Some amount of carbon dissolved into oceanic water remains as...

Carbon dioxide dissolved in oceanic water reacts with water to form carbonic acid, which dissociates into bicarbonate and carbonate ions. These carbonate ions react with calcium and magnesium ions present in seawater to form insoluble calcium carbonate and magnesium carbonate.

These insoluble carbonates precipitate and settle at the bottom of the ocean, forming sedimentary rocks such as limestone and dolomite. This process effectively removes carbon from the active carbon cycle and stores it as sediments.

Carbonates of sodium and potassium are highly soluble in water and do not form sediments. Carbonic acid remains dissolved in water.

Answer: carbonates of Ca and Mg