Astronomy and Astrophysics – Astrophysics – Earth and Planetary Astrophysics
Scientific paper
2012-03-30
Astronomy and Astrophysics
Astrophysics
Earth and Planetary Astrophysics
27 pages, 2 figures, submitted to Astrobiology
Scientific paper
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.
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