Earth's oxygen levels
Oxygen levels are generally thought to have increased dramatically about 2.3 billion years ago. 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. by ancient bacteria may have produced oxygen before this time. However, the oxygen reacted with iron and other substances on Earth, so oxygen levels did not rise to begin with. Oxygen levels could only begin to rise when these substances had been oxidiseChemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons.. In addition, early plants and algae began to release oxygen at a faster rate. Oxygen levels then showed a dramatic increase.
Why did carbon dioxide levels decrease?
Carbon dioxide levels decreased because of processes that included:
- dissolving in the oceans
- use by plants for photosynthesis
- formation of fossil fuelNatural, finite fuel formed from the remains of living organisms, eg oil, coal and natural gas. as plants died and their carbon compounds became locked up underground
- formation of sedimentary rockA type of rock formed by the deposition of material at the earth's surface. from the shells of ancient sea creatures
Types of evidence for the composition of the early atmosphere
Scientists cannot be sure about the composition of the early atmosphere. No measurements can be made, so scientists must analyse indirect evidence from other sources.
| Type of Evidence | What it tells scientists |
| Analysis of gases from modern volcanoes | Carbon dioxide and water vapour are released by volcanoes today. This means they are likely to have made up most of the early atmosphere, along with other volcanic gases. |
| Chemical composition of ancient rocks | Iron sulfide can only exist in rocks that were formed before there was oxygen in the atmosphere. Rocks containing iron oxide can only form if oxygen is present. Dating these rocks gives scientists evidence of when oxygen first built up in the atmosphere. |
| Fossils | Life processes can change the composition of the atmosphere. Fossils can tell scientists what living things were around at different times in Earth鈥檚 history. Evidence of early plants can tell scientists how oxygen started to be added to the atmosphere long ago. |
| Type of Evidence | Analysis of gases from modern volcanoes |
|---|---|
| What it tells scientists | Carbon dioxide and water vapour are released by volcanoes today. This means they are likely to have made up most of the early atmosphere, along with other volcanic gases. |
| Type of Evidence | Chemical composition of ancient rocks |
|---|---|
| What it tells scientists | Iron sulfide can only exist in rocks that were formed before there was oxygen in the atmosphere. Rocks containing iron oxide can only form if oxygen is present. Dating these rocks gives scientists evidence of when oxygen first built up in the atmosphere. |
| Type of Evidence | Fossils |
|---|---|
| What it tells scientists | Life processes can change the composition of the atmosphere. Fossils can tell scientists what living things were around at different times in Earth鈥檚 history. Evidence of early plants can tell scientists how oxygen started to be added to the atmosphere long ago. |
Question
Give four reasons why the amount of carbon dioxide in the atmosphere decreased over time.
- it dissolved in the oceans
- primitive plants used it for photosynthesis
- it became trapped underground during the formation of fossil fuels
- it became trapped underground during the formation of sedimentary rocks