"Any rock carries a memory of the environment in which it formed," Rosing said. "These ancient rocks that are about 3.8 billion years old, they actually carry a memory of the composition of the ocean and atmosphere at the time when they were deposited."
The critical part of the rocks' memory was the banding and that iron was found chemically bound to oxygen rather than CO2 in the bands. The alternating bands would only have been deposited if the carbon dioxide content of the atmosphere kept shifting back and forth across a threshold that controlled which mineral was deposited. But that also meant that the amount of carbon dioxide couldn't stray too far from that threshold. If there had been either substantially more or less carbon dioxide, only one of the minerals would have been laid down.
Another constraint on early carbon dioxide levels came from life itself.
In the days before photosynthetic organisms spread across the globe, most life forms were methanogens, single-celled organisms that consumed hydrogen and carbon dioxide and produced methane as a digestive byproduct.
But to thrive, methanogens need a balanced diet. If the concentration of either of their foodstuffs veers too far below their preferred proportions, methanogens won't survive. Their dietary restrictions, specifically the minimum concentration of hydrogen, provided another constraint on the concentration of carbon dioxide in the atmosphere, and it falls well below the level needed for a greenhouse effect sufficient to compensate for a weak early sun.
"The conclusion from all this is that we can't solve a faint sun paradox and also satisfy the geologic and metabolic constraints by having high carbon dioxide values," Bird said.
But the theory of a lower Earthly albedo meets those constraints.
"The lower albedo counterbalanced the fainter sun and provided Earth
|Contact: Louis Bergeron|