Photosynthesis pulled carbon dioxide out of the air
Definition
Photosynthesis
The process in which algae and plants use light energy to convert carbon dioxide and water into glucose, releasing oxygen.
- The early atmosphere was rich in carbon dioxide, CO2\text{CO}_2CO2, but its proportion fell steeply over time.
- A major cause was photosynthesis by algae and plants, which removes carbon dioxide from the air.
- In photosynthesis, carbon dioxide and water become glucose and oxygen: 6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_26CO2+6H2O→C6H12O6+6O2.
- The carbon taken in is built into the biomass of the organisms as they grow.
Key Idea
As oxygen rose, carbon dioxide fell: the same photosynthesis that added oxygen removed carbon dioxide from the air.
Oceans dissolved carbon dioxide and locked it in carbonate rock
Definition
Carbonate
A compound that contains the carbonate ion, CO32−\text{CO}_3^{2-}CO32−.
Definition
Sedimentary rock
Rock formed when layers of sediment are deposited, compacted and cemented together.
- Carbon dioxide is soluble, so large amounts dissolved into the oceans.
- In the oceans it formed carbonates, including calcium carbonate, CaCO3\text{CaCO}_3CaCO3.
- Many sea creatures used this carbonate to build their shells and skeletons.
- When they died, the remains sank and were compacted into sedimentary rock such as limestone.
- This locked carbon away for millions of years, keeping it out of the atmosphere.
Fossil fuels trapped carbon from ancient life
Definition
Fossil fuel
A finite fuel, such as coal, oil or natural gas, formed from the remains of ancient organisms over millions of years.
Definition
Biomass
Material from living organisms or organisms that were recently alive.
- Dead plants and tiny ocean plankton were buried before they could fully decay.
- Over millions of years, heat and pressure turned this biomass into fossil fuels.
- These fuels are coal, crude oil and natural gas, and the carbon of the organisms became trapped inside them.
- Burying this carbon removed still more of it from the carbon dioxide that had been in the air.
Common Mistake
- Do not give only one reason; carbon dioxide fell through photosynthesis, dissolving in oceans, and burial in rocks and fossil fuels.
- Burning fossil fuels today releases this stored carbon back as carbon dioxide, reversing the ancient removal.
Three routes that stored the carbon away
- Photosynthesis moved carbon from the air into living biomass.
- Dissolving and precipitation moved carbon into oceans and carbonate sediments.
- Burial locked carbon into sedimentary rock and fossil fuels.
Exam technique
- For full marks give more than one reason carbon dioxide fell: photosynthesis, dissolving in oceans, and storage in rocks and fossil fuels.
- Link the fall in carbon dioxide to the rise in oxygen whenever a question asks about both gases together.
Self review
- Which process removed carbon dioxide from the air and released oxygen?
- How did the oceans reduce the amount of carbon dioxide in the atmosphere?
- Name a sedimentary rock that stores carbon taken from ancient carbon dioxide.
- How did fossil fuels lock carbon away underground?
- Give three different ways carbon dioxide was removed from the early atmosphere.
