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.