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 the amount in the atmosphere decreased. Carbonate compounds were then precipitated as sedimentary rocks, eg limestone.
Carbon dioxide was also absorbed from the oceans when the first plants and algae carried out photosynthesis. Many of these organisms, and those in the food chains that they supported were turned into fossil fuels, eg crude oil, coal and natural gas, which all contain carbon.
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/algae used it for photosynthesis.
Plants make their own food by photosynthesis. In this process, carbon dioxide is reacted with water to produce glucose, with oxygen as a by-product:
carbon dioxide + water → glucose + oxygen
6CO2+ 6H2O→ C6H12O6+ 6O2
Scientists think that algae first evolved approximately 2.7 billion years ago, and soon after this oxygen began to exist in the atmosphere. Photosynthesis by primitive plants and algae released oxygen, which gradually built up in the atmosphere.
Eventually, the amount of oxygen present in the atmosphere enabled animals to evolve. Photosynthesis also decreased the amount of carbon dioxide present in the atmosphere.