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How was oxygen introduced into the atmosphere
Algae & plants produced the O2 that is now in the atmosphere by photosynthesis
Equation for photosynthesis
6CO2 + 6H2O -> C6H12O6 + 6O2
carbon dioxide + water –(light)-> glucose + oxygen
When did algae first produce oxygen
Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere
How have oxygen levels increased
Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere
How did Carbon Dioxide Levels decrease
Algae and plants decreased the % CO2 in the atmosphere by photosynthesis
How else did Carbon Dioxide decrease (fossil fuels)
-CO2 was also decreased by the formation of sedimentary rocks that contain carbon (e.g. limestone and coal)
-and by the production of fossil fuels from the remains of dead plants and animals when they decayed
Note 
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