Great Oxygenation Event - Timing

Timing

The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen was first produced by photosynthetic organisms (prokaryotic, then eukaryotic) that emitted oxygen as a waste product. These organisms lived long before the GOE, perhaps as early as 3,500 million years ago. The oxygen they produced would have quickly been removed from the atmosphere by the weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded-iron formations, shown for example in sediments in Minnesota. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE. Without a draw-down, oxygen could accumulate very rapidly: for example, at today's rates of photosynthesis (which are much greater than those in the land-plant-free Precambrian), modern atmospheric O₂ levels could be produced in around 2,000 years.

Another hypothesis is an interpretation of the supposed oxygen indicator, mass-independent fractionation of sulfur isotopes, used in previous studies, and that oxygen producers did not evolve until right before the major rise in atmospheric oxygen concentration. This hypothesis would eliminate the need to explain a lag in time between the evolution of oxyphotosynthetic microbes and the rise in free oxygen.

A third newly proposed hypothesis suggests that a sharp drop in mantle melting activity occurred approximately 2.5 billion years ago. This reduction in melting resulted in changes in the chemical makeup of basalts and other surface erupted rocks. Diminished melting in the mantle decreased the depth of melting in Earth's crust, which in turn reduced the output of reactive, iron oxide-based volcanic gases into the atmosphere. The new hypothesis states that when melting in the mantle is high, basalt contains greater concentrations of elements such as chromium and magnesium and iron that are ordinarily found in the mantle. Less intense melting, on the other hand, results in basalt with a higher content of elements such as sodium and potassium that are found closer to Earth's surface. Research has shown that there was a sudden drop in mantle melting and the depth of that melting. This supposedly allowed oxygen levels to rise much more quickly because this "iron sink" was no longer available to complex-up free oxygen. Blair Schoene and lead author C. Brenhin Keller, a Princeton faculty member and a geosciences doctoral student respectively, compiled a database of more than 70,000 geological samples to construct a 4-billion-year geochemical timeline that demonstrates this reduction in subsurface melting.

Either way, the oxygen did eventually accumulate in the atmosphere, with two major consequences. First, it oxidized atmospheric methane (a strong greenhouse gas) to carbon dioxide (a weaker one) and water, triggering the Huronian glaciation. The latter may have been a full-blown, and possibly the longest ever, snowball Earth episode, lasting 300-400 million years. Second, the increased oxygen levels provided a new opportunity for biological diversification, as well as tremendous changes in the nature of chemical interactions between rocks, sand, clay, and other geological substrates and the Earth's air, oceans, and other surface waters. Despite natural recycling of organic matter, life had remained energetically limited until the widespread availability of oxygen. This breakthrough in metabolic evolution greatly increased the free energy supply to living organisms, having a truly global environmental impact; mitochondria evolved after the GOE.