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Unformatted text preview: ! Protects non-aquatic life from sunlight Aerobes evolve (2 BYA) ! Aerobes use O 2 , give off CO 2 ! Developed mutualistic relationship with green plants to maintain steady-state atmospheric O 2 content UV O 2 + UV ! ! O 3 6 Pre-biotic (3.5 to 5 BYA) ! Outgassed by Earths surface ! Chemical reactions involving ammonia (NH 3 ) 3.2 BYA and later ! Denitrification: organic compounds react with nitrate ions (NO 3 ) to release molecular nitrogen (N 2 ), nitrous oxide (N 2 O), nitric oxide (NO) Nitrogen (N 2 ) 7 Atmospheric Catastrophes Oxygenation of the atmosphere Meteor/Asteroid strike on Earth about 65 MYA ! Impact raised dust clouds, set fires that produced global clouds of smoke ! Reduced sunlight intensity reduced Earth surface temperatures; large reptiles died...
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- Spring '09