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Carbon cycle
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-
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-
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150
1 020
38 100
700
Deep ocean
Dissolved
organic C
Sediments
Ocean surface
Marine
biota
3
4
6
6
0.2
40
50
100
a product of respiration in low oxygen environments, such as
stagnant marshes and the intestines of ruminants, including
cattle, sheep and goats. Methane in the atmosphere is eventu-
ally oxidised to produce carbon dioxide and water.
In the biosphere a significant amount of carbon is effectively
‘stored’ in living organisms (conventionally referred to as bio-
mass) and their dead, undecomposed or partially decomposed
remains in soil, on the sea floor or in sedimentary rock (fossil fu-
els are, of course, merely the remains of long dead organisms).
When the amount of atmospheric carbon fixed through pho-
tosynthesis is equivalent to the amount released into the at-
mosphere by respiring organisms and the burning of organic
carbon, then the living or biotic part of the carbon cycle is in
balance and concentrations of carbon dioxide and methane in
the atmosphere should remain relatively constant (although
their concentration will be affected by other parts of the carbon
cycle, notably volcanic activity and dissolution and precipitation
of inorganic carbon in water).
Often, however, the systemmay not be balanced, at least locally.
An area may be a
carbon sink
if carbon is accruing there faster
than it is being released. Conversely, an area is a
carbon source
if the production of atmospheric carbon from that area exceeds
the rate at which carbon is being fixed there. In terrestrial eco-
systems, whether an area is a sink or a source depends very
largely on the balance between the rate of photosynthesis and
the combined rate of respiration and burning.
The amount of carbon stored, the form that it is stored in and the
rate of turnover – that is the rate at which carbon is organically