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[For the theory on how the flying reptiles got so big, click When the oxygen level skyrocketed]
Our present atmosphere contains 20% oxygen, and it’s all too easy to assume that it always has. Even many scientists seem to do it. But there’s no earthly reason why the atmosphere’s oxygen should remain constant. And in fact evidence that it has varied widely, even over the past 400 million years, has been around for a long time. And yet I’ve only seen a couple of articles that mention this, both quite recent.
The most striking evidence is the fossils of giant dragonflies that flew in the ancient Carboniferous coal forests. They had wingspans of 2 feet (60 cm) or more across. This picture comes from ‘History of Life’ by Richard Cowen.
Giant dragonflies reappeared, though slightly smaller I think, during another period of high oxygen levels, some 150 million years ago.
But the entomologists tell us that insects don’t have a proper blood system to transport oxygen around their bodies. They rely on the oxygen ‘diffusing’ through their tissues of its own accord. So any insect, bigger than the largest ones of today, should simply not be possible.
And the aerodynamicists tell us that insects fly quite differently from birds; and that a giant insect shouldn’t be able to get off the ground.
Both these riddles are resolved if there was more oxygen around at the time. More oxygen would enable it to diffuse more quickly through insects’ bodies. And it would make the air denser (heavier) and overcome the aerodynamicists’ problem too. Incidentally I understand that, during the second period of high oxygen levels, they were actually less crazily high. This ties in with the dragonflies being slightly smaller.
But quite apart from that, oxygen is a terribly reactive gas. If you put it in contact with anything ‘combustible’, and warm it a bit, it reacts so violently that it all gets red hot. Even solid rock is hungry for oxygen, if it hasn’t enough.
Any oxygen that was originally hanging around in the environment would certainly have got used up very quickly.
We have oxygen to breathe today because cyano-bacteria and algae are pushing the stuff out in huge quantities. I was surprised to learn that plants have only ever been bit players in the oxygen production game. Oxygen consuming organisms, including animals, are using it up just as fast. And so we have a balance.
The photosynthetic organisms break down carbon dioxide from the atmosphere, using the energy from sunlight. They use the carbon as one of their vital building ingredients. The other main ingredients are hydrogen, oxygen and nitrogen (more). However they already get enough oxygen when they extract their hydrogen from water. So they chuck out this extra.
The non-photosynthetic organisms do the opposite. In essence they ‘burn’ the carbon, in the oxygen, to get energy that they need.
The oxygen story actually starts very early in the life of the planet – when the cyano-bacteria evolved in fact. Unfortunately no-one can tell us exactly how early this was.
However for thousands of years, their effect on the overall environment was little or nothing.
There’s a reason for this, and it’s the balance that we mentioned earlier. The available oxygen was freed by producing a certain amount of carbon. ‘Burning’ that amount of carbon will use up the same amount of oxygen, so in theory there will be exactly none left. As we’ll see later, it’s carbon burial that ensures there’s any left to oxygenate the environment. And in those days, the plate tectonic processes were far too turbulent to allow any carbon to stay buried for long.
There was something going on however, in the form of the Banded Iron Formations, or BIFs. BIFs need there to be a certain amount of oxygen in the water of shallow seas. As we keep saying, don’t expect things to be simple in this game.
So for the first couple of thousand years the overall environment (meaning the oceans) was totally ‘anoxic’.
Opinions differ about how and when oxygen started to appear. For want of a better guide, I’m following Professor Andrew Knoll of “Life on a Young Planet” fame. Knoll is a palaeontologist, so bugs figure largely in his thinking.
Knoll says that scientists used to believe that the planet oxygenated somewhere around 2.2-2.4 kMy ago. Before then there was little or none. But at some point there was a gradual transition to there being plenty of it.
But “now we know” that there was an intermediate state between 1.8 and 1.2 kMy – in which there was some oxygen around but not much. There was ‘moderate’ oxygen in the atmosphere and surface waters. But just below the surface, the oceans and other waters were anoxic. This could be because nitrogen was in short supply. This wouldn’t bother the cyano-bacteria because they can fix it. They are also dab hands at extracting nitrogen from dead remains. But by this time they may have been on the decline, with algae gradually were taking over (more). Algae can’t fix nitrogen, and they aren’t nearly as good as the cyanos at extracting it from their environment. Indeed, they can’t do it at all without molybdenum. Today moly is common throughout the oceans. But during this intermediate period, it would only have been available in the coastal waters, where it had been newly washed down. As soon as it reached the anoxic waters, the hydrogen sulphide would have reacted with it and taken it out of circulation.
So 1½ kMy ago, life may have been tough for the algae. However by around 1.2 kMy, the oxygen was beginning to spread onto the continental shelves and into the deeper waters. This gave the algae a larger habitat to exploit. And it enabled them gradually to expand oxygen production.
The creation of a fully oxygenated planet was a long slow process. But by the end of the Proterozoic, the entire underwater world had at least some. How much? Well enough to stop the oceans dissolving iron anyway. But Knoll reckons that there was no more than around 1% of present levels. There may have been much less.
However much it was, it paved the way for one last revolution in Biology (so far anyway), the rise of animals.
The amount was gradually increasing. Although the actual production was (and still is) all underwater, oxygen was gradually leaking out into the atmosphere. So when plants and animals were ready to venture on to dry land, there was oxygen ready and waiting for them. Indeed, so much carbon has been buried since life began that the entire environment, from the deep oceans up to the stratosphere, is now thoroughly oxygenated. Indeed some geo-physicists reckon that even the deep mantle has some of this oxygen in it.
I’ve seen little written about how the oxygen level varied from the Cambrian period onwards. But vary it certainly did.
It’s easy to assume that oxygen production is the key to how much there will be in the environment at any one time. But in fact, as we’ve hinted, carbon burial is even more important. High rates of carbon burial will lead to there being more oxygen around.
As we suggested earlier, it’s a balance. And there’s evidence that, as ‘recently’ as 350 million years ago, the balance tipped strongly in favour of oxygen. And the amount in the atmosphere more than doubled, to a crazy 35%. Dry wood ignites spontaneously at these levels.
These were ‘Carboniferous’ times – the time of the giant dragonflies. This was when the world’s supply of coal was being laid down. All round the world, massive forests were growing in swampy land. (Why all the swampy land? Because sea levels were high on account of Plate Tectonics. More)
The climate was good too, at least to start with. When the trees and other vegetation died, much of the dead wood was buried in the swamps – before the bugs, bacteria and fungi could get at it and recycle it. Huge amounts of carbon were being buried. This carbon came from the CO2 in the atmosphere, as we’ve seen. And all the spare oxygen went back into the atmosphere.
Let’s be quite clear about this. We owe the oxygen we breathe to carbon burial. If anywhere near all the buried carbon found its way back to the surface, it would claim its oxygen back and we would suffocate.
Reptiles and dinosaurs appear to have discovered flight during another period of high oxygen levels. Abundant oxygen also seems to be implicated in these guys getting so huge. Well we need some explanation!
In more recent times, the oxygen level may have gone down as low as 15% (if I remember rightly). We would probably cope quite well if it dropped that low again, though people might have to come down from the mountains. For normal low-altitude life, our lung capacity is considerably greater than we need.
Incidentally, you have heard similar arguments in relation to Global Warming. By burying the carbon we both prevent it getting back into the atmosphere as carbon dioxide, and help maintain the oxygen levels.
© C B Pease, February 08