Why did the Earth's oxygen levels increase?

Oxygen levels are generally thought to have increased dramatically about 2.3 billion years ago. Photosynthesis 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 oxidised. 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 fuels as plants died and their carbon compounds became locked up underground
  • formation of sedimentary rocks 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 volcanoesCarbon 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 rocksIron 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.
FossilsLife processes can change the composition of the atmosphere. Fossils can tell scientists what living things were around at different times in Earth’s history. Evidence of early plants can tell scientists how oxygen started to be added to the atmosphere long ago.

Give four reasons why the amount of carbon dioxide in the atmosphere decreased over time.

  1. it dissolved in the oceans
  2. primitive plants used it for photosynthesis
  3. it became trapped underground during the formation of fossil fuels
  4. it became trapped underground during the formation of sedimentary rocks