Oxygen in our atmosphere at 21% allows animals like humans to be active – when we choose to be! Oxygen powers a vibrant biosphere comprising plants, fungi and animals throughout the sea, land and air, on a biochemical foundation of bacteria and microbes. Oxygen is almost synonymous with life – but we know it wasn’t always there in the atmosphere. The early earth billions of years ago had only trace amounts of oxygen and early life was anaerobic.
So how did our atmosphere come to be one fifth oxygen – when at the start of earth’s history there was almost no oxygen in air? At first sight the history of the oxygenation of our atmosphere is confusing. We hear about the great oxygenation event – called a “catastrophe!” – happening two and a half to three billion years ago. This even formed the red stripes in rocks where most of the dissolved iron in the ocean deposited as red iron (Fe3+, or III) oxide in sediment and then rock. This gave us all the iron ore in places like western Australia. But then much later at the Cambrian explosion 540 million years ago we are told that the rapid emergence and diversification of multicellular animal life was triggered by increasing oxygen in the sea and air. But wasn’t it already there from the oxygen “catastrophe” 2 billion years earlier?
The answer to this conundrum is that both are correct – there was a step up of oxygen 2.5-2.8 billion years ago, then another one around the beginning of the Cambrian era 550 or so million years ago. These two oxygenation event actually serve to divide the earth’s 4.5 billion year history into three great epochs – first the Archaean (the “ancient”), then the Proterozoic (age of “first life”), and lastly the – present – Phanerozoic (the age of “visible life”). Oxygen levels started in the Archaean at 0.001% of present levels, then at the Proterozoic oxygenation event rose to between 0.1 and 10% of present levels, finally rising to our current 21% around 550 million years ago to usher in the Phanerozoic and the great Cambrian explosion of multicellular life.
So the child in us wants to know – why!? Why this two-step increase in oxygen which was so fortuitous for the emergence of vibrant animal and plant life? Well a really important paper has been published in 2016 giving a good answer to this question. It is titled “A theory of atmospheric oxygen” by T. A. Laakso and D. P. Schrag from the Department of Earth and Planetary Sciences at Harvard University, USA. Their model engages chaotic-nonlinear dynamics to show that the three great periods of approximate oxygen stasis in earth’s history were maintained by feedbacks and attractors in a compelling attractor landscape sustained by players such as volcanism, carbonate, pyrite and silicate weathering, organic burial, oxide burial, hydrothermal vents and others. Not to mention life itself, of course – the heroic role of photosynthesising cyanobacteria and their crowning role in primary production for the biosphere.
It’s always best to go straight to a paper rather than read someone’s opinionated digest of it – so here’s the abstract:
A theory of atmospheric oxygen.
Laakso TA, Schrag DP.
Geobiology 2017 May;15(3):366-84.
Geological records of atmospheric oxygen suggest that pO2 was less than 0.001% of present atmospheric levels (PAL) during the Archean, increasing abruptly to a Proterozoic value between 0.1% and 10% PAL, and rising quickly to modern levels in the Phanerozoic. Using a simple model of the biogeochemical cycles of carbon, oxygen, sulfur, hydrogen, iron, and phosphorous, we demonstrate that there are three stable states for atmospheric oxygen, roughly corresponding to levels observed in the geological record. These stable states arise from a series of specific positive and negative feedbacks, requiring a large geochemical perturbation to the redox state to transition from one to another. In particular, we show that a very low oxygen level in the Archean (i.e., 10-7 PAL) is consistent with the presence of oxygenic photosynthesis and a robust organic carbon cycle. We show that the Snowball Earth glaciations, which immediately precede both transitions, provide an appropriate transient increase in atmospheric oxygen to drive the atmosphere either from its Archean state to its
Proterozoic state, or from its Proterozoic state to its Phanerozoic state. This hypothesis provides a mechanistic explanation for the apparent synchronicity of the Proterozoic Snowball Earth events with both the Great Oxidation Event, and the Neoproterozoic oxidation.
Now here’s a link to the Journal article’s title and abstract:
And here is a link to the PDF of the paper from Harvard:
Of key importance in putting oxygen into the atmosphere was the production of oxygen by photosynthesis of the blue-green cyanobacteria which appeared early early in life’s history – maybe more than 3 billion years ago. Cyanobacteria remain key to most of the important photosynthesis to this day, since the chloroplasts in higher multicellular (and single celled) plants descend from endosymbiotic cyanobacteria, as discovered by the Russian botanist Konstantin Mereschkowski, and later by Lynn Margulis (Californian biologist and wife of Carl Sagan). So cyanobacteria deserve a picture in this post:
However another key player in oxygenating the atmosphere – perhaps more unexpectedly – is the process of burial of organic material resulting in carbon being separated from oxygen – termed “organic burial”. It turns out this is key to getting oxygen gas to accumulate in the atmosphere. Without organic burial, oxygen formed by photosynthesis would eventually recombine with carbon to make CO2, preventing a net build up of O2 in air. So important is organic burial to oxygenation of the atmosphere that in their paper Laakso and Schrag talk – in a shorthand way – of organic burial as “creating” atmospheric oxygen.
And that addition of oxygen to air from organic burial leads to another crucially important element of the author’s theory of oxygen – a network of feedbacks that maintain the atmosphere’s oxygen level at stable levels. The three periods with different oxygen levels were maintained at those levels by feedbacks such that those levels were stable. Remember – in the Archaean oxygen was at 0.001% or present levels, in the Proterozoic after 2.5 billion years ago it was at 0.1-10% and now in the Phanerozoic it is 100% – at 21% in the atmosphere.
One of those feedbacks involves organic burial. As we already discussed, organic burial liberates oxygen to accumulate in air. What happens if oxygen decreases in the atmosphere? Well, lower ambient oxygen actually increases organic burial since deposited sediments – such as at the ocean floor – are more likely to encounter anoxic (without oxygen) conditions which favour organic burial. If more organic carbon is sedimented and less is oxidised, then organic burial increases and oxygen also increases in the atmosphere, maintaining a steady level by feedback. Of course there are many other feedbacks also so what emerges is a complex attractor landscape.
OK – so if the Archaean super-low level was stable from feedbacks, then the late Proterozoic medium level of oxygen was also stable, and finally the current high 21% level in our Phanerozoic epoch is also stable then … how did the earth transition between these three “stable” oxygen regimes? What could have bumped up the oxygen level from one stable level – or attractor – to another? The answer given by Laakso and Schrag is … (long silence) … (tense music) … (close up on someone’s face) … (more silence) … GLACIATION. Yay!
There were major “snowball earth” glaciations at certain points in earth’s history. Two of them in fact. And these glaciations suspiciously occur just at the points in history when oxygen level jumps up from one level to another. The great Huronian glaciation happened 2.5-2.8 billion years ago. and “shortly” before the Cambrian and the Phanerozoic, 800-600 million years ago, there was a series of crushing global glaciations including the Sturtian and ending with the Marinoan, which are collectively known as the Cryogenian glaciation period. Now glaciation separates the ocean surface from the atmosphere. It also severely reduces weathering processes of silicate and other minerals which normally strongly influence atmospheric composition. An indirect result is a sharp increase in phosphorus concentration in seawater – vital to plankton and photosynthesis. So there is a boost to primary production and plankton photosynthesis increases. In absolute terms this does not amount to that much oxygen production since much of the ocean is ice covered. However – the sinks of oxygen removal are also greatly curtailed. So … oxygen starts building up and increasing in the atmosphere!
Carbon dioxide also builds up at the same time due to the reduced rock weathering, with the result that when the glaciation eventually ends, CO2 fuels an increase in photosynthesis. So a boom in primary production and photosynthesis of oxygen happens after the glaciation. Other things happen also including the higher oxygen removing more iron from the ocean. More details can be read in the paper, but the overall result of all these consequences of glaciation is that oxygen level is effectively bumped up from one attractor level to another.
So in short – blue-green bacteria and algae, glaciation, a lot of rocks, and the laws of chaotic-nonlinear attractor dynamics (always helps to remember those!) together gave us the oxygenation two-step by which our earth’s atmosphere became enriched in oxygen to 21%, its currently stable level, allowing us to breath and live. Here’s a picture commemorating those all-important glaciations: