3. CARBON CYCLE
• Carbon is present in atmosphere, mainly in
the form of carbon dioxide, thus it cycles in
this gaseous phase.
• Though it is a minor constituent of the
atmosphere, as compared to oxygen and
nitrogen, yet without carbon dioxide no life
could exist, for it is vital to the production of
carbohydrates through photosynthesis in
plants, the basic building blocks for other
organic compounds needed in metabolic
synthesis and incorporation of the carbon
with the protoplasm.
4. • There are two main sources of carbon in the
a biotic world :
• 1. The carbonaceous rocks such as chalk and
limestone containing carbon as calcium
carbon (CaCO₃).
• 2. The carbon dioxide of the air and that
dissolved in water.
• In addition, there is present large amounts of
carbon on fossil fuel (coal, petroleum, natural
gas, etc.) but this is not available to the
plants until and unless it is burned to
produce carbon dioxide.
5. • Carbon dioxide is released from
carbonaceous rocks through erosion and
weathering by rainfall, and so also by action
of acids formed during fermentation and by
bacteria that produce nitric acids sulphuric
acids.
• An insignificant amount of carbon dioxide is
also produced by activity of bacterium
Carboxydismonas oligocarbophila, which
oxidizes carbon monoxide to carbon dioxide.
6. • When Co₂ dissolves in water, some of it
reacts to form carbonic acid (H₂CO₃) which
immediately dissociates to form hydrogen
ions (H⁺) and bicarbonate ions (HCO₃).
• The bicarbonate ions in turn dissociate
further to form hydrogen ions (H⁺) and
carbonate ions (CO₃⁼).
• The carbonate ions may combine with some
positive ions such as Ca⁺⁺ to form calcium
carbonate (CaCO₃).
• This is insoluble in water and accumulates as
carbonaceous sedimentary rock.
•
7. • The reactions involved are as follows:
CO₂ + H₂ H₂CO₃ H⁺ + HCO₃
HCO₃ H⁺ + CO₃⁼
CO₃⁼ + Ca⁺⁺ CaCO₃
9. • The richest source of stored carbon today is
in the ocean.
• The oceans contain about 60 times more
carbon dioxide than in the atmosphere .
• This regulates atmosphere carbon dioxide
content level to 0.03% despite
photosynthetic uptake.
• Thus, there is a continuous exchange of
carbon dioxide between the atmosphere and
the organisms on the one hand and between
the atmosphere and the sea on the other
hand .
10. • However, the majority of ocean- dissolved
CO₂(HCO₃) is below the thermo cline and
inaccessible for rapid exchange with the
atmosphere.
• the immediate source of CO₂ for exchange is
thus restricted to relatively small quantity of
epilimnic CO₂.
• Sedimentary rocks, so formed by deposition of
CaCO₃.
• Through the process of , provide dissolution
CO₂ not only to the aquatic organism but from
there it may pass on to the atmosphere in the
process of regulation of CO₂ reserves:
12. The global cycling of carbon
• The carbon cycle involves a series of reactions in
which carbon is continuously criculated and
recycled in the environment, involving
interactions between organisms and their
surroundings.
• A simplified view of carbon cycle is given in
carbon from atmosphere pool moves to green
plants , then to animals and finally these to
bacteria, fungi and other microorganism of dead
organism matter.
13. • Some of this is also returned to the
atmosphere through respiration at various
levels in the food chain.
• It is estimated that half of the carbon fixed is
subsequently returned to the soil in the form
of decomposing organic matter.
• The atmospheric pool is very small as
compared to that of carbon in ocean and in
fossil fuels .
• Of the estimated 10 billion tons of CO₂
injected annually into the atmosphere from
the burning of fossil fuels and cultivation of
land for agriculture,
14. • Most of it is passed into the sea and stored in
the from of carbonates.
• Autotrophs in the process of photosynthesis
world-wide, use billion tons of CO₂.
• before the onset of industrial revolution CO₂-
flows among atmosphere, continents and
oceans were balanced.
• However, with industrialization and urban
development this equilibrium appears to be
disturbed as shown in dotted lines:
15. • Fossil fuel burning, forget fire, deforestation
as agriculture are some of the important
sources of new input.
• On the contrary , forests are important
“carbon sinks” as forest biomass is estimated
to contain 1.5 times and forest human 4
times the amount of carbon in the
atmosphere .
• Bolin et al..,1979, estimated that between 40
to 120 billion tons of CO₂ have been added to
the atmosphere through deforestation and
the expanding use of agriculture.
16. • Whichever of these estimates is considered
the amount of organic carbon is less than 1%
of that occurring in the geological component
of the environment.
• This point out that the major reservoir of
carbon is inorganic, with some 99% being in
in sedimentary form.
• Thus the major supply of carbon to on the
earth’s surface, where it becomes biologically
involved, is by erosion and metamorphism of
sediments.
17. • Carbon cycle is perhaps the most important
biogeochemical cycle in terms of human
impacts and environmental change.
• This is because CO₂ is a greenhouse gas,
which on increase would result into rise in
the temperature of the atmosphere, and
consequently rise in ocean level.
