level - any excess would simply
precipitate out as mineral crystals on the substrate.  We should
remember though, that limestone is little more than the recycled
shells of ancient marine organisms.  The changes from shells to
mineral deposit is largely physical, not chemical, and the resulting
limestone is still, chemically, primarily what the shell was -
calcium carbonate.  So, in the final analysis, dissolution of shells
is the source of calcium replacement, regardless of whether the
shells took a million year detour before dissolution or not.
 
Paul M.