composition and evolution of earths atmosphere

• gases in the atmosphere

  • for 200 million years, the proportions of gases in the atmosphere have been the same
  • 78% nitrogen
  • 21% oxygen
  • 0.04% carbon dioxide
  • >1% argon, water vapour
• earths early atmosphere - volcanoes

  • evidence is limited as there is a time scale of 4.6 billion years
  • intense volcanic activity - released gases that formed atmosphere and water vapour that condensed to form the oceans. similar to atmosphere of Mars or Venus, consisting of mainly carbon dioxide and little oxygen
  • volcanoes produced nitrogen which builds up atmosphere gradually. small proportions of methane/ammonia
• earths early atmosphere - oceans

  • when oceans formed carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of carbon dioxide in the atmosphere.
• how oxygen increased

  • algae and plants produced the oxygen that is now in the atmosphere by photosynthesis
  • 6co2 + 6h2o -> 602 + c6h1206
  • algae first produced about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere
  • over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve
• how carbon dioxide decreased

  • algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis
  • carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon
• changes in the atmosphere

  • oxygen increased by photosynthesis, carbon dioxide decreased by photosynthesis
  • ammonia decreased as it reacted with oxygen to form nitrogen and water vapour
  • methane decreased as it reacted with oxygen to make carbon dioxide and water
  • carbon dioxide locked up in sedimentary rock, absorbed by plants, dissolved in oceans
• how did carbon dioxide form sedimentary rocks

  • Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved to form soluble carbonatecompounds so its amount in the atmosphere decreased. Carbonate compounds were then precipitated as sedimentary rocks, eg limestone