Marine and freshwater geochemistry
The oceans are not saturated in sodium or chlorine. In fact, the chemical makeup of the oceans has remained fairly constant for hundreds of millions of years. Yet rivers and other sources continually bring sodium, chlorine, and other dissolved elements and compounds to the oceans. This means some processes must exist that remove constituent elements at more or less the same rate that the world's rivers and other sources supply these constituents.

It is true that the Earth's rivers bring substantially more calcium and carbonate ions to the oceans than they bring sodium and chlorine ions. Dissolved carbon dioxide from the atmosphere provides another source of carbonate ions. Yet the oceans are indeed substantially depleted of calcium and carbonate compared to the supply from rivers. So why is this?

The oceans are substantially saltier than are the rivers that supply the oceans. This is primarily a result of evaporation. Even with that, sodium and chlorine are but a fraction of their saturation levels in the world's oceans. This is not the case for calcium and carbonate. Carbonates in general have a low saturation level, and amongst common cations, calcium carbonate has a very low saturation level. Calcium carbonate is typically at or near the saturation level in the oceans, and oftentimes above it, particularly near the surface. This makes it very easy for calcium carbonate to come out of solution.

Because calcium and carbonate are at or above the saturation level, just about anything can cause calcium carbonate to form. The key "just about anything" process that causes calcium and carbonate to come out of solution is life.

This means that calcium and carbonate have a rather low residence time in the oceans. The processes that remove sodium and chlorine are much, much slower. The slowness by which sodium and chlorine are removed versus the rapidity by which calcium and carbonate are removed are why the oceans are "upside down" compared to rivers with regard to calcium, carbonate, sodium, and chlorine.