Oxygen, methane, and carbon

Plants photosynthesize and convert carbon dioxide and water into oxygen and organic matter, of composition approximately CH20. Animals and microorganisms consume most of this, using up the oxygen made by the plants and returning carbon doxide to the air. About 1 per cent of the organic matter is buried deep in the soil, where methanogens convert it to carbon dioxide and methane.
The methane escapes to the air, where it reacts with the remaining 1 per cent of oxygen to form water and carbon dioxide. A small proportion (about 0.1 per cent) of the buried organic matter escapes digestion by the methanogens. The carbon is buried deep in the sedimentary rocks and the equivalent oxygen is left free. Thus it is this small amount of buried carbon that accounts for the oxygen in the air. All the rest of the oxygen made by the plants is used up by animals and microorganisms, by reaction with methane and with rocks and gases during volcanic activity, and by weathering.

The evolution of the atmosphere

The involvement of life in Earth’s atmospheric composition is dramatically illustrated by the diagram above right. It shows the geophysiologist’s view of the evolution of the atmosphere during Gaia’s long life. The abundance of gases, shown on the vertical scale, is expressed as parts per million (ppm). (The scale is logarithmic, that is, in powers of ten: I means 10 ppm, and 5 means 100,000 ppm.) The horizontal axes shows the time scale expressed as eons before the present. The diagram is based both on geological evidence and on my computer models of Gaia.
It shows the progressive stages of decline of carbon dioxide levels from an abundance ot between 10 and 30 per cent (1-3 x 10⁵ ppm) before the birth of Gaia to the low level of 0.03 per cent (3 x 10² ppm) now, due to the intervention of life in the weathering process and carbon cycle. It also shows the early appearance of methane, generated by the methanogens (fermenters) of the Archean, and its subsequent decline alongside the rise of free oxygen. This complete change of state from a reducing to an oxidizing atmosphere heralded the rise of the consumers. You can see how oxygen climbed to around 1 per cent (10⁴ ppm) during the Proterozoic and remained there until, around the time of the evolution of larger land-based life, with greater carbon burial, it rose toward its current 21 per cent (over 2 x 10⁵ ppm). (see variation 19)