Marine Biogeochemical Cycles - Ocean in the Earth System - page 166

4.3.2.4. Long organic carbon cycle
The long organic carbon cycle links the photosynthetic production
of organic matter [ 12 ], its export to the ocean depths [ 6 ] and its burial
in soils [ 16 ] and in marine sediments [ 17 ] where it is slowly
transformed into fossil organic matter (including oil and natural gas,
accumulated or not in deposits). The processes of hydrocarbon deposit
formation and their natural oxidation during transfer to continents by
tectonic movements [ 8 ] correspond to a time scale of hundreds of
millions of years (which is the lifetime of the sea floor). As with the
calcination of CaCO 3 mentioned above [ 11 ], the evermore intense
exploitation of fossil organic matter deposits (coal and hydrocarbons)
by humans since the beginning of the industrial era (year 1750) [ 18 ]
shortens part of the long carbon cycle by releasing large quantities of
CO 2 into the atmosphere, at a much faster rate than the natural cycle.
This human activity is responsible for the majority of anthropogenic
CO 2 emissions. As the remainder of the long organic carbon cycle
remains unchanged in the short-term, there is a transitory increase in
CO 2 in the terrestrial system. It must be noted that getting access to
fossil organic matter has been the cause of numerous armed conflicts
between countries for over a century.
The brief summary above shows that the long and short cycles of
inorganic and organic carbon are strongly interconnected. This makes
predicting the exact effects of the current increase in atmospheric CO 2
on our planet over the next decades incredibly difficult (although the
longer-term global effect is known). Hence the climate models that
have been developed take into account an increasing number of
interactions between the different forms of carbon and between the
different reservoirs on Earth.
4.4. Oxygen: a poison that Earth cannot do without
Oxygen is the most abundant element in the Earth's crust, where it
is present in oxides of silicon, aluminum and iron, as well as in the