How carbon dioxide decreased
Formation of sedimentary rocks
Carbon dioxide is a very solubleAble to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution. gas. It dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. readily in water. As the oceans formed, carbon dioxide dissolved to form soluble carbonateSubstance containing carbonate ions. Most carbonates are insoluble (such as calcium carbonate) but sodium carbonate and ammonium carbonate are soluble in water. compoundA substance formed by the chemical union of two or more elements. so its amount in the atmosphereThe layers of gases that surround the Earth. The important gases in the atmosphere are nitrogen, oxygen and carbon dioxide. decreased. Carbonate compounds were then precipitateA suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction. as sedimentaryRocks that are formed through the deposition of sediments, eg limestone and sandstone. rocks, eg limestone.
Uptake by living organisms
Carbon dioxide was also absorbed from the oceans into photosynthesisA chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis. Algae subsumed within plants and some bacteria are also photosynthetic. algaeA group of organisms that photosynthesise but lack the complex range of cell types and organs found in land plants. Most algae are aquatic. All have chloroplasts. and plants. Many of these organismLiving entity, eg animals, plants or microorganisms., and the simple organisms in the food chains that they supported were turned into fossil fuels, eg crude oilMixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms., coal and natural gasA naturally occurring hydrocarbon gas mixture., which all contain carbon.
Coal is a fossil fuelNatural, finite fuel formed from the remains of living organisms, eg oil, coal and natural gas. 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 oxidationThe gain of oxygen, or loss of electrons, by a substance during a chemical reaction. 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.
Question
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.