Chemistry Reference
In-Depth Information
circulation of carbon through the atmosphere, hydrosphere, and biosphere
is usually known (see Fig. 62).
During the process of
photosynthesis
, atmospheric carbon is taken up
(as carbon dioxide) from the atmosphere by plants, and becomes a basic
component of all vegetable cells and tissues (see Textbox 53). Since its
chemical properties are identical to those of ordinary carbon, radiocarbon
also undergoes the photosynthesis process. Because the carbon isotopes
are
fractionated
(their relative abundance is changed) during the process,
however, radiocarbon becomes a part of the cells and tissues of plants in
a lower isotopic proportion than in the atmosphere. The radiocarbon
assimilated by the plants is, in turn, consumed by land and sea animals and
by humans. Radiocarbon becomes, therefore, uniformly distributed not
only in the atmosphere and hydrosphere but also throughout plants,
animals, and the whole of the biosphere, forming part of what is known
as a
carbon exchange reservoir
, the entire inventory of carbon in all its iso-
topic forms and chemical combinations. This includes the three isotopes
carbon-12, carbon-13, and carbon-14 in a wide variety of chemical combi-
nations, which circulate freely between the different components of the
reservoir. The time it takes for the isotopes to circulate and mix within the
reservoir varies greatly: only a few years, for example, are required for a
carbon isotope to circulate between the stratosphere and the troposphere,
but much longer periods of time are needed for it to do so between other
components (the hydrosphere and plants and animal tissues) of the reser-
voir. Whatever its length however, the time of circulation is much shorter
than the half-life of radiocarbon (5730
40 years).
The mean relative amount of the carbon isotopes on the earth's
surface, that is, the ratio carbon-12 : carbon-l3 : carbon-14, which is
known as the
isotopic ratio of carbon
, is known to be constant. There must
therefore be a sufficient amount of radiocarbon in the planet to ensure
that the rate of decay of the isotope balances its rate of formation. The
conclusion to be drawn from this is that radiocarbon in the earth is in a
state known as
secular equilibrium
, which means that for every radiocar-
bon atom created by cosmic radiation in the upper layers of the atmos-
phere, there is one decaying atom in the carbon reservoir of the earth (the
total amount of radiocarbon in the earth, incidentally, has been calculated
to be slightly above 80 tons). The equilibrium between the formation and
the depletion of radiocarbon is also maintained in living organisms, which
exchange carbon with their surroundings. The specific activity of living
matter - that is, the intensity of radioactivity due to radiocarbon per unit
mass (gram) - averages about 14 disintegrations per minute, equal to that
in the atmosphere.
The death of a living organism, however, stops the intake (into that
organism) of radiocarbon, and its dead remains are said to have been with-
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