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

Marine organisms play a crucial role in decreasing carbon dioxide levels by using dissolved carbonates to form shells and skeletons. When these organisms die, their shells contribute to sedimentation, effectively trapping carbon in sedimentary rocks and reducing atmospheric carbon dioxide.

A strong answer should directly address the approved learning objective to link carbon dioxide removal to changes in the carbon cycle. 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 carbon cycle, and keeps the wording specific to AQA GCSE revision.