11.3: Proterozoic Atmosphere

The Great Oxidation “Event”

In the early Proterozoic Eon, Earth transitioned from an oxygen-free atmosphere to one with a decent amount of free oxygen (\(\ce{O2}\)). Though this is called the Great Oxidation “Event,” [GOE] it is not a very sudden event at all. Traditionally, it is viewed as the span from 2.4 Ga to 2.0 Ga: that’s 400 million years, a very drawn-out “event.” Other names include the Oxygen Crisis, the Oxygen Catastrophe, and the Oxygen Revolution, each of which puts its own spin on interpreting the rise of this important gas.

Prior to the GOE, the planet’s atmosphere lacked significant free oxygen. We know this because of unusual chemical sedimentary rocks that could only form in an ocean that is largely oxygen free (banded iron formations [BIFs]), as well as grains of minerals like pyrite and uraninite in clastic sedimentary rocks. These signatures of the absence of oxidizing conditions imply a very different early Earth.

Figure \(\PageIndex{1}\): Deformed banded iron formation, Soudan, Minnesota. Chemical sedimentary rocks like these imply an oxygen-free atmosphere. (Callan Bentley photo)

Those anoxic conditions are preserved in Archean strata and those of the early Proterozoic. But by the middle of the Proterozoic, the BIFs and detrital pyrites peter out, because they were getting oxidized by atmospheric oxygen, and we see the first appearance of oxidized terrestrial sediments (red beds). These are river and floodplain sediments deposited in contact with copious oxygen – enough \(\ce{O2}\) to rust their iron content through and through.

Figure \(\PageIndex{2}\): Detrital pyrite in conglomerate of the Steyn Reef Placer, Witwatersrand Supergroup of South Africa. (Photo by Frances Deegan; reproduced with permission.)

Origin of Oxygen

So where did all this free oxygen come from, and what was the impact on the biosphere? Oxygen is a waste product of photosynthesis, so any photosynthetic organism produces it in proportion to the amount of glucose it generates. Of particular note are cyanobacteria, which are preserved from Archean and younger times as stromatolites. Long before true plants evolved, photosynthetic cyanobacteria was busy for billions of years pumping oxygen out into the oceans (and thence by diffusion into the atmosphere).

Figure \(\PageIndex{3}\): Modern stromatolites, Shark Bay, Australia. (Photo by Alicejmichel via Wikimedia)

This oxygen, being the highly reactive element it is, immediately reacted with whatever was suitable and available: carbon, for instance, making \(\ce{CO2}\). Or silicon, making \(\ce{SiO2}\). Or maybe iron dissolved in the seawater; that would oxidize and settle out as magnetite or hematite. But the rates of production matter: if oxygen was being churned out at a rate greater than the availability of these other reactive elements to buffer its rise, then it eventually began to accumulate in the atmosphere, available for reactions, but finding insufficient reactive elements to bond with. It built up and built up.

But while life was chemically fine for cyanobacteria sulfur-reducing bacteria, for instance (the ones implicated above in end-Permian euxinia) cannot survive in the presence of free oxygen; it’s a deadly poison to them. They are not alone: many groups of microbes cannot tolerate free oxygen. Some persist today in isolated low-oxygen settings such as swamp muck, but many others probably went extinct as the GOE took hold. How many exactly, and what were their characteristics? We’ll never know: microbes just don’t fossilize with the level of detail we would need to answer those questions. It seems reasonable to infer it was a great many, but we must become comfortable with the uncertainty about the specifics.

Definition: Term

• banded iron formation - unusual chemical sedimentary rocks that could only form in an ocean that is largely oxygen-free
• cyanobacteria - single or multicellular bacteria that use chlorophyll to absorb energy from light and were most responsible for constructing stromatolites
• detrital pyrite - rounded, transported pyrite found in sedimentary rocks, serving as an indicator of an oxygen-free Earth atmosphere
• stramatolite - layered, sedimentary rock structures formed by photosynthetic cyanobacteria trapping sediment in shallow water