Geoscience Reference
In-Depth Information
presence of carapaces, make the progressive adaptation of animals to
intermediate environments, where water periodically retreated,
possible. Some of these organisms probably developed the capacity to
briefly leave the water, then to move into an aerial medium, after
which point they would rejoin the water by their own means. In this
way, they perhaps promoted the adaptation of marine bacteria to
continental sunlight, notably those N
2
nitrogen-fixing bacteria that
terrestrial plant life needed. Unlike their marine equivalents, the
terrestrial plants are only slightly directly provided with nitrate and
they depend largely on the fixation of nitrogen (and transformation
into nitrate) for their photosynthesis. We note, however, that even in
the ocean we can find situations where nitrate is lacking, which leads
to the use of N
2
through this fixation.
The colonization of continental surfaces by life forms consequently
gave rise to new adaptations, but was also the source of an increased
influx of organic matter into the sedimentary reservoir of
the Earth. Rivers drained organic debris, essentially plant matter, from
the watersheds into the sediments of flood plains and deltas. On the
other hand, from the Devonian Period (around 400 million years ago),
the evolution of flora and the geographical characteristics of the
continents enabled the accumulation of large quantities of peat and
coal, the maximum flux having been attained in the Carboniferous
Period (around 350-300 million years ago).
As explained above, the sedimentary burial of organic matter
triggered a global accumulation of oxygen in the environment and it is
precisely between 400 and 350 million years ago that a new
acceleration in oxygenation occurred [BER 04, CAN 07]. It may have
increased the atmospheric O
2
level to around 30-35%, the most the
Earth system has ever known. However, it is necessary to note that
this value is considered too high by some paleoenvironmentalists,
since the risk of natural combustion exists at 13%, becoming a
substantial risk at 25%. Natural fires, of which we find a permanent
trace in sediments from the Devonian Period (carbonaceous particles
of the “fusinite” type), definitely limited the maximum oxygenation of
the terrestrial environment. Moreover, it is uncertain that the capacity
