C13.02: Our evolving atmosphere

• Algae and plants decreased the percentage of carbon dioxide in the early atmosphere by photosynthesis
• Carbon dioxide (and water) is taken in by plants and converted to glucose and oxygen in photosynthesis. The carbon in glucose is can then end up in new plant material. When animals eat the plants, some of this carbon is transferred to animal tissues, including their skeletons and shells.

  
  

  Over millions of years, the skeletons and shells of marine organisms built up at the bottom of oceans. They became covered with layers of fine sediment. Under the pressure, eventually the deposits formed sedimentary carbonate rocks such as limestone. A lot of the atmospheric carbon was decreased this way.
• Some of the remains of ancient living things were crushed by large-scale movements of the Earth and were heated within the Earth's crust over long periods of time. They formed the fossil fuels coal, crude oil, and natural gas.
• Coal is classed as a sedimentary rock, and was formed from thick deposits of plant material, such as ancient trees and ferns. When the plants died in swamps, they were buried, in the absence of oxygen, and compressed over millions of years.
• Crude oil and natural gas were formed from the remains of plankton deposited in muds on the seabed. These remains were covered by sediments that became layers of rock when compressed over millions of years. The crude oil and natural gas formed is found trapped beneath these layers of rock.
• Carbon dioxide was also removed from the early atmosphere by dissolving in the water of the oceans. It reacted, for example, with metal oxides, and made insoluble carbonate compounds. These fell to the seabed as sediments and helped to form more carbonate rocks.
• Volcanoes also produced nitrogen gas, which gradually built up in the early atmosphere, and there may have been small proportions of methane and ammonia gases.
• Any methane and ammonia found in Earth's early atmosphere reacted with the oxygen formed by the evolving algae and plants. This removed the methane and ammonia from the atmosphere. However, the levels of Nitrogen gas in the atmosphere could build up as nitrogen is a very unreactive gas.

  
• By 200 millions years ago, the proportion of gases in the Earth's atmosphere had stabilised.
• About 78% of the atmosphere today is nitrogen. About 21% is oxygen and 0.04% is caron dioxide. All the noble gases are found in air, with argon, the most abundant at 0.9%.