From Microbes to Cell Giants - Animate Earth: Science, Intuition and Gaia - page 142

less perfectly, oxygen cannot accumulate in the atmosphere if respiring organisms con-
sume all the sugars fixed by photosynthesis. But what if some of the sugars escape the
respirers and find their way into places such as thick, muddy sediments where there is no
oxygen? Then the oxygen zooms about in the atmosphere seeking the lost sugars which,
by eluding its attentions, ensure that the oxygen remains free in the atmosphere ( Figure
36 ) .
Figure 36: How oxygen builds up in the atmosphere due to photosynthesis and organic carbon
burial.
In Gaia's early years, any oxygen that was not buried with organic carbon reacted with
methane in the air, sulphur from volcanoes and iron in rocks. But eventually, around
2,500 million years ago, the very face of Gaia was gradually transformed beyond all
recognition as these oxygen 'sinks' became saturated and free oxygen lingered in the
atmosphere. Gone was the methane-rich air that had dominated the planet for 1,000 mil-
lion years, gone the pink skies and brownish seas. Oxygen turned the sky blue and drove
the methanogens deep into mud, ooze and slime where oxygen could not reach them.
Gone too was the warm, balmy climate produced by the methane-rich atmosphere, for