Rocks and rain interact to absorb carbon dioxide through a process called rock weathering that also eventually delivers carbon dioxide to the ocean.
Here’s how rock weathering works: some of the carbon dioxide in our atmosphere combines with water and falls to the surface of the earth in rain. The rain contains an acid - carbonic acid - that slowly dissolves away minerals in rocks. (Pure precipitation is naturally acidic. Water contains atmospheric gases as well as carbon dioxide, and when the carbon dioxide dissolves it forms carbonic acid.)
This weathering process releases the minerals such as calcium, magnesium, potassium, or sodium ions which are carried to the ocean by rivers. In the oceans, the ions and other compounds are digested by marine organisms such as corals and mollusks and converted into the solid calcium carbonate that makes up their shells and skeletons. As these organisms die, their remains settle onto the ocean floor and form layers of limestone and similar rock types. The carbon remains locked up there for millions of years.
Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. Since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Scientists are exploring ways to increase the amount of carbon dioxide stored in the ocean without increasing ocean acidification.
Nasa Earth Observatory, The Slow Carbon Cycle
World Resources Institute, Leveraging the Ocean’s Carbon Removal Potential