Question

Where was the calcium carbonate found in limestone was originally extracted from?

Answer 1

The carbonate in limestone was extracted from the dissolved carbon dioxide `CO2` in the ocean. The calcium was extracted from the dissolved solids carried to the oceans by streams and rivers.

Explanation:

Carbon dioxide dissolves in ocean water from the atmosphere. The carbon dioxide can combine with dissolved oxygen in the ocean to form the carbonate ion `CO3^-2`

Calcium is not very soluble in water but is more soluble in slightly acidic environments.

The corals create a slightly acidic environments where the calcium and carbonates are combined inside the corals to create calcium carbonate `CaCO3` that the corals use to build the limestone reefs.