The Great Oxygenation Event (GOE)—also referred to as The Great Oxidation Event—was a worldwide event during Earth's early history that marked a significant rise of oxygen in Earth's atmosphere. While it is difficult to attain a precise indication of when it took place, most experts believe that it occurred between 2.5 and 2.3 billion years ago, with the ends of the interval varying slightly between individual studies. This event transpired at the dawn of the Paleoproterozoic Era within the Proterozoic Eon, which occurred after much of Earth's early chaotic history during the Hadean and Archean Eons, but before the rise of noticeable life that we would see in the Ediacaran and Cambrian periods at the conclusion of the eon and the dawn of the Phanerozoic, respectively.
Prior to the rise in oxygen that the GOE would bring, the Earth was not as a hospitable of a place as it is today. In fact, before the event, Earth's atmospheric oxygen is thought to have been less than 0.001% of present atmospheric levels[2]. As a result, life during this time was simple, bacterial, and anaerobic. Life was not clearly detectable or easily visible, with primarily colonies of bacteria residing on Stromatolites: structures formed by the cementation of sedimentary grains by biofilms of microorganisms. These stromatolites were a staple of such early life, and were omnipresent for a colossal period of time.
The Earth remained this way throughout the duration of the Archean, stretching over a billion years, with oxygen levels remaining low. A planet with a "toxic" atmosphere was the norm, and would remain this way until the GOE would unfold, which would change the planet's atmosphere as we know it and pave the way for future life while inducing dramatic changes to the face of the planet and its atmosphere as a whole.
During the Archean, Methane (CH4) and Carbon Dioxide (CO2) dominated the atmosphere, with CH4 levels ranging between 10–2500 times its modern amount, and CO2 ranging between 100–10000 times its current concentration.[4] With such powerful greenhouse gases occupying the atmosphere in enormous qualities, the Earth was extremely hot from the impact of the greenhouse effect. As for oxygen, there was no oxygen gas on Earth, it resided only in compounds such as water and the atmosphere was essentially devoid of its presence.[5] Such conditions persisted until the emergence of oxygenic photoysnthetic cyanobacteria, which would bring about the GOE.
Cyanobacteria are microscopic organisms that are naturally found in both fresh water and salt water, and are colloquially known as blue-green algeae. When cyanobacteria first appeared on Earth is widely debated, but they are hypothesized to stretch as far back as 3.5 billion years ago, into the Archean.[7]. Cyanobacteria attain sustenance through photosynthesis, and for quite some period of time this process was anoxygenic, meaning that it did not produce any oxygen as a byproduct.
However, many scientists believe that just prior to the GOE, these microbes evolved and developed the ability of oxygenic photosynthesis.[8] Other scientists believe that this development happened earlier, with other restrictions hindering the massive production of oxygen. Regardless, with this development, these photosynthetic cyanobacteria were now capable of producing oxygen as a result of their processes. In turn, this would catalyze the feedback loop possible to engender the GOE and its monumental impact.
As mentioned previously, the atmosphere during this time was flooded with greenhouse gases, while photosynthetic cyanobacteria concurrently evolved oxygenic photosynthesis. Combined, these components were able to operate in unison for the reaction necessary to cause the Great Oxygenation Event. In a similar fashion to contemporary photosynthesis, these photosynthetic cyanobacteria at the time utilized the copious greenhouse gases in the air (namely CH4 and CO2) as part of their photosynthetic process and produced oxygen.
As a result, oxygen was first released into the ocean. At this time, there were large quantities of dissolved iron in the seawater, and the oxygen reacted with the iron to form iron oxide. This led to the deposition of iron oxide minerals being deposited on the sea floor, leading to the famous banded iron formations: a cornerstone of evidence for the GOE.[10] The formations are banded because exhibit the boom and bust cycles of the anaerobic photosynthetic cyanobacteria. When they created a lot of oxygen it would start to poison them and they would die off. Once they massively died off and oxygen decreased, the cyanobacteria would then thrive, but then would create too much oxygen and die off again. This process would repeat a multitude of times.
In addition to oxygen being released into the ocean, it was also released into the atmosphere. Here it was able to react with the CH4 in the atmosphere to create H2O and CO2. From here, the newly-created CO2 could be further used by the photosynthetic cyanobacteria for photosynthetic purposes. This allowed the cyanobacteria to produce even more oxygen, allowing it to eliminate even more methane from the atmosphere, producing more CO2 that it would take in, and so on. As a whole, this created a feedback loop[12] that resulted in a drastic reduction of greenhouse gases from the atmosphere, while simultaneously skyrocketing the proportion of atmospheric oxygen.
Prior to the GOE, atmospheric oxygen was virtually nonexistent, with the only trace of the element residing within water and the atmosphere being dominated by other gases. However, with the aforementioned feedback loop in full-effect, levels greenhouse gases such as CH4 and CO2 were drastically reduced while atmospheric O2 levels sharply increased. A common misconception though is that oxygen levels rose to modern concentrations, but that simply wasn't the case. From the GOE, oxygen levels only reached a fraction of its modern amount, with estimates ranging to being 1% of its modern equivalent to as high as 10% of present atmospheric levels.[14]
With the increase in oxygen during this time period, the number of anaerobic organisms declined, and they nearly reached the point of extinction. This was due to the fact that the anaerobic photosynthetic cyanobacteria proved to be their own worst enemy. Since they produced massive volumes of oxygen, they essentially poisoned themselves, which led to their intermittent die-offs throughout this duration of time. This is what led to the boom-and-bust cycles of their oxygen production that produced the banded iron formations mentioned earlier. In general, due to such an increase in oxygen, anaerobic life as a whole suffered, and it is estimated that this wiped out over 90% of life on earth.[15] This is why this event is sometimes referred to as the "oxygen crisis" due to the massive extinction that ensued. However, cyanobacteria managed to remain alive by residing in low oxygen environments, and life gradually evolved to become aerobic.
Due to the severe reduction in greenhouse gases, this led to the Earth as a whole becoming cooler. It was these gases that allowed the Earth to remain as warm as it did throughout the Archean, but with their mitigation from the cyanobacteria they were unable to trap heat on Earth as successfully as before. With such gradual reduction and subsequent cooling, the cyanobacteria did their job a little too well, and Earth's temperature consistently fell. Around 2.2–2.3 BYA, Earth's temperature plunged and became what is commonly known as a Snowball Earth event. Due to this happening around the same time as the GOE, many scientists contend that there was a causal relationship[17] between the two events.
This Snowball Earth in conjunction with the rise in oxygen nearly wiped out all life on Earth, and has been one of Earth's largest extinction events. Fortunately, however, life was able to survive this global freezing by relying on hydrothermal activity for warmth. As for Snowball Earth, it is hypothesized that this particular instance was able to conclude through greenhouse gases being released through tectonic activity and volcanoes[19]. This in itself showcases a quintessential interaction between the geosphere and biosphere of our planet.