[LINK] "Oxygen & Wildfires: Clues to Early Life"

[rfmcdonald]

Sometimes, when I want to make light in a conversation, I talk about that most neutral of topics, the atmosphere. "Wow! The atmosphere's great today! Enough oxygen to breathe, not enough to start spontaneous fires, and there's plenty of nitrogen in the atmosphere to serve as a buffer." (Yes, I know.) As Paul Gilster explains at Centauri Dreams, that pleasant atmosphere was hardly a given for Earth in much of its recent history (and likely not for Earth-like worlds, too).

Some people believe that gigantism in some animal groups was the result of higher levels of oxygen in the atmosphere, the consequence of the colonization of land by plants. And some would argue that higher amounts of oxygen allowed vertebrates to colonize land surfaces. Today we’ve got an oxygen level of 21 percent, but we know the levels have varied greatly in the past 400 million years and have declined steadily since the middle of the Cretaceous.

Studying this has been tricky because we use geochemical models to estimate the level of atmospheric oxygen, and various models produce differing results. But new work just published in Nature Geoscience now shows that the amount of charcoal preserved in ancient peat bogs provides a measure of how much oxygen was available in the past and may be the best way to measure the phenomenon. The variation is striking. Charcoal found in coal has remained at concentrations between 4-8 percent over the past 50 million years, indicating oxygen levels similar to today. But some earlier periods show a charcoal percentage as high as 70 percent.

That latter figure implies, according to Andrew Scott (Royal Holloway University of London) that high levels of atmospheric oxygen would have been available to spread frequent, large fires in both the Carboniferous and Permian periods, from 320-250 million years ago, and the Middle Cretaceous, approximately 100 million years ago. Says Scott: “It is interesting that these were times of major change in the evolution of vegetation on land with the evolution and spread of new plant groups, the conifers in the late Carboniferous and flowering plants in the Cretaceous.”

We may be dealing with a self-perpetuating mechanism: Frequent, hot fires resulting from high levels of atmospheric oxygen could have caused more erosion and thus greater burial of organic carbon, which creates a cycle keeping oxygen levels high.