Much of what we know about environmental changes that occurred during the Precambrian comes from the carbon isotope record kept in limestone (see Fig. 3.1). This record is relatively featureless except for two 0.3 billion year time intervals during which large variations occur. The first of these took place between 2.3 and 2.0 billion years ago – a time interval following the appearance of O2 in our atmosphere. The second occurred between 0.8 and 0.5 billion years ago – a time interval during which the Earth underwent two total freeze-ups. As the carbon isotope record quietened down between...

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