Calcium is present in seawater. One of our seafloor sediment types were calcareous oozes....or sediments composed of shells of organisms. Calcareous = containing calcium.

If water is a universal solvent, what might seawater dissolve in order for calcium to be present?

 

Limestone and calcareous shells are dissolved to present calcium ion in the sea water.

Step-by-step explanation

The limestone beds are having the calcium carbonate composition. When these beds near to coast or on sea bed may be dissolved to form the calcium and bicarbonate ions. These ions are used for the organisms to build their shells which are having calcium carbonate composition. These shells after the death of organism may sink to the sea bottom and dissolve under the calcium carbonate compensation depth. This also causes the calcium ions in sea water.