Describe the role of marine organisms in the decrease of carbon dioxide in the atmosphere.

Marine organisms contribute to the decrease of carbon dioxide in the atmosphere by using dissolved carbonates to form shells and skeletons. When these organisms die, their shells accumulate on the ocean floor, leading to the formation of sedimentary rocks, which sequester carbon over geological time.

A strong answer should directly address the approved learning objective to explain that some carbon was trapped in fossil fuels formed from dead organisms. 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 fossil fuels, and keeps the wording specific to AQA GCSE revision.