From what I understand the 'Great Oxidation Event' occurred around 2.4 billion - 2.3 billion years ago, when cyanobacteria flooded Earth’s atmosphere with oxygen.

The 'boring billion' period was around 1.8 billion years ago, during which it is thought not a lot changed on planet Earth. Low oxygen levels were thought to be 1-40% of modern oxygen levels during this time.

Why did the atmosphere go from 'flooded with oxygen' to 1-40% of modern oxygen levels. Was it because during the Mesoproterozoic era, the rate of burial of decaying organic carbon matter under marine sediments flatlined?

  • $\begingroup$ Doesn't the question contains an answer inside? ("when cyanobacteria flooded Earth’s atmosphere with oxygen"). Cyanobacteria can do oxygenic-photosynthetic (and seemingly the first oxygenic-photosynthetic organisms were cyanobacteria aka blue-green-algae). Are you telling about some other different event? $\endgroup$
    – user25568
    May 16, 2017 at 10:13
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    $\begingroup$ @AlwaysConfused there was a big drop in oxygen levels after the GOE, which led to the "boring billion". OP is asking why that drop happened. $\endgroup$
    – user24284
    May 16, 2017 at 10:59

2 Answers 2


Your question probably arises from a misunderstanding regarding the 'flooding' metaphor. I would not assume that 'flooded Earth's atmosphere with oxygen' means a lot of oxygen in the first place ... the metaphor means that oxygen levels rose from basically absent to noticeable.

If you look at the history of oxygen in Earth's atmosphere you see a general trend of gradual increase with a 1GY lasting stationary phase (Holland (2006), picture taken from above linked Wikipedia article - red and green lines are upper and lower estimates, respectively):

enter image description here

After initial atmospheric oxygenation about 2.5 GYa, Earth's oxygen sinks (mainly the oceans, after that landmasses and the ozone layer) were filled and buffered atmospheric oxygen levels, keeping them low. Once those sinks were saturated (approx. 850 MYa), the atmosphere started to oxygenise rapidly.

The constant (or as discussed below: drop in) oxygen concentration is caused by these sinks. The oxygen that is produced by photosynthesis was just not available to Earth's organisms as it was not atmospheric.

As @GerardoFurtado points out in the comment, modalities of the stationary phase are still debated. A review by Lyons et al. (2014) presents evidence that after the initial increase in oxygen levels, a drop in atmospheric oxygen preceded the stationary phase (see their Fig. 1, blue boxes). This does not change much as the newly inferred stationary phase, though probably a more fluctuating stationary phase than suggested by Holland (2006), is still higher than before the great oxidation event and lower than at present.

The major conclusions remains the same: There is evidence that the seemingly slow evolution 2-1 GYa was probably caused by oxygen limitation and oxygen levels were buffered by oxygen sinks. The change of oxygen concentration was determined by both the rate of oxygen production and the rate with which oxygen is stored in those sinks; the interplay of those rates caused the oxygen concentration to remain constant, drop or occasionally even rise, i.e. fluctuated.

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    $\begingroup$ I believe OP's premise is correct, and your answer is based on the wrong assumption that there was a "gradual increase with a stationary phase". There was a reduction in oxygen levels after the GOE, during the "boring billion". From Nature: "Earth’s early atmosphere has seen two major spikes in oxygen concentration [...] Rather than accumulating steadily, new findings suggest oxygen levels dropped precipitously soon after the first peak" (emphasis mine). Source: nature.com/news/… $\endgroup$
    – user24284
    May 16, 2017 at 9:48
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    $\begingroup$ @GerardoFurtado: Thanks for pointing that out. I do not think this changes much but incorporated a recent review that supports the claim of dropped oxygen levels and discussed this a bit. $\endgroup$ May 16, 2017 at 10:04
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    $\begingroup$ Good to see that, cheers. $\endgroup$
    – user24284
    May 16, 2017 at 10:57

Maybe the introduction of any amount of oxygen altered the availability of nutrients. The oxygen producers had evolved in low / no oxygen conditions and it was only when they reached critical mass that the great oxygenation event could occur.

Once it did occur, nutrients that had once been soluble and available were in short supply: specifically iron and reduced nitrogen.

Fe2+ is soluble and so before the oxygenation event there was probably a fair bit sloshing around. Once oxygen came the iron turned to Fe3+ and was much harder to come by. https://www.princeton.edu/~cebic/ironIIvsIII.html

Iron is still growth limiting in large areas of open ocean, as evidenced by this experiment supplementing the open ocean with iron. http://www.nytimes.com/2012/10/19/science/earth/iron-dumping-experiment-in-pacific-alarms-marine-experts.html

Nitrogen is the other nutrient whose availability would have changed.
The ancient enzyme used by nitrogen fixers is poisoned by oxygen and once oxygen was around, the activity of these organisms no doubt plummetted and with it, availability of reduced nitrogen. Lack of iron would be a double whammy for the nitrogen fixers in which the enzyme requires iron.

So: maybe the boring billion occurred because oxygen-induced nutrient starvation handicapped proliferation of organisms, and with low proliferation you have fewer chances to evolve something new.


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