The basis of the Archean model

Like Daisyworld, my model of the evolution of Earth’s atmosphere was a program—a set of mathematical formulae and instructions —run on a computer, and based on certain assumptions. The geochemistry of the Archean period was based on HD Holland’s book, The Chemical Evolution of the Atmosphere and the Oceans. Solar input at the start was taken to be 25 per cent less than now, and with carbon dioxide input from volcanoes three times higher than today.
Instead of dark and light daisies, rabbits, and foxes, the model had the three primitive ecosystems of the Archean — photosynthesizing bacteria drawing on the abundant carbon dioxide to manufacture organic rnafter; a few consumers in the pockets of free oxygen they generated; and fermenters (methanogens) using the decaying organic material from both.
The bulk of the atmosphere was assumed to be nitrogen, with carbon dioxide initially around 10 percent, giving a temperature of around 28°C. The Sun was allowed to warm up as it aged, and climate regulation was based on the greenhouse effects of methane and carbon dioxide, with the weathering sink for carbon dioxide increasing with the growth of organisms.
Organism growth was assumed to be slow below 5°C and above 50°C, and optimum near 25°C. It would also be dependent upon the abundance of carbon dioxide. Oxygen would have a positive effect on growth at low levels through its ability to increase the rate of release of nutrient elements from the rocks, but it would have a poisonous effect at high levels.

The results of the Archean model

The model results match what is known of the Earth’s early history.
The climate graph (lower) shows how, without life, the warming Sun would have led to a steadily warming Earth (dotted line). The solid line on the same graph shows how life changed all this. There is an abrupt fall of temperature after life starts, due to a rapid decline of carbon dioxide as it was used up by the photosynthesizers. The temperature then stabilizes, rising slowly throughout the Archean. The upper graphs of the gases and the bacterial populations show why — methane from the fermenters was accumulating in the atmosphere and its greenhouse effect replacing that of carbon dioxide. The temperature suddenly falls once more at the end of the Archean, when the sudden appearance of free oxygen marks the decline of methane. A rapid reduction of carbon dioxide abundance after life began is consistent with the Earth’s rock weathering record. The Earth’s temperature was fairly stable in the Archean but a cold glacial interlude about 2.3 eons ago may have marked its end. The model matches this pattern. Like Daisyworld, this model shows an abrupt change of conditions as soon as life starts. The organisms grow and change their environment and the atmosphere rapidly, until a steady state is reached and Gaia runs on in comfortable homeostasis. (see variation 19)