Describe the significance of carbonate formation in the context of carbon dioxide levels in the atmosphere.

The formation of carbonates by marine organisms is significant because it removes carbon dioxide from the atmosphere. As organisms use carbonates to create shells and skeletons, they sequester carbon, which contributes to a decrease in atmospheric carbon dioxide levels over geological time.

A strong answer should directly address the approved learning objective to explain that marine organisms used carbonates to make shells and skeletons. This question belongs to How carbon dioxide decreased within The composition and evolution of the Earth's atmosphere, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to carbonates, and keeps the wording specific to AQA GCSE revision.