Approximately 1,000,000,000 years ago ocean organisms evolved and began to photosynthesise. They used the carbon dioxide and gave off oxygen as a waste product. Then plants evolved and began to photosynthesise. More oxygen was added to the atmosphere.
Carbon dioxide disappeared from the atmosphere as it dissolved into the oceans and was used for photosynthesis. Thus, carbon dioxide was removed from the atmosphere and oxygen added. The carbon dioxide in the oceans was used and turned into calcium carbonate for shells by marine organisms. Oysters still have calcium carbonate shells today. When the marine organisms died, the shell fell into the sand and became sediment. The layers of sediment became compact and sedimentary rocks were formed. The link between limestone and carbon is that limestone is made from shells which are mostly calcium carbonate.
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