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and Lee Kump, both from Penn State University, and Dave Catling
from the University of Washington. They have all described, like Dick,
how the GOE likely represents a point in time when oxygen liberation
exceeded oxygen demand, and they, among others, have each presented
clever (but different) models to support this idea. I particularly like
Dick's model though, due to its simplicity and for the fact that the pro-
cesses he prescribes make good geological sense.
So, Preston Cloud had it right. The geologic record demonstrates
that around 2.3 billion years ago the oxygen content of Earth's atmo-
sphere increased dramatically. Since cyanobacteria likely evolved much
earlier, it does not appear that a well-oxygenated atmosphere is a neces-
sary or immediate consequence of the activities of oxygen-producing
organisms. Atmospheric chemistry is a slave to the dynamics of the
mantle, as the interior and exterior of the planet are connected in a pro-
found way. Indeed, it took half of Earth's history for the mantle to quiet
to point where oxygen could accumulate. This, however, represented a
watershed, a tipping point if you will, where the chemistry of Earth's sur-
face was forever altered. In the next chapter we will see what came next.
 
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