The Early Atmosphere
The first atmopshere was formed by volcanic gases:
- Lots of Carbon Dioxide
- Lots of water vapour
- Bits of other gases
The Earth cooled and the water vapour condensed to form oceans.
We can't be sure of the exact gases as humans didn't exist and the time machine hasn't been invented yet however we are relatively certain that there was no oxygen as iron compounds (such as iron sulfide and iron carbonate) can only be formed when there is no oxygen have been discovered.
A Changing Atmosphere
Approximately 1,000,000,000 years ago ocean organisms evolved and began to photosynthesise. They used the carbon dioxide and gave off oxygen as a waste product. Then plants evolved and began to photosynthesise. More oxygen was added to the atmosphere.
Carbon dioxide disappeared from the atmosphere as it dissolved into the oceans and was used for photosynthesis. Thus, carbon dioxide was removed from the atmosphere and oxygen added. The carbon dioxide in the oceans was used and turned into calcium carbonate for shells by marine organisms. Oysters still have calcium carbonate shells today. When the marine organisms died, the shell fell into the sand and became sediment. The layers of sediment became compact and sedimentary rocks were formed. The link between limestone and carbon is that limestone is made from shells which are mostly calcium carbonate.
The Atmosphere Today
In dry air, the atmosphere today contains approximately 78% nitrogen; 21% oxygen; 0.9% argon; 0.04% carbon dioxide and traces of other gases. Fossil fuels release carbon dioxide and sulphur dioxide back into the atmosphere because natural fuels have trapped carbon dioxide for many years and once this is burnt the trapped gases are released. Biofuels also contribute to this however as they are younger they have held less carbon dioxide and sulphur dioxide. Cattle and rice fields release methane into the air. Carbon dioxide is released through deforestation. This means that there are less trees to process the carbon dioxide and release oxygen. Volcanic eruptions release sulphur dioxide and carbon dioxide.
How The Amosphere Has Changed
4.1 billion years ago:
- Volcanoes released carbon dioxide and small amounts of hydrogen, carbon monoxide and nitrogen.
- Water vapour condenses to form oceans
1.8 billion years ago:
- Plants evolve and photosynthesise
- Reductions in carbon dioxide and increased amounts of oxygen
- Carbon dioxide dissolves into the oceans and becomes trapped in rocks
- Nitrogen increases as dead plants decay
- Carbon dioxide has decreased significantly
- There is more oxygen and nitrogen in the atmosphere.