Geoscience Reference
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for regulating the internal oxygen pressure of multicellular eukaryotes
[MAS03] could have adapted to a hyperoxygenation of the
environment. If it could not, the oxygenation would have been self-
limited by the drop in biological productivity of the eukaryotes
(notably that of plants).
After having reached its saturation point, the atmospheric oxygen
level diminished, probably due to a partial release of sedimentary
organic carbon accumulated in the course of the Devonian Period
(410-360 million years ago) and the Carboniferous Period (360-
295 million years ago). 200 million years ago, the atmospheric oxygen
level would fluctuate around a value close to the current one (21% in
volume), which leads us to think that the flux of oxygen consumed by
the oxidative destruction of recent or sedimentary organic matter,
which is principally due to aerobic respiration, became equivalent to
that of oxygen produced by photosynthesis. This also means that the
flux of organic carbon released from old sediments through erosion,
then oxidized, is balanced in the long-term by the flux of
photosynthetic organic carbon buried in new sediments. The Earth
system reached here its long-term regulation (millions of years), one
that simultaneously integrated exchanges of matter between the ocean,
atmosphere and sediments, the result of 3 billion years of biological
evolution, and the tectonic plates that enabled the recycling of
sediments
via
erosion and volcanic activity.
1.4. The regulation of the greenhouse effect by the ocean
1.4.1.
There is no life without a minimum greenhouse effect
Without CO
2
in the atmosphere, the atmospheric greenhouse effect
would be extremely reduced and the average temperature at the
surface of the Earth would be -18°C, much lower than it is today
(+15°C). Such conditions would have been unfavorable for the
presence of life forms on the surface of the Earth and would have
limited them to a few rare oases where they would have had every
likelihood of being destroyed by natural hazards (asteroid falls,
significant volcanic events, etc.). For life to persist for several billion
