Why does the air contain oxygen and where did it all originally come from? A new book ‘OXYGEN’ by Donald Canfield helps explain. No animal life from insects to fish to humans could have evolved or survive before oxygen first appeared in the atmosphere or dissolved in the oceans. Oxygen also protects animal life from damaging UV radiation from the sun by forming ozone in the stratosphere. The build up of oxygen in the atmosphere eventually resulted in the cambrian explosion of plant and animal life. This remarkable breakthrough was preceded by the evolution of cyanobacteria which finally cracked photosynthesis – the complex process powered by solar photons using H2O and CO2 to produce carbon compounds for cells while expelling O2 as a by product. The carbon fixation enzyme Rubisco is responsible for all food and fossil fuels on earth and the evolution of all multi-cellular organisms.
The complex biochemical process behind photosynthesis was evolved by cyanbacteria from two previous “anoxygenic” processes in iron or sulfur environments. The cyanbacteria revolution was important because now the only environment needed was water, air and sunlight so they rapidly spread all around the world, and especially across all the oceans. Suddenly oxygen was being generated by photosynthesis whose imprint is clearly seen in the geological record beginning with the Great Oxidation about 2.3 billion years ago. However the long term build up of oxygen in the atmosphere is a very subtle effect, that is still not fully understood. Essentially 99.5% of emitted oxygen is consequently reabsorbed, but its relationship to CO2 levels is particularly interesting. Some interesting facts about photosynthesis that need to be explained are the following.
- Current levels of photosynthesis on earth would deplete all CO2 in the atmosphere in just 9 years.
- Photosynthesis in the Oceans depletes all available phosphorous needed by aquatic plants and algae in just 86 years.
Most of the CO2 absorbed by plants is soon liberated to the atmosphere when they die or are eaten by animals, while only a tiny amount of carbon is buried in sediments. Even by including this recycling effect we still find CO2 depletion of the atmosphere takes a mere 13,000 years while phosphorous depletion takes only 29,000 years. So what are we doing wrong?
The incredible story is that these trapped sediments are not lost from the environment for ever because plate tectonics recycles material over very long timescales today. Subduction, mountain building and sea level change continuously re-exposes the raw materials for life through weathering. Plate tectonics is essential to re-cycle the raw materials for life on earth !
CO2 re-enters the atmosphere from the mantle through out-gassing of Volcanoes and also through deep ocean vents near mid ocean ridges. CO2 is removed from the atmosphere by weathering due to the abundance of water on the earth. Such weathering does not happen for example on Venus. The ‘natural’ carbon cycle essentially controls the temperature on earth because weathering by liquid water is a temperature dependent phenomenon.
The total content of Oxygen in the atmosphere is equal to the total buried carbon in the sediments. This results in the current 21% oxygen content. The total CO2 content in the atmosphere is instead fine tuned to the temperature of the earth. This is roughly how it seems to work.
CO2 enters the atmosphere from volcanoes and under-sea vents driven by plate tectonics, the result of convection from the hot interior energy of the earth. CO2 exits the atmosphere through weathering of rocks driven by the climate of the earth and the oceans. If the atmospheric temperature gets too hot so the rate of weathering increases and CO2 is removed from the atmosphere. If temperatures get too cold weathering slows down causing a build up of CO2 in the atmosphere. This eventually increases temperatures to their optimum level which coincides with liquid oceans. CO2 acts as a negative feedback to keep the climate stable and the oceans cool and liquid. This may be the primary explanation for the faint sun paradox, although the water cycle of evaporation and cloud formation must also play a stabilizing role.
Once photosynthesis and multi-cellular life evolved, so this balance of CO2 levels must have shifted coincident with the rise in O2. Firstly it now became possible to bury underground large quantities of biotic carbon including fossil fuels. Secondly there was an enhanced formation of sedimentary rocks caused by the compression of dead sea creatures. Thirdly the large amount of oxygen in the atmosphere now led to the formation of ozone in the upper atmosphere which is also a strong greenhouse gas. The explosion in plant and animal life coincident with a huge rise in oxygen content occurred during the Cambrian period about 540 million years ago. Since then the optimum temperature climate control for CO2 has shifted because the carbon cycle now includes life.
Current interglacial climate conditions on Earth all have an average surface temperatures of ~288K with a stable CO2 concentration of around 300 ppm which is nearly 1000 times less that the oxygen content of the atmosphere. Can we understand how these very low value of CO2 could arise from natural-bio temperature control ?
About a year ago I wrote a simple model of radiative transfer from the surface to space covering the dominant CO2 15 micron band. I found that for a surface temperature of 288K the maximum radiative loss of heat in the atmosphere occurs at exactly 300ppm. I cannot believe this can be a coincidence. CO2 levels are fine tuned such that the atmosphere cools at its maximum rate in accordance with the second law of thermodynamics.
What is this result really saying ? The thermostat that controls the temperature of the earth is a mix of geochemistry and life. The earth lies within the habitable zone of our sun and has maintained liquid oceans for 4.5 billion years while the sun brightened. Initially the natural carbon balance between the weathering sinks due to both water and CO2 was perfectly balanced with volcanic emissions from the earth’s crust to maintain the oceans. This was thanks to the internal energy. The evolution of photosynthesis caused an explosion in life across the surface of the earth releasing huge amounts of free oxygen and sucking out CO2 as bio-carbon was drawn out of the atmosphere and buried in the ground. Life today needs carbon, water, oxygen and some essential minerals like phosphorous. If we assume a constant rate of CO2 venting from volcanoes and under-sea vents that we can deduce the following. Too little CO2 in the atmosphere and cooler temperatures lead to both photosynthesis and the weathering of rocks to slow down thereby causing CO2 levels to increase and the climate to warm. Too much CO2 and consequent higher temperatures lead to a reduction in the long term weathering of rocks with a consequent fall in CO2 levels causing the planet to cool. Underpinning everything is plate tectonics because carbon and the raw materials for life must eventually be recycled to maintain the balance. Such balance will continue to stabilize the climate so long as the earth’s internal energy continues to drives plate tectonics. Life is therefore probably safe for another billion years.