Carbon dioxideand the carboncycle 37What we can learn from carbon isotopes
Isotopes are chemically identical forms of the same element but with dif-
ferent atomic weights. Three isotopes of carbon are important in studies
of the carbon cycle: the most abundant isotope^12 C which makes up
98.9 per cent of ordinary carbon,^13 C present at about 1.1 per cent and
the radioactive isotope^14 C which is present only in very small quantities.
About 10 kg of^14 C is produced in the atmosphere each year by the action
of particle radiation from the Sun;half of this will decay into nitrogen
over a period of 5730 years (the ‘half-life’ of^14 C).
When carbon in carbon dioxide is taken up by plants and other
living things, less^13 C is taken up in proportion than^12 C. Fossil fuel
such as coal and oil was originally living matter so also contains less(^13) C (by about eighteen parts perthousand) than the carbon dioxide in
ordinary air in the atmosphere today. Adding carbon to the atmosphere
from burning forests, decaying vegetation or fossil fuel will therefore
tend to reduce the proportion of^13 C.
Because fossil fuel has been stored in the Earth for much longer
than 5730 years (the half-life of^14 C), it contains no^14 C at all. Therefore,
carbon from fossil fuel added to the atmosphere reduces the proportion
of^14 C the atmosphere contains.
By studying the ratio of the different isotopes of carbon in the at-
mosphere, in the oceans, in gas trapped in ice cores and in tree rings, it
is possible to find out where the additional carbon dioxide in the atmo-
sphere has come from and also what amount has been transferred to the
ocean. For instance, it has been possible to estimate for different times
how much carbon dioxidehas entered the atmosphere fromthe burning
or decay of forests and other vegetation and how much from fossil fuels.
Similar isotopic measurements on the carbon in atmospheric
methane provide information about how much methane from fossil fuel
sources has entered the atmosphere at different times.
the effects ofincreased use of nitrogen fertilisers and of some changes in
climate. The magnitudes of these contributions (Table 3.1 and Figure 3.3)
are difficult to estimate directly and are subject to much more uncertainty
than their total, which can be inferred from the requirement to balance
the overall carbon cycle budget.
A clue to the uptake of carbon by the land biosphere is provided from
observations of the atmospheric concentration of carbon dioxide which,
each year, show a regular cycle; the seasonal variation, for instance, at
the observatory site at Mauna Loa in Hawaii approaches about 10 ppm.
Carbon dioxide is removed from the atmosphere during the growing sea-
son and is returned as the vegetation dies away in the winter. Since there