How carbon dioxide decreased

Formation of sedimentary rocks

Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved to form soluble carbonate compounds so its amount in the atmosphere decreased. Carbonate compounds were then precipitated as sedimentary rocks, eg limestone.

Uptake by living organisms

Carbon dioxide was also absorbed from the oceans into photosynthetic algae and plants. Many of these organisms, and the simple organisms in the food chains that they supported were turned into fossil fuels, eg crude oil, coal and natural gas, which all contain carbon.

Coal is a fossil fuel which was formed from trees which were in dense forests in low-lying wetland areas. Flooding caused the wood from these forests to be buried in a way that prevented oxidation taking place. Compression and heating over millions of years turned the wood into coal.

Crude oil and natural gas were formed from simple plants and tiny animals which were living in oceans and lakes. These small organisms died and their remains sank to the bottom where they were buried under sediments. The lack of oxygen prevented oxidation from occurring.

Over millions of years, heat and pressure turned the remains of the organisms into crude oil and natural gas. Natural gas contains the smallest molecules and is often found on top of crude oil, trapped under sedimentary rock.


Describe two reasons why the amount of carbon dioxide in the atmosphere decreased over time.

Carbon dioxide dissolved in the oceans, and primitive plants used it for photosynthesis.

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