Thinking about Air Composition

As someone with a background in physics, I tend to look at what's happening around me from first principles. Or basic principles.

Today there is a lot of discussion about climate change. Thinking about the conservation of matter - ie that basically the earth contains the same materials now as it did billions of years ago - I have often wondered how the earth's atmosphere has changed over millenia. I was curious because there are lots of references to the "primeval soup" that was around when the earth "started". So I had a hunt - as you do - on Google and here's the results of that "research". Interesting....

This graph gives a picture of the composition of earth's atmosphere over the last 4.5 billion years. The important thing to notice is that there was a lot of carbon dioxide (25%) and water vapour (25%) a long time ago in the eary's atmosphere. There was a little bit of methane and the rest was nitrogen. We only got oxygen in our atmosphere 2 billion years ago, and as I understand it this was because the carbon dioxide in the air, combined with water, lightning and - literally - God knows what else, to form rudimentary plant life in water. That life photosynthesized carbon dioxide and produced oxygen in the air, and deposited carbon into water and the ground as by-products (plant litter, soil.... these later metamorphosed into coal and oil). The other thing that happened was carbon dioxide dissolved in water (mostly seawater), reacted with calcium that was dissolved in seawater, and formed calcium carbonate. As the scientists put it: "Calcium carbonate exists in several forms with different levels of stability. The first stage is noncrystalline, amorphous calcium carbonate. It forms when carbon dioxide mixes with calcium dissolved in water, either in the soil or in the oceans. Animals such as sea urchins and shellfish also make amorphous calcium carbonate and use it as a first step to build their spines and shells....". When these animals die, their shells go to to the bottom as sediment. We see a lot of this material in limestone (as in Oamaru Stone), and when it is metamorphosed by heat and pressure, we see it in Marble. So I guess that carbon dioxide is fairly "locked in"...

This chart shows the same sort of thing in a different way. Though it seems to assume that there would be a lot less nitrogen in the "standard atmosphere of a terrestrial planet". The presence of water is what allows the process of photosynthesis, and the production of oxygen, and the conversion of carbon into solid stuff. Plants that decay and keep carbon out of the air. Forming coal and oil over millenia. Underground. Not in air.

This one looks into oxygen in more detail, where it came from, where it went. Again, the emphasis is on photosynthesis. And gradually we got an atmosphere with enough oxygen to support life on land.

And this one brings some of those ideas together, raising the question about what happens to all that carbon that went to forming coal, lignite, oil and peat - if we continue digging it up and burning it, will we end up with a similar sort of atmosphere that the earth had before photosynthesis took the carbon out of it? Back into the Primeval Soup.