A BREATH OF FRESH AIR: HOW GREAT WAS THE GREAT OXIDATION EVENT?

A fundamental change in the oxidation state of the Earth's biosphere at ~2.2 Ga is marked in the stratigraphic record by a large (>10‰) positive C-isotope excursion. This C-isotope excursion records increased photosynthetic production and burial of carbon as organic carbon, and the subsequent release of oxygen to the biosphere. Increased biospheric oxygen is further supported by the disappearance of mass-independent fractionation, and a concomitant increase in the mass-dependent S-isotopic fractionation, recorded in sedimentary sulfides. Unfortunately, mass-independent fractionation disappears at oxygen levels as low as 10^-5 times PAL, so it remains unclear to what extent the biosphere was oxygenated by this event.

Because sulfate is delivered to the oceans by the oxidative weathering of crustal rocks, marine sulfate reservoir size can be used as a proxy for atmospheric oxygen levels. Detailed examination of the S-isotope record of marine sulfates (via carbonate-associated sulfate) may therefore provide the best means to determine the ultimate fate of the oxidizing potential recorded in the Paleoproterozoic marine C-isotope record. In particular, modeling the marine sulfate reservoir as a non-steady-state system, which is dependent upon both the mass and isotopic composition of reservoir size as well as input and output fluxes, permits an unprecedented glimpse into the possible evolution of the Precambrian ocean-atmosphere system.

Prior to 2.2 Ga, sulfate concentrations are inferred to have been <200µM, based on limited S-isotope variability preserved in sedimentary sulfides and by experimental data showing suppressed isotopic fractionation at extremely low sulfate concentrations. Non-steady state modeling of Proterozoic S-isotope records (Kah et al. 2004), however, suggest that sulfate levels remained between 1.5-4.5 mM, or ~5-15% modern values, for >1000 million years after the Great Oxidation Event, and may have reached levels of only 0.5-0.9 mM, or ~1-3% modern values, by the end of the Great Oxidation Event. This data suggests that, during the Great Oxidation Event, the capacity of volcanogenic/crustal oxygen sinks likely far outstripped net biospheric oxygen production.