Roland,
 
The CRC Handbook of Chemistry gives the following solubilities for CaCO3 in
water in grams/100 ml at 25C:
 
aragonite 0.0015
calcite   0.0014
 
Using the value for aragonite, I calculate the equilibrium concentration of
calcium in a saturated solution to be 6 mg/L. so if you put a shell in a
liter of water kept at 25 C & waited long enough to attain equilibrium, you
wouldn't have more than 6 mg of Ca dissolved regardless of how large the
shell was. (I have no idea how long it would take to reach equilibrium, i.e.,
saturation in this case.)
 
The same source gives the Ca content of sea water as 400 mg/L. Therefore, sea
water already contains more Ca than there is in a saturated CaCO3 solution.
Consequently, a shell in sea water would not dissolve at all.
 
So far, so good. After I did this calculation yesterday, I went home & looked
up the subject in Vermeij, A Natural History of Shells, pp.40-50. It turns
out that Ca behaves differently in sea water. In ordinary water, more Ca
dissolves as temperature increases. But Vermeij claims that in sea water it
is the other way around: solubility of Ca increases as temperature goes down.
This is what he says: "Seawater in most of the world's oceans is
supersaturated with calcium and dissolved carbon dioxide, so that
availability of raw materials is not a significant limitation in the design
of shells." (He is discussing the evolutionary economics of shell design).
But later, he adds: "Cold seawater, deep waters and many freshwaters are
undersaturated with calcium carbonate, so that molluscs living there may be
affected by a limitation of mineral supplies. Calcium carbonate is unusual
among mineral salts for being more soluble at low than at high temperatures
in sea water." Further on (pp.45-48), he states: "In cold water as well as
under high pressure, however, shells built of calcium carbonate are
susceptible to dissolution." According to him, one of the adaptations shells
have come with to slow down dissolution is to shield the calcium carbonate
layers with a peristracum.
 
What is the moral of the story? It appears that in warm waters shells may not
dissolve on their own at all. So where do the empty shells go? (1) Shells in
shallow waters can be moved around by waves and currents & gradually worn
away from the abrasion. (2) Snails may consume dead shells to obtain calcium
carbonate. (Land snails apparently do this.) Even if the water is saturated
with calcium carbonate, direct consumption of calcium carbonate may be a
faster way of getting calcium. Finally, Vermeij mentions a third mechanism:
drilling predators (for example, octopuses) & bioeroders abrade shells. For
example, bryozoans, polychaete worms excavate tunnels & depressions in
shells. Do the octopuses in Puget Sound drill?
 
I suppose the water in Puget Sound would be considered cold. There shells may
indeed slowly dissolve away. I don't know how quickly that would happen
though.
 
A.