GEO 102 Lecture 25 Outline of Last Lecture I What is life Outline of Current Lecture II Introduction of free oxygen III Life and atmosphere IV What were these organisms V stromatolites Current Lecture Introduction to free oxygen two processes introduced free oxygen into the atmosphere o Hadean Archean 1 Photochemical dissociation involves ultraviolet radiation in the upper atmosphere a The radiation disrupts water molecules and releases their oxygen and hydrogen b This could account for 2 of present day oxygen c But with 2 oxygen ozone forms creating a barrier against ultraviolet radiation 2 More important were the activities of organisms that practiced photosynthesis Life and atmosphere The first process making ozone helped to make later life o UV light destroys key building blocks of life The second process could only occur because of life o Irony is that oxygen is deadly to the organisms that produced the oxygen What were these organisms Single cell true bacteria Not many organelles simple cells Photosynthetic and chemosynthetic That produced oxygen as a metabolic product First fossils These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Photomicrographs from western Australia s 3 3 to 3 5 billion year old warrawoona group Stromatolites Present day stromatolites grow as sediment is trapped on sticky mats of blue green algae cyanobacteria o Now restricted to environments where snails cannot live The oldest know undisputed stromatolites are found in rocks in South Africa o 3 0 billion years old o Probable ones from the Warrowoona Group in Australia o 3 3 to 3 5 billion years old
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