Why is the presence of limestone in the geological record considered evidence that atmospheric carbon dioxide levels were lower during the time it was deposited?

Limestone forms when dissolved carbon dioxide reacts with calcium ions in seawater to produce calcium carbonate precipitates. When atmospheric CO₂ is low, less CO₂ dissolves in the oceans, reducing the amount of carbonate that can precipitate. Therefore, extensive limestone deposits indicate periods when CO₂ levels were relatively low, allowing carbonate to accumulate.

A strong answer should directly address the approved learning objective to interpret simple evidence for changes in atmospheric oxygen over time. This question belongs to How oxygen increased 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 oxygen, and keeps the wording specific to AQA GCSE revision.