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ATMOSPHERE
Lesson 5 - Page 3

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There is evidence of free oxygen in rocks, like the banded iron formation. The red is caused when iron reacts and forms oxides. Weathering became an oxygen consumer, taking free oxygen and combining it with elements. This must have been the start of an abundant supply.

Without molecular oxygen life could not have evolved to its present form. It probably originated with cyanobacteria (like present day stromatolites) creating free oxygen as a by-product of photosynthesis.

During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10%. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 billion years ago. Present levels of O2 were probably not achieved until about 400 million years.


Stromatolites in Australia


Banded iron from Wisconsin

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