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thermocline), but also experience exchanges with the atmosphere at its
upper surface, in the case of the ocean surface, and with the seafloor, in
the case of ocean's interior (underwater volcanic activity,
sedimentation, early diagenesis, circulation of hydrothermal fluids,
etc.). Similar distinctions exist between the upper and lower
atmosphere, or between the geosphere in contact with the atmosphere
(soils, outcrops, lavas and basalts on continents and islands) and that
which is in contact with the ocean (sediments on continental margins
and abysses, lavas and basalts).
1.1.5.
What is the biosphere?
The notion of a “biosphere”, in parallel with the
large components that we have just described above, brings us
back to considering the sum of all life forms as simply an additional
component. Evidently, this would not correspond to the reality and
would moreover be fundamentally dismissive in regards to the
importance and complexity of the role of life in the functioning and
evolution of our planetary system.
Firstly, life forms are neither homogenous nor continuous. They
have effectively colonized a great number of niches, whether that is
the surface of the geosphere (sediment), the oceans, continental water
or the surfaces of continents. Moreover, although they represent only a
slight mass (biomass) compared to that of the large compartments of
the Earth system, life forms are the mediators of an important part
of the flux of matter between compartments through the phenomena of
photosynthesis and respiration, the mobility of organisms, the
phenomenon of bio-mineralization leading to the construction of
internal skeletons and shells, and the production of organic and
mineral waste (senescence, death, production of feces and urine, etc.).
A life form is therefore not, strictly speaking, a reservoir of the Earth
system, but rather a motor, or mediator, of its internal interactions. But
the definition of the biosphere does not stop there. When we speak of
the evolution and regulation of the Earth system across large scales of
time, life is also characterized by its endlessly renewed diversity, from
which its adaptive plasticity develops under the pressure of natural
selection. It is this plasticity of life forms that, via its role in the
