Geoscience Reference
In-Depth Information
be quite active, but oxygen will not accumulate to appreciable amounts
because it is reacted out of the atmosphere by volcanic gases. As dis-
cussed in the previous chapter, we don't quite know when cyanobacte-
ria first evolved. If they evolved well before 2.5 to 2.6 billion years ago,
then volcanic outgassing of oxygen-reactive gases was clearly in excess
of the oxygen liberation rates for quite some time. By 2.5 to 2.6 billion
years ago, however, the rates of oxygen liberation and rates of oxygen
consumption by volcanic gases seemed to be nearly balanced. Sometimes
the balance shifted toward oxygen liberation, generating an oxygen
whiff, and sometimes the balance shifted toward oxygen consumption,
and oxygen disappeared.
The question then becomes, when and how did oxygen become more
than a whiff and a permanent feature of Earth's atmosphere? This will
be a focus of the next chapter.
