RISES AND FALLS OF THE ATMOSPHERIC OXYGEN IN EARLY PALEOPROTEROZOIC

Presence of mass-independent fractionation (MIF) in sulfur isotopes prior to ~2.45 Ga and its absence in rocks younger than ~2.0 Ga strongly suggest that oxygenation of the Earth’s atmosphere occurred in this period of time. However, details of such a dramatic change in the Earth’s environment are still under debate. It has been suggested that by ~2.3 Ga photosynthetic oxygen flux overcome the flux of reduced volcanic gases and the atmosphere started to accumulate oxygen. In such scenario the oxygenation of the atmosphere should have been quick and irreversible. Furthermore, accumulation of oxygen results in the dramatic decrease of the atmospheric methane abundance and therefore could trigger Huronian glaciations.

Although such scenario is broadly plausible, it leaves at least two major questions unanswered: 1) Why were there three instead of one glaciations between 2.45-2.22 Ga ; 2) Why did it take ~200 Ma years to accumulate oxygen in the atmosphere.

Here we propose the following scenario. The first Paleoproterozoic glaciation was triggered by the decrease in methane abundance due to initial rise of the atmospheric oxygen. However, once Snowball conditions established atmospheric oxygen abundance should have decreased dramatically due to suppressed biogenic O2 flux and continuous flux of H2 and H2S/SO2 from volcanoes. The duration of the Paleoproterozoic Snowball glaciations depends on the time of CO2 accumulation to the (bars) levels necessary to deglaciate the planet. By the time CO2 would accumulate to such levels, atmosphere would become virtually anoxic again and allow high H2/methane concentrations to build up. Once the Earth’s biosphere was fully recovered, oxygen would accumulate again depleting methane and forcing the next Snowball Earth glaciation. Such cycles could take place several times producing oxygen-poor conditions between glaciations and explain observed presence of small MIF in sulfur isotopes between the first and the second Huronian glaciations.

Positive carbon isotope excursion in carbonates between the second and the third glaciations suggests an increase in oxygen production, which could have helped to break out of the Huronian Snowball Earth cycles and changed Proterozoic atmosphere irreversibly.