Carbon is the backbone of life on the Earth
and, possibly, in the Universe. (According to the Carbon Chauvinism hypothesis,
due to the unique chemical properties of carbon, life can only exist on the
planets where it could be evolved from carbon based structural units.) Carbon’s
unique capacity for forming multiple bonds and long-chain molecules
(biopolymers) makes life possible; carbon comprises about half the dry weight of
most living organisms. Starting from the discovery of fire, our civilization
vitally depends on carbon for its livelihood. Carbon-based fossil fuels powered
the Industrial Revolution and brought about the rise in the standard of living
we currently enjoy. Almost everything we get energy from, whether through
food (carbohydrates) or through fuels at power
stations (gas, coal) and transport (gasoline, jet, and diesel fuels), is based
on one form of carbon-based compounds or another. For this very reason, our
civilization is rightfully called “Carbon Civilization.”
Carbon’s abundance on our planet is
surprisingly low: the lithosphere has only 0.032 wt.% of carbon in all its
forms (for comparison, iron’s abundance is 5 wt.%).
Over geologic timescale, most of the carbon on
the Earth became locked up in sedimentary rocks as carbonates and fossil fuels,
and significant part of it got dissolved into the oceans as CO2, carbonate (CO3
2−), and bicarbonate (HCO3 −) ions.
Available data show that the atmospheric CO2 concentration
gradually reached the level of about 0.02–0.03 vol.% and fluctuated within this
range for about half a million years.
Carbon is stored on our planet in the following
major carbon reservoirs:
•
Carbonates and other sedimentary rock deposits in the lithosphere
•
Dissolved CO2 and carbonates in the ocean
•
Soil organic matter
•
Fossil fuel deposits
•
Living and dead organisms in the biosphere
•
CO2 in the atmosphere
The Figure depicts
the relative abundance of the major carbon reservoirs on the Earth.
Inorganic deposits of carbon in the lithosphere
in the form of limestone, dolomite, chalk, and other carbonates (representing
the most thermodynamically stable form of carbon) constitute the largest
reservoir of carbon on our planet.
Organic forms of carbon, e.g., carbon in
biosphere (plants, living organisms) and soil organic matter (e.g., humus),
represent significantly lesser share of the total carbon inventory compared to
inorganic forms of carbon. The amount of carbon in the form of carbon-bearing
fossil fuels, i.e., coal, oil, natural gas
(NG), peat, tar, and bitumen, is estimated at about 5,000 Gt, however, if the
potentially recoverable resources of methane hydrates would be factored in this
figure would increase by almost one order of magnitude (Gt is gigaton or 109
ton).
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