  Competing geophysical/geochemical hypotheses for how Earth's surface became
oxygenated - organic carbon burial, hydrogen escape to space, and changes in
the redox state of volcanic gases - are examined and a more biologically-based
hypothesis is offered in response. It is argued that organic carbon burial is
of minor importance to the accumulation of oxygen in a mainly anoxic world
where aerobic respiration is not globally significant. Thus, for the Great
Oxidation Event (GOE) ~ 2.4 Gyr ago, an increasing flux of O2 due to its
production by an expanding population of cyanobacteria is parameterized as the
primary source of O2. Various factors would have constrained cyanobacterial
proliferation and O2 production during most of the Archean and therefore a long
delay between the appearance of cyanobacteria and oxygenation of the atmosphere
is to be expected. Destruction of O2 via CH4 oxidation in the atmosphere was a
major O2 sink during the Archean, and the GOE is explained to a significant
extent by a large decline in the methanogen population and corresponding CH4
flux which, in turn, was caused primarily by partial oxygenation of the surface
ocean. The partially oxygenated state of these waters also made it possible for
an aerobic methanotroph population to become established. This further
contributed to the large reduction in the CH4 flux to the atmosphere by
increasing the consumption of CH4 diffusing upwards from the deeper anoxic
depths of the water column as well as any CH4 still being produced in the upper
layer. The reduction in the CH4 flux lowered the CH4 oxidation sink for O2 at
about the same time the metamorphic and volcanic gas sinks for O2 also
declined. As the O2 source increased from an expanding population of
cyanobacteria - triggered by a burst of continent formation ~ 2.7-2.4 Gyr ago -
the atmosphere flipped and became permanently oxygenated.
