Geoscience Reference
In-Depth Information
C
OMMENTARY ON
F
IGURE
1.5.- The majority of organic matter
produced is recycled in the mixed layer through respiration in the
trophic network. A small part is exported out into the interior ocean,
where it is almost entirely degraded, always by respiration of the
trophic network, whether it is deep pelagic or benthic. A very small
fraction escapes degradation and fossilizes in the sediments, from
where it is then integrated into the long-term geological cycle. When
the nitrate is not limited relative to the phosphate, photosynthesis uses it
preferentially and the flux of organic matter exported is at saturation,
which consumes the oxygen in the inner ocean and creates anoxic
environments. But these favor the denitrification and exportation of
nitrogen into the atmosphere, thus reducing the oceanic stock of
nitrate. If the nitrate is limited relative to the phosphate,
photosynthesis calls upon species capable of fixing atmospheric
nitrogen, whom then restock the oceanic nitrate through the
process of exporting organic matter into the deep ocean and breaking
it down.
1.6.
Conclusion
1.6.1.
The ocean in the Earth system
The few examples that have just been given, which
are summarized in Figure 1.6, do not exhaust the complexity of the
mechanisms of the Earth system. The previous section has shown
the central role that the ocean plays in this system, notably through the
following:
- plate tectonics and the related geological recycling of biogenic
elements;
- the abiotic regulation of the greenhouse effect due to the
chemical system of inorganic carbon dissolved in the ocean;
- the long-term increasing enrichment in oxygen O
2
of the
environment and its self-regulation, both being due to the Darwinian
evolution of respiratory metabolism in the ocean;
- the short-term self-regulation of the N/P ratio in the ocean due to
oceanic circulation and the dynamic of phytoplankton species.
